Thousands of years ago duplicate metal structures were cast from an original sculpture by the lost-wax casting process. Investment casting is one of the oldest metal-forming techniques based on this lost-wax casting process. A beeswax pattern is coated with refractory ceramic material, which then hardens and takes the shape of the casting. The ceramic mold is then heated to melt the wax and remove it. Molten metal is then poured into the cavity. The metal solidifies and the ceramic mold is broken. This casting method allows the production of components with accuracy, repeatability, versatility, and integrity in a variety of metals and high-performance alloys.
Aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel are the most common metals used in investment casting. For casting metals with high melting temperatures which cannot be molded in plaster or metal, this method is useful. Parts with complex geometry such as turbine blades or firearm components are generally made using investment casting. High temperature applications are also common, which include parts for automotive, aircraft, and military industries.
The biggest benefit of investment casting is that it can produce complicated parts with very fine detail and excellent surface finish that would otherwise have been difficult with a die casting method.
The manufacturing process
There are two ways for making casts: using the wax model itself in the direct method, or creating a wax copy of a model that necessarily not be of wax, with the indirect method. The following are the steps for the indirect process which can take two days to one week to complete:
The wax patterns are formed as one piece with cores being used to create any internal features on the pattern. Several wax patterns are then attached to a central wax gating system to form a tree-like assembly. The gating system has channels through which the molten metal will flow into the mold cavity.
The wax pattern tree is then dipped into a refractory slurry composed of extremely fine grained silica, water, and binders. Slurry then hardens to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is hard enough to withstand the high temperature of molten metal. The ceramic shell is then placed in an oven to heat it in order to melt the wax out, leaving a hollow ceramic shell.
Read more: Investment Casting 101
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