Page 159 - Injection Molding Advanced Troubleshooting Guide

16.3 Blush Troubleshooting 147

When evaluating hot runner systems verify there is an adequate number of heat-
ers and thermocouples and that they are properly located. Also verify that heaters
and thermocouples are properly installed and tightened to ensure accurate results.
Verify that the hot runner zones are all reading correctly. Look for zones that are
running without thermocouple control as they will not provide accurate closed
loop temperature control. The hot runner zones should be checked for amp draw to
establish that they are heating correctly. Examination of the thermocouple location
on the drop may need to be conducted to understand where the actual drop tem-
perature is being measured.

16.3.2.3„ Mold: Cold Slug Well
A cold slug well is located in a mold at the end of the sprue entering the runner
system. The cold slug well is intended to catch any frozen slugs of material from
the nozzle tip. If a cold slug well is not available or designed to be as large as the
sprue it will not be effective in trapping any cold slugs that are injected from
through the sprue. Figure 16.3 shows an example of a cold slug well with a trapped
cold slug; in this case the cold slug was actually a hot tip vestige that had stuck at
the hot drop. If a slug of material like this should make it to the part and it makes
it through the gate there will be flow defects that may appear as blush. Runner
systems should have a cold slug well to prevent this material from making its way
to the part. Using the STOP method will help avoid processing around a mold that
does not have an adequate cold slug well.

Figure 16.3 Cold slug trapped in cold slug well