Page 160 - Injection Molding Advanced Troubleshooting Guide
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148 16 Blush
16.3.2.4 Mold: Gate Location
When gating directly into a class “A” surface there will be struggles with appear-
ance. One of the most effective ways to avoid problems with gate appearance is to
locate gates in an area of the part that is not class “A”. To accomplish this, look for
opportunities to gate into tabs, ribs, side walls, or other part details to provide a
well-established melt front to hit the show surfaces of the part.
Frequently, when a cashew gate is coming up directly behind the class “A” surface
of the part rings of blush will be apparent, even looking like a bull’s-eye in severe
cases. This design will often result in narrow process windows to yield acceptable
parts. Locating the gate in a less objectionable area would have resulted in a better
process window and less overall problems.
16.3.3 Blush Troubleshooting Machine Issues
Machine contributions to blush may include:
Nozzle
Machine performance
16.3.3.1 Machine: Nozzle
Verify that the machine has the correct nozzle setup. The nozzle length, style, tip
type, tip orifice, and heaters should all match the documented process. This an
area that some processors may consider overkill to document but frequently in in-
jection molding troubleshooting it is the small things that can lead to many hours
of time wasted. Thorough documentation will allow for a much easier job for 4M
troubleshooting.
Verify that all of the nozzle components are set up to the required standard. Use a
nozzle orifice gauge to verify that the tip orifice is correct. Check for complete
coverage of nozzle with heater bands. An additional item to check is the nozzle
thermocouple; ensure that it is correctly installed within the nozzle. Loose thermo-
couples can give wrong temperature readings and cause a nozzle to run at the
wrong temperature. Verify that the nozzle tip is not the wrong type, specifically a
mixing tip or a nylon tip when a full taper is called for.
Watch for temperature variation across the length of the nozzle. There are times
when the thermocouple at the rear of the nozzle is not providing an accurate mea-
sure of the actual nozzle temperature. Use of a thin immersion thermocouple can
allow detection of variation in nozzle temperatures when troubleshooting.
