Page 226 - Injection Molding Advanced Troubleshooting Guide
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23.3 Contamination Troubleshooting 217
Anti-seize is an item where “if a little is good a lot is better” does not apply. Teach
processors to use an appropriate amount of anti-seize to avoid running scrap at
startup.
23.3.3.3 Machine: Robot Contamination
Take a look at the robots in the shop. Are they dusty with grease drips in various
locations? When a robot enters the mold to pick a part it can contaminate the mold.
One example of this is when a dirty air line brushes up against the cavity, because
this will transfer some of the dirt and potentially lead to a cosmetic defect.
Another factor to consider with the robot is the condition of the end-of-arm tooling.
If it is very dirty the vacuum cups can leave a surface contamination on the part.
Also if components from the end of arm tooling touch the mold surface they may
leave behind contamination that can end up in the cavity.
Case Study: Robot Contamination
This example was on a single-cavity ABS part that was experiencing what
appeared to be splay. Examinations of drying conditions, moisture content,
venting, process settings, and melt temperatures all led nowhere. Utilizing
the STOP methodology and observing the process showed that when the
robot picked the part it backed away from the ejector and a dirty air line
brushed against the cavity. The cycle was stopped and a small smudge of
grease/dirt was detected on the cavity. The part was molded and it was
scrap due to the contamination. A couple of cable ties and some cleaning
resolved the issue. In this case utilizing the STOP methodology earlier would
have led to quicker effective troubleshooting.
23.3.4 Contamination Troubleshooting Material Issues
There are many ways that a material can become contaminated including:
Improper storage
Regrind
Incoming contamination
Mixed materials
23.3.4.1 Material: Improper Storage
In every molding shop there are a wide variety of foreign contaminants including:
Dirt, dust, and pollen
Other plastics
