Page 132 - Injection Molding Advanced Troubleshooting Guide
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13.2 Cavity Thermocouples 119
The data obtained from cavity thermocouples are not as detailed as is the case with
cavity pressure; however, thermocouples will provide the following:
Actual mold steel temperature
Actual fill time to the thermocouple location
Change to melt temperature or cooling rate
An advantage of cavity thermocouples is that they are extremely fast-acting, which
makes them very useful for setting up valve gate actuation. By using cavity ther-
mocouples positioned throughout the mold, sequential valve gate actuation can be
very accurate, and the gate does not actuate until the plastic has reached a specific
point in the cavity. The equipment can be set up to delay firing based on time after
a temperature rise at a given thermocouple. This is a very repeatable way of setting
up sequential valve gate actuation.
In Figure 13.8 process data for a variety of sensors is shown including cavity tem-
perature, cavity pressure, machine pressure, and screw movement. Notice that the
mold temperature curve stays horizontal until the plastic hits it. The temperature
rises quickly once the plastic contacts the thermocouple. In this example the ther-
mocouple appears to be very near the end of the cavity because the temperature
seems to rise just after transfer. This fact would allow detection of short shots eas-
ily, because if there was no temperature rise at the end-of-cavity thermocouple
there was no plastic present, and therefore a short shot occurred.
Mold temperature
Figure 13.8 Process monitoring data showing cavity temperature, cavity pressure, machine
pressure, and screw movement
