August 2009
A prototype can help you to see whether your invention works, to identify ways to improve its appearance and function, and to communicate it to the world of business and law. A prototype made of plastic is attractive and affordable, and can be made quickly. Mr. Andrew Nehrt (812-522-4433, proformplastics@verizon.net) of Pro-Form Plastics, Inc. (manufacturer of heavy-gauge [.03-.5" thick] molded plastics, non-metallic material [foam, felt, plastic, etc.] cut by steel-rule die, and customized wood shipping containers) spoke to us about the different designs, methods, plastics, and tools he uses to build prototypes.
The method of thermoforming uses a vacuum to fit a rectangular sheet of heated plastic (3x4' to 5x9') to a mold of a product part (remember the Vac-u-form toy from years ago?). Before being fit to a deep mold, the plastic is stretched with a felt-covered plug. Plastic shrinks while cooling on the mold, so designing the mold with at least 2 degrees of draft angle makes separation of the plastic from the mold easier and less expensive.
Several kinds of plastic, each with its own advantages and disadvantages, can be thermoformed.
- HDPE/LDPE/PP (high density polyethylene/low density polyethylene/polypropylene) can be used to make things like a gasoline tank. It is tough, resists chemicals and weather, and is relatively inexpensive. However it is dull and pliable.
- ABS (acetonitrile butadiene styrene) is durable, stiff, glossy, and can be used for detailed parts However it cracks under stress, does not resist chemicals, and is expensive.
- HIPS (high impact polystyrene) provides good detail, stiffness, and formability, and is the least expensive of these plastics. However it is brittle, unsuited for stress, and does not resist chemicals.
- Polycarbonate (Lexan) is clear, stiff, tough, and resists scratching. However it is amorphous (needs to dry), hard to form, and expensive.
- Acrylic (Plexiglass) is easy to form, somewhat clear, and relatively inexpensive (half the cost of polycarbonate). However it is brittle, scratches easily, and can't be used for detail.
- Wood (especially mahogany, which lasts and doesn't leave an imprint of its grain in the plastic product) is good for prototypes and small production runs of products made of ABS/styrene.
- Aluminum (machined or cast) allows temperature control during cooling, is an inexpensive (~ 30% the cost of an injection mold) way to make a long-lasting tool, and is good for making prototypes out of any of the listed plastics.
- Epoxy/Ren/Gypsum performs a little better than wood, is significantly more expensive than wood, and is good for limited production using ABS/styrene.