Page 391 - Injection Molding Advanced Troubleshooting Guide
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40.3 Short Shot Troubleshooting 389
Figure 40.7 Leaking hot runner manifold
A common reason for leaking hot runner manifolds is cold starts. A hot runner
must be allowed to achieve temperature and soak to ensure that all of the plastic in
the manifold is melted and will move when a shot is injected through the mold.
There are cases of leaking manifolds that are severe enough that the mold is smok-
ing due to burning plastic. Early detection is vital to dealing with this problem.
40.3.3 Short Shot Troubleshooting Machine Issues
Many machine concerns can contribute to short shots including:
Non-return valve leaking
Wrong nozzle
Worn barrel
Machine performance
Nozzle leak
40.3.3.1 Machine: Non-return Valve Leaking
The job of a non-return valve on the screw tip is to keep material from flowing
backwards during injection. If a non-return valve becomes worn or damaged the
valve will not seal properly and plastic will squeeze backwards through the non-
return valve. Figure 40.8 shows a typical sliding-ring-style non-return valve and
Figure 40.9 shows a ball-style non-return valve.
