MCE 332L

Material and Manufacturing Process

Laboratory

Lab.6
Sand Casting
 Contents
 Manufacturing Processes.
 Shaping Processes
 Casting Processes
 Advantages and Disadvantages of Casting
 Metals for Casting
 Classification of Casting Process
 Sand Casting
 Sand Casting Mold
 Steps of Sand Casting
 Heating and Pouring
 Solidification and Cooling
 Casting Quality and Defects
 Procedure
 Discussion and Results

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 Manufacturing Processes
 Are the methods and steps in which raw materials are
transformed to products.

 Manufacturing Process can be broadly classified into

three categories:
1. Shaping processes.
2. Joining processes.
3. Finishing processes.

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Manufacturing Processes

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 Casting Processes
 Casting is a process in which molten metal flows by gravity or
other force into a mold where it solidifies in the shape of the
mold cavity.

 The term casting is also applied to the part that is made by

this process.

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 Casting Processes
 Most of the manufactured parts start
its journey with casting process.

 Casting includes both the casting of

ingots and the casting of shapes.

 The term ingot is usually associated

with the primary metals industries; it
describes a large casting that is
simple in shape and intended for
subsequent reshaping by processes
such as rolling or forging.

Aluminum ingots 6
 Casting Processes
 Continuous Casting:
The process is replacing ingot casting because it dramatically
increases productivity.

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 Casting Processes
 Slabs, blooms and billets can be produced from ingots

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 Casting Processes
 The metal casting process
involves three sequential steps:
• Liquefying of metallic
material by properly heating
it in a suitable furnace.
• Pouring of hot molten metal
into a previously made
colder mould cavity.
• Extraction of the solidified
cast from the mould cavity.

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 Casting Processes
 Casting is usually performed in a
Foundry.

 A Foundry: is a factory equipped for

making molds, melting and handling
molten metal, performing the casting
process, and cleaning the finished
casting.

 Workers who perform casting are

called Foundrymen

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 Advantages of Casting
 Casting can be used to create complex part geometries, including both
external and internal shapes.

 Some casting processes are capable of producing parts to net shape. No

further manufacturing operations are required to achieve the required
geometry and dimensions of the parts. Other casting processes are near
net shape, for which some additional shape processing is required (usually
machining) in order to achieve accurate dimensions and details.

 Casting can be used to produce very large parts. Castings weighing more
than 100 tons have been made.

 The casting process can be performed on any metal that can be heated to
the liquid state.

 Some casting methods are quite suited to mass production.

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 Disadvantages of Casting
 Limitations on mechanical properties.

 Porosity and other defects.

 Poor dimensional accuracy and surface finish for some casting processes.

 Safety hazards to humans when processing hot molten metals.

 Environmental problems.

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 Metals for Casting
 Most commercial castings are made of alloys
rather than pure metals
– Alloys are generally easier to cast, and properties of
product are better

 Casting alloys can be classified as:

– Ferrous
– Nonferrous

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 Classification of Casting Process
 The casting processes can be classified into two broad categories :
• Expendable mold casting processes: mold is usually broken to
free the solidified cast. These molds are made out of sand,
plaster, or similar materials, whose form is maintained by using
binders of various kinds. Sand casting is the most prominent
example of the expandable-mold process.
• Permanent mold casting processes: the mold can be reused.
Mostly, it is made of metal.

 Also, the pattern used to prepare the mold can be expandable or

permanent.

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 Sand Casting
 Sand Casting consists of placing a pattern (having the shape of
desired casting) in sand to make an imprint, incorporating a
gating system, filing the resulting cavity with molten metal,
allowing the metal to cool until it solidifies, breaking a way the
sand mold and removing the casting.

 Typical part made by sand casting: machine-tool bases, engine

blocks, cylinder heads and pump housing.

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 Sand Casting Mold
 In general, the main tool in casting is the
mold (or the die in case of die casting).

 The mold could be open as in (a) or closed

as in (b).

 In an open mold, the liquid metal is simply

poured unit it fills the cavity .

 Where in a closed mold, a passageway

called the gating system allows the molten
metal to flow into the cavity.

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 Sand Casting Mold
 Basically, sand casting mold consists of the following elements: Flask,
sand, cavity, core, runner, riser, pouring cup, parting line.

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 Sand Casting Mold
 Flask: is the box that contains mold parts.

 If the mold is open type the flask is formed from

one part. As in (a).

 If the mold is closed type the flask is formed from

two halves. As in (b).

 The upper half is called the Cope.

 The lower half is called the Drag.

 The two halves are separated at the Parting Line.

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Sand Casting Mold
 Sand Casting Mold
 Sand: Most sand casting operation use silica sand (SiO2) or silica mixed
with other additives.

 Sand is inexpensive and can resist high temperatures.

 There are two general types of sand: naturally bonded (bank sands) and
synthetic (lake sands) which is more preferred because its composition can
be controlled more acutely.

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 Sand Casting Mold
 Sand is held together by a mixture of water and bonding clay. Typical mix
is: 90% sand, 3% water and 7% clay.

 Clay is used as a cohesive agent to bond sand particles and give sand
strength.

 Bonding agents, other than clay, include:

• Organic resins (phenolic resins)
• Inorganic resins (sodium silicate and phosphate).

 Additives are sometimes combined with the mixture to increase strength

and/or permeability.

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 Sand Casting Mold
 Sand affects the quality of the sand mold. Following are indicators of mold
quality:
• Strength: the mold’s ability to maintain its shape and resist erosion
caused by the flow of molten metal; it depends on grain shape, adhesive
qualities of the binder, and other factors
• Permeability: allow gases and steam evolved during casting process to
escape easily.
• Thermal stability: ability of the sand at the surface of the mold cavity to
resist cracking and buckling upon contact with the molten metal.
• Collapsibility: ability of the mold to give way and allow the casting to
shrink without cracking the casting and the ability to remove the sand
from the casting during cleaning.
• Reusability: can the sand from a broken mold be reused but after
treatment.

 When selecting sand type certain tradeoffs need to be made, for example fine
grained sand enhance mode strength but has lower permeability.

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 Sand Casting Mold
 Sand molds can be classified according to sand conditions to three main
types:
• Green sand mold:
 Mixture of sand, water and clay.
 Green means the mold contains moisture at the time of pouring.
 Posses sufficient strength, good collapsibility, good permeability,
good reusability and it is least expensive.
 Moisture in the sand can cause defects.

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 Sand Casting Mold
• Dry sand mold:
 Made using organic binder rather than clay.
 The mold is baked to improve strength.
 Provider better dimension control compared to green sand mold.
 More expensive.
 Lower production rate due to drying time.
 Application is limited to medium and large castings in low to
medium production rate.

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 Sand Casting Mold
• Skin dried mold:
 Getting the partial advantages of dry sand mold by drying the green
sand mold to a depth of 10 – 25mm at the mold cavity surface using
a torch or heating lamp.

 The one we are using in our lab.

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 Sand Casting Mold
 Cavity: to form the cavity a pattern is needed.
 The pattern is a full sized model of the part.
 The mold cavity is formed by packing sand around the pattern which has
the shape of the part, then separating the mold into two halves and
removing the pattern.

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Sand Casting Mold

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Sand Casting Mold

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 Sand Casting Mold
 The pattern is enlarged (oversized) to account for shrinkage and machining
allowance in the final casting.

 Only in cast Iron with high carbon content expands and does not shrink
due graphitization during final stages of solidification.

 Pattern materials:
• Wood: common material because it is easy to work with but it warps
and abraded by the sand compacted around it.
• Metal: Last much longer but more expensive.
• Plastic: Compromise between wood and metal.

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 Sand Casting Mold
 Pattern types:
• Solid pattern:
 Made of one piece
 Easiest to fabricate.
 Could have problems when making the sand mold like: determining
the location of parting line between the two halves and
incorporating the gating system and sprue.

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 Sand Casting Mold
• Split patterns:
 Consists of two pieces.
 Appropriate for complex part geometries.
 The parting line is determined by the two halves of the pattern,
rather than by operator judgment.
 Moderate production quantities.

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 Sand Casting Mold
• Match plate patterns:
 The two pieces of the split pattern are attached to opposite sides of
a wood or metal plate.
 Holes in the plate allow the cope and drag sections of the mold to
be aligned accurately.
 Higher production rate.

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 Sand Casting Mold
• Cope and drag patterns:
 The split pattern halves are attached to separate plates, so the cop
and drag can be fabricated independently.

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 Sand Casting Mold
 Core: is the full scale model of the interior
surfaces of the of the part.

 Pattern defines the external shape of the cast

part. If the cast has internal surfaces a core is
needed. It is placed inside the mold cavity.

 Usually made of sand compacted into the

desired shape.

 It must include allowance for shrinkage and

machining.

 The core may or may not need supports to

hold it in position in the mold cavity during
pouring.

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 Sand Casting Mold
 These supports called chaplets.

 They are made of a metal with a higher

melting temperature than casting metal.

 On pouring and solidification the chaplets

become bonded into the casting.

 The portion of chaplet protruding from the

casing is cut off.

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Sand Casting Mold

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 Sand Casting Mold
 Gating system: It is the channel, or
network of channels, by which molten
metal flow into the cavity from outside
of the mold.

 Consists of:
• Downsprue (sprue): way from
outside to runner.
• Runner: leads metal to main cavity.
• Pouring cup: at the top of the sprue
to minimize splash and turbulence
as metal flows into sprue.

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 Sand Casting Mold
 Riser: it is a reservoir in the mold that
serves as source of liquid metal for the
casting to compensate for shrinkage
during solidification.
It should be designed to freeze after the
main casting in order to satisfy the
function.

 Vent: in sand casting the natural

porosity of the sand mold permits the air
and gas to escape through the walls of
the cavity.
In permanent metal molds, small vent
holes are drilled into the mold to permit
removal of air and gases.

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Sand Casting Mold

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Sand Casting Mold

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 Steps of Sand Casting
 Basic steps in sand casting:
• Pattern making
• Mold making
• Melting and pouring
• Cleaning

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 Steps of Sand Casting
Detailed steps of sand casting
1. Place pattern on molding board. 18. Separate cope from drag.
2. Place drag parting surface down on molding 19. Moisten drag mold edges with swab
board. 20. Use draw spike to loosen pattern.
3. Riddle sand over pattern until covered. 21. Remove the pattern.
4. Press sand around pattern with fingers. 22. Cut gate from sprue to pattern cavity.
5. Completely fill drag with sand. 23. Cut riser in cope to channel hot metal
6. Use a ram to pack sand. 24. Spray, swab, or dust the mold surfaces
7. Remove excess sand with strike rod. with coating material.
8. Make vent holes for gases to escape. 25. Re-assemble cope and drag to prepare
9. Place bottom board on drag. for pouring.
10. Turn over drag and remove molding board. 26. Weight cope to prevent seepage at
11. Smooth molding sand. parting line.
12. Add fine coat of parting sand. 27. Pour the metal.
13. Place the cope on the drag. 28. Allow to cool.
14. Add sprue pin ~ 1” to side of pattern. 29. Separate and clean casting.
15. Fill, ram, & vent cope as done. 30. Reclaim the sand & clean the flask.
16. Withdraw sprue pin.
17. Create a funnel opening.
Check this out: http://foundry101.com/1.htm

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 Heating and Pouring
 Heating the metal
Heating furnaces are used to heat the metal to molten temperature
sufficient for casting.

The heat required for casting is the sum:

1. Heat to raise temperature to melting point.
2. Heat of fusion to convert from solid to liquid.
3. Heat to raise molten metal to desired temperature of pouring.

For Aluminum,
Melting Temperature is 660oC
Pouring Temperature is 700oC-750oC

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 Heating and Pouring
 Pouring the molten metal
For this step to be successful, metal must flow into all regions of the mold
before solidifying.

Factors affecting pouring operation:

1. Pouring temperature: is the temperature of molten metal as it is
introduced to the mold.

Superheat is the amount of heat must be removed from the molten

metal between pouring and when solidification commences. It is
equal to the difference between pouring temperature and the
temperature at which freezing begins (melting point).

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 Heating and Pouring
2. Pouring rate: if the rate is too slow the metal will freeze before filling
the cavity and if it is excessive, turbulence can become a serious
problem.

3. Turbulence: it should be avoided for the following reasons:

• It accelerates the formation of metal oxides which entrapped
during solidification.
• Increase wear and erosion to the mold.

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 Solidification and Cooling
 Solidification of Metals
The solidification process differs depending on
whether the metal is a pure element or an alloy.

Pure Metals
Solidifies at a constant temperature (melting
point)

Local solidification time: is the time of actual

freezing during which the metal’s latent heat of
fusion is released into surrounding mold.

The total solidification time: is the time taken Cooling curve

between pouring and complete solidification

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 Solidification and Cooling
Because of the chilling action of the mold wall, a thin skin of solid metal is
initially formed at the interface immediately after pouring. This cooling
action causes the grains in the skin to be fine and randomly oriented.

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 Solidification and Cooling
Solidification progresses inward toward the center of the cavity and grain
formation and growth occur in a direction away from (opposite to) the
heat transfer as needles or spines of solid metal.

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 Solidification and Cooling
Grains that have a favorable orientation, called columnar grains, grow
preferentially. Grains that have substantially different orientations are
blocked from further growth. Grain development is known as
homogeneous nucleation meaning that the grains (crystals) grow upon
themselves starting at the mold wall.

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 Solidification and Cooling
As these spines enlarge, lateral branches form, and as these branches
grow, further branches form at right angles to the first branches. This type
of grain growth is referred to as dendritic growth. It occurs in both pure
metals and alloys.

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 Solidification and Cooling
These treelike structures are gradually filled-in during freezing, as
additional metal is continually deposited onto the dendrites until complete
solidification has occurred

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 Solidification and Cooling
Most Alloys
Most alloys freeze over a temperature range rather than at a single
temperature.

As temperature drops, freezing begins at the temperature indicated by the

liquids and is completed when the solidus is reached.

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 Solidification and Cooling
The start of freezing is similar to that of the pure metal.

Due to the temperature spread between the liquidus and solidus, both
liquid and solid metal coexist during the dendritic growth.

Dendrite structures that have formed sufficiently is the solid portion that
traps small islands of liquid metal in the matrix.

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 Solidification and Cooling
This solid–liquid region is called the mushy zone

The mushy zone can be relatively narrow, or it

can exist through out most of the casting
depending on factors like heat transfer rate out
the hot metal and the difference between
liquidus and solidus temperatures.

Slow heat transfer out of the hot metal and a

wide difference between liquidus and solidus
temperatures promote larger mushy zone.

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 Solidification and Cooling
Segregation
Dendrites as they start to form favor the metal with the higher melting
point.

As freezing continues and the dendrites grow, an imbalance in composition

between the metal that has solidified and the remaining molten metal is
developed.

This composition imbalance is finally manifested in the completed casting

in the form of segregation of the elements.

The segregation is of two types, microscopic and macroscopic.

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 Solidification and Cooling
At the microscopic level, the chemical composition varies throughout each
individual grain.

This is due to the fact that the beginning spine of each dendrite has a higher
proportion of one of the elements in the alloy.

As the dendrite grows in its local vicinity, it must expand using the remaining
liquid metal that has been partially depleted of the first component.

Finally, the last metal to freeze in each grain is that which has been trapped by
the branches of the dendrite, and its composition is even further out of
balance.

Thus, we have a variation in chemical composition within single grains of the

casting.

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 Solidification and Cooling
At the macroscopic level, the chemical composition varies throughout the
entire casting.

Since the regions of the casting that freeze first (at the outside near the mold
walls) are richer in one component than the other, the remaining molten alloy
is deprived of that component by the time freezing occurs at the interior.

Thus there is a general segregation through the cross-section of the casting,

sometimes called ingot segregation.

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 Solidification and Cooling
Eutectic Alloys
Eutectic alloys constitute an exception to
the general process by which
alloys solidify.

A eutectic alloy is a particular composition

in an alloy system for which the solidus and
liquidus are at the same temperature.
Hence, solidification occurs at a constant
temperature rather than over a
temperature range.
Eutectic temperature

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 Solidification and Cooling
 Solidification Time
Solidification takes time. It depends on:
 Size and shape of casting
 Mold material
 Thermal properties of casting metal
 Pouring temperature relative to melting point

Chvorinov’s rule shows that a casting with a higher volume-to-surface

area ratio cools and solidifies more slowly than one with a lower ratio.

This principle is important in the design of the riser. Design the riser to
have a larger volume-to-area ratio so that the main casting solidifies first
to minimizes the effects of shrinkage

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 Solidification and Cooling
The desired directional solidification is achieved by the following methods:
1. Observing Chvorinov’s rule in the design of the casting itself, its
orientation within the mold, and the design of the riser system that
feeds it.

For example, by locating sections of the casting with lower V/A ratios
away from the riser, freezing will occur first in these regions and the
supply of liquid metal for the rest of the casting will remain open until
these bulkier sections solidify.

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 Solidification and Cooling
2. Use chills (internal or external) heat sinks that cause rapid freezing in
certain regions of the casting.

Internal chills are small metal parts placed inside the cavity before
pouring so that the molten metal will solidify first around these parts.
It has a chemical composition similar to the cast itself.

External chills are metal inserts in the walls of the mold cavity that can
remove heat from the molten metal more rapidly than the
surrounding sand in order to promote solidification

With external chills Without external chills

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 Solidification and Cooling
3. A void premature solidification in sections of the mold nearest to the
riser. Specifically the passageway between the riser and the main
cavity.

Even it is desirable to minimize the volume in the connection (to

reduce wasted metal), the cross-sectional area must be sufficient to
delay the onset of freezing.

The passageway should be made short in length, so that it absorbs

heat from the molten metal in the riser and the casting.

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 Solidification and Cooling
 Riser Design
Riser feeds liquid metal to the casting during freezing in order to
compensate for solidification shrinkage

To function, the riser must remain molten until after the casting solidifies.

The riser represents waste metal that will be separated from the cast part
and remelted to make subsequent castings.

It is desirable for the volume of metal in the riser to be a minimum.

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 Solidification and Cooling
It could be side riser, top riser, open or blind (entirely enclosed within the
mold)

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 Casting Quality and Defects
 Defects could be :
• General defects common to all casting processes
• Defects related to sand casting process

 Defects common to casting processes:

Misruns: A casting that has solidified before completely filling mold cavity

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 Casting Quality and Defects
Cold Shut: Two portions of metal flow together but there is a lack of fusion
due to premature freezing

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 Casting Quality and Defects
Cold Shot: Metal splatters during pouring causing the formation of solid
globules of metal that become entrapped in casting.

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 Casting Quality and Defects
Shrinkage Cavity: Depression in surface or internal void caused by
solidification shrinkage that restricts amount of molten metal available in
last region to freeze

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 Casting Quality and Defects
Microporosity : consists of a network of small voids distributed throughout
the casting caused by localized solidification shrinkage of the final molten
metal in the dendritic structure

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 Casting Quality and Defects
Hot tearing: also called hot cracking, occurs when the casting is restrained
from contraction by an unyielding mold during the final stages of
solidification or early stages of cooling after solidification

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 Casting Quality and Defects
 Defects related to sand casting:
Sand Blow: Balloon-shaped gas cavity caused by release of mold gases
during pouring

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 Casting Quality and Defects
Pinholes: Formation of many small gas cavities at or slightly below surface
of casting

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 Casting Quality and Defects
Sand wash: which is an irregularity in the surface of the casting that results
from erosion of the sand mold during pouring

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 Casting Quality and Defects
Scabs: rough areas on the surface of the casting due to encrustations of
sand and metal

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 Casting Quality and Defects
Penetration : When fluidity of liquid metal is high, it may penetrate into
sand mold or core, causing casting surface to consist of a mixture of sand
grains and metal

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 Casting Quality and Defects
Mold shift : A step in cast product at parting line caused by sidewise
relative displacement of cope and drag

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 Casting Quality and Defects
Core shift : is similar to mold shift, but it is the core that is displaced, and
the displacement is usually vertical

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 Casting Quality and Defects
Mold crack: occurs when mold strength is insufficient, and a crack
develops, into which liquid metal can seep to form a ‘‘fin’’ on the final
casting.

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Procedure

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Procedure

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 Discussion and Results
 Discuss the effects of molding sand on the finished part.
 Discuss the locations of the sprue and riser pins on the finished
casting.
 Discuss the quality of the finished casting and report any defects
observed.
 Discuss possible preventing measures.
 Discuss all possible casting defects.

Support your discussion with pictures.

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 References
 M F Ashby, Material Selection in Mechanical Design, Butterworth-
Heinemann, 1999.

 Mikell P. Groover, Fundamentals of Modern Manufacturing,

Materials, Process, and System. Hoboken, NJ: John Wiley & Sons,
Inc., 2010.

 Serope Kalpakjian, Manufacturing Processes for Engineering

Materials. USA: Addison-Wesley Publishing Company, 1991.

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