core and core making

7/29/2019 Core and Core Making

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Core and core making

Introduction1) A core is essentially a body of materials which forms components of the mold. It

possesses sufficient strength to be handled as an independent unit.

2)

Core is an obstruction which when positioned in the mold, naturally does not permitthe molten metal to fill up the space occupied by the core. In this way a coreproduces hollow casting

3) Cores are required to create the recesses, undercuts and interior cavities that areoften apart of castings.

4) Cores are employed as inserts in moulds to form design features that are otherwiseextremely difficult to produce by simple molding.

5) A core may be defined as a sand shape or form which makes the contour of acasting for which no provision has been made in the pattern for molding.

6) Cores are made up of sand, metal, plaster or ceramics.7) Cores are used to:a. Form the air-space between the cooling fins of an air cooled engine cylinderb. Make the water cooling chamber in internal combustion engine.

Different functions, purpose of cores:1. For hollow casting, core provides the means of forming the maininternal cavities.

2. Cores may form a part of green sand mould.

3. Cores may provide external undercut feature

4. Cores may be employed to improve the mould surface.

5. Cores may be inserted to achieve deep recesses in the castings

6. Cores may be used to strengthen the mould.

7. Some times the mould may be completed simply by assemblingthe core pieces or core.

8. Cores may be used to form the gating systems of large sizemoulds

Essential characteristics of (dry sand) cores:Cores may possess:

1. Sufficient strength to support itself and to get handled withoutbreaking.

2. High permeability to let the mould gases escape through themould walls.

3. Smooth surface to ensure a smooth casting.

4. High refractoriness to withstand the action of hot moltenmetal(metal penetration).

5. High collapsibility in order to assess the free contractor of thesolidifying metal.

6. Those ingredients which do not generate mold gases.

Core making procedures:

1. Core sand preparation

2. Making core


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3. Baking core

4. Finishing core

5. Setting core

Making the core:1. Small cores can be made manually in hand rammed core boxes.

2. Cores on mass scale are rapidly produced on a variety of coremaking machines, to name a few,

a. Jolt machine.

b. Core roll over machine

c. Sand slinger

d. Core extrusion machine

e. Core blower

f. Shell core machine

Core box:1. A core box is basically a pattern for making cores

2. Core boxes are employed for ramming cores in them

3. Core boxes impart the desired shape to the core sand.

4. Core boxes range from simple wooden structures to precisionmetal assemblies which possess long life under extracting condition.

Types of core boxes:a. Half core boxb. Slab or dump core boxc. Split core boxd. Left and right hand core boxe. Strickle core boxf. Loose piece core box.g. Gang core box.

Equipments used for the baking of cores:A. Core ovensa. Batch type

i. Drawer typeii. Rack type

b. Continuous typeB. Dielectric bakersC. Radiant bakers

Finishing of cores:

1. Baked cores are finished before they can be set in the mald.2. Core finishing consists ofa. Cleaning

i. Trimmingii. Brushing

iii. Coating


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iv. Muddingb. Sizingc. Core assembly

Setting of cores:

1. Core setting means placing cores in the mold2. In order to obtain correct cavities in the castings the cores shouldbe accurately positioned in the molds.

3. Cores in the moulds should be firmly secured so that they canwithstand the buoyancy effect of the being poured molten metal.

4. Small cores are set in the moulds by hand whereas big coresmay required a crane for the purpose.

Types of coresA. The state or condition of core

a. Green sand coreb. Dry sand core

B. The nature of core materials employed

a. Oil bonded cores

b. Resin bonded cores

c. Shell cores

d. Sodium silicate cores

C. The type of core hardening process employed

a. Co2 process

b. The hot-box process

c. The cold set process

d. Fluid or castable sand process

e. Nishiyama process

f. Furan no-bake system

g. Oil no-bake system

D. The shape and position of the core

a. Horizontal core

b. Vertical core

c. Hanging or cover core

d. Balanced core

e. Drop core or stop off core

f. Ram up core

g. Kiss core

1. Green sand core:

a. Green sand cores are formed by pattern itself.

b. A green sand core is a part of the mold.

c. A green sand core is made out of the same sand fromwhich the rest of mold has been made i.e molding steel.


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2. Dry Sand cores

a. Dry sand cores, unlike green sand cores are not producedas a part of the sand.

b. Dry sand cores are made separately and independent of

that mold.

c. A dry sand core is made up of core sand which differsvery much from the sand out of which the mold is constructed.

d. A dry sand core is made in a core box and it is baked afterramming.

e. A dry sand core is positioned in the mold on core seatsformed by core print on the patten.

f. A dry sand core is inserted in the mold before closing thesame.

1) Oil bonded cores:a) Conventional sand cores are produced by mixing silica sand with a small

percentage of linseed sand.

2) Resin bonded cores:a) Phenol resin bonded sand is rammed in a core box.b) The core is removed from the core box and baked in a core oven at 375 to 450 f to

harden the core.

3) Shell cores:a) Shell cores can be made manually or on machines.b) The procedure of making shell cores is as follows:1) The core box is heated to temperature of the order of 400 to 600 F.2) Sand mixed with about 2 to 5 % thermosetting resin of phenolic type is either

dumped or blown into the preheated metal core box.3) Where sand blowing is employed, it is preferred to use resin precoated sand to

avoid resin segregation.4) The resin is allowed to melt to the specified thickness.5) The resin gets cured.6) The excess sand is dumped and removed.7) The hardened core is extracted from the core box.8) Cores thus produced needs no further baking.c) Shell core posses very smooth surface(3125 micro mm root mean square) and

close tolerance .(+_ 0.003 mm/mm)d) Shell core making process can be mechanized and several core making machines

are commercially available.e) High permeability is achieved in shell core making . f) Shell cores can easily stored for future use.g) Shell cores are costly as compared to cores produced by other methods.

4) Sodium silicate CO2 cores:a) These cores use a core material consisting of clean, dry sand mixed with a solution

of sodium silicate.b) The sand mixture is rammed into the core box.


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c) The rammed sand while it is in the core box is gassed for several seconds with CO2gas. As results a silica gel forms which binds sand grains into a strong solid form.

Na2SiO3+CO2Na2CO3+SiO2 (silica sand)d) Cores thus produced usually need no baking.e) Cores thus formed possess more strength than the oil /resin bonded cores.f) Unhardened cores are not handled so that there is no chance of braking or sagging

of cores.g) Core dryer is not required.h) Core formed by CO2 process are used in the production of cast iron, steel,

aluminum and copper base alloy castings.i) The used sand mixture however can not be recovered and reused.

5) Hot box process :a) It uses heated core boxes for the production of cores.b) The core box is made up of cast iron, steel or aluminum and possesses vents and

ejectors for removing core gases and stripping cores from the core box respectively.c) Core box is heated from 350 to 500 F.d) Heated core box is employed for making shell cores from dry resin bonded mixtures.e) Hot core boxes can also be used with core sand mixture employing liquid resin

binders and a catalyst.

6) The cold set process :a) While mixing the core sand, an accelerator to the binders.b) The sand mixture is very flowable and is easily rammed.c) Curing begins immediately with the addition of accelerators and continues until the

core is strong to be removed from the core box.

d) A little heating of the cores hardens it completely.e) Cold set process is preferred for jobbing production.f) Cold set process is employed for making large cores.

7) Castable sand process :i. A setting or hardening agent such as dicalcium silicate is added to sodium silicateat the time of core sand mixing.ii. The sand mixture possesses high flowability and after being poured in the corebox, it chemically hardens after a short interval of time.

iii. As compared to CO2 process, where it may not be possible to gas the full coreuniformly and to obtain uniformly hardened cores, castable sand process produces

much better and uniform results.iv. Castable sand process is best suited for large jobbing work.

1) Nishiyama process :a) Nishiyama process uses sodium silicate bonded sand, which is mixed with 2% finely

powered ferrosilicon.b) Hardening occurs because of exothermic reaction of silicon with Noah produced by

hydrolysis in the solution of sodium silicate.c) Cores thus made posses short bench life.

2) Furan no-bake system :

a)

The core sand mixture contains washed sand dried sand with clay content less than0.5%, furan no-bake resin 2% and activator (phosphoric acid) 40%.


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b) The basic reaction between the furan resin and phosphoric acid results in an aciddehydration of the resin.

c) The core sand mixture has high flowability and needs reduced rodding.d) Uniform core hardness, exact core dimension, better fitting cores, lower machining

and layout costs and reduction of oven baking are some of the good characteristics

of cores made by this system.

3) Oil no-bake process :a) The process employs a synthetic oil binder which when mixed with basic sands

activated chemically produces cores that can be cured at room temperature.b) The sand may consists of:- Washed and dried sand 500 kg- Oil no-bake binders and catalyst 7 kg- Oil no-bake cross linking agent 1.4 kgc) In oil no-bake process, the polymerization reaction results in a complete and uniform

setting of the complete core sand mass.

d) This process assures better depth of set, fast baking, easier core withdrawal andlower production cost as compared to furan or oil bonding process.

4) Horizontal core :

5) Vertical cores :

6) Hanging or cover core :
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7) Drop or stop off core:

8) Balanced core :

9) Ram up core :

10) Kiss core :
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Core applications :1) Core and core forms greatly increase the versatility of molding and casting

operations.2) Before being used for forming recesses and holes in the castings, cores are also

employed:a) As a strainer, gates and pouring cups.

b)

As riser core

c) For making moldsd) As core mold in centrifugal casting processe) As slab core for increasing casting output from one mold.f) For increasing production from mold plate molding.
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core and core making

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