Designing for Injection Molding: Shapes and Parts

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injection molding parts

photo by David Burton

Injection Molding Parts

By its very nature, injection molding cannot be used to make hollow objects. One option to get around this limitation is to make your part in two different pieces and connect them later.

This is a pretty popular option…keyboards, TV remotes, and my Otterbox case are all made this way.

All of these objects are designed to hold something, but even if you just want a hollow part to alleviate weight or sink problems, it’s still impossible to make a hollow item without molding the two pieces separately.

If you’re willing to separate your pieces, injection molding will be the very best way to make your part. Although it might take a little longer, it’s much more cost effective for production than 3D printing (which can make a hollow piece, but not one with an accessible center).

If your part has rotational symmetry—that is, half of it can be either the top or the bottom—you can even use the same mold for both halves, saving production costs (you’ll only have to make one mold instead of two). Sometimes just a few tweaks can make your part rotationally symmetrical. If you have questions about whether or not your part could have rotational symmetry, feel free to contact us with any questions.

You can view this video for an explanation of rotational symmetry, using a cube as a rather simple example:

If you need a part to be air- or fluid-tight, we can easily add a seal to your mating surfaces to meet your needs.

Another type of part can also be difficult to mold properly: C-Shaped objects.

Molding C-Shaped Objects

Objects that are shaped like a C are particularly prone to warpage, especially if you choose the wrong material.

This particular shape is so problematic because plastic fibers tend to align themselves along the direction of their flow—this means that the different arms of the C will have plastic fibers flowing in different directions instead of following in a more unified way. This lack of cohesion means that your part is more likely to warp as it cools.

Remember when we talked about adding a radius to your corners? This is a perfect example of why. The inside of the angle will cool slower, pulling on the outside of the angle. If you radius the corners, you can minimize this warping, but you won’t eliminate all of your problems.

If you use an O shape instead of a C shape, the extra material acts as a support and prevents warping. This can be accomplished by adding a removable brace until the part has been cooled and stabilized. You’ll also probably want to choose a shrink-resistant resin to prevent your part from changing too much while cooling.

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Little Details | Shapes and Parts | Finishing

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