Prototyping Methods Available
Prototype parts are usually either machined from gravity cast material or directly cast using a gravity process. Sand casting, and investment casting (lost wax) are most frequently used but plaster mould casting is also used to very good effect. Patterns for both sand casting and the production of investment wax moulds can be produced by any desired route. So called rapid prototyping techniques such as stereolithography, fused deposition modelling (FDM) and laminated object manufacture (LOM) are becoming increasingly common. In many cases however machining or hand crafting of patterns is still the most cost effective route.
Plastic patterns including rapid prototyping models can be used directly for investment casting but it is much safer to cast a silicon rubber mould from it, to enable the production of any desired quantity of waxes. Whilst the investment casting process itself is reasonably accurate, the precision of the casting depends upon the precision with which the waxes are made. It is unlikely that the investment casting process using rapid techniques can approach the kind of precision available from zinc alloy pressure diecasting. A possible exception is precision machining of a master followed by the casting of a rigid epoxy mould for waxes.
When a component with a configuration similar to the desired end product exists, it may often be modified and used as a pattern for the plaster mold process. Pattern features may be modified by machining or by building up with modeling clay or wax. The plaster mold process is especially useful when the pattern must be slightly enlarged to compensate for solidification shrinkage. The intermediate rubber tool can be “grown” (expanded), within limits, to provide for the shrinkage
Sand casting is often regarded as a fairly crude process only capable of producing parts with fairly wide tolerances and with rough finishes. However specialist precision foundries are capable of at least achieving good surface finishes, although tolerances will inevitably be much wider than available from pressure diecasting and wall sections below about 1.8 mm will not be achievable.
Other proprietary and closely guarded prototype casting methods are in use, and good examples of their capabilities have been seen. These processes all involve the production of mould tooling and are good for 1000 or more castings.
Whichever casting process is used it is likely, although not inevitable, that some finish machining of the casting will be required to achieve a prototype which conforms to specified component dimensions.