Even seasoned engineers often overlook rules and make mistakes when designing plastic parts. Such errors can result in excessive cost, extended schedules, or reduced output, according to UFE Inc., Stillwater, Minn., (ufeinc.com). The company's Big 6 Design Do's & Don'ts brochure lists several things you should consider to avoid the problems. A few include:

Avoid sharp corners

Stresses are concentrated in sharp corners as the plastic cools and shrinks. These “molded in“ stresses cause warping, cracking, or outright breakage. Using generous radii for corners avoids the problem. A good rule of thumb for inside-corner radii is to equal or exceed 30% of the wall thickness (with a minimum of 0.5 mm). And the outside-corner radius should be equal to or greater than the inside radius plus the wall thickness.

Don't overengineer

CAD programs make it easy to maintain the same number of significant digits for all dimensional tolerances. But this typically increases the cost of your tools and parts. And it makes it difficult for your mold builder and processor to determine critical features. Overspecifying is a common, natural tendency that can be avoided through consultation with your molder and mold builder.

Provide adequate draft

Parts must eject from the tool without damage or long cycle time. Even ejecting relatively short parts with no draft can introduce stress and sometimes outright damage to the plastic part, because plastic shrinks as it cools. These stresses typically result in warpage, or worse. Your molder and mold designer can readily identify draft issues before your part design is completed.

The complete brochure can be downloaded from http://big6pr.ufeinc.com