Design Plastic Stuff Right, Part 2

Posted March 27th, 2018 by Bailey Jones

SINK: How to Avoid it

If you hunt around the house and inspect some plastic things, you will see that many of them are basically a hollow cup on the inside. You’ll also notice that the inside is full of little features such as ribs, hooks, and screw bosses. These features must follow special design principles in an attempt to prevent bulky, thick areas of plastic that will deform the part. If designed too thick, the little features will tend to create sink, which is a divot of imperfection on the cosmetic exterior of the part caused by non-uniform shrinking as the hot plastic cools.

Back to our sandcastle form, you’ll see a new thick rib. The rib is improperly designed. It adds such a localized mass of material that the outside surface will sink in as the part cools in the mold. This forms an unacceptable, and unnecessary, imperfection in our part. Thick sections like this can also cause functional problems if the sink is severe enough to cause the part to warp.


With plastic injection molding there are many competing priorities to balance. But the beauty of the process is that all this effort to get the plastic design right results in elegant, inexpensive parts. When you are producing 100,000 parts, the up-front fixed cost becomes a small component of the final piece price. We’ve just scratched the surface here. Go a bit deeper and check out The 9 Fundamental Plastic Design Principles 

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