Page 381 - Injection Molding Advanced Troubleshooting Guide

40.3 Short Shot Troubleshooting 379

ƒ No back pressure
ƒ Low melt temperature
ƒ Low mold temperature

40.3.1.1„ Molding Process: Light Fill Only Weight
When using RJG Decoupled Molding techniques the mold should be filled during
®
first-stage injection to 95–98% full. If the mold is not filled far enough during first
stage the mold may end up with a short shot.
Verify that the fill only weight matches the documented process. If the fill only
weight is too light investigate potential causes, which can include:

ƒ Shot size too small
ƒ Transfer position too large
ƒ Leaking nozzle
ƒ Leaking non-return valve
ƒ Leaking manifold
ƒ Timing out on first-stage injection time
Before adjusting shot size or the transfer position, it is critical to determine that
the plastic is not going somewhere where it does not belong. There are many cases
of someone fixing a short shot by adding plastic to the shot size only to increase
the amount of plastic leaking into the hot runner manifold (see Figure 40.7).
Fill only weight is critical to ensuring that the mold is filled reproducibly. If the fill
only weight is too light the volumetric flow rate may have changed during mold
filling.

40.3.1.2„ Molding Process: Pressure Limited
A molding process is pressure limited when there is not enough pressure available
during first-stage injection to achieve and maintain set velocity. A process can be
pressure limited due to a number of factors including:
ƒ Machine capability. The machine may not have enough pressure available.
ƒ Peak injection pressure setting set too low. Make sure that this is set approxi-
mately 20% above the required peak first-stage pressure. This will allow for vari-
ation in viscosity.
ƒ Plugged feed system. A piece of metal or other contaminate may be trapped in
the nozzle tip or the hot runner manifold causing pressure to spike.
ƒ Cold temperature somewhere often in the nozzle or hot runner
ƒ High pressure drop through nozzle, runner gate, or part
If a process is pressure limited the first-stage velocity will decrease during mold
fill. Figure 40.2 shows process monitoring curves for a pressure-limited process;