Page 308 - Injection Molding Advanced Troubleshooting Guide
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302 31 Flow Lines
the machine or molding process and recheck the melt temperature. If the melt
temperature is running correctly try raising the melt temperature to determine
impact on the defect.
Very low melt temperatures will also potentially lead to a record grooving type of
defect on the part. These record groove ridges can be a good indicator that the melt
temperature of the material should be raised. Record groove defects are often
accompanied with a short shot at end of fill.
Low melt temperatures will also impact the quality of meld lines. Always try to
determine if the flow line is actually a meld line downstream from the actual loca-
tion where the material separated. Meld lines can show up literally anywhere from
the formation location to the end of the part. Warmer melt temperatures will allow
a better-quality meld line.
31.3.1.4 Molding Process: Mold Temperature
Cold mold surface temperatures can lead to a variety of flow-related defects. Long
flow lengths in cold molds can lead to hesitation due to the material freezing exces-
sively. This impact is compounded by flow length, thinner wall stock, and first-
stage velocity. Thin wall stocks with long flows will experience more flow limita-
tions and slow fills will see a greater impact of the frozen layer of material that has
contacted the mold.
Check the process to determine if the mold temperature is set correctly, and make
the required adjustments if it does not meet specification. Watch the defect; if it
only occurs on startup parts it may be an indication that mold temperature needs
to be raised. Remember that prior to injecting plastic the mold surface will be very
close to the incoming water temperature but will increase as plastic is injected into
the cavity. This increase may be slight but it still may point to a direction in trou-
bleshooting.
31.3.2 Flow Lines Troubleshooting Mold Issues
Mold-related issues can include:
Wall stock
Flow pattern
31.3.2.1 Mold: Wall Stock
Look for areas in the molded part where the wall stock chokes off the flow front.
Such areas often will result in a flow line from the melt hesitating as it hits the re-
striction. Some thin sections will end up being backfilled well after the flow front
has progressed past the location of the thin area. These backfilled locations can
