November 2003 Design Tip

Don't undercut your part design effort.

What is a straight-pull mould?

Protomold is continuously working to expand the size and geometric capabilities of our rapid injection moulding process. However, it is still important to avoid undercuts in your part design because our rapid turnaround is still limited to straight pull moulds.

The good news is that there are a number of design techniques that can be used to convert an undercut on your part into a geometry that can be produced using a straight-pull mould.

A through-hole can be redesigned as a feature
mouldable using telescoping shutoff surfaces in
a straight pull mould.

As one example, consider a part design where a through-hole is required. In the figure to the left we show how the part can be redesigned to achieve the same function by converting the through-hole into a set of three opposing features, each of which can be formed within the opposing halves of a straight-pull mould through the use of telescoping shutoffs.

An access hole beneath the undercut allows the
mould to protrude through the part and provide
the needed latch shutoff geometry.

As another example, consider the undercut that seems to be required for a typical latch mechanism. The figure to the right illustrates how the latch design can be changed to eliminate an undercut once again through the use of telescoping shutoff surfaces. Note also the addition of draft to the part geometry that will enable the mould to open and close with minimal sliding surface friction.

An undercut can also be added after the
part is made through the use of a
secondary operation like drilling a hole.

Of course, the simplest approach may be to just modify the part after it has been injection moulded. The example to the left illustrates how the through-hole noted above can be added through a secondary operation such as drilling.

See the Protomold Design Guide for other helpful design information.