Download Types of Casting

Types of Casting
By Akhileshwar Nirala
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Sand casting
Shell mold casting
Investment casting
Plaster mold casting
Ceramic mold casting
Die casting
Centrifugal casting
Sand casting is defined as pouring of molten metal
into a sand mold (molds are generally provided
with a cavity of the shape to be made) and allowing
it to solidify inside the mould. Various patterns are
used to create cavity in the molds wherein, pattern
can be said as the replica of the final object to be
made with some modifications. Depending on
production quantities, different pattern materials
namely wood, aluminum, ferrous metals are used
in practice
Casting Terminology
Casting
Casting =
Pattern making + Mold making + Pouring
Molten Metal + Cooling + Removal + Inspection
Sand Casting :
Expendable Mold casting process uses sand as
molding material
Mold Making
 Consists : Sand Preparation and Mold Making
 Sand Properties :
Refractoriness (high Temp resistance), Cohesiveness,
Permeability, Collapsibility
 How to get these properties?
Mold Making Methods:
Hand Ramming, Sand Slinger, Jolting, Squeezing,
Combination
Pouring
Pouring System:
Pouring cup/Basin:
Rectangular Shape-No
vortex-No suction of
Gases
Sprue: Short-Less
KE –Less
turbulenceTapered shape
Sprue Well :
Dissipate KEPrevent splashing –
less turbulence
Pouring
Riser
 Need :
Directional Solidification
Feeding molten metal to compensate for solidification
shrinkage
Criteria: Riser should solidify after the casting.
Types:
Image Courtesy: wiki
Cooling/Solidification
Cooling Curve for a Pure Metal:
Pouring Temp.: Temp of first liquid
entering Mold cavity
Alloy-Distinct M.P.-freezing
range
High Cooling Rate-fine
structures-good
mechanical properties
Start of
Solidification
End of
Solidification
Chvorino’s rule :
ts=B(V/A)n
B=Mold constant
n=1.5 to 2
Effect of Cooling rate:
Process
Sand Casting
Permanent Mold
Die Cast
Solidification
Yield
Strength(MPa)
55
62
110
Tensile
strength(MPa)
131
158
227
Elongation(%)
8
10
9
Cast Structure:
Image Courtesy: wiki
Removal of Casting:
Shake out-vibrating machine
Shot blasting-Removal of internal surface sand
Trimming:
Excess material-manual cutting, sawingTrimming Press
Inspection:
Nobody is perfect…Sand Casting too…Defects…
Casting Defects:
Image Courtesy: IITM, Manufacturing Processes, Lecture notes
Different Sand Casting Processes:
Green Sand Casting:
88% silica sand, 9% clay,3% water
CO2 Molding:
Sodium Silicate(3-6%)-CO2 flow- Strength Gain
Shell Molding:
Silica Sand + Thermo setting phenolic resin+ heat sensitive liquid catalystshot on preheated metal pattern-shell formation
Shell mold casting is a metal casting process similar to sand
casting, in that molten metal is poured into an expendable
mold. However, in shell mold casting, the mold is a thinwalled shell created from applying a sand-resin mixture
around a pattern. The pattern, a metal piece in the shape
of the desired part, is reused to form multiple shell molds.
A reusable pattern allows for higher production rates,
while the disposable molds enable complex geometries to
be cast. Shell mold casting requires the use of a metal
pattern, oven, sand-resin mixture, dump box, and molten
metal.
The shell mold casting process consists of the following steps:
Pattern creation - A two-piece metal pattern is created in the
shape of the desired part, typically from iron or steel. Other materials
are sometimes used, such as aluminum for low volume production or
graphite for casting reactive materials.
Mold creation - First, each pattern half is heated to 175-370°C
(350-700°F) and coated with a lubricant to facilitate removal. Next,
the heated pattern is clamped to a dump box, which contains a
mixture of sand and a resin binder. The dump box is inverted,
allowing this sand-resin mixture to coat the pattern. The heated
pattern partially cures the mixture, which now forms a shell around
the pattern. Each pattern half and surrounding shell is cured to
completion in an oven and then the shell is ejected from the pattern.
Mold assembly - The two shell halves are joined together and
securely clamped to form the complete shell mold. If any cores are
required, they are inserted prior to closing the mold. The shell mold is
then placed into a flask and supported by a backing material.
Pouring - The mold is securely clamped together while the molten
metal is poured from a ladle into the gating system and fills the mold
cavity.
Cooling - After the mold has been filled, the molten metal is allowed
to cool and solidify into the shape of the final casting.
Casting removal - After the molten metal has cooled, the mold can be
broken and the casting removed. Trimming and cleaning processes are
required to remove any excess metal from the feed system and any
sand from the mold.
Introduction:
• Investment casting is an industrial process and it
is also called lost-wax casting . This is one of the
oldest known metal-forming techniques from 5000
years ago.
• This castings allow the production of components
with accuracy, repeatability, versatility and integrity
in a variety of metals and high-performance alloys.
• It is generally more expensive per unit than die
casting or sand casting, but has lower equipment
costs.
Process:
Moldmaking:
The first step in investment casting is to manufacture the wax pattern for the process.
A mold , known as the master die, is made of the master pattern. The master pattern
may be made from a low-melting-point metal, steel, or wood. Rubber molds can also
be cast directly from the master pattern.
Produce the wax patterns:
The pattern for this process may also be made from plastic; however it is often made of
wax since it will melt out easily and wax can be reused.
Assemble the wax patterns:
The wax pattern is then removed
from the mold . Depending on
the application multiple wax patterns
may be created so that they can all be
cast at once. In other applications,
multiple different wax patterns may
be created and then assembled into
one complex pattern. The multiple
patterns are attached to a wax sprue ,
and then it make a tree. Finally it is
dressed, which means any other imperfections
are addressed so that the wax now looks like
the finished piece.
Investment:
• The metal casting pattern is then dipped in a refractory slurry whose
composition includes extremely fine grained sand, water and binders. A
ceramic layer is obtained over the surface of the pattern. The pattern is
then repeatedly dipped into the slurry to increase the thickness of the
ceramic coat.
Dewax & burnout:
The investment is then allowed to
completely dry, which can take 16 to 48
hours . Drying can be enhanced by
applying a vacuum or minimizing the
environmental humidity. Most shell
failures occur at this point because the
waxes used have a thermal expansion
Coefficient., so as the wax is heated it
expands and induces great stresses. The mold is then subjected to a burnout,
which heats the mold between 870 °C and 1095 °C to remove any moisture and
residual wax, and to sinter the mold.
Pouring:
The investment mold is then
placed cup-upwards into a tub
filled with sand but if there are
thin sections in the mold it may
be filled by applying positive air
pressure or pressure assisted
Pouring .Then after pouring the
mold is left for cooling.
Removal:
The shell is hammered, vibrated, water jeted , or chemically
dissolved to release the casting. The sprue is cut off and recycled.
Advantages of Investment casting:
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Many Intricate forms with undercuts can be cast.
A very smooth surface is obtained with no parting line.
Dimensional accuracy is good.
Certain unmachinable parts can be cast to preplanned
shape.
• It may be used to replace die-casting where short runs are
involved.
• With this casting we can get the exact shape of the pattern.
Disadvantages of Investment casting:
• This process is expensive, is usually limited to small
casting, and presents some difficulties where cores
are involved.
• Holes cannot be so small.
• Investment castings require very long productioncycle times versus other casting processes.
• This process is practically infeasible for high-volume
manufacturing, due to its high cost and long cycle
times.
Application:
• Investment casting is used in
the aerospace and power generation industries to
produce turbine blades with complex shapes or
cooling systems.
• Investment casting is also widely used by firearms
manufacturers to fabricate firearm receivers,
triggers, hammers, and other precision parts at low
cost.
• Industries that use standard investment-cast parts
include military, medical, commercial and
automotive.
Plaster mold casting is a metalworking casting process similar
to sand casting except the molding material is plaster of
Paris instead of sand. Like sand casting, plaster mold casting is an
expendable mold process, however it can only be used with nonferrous materials.
Process
First, the plaster is mixed and the pattern is sprayed with a thin film
of parting compound to prevent the plaster from sticking to the
pattern. The plaster is then poured over the pattern and the unit
shaken so that the plaster fills any small features. The plaster sets,
usually in about 15 minutes, and the pattern is removed. The mold is
then baked, between 120 °C (248 °F) and 260 °C (500 °F), to remove
any excess water. The dried mold is then assembled, preheated, and
the metal poured. Finally, after the metal has solidified, the plaster is
broken from the cast part. The used plaster cannot be reused.
Mold:
• The mold is made of special die steels.
Die: The mold made of metals is called “die”.
• These steels have very high melting point and must resist very high
temperatures
• This mold is only preferred when a large no. of castings are to be
prepared
Reason: This is a expensive mold.
• The die is in two halves.
• One half is fixed while other is moveable to allow the casting to be
removed.
Process:
A die-cycle is completed in following steps:
• The ladle brings the molten metal from the furnace and pours it into
the shot sleeve.
• Plunger pushes the molten metal into the cavity in the die, with a
pressure of nearly 9800 psi.
Die Casting
• When metal is filled in the neck; incase of air-operated m/c, it
is raised up to the die and locked in contact with the die
opening.
Die Casting
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Plunger is pushed down by a pressure of nearly 5000 psi to
inject the molten metal into the cavity in the die.
After the die is filled, metal is left to solidify.
Pressure is maintained during solidification by a plunger or
compressed air.
After solidification, pressure is removed, the die is opened
and knock-out/ejector pins are used to eject the casting out.
Die Casting
Die cycle:
 A cycle is a complete set of events in which one casting is
produced.
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It consists of following steps:
 Closing the die.
 Shot: Injection of molten metal in die.
 Solidification of metal under pressure.
 Opening the die.
 Ejection of casting from the die.
Die Casting
Types/methods:
Die casting is done by two methods:
1. Gravity Die Casting/Permanent die casting
i- Slush casting
ii- Low pressure die casting
1. Pressure Die Casting
i- Hot chamber die casting
ii- Cold chamber die casting
iii- Centrifugal casting
Die Casting
Gravity Die Casting/Permanent die casting:
• Molten metal is poured under gravity (i.e. without pressure).
Die Casting
Pressure Die Casting:
• Molten metal is poured under pressure.
• There are two types of pressure die casting.
– Hot-Chamber die casting
– Cold-chamber die casting
Die Casting
Gravity Die Casting:
• In gravity die casting or Permanent Mold Casting, fluid metal
is poured by hand into metal molds.
• Both ferrous and non-ferrous metals can be casted.
• Production rate is slow.
• The casting is not so smooth or dimensionally accurate.
Die Casting
Cold-Chamber Die Casting:
 In this pressure die casting process, the basin of molten metal is not a
part of the m/c.
 Molten metal is poured from an external melting container and a piston
is used to inject the metal under high pressure into the die cavity.
 Injection pressure used in this machine typically 14 to 140 Mpa.
 High melting alloys of brass, aluminum, and magnesium are casted in
cold-chamber m/c.
Hot-Chamber Die Casting:
• In this pressure die casting process, the basin of molten metal
is a part of the m/c.
• Low melting (less than 700oC) alloys of zinc, tin, and lead are
casted in hot-chamber m/c.
• It is mainly used for small castings (0.03kg to 40kg).
• The injection pressure are 7 to 35 MPa.
Die Casting
The actual mold life varies:
 Alloy being cost: as higher the melting point, shorter the mold life
 Mold material: Grey cast iron has the best resistance to thermal fatigue and
machines easily.
 Pouring temperature: High pouring temperature reduce the mold life, increases
shrinkage problems and induce longer cycle time.
 Mold temperature: Low temperature produce misruns and high temperature results
the mold erosion.
 Mold configuration: Difference in section size of mold and casting can produce
Die Casting
Advantages:
• High production rates can be possible
• Good surface finish, smooth cavity produces smooth castings.
• Large number of castings can be produced by metal mold and
economical justification for large production.
• The process is fully or semi-automatic, which reduces labor
cost.
• Rapid cooling provides small grains size and good strength.
Die Casting
• The process is very fast and can produce large number of
castings in a small time.
• As casting is done under high pressure so the grains are highly
compacted, increasing the strength of the casting.
• High dimensional accuracy.
Die Casting
Disadvantages:
• Metal dies are very expensive so high volume production is
usually required to justify the expense.
• Die casting machines are expensive too.
• Machining for producing the cavity is expensive.
• Nozzle and piston both require replacements and repairing.
• High melting alloys cannot be prepared in hot chamber m/c.
Die Casting
• Expert metallurgical and production control is required for
perfect castings.
• Careful gating and vent systems are to be considered.
• Although cold chamber process is applicable to most of alloys
but due to ladling operation its production speed is slow.
• The hard and strong metals such as iron and steel cannot be
die-cast.
• Centrifugal casting is also known as rotocasting, is a casting
technique that is typically used to cast thin-walled cylinders. It
is noted for the high quality of the results attainable,
particularly for precise control of their metallurgy and crystal
structure. Unlike most other casting techniques, centrifugal
casting is chiefly used to manufacture stock materials in
standard sizes for further machining, rather than shaped parts
tailored to a particular end-use.
• Centrifugal casting was the invention of Alfred Krupp, who
used it to manufacture cast steel tyres for railway wheels in
1852.
How is centrifugal casting done?
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In centrifugal casting, a permanent mold is rotated continuously
about its axis at high speeds (300 to 3000 rpm) as the molten
metal is poured. The molten metal is centrifugally thrown
towards the inside mold wall, where it solidifies after cooling.
The casting is usually a fine-grained casting with a very finegrained outer diameter, owing to chilling against the mould
surface. Impurities and inclusions are thrown to the surface of
the inside diameter, which can be machined away.
Casting machines may be either horizontal or vertical-axis.
Horizontal axis machines are preferred for long, thin cylinders,
vertical machines for rings.
 Most castings are solidified from the outside first. This
may be used to encourage directional solidification of
the casting, and thus give useful metallurgical
properties to it. Often the inner and outer layers are
discarded and only the intermediary columnar zone is
used.
Features of centrifugal casting:
• Castings can be made in almost any length, thickness
and diameter.
• Different wall thicknesses can be produced from the
same size mold.
• Mechanical properties of centrifugal castings are
excellent.
• Only cylindrical shapes can be produced with this
process.
• Size limits are up to 3 m (10 feet) diameter and 15 m
(50 feet) length.
• Wall thickness range from 2.5 mm to 125 mm (0.1 5.0 in).
BENEFITS
• Cylinders and shapes with rotational symmetry are
most commonly cast by this technique
• Thin-walled cylinders are difficult to cast by other
means, but centrifugal casting is particularly suited to
them. To the rotation radius, these are effectively
shallow flat castings and are thus simple.