Page 475 - Injection Molding Advanced Troubleshooting Guide

47.3 Weld Lines Troubleshooting 475

Verify pack pressure to the documented process. Make sure to account for intensi-
fication ratio as required. If weak weld lines are a problem try increasing the sec-
ond-stage pressure to check for improvement.

47.3.1.4„ Molding Process: Mold Temperature
Normally higher mold temperatures will allow for better formation of a weld line.
With a warmer mold the plastic will pack the knit easier and the appearance of the
weld line will reduce. Using a higher mold temperature will allow better chances
for the molecules to become entangled at the weld line.
Increasing the mold temperature will improve the appearance and strength of
many weld lines. If weld lines are a concern try sampling the mold at a higher
temperature.

There are rapid mold heating systems on the market that can provide a much
higher mold temperature. These technologies will do a nice job of eliminating the
visual aspect of the weld line and strengthen the weld line. If weld lines are of
critical concern investigating a rapid mold heating technology may be the best
solution.

47.3.2„ Weld Lines Troubleshooting Mold Issues

Key impact items for the mold include:
ƒ Venting
ƒ Wall stock
ƒ Gate location

47.3.2.1„ Mold: Venting
Venting is one of the main culprits of bad weld lines. When two flow fronts meet,
the gas that is traveling ahead of the front must be vented to provide a high-quality
weld line. If gas is trapped between the flow fronts the weld line will be weak and
visually obvious. The gas acts as a buffer between the flow fronts and inhibits the
chance for the material to actually bond together.
Even with meld lines poor venting can lead to visual defects. There are cases when
what appears to be a flow line will appear along a part. Running short shots will
sometimes show that these lines are actually a meld line that is occurring from a
portion of the melt front hesitating. The initiation of this defect can sometimes be
seen as a slightly skewed flow front. Running short shots is a very important
troubleshooting tool that will help identify the evolution of many defects. These
flow front hesitations can occur as a portion of the melt stream runs along a radius
while the rest of the flow front travels in the nominal wall.