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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
Moulding Sand Properties
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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
Moulding Sand Properties
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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
Moulding Sand Properties
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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