Undercuts are recessed features in a thermoformed plastic part that, in thermoforming, prevent its ejection from the thermoform mold. These part features commonly provide a fastening point or locating edge for mating parts, such as with snap-fit assemblies, lids, or panels. Another common use for undercut features is to improve cosmetics and aesthetics by hiding the trimmed edges of finished parts, such as with displays, kiosks, and handheld devices.
This thermoformed part has a stepped-in double undercut for secure closure

Undercut features provide the type of clean look that’s generally associated with injection molding and can be used to reduce the ingress of water, chemicals, and other liquids. In many cases, undercut features also increase the overall strength and rigidity of the part. When mating components must fit together tightly and without a large radius between different sections, an undercut feature can relieve the stress concentration.

Mold Complexity and Costs

Undercuts change the way that thermoformed parts are ejected, and that’s something for part designers to consider in terms of mold complexity and costs. During the design for manufacturing (DfM) process, an experienced thermoformer can explain which part features will be undercuts so that the designer can determine whether to keep them or remove them.

If undercuts are essential for the design, different part-release methods can be used. For example, an aluminum tool can use a pneumatic slide, or action, to release the undercut feature. Compressed air moves the slide, which may contain pins for alignment as well as Hinges or flippers, which are another part-release option for certain part geometries. They use air pressure to lock the part in place and then remove this pressure to release the undercut feature.

This aluminum mold uses pneumatic slides to release undercut features

In terms of appearance, undercuts usually look like inward-facing flanges. Sometimes, they have a stepped-in appearance that resembles a stair. Regardless, undercuts prevent the straight ejection of the thermoformed part in the direction of the mold’s draw. Adding slides, push-in pins, or hinges increases the cost of the mold, but this tooling is still less expensive than what’s used for injection molding or structural foam and it greatly adds value to the final part.

Part Design and Best Practices

To account for undercuts, designers need to plan for the appearance of a parting line, or witness line, where the dynamic part of the thermoforming tool retracts. This thin seam, or lip, may require removal for aesthetic purposes. Also, it’s important for part designs to avoid undercuts that are too deep and narrow that there’s no way to release the part.

Undercuts can support parts mating and typically look like inward-facing flanges

Finally, because undercuts may not be easy to produce, it’s important to partner with an experienced thermoformer, especially one that is experienced in pressure forming to produce detailed and consistent undercuts. Pressure forming is similar to vacuum forming but applies significant positive air pressure to the backside of the part that is formed. Profile Plastics is ready to help you with your next plastics projects and invites you to contact us.