Page 114 - Injection Molding Advanced Troubleshooting Guide

100 11 Hot Runners

the flow channel. Gently move the thermocouple back and forth against the steel to
get an accurate temperature. You will get a false reading if you just probe inside
the drop because the plastic will form a skin and affect the temperature reading. If
you find you have a thermocouple issue and are not getting the proper set tempera-
ture you have the option of running on percentage with most controllers. If you
have a matching zone with the same wattage heater you can set the percentage
until it matches the same current (amp) draw of the matching zone. You must also
consider that the amp draw will change as you are running parts because the hot
plastic running through the hot runner will reduce the current needed with the
plastic releasing heat so the heater will not have to work as hard. If you have a zone
hotter than the set temperature on the controller it is more than likely a thermo-
couple issue. If the thermocouple is loose or not making steel contact with the hot
runner or hot drop it will be reading the air temperature, which would be cooler
than the steel of the hot runner. It would thus tell the controller to supply more
current to the heater, which would then lead to a much higher temperature than
the set temperature on the controller.
Heaters are in most cases like a light bulb, either good or bad. But in rare situa-
tions if there are damaged wires where some of the strands are broken a heater
may not be able to manage all the current required to bring the heater up to tem-
perature. Heaters can be checked for proper function with resistance using a mul-
timeter in the ohms setting. If you know the voltage and the watts on the heater
you can calculate what ohms that heater should be at. Take the volts squared
divided by the watts to figure out the ohms. For example, if you have a 240-volt
heater with 1000 watts the ohms should read 57.6 (i. e., 240 × 240/1000).
Water leaks where water gets down into the hot runner system can also cause is-
sues. With most hot runner designs today these do not cause major issues with
blown heaters like they would with older designs. In most cases you just need to
drain the water out, and dry the system out, and put it back into operation. One
sign of a water leak is with zones struggling to get up to, or not achieving, the set
temperature.