Prototypes
A prototype plastic part cannot duplicate exactly the performance of an injection-molded production part unless it is molded in a production environment in a production mold. This is normally impractical, so the best that can be expected is an approximation of a production part.

The fastest and most economical way to produce plastic prototypes is to machine them from slab or bar stock. But stock forms of plastics are made by extrusion, not injection molding. Besides not having the same flow orientation of a molded part, extrusions of a given material are usually made from a higher molecular-weight grade than that used for injection molding. Consequently, properties such as impact strength, creep resistance, and chemical resistance tend to be higher in an extruded material. These differences are particularly significant in the crystalline plastics such as nylon, acetal, and polyethylene. The variations are usually smaller in amorphous materials such as ABS, polycarbonate, and polystyrene, but even minor differences can be critical in some applications.

Molding, rather than machining, of prototypes generally provides a better approximation of a production part, but here too, a number of differences in conditions can cause misleading results. For example, if the prototype mold is made from epoxy resin, the molded part will cool at a much slower rate than it would in a production (steel) mold. And cooling rate can affect tensile and impact strength as well as heat and chemical resistance, elongation, and stiffness -- particularly in crystalline plastics.

Making prototypes in an aluminum mold improves their similarity to production parts, but this method also has drawbacks. Here, because of the high thermal conductivity of aluminum, faster cooling is the problem that alters properties from what they would be in a part made in a steel mold. Also, there is difficulty in getting the resin to flow into a mold that cools rapidly. This problem can be offset by higher injection pressure, but the greater density that results causes other variations.

The closest duplication of a production part is produced by injection-molding prototype parts in steel molds. A relatively soft steel can be used for prototypes, so that machining is not difficult. But even here, because certain shortcuts are usually made (in polishing surfaces or in simplifying cooling passages, for example), the quality and accuracy of the resultant moldings are something less than what would be expected in production moldings. Nevertheless, steel prototype molds produce parts that most nearly duplicate production parts. Although this is the most expensive prototyping method, it may be the most economical in that it provides the surest way to avoid expensive changes in production molds.

Computer-aided design
In order to reduce the number of prototypes required, designers are turning to finite-element analysis. FEA programs allow parts design and structural analysis before tooling is cut. Related software, such as moldfilling analysis and warpage programs, help to eliminate processing problems before they start. Many of these programs depend on materials databases that organize various properties into a manageable format. Integrating FEA and molding programs with a database often optimizes the use of capital-intensive CAD/CAM equipment. Databases also enable users to conduct a fast and efficient materials search.

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