3. Permanent mold processes
Mold is made of metal and can be
used to make many castings.
Advantage: higher production
rates.
Disadvantage: geometries limited
by need to open mold.
4. Permanent Mold Casting Processes
Economic disadvantage of expendable mold casting:
a new mold is required for every casting
In permanent mold casting, the mold is reused
many times
The processes include:
i. Basic permanent mold casting
ii. Squeeze casting
iii. Die casting
iv. Centrifugal casting
v. Slush Casting
vi. Pressure Casting
vii. Semi solid metal forming
5. The Basic Permanent Mold Process
Uses a metal mold constructed of two
sections designed for easy, precise (Exact)
opening and closing.
Molds used for casting lower melting point
alloys are commonly made of steel or cast
iron .
Molds used for casting steel must be made
of refractory material, due to the very high
pouring temperatures.
6. Figure - Steps in permanent mold
casting:
(1) mold is
preheated and
coated.
Figure - Steps in permanent mold casting:
(2) cores (if used) are inserted
and mold is closed.
7. Figure - Steps in permanent mold casting:
(3) molten metal is poured into the mold.
9. Slush Casting
Generally this process involves Low
temperature melting alloys such as (Zinc,Tin &
Lead Alloys. )
The principle involves pouring the molten metal
into a permanent mould.
After the skin has frozen (Cold).
The mould is turned up side down or slung
(Hanged) to remove the metal still liquid.
A thin walled casting results, the thickness
depending on the chilling effect from the mold
and the time of operation.
10. Slush casting process it is used for small productions like
Ornamental & Decorative objectives & toys from low melting point
metals, such as Zinc,Tin & Lead Alloys.
14. Pressure Casting
This process is also called as Pressure pouring or
Low-Pressure casting.
In this process the molten metal is forced
upward by gas pressure into the graphite or
metal mold.
The pressure is maintained until the metal has
completely solidified in the mold.
The molten metal may also be forced upward by
vacuum, which removes dissolved gases and
produces a casting with lower porosity as well.
15. (a) The bottom-pressure casting process utilizes graphite molds for the production of
steel railroad wheels.
(b) Gravity pouring method of casting a railroad wheel. Note that the pouring basin
also serves as a riser.
Pressure Casting
16.
17. Properties And Considerations Of Manufacturing By Pressure Casting
Pressure casting manufacture can be used to produce metal castings
with superior mechanical properties, good surface finish, and close
dimensional accuracy.
Like in other permanent mold methods, the mold needs to be able to
open and close for removal of the work piece. Therefore, very
complicated casting geometry is limited.
Since the refractory tube is submersed in the molten material, the
metal drawn for the casting comes from well below the surface. This
metal has had less exposure to the environment than the material at
the top. Gas trapped in the metal as well as oxidation effects are
greatly reduced.
The high setup cost makes pressure casting not efficient for small runs,
but an excellent productivity rate makes it suitable for large batch
manufacture
19. Die Casting
A permanent mold casting process in which molten
metal is injected into mold cavity under high
pressure.
Pressure is maintained during solidification, then
mold is opened and part is removed.
Molds in this casting operation are called dies;
hence the name die casting.
Use of high pressure to force metal into die
cavity is what distinguishes this from other
permanent mold processes.
20. Die Casting Machines Types
Classifications of Die Casting:
1. Hot-chamber machine
2. Cold-chamber machine
21. Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects
liquid metal under high pressure into the die
High production rates - 500 parts per hour
not uncommon.
Applications limited to low melting-point
metals that do not chemically attack
plunger and other mechanical components
Casting metals: zinc, tin, lead, and
magnesium
22. Figure - Cycle in hot-chamber casting:
(1) with die closed and
plunger withdrawn, molten
metal flows into the
chamber.
Figure - Cycle in hot-chamber casting:
(2) plunger forces metal in
chamber to flow into die,
maintaining pressure during
cooling and solidification.
23. Cold Chamber Die Casting Machine
Molten metal is poured into unheated chamber
from external melting container, and a piston
injects metal under high pressure into die cavity
High production but not usually as fast as
hot-chamber machines because of pouring step
Casting metals: aluminum, brass, and
magnesium alloys
Advantages of hot-chamber process favor its use
on low melting-point alloys (zinc, tin, lead)
24. Figure - Cycle in cold-chamber casting:
(1)with die closed and
ram withdrawn,
molten metal is
poured into the
chamber.
Figure - Cycle in cold-chamber casting:
(2) ram forces metal to
flow into die,
maintaining pressure
during cooling and
solidification.
25. Molds for Die Casting
Usually made of tool steel, mold steel, or
maraging steel.
Tungsten and molybdenum (good
refractory qualities) used to die cast steel
and cast iron
Ejector pins required to remove part from
die when it opens
Lubricants must be sprayed into cavities to
prevent sticking
27. Centrifugal Casting
Molten metal is poured into rotating mold to
produce a hallow tubular part.
In some operations, mold rotation
commences after pouring rather than before
Parts: pipes, tubes, bushings, and rings
Outside shape of casting can be round,
octagonal, hexagonal, etc , but inside shape
is (theoretically) perfectly round, due to
radial symmetric forces.
33. Squeeze casting
Molten metal is poured into a
metallic mold or die cavity
with one-half of the die
squeezing (Pressing) the
molten metal to fill in the
intended cavity under
pressure until solidification is
complete.
34. Cont…
used to produce automobile pistons.
However, squeeze casting is limited only
to thin part or part with smaller
dimensions.
35. Principle of the squeeze casting process for solid and hollow forms
38. Semi Solid Metal Forming
This process also called semisolid metal working.
It’s developed in 1970’s.
When the metal or alloy enters the die or mould,
nondentric, roughly Spherical, & fine grained
structure.
The alloy exhibits it’s thixotropic behaviour, that
is, it’s viscosity decreases when agitated
(Disturbed).
Hence this process is known as Thixoforming or
Thixocasting.
39. Thixo molding Process
In detail and referring to Fig. alloy
granules are fed through a hopper into
a heated barrel within which operates
a rotating and reciprocating screw.
Barrel heating to about 600ºC
partially melts the granules, and the
screw rotation homogenizes the semi-
solid mixture and may refine and
round the solid phase.
40. Cont…
Periodically the screw injects the
thixotropic alloy into a heated die,
conventionally through a nozzle,
sprue, and runners.
The melt is protected by argon in the
barrel and vacuum can be used in the
die, as both practices minimize oxygen
pickup.
41. Cont…
Machine sizes range from clamp
tonnages of 100 to 1000 tons; the
smaller sizes for cell phones and
the larger sizes for auto seat backs.
Injection times can be as short as
10 seconds on the small machines
but as long as 60 seconds on the
larger machines.
