Design Plastic Stuff Right, Part 1

Posted March 13th, 2018 by Bailey Jones

Injection molding may be the most common plastic production process. This popular method can accommodate complicated shapes and a wide variety of features such as snaps, screw bosses and ribs that are all formed at once as molten plastic is injected into a mold under high pressure. And it requires careful design so that the parts will come out right.

A good analogy to this manufacturing process would be using a form to make a sandcastle at the beach. You’ll see that the form for the turret has angled sides that allow the sand to slip out of the mold. All “vertical” walls of a plastic part must have this angle, which is called draft.

Also consider that a large mass of plastic, which is hot as it goes into the mold, will warp and shrink as it cools. To mitigate those effects, plastic parts should incorporate a uniform wall thickness. Our plastic sandcastle form is a good example of this. The inside shape precisely follows the curves and contours of the outside shape.

Our plastic part, whatever it is, has likely ended up being some sort of cup shape at this point. 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.

We’ve just scratched the surface here. Go a bit deeper and check out The 9 Fundamental Plastic Design Principles. Also, stay tuned for Part 2 where I discuss sink and how to avoid it.

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