Page 94 - Injection Molding Advanced Troubleshooting Guide
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78 8 Machine Performance
If a heater zone is overriding the setpoint there may be a PID tuning issue with
the machine or excess shear may be overheating the barrel. If a zone is overrid-
ing the zone is not being controlled by the machine. Try running a PID tuning
cycle on the zone, and if it does not help it may be necessary to raise the barrel
temperature setpoint to allow the machine to control the zone.
Other potential concerns relate to thermocouples. If the wrong type of thermo-
couple (Type J instead of Type K or opposite) is used the temperatures will not be
read accurately. Table 8.1 shows the color coding of wires on the typical thermo-
couples used in injection molding.
Table 8.1 Color Coding of the Most Common Molding Thermocouples
Minus (−) Plus (+)
Type J red wire white wire
Type K red wire yellow wire
Positional control: How well a machine hits position setpoints need to be watched
over time. If a machine is showing variation in shot size or transfer position the
fill only weight injected during first-stage fill will be impacted.
If a machine does not open to the same mold open position robot handling will be
compromised. If frequent adjustments are being made to the clamp or the robot
it is likely that the mold open position is varying. Ensure that the clamp has an
adequate clamp open slow down setting; the machine may be trying to stop too
abruptly.
Verify repeatability for the ejector stroke, as this will also impact the robot han-
dling of the parts. Also, if the ejector stroke is varying on a mold that needs full
stroke distance to clear the lifters there will be issues with the part sticking on
the lifters.
Remember that the molding machine should “do what we want when we want”
shot after shot. Machine performance is one of the keys to successful molding
operations. Without a picture of how the machine is reacting troubleshooting will
be much more challenging and a feeling of working in the dark can be the result.
8.7 Machine Documentation
It is important to maintain reliable records for all molding machines. Key items
that need to be known include:
Platen size
Ejector pattern
