Page 277 - Injection Molding Advanced Troubleshooting Guide

28.3 High Fill Pressure Troubleshooting 269

Also keep in mind that restrictive hot runner tips can become plugged with con-
tamination such as alternate materials, metal, or other foreign materials. There
have been many hot runners that needed to be torn apart to have contamination
cleaned out. In some cases even the internal spreader tip on the hot runner can
break loose and plug the orifice.

28.3.2.4„ Mold: Plugged Gates
Whether with hot drops or standard gates there can be spikes in pressure due to
material plugging the gate. Sometimes partially melted pellets will make their way
to the gate and create a spike in pressure before they bust through the gate into the
cavity.
If a process is experiencing intermittent spikes in pressure check for melt con-
sistency. Sometimes when using a large percentage of the barrel the melt will be of
poor quality, especially when running a semi-crystalline material. Unmelted pel-
lets plugging gates will many times lead to short shots but they also may lead to
high fill pressures.
Another possibility for plugging gates or drop tips is when a piece of frozen mate-
rial does not get trapped in a cold slug well. These frozen cold slugs can easily plug
a gate or drop orifice until the pressure reaches a point that the cold slug is forced
through the tip.

28.3.2.5„ Mold: Wall Stock
Thin wall stock can lead to high fill pressures. In recent years there has been a
push to have more injection pressure available on molding machines to fill thinner
wall stocks. Check with the material supplier to confirm that the wall stock to flow
length is acceptable. Early CAE studies will help determine if a part can be filled.

28.3.3„ High Fill Pressure Troubleshooting Machine Issues

Machine-related concerns are:
ƒ Nozzle tip style or orifice
ƒ Nozzle style
ƒ Plugged tip or nozzle
ƒ Machine performance

28.3.3.1„ Machine: Nozzle Tip Style or Orifice

The same tip style and orifice must be used for every run with a given mold. If a
nylon-style tip is used instead of a full taper tip there will be a very different pres-
sure drop through the tip. The extra restriction in a nylon-style tip will drive up fill