Page 198 - Injection Molding Advanced Troubleshooting Guide

188 20 Burns

Examination of the barrel temperature readings will indicate if there are zones
that are overheating. Often the zone may actually be overriding from the shear
heating during screw recovery. If the zone is running over the set point but not
actually calling for heat one of two possibilities exist:
1. The shear created from the rotating screw is generating heat and higher tem-
perature than the set point and the barrel heaters are no longer being used to
control the barrel temperature at all. If this is the case look at reducing the
screw rotation speed to allow the screw to recover 2–3 seconds prior to mold
open, or examine the back pressure being applied to the screw.
2. There is a remote possibility that the thermocouples have been installed in the
incorrect locations. This is more likely a potential cause if heaters or thermo-
couples had been recently replaced on the machine. If this is the case you will
see a zone that is running hot without calling for heat and another zone will be
running cold while calling for heat continuously. This is a good indicator that
the thermocouples have been cross wired.
IR photos can be used to discover hot spots in the nozzle as shown in Figure
20.4.

Figure 20.4 IR photo of nozzle looking for hot spots

20.3.3.4„ Machine: Damaged Components
Damaged components in the screw, barrel, check valve, end cap, nozzle, or tip can
lead to areas of high shear and or buildup locations. The damaged components can