Undercut Part Design for Injection Molding

Undercuts are part features that can complicate the ejection of injection molded parts. Examples of these protrusions and indentations include locking tabs, slots for switches, and screw threads. When a straight-pull mold is released, undercuts can interfere with the separation of the core and cavity. An injection molder can use additional tooling to facilitate this separation, but extra engineering and machining adds costs to your projects. Cycle times take longer, too. That’s why design for manufacturing is so important.

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.

One of the easiest approaches to working with undercuts is to move the mold’s parting line so that it intersects with the part feature. On some injection molded parts, this dividing line between the core and the cavity is right down the middle of the mold. With more complex shapes, determining the location of the parting line is more complicated. That’s because the location of the parting line determines the direction in which features must be drafted for ease of ejection.

If multiple undercuts are required, the parting line can zigzag to intersect with each feature. Remember, however, that parting line placement is also a function of part geometry, material flow, and other considerations. With liquid silicone rubber (LSR), don’t locate the parting line on a sealing surface. Parting line location also affects the cost of the mold and any secondary operations that are needed to finish the injection molded part. If changing the parting line isn’t an option, consider bump-offs and side actions.

Bump-offs or stripping undercuts can be used when a part feature is flexible enough to deform over the mold during ejection. Designers need to follow some rules, however. First, locate the bump-off away from any stiffening features such as ribs or corners. The stripping undercut requires a lead angle of 30° to 45°, and the injection molded part needs enough space to support expansion and deformation. Flexible plastics like polypropylene are good candidates for bump-offs but reinforced plastics are not.

Side actions are mechanisms that slide in as the mold closes and slide out when it opens. They support part ejection with undercuts but also impose some design rules. First, there must be enough space for the core to move in and out. There also needs to be enough draft to accommodate the movement of the side-action core. Finally, it’s important to understand that side actions move perpendicularly. Moving them at any other angle adds complexity and costs to your project.

Do you have questions about undercut part design for injection molding? Extreme Molding of Watervliet, New York (USA) is a leader in custom injection molding with plastic and silicone. To learn more about us and how we can help you, contact us.