Bump-Off: Undercut in Plastic Part Design

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Do The Bump!
By Protomold - Nobody’s Faster In The Short Run.®

A bump-off is a small undercut in a part design that can be safely removed from a straight-pull mold without the use of side actions. It works like the snaps on the wind flap of a parka or the snaps on a western style shirt—push to close, pull to open. If you look very closely at the two parts of a snap, you will see that there is slight deformation of the material when the snap is opened or closed. The choice of material and design of the snap’s components allows that deformation to take place without damage or significant wear to the mating parts. This is exactly what happens during the ejection of a part with a bump-off feature.

In the case of a plastic part in an aluminum mold, any deformation that takes place at ejection will be entirely in the plastic part, not in the mold. A number of factors determine the ability of a part to be “bumped-off” without damage. These include the shape of the undercut itself, the resin used to form the part, the geometry of the area surrounding the undercut, and the design of the mold.

Figures 1 and 2 address the shape of the undercut feature. In order to be successfully bumped-off, the leading edge of an undercut must provide a “ramp” or radius like that in Figure 1 rather than a hook or sharp edge as shown in Figure 2. If other factors are correct, during ejection the ramp-shaped lip inside the cylinder in Figure 1 can ride up over the edge of the groove in the mold that formed it, much as a car rides over a speed bump. Conversely, the hook in Figure 2 will remain lodged in the groove that formed it and either prevent ejection entirely or be torn off when the part is ejected.

Figure 1

Figure 2

Resin choice is critical to the success of a bump-off. Depending on the shape of the part, the resin will need to stretch and/or bend during ejection and then return to its original shape and size. A resin like TPE or unfilled polyethylene is flexible enough to bump off. Glass-filled nylon, on the other hand, is very rigid and is not likely to work very well.

Finally, we consider the shape of the part and the molding technique. If Figure 1 is, for example, a snap-on cap for a bottle, Figure 3 shows cross sections representing different ways in which that cap could be made. Figure 3.1 and 3.2 show ways in which the part could be molded as a rib. In this approach, both the outside and inside of the cap are formed by the B-side mold half; the A-side is simply a flat surface that forms the bottom of the upturned cap. Figure 3.3 shows the part as it would be produced by core-cavity molding, the outside formed by the A-side and the inside by the B-side mold halves.

Figure 3 - Represents 3 different ways in which a part with an undercut feature could be made

The approach shown in Figure 4 is problematic because during ejection the bump will have to squeeze out through the thinner rib section below it. This requires that the resin be compressed during ejection, and few resins other than low durometer TPEs or TPUs are that compressible. A better approach is shown in Figure 5. Here the raised ring is replaced by an inward bend in the vertical wall of the part. Instead of being compressed, the wall of the part can be “snaked out” during ejection, requiring only that it bend and stretch. This is easier for most resins to do than to compress. You can see an example of this bump-off in our new Protomold Torus complex feature sample. (To request a Torus, click the banner in this Design Tip.)

Figure 4 - Undercut will have to squeeze out through rib (harder approach).

Figure 5 - Undercut can snake out during part ejection (easier approach).

A third approach is shown in Figure 6. Here the part is formed between a cavity in the A-side mold half and a core in the B-side. After cooling, the mold opens, leaving the part on the core. With nothing pressing against the outside of the part, the part wall is free to stretch as it is ejected off the B-side core, releasing the part with the inside ring intact.

Figure 6 - Mold can be made core-cavity, allowing room for the part to "bump-off" after the mold opens.

If you have questions about the feasibility of bump-offs for small undercuts in your part design, Proto Labs customer service engineers are here to help and can be reached at 877-479-3680.