Page 390 - Injection Molding Advanced Troubleshooting Guide

388 40 Short Shot

40.3.2.5„ Mold: Hot Runner Temperature
Low hot runner temperature can result in an increased pressure drop as the
mold fills. Drop and tip temperature seem to have the biggest impact on flow of the
plastic.
Check all hot runner zones and determine that everything is set correctly. Also
check to make sure that all zones are drawing amperage and the thermocouples
are reading correctly. If the hot runner controller is set correctly but the zone does
not want to heat up check the following items:
1. Hot runner cables
Verify that the correct number of zones are connected properly, and check for
pins not pushed in.
2. Hot runner plugs on mold
Verify that plugs are not damaged and cables are connected tightly.
3. Try another cable
If another cable fixes the problem check the bad cable for continuity from pin to
pin on the plugs.
4. After verification of the process the mold heaters and thermocouples will need
to be checked.
A hot runner that is overheating due to a malfunction can lead to degraded mate-
rial, which can in turn lead to short shots. Verify that the thermocouple is installed
correctly and reading accurately. Also verify that the hot runner zones in question
are providing accurate heating and not overriding set temperatures.

40.3.2.6„ Mold: Leaking Hot Runner Manifold
If plastic is not making its way into the cavity one place where it can leak is within
the hot runner manifold. Damage to the manifold can result in a plastic leak (see
Figure 40.7). This is a perfect example of why the STOP mentality is so important:
if the manifold is leaking and the technician adds shot size all they are doing is
compounding the problem (a good lesson for a technician who does this is cleaning
the plastic from the manifold). Such problems can become expensive and time-con-
suming repairs that always seem to occur when the mold is behind on production.
Catching this problem early through cushion monitoring may reduce the amount
of damage within the manifold and make for an easier repair.