An injection molded part needs to come out of the mold without damage or too much resistance. To avoid these issues, you want to angle the walls of the part from the parting line (drafting). Generally speaking, there should not be any surface of the part that is exactly 90 degrees to the line of draw on the mold. Not drafting a part can cause defects like drag marks and create difficult ejection. In the image below, the A-side tooling is colored blue, the B-side is red, and the part is yellow. Notice the small amount of draft (referenced by the angled lines) that allows the part (yellow) to break free from the tooling block (red).

The amount of draft you should apply to the part depends on the application. The general rule is to have at least one degree for every inch of depth. Below is a list of different design considerations where you want to add to the amount of draft.

Design Note: Having to “add plastic” to a molded part means removing steel from the mold. Machining away material in the mold is much cheaper and easier than welding and machining. We call this being “steel safe.” It’s common to stay steel safe on critical dimensions, test run the mold, check dimensions, and then remove steel to finalize that dimension.

To avoid thick sections of a part, you can add coring and ribbing. These features reduce cycle time, reduce part weight, and could make the part stronger. composing your Mold Design these features into the B-side of the part is common and is the best practice. These features can help pull the part to the B-side or ejection side of the mold; they would then be on the non-show side of the part. However, it’s possible to have them on the A-side of the tooling as well.