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Manufacturing:
Metal Casting
Metal
Casting Techniques - Expandable
Pattern Casting (Lost Foam)
Also known
as Expanded Polystyrene Molding or Full
Mold Process, the EPC or Lost Foam process
is an economical
method for producing complex, close-tolerance
castings using an expandable polystyrene
pattern and unbonded sand.
The EPC process
involves attaching expandable polystyrene
patterns to an expandable polystyrene gating
system and applying a refractory coating
to the entire assembly. After the coating
has dried, the foam pattern assembly is
positioned on several inches of loose dry
sand in a vented flask. Additional sand
is then added while the flask is vibrated
until the pattern assembly is completely
embedded in sand.
A suitable
downsprue is located above the gating system
and sand is again added until it is level
to the top of the sprue. Molten metal is
poured into the sprue, vaporizing the foam
polystyrene, perfectly reproducing the pattern.
Gases formed from the vaporized pattern
permeate through the coating on the pattern,
the sand and finally through the flask vents.
In this process,
a pattern refers to the expandable polystyrene
or foamed polystyrene part that is vaporized
by the molten metal. A pattern is required
for each casting.
Advantages
- No cores are required.
- Reduction in capital
investment and operating costs.
- Closer tolerances and
walls as thin as 0.120 in.
- No binders or other additives
are required for the sand, which is reusable.
- Flasks for containing
the mold assembly are inexpensive, and
shakeout of the castings in unbonded sand
is simplified and do not require the heavy
shakeout machinery required for other
sand casting methods.
- Need for skilled labor
is greatly reduced.
- Casting cleaning is minimized
since there are no parting lines or core
fins.
Disadvantages
- The pattern coating
process is time-consuming, and pattern
handling requires great care.
- Good process control
is required as a scrapped casting means
replacement not only of the mold but the
pattern as well.
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