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Gabriele Galante Ovidio Michilli Ruggero Maspero No-Bake AS WE SEE IT 1

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Page 1: NO BAKE II

G a b r i e l e G a l a n t e

O v i d i o M i c h i l l i

R u g g e r o M a s p e r o

No-BakeAS WE SEE IT

1

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INDEX

PREFACE page 11

1. THE NO BAKE PROCESS page 13

1.1 Process comPatibility page 13

2. THE RESINS AND THEIR POLYMERISATION page 15

2.1. the resin families page 16

2.1.1. General characteristics page 16

2.2. the classification of foundry resins page 19

2.2.1. first GrouP page. 19 Furan resins page 19 Phenol resins and Furan-Phenol resins page 26 urea-Phenol and urea-Furan resins page 27

2.2.2. second GrouP isocyanates -urethane system page 28 The Three comPonenTs TyPe page 28 The Two soluTions TyPe page 29

2.2.3. third GrouP

alkaline Phenol resins page 30

2.2.4 resin aGeinG page 32

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2.3 additives page 33 siilanes page 33 waTer page 33 iron oxide page 33

2.4 Physical and chemical checks on resisns page 35 ViscosiTy - densiTy page 36 reFracTiVe index page 36

3. CATALYSTS AND HARDENERS page 39

3.1 catalysts page 39

3.2 hardeners page 42

3.2.1 esters page 43 The use oF esTers in The alkaline no-Bake sysTem page 43 The Pouring Process page 43 The regeneraTion Process page 44

4. SODIUM SILICATE page 45

4.1 the basic PrinciPles of the Process page 45 The silicaTe-esTer reacTion page 45

4.1.1 settinG times page 46

4.2 the tyPe of sodium silicate page 47

4.3 the tyPe of ester page 47

4.4 additives page 49

4.5 carryinG out the work page 49

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4.5.1 mix PreParation page 49

4.5.2. mouldinG page 50

4.6 silicate checks page 50 chemical checks page 51 Physical checks page 51 mechanical checks on TesT Pieces oF Bonded sand page 51

5. THE SANDS page 55 silica sand page 57 oliVine sand page. 57 chromiTe sand page 57 Zircon sand page 58

6. THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES page 59

6.1 sand characteristics page 59 granulomeTry and Fineness index page 59 sPeciFic surFace area oF The grains page 60 moisTure page 61 Fines’ FracTions page 61 loss on igniTion page 62 The acid demand Value (adV) page 62 The Base demand Value page 62 clay page 63 ooliTe conTaminaTion page 63 TemPeraTure page 63

6.2 the hardeninG Phases page 64

6.2.1. the work time page 64

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6.2.2. the striP time page 65

6. 3 mould aGeinG page 66

6.4 the siGnificance of sand-binder mixtures’ quality control page 66

6.5 environmental and hyGiene considerations page 67

6.6 the comPatibilty of sands, binders and metals page 69

7. RELEASE AGENTS page 75

8. PAINTS page 77

8.1 water based Paints page 77 comPonenTs page 78 Physical requiremenTs page 79

9. NO-BAKE ADVANTAGES AND PROBLEMS page 83 dimensional Precision page 83 Flasks page 83 moulding page 84 The surFace aPPearance oF casTings page 84 core assemBly page 85 residual sTresses page 85 cosTs page 86 casTing qualiTy page 87

9.1 the no-bake contribution to the reduction of faults due to shrinkaGe in cast iron castinGs page 87

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9.1.1 cast iron solidification page 88 The VolumeTric change in casT irons page 89 no-Bake and ducTile iron casTing page 91

9.2 the contribution of thermal analysis

to the evaluation of the tendency of

cast irons to shrink page 93

9.3 mouldinG by Pressure shootinG page 97

9.3.1 the Process chemistry page 98

9.3.2. descriPtion of the mould shootinG Plant page 101 The mixer page 101 The moulding PlanT page 101 gassing - Purging page 106 The BuFFer PosiTion page 106 mould sTriPPing page 107 Passing The moulds To The Pouring lines page 107 horiZonTally Poured moulds page 107 VerTically Poured moulds page 107

9.3.3. the advantaGes of mouldinG with a mould shooter page 108

9.3.4 the advantaGes of vertical PourinG page 109

9.3.5. fields of aPPlication page 109

10. SAND RECLAMATION (REGENERATION) page 111

10.1 the deGree of reGeneration page 114

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11. PATTERN MAKING page 119

11.1 tyPes of construction page 121

12. CASTING DESIGN FOR THE NO-BAKE SYSTEM page 125

12.1 mould striPPinG in the no-bake system

examPles of correct castinG desiGn page 125

APPENDIX definition of no-bake chemical

comPounds page 131 definition of no-bake Physical

ProPerties page 147

Glossary page 151

BIBLIOGRAPHY page 159

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7

CAV. GALANTE GABRIELE

Gabriele Galante belongs to a family which, in the best traditions of the Luino industrial class, continues to carry out a very significant role.His grandfather founded a construction company and his father started his own foundry where young Gabriele, learnt the basic techniques founding stet, once his studies has been completed.Since 1972, when IMF was founded, he has demonstrated his innate design and entrepreneurial capabilities, through the development of the technology, which is the hall mark of IMF in today’s world markets.

As President of IMF he can offer machines and equipment for the application of proven processes, marked by precision, flexibility, modularity and adaptabi-lity: suitable for a wide variety of operating conditions.His successful commercial strategies have led to expansion abroad, and he is also the President of EPF, the French subsidiary; and President of IMF North America, the USA subsidiary.

AMAFOND, the Italian Association of Foundry Machinery Makers, elected him as Association President from 1983 to 1987, a period of integration with analo-gous associations, within a wide International context.He followed this success by becoming President of the European Committee of Foundry Materials’ Producers (CEMAFON) from 1988 to 1991. In this posi-tion he took a broad view and forged connections with similar organisations outside Europe, thus creating wider horizons for exports.

Today, as a member of the Executive Commission of AMAFOND, he is respon-sible for the development of its representative role within CEMAFON, at a cru-cial moment in the process of industrial globalisation.

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8

“No-Bake” as we see it - Part One

DR. OVIDIO MICHILLI

He was born in S. Valentino (Pescara), on the 9th July 1925.

In 1943 he joined the chemical laboratory of the “Fonderia Ansaldo” in Genoa. In 1944 he transferred to the melting departments of the section concerned with cast-iron, light alloys and copper alloys. He contributed on the perfection of the process for the spheroidisation of graphite, through the introduction of magne-sium metal. This process resolved the serious problem of spheroidisation.His innovative approach was a great success both in Italy and abroad.In 1952 he joined the “Fonderie Getti Speciali Colombo Giuseppe di Carlo” at S. Giorgio Legnano.Under the competent management of the owner, and with the professional capacity of Dr. Michilli, this foundry became highly proficient in the production of special cast-iron castings. The metallurgic techniques employed and their originality, became standards for the industry, both in Italy and abroad.In 1956 he was awarded a degree in Industrial Chemistry at the University of Pisa.In 1980, he started his activity as a consultant, both in Italy and abroad.Characteristics: during his working career, entirely spent in the foundry sector, he has worked with enthusiasm and competence, to progressively free foundry techniques from empiricism.

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9

DR. RUGGERO MASPERO

He was born at Carimate (Como), on the 4th May 1932.

He was awarded a degree in Industrial Chemistry at the University of Bologna.Following his military service, he worked in Duesseldorf, Germany, with the Huetteness-Albertus GmbH (at that time Gebrueder Huetteness), until 1972. He was initially a research worker, and later was the manager of the Research and Control Laboratory.After returning to Italy, he directed the Technical Laboratory of the Research and Development sector of the Satef Huettenes Albertus SpA, from 1972 to 1994. In this period he edites several technical publications and was a speaker at many Congresses and Fairs.In 1992 he was awarded the “A Dacco prize, for Italian foundry work”, following which he addressed the International Congress held at the Hague (1993), giving the official Italian paper.

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10

“No-Bake” as we see it - Part One

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11

PREFACE

PREFACE

In the face of a continuous and increasing pressure to produce quality castings; the foundry technician has a daily need to reduce production costs. He is also faced with a lack of skilled labour. The empirical approach, which nowadays is increasingly being repla-ced by technology, is the uncertainty element still typical of found-ry work.The “No-Bake” moulding process has made a great contribution to resolving these uncertainties.IMF has been working in the plant sector of “No-Bake” technology, as a partner of casting foundry technicians, for more than twenty years. This “partnership” extends from the foundry floor to the technical offices, and has resulted in technical solutions and plants which are widely recognised for their quality.Through this “practical guide” IMF aims to widen its contribution, by classifying its experiences and by uniting them with the latest spe-cific publications in the field. Through this work IMF also expects to make a significant contribution to the training of foundry techni-cians.The manual has three parts, each contained in a separate volume. They are all easily consulted and are complementary to one ano-ther.The first volume is instructional. It contains the essential basic theo-retical concepts and the more important technical subjects. This volume will be of most interest to foundry engineers, to methods’ office technicians and to those students who intend to specialise in the foundry sector.(1)

The second volume is a practical manual, which contains the most important technical information connected directly or indirectly with the “No-Bake” process.It is divided into two parts; the first of these describes the charac-teristics of typical materials and their uses, the second consists of easy reference technical schedules.The third volume deals with equipment.We have left out some technical information on the basis that it is common knowledge, whilst other information has been deliberate-ly repeated, partly in order that it should be completely understood and partly to make the various parts of the manual independent of

(1) In the appendix which follows the Part I text, there is a glossary of chemical terms and com-pounds, and of physycal phenomena, connected with the “No-Bake” process.

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12

“No-Bake” as we see it - Part One

one another. The on-going developments in the field of foundry binder chemistry, mean that this treatise cannot be considered to be final; nevertheless we believe that its contents will enable you to follow the development of “No-Bake” technology correctly.

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13

THE NO BAKE PROCESS

1. THE NO BAKE PROCESS

Moulding using the “No-Bake” process started in the sixties, using sand cold bonded with either urea-furan resins, or inorganic bin-ders of the silicate type. Its introduction made important changes in the production of cores; and later in the production of flasks and flaskless moulds.The first obvious effect, much appreciated by the foundry operator, was the “Increase in Production” by 40 to 60%, especially in the moul-ding process for large castings. The second advantage was in the “Quality Factor” and the increased certainty that the casting would be a good one, despite the fact that the typical moulding defects were replaced by a series of new and for the moment, unrecognised faults.The third advantage was the possibility, in general, of using less skilled labour.

1.1 Process comPatibility

Moulds can be made using sand and binders with different characteristics and also, mixed with sand reclaimed from cores. It is therefore necessary to check the compatibility of the processes, especially when regenerated sand is used, or when new processes are introduced. The characteristics of the sands from the different processes must be assessed, especially for the amount of fines and the degree of acidity or basicity in the sand.The morphology of new sand needs to be evaluated for grain fragility characteristics, to avoid the need to use greater amounts of binder and hardener as the number of reclamation cycles increases. In fact, if the sand has sharp angles and is fragile, there is a saving in the use of binder at first, due to the small interstitial volume. As the number of regeneration cycles increases, the amount of binder used increases due to grain break down and the increased fines fraction. In a well regenerated sand this problem is much reduced.

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“No-Bake” as we see it - Part One

You are referred to the technical schedules R9/a/b/c, S2, S3, in Part II, for a check of process compatibility.

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15

THE RESINS AND THEIR POLYMERISATION

2. THE RESINS AND THEIR POLYMERISATION

Foundry resins are organic compounds, normally liquid, and have molecules which mainly consist of carbon, oxygen, hydrogen and nitrogen atoms. These molecules, called monomers, are simple molecules which can be likened to rings. In the resin production phase active centers form and the molecules join together in long, mainly two dimensional chains, under the combined action of heat and a catalyst. In the application phase this reaction continues due to the addition of a second catalyst; and a rapid and three dimen-sional chain formation is achieved; resulting in a rigid and dense network.The macromolecules thus formed have a very high molecular weight and are known as polymers (if they are formed from identical molecules) or copolymers (if they are formed from more than one type of molecule). Their configurations give rise to the term reticula-tion.When this reaction takes place in a sand, the network formed holds the sand grains together in such a way that a rigid skeleton is formed.The reaction of chain formation by monomers as described above, is called “polymerisation”.The resins mainly used in the “No-Bake” process are formed by a polymerisation of monomers (poly-condensation) and the co-polymers formed are phenoplasts (from phenol and formaldehyde), aminoplasts (from urea and formaldehyde) and furfu-ryl copolymers (from furfuryl alcohol, phenol, urea and formaldeyde). According to the type of polycondensate, a further polymerisation is necessary at the point of use in the foundry; and this may be one of two types :• Addition polymerisation : also known as poly-addition, is the pro-

cess in which the reaction product repeats the monomer unit and the molecular weight of the product is equal to the sum of the number of monomer units which form the poly-mer.

• Condensation polymerisation is the process in which organic molecules with individual low molecular weights (mono-mers), form heavy macromolecules (polymers). The mono-mer units which are repeated in the polymer chain contain

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16

“No-Bake” as we see it - Part One

fewer atoms than the original monomer molecules. This is due to the elimination of subsidiary compounds at chain formation, usual-ly water.Polymerisation starts slowly when the resin is mixed with the sand and speeds up continuously until the reaction is completed.The polymerisation process is disturbed by the movement of the sand particles due to the mix being handled, as these movements break parts of the polymerised mesh during its formation. This wastes resin, reduces the flow characteristics of the sand mixture and reduces the mechanical strength of the cured mould.It is therefore advisable to control the polymerisation (or hardening) process during the mixture preparation phase, in order to prevent premature resin chain formation. This means that the catalyst addition needs to take account of the mixture preparation time, so that the polymerisation takes place in harmony with the several phases of moulding.

2.1. the resin families

The rapid growth of the number of types of binders, coupled with the plant developments in the traditionally complex and divergent foundry sector, makes it impossible to give precise indications or off-the-shelf solutions, for the choice of the most suitable binders.This statement is also true in the context of this publication, given its instructional and informative nature.Nevertheless, the data and information given below make a useful contribution for the best choice of products and plant, having taken account of specific factors such as : moulding materials, the type of casting, the production rate required, the equipment, the skill of the labour force and environmental impact.

2.1.1. General characteristics

The choice of binders is largely determined by the required produc-tion rate and the dimensions of the mould and/or the core.

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High production rates require rapid and constant hardening times, whilst diametrically opposite conditions are required by large and heavy moulds and cores. These require long initial hardening times and therefore a long “working life” (bench life) to enable the mould to be filled in an acceptable working time, that is before the polymerisation has appreciably progressed towards completion.The flow characteristics of the sand/resin mixture must allow the pattern to be copied faithfully and allow a satisfactory level of com-paction. In the case of complicated pattern models, with so-called “shadow zones” (parts where the sand compaction is not easy), it is advisable to use vibrating compaction tables. Good compaction enables the percentage of binder to be reduced without reducing the mechanical strength of the mould.Resin viscosity plays an important role as it governs the capacity of the binder to cover the grains of sand.The inability to “Rap” the pattern, except by vibration, means that special removal equipment is needed. This enables the maximum use to be made of one of the “No-Bake” system advantages, the minimum deformation of the mould cavity impression.The decomposition rate of the resin during pouring determines the amount of gas produced in the mould cavity.The gas quantity cannot be easily controlled due to the organic nature of the binder. It is therefore necessary to minimise the binder quantity and the gas contact with the liquid metal (see fig. 2). This is particularly important when hydrogen and nitrogen are present as they are in the very reactive “nascent” state. In these conditions they are easily absorbed by the liquid metal, and may cause small blow holes in steel and cast iron castings.Sulphurous gases arising from the decomposition of the catalysts, may cause morphological changes of the graphite on the surface of ductile iron castings.However, not all cast alloys are affected to the same degree by gases.Environmental considerations require that the bonded sand has to be reclaimed (regenerated) after use and recycled. This process consists of removing the hardened resin film which covers the sand grains.If this is to be carried out by a attrition process, it is essential that the film should be easily removed. This requirement must be borne in mind when choosing the type of

17

THE RESINS AND THEIR POLYMERISATION

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resin to be used.The possible need to use a reinforcing framework, is a further fac-tor to be added, when selecting the most suitable binder.A good resin does not normally require the mould to go through a drying stage.The safety and environmental protection problems arising from the several stages; from moulding, to manipulation, to pouring and knocking out, must be minimised. In any event, safety precautions and environmental pollution levels must conform to the legal limits and regulations.In the appendix of Part II, the technical schedules and their usage instructions are given, both for the resins described and the sand/resin mixes; together with advice for their best use.The same schedules also give the safety precautions to be taken during the handling of the binders, together with other information about safety and environmental problems. Finally, there is also information and data about release agents and paints, to assist in selecting types which are compatible with every type of binder described.

18

“No-Bake” as we see it - Part One

Fig. 2 - Gas evolution at 1,010°C by different resin types

1. 1,5% alkyd resin/20% isocyanate2. 1,5% polyurethane resin3. 1,5% furan resin/30% toluene sulphonic acid4. 1,5% phenol resin/30% toluene sulphonic acid5. 1,5% alkaline phenol resin

cc O

F G

AS

PE

R G

RA

MM

E O

F S

AM

PLE

TIME, in seconds

15 SECONDS CONTACT

1. alkyd resin

2.polyurethane resin

3. furan resin4.phenol resin with ind. acidalkaline phenol resin

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19

THE RESINS AND THEIR POLYMERISATION

2.2. the classification of foundry resins

The most popular resins used for the “No-Bake” process can be classified into three groups:• Thefirstoftheseiscomposedofresinscatalysedbyacids,

such as furan resins (see fig. 2.1), phenol resins and phenol-furan resins (see fig. 2.2), and urea-phenol resins (see fig. 2.3). These can be used singly, or as combinations if speci-fic characteristics are required to meet production needs.

• Thesecondgroupiscomposedofisocyanates,whichpoly-merise by addition with poly-alcohols to form polyurethanes (see fig. 2.4).

• Thethirdgroupofresins,hasonlybeen inuseforashorttime and consists of alkaline (basic) phenol resins (see fig. 2.5). This group completes the range of the resins most commonly used in the “No-Bake” process; and any type of sand can be used with them including olivine sand (given the basic nature of this sand it cannot be used with resins catalysed by acids).

2.2.1. the first GrouP

Furan, phenol and phenol-furan resins, are those most commonly used in the “No-Bake” process.These definitions are only generic and indicate the type of basic resin components. These are respectively furfuryl alcohol, phenol and mixtures of these. Normally other compounds are also used to complete the formulation and modify the resin, to obtain the requi-red characteristics in the final product.

Furan resins

The adjective “furan” describes the basic component of the resin. This is furfuryl alcohol and its polymerisation reaction (condensation) is shown in fig. 2.1. It is soluble in water and has a low viscosity. It is therefore easily mixed with sand and gives optimum coverage of the sand grains.

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20

“No-Bake” as we see it - Part One

Fig. 2.1 - Furan resin formation by the polymerisation of furfuryl alcohol. The furan nuclei are connected by methy-lene links to form linear chains. The condensation reaction is exothermic

CH

CH

HC

CH

C

CH

CH

2OH

O

O

O

O

O

O

OC

C CH

HC

CH

HC

CH

H2C

CH

2H

C

HC

CH

C

CC

CH

2

CH

CCH

2

HC

CH

2C

HH

CC C

H

O

O

CH

2

C

CH

2nx

+ C

AT-

H2O

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21

THE RESINS AND THEIR POLYMERISATION

2

m

HO

H2C

H O CH 2 OHC

H 2O

H +

OH H

CH 2

OH

n

HO

H2C

+ C

AT-

H2O

OH

H H O CH 2

HO

H 2C

HO

H 2C

OH

CH 2

OH

CH 2

OH

OH

H OH

HO

H 2C

HO

H 2C

CH 2

O H

CH 2

OH

CH 2

OH

OH

H H O CH 2

HO

H 2C

HO

H 2C

OH

CH 2

OH

CH 2

OH

OH

CH 2

OH

+C

H 2O

H+

CAT

O

OH

CH 2

CH 2

OO

H2O

Fig. 2.2 - Polymerisation by condensation reactions: A) phenol resin=polymerisation of phenylmethanol(formed by the condensation of phenol with formaldehyde). B) phenol-furan resin=polymerisation of phenylmethanol with furfuryl alcohol.

A B

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22

“No-Bake” as we see it - Part One

Fig. 2.3 - Urea-phenol resin formed by the polymerisation of mono methyl urea and phenylmethanol

CH 2

OH

OH

++

CAT

N CO

CH

2OHC

H2

NH

CO

CH

2

NO

H

CH

2OH

+H

2O

HN

CH

2

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23

THE RESINS AND THEIR POLYMERISATION

Fig. 2.4 - Polymerisation reaction by addition between a resol and an isocyanate.

+C

AT[C

2H5] 3

N

R

R 1

OH

+ O

CN

R 2

R

R 1

OC

H N

O

R 2

Pol

y-is

ocya

nate

Pol

yure

than

eP

heno

l res

inb

enzy

leth

er t

ype

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24

“No-Bake” as we see it - Part One

OK

HO

CH 2

OK

OH

OK

CH 2

CH 2

O

OH

CH 2

CH 2

O

CH 2OK

O CH 2

CH 2

CH 2

CH 2

O

CH 2

OK

OC

H 3

+ H

CO

HO

CH 2

OH

OH

OH

CH 2

CH 2

O

OH

CH 2

CH 2

O

CH 2OH

CH 2

CH 2

O

CH 2

OH

CH 2

CH 2

O

+ C

H3

OH

+

HC

OK

O

Fig. 2.5 - The hardening reaction of an alkaline resin with methyl formate (procedure using a gaseous ester) and the pH changes in the several phases.

a) In

dic

ativ

e re

actio

n

Alk

alin

e re

sol

Met

hyl f

orm

ate

Pota

ssiu

m fo

rmat

eM

etha

nol

Inso

lub

le m

acro

-mol

ecul

e

b) S

chem

atic

pha

se r

eact

ions

M

+O

PF-

+

R

OO

CH

p

H v

alue

A

lkal

ine

resi

n

Est

er

12 -

14

tran

sitio

n p

hase

12 -

14

(H

OP

F) n

+

M

+O

OC

H-

+ R

OH

7

p

olim

er

al

kalin

e sa

lt

alco

hol

➙ C

omp

lex

OK

HO

CH 2

OK

OH

OK

CH 2

CH 2

O

OH

CH 2

CH 2

O

CH 2OK

O CH 2

CH 2

CH 2

CH 2

O

CH 2

OK

OC

H 3

+ H

CO

HO

CH 2

OH

OH

OH

CH 2

CH 2

O

OH

CH 2

CH 2

O

CH 2OH

CH 2

CH 2

O

CH 2

OH

CH 2

CH 2

O

+ C

H3

OH

+

HC

OK

O

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25

Titolo

To make the furfuryl alcohol suitable for use as a foundry binder it is reacted with phenol, or urea or formaldehyde, according to the formulation. The copolymers which are formed give resins with better characteristics, as follows : • betterheatresistance• lessgasformationwhenpouringtakesplace• betterstabilityofmoulds’propertiesovertime

• longerpolymerisation(setting)times

Urea in particular, reduces excessive hardness and fragility and also assists in heat decomposition. However, the quantity of urea that can be used is limited by the nitrogen level which can be tole-rated in the metal casting.The furan resins are classified into three groups, according to the percentage of nitrogen contributed by the urea and by their water contents :

The first type is normally used for casting steel and copper based alloys which will not tolerate nitrogen, as this gas gives rise to “pin-hole” defects (see figs. 6.3 to 6.6 page 71-72).Resins with a low nitrogen content are generally used for casting spheroidal cast iron, however, always taking into account the ten-dency of these cast irons to absorb nitrogen.Resins with a medium to high nitrogen content are used for casting aluminium and for casting common cast iron, as these do not have special requirements.The furfuryl alcohol acts as a solvent for the urea and formaldehyde compounds, in addition to its main function as the monomer which binds the sand through its polymerisation.Furan resins have a very wide field of applications. This is because the proportions of the furfuryl alcohol and urea-formaldehyde com-ponents in the initial formulation, can be varied to achieve the best binding characteristics for the particular application.

RESIN TYPE N2 H2O

Without nitrogen 0% 0,5%

Low nitrogen 0,1 - 2% 5 - 15%

Medium nitrogen >2% >15%

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26

“No-Bake” as we see it - Part One

We list their main characteristics below :• Thesand/furanresinmixturehasexcellentflowcharacteri-

stics;• There is a low content of chemical reaction water. This

means that the curing time is less severely affected than with other resins, both as regards the quantity and type of catalyst required and the quality and temperature of the sand. The tendency for the mould to skin harden, is also reduced. This skin hardening phenomenon is due to loss of water by evaporation. This happens more quickly at the sur-face than in the centre of the piece;

• Themouldsbondedwithfuranresinmaintaintheirmecha-nical characteristics well even when hot. This enables the ratio of sand to casting to be improved with a consequent cost saving. This partly offsets the greater cost of this type of resin.

• Thepolymerisedresinfilmonthesandgrainsiseasilyremo-ved during mechanical regeneration.

Phenol resins and Phenol-Furan resins

The phenol resins used in the “No-Bake” process are produced by a poly-condensation reaction between phenol and formaldehyde under basic conditions. The formaldehyde is in a small excess; and this leads to the initial formation of phenylmethanol (phenoplast). The resol formed polymerises by further condensation, due to the addi-tion of an acid catalyst in the foundry. This gives a polymer (resin) with excellent mechanical and heat resistance characteristics.These resins were initially used in the hot process; and their use was later extended to the “No-Bake” process in cost competition with the furan resins. This was the result of a series of modifications which improved their technical and environmental characteristics. Specifically, the viscosity was reduced to enable the sand grains to be coated more easily, the free phenol was reduced to around or below 5% and the free formaldehyde to below 0.5%. These resins give off an unpleasant smell.Compared with the furan resins, their polymerisation and harde-ning are affected by several factors, principally any temperature variation of either the sand or the pattern plates, or by exposure of

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27

THE RESINS AND THEIR POLYMERISATION

the mould to air.This is because the resin sets very quickly and the setting time is strongly affected by temperature changes. It is therefore difficult to control the setting time, or to keep it constant. Again, the surface of the mould sets more quickly than the internal parts.Good phenol resins do not contain any nitrogen (which cannot the-refore increase when the sand is regenerated), and they are also relatively cheap. Phenol resins may contain furfuryl alcohol as a solvent in many formulations. This improves the mixing characteri-stics. When furfuryl alcohol is used, it also acts as a binder due to its monomeric nature, whilst if it is used in appropriate concentra-tions in the resin formulation, it forms compounds known as phenol-furan resins. These resins combine the characteristics of each com-ponent, and are therefore nitrogen-free, and withstand heat well.Condensation with urea and addition of silane improves cold strength and therefore the knock-out characteristics as well.When the absence of nitrogen must be matched with low cost, either phenol resins or phenol-furan resins with a low urea content are indicated.Phenol resins are slightly hygroscopic and withstand heat well. However, the expansion of the sand may not be contained and this may lead to surface cracks in the mould.These resins have high mechanical strength when cold and this reduces the friability of the mould. However, this may create pro-blems when knocking out. The quantity of gas produced and the speed at which it is produced at pouring, on colling and at sha-keout, are modest. The phenol resins have their maximum gas production slightly later than that of furan resins.The polymerisation reactions of phenol and phenol-furan resins are both shown in fig. 2.2 (page 21).

urea-Phenol resins and urea-Furan resins

Urea resins are formed by the reaction between urea and formalde-hyde. This gives mono and di-methylurea (both anhydrous and hydra-ted), and these polymerise through reciprocal and complex reac-tions to give urea resin.Phenol and furan poly-condensed are normally part of the formu-lation in which they form urea-phenol (see fig. 2.3 page 22) and urea-

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“No-Bake” as we see it - Part One

furan co-polymers.These resins have a relatively low heat resistance and also develop nitrogen-rich gases. Figures 6.3, 6.4, 6.5 and 6.6 (page 71-72), show casting defects which can be caused by nitrogen.

2.2.2. the second GrouP: isocyanates-urethane system

The second group of resins is made by reacting poly-isocyanates with polybenzylphenylether (a resol), using pyridine or an amine as catalyst. The polymerisation reaction is an addition reaction (poly-addition); a resol of the benzylether type reacts with the isocyanate to form polyurethane, without any secondary products being formed (see fig. 2.4 page 23). The name polyurethane resin is derived from this compound.There are two types of formulations on the market : one with three separate components, one with two solutions.

The Three seParaTe comPonenTs TyPe

The resin, the isocyanate and the catalyst, are supplied separately and are added to the mixer through three separate metering pumps, one for each component (see fig. 2.4/A page 29).This presentation is the one which is most widely used as it gives very flexible moulding cycles. It therefore enables the widest range of requirements to be satisfied. For example :- different types of patterns;- different types of alloys;- different production cycles from moulding to pouring;- climatic variations both seasonal and daily;- variable moulding programmes (automatic, semi-automatic, and manual);The amount of catalyst to be used is always small, and the mete-ring pump for it must be a high precision type.

The Two soluTions TyPe

The resin and the catalyst form a separate solution to that of the

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THE RESINS AND THEIR POLYMERISATION

isocyanate. The procedure with two solutions requires constant moulding conditions: for the production cycle, the type of casting and the environment.The reason for this uniformity, lies in the polymerisation time and this governs the production cycle. The quantity of catalyst added to the binder must also be standardised. The “work time” is there-fore fixed by these when the binder is used.The two solutions system is recommended when the plant is not equipped with a high precision metering pump for catalyst addi-tion, however, but this restricts the system flexibility.Normally, the two solutions method requires as many resin types as there are moulding cycles, or temperature variations.The supply programme must take account of the fact that whilst the resin appears to stop the action of the catalyst, over a longer period the polymerisation continues to completion, prior to use.The reaction of the two parts begins slowly and leads to the forma-tion of polyurethanes, after they have been added to the sand. There are therefore a few minutes during which the mixture runs well. The initial slow reaction rate then accelerates quickly and the hardening occurs almost simultaneously at the surface and in the

Fig. 2.4/A - Plant layout for the urethane “No-Bake” system, showing the three separate compo-nents’ storage and components feeds.

From the tank

Continuous mixer

Dosing pump

200 l. drum of catalyst

Gear pumpsIsocyanate

Resin

From the tank

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“No-Bake” as we see it - Part One

centre of the piece. This means that the pattern stripping leaves a perfect mould; and that the work time / pattern stripping time ratio is very good.Total polymerisation, that is the attainment of the maximum mecha-nical strength, takes less than one hour. During the pour, the polyu-rethane mould releases “lustrous carbon” and this may lead to an increase of carbon in the surface layers of cast steels. This problem can be overcome by adding 2 to 3% of black iron oxide to the sand mixture, when the sand is being mixed.Isocyanate releases nitrogen at pouring and this may cause “pinho-les” in cast-iron and steel castings.The characteristics of the sand / urethane resin mixture are :• Productionflexibilityduetotheexcellentratioofworktime

to pattern stripping time;• Themouldisnotaffectedbymoisture,norbywaterpaint,

impurities, variation of temperature or the pH of the sand;• Ithasagoodresistancetoheat;• Itiseasilyknockedout.Specific isocyanate solvents are compatible with polystyrene. Therefore polystyrene patterns can be used with the isocyanate process. The flow properties which the binder confers to the sand need to be improved by using a vibrating table.We recommend that 2 to 3% of black iron oxide or 1 to 2% of red iron oxide should be added to the mould mixture, to reduce the risk of gas defects in the casting when pouring steel. The binder should be considered to be toxic.

2.2.3. the third GrouP: alkaline Phenol resins

The components used in the so-called “alkaline resin” process are an ester and an alkaline resol and they are basic. The resol is a resin which is formed at the first stage of the conden-sation process and it is a complex mixture of isomers and/or other compounds.The chemical reaction of these components is not catalysed. In contrast to catalysed reactions therefore, the amount of reaction

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THE RESINS AND THEIR POLYMERISATION

product and the speed of its formation are directly linked to the quantity and the type of the reagents used. The reaction products are a phenol resin, an alkaline salt and an alcohol.The phenol resin formed polymerises partially at normal tempera-tures and this action is completed by the heat of the pour.In fig. 2.5 (page 24), the development of this reaction is shown, together with the pH of the phases.The special characteristics of basic resins are :• Theyarelessaffectedbytheacidoralkalinenatureofthe

sand than acid catalysed resins. They can therefore be used without encountering problems, even with olivine sand.

• Thesettingtimevarieswiththequalityandnotthequantityof the hardener, as the chemical reaction which gives harde-ning is not a catalysed reaction. It follows therefore that the dosage of the hardener need not be precise, within reaso-nable limits. Again the hardening reaction can be started with a wide range of hardeners and these enable the harde-ning process reaction to be controlled. In practice, the “work time” of the sand mixture can be fixed at value betwe-en a few minutes and an hour.

• Themouldisnotrigidasthetotalpolymerisationonlyhap-pens when the mould is strongly heated by the casting pour. The poured metal therefore finds the mould in a thermopla-stic condition.

This has the following advantages :• Themixerscanbecleanedmoreeasily;• Optimumknockingout as themouldstill hasadegreeof

flexibility;• There iscompensationfor theheatexpansionof thesand

and consequently there are fewer of the defects called “fins or veins” caused by superficial cracks in the mould;

• There is improved resistance to erosion, thanks to theinstant rigidity of the surfaces when they come into contact with the liquid metal;

• Theamountofgasformedatpouringandthespeedofitsformation are lower than with traditional resins; and the gases do not contain either nitrogen or sulphur. These cha-racteristics make this binder ideal for moulds used for casting steel and spheroidal cast-iron.

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We wish to stress that as the moulds only harden completely when metal pouring takes place, they are not very strong mechanically. The mechanical regeneration of the sand used requires a very effective regeneration plant as the alkaline residues stick firmly to the sand grains.

2.2.4 resin aGeinG

As we have already said, resins are solutions of macromolecules, and in the solvent mixture there are molecules which continue to polymerise slowly at normal temperatures.When the resins are used, the poly-condensation reactions are stimulated by the catalyst used and speed up.It is therefore essential to comply with the suppliers’ warnings and storage advice to prevent the resins polymerising before they are used.The most obvious marker of polymerisation, is that the resin beco-mes more viscous.The drums must be sealed hermetically to prevent solvent loss to air.The following analyses enable a rapid check to be carried out to determine the state of the stored resin :• Refractiveindex;• Viscosity(toregisteranychanges);• Decrease of the bending resistance over 24 hours, of a

sand/resin mixture, when compared with a standard mixtu-re;

• Amixingchecktoverify that theresinmixeswellwiththesand.

As ageing changes the concentration of free formaldehyde and water, it is advisable to determine their concentrations chemically.The changes registered when a resin ages are irreversible.Furan resins age more slowly than the other types.

2.3 additives

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THE RESINS AND THEIR POLYMERISATION

A number of additives are used in the “No-Bake” system formula-tions. They are used to meet technical and cost requirements, by altering the characteristics of the basic components of the mix. The most important of these are listed below, with indications of their action and their use.

silanes

The silanes used in foundry resins have a common formula of R.Si (OR’)3.They are added to reduce the hygroscopic characteristics of the mould and to improve the resin wetting of the sand grains. The latter improves the mechanical strength of the finished mould.

waTer

Whilst there has to be water present as it is a product of polymeri-sation, it must be kept as far as possible, within the limits imposed by the process. Therefore any increases due to the addition of moi-sture with other essential materials, must be kept to the absolute minimum.Apart from the cost aspects, water reduces the hardening time of the mixture, can create blow hole defects in the castings and dra-stically reduces the mechanical strength of the moulds (see fig. 2.6).If follows that moulds tend to harden more rapidly at the surface than in the centre due to the loss of water through evaporation (see fig. 2.7).The application of water based paints must also be carried out as late as possible, in order to guarantee the greatest degree of poly-merisation of the resin.

iron oxide

Iron oxides are mixed with the sand to reduce the occurrence of the following defects in castings :• “Pinholes”duetogasabsorptioninthesurfacelayers;• Cracksduetotheheatexpansionofthesand;

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“No-Bake” as we see it - Part One

Fig. 2.7- The differences of hardening time in surfaces exposed to air and not exposed to air in products using different amounts of binder.

Har

dne

ss G

F

Mixture D Mixture C Mixture BMixture with self setting

oil

Time in minutes from packing the coreHardness of exposed surfacesHardness of surfaces against the corebox

Fig. 2.6- The effect of water on the resistance to bending stress, of a sand mixture containing 1.3% of resin.

Tran

sver

se s

tren

ght

lbf/

in2

H2O%

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THE RESINS AND THEIR POLYMERISATION

• Carbonenrichmentinthesurfacelayersduetotheforma-tion of “lustrous carbon”.

The action of iron oxide is the subject of some debate. The widest held theory is that a compound which is highly resistant to heat is formed on the surface of the sand grains. It is also believed that Fayalite is formed due to a reaction with the iron oxide arising from the oxidisation of the casting metal. This oxide is always present in regenerated sands. The compound formed has a low melting point and fills the spaces between the sand grains, thus preventing penetration by the poured metal.The iron oxide used in the production of cores and moulds can be one of two types :Magnetite (Fe3O4), which is black. 2 to 3% is added with respect to the amount of sand.Haematite (Fe2O3), which is red. 1 to 2% is added with respect to the amount of sand.In choosing the most suitable type of iron oxide to use, there is a preference for using the black oxide, especially for the production of large steel castings.

2.4 Physical-chemical checks on resins

The uniformity of the resins’ characteristics is clearly important for the maintenance of production quality and production rates. It is therefore necessary to determine both the tests to be carried out and their frequency. It is also necessary to agree the methods and the acceptable test results’ variations with the supplier, so that there is agreement on an acceptable quality level.This is indispensable given the variety of products, their different uses and the differences between the analytical methods emplo-yed. The checks carried out are both physical and chemical and some of them require special equipment and have complex proce-dures.Simple checks are given below, which enable sufficient information to be gathered to judge whether essential characteristics conform to requirements.

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ViscosiTy-densiTy

It is advisable to carry out the determination of the viscosity of very viscous resins (above 1100 m. Pas.) by using a roto-viscometer instead of Ford’s cup.Variations in excess of 10% above the specified viscosity value, and in excess of 3.5% above the specified density, constitute unacceptable quality.The density is read using an instrument with an appropriate scale.

reFracTiVe index

The refractive index is one of the significant characteristics of resin quality. It is a very useful indicator for evaluating the degree of poly-merisation, the impurities and the amount of water (see figs. 2.8 and 2.9).The simplicity of the instrument and of the analysis, means that an initial selection of products can be easily carried out. Variation in the refractive index of more than +/- 0.05 units, indicates the need to carry out further tests.

Fig. 2.8 - The change in the refractive index of resin, as a function of condensation, measured at constant temperature.

Ref

ract

ive

ind

ex 2

0° C

ηD

Time in minutes at 70°C ± 3

resin 1

resin 2

resin 3

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THE RESINS AND THEIR POLYMERISATION

The refractive index measurements are carried out with an Abbe’ refractometer fitted with a constant temperature prism.

Fig. 2.9- Changes in the refractive index of resin, as a function of the percentage of water.

% water in the resin

Ref

ract

ive

ind

ex (2

0°C

ηD

)

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“No-Bake” as we see it - Part One

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CATALYSTS - HARDENERS

3. CATALYSTS - HARDENERS

Moulding with the “No-Bake” process is used for a wide variety of types and quantities of castings. Castings may be very small or very large, they may be “one-off” or in long series made on high rate automated equipment. The binders are therefore required to have constant hardening times which can be synchronised with the various phases of the moulding production cycle.The catalysts and the hardeners produce this conformity, and take into account the external variables such as temperature, environ-mental humidity, etc.

3.1 Catalysts

The term catalysis is used to describe the influence of substances called catalysts on the activation energy value and consequently on the speed of chemical reactions. The action of the catalyst does not alter the free energy (heat) involved in the reaction in any way, even though they probably take a direct part in the reaction. At the end of the reaction the catalyst is unchanged(2). The term “catalyst” is often replaced (incorrectly !) by a synonym such as “acid” or “hardener” or “accelerator”. In practical terms the catalyst carries out an important function in speeding up the poly-merisation of the binders, and in its effect on specified technical moulding times.The temperature of the sand, of the patterns and the environment (if high), all act synergically with the catalyst on the speed of the reaction, whilst the quantity of water present acts in opposition to the effect of the catalyst (see fig. 2.6 page 34).The action of both factors must therefore be taken into considera-tion when determining the type and quantity of catalyst to be used, to achieve a polymerisation rate compatible with the phases of the productive process; and especially with the moulding process. Remember that a too rapid polymerisation leads to premature har-dening and the mould will be friable, especially at the external angles. In jargon this is called “burnt”.The catalyst needs to be added to the sand before the resin and it

(2) - The action of positive catalysts on particles, is comparable to that of lubricating oils: they reduce their resistance to motion.

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must be mixed very evenly throughout the mass. Never mix resin and catalyst without sand as the exothermic reac-tion is so violent, it is like an explosion.If the catalyst is diluted this makes the distribution easier, however this slows down the reaction rate. If it is necessary to dilute the catalyst, always add the catalyst to the water, never the contrary. The best solution is to buy the catalyst at the correct dilution.An excess of acid can accumulate in sands used many times without correct regeneration.In addition to its effect on the mould hardening process, it can cause superficial defects of the type shown in fig. 3.1, due to a reaction between the metal and the mould. As can be seen, the upper part of the casting has a normal appearance (it was made

Fig. 3.1- Superficial casting defects (known as the orange peel effect) due to reaction between the metal and the mould, arising from the use of sand which has not been correctly regenerated.

using new sand

using highly contaminated

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with new moulding sand), whilst the lower part is pitted (it was made with poor quality recycled sand).Figs. 3.2 and 3.3 show the variations in mechanical strength of a sand mixture as a function of time, when different percentages of

41

CATALYSTS - HARDENERS

Fig. 3.2- Changes in mechanical strength with different catalyst levels, as a function of time (PTSA=paratoluensulphonic acid).

Fig. 3.3- The mechanical strength of samples (all made from the same sand resin mixture., (1) with 30% catalyst at 25°C - (2) with 80% catalyst at 8° C), as a function of time.

Time in hrs

Time in hrs

30% catalyst at 25°C

80% catalyst at 8°C

Com

pre

ssiv

e st

reng

ht lb

/in2

Com

pre

ssiv

e st

reng

ht lb

/in2

(I)

(II)

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catalyst are used, both at constant temperature and variable tem-perature.Each lot of catalyst must have its concentration checked, either by direct titration, or indirectly by measuring the density. The dosage must always be checked at the time of use. This is essential given the determining effect of the catalyst on the poly-merisation process and on the uniformity of the production process rate.Organic sulphonic acids are the most commonly used catalysts with furan and phenol resins, either paratoluenesulphonic acid or benzene-sulphonic acid. These are replacing the use of mineral acids such as phosphoric acid or sulphuric acid to an ever greater extent.Phosphoric acid at a concentration of 70 to 80% is still used with furan resins, however its use is dying out due to the difficulty of removing it both by heat (starting with the pour) and by the regene-ration processes. Furthermore, it has longer reaction times than those which can be achieved using organic acids.Sulphuric acid is used as an activator in synergic combination with other acids.All sand types are compatible with acid catalysts, except olivine sand (due to its basic characteristics).The urethane resins are catalysed by pyridine derivatives (a basic organic compound) and these can be added to the resin at the binder production phase. This determines the speed of reaction and it cannot be altered during use.The viscosity of the “phenol resin/catalyst” mixture is affected by temperature; and it is recommended that when it is used there should be temperature control. It is also recommended that it should be placed in the mixer before adding the isocyanate.The technical schedules in Part II give the characteristics of the catalysts normally used with the resins described. The normal pre-caution in handling acids should be followed for catalysts.

3.2 hardeners

Hardeners are chemical compounds which, unlike catalysts, take part in the chemical reaction as specific components. They are

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CATALYSTS - HARDENERS

used in the correct stoichiometric amounts to form a solid compound with the other components. Esters are the most commonly used hardeners.

3.2.1 esters

The combination of alkaline phenol resin - ester appeared during the mid 80’s and took a small share of the “No-Bake” market. Its market share was limited by the limited mechanical strength of the moulds made with it. This was due to the low concentration of the resin in its natural solvent (water).The typical arrangement of the atoms in an ester is shown by the following formula :

R-C-0-R1

II 0

R and R1 are aliphatic radicals whose precise nature can be varied. Esters which are typically used are : acetates (di-acetates and tri-acetates of glycols and triacetin) and propylene carbonate. Generally speaking, esters are formed by the reaction of an alcohol or glycol with an organic acid, and water is produced as a by-product.

The use oF esTers in The alkaline “no-Bake” sysTem

The quantity of ester used is 15 to 20% of the quantity of resin. The quantity of resin used is between 1,4 and 1,8% of the quantity of sand.The types of hardener which can be used, enable the mould to be stripped out from 12’ to 15’, at 20°C.

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The Pouring Process

The heat given out by the metal breaks down the resin andhardener molecules. The combustion products are those normally produced by the strong oxidation of organic binders : carbon-di-oxide, carbon-mon-oxide, water vapour, saturated and unsatura-ted hydrocarbons (both aliphatic and aromatic).It is not possible to make either qualitative or quantitative forecasts, it is necessary to make specific analyses. These compounds are volatile and leave the system, while the alkaline ions from the ester salts remain behind. These diminish the refractory properties of the silica sand and make its regeneration difficult.

The regeneraTion Process

Heat regeneration is not possible as the alkaline ions do not leave the system and are therefore not eliminated. The alkali stays atta-ched to the fissures and roughness of the sand particles and redu-ces their refractory characteristics.Concentrations of these alkaline oxides must not exceed 0.12% as this would affect the work time of successive cycles.Alkaline sand can be regenerated mechanically, and wet regenera-tion is also possible. It needs to be remembered that the wet pro-cess produces contaminated water and this must be treated befo-re discharge.

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SODIUM SILICATE

4. SODIUM SILICATE

Sodium silicate with esters is an inorganic binder used in the “No-Bake” process, as an alternative to organic binders.This binder is compatible with all the casting processes. It is espe-cially suitable for low carbon steel castings as these readily absorb carbon when organic binders are used (the casting surface absor-bs the carbon released by the heat “cracking” of the binder, and a layer of several millimetres may be affected).Another reason why sodium silicate is an ideal binder for use with steel castings, is because the binder is thermoplastic and the pour heat induces the inward completion of the binder set, consequen-tly there are reduced obstacles to cast contraction.

4.1 the basic PrinciPles of the Process

The basic principles of the process are :

The silicaTe-esTer reacTion

The sodium silicate solution has a strongly alkaline pH, and it has the following empirical formula :

(x Na2O, y SiO2, z H2O).

The reaction is based on the reduction of the pH to values in the range 5 to 7, following the displacement of the weak siliceous acid by a stronger acid. This acid, derived from the ester, salifies with the sodium ions. The neutralisation of the soda brings down the pH value, the silicate gels and its particles pass from the sol state to the gel state and aggregate as siliceous acid, silica gel or hydrated silica.

The reaction phases are the following:1) The reaction of the silicate ions.

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The reaction can be represented as follows :

2Na+ + SiO3-- + H2O ➞ SiO2 +2Na+ + 2 (OH-)

The siliceous acid has a very low dissociation constant, so when its negative charge is neutralised it tends to precipitate as a colloid. That is to say it precipitates in an extremely dispersed state within the sand mass and finally coagulates.

2) The ester-soda ionic reaction.For the sodium silicate-ester reaction the ester has been chosen for its solubility in the silicate. The hydrolysis of the ester proceeds rapidly as shown below and liberates its salt and alcohol. Both of these exert a strong gelling action :

R- COO - R1 +NaOH ➞ R COO Na+R1 OHester salt alcohol

The dynamics of this reaction are as follows :• DecreaseofthepHvalueduetotheremovalofsodiumions

by salt formation• Transformation of the silicate ions into poly-siliceous acid

which precipitates, due to its high instability• Chemicaldryingduetotheremovalofwaterbythealcohol.

This reduces the amount of water available for silicate dilution and leads to a stronger agglomeration of the colloid

• Strongsilicagelstabilityduetothedehydrationbythealco-hol

4.1.1 settinG times

The silicate neutralisation is not instantaneous therefore the setting reaction is progressive and gives a long “work time” to the sand mixture (that is a long period in which it can be worked).The mould hardening reaction is affected by external factors such as the sand temperature, the environment temperature and the humidity. In the case of the manufacture of widely differing moulds it may be opportune to control the hardening process by blowing

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SODIUM SILICATE

with hot air, possibly enriched with CO2.4.2 the tyPe of sodium silicate

Sodium silicate is an amorphous mineral substance consisting of silica, soda and water in varying amounts. Its composition is defi-ned by its modulus or weight ratios of SiO2 / Na2O. For example, a silicate with the following composition :SiO2=30%; Na2O=10%; H2O=60%This is a silicate with the following characteristics : Modulus, or weight ratio : SiO2 / Na2O : 30/10 = 3Dry extract = 40%Foundry practice shows that in the silicate-ester process, the sili-cates with a modulus higher than those used with the silicate-CO2 system give the best results.In choosing the correct modulus for the type of production to be carried out it must be remembered that as the modulus value incre-ases, the setting rate increases, the initial mechanical strength of the mould increases, and therefore the difficulty of knocking out increases.The amount of silicate to be mixed with the sand is determined by a number of factors including : • Themodulus• Thefinenessofthesand(inrelationtothesurfaceareaofthe

grains to be coated);• Thetypeandquantityoftheadditives• Thetemperatureandhumidity• Theeventualpresenceofclayinthesand(ifthisismorethan

1% it means that more dilute silicates with a lower modulus must be used).

4.3 the tyPe of ester

The silicate hardening can be achieved with two procedures which use different hardeners. Briefly the processes are :• Variationofthesilica-sodaratiobyneutralisationofpartofthe

free soda using either an acid ion or radical. Substances which can be used for this are : carbon-di-oxide, silicon,

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esters, zirconium fluosilicate and similar substances and glyoxal;

• Removaloftheformulationwaterandthereforethedehydra-tion of the silicate, using cement, blast furnace ash, plaster, calcined dolomite and also by the action of carbon-di-oxide.

In the “No-Bake” moulding process the use of esters is preferred as these are the best compounds for the neutralisation of the free soda, they increase the silica-soda ratio (by the ester free radical), and remove water from the silicate.Several formulas are used, with a well established preference for using mixtures of acetates of poly-alcohols.For example we show the reaction of glycerol triacetate with soda:C3H5(OO CC H3)3+ 3NaOH ↔ 3 CH3 COONa+ C3H5(OH)3Setting takes place in the cold and the setting time depends on the type of acetate used. Glycerol di-acetate gives rapid hardening whilst glycerol tri-acetate is relatively slow (see fig. 4.1).Using measured mixtures of the two reagents, a series of interme-

Fig. 4.1- Changes in the mechanical strength of sand bonded with silicate as functions of time and the type of hardener used.

Time in hrs

Group 1

Group 2 Group 3

Type A

Type A

Rc,

in d

aN/c

m2

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SODIUM SILICATE

diate setting times can be established.4.4 additives

The method of binding the sand with silicate may have the fol-lowing drawbacks :• Maintenanceofmouldcharacteristicsduringstorage• Difficultmouldshake-outforcertaintypesofcasting• Theneedtoimprovethesurfacequalityofcastingswith certain types of alloys The size of the problems inherent in this procedure mainly relate to the type of metal cast in the mould. At pouring, the sand grains become coated with a film of glass consisting of anhydrous sodium silicate and this effectively sinters the sand grains.This sintering is determined by the intensity of the heating to which the sand grains are subjected, the length of the heating time and the size of the sand grains.In contrast to many organic binders, silicate increases its cohesive properties after heating. Therefore, to make knocking out easier, various organic additives are used. These burn and leave disconti-nuities in the glassy mass. This device is particularly important to make the break-down and removal of “closed” cores easier.Additives can therefore be divided into knocking-out additives and additives with binding properties :• Knockingoutadditivesmaybe:coal,pitch,sugar,molasses,

glucose and sawdust• Additivesforincreasingbindingproperties:speciallytreated

phenolic resins increase the storage strength and improve the flow properties of the sand mixtures

• Black ironoxide improves themould’s resistance tomoltenmetal penetration

4.5 carryinG out the work

4.5.1 mix PreParation

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The silicate-ester process requires the use of dry sand which is not hot, or which is at least at a controlled temperature.The addition sequence is as follows :• additives• esters• silicate:asthesilicateisrelativelyviscousitisrecommen-

ded that it should be dosed using a volumetric pump, and that it should be mixed effectively, whilst avoiding overhea-ting the sand to prevent water loss. In hot weather a little water may be added to compensate for the evaporation which takes place during mixing. Alternatively a less con-centrated silicate can be used.

4.5.2 mouldinG

During hot weather, it may be advisable to add a little water (0.5 to 1.0%), to compensate for evaporation, or to use a less concentra-ted silicate solution.The hardened mould has very little elasticity, therefore the pattern must have a carefully designed draft angle and smooth surfaces.The relatively poor flow properties of the sand-silicate mass, means that there must be an adequate compacting action to ensure that the mixture fills all the space around the pattern.Painting moulds with water based paints weakens the strength of the painted surfaces. Alcohol based paints are more suitable for this purpose..Moulds exposed to the air deteriorate, as their mechanical charac-teristics are weakened by water absorption (due to their hygrosco-pic nature).

4.6 silicate checks

The maintenance of regular production cycles requires that in addi-tion to the silicate checks (physical and chemical), the sand quality must be checked. This is particularly important if regenerated sand

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is being used. The checks to be carried out are :• Thesandtemperature• Thealkalinity• Thequantityoffinesinthesand

chemical checks

The determination of SiO2 and Na2O to calculate the modulus of the sodium silicate, can be quickly carried out by a simple titra-tion.The method is acceptable for quality conformity checks. The deter-mination of the amount of dry weight, is carried out by determining the difference of weight before and after ignition at 600°C.

Physical checks

Sodium silicate is composed of three compounds. However, the determination of any two of the characteristics listed below will be enough to establish the conformity of the product :• Viscosity;• Density(degreesBaume’);• Solidscontent;• Thesilicatemodulus,thatistheratioSiO2 / Na2O;• Thesilicacontent(SiO2);• Thesodiumoxide(Na2O) content.The diagrams shown in figs. 4.2 and 4.3 show the relationships between density, degrees Baumé, the sodium content, the SiO2/Na2O ratio of the sodium silicate and the solids’ content.You are referred to Part II for the analytical methods (see schedules M6 and M7).

mechanical checks on TesT Pieces oF Bonded sand

The technical tests carried out to determine the mechanical cha-racteristics of the various mixtures, enable indirect evaluation of the silicate characteristics; together with direct evaluation of its mixture with sand.To carry out a correct evaluation, the mixture must be made without

51

SODIUM SILICATE

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any heating and ensuring that no water is lost by evaporation during the various stages. This means that the sample of the mix-ture must be quickly placed in a sealed container, and that the test pieces must be prepared rapidly.An exposure to air of about two minutes may cause hardening to

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Fig. 4.2- The viscosity of sodium silicate as a function of the R modulus, for different concentra-tions of dry material (MS%).

R modulus

N

on v

isco

us li

quid

s

Vi

scou

s liq

uids

pa

stes

solid

Vis

co

sit

y i

n p

ois

es

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53

SODIUM SILICATE

begin, due to the evaporation of water and absorption of atmos-pheric CO2.

5. THE SANDS

Fig. 4.3- The relationship between density, the Baumé degrees, viscosity, the dry material content, the soda level and the silica/soda ratio in the sodium silicate.

Modulus SiO2or Ratio ------- Na2O

Dry

mat

eria

l con

tent

as

a %

age

Density Degrees Baumé

Viscosità

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55

THE SANDS

The sand is the mould skeleton which supports the metal and the binder gives it the necessary rigidity.In this section we shall look at the most important aspects of the use of the different sand types. You are referred to Part II for the specifications.

silica sand (si02)Silica sand is the most commonly used moulding sand, as with very few exceptions, it is compatible with all binders and all alloys. One of the exceptions is steel with 12% manganese (Mn), as this reacts with the mould. This alloy gives rise to an oxidation-reduc-tion reaction between the SiO2 and the Mn, with the formation of a low melting point eutectic mixture (FeO MnO-SiO2) which allows casting metal to penetrate into the mould structure. This means that extensive surface defects are formed on the casting. Paints based on magnesite or zircon can be used to prevent this fault.Whenever the refractory characteristics of the sand are insufficient to prevent sintering defects on the surface of castings, the use of protective paints offers a wide degree of protection.Quartz undergoes change into tridymite at 870°C, this means that there is a marked change in volume. If the resin binder is unable to contain this expansion there will be surface cracks in the mould and also mould deformation. These cause superficial defects in the castings, such as fins and metal penetration (see fig. 5.0).The addition of iron oxides to the sand : 2 to 3% of magnetite (Fe3O4; black colour) or 1 to 2% of haematite (Fe2O3; red colour), either reduces or eliminates these defects.The explanation of this action is still a matter for debate. The most convincing explanation suggests that the amount of oxide which forms a low melting point compound, acts as a binder on the sand grains and makes the system more rigid.In practice, the irreversibility of the above allotropic change in the quartz occurs in recycled sand because of several pouring cycles and of mineral oxyde build up. Therefore, moulds made with recycled sand have a lesser tendency to suffer from the defects caused by tridymite expansion.The degree of purity of sand used in mould manufacture is extre-

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mely important for the “No-Bake” process . The acid catalyst can react with any alkaline compounds which may be present in sea sands, or with metallic particles or oxides. In this event even an excess of acid will not prevent the unwelcome consequences ari-sing from their presence. This is due to the following reasons :• ThedevelopmentofCO2 due to the decomposition of car-

bonates, may break the hardened resin film.• Theacid-oxidereactionisslowerthanthesettingactionof

the binder and the newly formed resin film will be damaged by it.

The presence of clay reduces setting times and also lowers the mechanical strength of moulds.

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Fig. 5.0- Crests due to sand expansion not contained by the binder content.

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THE SANDS

A dry sand with rounded grains, a low percentage of fines and a clay content of less than 0.2% is acceptable for the “No-Bake” moulding process if proper allowance is made.The optimum sand characteristics, both new and regenerated, for use in mould and core production; are given in table A (page 117).

oliVine sand

This material is 93% magnesium orthosilicate (known as Forsterite). Its chemical composition is Mg2SiO4.Olivine sand is the main sand used for casting steel containing 12% manganese, as it does not support a metal-mould reaction. The castings do not therefore suffer from the surface defects which are so typical when silica sand moulds are used.This sand is basic (pH = 9 approximately). It cannot be used with acid catalysts, as these attack it, even in the diluted state. The acid consumed by the attack would be removed from its role as a catalyst and this would affect the hardening rate of the resin.This sand has optimum refractory characteristics and these make it ideal for steel casting.Its grain fragility limits the number of times it can be regenerated mechanically.

chromiTe sand Chromite sand is a sand with very angular grains. It consists of a mixture of spinels :

FeO Cr2O3, MgO Cr2 O3, MgO Al203

This sand has a very high thermal conductivity, a low thermal expansion and has excellent refractory properties.It is mainly used to solve metal penetration problems and as a chill, in those parts of the casting which might be most affected by microporosity.It is also used for making cores whenever the use of silica sand might lead to difficulties of removal.Whilst it is slightly basic (pH = 7 to 8), it is compatible with all types of binder as it has a marked chemical inertia.

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Chromite reacts with silica both in sand and in paints, and is itself changed if polluted with silica sand. This is due to the following reaction which takes place at tempera-tures above 600°C FeO Cr2O3 decomposes into FeO and Cr2O3. Part of the FeO oxidises to Fe2O3. This in its turn forms a solid solution with the Cr2O3. This forms a sealant barrier against metal penetration and eliminates defects such as metal penetration and fins.If any SiO2 is present it can react with the above iron oxide to form Fe2SiO4 (fayalite) which is a low melting point compound. Large amounts of fayalite at high temperatures may cause sand encru-sted castings.Therefore, moulds in chromite sand protected with quartz based paint and chromite sand contaminated with silica sand in excess of 2%, can give rise to castings with sintered surface adhesions. (See Vol. II, S-5).Fayalite formation also occurs when silica sand is contaminated with chromite sand. These reciprocal contamination problems must be remembered when designing sand regeneration plants.

Zircon sand This is a sand composed of zirconium silicate (ZrSiO4). This is che-mically and thermally inert and does not react with metals.It has a high thermal conductivity and a high thermal capacity due to its density. This increases the cooling speed of castings to about four times the rate when silica sand is used.The grains are rounded and there are no fines. This means that the moulds have high mechanical strength. However, zircon sand is extremely uniform in grain size, these being distributed across very few mesh sizes. This can cause metal penetration defects.

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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

6. THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

This section gives the characteristics common to all the sand mix-tures, which affect the properties of moulds/cores and the quality of castings.You are referred to Part II for the control methods.

6.1 sand characteristics

The morphology and composition of the sand strongly affect the quantity of binder required and its polymerisation process. This has a marked effect on the mechanical strength of the mixtures. The acidity or basicity of the sand is a very important factor, whether due to its original nature or induced by accumulated impurities.Olivine sand is a good example - it is not compatible with acid catalysed binders due to its basic nature.Angular sand grains of the same grain size as rounded grains, have a greater surface area and therefore require more binder, they have worse flow characteristics; and due to the breakage of the grain projections they produce more fines’ fractions.

granulomeTry and The Fineness index The fineness index of a sand has significance when examined toge-ther with the granulometric spectrum (see fig. 6.1). In fact, if two sands with the same fineness index are compared, and one has a grain distribution across only two sieves, and the other has a grain distribution across four or five sieves, the former will require less binder. This is because it has a smaller surface area to be covered. It should be remembered that a variation of five units of fineness, can result in a fall of a few kg./cm.2 in the value of the mechanical strength of a sand/binder mixture. This variation can easily be attri-buted to an instrument error, by mistake.

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The sPeciFic surFace area oF The grains

It is important to extrapolate the total surface area of the sand grains from the granulometric values. This enables the binder requirement to be assessed (see the technical schedule MO1 in Part II).The influence of the specific surface area of the grains, and there-fore of the fineness index on the consumption of binder is quite clear. The calculation enables us to follow the deterioration of a sand through its regeneration cycles; and the following formula enables us to determine the effect of the variation of the fineness index of a sand, on the consumption of binder.

Where :AR = the percentage of binder on the recovered sandAN = the percentage of binder on the new sandIFR = the fineness index AFS of the regenerated sandIFN = the fineness index AFS of the new sand

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Fig. 6.1 - Sands with the same fineness index but with very different granulometric distribution.

FINENESS INDEX: 67,70 FINENESS INDEX: 67,70

IFRAR=AN• IFN

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moisTure

Moisture values above 0.1% are very detrimental to the mechanical properties of the binder mixture (see fig. 2.6-page 34)

Fines’ FracTions

The sand impurities are usually concentrated in the fines. The fines may interfere with chemical reactions and certainly increase the binder requirement due to the increase of surface area, which needs to be covered with binder. If the quantity of binder is kept constant, the fines reduce the mechanical strength of the moulding mixture (see fig. 6.2).The fines increase in sand with poor thermo-mechanical characte-ristics (i.e. a fragile sand). This increase is made worse by pneuma-tic transport and by regeneration processes.A poorly regenerated sand which has a high fines’ content, is also probably contaminated with acid residues, oxides and oolites. These may cause casting defects, due to reactions between the poured metal and the mould, as shown in fig. 3.1 (page 40).The flow properties and the permeability, of a sand, both decrease as the fines’ fractions increase.

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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

Fig. 6.2 - Loss of resistance to bending of a mixture with 1.2% of resin, as the percentage of fines varies.

Fines %

Resistance to bending

Resi

stan

ce to

ben

ding

lb/in

2

bar

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loss on igniTion

Sand recovery is necessary both for cost and environmental rea-sons. It is essential that recovered sand has a low percentage of fines. As already mentioned, these consume part of the binder and reduce the mechanical strength of the mould. Moreover, at pouring the volatile component of the fines develops gases which can be absorbed by the metal. The volatile component consists of unburned or cracked binder and catalyst from the previous cycle(s). One of the parameters for the evaluation of volatile residues is the loss on ignition (LOI). The type of metal being cast, the composition of the above residues and their nitrogen and hydrogen contents, set the limit for acceptable loss on ignition.

The acid demand Value

It is important to know the degree of alkalinity of the sand, in order to assess the extra acid to use, over and above that required by the resin. In other words it is necessary to know the acid demand value (ADV) to neutralise the sand.The pH value, or the acid demand value, enable us to evaluate the degree of sand contamination with basic materials. These neutrali-se part of the catalyst and slow down the hardening process (as the catalyst is normally a weak acid).The acid demand value of a new sand should be below 0.5 cc of N/10 HCl per 100 g. of sand. The acid demand value of a regene-rated sand should be less than 5 cc of N/10 HCl per 100 g. to give a pH of 6 to 8.The acid demand value of a sand must be continually monitored, in order to be sure that the amount of catalyst used will lead to hardening in the required time.

The Base demand Value

The above considerations also apply, to evaluating the degree of sand contamination with acids. These acids speed up the harde-ning process.

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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

clay

A clay content above 0.1% is enough to alter the “work time” of a sand mixture. If the sand is washed with recycled water with a high clay content, the sand grains become coated with a clay film which is very difficult to remove. In this event it is necessary to install equipment for moving the sand mass so that this film can be bro-ken by attrition.

The degree oF ooliTe conTaminaTion

Sand which has been regenerated several times, has a degree of contamination such that “oolites” may form due to the heat at pou-ring and at heat regeneration. These oolites may be siliceous, side-riferous (iron carbonate), phosphatic (tri-calcium phosphate) or ferruginous (iron silicates and oxides).All these compounds have low melting points and react with cer-tain metals. They therefore cause surface defects on castings due to a metal-mould reaction.

TemPeraTure

The rate of chemical reactions, including catalysed reactions, is directly related to the temperature of the reagents. This means that the uniformity of the various hardening stages in mould forming, is absolutely dependent on temperature control and stability. This is true for the sand, the pattern plates, the reagents and the work environment. The degree of response to variation in reaction temperature, is a characteristic of each binder and each catalyst.To stress the importance of temperature control : one can make a general statement that a 10°C. increase will halve the hardening time, and a 10°C. fall will double it.Fig. 3.3 (page 41) shows the relationship of time to mechanical strength, in mixtures with different percentages of catalyst, added to mixtures at different temperatures. The diagram clearly shows that the effect of temperature on mechanical strength is greater than that of the amount of catalyst used.

6.2 the hardeninG Phases

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The resin hardening reaction must never be disturbed and this is the reason why the sand mixture must be made and used as quic-kly as possible, before the polymerisation really takes hold.To assist processing, continuous mixers are recommended. The components are added to these in the sequence : sand, additives, hardener and finally resin.The addition process for the hardener should be automated, com-puterised if possible.Vibrating tables or shooting machines ensure that the sand is com-pacted as quickly as possible. The critical phases of the hardening process are : the “work time ” (bench life) and the “strip time”.

6.2.1. the “work time”

The resin hardening reaction must be slowed up as much as pos-sible in its early phase, to enable the mould to be completed. The parameters which govern the reticulation process must therefore allow a sufficient “work time” (bench life).The evaluation of the “work time” fixes the maximum time for using the resin mixture and it should not be used beyond this. Use after this time will result in very poor of flow properties and excessive reduction of the mechanical strength of the final product. The work time shortens as the polymerisation of the resin-catalyst mixture becomes faster at normal environmental temperatures.The “work time” of a sand resin mixture is evaluated in the labora-tory and is controlled empirically in the moulding shop.There are two laboratory methods :• byreactometry(seethetechnicalscheduleM4inPartII). • bymeasuring the reduction of the resistance to bending.

The test consists of making a series of test pieces from the mixture, at regular time intervals, under constant temperatu-re conditions. After 24 hours the test pieces’ resistance to bending is checked and the reduction trend is noted. The time between the preparation of the mixture and the time when the test piece is prepared, which corresponds to a

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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

pre-selected value of resistance reduction (normally around 30% reduction compared with the maximum value obtaina-ble), is the “work time” of the mixture being examined, under those conditions.

Refer to technical schedule R for the data on work times of sand-binder mixtures, with the most commonly used resins.The evaluation of the work time can be carried out empirically by watching the changes in a lump of mixture exposed to the air under dry ventilated conditions. The time taken for the surface of the lump to form a weak crust, which is slightly resistant to the touch, but is clearly evident, is the work time.

6.2.2. the “striP time”

The second characteristic of a good sand resin mixture, is that it should harden quickly to enable the mould to be stripped. This length of time is called the “strip time”. Once the mould box has been filled, the mixture should set as quickly as possible to a consistency which enables the mould to be stripped without causing any dimensional changes or excessive deformation of the piece.Clearly there is a conflict between the requirements of “work time” (bench life) and “strip time”. The ratio between these must be as high as possible, as the moulding phase normally requiresmore time than the preparation and stripping.This ratio depends on the resin type, on the catalyst type and quantity, on the sand quality and temperature, and is a key factor for the production rate.In practice an evaluation of the “strip time” can be made by asses-sing the resistance to penetration by a wire probe.The handling time of the mixture is also affected by its flow charac-teristics, and by the mechanical equipment (vibrating tables or shooting machines).The binder should therefore have properties which permit the sand mix to form the precise shape of the model and to fill the most inaccessible corners of the mould quickly. A mould or core which

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has been well compacted has a better chance of lasting unchan-ged, even during a long storage period.

6. 3 mould aGeinG

It is advisable to store moulds before use, to ensure that the poly-merisation process is complete and that the excess solvent has evaporated. This precaution also reduces the amount of gas pro-duced during pouring. In automated moulding cycles, it is therefo-re necessary to ensure that there is a sufficient time lapse between moulding and pouring.It is also necessary to take into consideration a possible degenera-tion of the mould or core characteristics during ageing. When the air has a high relative humidity and its temperature is low, the resin reticulation deteriorates. The degeneration phenomenon is more marked when there is a low percentage of binder in the mix.The damage caused by moisture absorption is irreversible. For more details see point 3.2 in the second volume. A number indica-ting the hygroscopicity of a binder, is given by the time it takes to halve the mechanical strength of the mould. The test consists of placing samples in a greensand mould and testing the deteriora-tion of their mechanical strengths over time, or alternatively, by holding the test pieces under saturated humidity conditions, in the laboratory.

6.4 the siGnificance of sand-binder mixtures’ quality control

The quality of moulds and castings and the uniformity of the pro-duction rate, are assured by strict control of the sand mixture cha-racteristics.An equally important aspect, is that these checks also confirm the efficiency of the plant. Any mixture anomalies, can be an indication that urgent maintenance is required, to restore normal plant wor-king conditions.

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You are referred to Part II for the routine control methods mentio-ned above, both for laboratory and factory use.Systematic quality control must also be carried out by suppliers; and will be reflected in the quality of the castings (for the defects which can occur in the “No-Bake” process).Suppliers must therefore be “qualified”, and each supply lot must be certified, especially for those tests which the foundry cannot carry out in its own laboratories. It is essential to be aware of any quality problems before materials supplied are used, rather than later, when rejected moulds or castings result from using sub-standard materials.The above checks become significant when they are carried out immediately and systematically and when the data obtained can be used to provide the greatest amount of useful information. The method used to achieve this aim is known as “statistical quality con-trol”. It enables any factor affecting the process to be identified quickly, and also enables the effectiveness of the counter measu-res, to be assessed.In a process like casting, in which there is still a certain “know-how” due to empiricism and personal experience; it is extremely important to be able to distinguish chance deviations from the standard, from those due to technical factors which are out of con-trol.Money and time spent in continuous quality control, enables pro-blems to be foreseen and avoided. It also gives great cost and psychological returns, due to the reduction of the founders’ daily problems and uncertainties.

6.5 environmental and hyGiene considerations

When selecting a binder, it is important to take into account the effects it may have on the environment, any handling effects on staff; and the costs involved to solve these problems. It is therefo-re essential to know the toxicological characteristics of the pro-ducts which are to be used.Furthermore, do not forget any problems which might arise in con-nection with the disposal of used “No-Bake” products and non-

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recyclable moulds, etc.The production rate and the number of air changes in the workpla-ce, may increase or decrease the potential risks to personnel. It is therefore essential to make surveys and systematic measurements at the various working positions; and to consider these as an inte-gral part of the general production checks carried out on binders and mixes.The admissible exposure time for operators, and therefore their working time, is one of the key factors in defining the danger to their health.The type of combination of different toxic materials, determines the concentration limit which is acceptable for each material. The fol-lowing formula establishes the threshold limit for materials :

The following table gives the maximum legal concentrations for the most commonly used components of the “No-Bake” process. The differences between countries and the rate of change in their legi-slations, means that the table data is indicative, and specific checks should be made to determine the current state of any legi-slation.

To make the calculation clear, an example is given below: take the case of formaldehyde fumes on their own:• theconcentrationdeterminedis:C1=0,4 ppm• the ratio between the concentration determined and the

maximum permitted concentration is:

TYPE MAXIMUM ALLOWABLE CONCENTRATION IN THE AIR

formaldehyde G1 = 0,5ppm = 0,6 mg/m3

furfuryl alcohol G1 = 50ppm = 200 mg/m3

phenol G1 = 5ppm = 19 mg/m3

benzene G1 = 5ppm = 16 mg/m3

i = n Ci Ci = the value in ppm of a material (compound)∑=CI≤1wherei =1 Gi Gi = the corresponding maximum concentration in ppm permitted for the material on its own CI = concentration index

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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

CI = C1/G1=0,4ppm/0,5ppm=0,8As the value of the concentration index (0.8) is less than 1, the environmental conditions are acceptable from a Health and Safety viewpoint. Take the case of formaldehyde plus phenol fumes: the con-centration of the formaldehyde and its index are as given above, therefore we have: C1/G1=0,8the phenol concentration is determined as 2 ppmthe concentration ratio = C2/G2=2 ppm/ 5 ppm=0,4tne concentration index (CI) = the sum of the values of the respe-ctive concentration ratios. Therefore we have: CI = 0,8 + 0,4 = 1,2As this value is greater than 1, the environmental conditions are unacceptable.The use of acids and harmful substances require the application of well known industrial safety precautions; however, as these pro-blems are new ones for foundries, particular attention should be given to them to avoid accidents.Over and above the application of the safety regulations, it must also be remebered that there are persons who are particularly aller-gic to some of the products used in the No-Bake process. As foundry working conditions have greatly improved, and as the No-Bake process has made a significant contribution to this impro-vement; it would be a pity to cancel these benefits by not taking simple precautions.

6.6 the comPatibility of sands, binders and metals

The heat decomposition of the binder must occur under equili-brium conditions, in the sense that there must be a reducing atmo-sphere in the mould during pouring to prevent the oxidation of the metal. The oxides would attack the quartz grains and reduce their refractivity and increase the tendency for the metal to penetrate between the sand grains.It is also important that the amount and the pressure of the gas produced, does not create local gas saturation in the metal. This

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would lead to bubble formation and these might be trapped in the casting. The higher the gas pressure, the smaller the trapped gas bubbles. The type of gas produced is also important. It has been known for some time that both hydrogen and nitrogen are respon-sible for the blowholes known as “pinholes” in cast-iron and steel castings. These gases are formed in the atomic state and only form molecules later. The atomic state is also called the “nascent state” and is the condition of maximum reactivity. The urea contained in some resins, probably decomposes into ammonia first and this then dissociates into hydrogen and nitrogen. It certainly plays a role in the formation of the defects shown in figs. 6.3 to 6.6.Lustrous carbon, formed by the cracking of polyurethanes, can change the carbon content in the surface layers of steel castings. Phosphor and sulphur from catalysts, may cause the formation of a eutectic phosphorous compound and the degeneration of gra-phite nodules respectively, in the surface layers of spheroidal cast-iron.The addition of between 0.05 and 0.1% of titanium inhibits the formation of pinholes; and the addition of between 1 and 3% of black iron oxide prevents the surface carburation of steel castings by lustrous carbon. It also reduces sand expansion and the related surface finning or veining defects (see fig. 5.0 page 56).Recycled sand which is not regenerated, or which is regenerated without effective fines’ removal, may contain an excessive quantity of residual acids and oxides. These give rise to a mould-metal reaction and surface defects known as the “orange peel effect” (see fig. 3.1 page 40).As production rates have increased these defects have increased too, due to the use of larger amounts of catalyst to reduce harde-ning times. This has therefore required the improvement of attrition cleaning and the improvement of suction equipment to remove fines, in the sand regeneration plants.On the other hand, the replacement of mineral catalysts with orga-nic catalysts which are destroyed at much lower temperatures, has contributed to the solution of the acid residue problem.Paints can be used to form an effective barrier between the resin and the metal. To recapitulate briefly, silica sand is not recommended for casting manganese steel as the silica (SiO2) reacts with the manganese and is reduced to silicon which passes into the bath. The manga-

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Fig. 6.3 - Defects called “pinholes” in a cast iron casting, due to small quantities of hydrogen and nitrogen acting synergically with aluminium contamination. The defect was discovered during machining operations.

Fig. 6.4 - A steel casting with “pinholes” below the surface, discovered at the machining stage.

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Fig. 6.5 - A cast steel pump body with “pinhole” defects below the surface, discovered at the machining stage.

Fig. 6.6 - Cracks due to a high nitrogen level (pump body).

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nese oxide produced by this reaction attacks the surface of the casting and this becomes badly pitted. Zircon based paints are a possible remedy, whilst the use of olivine sand drastically reduces the problem.

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RELEASE AGENTS

7. RELEASE AGENTS

Release or separation agents, reduce the energy necessary for separating the pattern model from the mould surface. This means that there is less stress/breakage of the mould surface and means that the mould is a better reproduction of the pattern model in all its detail.The release agent separates the pattern from the sand and pre-vents the interaction of the pattern material and the binder by reducing the interface forces; and by neutralising the effect of the binder’s surface tension. In fact for all binders, the adhesion phe-nomenon depends upon their capability to wet the pattern mate-rial. When the release agent comes between the mould and the pattern, the materials do not inter-react and release is easier.However, liquid release agents must not “wet” the pattern either, and the contact angles must be greater than 90°.The choice of the type of release agent used, must take account of the degree of plasticity given to the mould by the binder and also its pH. This is necessary to avoid unwanted reactions at the pattern -mould interface. Therefore a basic release agent should not be used if the binder has an acid catalyst, and vice versa.The pattern material itself may also require attention if it is made of plastic. At regular intervals this needs to be cleaned to remove release agent residues, which may have accumulated in layers. Remember that release agents should only be used in the smallest amounts consistent with effective separation. In cleaning the pattern care must be taken to avoid it becoming scratched or marked, as this increases the adherence of the bin-der/sand mixture.The static electric charges of plastic attract the binder to the pat-tern; it is necessary to verify the reciprocal compatibility of binder, release agent and plastic used for the pattern.

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8. PAINTS

Paints are used to reduce the roughness of the mould surface, to prevent mould-metal reactions and if necessary, to give protection to the sand grains against heat.Paints can be schematically divided into water-based and alcohol-based paints, however both types must have the following charac-teristics :• therefractorypowderinthepaintmusthaveahighmelting

point, so that it forms a protective heat barrier and ensures separation of the mould from the metal;

• itmustadherewell.Thereforeitmusthaveathermalexpan-sion which is compatible with that of the sand;

• itmustcoverwellandthereforeitneedstohavealowsur-face tension

• itmusthaveahighresistancetometalpenetration;• itneedstohaveexcellenterosionresistance;• itmusthavealowcontentofmaterialswhicharevolatileat

casting temperature. It must have a granulometry which ensures that the refractive material stays in suspension for a reasonable period of time;

• itneedstohaveexcellentthixotropiccharacteristics,sothatafter settling out it can quickly be re-suspended by mixing;

• itmustnotbeliabletobacteriologicaldegradation;• itmustbeenvironmentally“friendly”;• itmustbecompatiblewiththemetaltobecast.Safety and environmental considerations, mean that water based paints are those normally used.

8.1 water based Paints

The “paint problem” arises from two conflicting requirements: envi-ronmental requirements mean that water based paints are increa-singly used, yet the growing number of high productivity “No-Bake”

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plants require that the paint used should not need a long drying time. The paint must therefore dry quickly.The most important characteristics of paints are given below, and an assessment of these will enable the best choice to be made for specific uses.

comPonenTs

The refractory materialZircon flour, aluminium silicate, magnesite, graphite and silica flour, are the refractory powders most commonly used in paint pro-duction.The grain size and the specific weight of the refractory powder are important.In addition to the refractory properties, lack of reactivity with the metal to be cast is essential to avoid unwanted paint-metal interac-tion. In this context, remember that carbonaceous materials can carburize the surface layers of steel castings; and silica cannot be used with manganese steel castings.Graphite, in addition to its refractory properties, can be mixed with other refractories (e.g. silica) to improve the separation of cast iron from the mould.

WaterThe choice of the suspending liquid for the refractory powder, is governed by environmental factors, by cost, by the production rate, by the type of plant used and by the type of mould binder used.In contrast to quick drying organic vehicles, water needs time to dry well. The water paints which are best from this viewpoint, are those with a very low water content. These are called, somewhat euphemistically, “self-drying”. These paints fluidise easily despite having a relatively high density. They form a thin protective layer and therefore dry rapidly.The composition of the water, its hardness and its pH, must be taken into consideration: these could interfere with the action of additives used to optimise the suspension characteristics, the covering power of the refractory component, the drip speed and adherence to the mould surface.

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BindersThe binders can be organic or inorganic, provided they guarantee that the refractory particles adhere well to the mould surface.Amongst the inorganic binders, bentonite and kaolin are those most widely used, especially for high temperatures. The bentonites have an excellent fixing power, due to their exceptional swelling capability. This increases their surface area and therefore enables them to cover a large quantity of refractory grains. This swelling capability, which can be up to ten times the initial volume, means that they also have high water retention and therefore long drying times. It follows from this that organic binders of the polymer type, supplemented with dextrins, polysaccharides and silicates, are preferable for water paints, as they do not retain water.Generally speaking, an excessive quantity of additives prevents easy paint application.

Suspension agentsThe suspension agents give uniformity of solids’ suspension and aid retention of particles in suspension. They normally form a gel which makes a network to which the material particles adhere. In water paints, the most commonly used suspension agent is ben-tonite, as this has good refractory properties and is capable of forming a voluminous gel. This is capable of retaining a large amount of refractory material in fine suspension. Celluloses and alginates are also used.

Physical requiremenTs

ThixotropyA good water paint, when undisturbed, seems to be a semi-fluid gel capable of maintaining a large quantity of covering material in suspension, in a uniform manner, for a long period. On mixing, either by the action of the compressed air of the paint sprayer, or by brush action, it must be extremely fluid. Both mixing methods transmit kinetic energy to the liquid particles. This energy will be lower, the greater the thixotropy of the paint. To summarise: a thi-xotropic paint has a low water content; but has excellent fluidity at

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application.

Surface tensionThe molecules of a drop of liquid exercise a reciprocal attraction and tend to form a sphere, as this form requires the least energy. This attractive force is called “surface tension”.An idea of the magnitude of this aggregating force can be gained by observing the movement of insects or the behaviour of a razor blade on a water surface. Their weights are insufficient to separate the water molecules and they stay afloat. This aggregating force can be reduced by the addition of a liquid consisting of compoun-ds called “surfactants”. This reduction is appreciable, as can be seen by looking at the meniscus which the liquid forms in contact with the surface containing or supporting it. The practical conse-quence of this phenomenon, is that a foundry paint containing surfactants tends to spread easily and to wet the surface of the mould well. This gives the following benefits :• paint saving due to the reduction of the thickness of the

paint film• excellentdrainingcharacteristics, acharacteristic required

in plants with a painting station;• moreprecisecastingas there is lesspaint in thecorners;

and a thinner coating everywhere;• fasterdryingtimes.Non-ionic (i.e. neutral) surfactants are unaffected by the degree of hardness of the water.

DensityPaint density, that is its weight per unit volume, is usually measured with a gravimeter, and the values obtained are expressed in degre-es Baumé. The measurement of the density enables the following assessments to be made within the limits imposed by the type of paint :• a preliminary judgement about its quality conformity, by

comparing the density with the standard sample;• controlofadilutionoperation,tothevaluerequiredbythe

specific application;• assessmentofthesedimentationrate,bycarryingoutmea-

surements at time intervals and also at different sediment

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heights in a tall container.However, density measurements do not detect small changes in the composition of the paint. These may be important although they do not noticeably affect particular characteristics of the pro-duct.For example the Baumé measurement is not suitable for use with very dense water paints which are highly thixotropic. The density value does not give any indication of the paint workability at appli-cation.

ViscosityViscosity is the resistance of a fluid to flow. Viscometry consists of measuring the time it takes to empty a given amount of liquid through an orifice of given dimensions at a given temperature.A method which is better for dense thixotropic liquids, measures the resistance of the liquid to the rotation of a disc or cylinder.Even more sophisticated methods enable the viscosity gradient to be measured. The resulting curves give the best indication of any quality changes.

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9. “NO-BAKE” ADVANTAGES AND PROBLEMS

The “No-Bake” system has made great improvements in mould construction. The many advantages of this system and its pro-blems, can be summarised as follows :

dimensional Precision

The rigidity of the mould and the uniformity of compacting enable the design dimensions of castings to be produced more precisely. However, this rigidity requires provision to be made when the pat-tern models are produced, especially in setting the draft angles, as these must take into account the relatively inflexible state of the mould when the pattern is being removed. It is not practical to rap the pattern and this makes it advisable to make the widest possible use of vibrating devices to make pattern separation easier.Release agents have an important role in draft angles reduction.Thanks to these measures, castings can be produced with high dimensional precision, and patterns can be kept in good condition. The “No-Bake” system has enabled the regulations on dimensional tolerances and casting margins, to be revised, and a new more precise class to be added (see schedule MO in Part II).The system requires patterns to be made more carefully, to avoid increased sticking in the mould. An eventual increase in the draft angle, does not significantly increase the total costs of the mecha-nical work, as the casting tolerances can be tightened to reduce machining allowances. In fact, the “No-Bake” process has elimina-ted the typical surface defects of green or dry sand moulding.

Flasks

The traditional moulding materials require a high compaction pres-sure to ensure that they form a close fit to the pattern. This means

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that it is necessary to use rigid and barred flasks. This creates problems at mould removal and also maintenance problems. The “No-Bake” system has reduced these problems to the extent that in most cases moulding now uses flaskless moulds, which are only slightly larger than the patterns themselves. moulding

The high pattern covering capability of the sand-resin mixture must not lead one to underestimate the requirement for uniform com-pacting. The diagram shown in fig. 9.1 shows the variation of mould density as the compacting pressure is changed. It is equal-ly important to stay within the “work-time” when handling the sand resin mixture; and this should allow more time than is strictly necessary for the work to be accomplished.

This is to ensure that the sand has the required mechanical streng-th due to an effective polymerisation process; and that this is not decreased due to the initial polymerisation phases being disrupted by sand movement. These precautions also reduce the defects due to abrasion and sand metalisation (see fig. 9.2).

Fig. 9.1 - The increase in sand density as a function of the tamping pressure.

Core box smooth and clean

Core box badly maintained

Pressure

Den

sity

g/c

m2

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The surFace aPPearance oF casTings

When good quality moulding materials are used and when alloys

are cast which do not need very high pouring temperatures, it is possible to do away with mould painting. This requires that mould compaction is carried out to a high and uniform standard, making maximum use of the flow characteristics of the sand-resin mixture. The elimination of mould surface painting also makes a contribu-tion to the dimensional precision of the casting.

core assemBly

A rigid mould gives a greater guarantee of a successful core assembly operation and also enables the core prints to be reduced by up to 50%, compared with the traditional moulding systems.

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Fig. 9.2 - An example of the metalisation of a core.

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residual sTresses

Mould rigidity may interfere with the linear contraction of the casting and induce stresses, distortions and even breakages, in alloys with low hot flow characteristics.In contrast, in castings of alloys with good hot flow characteristics, the linear contraction may be up to 0.3% less than normal.The problem must be allowed for at the casting or pattern design stage. It is advisable, for example, to make those parts which could be retention points during casting shrinkage, as core elements.The following factors are some of those which can contribute to the formation of residual stresses in castings :- excess resin- high resin heat resistance- flash- runner bars which are too long- opposing risers

cosTs

“No-Bake” moulding gives cost savings, for the following reasons :• lessskilledlabourcanbeused;• themouldsdonotneeddrying;• flaskelimination;• shortmould forming times even for large casings, as the

sand does not need to be rammed, only vibrated;• itusessmallervolumesofsandasitispossibletocontour

the flask to the pattern profile and to reduce the intermedia-te space. Large spaces can be filled with lumps from the break down of old moulds;

• themixingmillcanbesimplerandcanbecontinuous;• thesandtransportandconditioningsystem,andthefines’

removal system can be simpler and more compact, as the system does not have any water or water vapour;

• plantcleaningiseasier,astherearenodeeptrenches,ele-vators, long conveyor belts, extractors, etc.;

• therearefewermovingparts,thereforeareducedpossibilityof unplanned plant stoppages;

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• thereislesssandintheproductioncycle;• thequantityandvolumeofadditivesaresmaller,andtheir

handling creates fewer problems;• thereislessenergyconsumption;• theinvestmentisasmallerburdenandcanbeplanned,as

the plant can be built in phases. casTing qualiTy

The casting system using flaskless moulds, without the restriction of the flask dimensions, enables very flexible plants to be created. These can handle patterns which are different in size and number. This means that productivity is improved, there is a better ratio of gating/risers to castings and fewer restrictions in the arrangement of pouring systems and feed systems.However, the most interesting feature of the “No-Bake” system is the improved production quality given by the improved rigidity of the moulds and the absence of any water.The advantages resulting from these can be summarised as fol-lows :• therigidityofthemouldmeansthattherearefewerdimen-

sions which diverge from the design drawings, there is less shrinkage and consequent saving in setting up feed heads for the casting (see point 9.2);

• theabsenceofwatereliminatesthetypicaldefectsofgreensand moulding such as rat tails, scabs, veining, hard spots, blowholes, etc. This means that the casting machining allo-wance can be reduced. This and the reduction of the dimen-sional variations, means that there is a reduction of machi-ning costs;

• flasks require constantly ongoingmaintenance to preventmisaligned castings. In the flaskless “No-Bake” system the problem is reduced to moulding frame maintenance;

• thequalityof themould is lessaffectedbytheskillof themoulder.

Any modification of the sand mixture characteristics can be carried out immediately, with direct benefits to the quality of the castings, as the amounts of sand in the process cycle are smaller.

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9.1 the no-bake contribution to the reduction of faults due to shrinkaGe in cast-iron castinGs

Shrinkage is the volume change which occurs when cast metal solidifies.These volume variations are due to metallurgical phenomena, but can also be caused by the deformation of the internal walls of the mould.The rigidity of the “No-Bake” mould excludes the latter and ena-bles the total exploitation of graphitic expansion. This enables the feed heads setting-up to be reduced; and for high graphite cast iron and certain other types of casting, to be eliminated altoge-ther.The phenomena of solidification and graphitic expansion, in cast-iron castings; especially in spheroidal graphite cast-iron, is worth further explanation. This will clearly show the benefits gained from the application of “No-Bake”, in the elimination of casting defects due to shrinkage.

9.1.1 cast-iron solidification

The cooling of steel, white cast-iron and non-ferrous metals, cau-ses an initial contraction called a “volumetric” contraction. This starts at the pouring temperature and ends at the temperature when solidification is complete. It is followed by a contraction cal-led a “linear” contraction, which starts at the completion of setting and ends at normal ambient temperature (see fig. 9.3).The process of volumetric contraction of steel and white cast-iron, differs from that of grey iron and ductile cast-iron. To understand this difference in behaviour it is necessary to make an introductory statement :Steel and cast-iron in the liquid state, contain carbon dissolved in the iron. In other words they are liquid or solid solutions of carbon in iron. As the temperature falls to the solidification temperature, the capacity of the iron to hold the carbon in solution decreases. The concentration of carbon which saturates the iron (beyond which it is thrown out of solution), distinguishes the steels from

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cast irons.In the spheroidal cast irons the separated carbon precipitates in the form of nodules, whilst in the grey cast irons, the carbon disperses in the form of flakes of graphite, as shown in fig. 9.4.These flakes are at first immersed in liquid iron and are finally set in solid iron. They are arranged in a way that calls to mind flower petals immersed in water or ice. The VolumeTric change in casT-irons

It can be easily understood how the separation of graphite can compensate wholly or partly for the volumetric contraction of the iron, altering its effect, according to the amount and the form of the graphite itself.The volumetric changes of grey cast-irons or the spheroidal graphi-te cast-irons, as a function of temperature, when graphed; give similar or intermediate curves to those shown schematically in fig. 9.5; according to the amounts and the form of the graphite they contain.The curves show the volumetric changes on cooling, from the pou-ring temperature, of two cast-irons. The sequence of the volume-tric changes is the following :• startingfromthepouringtemperature,thereisacontraction

which is shown for the two cast-irons by a1 and a2;

Fig. 9.3 - Diagram of the volume change on cooling, for steel, white cast iron, and non-ferrous castings.

Specificvolume(cm3/g)

Pouringtemperature

Primaryliquidcontraction

Solidificationcontraction

Rate of contractionof the solid

Temperature interval atsolidification

Temperature (°C, °F)

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Fig. 9.4 - The volumetric arrangement of the graphite flakes (a) and in a graphite nodule cross sec-tion (b).

Section as seen under the microscope

(a)

(b)

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• at the eutectic temperature, due to the separation of thegraphite, the liquid expands until the solidification of the eutectic mixture is almost complete; the degree of expan-sion of the two cast-irons is shown as b1 and b2;

• theremainingpartoftheeutecticmixtureisdepletedincar-bon and it solidifies like steel not like cast-iron giving a fur-ther secondary contraction shown by c1 and c2. In cast-irons with a high graphite content the graphitic expansion creates a pressure against the mould. In spheroidal cast-irons the pressure against the mould walls, is greater than that created by the grey cast-irons.

There are two reasons for this :- there is a greater amount of eutectic graphite;- there is a different solidification process, as explained below.

no-Bake and ducTile iron casTing

The solidification of the grey cast-irons starts against the mould

Fig. 9.5 - General diagram of the volume changes in spheroidal graphite cast-iron, and grey cast-iron. a) liquid contraction - b) expansion - c) secondary contraction

Temperature °C

∆V

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walls and extends gradually towards the thermal centre. The first solid layers therefore help the mould walls to resist the pressure arising from the graphitic expansion (see fig. 9.6).In the speroidal graphite cast irons however, the solidification takes place more slowly with more phase and temperature uniformity throughout the pour. This is due to the fact that spheroidal graphi-te cast-iron has a lower heat conductivity and also a narrower solidification interval.Under these conditions the mould walls must sustain all the expan-sion pressure on their own. The “No-Bake” moulds are able to contain this pressure without noticeable deformation, so the mould volume does not increase. The liquid therefore exits into the fee-dheads and penetrates into the interdendritic pores. The effect is such that it is possible to produce castings in spheroidal graphite

Fig. 9.6 - The solidification pressure as a function of the carbon equivalent and the modulus of cooling (the casting’s ratio of volume to surface).

Carbon equivalent

Pressure(Kg/cm

2 )

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cast-iron without feeding, or with only small feedheads to allow feeding up to take place at the first contraction phase.The essential conditions are that the mould must be rigid, the equi-valent carbon must be almost at eutectic concentration and the graphite contribution must be as great as possible. This is to ensu-re that the amount of graphite is sufficient to compensate for the volumetric contraction of the iron.The metallostatic pressure tends to force the mould walls apart and therefore to vary the entity of the volumetric contraction. In the case of the cast-irons described above, there could also be an additional pressure due to the separation of the graphite.From the above, the importance of a rigid mould is clear. This rigi-dity is provided by the “No-Bake” system, as is the solution to the problem of feeding the castings.The control of graphitic expansion can be achieved through ther-mal analysis.

9.2 the contribution of thermal analysis to the evaluation of the ten-dency of cast-irons to shrink

The metallurgical characteristics of a cast-iron casting depend on its content of carbon and silicon. These control the formation of the graphites and the degree of undercooling below the temperature of complete solidification, that is, the tendency of the iron to throw down the carbon as cementite instead of graphite at this tempera-ture. All the above elements are necessary to assess the amounts and the arrangement of the graphites, both for their effect on the metallurgical characteristics of the cast-iron under examination and for their effect on its volumetric contraction.The analyses of the parameters must be carried out on the metal in the ladle, before the casting is poured.Given the immediacy of thermal analysis, it is suitable for this pur-pose.The apparatus consists of two small crucibles each fitted with a thermocouple (see fig. 9.7). The thermocouples are connected to a pyrometer and to a thermal data processor, which calculates the carbon and silicon percentages and the extent of undercooling,

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expressed as degrees. The first determination is carried out with some tellurium in the crucible to make the thermal data easier to understand.The basic principal of thermal analyses, is as follows :The cooling of an alloy is marked by periods in which the cooling speeds up or slows down. These occur each time there are struc-tural transformations which are respectively endothermic or exo-thermic.These thermal variations are seen in the trace of the cooling curve, an example of which is shown in fig. 9.8. This schematic trace shows the cooling of a cast-iron and starts at the pouring tempe-rature Tp, in the crucible containing the tellurium. It cools at a uni-form rate until temperature T1 where there is a slowing down in cooling. This is due to an exothermic reaction taking place in the alloy in which the first austenite crystals are forming with composi-tion A.

Fig. 9.7 - Crucible fitted with a thermocouple.

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Cooling continues steadily until temperature T2 is reached. At this temperature the eutectic mixture B, composed of a mixture of austenite and cementite, starts to solidify. The formation of this mixture is also exothermic so there is a brief pause in cooling. The thermal history below temperature T2 does not interest us in our current context. The processing of the thermal data enables the percentages of carbon and silicon to be determined. To this end, the temperatures are transferred to a specific section of the Fe-C-Si state diagram (see fig. 9.9). As an example take the cooling curves of the two cast-irons which have 3.0 and 3.8% carbon respectively (see part A of fig. 9.9) and transfer them onto the state diagram shown schematically in part B of the same figure. The temperature TLB at point B of the curve AO shows that at temperature TLB crystals of primary austenite precipitate, and give rise to the irregularity shown in curve 1. This point corresponds to

Fig. 9.8 - A cooling curve for cast-iron

TIME

liquid

primary austenite

eutectic point

phosphorous eutectic

eutectic point

TEM

PE

RAT

UR

E

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3% carbon read from the abscissa of the state diagram. In curve 2 no thermal irregularity has been noted at this stage.A second thermal step is shown at temperature TSE and is shown in both curves. This is due to the exothermic reaction of the forma-tion of the eutectic austenite-cementite. The temperature at which this occurs is a function of the percenta-ge of silicon present and this indicates the silicon content. Point O in diagram B is also a function of temperature TSE, and if we look at curve 2, this point as it belongs to the curve AO allows us to read the curve 2 cast-iron carbon percentage (3.8), from the abscissa of the state diagram. The percentage of carbon does not tell us anything about the quantity of graphite, that is to say, whether or

not cementite is absent.This information can be obtained by a second thermal analysis carried out using the second crucible which does not contain tellu-rium. This enables us to obtain cooling curves of the type shown in

Fig. 9.9 - An example of the transfer of thermal information onto the Fe-C-Si State diagram for the determination of the percentage of carbon and silicon.

time concentration %C

liquid

Curve 1 = example with 3% CCurve 2 = example with 3,8%

primaryaustenite

+liquid

austenite p.+

eutectic

graphite p.+

eutectic

tem

per

atur

e

The cooling curve Diagrammatic section of the STATE DIAGRAM

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fig. 9.10. These refer to the same basic cast-iron, but inoculated with different materials, so that there are three different graphite formations. Therefore three different values are obtained for the volumetric contraction.The required information is given by the size of the undercooling ΔT below the eutectic temperature TE. The least volumetric contraction, therefore the greatest graphite release and the best mechanical characteristics, occur when the values of ΔT are not greater than 4°C.

9.3 mouldinG by Pressure shootinG

The characteristics of gravity moulding in automatic “Fast-loop” type plants, are well known (see figs. 9.11, 9.11/A, 9.13/A). They are appreciated by founders, especially for the flexibility of use with different patterns, the easy changing of patterns and the possibility of process control by computer.

Fig. 9.10 - Different undercooling values obtained with the same cast-iron, with three different types of inoculation.

GRAINS TYPE I GRAINS TYPE II GRAINS TYPE III

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In addition to the system’s many advantages, this has enabled increases in productivity to be achieved within the intrinsic limits of the system.These limits were exceeded when shot moulding was applied to the system.The experience gained in using core shooting machines, combined with the installation of computer control, enabled the combination of the two concepts ; that of Fast Loop moulding and that of core shooting, to produce the process of flaskless mould formation by pressure shooting.In practice, to achieve the same Fast Loop flexibility when using a “core shooter” in the moulding area, it was necessary to design a core shooter which could be adjusted each time to the dimensions of the pattern and the “mould box” presented at the filling (shooting), gassing and purging stations. It was therefore necessary to design a machine capable of matching the performance of each station to the pattern in use, at a given time. The shooting head automatically adjusts the shooting pressure, the shot time and the number of shots. The gassing head adjusts the gassing time and the purging station likewise.

9.3.1 the Process chemistry

In the shot moulding system, hardening is achieved by using a gas which is blown into the mould box. Specially designed vents in the pattern plate ensure the gas penetrates effectively into the most inaccessible cavities, ensuring that mould hardening takes place completely and rapidly throughout.The resins most commonly used are the phenol resins, the uretha-ne resins and the basic phenol resins.The first two categories are hardened by triethylamine (which has a less unpleasant smell than diethylamine), or by diethylamine which has a higher vapour pressure and therefore evaporates more easily.The basic phenol resins are hardened using methylformate (see fig. 2.5 page 24).These hardeners are liquids vapourized in a generator and driven

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Fig.

9.1

1 -

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Fig. 9.11/A - Fast Loop moulding area (continuous mixer, moulding area, mould boxes rollover-stripping unit, Fast Loop).

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by inert gas pressure. The generator can be an injector which ato-mises the liquid and the gas formed can be injected directly into the shooting system. In another type of generator, the vaporization of the catalyst, or of the second reagent, is carried out by bubbling an inert gas through it in the liquid state. The gas formed is then distributed to the several shooting systems. A third type of genera-tor heats the liquid amine (below its boiling point).The epoxy resins and the furan resins, when combined with oxidising agents, poly-merise in the presence of sulphur dioxide. These resins have a long “work time” and break down readily at mould shakeout. They are therefore especially suitable for use in aluminium casting.All the above processes require the moulds to be purged with inert gas to remove catalyst residues. The workplace must be well ven-tilated and all the necessary precautions must be taken for staff safety and environmental protection.

9.3.2. descriPtion of the mould shootinG Plant

A mould shooting plant has the following parts: (see figs. 9.12 - 9.12/A - 9.13 - 9.13/A).

The mixer

The sand-binder mixture is prepared in a type T. 36/15 mixer capa-ble of mixing 15 - 20 T./h. This is fed from a two compartment silo (to enable two sand types to be used).The metering of the different types of binder is accomplished using electronically controlled variable speed metering pumps. This ena-bles the flow to be quickly corrected to the rate required for the programme to be used.The binder can only be added when a probe indicates the presen-ce of sand in the feed tube, and after the sand feed box has ope-ned.This protective device eliminates the possibility that binder can be added to the shooting chamber on its own, and the reasoning is clear, given that the mixer is completely automatic.The mixer discharge port is fitted with a shutter gate, which at the

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1

1A

2

2A

108

9

34

5

67

7A

1

- N

ew

and

recla

imed

sand

silo

s

1A

-

Sand

+ c

ata

lyst

+ b

ind

er

mix

er

2

- P

att

ern

pla

tes F

ast

loo

p

2A

-

Mo

uld

sho

oting

unit

3

- M

ould

ro

llover/

str

ip s

tatio

n

4

- P

ain

ting

sta

tio

n

5

- D

ryin

g t

unnel

6

- C

ore

s’

sett

ing

lin

e

7

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ould

clo

sin

g s

tatio

n

7A

-

Mo

uld

tra

nsfe

rrin

g d

evic

e

8

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ouring

and

co

olin

g a

rea

9

- S

hakeo

ut

10

- S

and

recla

imin

g p

lant

Fig.

9.1

2 -

Layo

ut a

of F

ast

loop

pla

nt w

ith m

ould

sho

otin

g sy

stem

and

ver

tical

ly p

oure

d m

ould

s.

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9

34

5

67

8

8A

1

1A

2

2A

11

10

1

- N

ew

and

recla

imed

sand

silo

s

1A

-

Sand

+ c

ata

lyst

+ b

ind

er

mix

er

2

- P

att

ern

pla

tes F

ast

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uld

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ould

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sta

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ore

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pili

ng

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e

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Mo

uld

pile

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ner

9

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ouring

and

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olin

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rea

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and

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Fig.

9.1

2/A

- L

ayou

t of

a F

ast

Loop

typ

e p

lant

with

a

shoo

t m

ould

ing

unit

and

ver

tical

ly p

oure

d m

ould

s

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104

Fig. 9.13 - Detail of moulds shooting station

1

2

3

4

5

6

1 - Sand + catalyst + binder mixer 2 - Loading hopper 3 - Mould box 4 - Mould shooting area 4A - Mould shooting unit 5 - Gassing station 6 - Purging station

4A

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105

Fig.

9.1

3/A

- L

ayou

t of

fou

ndry

eq

uip

ped

with

a m

echa

nize

d N

o-B

ake

mou

ldin

g p

lant

(gra

vity

sys

tem

)

12

4

5

6

7

8

10

12

13

9

3

1

- N

ew

and

recla

imed

sand

silo

s

2

- S

and

+ c

ata

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+

bin

der

mix

er

3

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ould

sett

ing

and

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ern

pla

tes F

ast

Lo

op

4

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ould

ro

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ip s

tatio

n

5

- P

ain

ting

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6

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unnel

7

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ore

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sett

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lin

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8

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ould

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9

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lectr

ic m

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13

- D

ust

co

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r

14

- F

ett

ling

sho

p

11

14

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“No-Bake” as we see it - Part One

beginning of each mixing cycle holds back the sand for a few seconds, to ensure that the mixture is uniform.

The moulding PlanT

The moulding plant has the following parts :- A loading hopper with a capacity of 80 to 250 kg. The capacity can be adjusted by means of photocells.- A shooting system, “FOMES Patent” consisting of :a. A rapid opening shooting valve, positioned directly above the cartridge;b. A holding cartridge for the sand mixture. This has a device which enables the sand to be fluidised in the shooting phase;c. A shooting head consisting of an aluminium plate with a network of holes. The holes have inter-changeable steel bushes, to enable their diameters to be altered, or for them to be completely closed, as necessary. There are air vents round the edges of the plate.The shooting head moves from the loading position under the loa-ding hopper to the shooting position. If the amount of sand requi-red for the mould exceeds the quantity that can be held by the cartridge, the filling operation and the shoot need to be repeated, whilst maintaining a maximum cycle time of 30 seconds.The mould box arrives under the shooting head in a programmed sequence, as it arrives the upper part is cleaned with a rotary brush, to give a better seal against the shooting head.When this cleaning is completed, the mould box base plate rises and presses the mould box opening against the shooting head. This makes an effective seal between the moulding box and the shooting head and between the cartridge and the valve.The sand is shot at a pressure in the range between 2.5 and 5.0 bar, for a variable time. The pressure and the time both depend on the configuration and size of the pattern.

gassing - Purging

The filled mould box moves automatically to the gassing station and is raised up to make a seal against the gassing head. Following this it is lowered and moves towards the purging station.The gassing and purging operations can also be carried out in the same position

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“NO-BAKE” ADVANYAGES AND PROBLEMS

The BuFFer PosiTion

The mean time of each operation is 30”.There may be some variation, either more or less than this mean time. A buffer zone has therefore been provided, with four buffer positions, between the purging zone and the mould stripping area.

mould sTriPPing

From the buffer position, each mould box passes to the rollover. The rollover is used for mould stripping and it has a hydraulically operated stripping plate. This plate works the ejector pins under the pattern plate.The ejectors act when the mould box is turned over. This ensures that the mould box is emptied, even if mould setting is incomplete. The device ensures that the mould box is emptied within the cycle time.

Passing The moulds To The Pouring lines

The shooting system of mould making enables a higher production rate than gravity moulding. This means that putting the moulds in order, and passing them to the pouring lines, needs to be automa-ted.The type of automation differs according to whether the mould is poured horizontally or vertically.

horiZonTally Poured moulds

After the half moulds have been formed, painted, any cores set and the moulds assembled they are sent to an automatic handler, are sorted automatically and passed to the pouring lines.

VerTically Poured moulds

The moulds are closed by an automatic handler on an oscillating platform after being formed and painted. A second handler takes the moulds and stacks them in a container.This container is arranged vertically and the moulds are placed in one at a time until it is full. The cut-off point is determined by a

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“No-Bake” as we see it - Part One

photocell.When the container is full, an automatic screw clamp locks the stack of moulds in place.The container, full of well clamped moulds is turned to the horizon-tal position and is transferred to the pouring line in conformity with a programme.There are always two containers at the pouring station, to prevent lost time between one pour and the next, and in the case of two different types of alloy, to allow the moulds in the two containers to be arranged according to the alloy to be used. After pouring and cooling, the containers are transferred to above the shake out, are unlocked and overturned to empty them.

9.3.3. the advantaGes of mouldinG with a mould shooter

The advantages brought to traditional Fast Loop mould forming, by the shooting system, can be summarised as follows :• theoperationsarecompletelyautomaticthroughout. Operators are only used for painting or core setting;• the almost complete elimination of excess sand even in

small moulds;• extremelyeasystrippingusingmechanicalejectors;• themouldshaveahigherdegreeofcompactioncompared

to vibrated moulds, due to the use of the patented Fomes shooting head. In this respect it is essential that there should be air vents at the mould points which are most

difficult to fill with sand;• byusingahighlyrefractorysand,thehighdegreeofcom-

pacting enables mould painting to be eliminated in specific cases. To achieve this end, a fine sand can be used as a flaskless mould enables the gas to escape easily;

• thewholeproductioncanbeautomaticallycontrolledbyacomputer;

• moulds stacked vertically can be clamped into positionautomatically;

• it ispossible tousestackedmouldswithsingleordoublesided impressions and for pouring to be horizontal or verti-cal.

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“NO-BAKE” ADVANYAGES AND PROBLEMS

9.3.4 the advantaGes of vertical PourinG

Pouring castings “vertically” offers several advantages. The use of flaskless moulds separated vertically requires that the design of the gating system and risers is different to the design of those used in horizontal pouring.If there are impressions at different levels, the design of the gating system must take into account each impression at each level, even if these are reciprocally connected.The basic premise is that all the cavities at each different level should be filled at the same rate, that is in the same pouring time.The pouring rate of the castings in the bottom half of a mould should be the same as the pouring rate of those in the top half of a mould. If the pouring rate of the lower half is too high there may be faults due to turbulence. The commonest faults are penetration, “pinholes” and inter-dendritic gas porosity.The correct sequence for filling flaskless mould cavities, requires the adjustment of the gating system for each pattern.The advantages of the vertical pouring system are as follows :• thereisawiderchoiceofmetalflowarrangements;• thepouringtimeisreduced;• grindingofthegatesisreducedtoaminimum;• thegasescanberemovedmoreeasily;• insomecases,thereiseasiercastingsfeeding;The limiting factors of mould shooting are :• themaximumsizeofthemouldis 880 x 880 x 310 x 310 mm.;• themouldboxmustbemadeofaluminiumduetotherough

handling, and to guarantee the mechanical strength to resist the forces in play at the time of shooting;

• thepatternsmaybemadeofwood,orof resin for limitedproduction runs.

9.3.5. fields of aPPlication

The mould shooting system enables the automation of the entire “No-Bake” mould making cycle.

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“No-Bake” as we see it - Part One

The procedure has been applied successfully in the following fields :• steel castings (the process described eliminates pinhole

faults, as the resin is almost nitrogen-free);• highqualityaluminiumcastings;• shortrunsofhighqualitycastings;• prototypes;• castings inductilecast-iron(astheseare improvedbythe

highly compacted and rigid mould, and the reduction of the shrinkage tendency).

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10. SAND RECLAMATION (REGENERATION)

Sand regeneration has been part of the casting cycle for some time as an intrinsic and indispensable stage, due to the following moti-ves :• therequirementtohavemoreheatstability;• thescarcityofgoodqualitynewsand;• thecostof newsand, continues to rise, forpurchase, for

storage and for the transport required;• thecostofdisposalofusedsandisalsorisingrapidly;• thedifficultyoffindingsuitabledisposaltips;The thermal expansion of silica due to the morphological changes in the quartz, are pratically irreversible in the re-cycled sand owing to the repeated heating and to the build up of the mineral oxides. The use of this sand reduces the superficial defects in castings caused by heat deformation of the mould surfaces.The regeneration of contaminated sand is a recovery and re-con-ditioning process, which uses the following operations :• cleaningthefilmofhardenedbinderandcatalystfromthe

surface of the grains, to maintain these at acceptable levels;

• removalofmetalsandotherforeignbodies;• removalofinertmaterials,andfinesduetothebreakageof

sand grains, using a system which does not cause any fur-ther breakage of the grains;

• coolingthesand.The mechanical regeneration of sand agglomerated with sodium silicate requires a preliminary operation of heating the sand to a temperature which may be as high as 300°C. This is necessary to dehydrate the silicate and remove both the dilution water and the combined water.Furthermore, the intrinsic hygroscopicity of sodium compounds needs to be taken into consideration when designing the plant.Fig. 10.1 shows diagrammatically the processes for the recovery of sand using heat, mechanical and wet, processes.The efficiency of a sand recovery plant and its state of maintenan-ce, can easily be determined by carrying out the following checks on the regenerated sand :• determinationofthegranulometricspectrumandtheindex

of fineness;

111

SAND RECLAMATION (REGENERATION)

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“No-Bake” as we see it - Part One

Fig. 10.1 - Block diagram for processing used foundry sand (sand recovery).

MOULD

Breaking out with/without mould breakage

Used sand, core residues, lumps

Preparation of the sand, removal of contaminants,

size reduction

Removal of binders and additives

Final treatment

Removal of metals

Breakdown of lumps and size reduction

Removal of metals

Sieving

Wet regenera-tion, stirring,

solvent, neutrali-sation of the

water

Heat treatment, removal of vola-

tiles, combu-stion of combu-

stibles, size reduction

Mechanical regeneration, by milling, grinding & mechanical/

pneumatic beating

Pre-coolingContact, vibrating bed, steam cascade

Pre-coolingContact, vibrating bed, steam cascade

Drying

Regenerated sand

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SAND RECLAMATION (REGENERATION)

• determinationofthemechanicalcharacteristicsofthesandmixture before re-use, compared with those of a new sand mixture. A reduction of the resistance to deflection is accep-table, provided that it is not more than 20%;

• determinationofthesinteringtemperature.Theimportanceof this is illustrated by the following : a sand which sinters at 1,450°C. and which has a quartz content of 99%, sinters at 1,250°C when the quartz content falls to 96% due to contamination with low melting point compounds;

• determinationofthelossonignition;• determinationoftheacidorbasecontent.

In figs. 10.2 and 10.3 the changes to compression strength and bending strength resistance are compared, for a sand which has 100% new sand and one in which 80% is reclaimed sand with only 20% new sand.Table A lists the optimum characteristics for a new foundry sand;

Fig. 10.2- Development of the mechanical strength of 100% new sand mixtures and 80% recove-red sand mixtures, over time.

Time

Com

pre

ssio

n st

reng

th (N

/cm

2 )

80/20 recycled sand/new sand

new sand

1% resin0,3% catalyst

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“No-Bake” as we see it - Part One

and for a regenerated (reclaimed) sand, for use in both mould and core making. Changes in these values depend on the specific use conditions.

10.1 the deGree of reGeneration

The degree of regeneration is given by the reduction of the quanti-ty of undesirable substances in the sand per operating cycle. In practice, these are binder residues and catalyst residues. The value is expressed as a percentage of L.O.I. reduction.The regeneration plant plays a role in achieving a specific degree of foundry sand regeneration, this achievement is summed with that due to the heat effect at pouring.A major contribution towards achieving a given degree of regene-

Fig. 10.3 - Change of bending resistance of 100% new sand mixtures and 80% recovered sand 20% new sand mixtures, over time.

Time

1% resin0,25% catalyst

80/20 recovered sand/new sand

new sandB

end

ing

resi

stan

ce (N

/cm

2 )

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SAND RECLAMATION (REGENERATION)

ration, is due to the following factors :• the type and percentage of resin used and the type of

catalyst;• a low ratio of sand to casting (for a high degree of self-

regeneration);• thequantityofnewsandadded.The acceptable level of regeneration is defined by the quality and quantity of gas produced by the decomposition of the binder and the catalyst; and the acceptability of its effect on the alloy being cast. In analytical terms, it is necessary to define for every alloy, the acceptable level of the loss on ignition at saturation. This is the value of the maximum percentage of volatile residues in a recycled sand, irrespective of the number of times it has been recycled.The percentage loss of a new sand mixture on burning in relation to the ratio of sand to casting, is given in fig. 10.4. The values show the effect of this ratio on the degree of regeneration. It is very important to know the relationship between the acceptable loss on ignition at saturation and the self-regeneration contribution. In fact the difference enables the required characteristics of the regenera-tion plant to be defined.The curves of the graphs shown in fig. 10.5 clearly show how the quantity of unburnt resin in a recycled and regenerated sand tends to level off at a maximum value, called the saturation value. The data were derived from a series of recycled sands with different degrees of total regeneration, including the part due to the pouring and the part due to the regeneration process.The graph refers to reclaimed sand, which has been continuously recycled.It is possible to calculate the maximum value of the organic mate-rial at saturation, as a function of the degree of regeneration of the system. The following formula is valid for regenerated sand to which no new sand has been added : Z•100I max = _______ - Z rWhere: I max = the loss on ignition as a maximum value (i.e. at saturation) Z = the percentage of resin plus catalyst r = the degree of regeneration expressed as a percentage of

the “loss on ignition” reduction each cycle, following pou-

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Fig. 10.4 - Percent variation of the loss on ignition as a function of the sand to metal ratio.

Ratio of sand to cast

Pouring temperature in °C

Loss

on

igni

tion

%

Fig. 10.5 - The increase in the loss on ignition as a function of the number of recyclings and for different degrees of regeneration, in total r%. The illustration is a sand mixture with 2% resin and an organic catalyst.

Number of times the sand is recycled

Total degree of regeneration in %Z = 2% resin pluscatalyst

Loss

on

igni

tion

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SAND RECLAMATION (REGENERATION)

ring and eventual regenerationAs an clarifying example, we give an example of the calculation of the ignition loss at saturation:Z = 1,5 (the percentage of resin plus catalyst)r = 30% (the degree of total regeneration)

(1,5•100)I max = ________ - 1,5 = 3,5% 30

The formula has to be modified as follows if any additions of new sand are made :

Z•100100-NI max = ( _______ - Z )•______

r 100

N = the percentage of new sand added at each cycle

(1) - It is very important to keep the acid demand value constant .

table a: foundry sand characteristics

NEW REGENERATED

SiO2 > 99 % > 99 %

Sintering point > 1500 ° C -

Moisture 0,1 % 0,1 %

Clays 0,1 % 0,3 %

Loss on ignition 0,2 % max 0,6 %

pH 6 - 8 5-6 (1)

Acid demand in cm3 of

N/10 HCl/50 g. sand 1 max (1)

at pH 4,5

Oolite contamination level - 3 % max

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“No-Bake” as we see it - Part One

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PATTERN MAKING

11. PATTERN MAKING

Collaboration between the customer, the founder and the pattern maker, is an essential premise for the construction of a pattern with an acceptable life, which meets the needs of a given method of casting and which has the necessary accessories required by the foundry technology. Whilst the involvement of the founder is expressly cited in paragra-ph 5.2 of the UNI 473 regulation and other standard regulations, it often happens that he is not involved at the pattern design stage. He is therefore required to correct or compensate for errors of pat-tern design, and/or construction.The pattern maker must be able to assimilate all the information and requests given him by the founder, to be aware of the intrinsic requirements of the casting process in general and of those speci-fically required by the “No-Bake” process.The first thing to bear in mind, is that the “No-Bake” system of moulding will accept a pattern which is not as strong as those required by the traditional moulding process. This is due to the flow characteristics of the sand mixture, and the fact that this system does not require such a forceful sand compaction as the traditional method. The cost savings due to the lighter construction compen-sate for the extra costs, due to the greater effort required to make a pattern suitable for this moulding system. For example soft wood can be used, retaining hard wood only for those parts which are stressed during pattern removal from the mould.Against this, the low flexibility of the sand mixture requires a pat-tern without undercuts which would require forcing at the pattern stripping stage. Forcing might not damage the pattern, but would certainly damage the mould. The same considerations apply to the rigorous attention required to setting the pattern draft angles, to ensure easy pattern stripping.The construction technique must therefore ensure that the “seg-ments”, the inserts and the connections generally, must not move due to the wood flexing or to moisture, or the handling it receives.The “No-Bake” mould becomes somewhat flexible during the har-dening phase, but never to the extent that it enables the pattern to be “rapped”.

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“No-Bake” as we see it - Part One

To carry out the removal of the pattern therefore, the draft angles are normally 2 to 3° according to the height of the parts [see the rules of the pattern making technical schedule in Part II]. Draft angles are very important for pattern stripping.The quality of the release agents is also important as is the vibra-tion of the pattern plates. The pattern surface finish is most impor-tant both as regards the construction and its painting. All the cor-ners must be well radiused and the pattern must be very securely fastened to the plate to prevent sand infiltration between it and the pattern.The loose pieces for undercuts must be very limited, as they may move during the sand mixture compaction process, and form so-called “shadow” areas which do not permit the sand mixture to be easily compacted. Furthermore, the joints of these parts can easily become set with use, and they then become mould retention points.On this basis, it is preferable that the parts of the design which create the undercuts, should be produced with cores.That is, unless the design of the piece can be modified to remove the problem.In defining the amount of linear contraction and the machining allo-wance, it is essential to assess the possibility that parts of the casting will be restrained by the rigid mould, on cooling. This can be caused by differences of as little as 13 per 1000 less than the theoretical contraction shown in the UNI standards (see Part II - pattern making technical schedules).In addition to the geometry of the casting, the extent of the phenomenon is a function of the hot flow properties of the metal. With rigid alloys there can be a build-up of casting stress to breaking point, either when cold, or even in the mould.In this event it is necessary to make the above parts of the casting using cores made with less rigid binders. In any case, the play of the core in the print makes that point less rigid. This reasoning is valid and applicable, if the design of the piece cannot be modi-fied.Pattern construction for “No-Bake” casting can be carried out using the materials specified in UNI 473.The “No-Bake” process is particularly suitable for use with one-time patterns made of expanded polystyrene, given that it does not require more than a light sand compaction and this would not

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PATTERN MAKING

affect the pattern. It should be remembered that the use of metal patterns and plates can slow down resin setting if they are cold, due to their greater heat conductivity compared with wood. It should also be remembered that wooden master patterns used to make metal patterns, must be constructed to compensate for dou-ble contraction.If resin models are used, it is essential to check that they are com-patible with both the release agent and the resins to be used in the mould.Consult the supplier about this.The parts of wooden patterns which are likely to suffer hard wear, or which are delicate, can either be made in metal, or constructed so that they can be easily replaced.Patterns on plates must be well fastened to the supporting plate. Pattern plates of plywood are recommended as they are light and keep the system rigid at pattern stripping. In the event of long sto-rage periods they also significantly reduce the possibility of the pattern becoming distorted.When chills are used, they are heavy and have fins to improve their cooling capacity and their contact with the sand is improved. This prevents them, moving due to their own weight, before the sand sets.Clearly, other fastening systems can also be used.“No-Bake” moulding enables castings to be made with very close tolerances compared to the drawings. This may result in pattern costs being increased by up to 30%, due to the increased preci-sion/accuracy required. This extra cost is recovered later during the machining phase, due to the reduction of the machining allo-wance.

11.1 tyPes of construction

Large items, to be produced as single pieces, or as a short run, can be hand made with a skeleton model or with profiled green sand, or using the “No-Bake” system with the normal technique used for moulding using dry sand. The “No-Bake” system is not suitable for moulding with intermdiate part moulds and the pattern must be constructed for moulding with two part moulds. The central parts which cannot be stripped, are made of several core parts, to pro-

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“No-Bake” as we see it - Part One

duce the intermediate shape required. (see fig. 11.1).If the bottom ingate must be cut through the core, the appropriate core print must be made in the pattern (see fig. 11.2). The sand plug and the forming of a stepped parting line, will be replaced by the core system, therefore the pattern must have the required core prints.Hand moulding using complete piece patterns which need a false print, can be made by the usual technique, and also by the “No-Bake” process.Clearly, the part which must receive the pattern provisionally, that is the “odd-side”, should be made with green sand for greater con-venience.Patterns which have a parting line which is not horizontal, require a false print, which is usually difficult to produce using the “No-Bake” process.

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PATTERN EQUIPMENT

Fig. 11.1 - Example of a mould change from several sections to two parts.

(a) ORIGINAL DESIGN

Mould section

Mould section

Mould section

Mould section

Mould section

Mould section

Mould section

Mould section

Flas

ksFl

asks

Patternsection 5

Patternsection 4

Pattern section 3

Patternsection 1

Pattern

Core 3

Core 2

Core 1

Cope core print

Pattern

(c) SOLUTION FOR TWO PART MOULDS

Coren. 1Core

n. 2

Mould cope half

Mould drag half

(b) ORIGINAL DESIGN

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“No-Bake” as we see it - Part One

Fig. 11.2 - Example of a pouring device installed in a core. The device is necessary due to the change in the mould system from several parts to two parts.

Cope

Drag

Runner bar

Parting line

Core insert for gating system

Back ingates

Back ingates

AA

Core insert for gating system

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CASTING DESIGN FOR THE “NO-BAKE” SYSTEM

12. CASTING DESIGN FOR THE “NO-BAKE” SYSTEM

The “No-Bake” system does not require skilled operators, as the moulding operation can take place almost without the operator needing to touch the mould. The essential conditions for exploiting this advantage are, that the equipment is correctly laid out and that the casting design takes account of the process requirements.It must be remembered that the plasticity of the sand-resin mixture is so low that it is not easy to extract pattern parts such as ribs, hubs, flanges, guides, etc., or to mould manually to form plugs. Moulding in three or more parts is almost impossible (unlike green-sand), when it is necessary to make undercut profiles. When using the “No-Bake” process many parts must be made with cores to enable the castings to be removed from the mould. This means increased costs, for both equipment and labour, and these may be sufficient to cancel out the advantages of the “No-Bake” system. In this event it is necessary to take corrective action by applying good practice in casting design, which takes account of the requi-rements of the “No-Bake” process.

12.1 mould sTriPPing in The “no-Bake” sysTem: examPles oF correcT casTing design

Pieces can always be moulded. However, the moulding cost is lower, the more good design practice imposed by foundry practice is taken into account. These practices are even less flexible when moulding with the “No-Bake” process.The examples given below show how critical it is and at the same time how simple it is, to adopt correct profiles for easy mould removal.The diagrams show this elegantly and do not need detai-led explanations.

Example No. 1, fig. 12.1

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“No-Bake” as we see it - Part One

A) The two loose pieces of the pattern for the shape of the undercut, must be removable to enable the pattern to be extracted. They are extracted after the main pattern is removed.b. In this solution, the shape of the projection is not undercut and does not interfere with the extraction of the pattern. The pattern for the projection is therefore attached to the main pattern body.

Example No. 2, fig. 12.2In fig. A the undercut parts must be moulded with loose pieces on

Fig. 12.1 - Modification of the shape of a projection to eliminate an undercut and to make mould removal possible.

undercut

Fig. 12.2 - Simplification of a baseplate mould to allow pattern extraction.

Parts of the pattern which cannot be removed

Pattern removal difficult

Poor design

Correct design

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pattern which are be extracted from the mould after the main pat-tern is removed. The alterations to the shape of the casting shown in B simplify pattern construction and the removal operation.

Example No. 3, fig. 12.3The undercut parts have been transferred to the core, without alte-ring the casting profile significantly.

Example No. 4, fig. 12.4The shape shown in A can only be moulded horizontally.Foundry techniques, of which the designer may be unaware, indi-cate solution B as the one to be carried out, as it permits vertical casting and pattern extraction, in the direction shown by the arrow. This second solution also reduces the risk of the cast containing inclusions; and of the core moving under flotation forces. “No-Bake” prescriptions need to be added to these generic ones. Solution A requires the use of a balance print, to ensure that the core does not float under the upward thrust of buoyancy, unless supports are used. This would mean increasing the sand quantity, to fill a larger flask.

Fig. 12.3 - Elimination of mould removal problems.

Transfer to the interior of details which cause removal difficulties

Incorrect Correct

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Example No. 5, fig. 12.5This is an example of an undercut boss. With green sand or dry sand moulding this can be shaped using loose pieces, or a sand plug. Using “No-Bake” moulding the boss needs to be core formed (see a), or extended up to the flange (see b). For a casting without a flange (see c), the boss needs to be lengthened as shown in (d).

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Fig. 12.5 - The undercut boss prevents mould removal; and this means that a core will be needed for the application of the “No Bake” technique. (see a); the best solution is to lengthen the boss till it meets the flange (see b). For (c) the solution is given in (d).

Fig. 12.4 - Design changes to simplify mould removal.

direction of mould removal

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CASTING DESIGN FOR THE “NO-BAKE” TECNIQUE

Example No. 6, fig. 12.6If the pattern is large or medium sized, the webs shown in fig. A can be made using loose pieces on pattern. If they have the correct draft and appropriate rapping they can be extracted from the mould, provided that it is slightly plastic, as when made with green sand or dry sand.This is practically impossible with a “No-Bake” mould, which requi-res that the webs must be arranged in the core box. Figs. B and C show economical alternatives which ca be used with any type of

moulding.

Example No. 7, fig. 12.7The moulding for a complete piece pattern as in A, requires a par-ting line which is not level but is stepped. This pre-supposes that this can be achieved in making the mould. This type of moulding is not possible with “No-Bake”, unless a core is used, and the design needs to be changed, as shown in B. This solves the problem.

Fig. 12.6 - Changing the orientation of the webs in fig. A, to those shown in figs. B and C allows mould removal.

Fig. 12.7 - Replacement of an offset joint with one in the same plane.

An unacceptable design A much better

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Example No. 8, fig. 12.8In fig. A the feet are connected in pairs in the two half patterns, using dovetail joints.They are held in the mould when each half pattern is removed, and are later extracted inwards.The simplification offered by solution B is very clear.

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Fig. 12.8 - Projections joined together eliminate the need for a core.

incorrect

correct