October 2007 Design Tips

Sliding Shutoffs (Again)

We've mentioned sliding shutoffs before, but they are both important enough and tricky enough to deserve closer attention. Done right, they can give you a lot of design flexibility; done wrong, they can easily destroy a mould.

Figure 1 shows the feature we are moulding: a clip rising from a flat surface.

Fig. 1

Fig. 1

The bottom of the clip's "hook" and the blue face of the clip's shaft will be formed by an extension (shown by yellow lines) of the A-side mould half, which protrudes through a hole in the base of the part. The rest of the clip is formed by the B-side mould half.

Figure 2 shows a section view of the feature in the closed mould.

Fig. 2

Fig. 2

In this 2D diagram, red indicates sliding contact between metal surfaces from the two mould halves. (In the actual mould, there would be three flat faces of the extension from the A-side mould half making sliding contact with the B-side mould half.) This is called a sliding shutoff, telescoping shutoff or a pass-through shutoff.

As you can imagine, if these surfaces are parallel to the direction of mould closing, they will rub against one another along their entire length as the mould closes. Since the fit of the two mould halves must be tight to prevent "flash," there will be considerable friction and wear along these faces as the mould opens and closes, quickly ruining the mould. This causes flash on the plastic parts under the clip head, interfering with the operation of your clip.

The solution is to draft the faces by at least three degrees, so the faces approach one another as the mould closes but do not actually touch until the mould is fully closed. See figure 3.

Fig. 3

Fig. 3

Sliding Shutoff Demo

If you'd like to see sliding shutoffs, both poorly and well designed, in action, click here.