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Plastic Parts Design

Somewhere, I have a copy of what was an internal manual for an international manufacturer of kitchen and personal care products. It concentrates a great deal on the "B"-side features used for holding motors, circuits boards and the like. What is unique about it is the way in which at least two options are given for a given functional feature, with tooling implications (ease of manufacture & therefore cost, robustness and maintenance). It also includes relevant sections of ISO standards on tolerance values for both injection molds and molding materials, so helped (maybe still helps) their designers keep their feet on the ground with what is feasible and at what cost on the interior of products, even though their heads might rightfully be in the clouds when shaping the "A" surfaces. It is my intention that once I unearth it in my mammoth collection, I will seek permission to get it published for the benefit of the global plastics design community.

In 2005 I gave a paper in Berlin at the Rapra Conference "The Art of Plastic Design," my general theme being the huge pitfalls of designers concentrating on marvelous aesthetics — made even more seductive by photorealistic renderings of their creations — but not understanding the many devils (not just one, sadly!) in the details. My title, "Beauty, not just Skin Deep," was chosen to reflect this. Full sets of papers are still available, I believe and, no, I don't get royalties! If I can get permission from the organizer, I may be able to release just my paper.

Some of the details appearing in some of the GE Guides (Assembly Manual and some illustrations in Product Design) emanated from yours truly, they having bought Borg-Warner Chemicals, for whom I worked until just before the take-over.

First there are the general rules for joining the individual features of the part like reinforcement ribs, bosses, functional features, etc.
1. Too thick ribs causing sink marks.
2. Inconsistent/incorrect wall.
3. Incorrect draft angle or the combination with the defined texture roughness for proper demolding (specialty in astheatic parts).

On the other hand and more important there is combining these features into one part defining the way of demoulding (undercuts, etc) and the split line. This often is unknown by plastic part designers not familiar with injection mold design. It can cause a higher mold price than needed and/or more vulnerable mold inserts. Tooling must be involved in plastic part design in the early stage to correct demolding difficulties before lead time becomes critical.

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