Page 418 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 417
ensure that minimal material is allowed to be outside the drying hopper as it is
transported to the press. This can be accomplished in a couple of different ways
including:
Use of small one-shot hoppers on the molding machine. This will keep the vol-
ume of material outside of the drying hopper minimized. Set the proximity
switch on the one-shot hopper according to the required shot volume.
Place the drying hopper on the feed throat of the machine. This will eliminate
any issues with external transfer of the material from the drying hopper to the
machine hopper. This works reasonably well with small hoppers but as they get
larger cleaning them out during material changes can be more challenging.
Case Study: Drying
TPU is a very moisture-sensitive material and must be under 0.02% moisture
content to process correctly. TPU will undergo a hydrolysis reaction that will
lead to molecular weight reduction. When this degradation occurs the vis-
cosity of the material will drop significantly, which will show in the transfer
pressure on the machine. TPU also becomes extremely sticky when it is run
wet and will be very difficult to remove from the mold. Past experience with
TPU using both Karl Fischer titration and loss on weight moisture analysis
showed that during humid months 5 minutes was the maximum time out-
side the dryer before the material was above the 0.02% specification! Make
sure that everyone understands how critical it is to keep the material dry.
42.3.1.2 Molding Process: High Melt Temperature
Thermoplastic materials have a given temperature range within which they can be
processed. If a material is processed outside of its specified temperature range
thermal degradation can occur. When a material degrades it will create an off gas
that can be trapped in the melt stream resulting in splay on the surface of the part.
Figure 42.3 shows a degraded piece of material that created a splay streak down
flow.
One of the first things to check relative to a temperature issue is the barrel set
points. Verify that the barrel set points are set correctly (match setup sheet). Sec-
ondly verify that the actual thermocouple readings are within 5 °F of the set point.
If a zone has a reading outside of the norm it should be checked to determine if the
heater band is working. Often a barrel temperature will be running 20–30 °F above
the set point, which could indicate that there is high shear in the barrel creating a
temperature override. This high shear may be coming from excessive back pres-
sure, high screw recovery rates, or improper barrel temperature profiles. When a
heater zone is overriding the set point the process is running out of control.
