Design Tip

Creating Complex Undercuts with Bumpoffs

Incorporate bumpoffs, or small undercuts, to help form complex features on moulded parts

A bumpoff is a small undercut in a part design that can be safely removed from a straight-pull mould 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 bumpoff feature.

Injection moulding bumpoff ramp
Figure 1: Bumpoffs require a ramp to help the part smoothly eject from the mould.

In the case of a plastic part in an aluminium mould, any deformation that takes place at ejection will be entirely in the plastic part, not in the mould. 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 mould.

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 mould 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.

Injection moulding design with hook
Figure 2: Features like the hook on this moulded part will prevent the part from ejecting properly.

Resin choice is critical to the success of a bumpoff. 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 moulding 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 moulding, the outside formed by the A-side and the inside by the B-side mould halves.

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.

A third approach is shown in Figure 6. Here the part is formed between a cavity in the A-side mould half and a core in the B-side. After cooling, the mould 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.

If you have questions about the feasibility of bumpoffs for small undercuts in your part design, Protolabs' applications engineers are here to help and can be reached at +44 (0) 1952 683047.