Undercuts

Undercuts are features in a plastic part that cannot be ejected from the mold in the direction of mold opening. Some examples of undercuts, in some cases, are threads, holes in the side wall of the part, and cantilever snaps. Undercuts should be avoided whenever possible because they can greatly complicate the mold design. The features that must be added to the mold will usually increase its initial cost, increase mold maintenance, interrupt the cooling lines and ejection system, and increase the cycle time. Figure 1 shows a part with a hole in the side wall that will require side action to eject from the mold.

There are many techniques available to avoid undercuts. Figure 2 shows two different potentially problematic features. On the right side of the part, the round hole in the side wall will require side action. A solution would be to use one of the two types of slots instead of the hole, because the slots do not require side action. On the left side of the part, there are two cantilever snaps. The left snap will require a special mold component called a lifter to eject it from the mold. The right snap has a hole at its base, eliminating the undercut since the snap's end can be formed by the other side of the mold. This common technique can only be used if a hole is functionally and aesthetically suitable.

Figure 3 shows a method to create a hole in the side of a part without any side action. Because the walls have large draft, bypass shutoffs can form the hole. This technique can only be used if the part has a sufficiently angled side.