Page 133 - Injection Molding Advanced Troubleshooting Guide

120 13 Cavity Instrumentation

An unusual application of thermocouples is represented in Figure 13.9. In this
particular case auxiliary thermocouples were installed in a hot runner manifold to
measure the temperature variation across the manifold. The first thing to note on
this data is that even though all of the hot runner zones were set at 510 °F the
actual temperatures at the shot the cursor is highlighting varied from 483 to
522 °F. Also notice that each of the curves has a large amount of variation over
time. In this particular case there was an ongoing issue with burning and splay
types of defects in one area of the part. After the thermocouples had been installed
large swings in temperature were detected over time with specific zones changing
over 200 °F while the mold was running. Observation of this variation showed no
correlation to readings on the hot runner controller, but 100% correlation with the
burning and splay scrap. The hot runner controller was replaced with another unit
which resolved the issue. Without utilizing the capabilities of the process monitor-
ing equipment this problem would not have been resolved because there was no
reason to suspect the hot runner controller.

Figure 13.9 Six auxiliary thermocouples installed on hot runner manifold

Another valuable piece of data that can be obtained from in-mold thermocouples is
actual mold temperature. Without thermocouples in the mold the best way to check
steel temperature is to stop the machine after running on cycle and use a surface
pyrometer to check the steel temperature, but as soon as the machine is stopped
the mold is cooling from its true running temperature. With a thermocouple