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 mold.

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

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 mold half, which protrudes through a hole in the base of the part. The rest of the clip is formed by the B-side mold half.

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

Fig. 2

In this 2D diagram, red indicates sliding contact between metal surfaces from the two mold halves. (In the actual mold, there would be three flat faces of the extension from the A-side mold half making sliding contact with the B-side mold 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 mold closing, they will rub against one another along their entire length as the mold closes. Since the fit of the two mold halves must be tight to prevent "flash," there will be considerable friction and wear along these faces as the mold opens and closes, quickly ruining the mold. 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 mold closes but do not actually touch until the mold is fully closed. See figure 3.

Fig. 3

Sliding Shutoff Demo

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

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Plastics Trivia Question

Scientists at University of Durham in the UK have found that a blend of emeraldine base polyaniline (PANi) and tetracyanoquinodi-
methane (TCNQ) plastics develops what property over time?:

 A. Selective gas permeability

 B. Osmotic permeability

 C. Semiotic impermeability

 D. Transparency

 E. Magnetism

(Honor System: No Googling, Yahooing, or Dogpiling until after you've submitted your guess.)

Last month's question/results:
Original pink flamingo plastic lawn ornaments:

A. are still being manufactured by Union Products

B. were sculpted by artist Charles Motherwell

C. were first introduced in 1951

D. were dubbed by their creator "Haliaeetus leucocephalus plasticus"

E. won an Ig Nobel prize in 1996

The correct answer is E. won an Ig Nobel prize in 1996.

The responses are represented in the following chart:

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