The Golden Rule of plastic part design

Published On: January 15th, 2014|Categories: Blog|

Much has been said and written about this “golden rule,” but based on the drawings and CAD files we receive, it bears repeating. Part walls must (should) be kept uniform as possible.

A designer can see cause for non-uniform walls. The mold designer and mold builder can certainly design and make a mold that will mirror the design’s non-uniform walls, (even more so if they are “throwing it over the wall” to an unrelated molder). In the end, the production floor can only do so much when trying to accommodate design considerations, but they certainly cannot defy the laws of physics.

Plastics parts shrink as they cool and the laws of heat transfer dictate how that shrinkage will occur. Thicker areas take longer to cool than thinner areas. Where those two meet, stresses develop and show up in the form of shrink marks, bow, and twist (warping). The bigger the part, the larger the appearance and more pronounced the defect. You can try to combat the phenomenon with longer cooling times. That may help with warping, but sink marks are hard to process out AND that will cost you more. In addition, if your product end use sees temperature cycling, it is very likely that the bow and/or twist will appear in your customers’ hands.

On a side note, even IF the walls are uniform, the mold design needs to take into consideration the part design regarding cooling. The mold designer must supply Adequate cooling must be supplied to concentrated heat areas, or differentials in cool rates will be seeing and you may end up with warping as well.

Therefore, if you are talking molding and we ask you if you have obeyed the golden rule, you can faithfully answer yes, my wall ARE all uniform in thickness! It will save everyone time and money, not to mention gets your product to market FASTER.


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