How to Design the Right Plastic Injection Molding Parts

Plastic Injection Molding Machine

Designing plastic injection molding parts? How should you do it in order to get “the right” parts you need?

Material Considerations

First, figure out what material the mold will be made from (steel? aluminum?) and what your intended production volume will be. Pick out a final surface finish for your design. Maybe you want a mirror or lens finish, or perhaps the parts need to be etched with a logo? Think about the kind of finish and texture you want, keeping in mind visual appeal, too.

Molding Parts

Next, consider the specifics of plastic injection molding parts. For instance, there are draft angles which allow parts to be released from the injection mold. If you don’t want to have to pry stuff out, use at least 1 degree on an untextured mold and at least 3 degrees for a textured mold surface. You’ll also want to avoid having sharp corners, since they are difficult to completely form in a mold– instead, add a radius wherever possible. When designing parts, the resin should flow from thick to thin sections. Keep in mind that molten resin loses pressure and temperature as it flows through the injection mold, so that’s why it should flow to thinner sections last.


Sometimes there are defects. Decide ahead of time which molding defects are acceptable to you. If possible, design your parts so they won’t easily allow for defects. Design “around” limitations as best you can.

Rib Strength

Strengthening ribs involve the base thickness, rib height and overall thickness. Base thickness should be designed at 60% or less of the wall thickness. Rib height should be as short as possible (ideally less than 3 times the part thickness). And overall thickness should be smaller than the base thickness. Use strengthening ribs sparingly.

Designing With Assembly

Finally, when designing “the right” plastic injection molding parts, keep in mind that most parts are made to be incorporated into something bigger than themselves. So, design parts with assembly in mind. Consider that parts will be assembled the same way every single time.

If you need help designing the right plastic injection molding parts for your needs, contact Bridgville Plastics today.