Injection molding design guides say to avoid undercuts, but what if your part requires them? In medical device manufacturing, for example, undercuts are used to create snap-and-latch features. These interlocks support clamshell-style housings for quick and easy assembly. Undercuts are also used with vertical threads, barb fittings, and holes that accommodate buttons or wiring. In addition, undercuts are used to core-out thick sections and prevent sinking and warping.
Unless your design requires them, however, it’s best to avoid undercuts because they add costs, complexity, and maintenance requirements to molds. If undercuts are necessary, consider redesigning your part using shutoffs. With a shut off angle, one part of the mold closes against the other to form a slot or hole. If the shutoff surfaces are parallel to the direction that the mold opens, be sure to add enough draft. Otherwise, parts of the mold may grind against each other and shorten the life of the tooling.
There are other ways to account for undercut part design, too.