Page 409 - Injection Molding Advanced Troubleshooting Guide

408 41 Sink

41.3.2.8„ Mold: Pull Sink
Sometimes during ejection, a runner and gate will pull against the part and actual
create a sink at the gate. This is more typical with sub gates or cashew gates.
If the gate is pulling a sink during ejection first verify gate seal. If the gate is sealed
it is possible that the part is being ejected before it has cooled enough. Try adding
cooling time to see if the defect is improved. If cooling time improves the sink look
for ways to improve the cooling in the gate area and runner to shorten the cycle.
This is another area that IR thermal imaging is useful.
Another possibility is that the gate is too thick to separate well during ejection. A
thick sub gate or cashew gate will tend to not break as easily which can lead to a
pull sink. If the gate is too thick it should be welded up and recut. Often thinner
and wider gates will provide just as much volume throughput but freeze faster and
break better.

Also if details such as bosses are sticking in the mold they may pull a sink opposite
to their location during ejection. If the sink is directly opposite a boss check to
ensure that the boss is polished correctly and does not have residual machining or
electrical discharge machining (EDM) marks.

41.3.3„ Sink Troubleshooting Machine Issues

Possible machine-related causes include:
ƒ Machine performance
ƒ Velocity to pressure transfer
ƒ Inconsistent non-return valve
ƒ Nozzle tip size/type
ƒ Gas assist not working

41.3.3.1„ Machine: Machine Performance
See Chapter 8 on machine performance.

41.3.3.2„ Machine: Velocity to Pressure Transfer
Velocity to pressure transfer is when the machine transfers from first-stage velocity
control to second-stage pressure control. If the machine struggles to achieve the
set second-stage pressure after transfer the part may have sink. Figure 41.6 shows
process monitoring curves for a machine running with poor velocity to pressure
response. Notice how the machine pressure curve drops down in pressure after
transfer and then takes several seconds to reach the set pressure. The cavity pres-
sure curve drops in proportion to the machine pressure. In this particular case the