Designing for Injection Molding: Finishes

Written by RevPart

injection molding finishes

After reading all of the other posts in our series, the last detail you need to think about is which of the injection molding finishes you want to choose for your design.

Although smooth finishes are the easiest to get out of molds, they’re not always the most visually interesting. Getting a smooth finish out of an injection mold is accomplished by hand polishing the metal before beginning production.

Other textures are achieved by applying abrasion, cutting, or acid etching to the mold before the first injection. The exact method depends on how you want your part to look in the end, but no matter what the finish, it’s applied directly to the mold, not to the finished part.

If you’re adding a texture, there’s something else you need to think about: if not properly designed, the textured plastic part can have a hard time getting out of the textured metal mold, even with ejector pins.

Think of it this way: have you ever run your fingers over bricks? There’s a lot of friction between your fingertips and the rough surface of the bricks—you can’t pull your fingers across it without hurting them. The same applies to pushing a plastic part out of a textured mold. If the walls of the mold are straight, the plastic part may come out with some ugly drag lines, ruining your part and proving the mold useless. This is a waste of both time and money.

This is an easy fix though: to draft your walls a couple of degrees. When the mold comes way from the part, the space between the two materials increases as the angle does, ensuring that your part escapes the mold unharmed. We’ll let you know if this may be a problem long before we begin building your mold though, so don’t worry.

You’ll also have to think about how the outside of your part is designed. If you have narrow ribs of plastic on the outside, the texture might not necessarily reach into the narrow parts of the mold. You might want to change your design a bit or not use a texture at all, but we can help you with this.

Now that you’ve read all of our tips, what type of part are you planning on making? Are you at the beginning of the design process at the end? Ready for a quote or just getting started?

Please leave us a comment with your questions about the design process or start this series over from the beginning.

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Shapes & Parts | Finishes

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