The Plastic Engineer’s Undercut

By Shamilah Faria, Applications Engineer

What keeps a plastic engineer up at night? Sometimes they want to solve problems but cannot find that perfect solution without drawbacks. In the real world, each solution introduces new risks, and they must play a balancing act with the tradeoffs.

For example, what happens when a part’s functionality hinges on an intentional undercut? Undercuts can grab onto the core side of an injection mold, preventing clean part ejection. Undercuts typically add complexity and cost to manufacturing but can hold value for many applications. Common undercut features include snaps, threads, side windows, and horizontal bosses. You can read more about removing undercuts here.

In the image below, you can see the snap feature that creates an undercut. Imagine the steel that would have to form that feature. Then imagine how the plastic engineer would move that steel out of the way to remove the part from the mold. Without forethought the part would be stuck inside the mold unable to be removed.

The plastic engineer wants to consider both part design and tool design before incorporating undercuts. At the part design level, discerning the role of the undercut can allow for flexibility with its design.

Thinking about the part design with the tool in mind can allow for undercuts to be avoided or formed cleverly. Both must be considered to accurately weigh the tradeoffs and choose an appropriate solution. An example case study illustrates the point.


A medical customer needed an onboard buffer reagent for a diagnostics test. The plastic engineer designed a polycarbonate sample collector which fit inside a polypropylene buffer storage component.

The sample and buffer needed to remain separate during sample collection. The plastic engineer had to achieve a stable assembly lock of the components prior to use. The application did not require a hermetic seal, but it needed to minimize the risk of leakage.


The plastic engineer initially designed the subassembly with a diametric interference fit. The design posed the risk of blood leakage during use. The tight tolerances meant slight variations could result in the components either popping apart or not joining in the first place.


Because cams were already forming the external face of the internal component, the plastic engineer opted to utilize cam functionality to add an undercut locking feature on that face.

For the external component, options for forming the mating undercut included lifters. These angled mold ejection components move laterally to form undercuts. During ejection, the lifters move out of the way to allow for a clean release. The plastic engineer could not use lifters because they would require more lateral space than was available within the inner diameter of the part.

Another option to form undercuts includes collapsible cores. Collapsible cores contract radially to form undercuts. The plastic engineer did not have adequate space to use collapsible cores either.


For the internal component, the plastic engineer added a radial ring to create interference with the internal face of the external component. For the external component, they designed rice grain features into the inner diameter.


The plastic engineer reduced the chance of leakage during use. The durometer of the external part’s material allowed it to bump off the tool during ejection without added mold features. The undercuts gripped onto the mating component’s radial ring, which allowed the two components to engage securely.

Undercuts can add useful functionality to a part, but the plastic engineer must consider undercuts in conjunction with both part and tool design. Looking at different options to achieve the intended function can allow for an informed, if not perfect, solution.

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