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Investment Casting :
Investment casting is a
method of making small, simple or intricately shaped parts
to a relatively high degree of accuracy and with a smooth
surface. The first step in this process is the manufacture
of an aluminum die from which the wax patterns are produced.
Wax is injected into the die under pressure to give a
replica of the finished components. The wax patterns are
assembled onto a wax runner or tree. For small items it is
possible to have several hundred patterns on a tree, which
can be cast in one pour, whereas the larger items will be
made with only one per tree.
The complete assembly is dipped (or ‘invested’) into a wet
ceramic-based slurry and coated with a find dry refractory.
Once dry the coating operations are repeated several times,
using coarser grades of refractory, until the mould has
sufficient thickness to withstand the force of the molten
metal during the pouring operation.
After drying the ceramic shell is placed in a steam oven and
the wax is melted out – hence the name ‘lost wax
process.’ The shell is then fired at 1000 0 C to burn
off any residual wax and to strengthen the mould.
Aluminum alloys are melted in bale-out furnaces, whilst all
other alloys are melted in high-frequency induction
furnaces. The metal is poured into a hot shell, enabling the
material to flow into thin wall sections giving fine furnace
detail.
After solidification and cooling, the ceramic is broken away
from the cast and metal and the castings are cut off the
tree. Careful fettling and finishing of the investment
castings is then carried out, process such as heat
treatment, machine and electro-polishing, before final
inspection in accordance with customer requirements.
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