Page 164 - Injection Molding Advanced Troubleshooting Guide
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17.3 Brown Streak Troubleshooting 153
17.3.1.1 Molding Process: Barrel Temperatures
The most common process cause of brown streaks or degraded material comes
from running barrel temperatures too high. Each plastic material has a maximum
upper temperature that it is able to withstand and if it is exposed to temperatures
higher than this upper limit degradation will occur. The first thing to check when
dealing with brown streaks is that the barrel temperatures are set correctly and
match the specification (either setup sheet or manufacturer's recommendation).
When reviewing barrel temperature set points also ensure that the actual tempera-
tures on each zone are holding at the set point; they should be maintained within
±5 °F. A barrel heater zone that is overriding can be a sign of either a controller
issue (see Section 8.6.1) or an issue with excessive shear heating causing the
heater zones to override the set point.
17.3.1.2 Molding Process: Hot Runner Temperatures
If the mold has a hot runner system it should be treated as an extension of the
barrel heating and must be controlled within the requirements for the material.
Hot runner temperatures should be verified along with the barrel temperatures to
ensure that the temperatures are set correctly and not overriding.
Be aware with hot runner systems that there are multiple potential points of failure
beyond just the hot runner manifold. These failure points include:
The hot runner controller
Hot runner cables
Hot runner connector plugs
The hot runner controller can have zones that are failing to maintain temperatures
at the required set point.
Case Study: Hot Runner Controller
In this particular case a mold was running with high levels of scrap for brown
streaks and splay. Detailed observation of the scrap showed that the scrap
tended to occur in bunches. Everything on the mold and hot runner checked
out as working correctly but all indications were that the material was being
overheated. To gain further understanding of what was occurring within the
manifold additional thermocouples were added to determine what was differ-
ent on the end of the part at which the brown streaking was occurring. These
secondary thermocouples were connected to a process-monitoring system to
record the manifold temperatures over time. During a period of high scrap,
the data on the process-monitoring system was reviewed to determine if
anything was changing. The monitoring thermocouples showed that the end
of the part with the degraded material was seeing temperature variation of
over 200 °F while the scrap was being produced but maintaining temperature
when acceptable parts were being produced.
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