Page 106 - Injection Molding Advanced Troubleshooting Guide
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10 Purging
10.1 Purging Impact
When a molding machine is changed over from one color or material to another it
is critical to do a thorough changeover and purge. If a changeover is not done cor-
rectly hours of scrap can be produced. Every molding shop should establish proce-
dures for a proper changeover of materials or colors of material.
Poor changeovers can result in:
Color streaking
Contamination
Delamination
Splay
Black/brown streaking
Black specks
Cloudiness
Plugged hot tips
Note that scheduling can be a major help to changeovers. One very helpful hint is
to keep like materials and colors grouped in specific presses as much as possible.
Downtime can be reduced by grouping materials of similar process temperatures,
which reduces waiting for barrels to cool down. Also, if making color changes try to
go from light colors to dark whenever possible. The purge in Figure 10.1 shows
the transition from black to gray; also note that this machine appears to have had
extended downtime prior to purging due to the presence of degraded material.
