July 14, 2022

The depth/width of ribs on plastic parts

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The plastic part designers are not necessarily wise in the design of plastic parts, or of manufacturing technicalities. Lots of people look at plastic parts and don’t have a clue as to how they are made. My advice is to get over it and educate (politely) your contact a little bit.

There are several simple rules of thumb regarding the depth/width of ribs on plastic parts. Most plastic material makers have very good rules for their specific polymer.

First, plastic parts SHRINK after they are molded. If the ribs in question are not in an aesthetic (visible) area, then maybe they can live with some sink marks on the plastic part. This can possibly be covered up by a texture on the visible parts.

Second, ribs that are deep should be quite narrow with as much draft as you can get. Use you tolerances and DRAW POLISH the mold. The more and deeper the ribs, you have to have that much more ejection area just to get the plastic part out without marks or distortion of the part, just based on friction of the shrinking part and the steel; and the likely result of not filling the rib at all (even more shrinkage). As a rule of thumb, the rib should be no thicker than the wall that it is attached to (sink marks), in fact only 50% of the wall and 2-3X wall for the depth would be a lot better.

The basic line is this: What is an ACCEPTABLE plastic part? Tolerances on a part print are a little leeway from absolutely perfect form and to give the manufacturer as much room as they can get to make an ACCEPTABLE plastic part. In the computer screen world everything fits together, that is not real life.

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