mold & mold making

8/2/2019 Mold & Mold Making

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SAND CASTING

MOLD AND MOLD

MAKING


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Schematic of foundry sand processes and material flows


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Important features of foundry sands:

Good refractory properties - capacity to endure high temperatureswithout melting

Grain size - small better surface finish on the cast part- large more permeable, allowing gases to escape during

pouring

Distribution of grain size in the mixture

Shape of the individual grains irregular shapes is stronger because ofinterlocking, compared to round grains

Disadvantage: interlocking tends to reduce permeability

Most common sand to usedSilica (SiO2)or silica mixed with other minerals

The Foundry Sand


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Irregular grain shapes produce stronger mould

Larger grain size ensures better permeability - Allow gases and steam escape

Smaller grain size results in a good surface finish

Selection of Sands


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Mold materialThe grains of sand are held together by a mixture of water and bonding clay

Ingredients of Moulding Sands

Differ in: Average grain size,grain distribution, andgrain shape Chemical composition

Refractoriness andthermal stability

Bonding agent Source of the strengthand plasticity, with suitablewater content

e.g. of clay minerals:bentonites

Control of the moisture content ofthe molding sandCoats the clay flakes surface,help in bonding develop strength

Sand(SiO2)

90%

Clay

7%

Water

3%


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Properties required in moulding material used to determine the quality of the sand mould


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Moulding Sand Properties

The properties depends on several factors:

The sand ingredients The methods of preparing the sand for molding The method of molding employed in using the sand Variables related to the casting e.g. weight, shape, kind of

casting alloy, and gating design

Effect of the Ingredients:

Each of the ingredients can have

important effects in the properties

Principal ingredient

Silica SandGrains

Effects of the Sand Grains:

Casting surface finish, mold

permeability, and strength,

refractoriness, and expansion

characteristics are all influenced by

the sand grain portion of the

mixture

Sand-grain contour of the mold

cavity

Fine grains smooth wall at the

metal interface


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Sand Grains and

Permeability:

Coarser sand greater

permeability

Finer sand lower

permeability

The grain size distributionhas a pronounced effect

on permeability

Narrowest distribution

range and minimum of

fine materials such as silt

or clay presentmost

permeable Permeability is also

promoted by the rounded

type of sand grain

Sand Grains and

Strength:

Strength surface area

of sand grains available for

binding

Fine sands present more

surface area and candevelop high strength, but

of course more clay is

required

Size distributionwide

size distributions favour

strength, while narrow

distributions reducestrength

Angular sand grains

More strength

Sand Grains and

Refractoriness:

Refractoriness - ability of

the molding material to

resist temperature of the

liquid metal so it not get

merged with the metal High refractoriness

sand grains ofmaximum

purity and size

Impurity which discolour

silica lower its fusion point

Finer grains appear to be

more easily merged thancoarser ones



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Sand Grains and

Expansion:

Wide size distribution

dense packing of the

grainscause expansion

problems

Fine sands also expands

more

Clay and Sand Strength:

For a given clay type and

content, there is an

optimum water content

The effects of the clay on

dry and hot strengths are

quite important

Too low and dry strength

permits washing of the

sand by the metal, and dirt

in the castings

Too low or too high a hot

strength is also

undesirable Aluminium castings

only green and dry

strength have appreciable

importance since casting

are poured at low

temperatures

Clay Content and

Permeability:

Permeability are reduced

by fine material in the

sand

Increasing clay content

lowers permeability

Higher clay content alsorequire more tempering

water, and hence more

steam is formed when the

metal is poured



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Clay Content & BulkDensity:

Clay content also influence

the bulk density achieved

by the sand during

ramming A sand having minimum

bulk density has much

void space and will have a

good permeability

adequatewith its sand

grain characteristics

Clay Content &Expansion:

Clay content of 10 to 14 %

in the sand mixture are

accompanied by minimum

confined-expansion value,0.03 to 0.04 in. per in. as

measured 2500 F

High clay contents

together with the proper

amount of water and

ramming if the sand thus

favour thermal stability

Clay Content andOther Properties:

High hot and dry strengths

are developed by

bentonite and bentonite-

fire clay mixtures lesscollapsibility

Combustible materials

may be added to promote

collapsibility

Excess clay clay balls

Good mixing is required



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- Mixture of sand, clay, and water

- green sand in the mold is moist

- Least expensive method of making molds

Advantages:

- Sufficient strength for most applications- Good collapsibility

- Good permeability

- Good reusability

Drawbacks:

- Moisture in the sand can cause defects insome casting

- Also called cold box mould

- Made using organic binders rather than clay

- The mold is strengthen by baking it in an

ovenAdvantages

- High dimensional accuracy

Drawbacks

- More expensive

- Lower production rate because of drying time

- The mold begins like a green sand mold, butadditional bonding materials are added

- Mold cavity surfaces are dried by torch orheating lamp to increase mold strength

- Used for large castings- Better dimensional accuracy and surface

finish

Drawbacks:

- Distortion to the mold is greater

- Castings susceptible to hot tearing

- Slower production rate

- Synthetic liquid resin is mixed into the sand

- Hardens at room temperature

- Good dimensional accuracy control in highproduction

Types of Sand Mould

Green sand mold Dry sand mold

Skin-dried mold No-bake mold


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Open Molds and Closed Molds

Two forms of mold: (a) open mold, simply a container in theshape of the desired part; and (b) closed mold, in which the moldgeometry is more complex and requires a gating system

(passageway) leading into the cavity.


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The Sand Casting Mold


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POURING CUP: This is where the metal is poured into the mold.

SPRUE: The vertical channel from the top of the mold to the gating and riser system. Also, a

generic term used to cover all gates, runners and risers.

RUNNER: The portion of the gate assembly that connects the sprue to the casting in gate or riser.

GATE: The end of the runner in a mold where molten metal enters the mold cavity.

RISER: A reservoir of molten metal provided to compensate for the contraction of the metal as it

solidifies.

BLIND RISERS:Risers that are completely contained within the mold.

OPEN RISERS:Risers that are open at the top to the outside environment.MOLD CAVITY: The impression in a mold produced by the removal of the pattern. When filled

with molten metal it forms a casting.

COPE: Upper or top most section of a flask, mold or pattern.

PARTING LINE: A line on a pattern or casting corresponding to the separation between the partsof a mold.

DRAG: Lower or bottom section of a flask, mold or pattern.

VENTS: Vents help to assist in the escape of gases that are expelled from the molten metal during

the solidification phase of the metal casting process.

INGATE/CHOKE AREA: Once at the sprue base the molten material must pass through the ingate

in order to enter the inner area of the mold. The ingate is very important in flow regulationduring the casting operation.

Elements of a Gating System:


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The pouring cup, downsprue, runners, etc., are known as the mold gatingsystem, which serves to deliver the molten metal to all sections of themold cavity. The aims are to providing defect-free casting by:

1. Rapid mold filling

2. Minimizing turbulence

3. Avoiding erosion

4. Removing inclusions

5. Controlled flow and thermal conditions

6. Minimizing scrap and secondary

operations

Gating System


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-As a reservoir from which the moltenmetal will moves smoothly into the sprue- Main function:

-To reduce momentum of the liquidflowing into the mold by settling first

into it- To minimize splash and turbulenceas the metal flows into downsprue

- Main requirements for pouring cupdesign:

-One of the wall should inclined about45 - to absorb metal momentum andavoid vortex formation-Deep enough to avoid turbulence- During pouring, the pouring cupshould be kept full to avoid vortexformation

Elements of a Gating System

Pouring Cup

Proportion ofpouring cup dimension


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- A channel through which the molten metal is brought into the partingplane where it enters the runners and gates to ultimately reach themould cavity- Require smaller area of cross section at the bottom to gain velocity toflow-Tapered to gradually reduce the cross section to eliminate problem

of air aspiration

Sprue

Proportion of sprue height


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- A reservoir for metal at the bottom of the sprue- To reduce momentum of the molten metal and flow through therunner in a uniform wayprevent splashing- Dimension- general guideline :

-Well area = 5 X sprue choke area-Well depth = runner depth- well diameter (for a narrow and deep runner)

= 2.5 X runner width (in two runner system)= 2 X runner width (in one runner system)

Sprue Base Well


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- Connect to sprue allowing molten metal to enter mold cavity- Design consideration:

-sharp corners or sudden change of section should be

reduced to avoid turbulence and gas entrapment-circular cross section is preferable to avoid heat loss- width ~ 2 X runner depth- the entry into runners from sprue base well should be madeas smooth as possible

Runner

- The runner is extended a little further after it encounters the in-gate- To trap contaminants in the molten metal

Runner Extension

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