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The advantages in producing accurate press-tools easily,
quickly and cheaply from a fusible alloy are great in comparison
with the traditional, and laborious, manual methods. Both
punch and die can be formed by casting one against the other,
the necessary separation being achieved by layering with a
special sheet wax. The process is even simpler when an existing
pressing is available as a pattern.
Most often used in this context is MCP 137, an alloy of bismuth
and tin. The viscosity and surface tension of the alloy are
very low, making for easy flowing and accurate reproduction
of detail. It is of good dimensional stability after casting,
but a more significant advantage is that there is very little
change in volume as the alloy solidifies, meaning that actual-size
patterns can be used without special modification. It is occasionally
advantageous to use the related MCP 138 alloy, a modification
with minor additions intended to reduce the wear-rate; a requirement
for lower working temperatures can be met by MCP 70, containing
lead and cadmium in addition to bismuth and tin.
The Brinell hardness of about 23 combines with sufficient
compressive strength to produce practical tooling that can
be used in suitable cases for runs of several hundred pressings
before wear or deformation becomes significant. The life of
the tools does depend to some extent on the peculiarities
of the pattern, with sharp comers being more liable to lose
shape than rounded shoulders.
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With proper attention to usage, the MCP low melting-point
alloys can be re-used continually over periods extending into
years. If the composition is found to have drifted away from
the original specification, it can usually be corrected in-house
by the user. For this purpose, a testing service is available
at the Wellingborough production laboratory. The corrective
amounts of constituents can be supplied as a specially formulated
alloy.
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