Page 479 - Injection Molding Advanced Troubleshooting Guide

47.3 Weld Lines Troubleshooting 479

Some fillers will establish orientation as the plastic travels through the mold. This
orientation may change direction at the weld line leading to a more obvious weld.

Figure 47.5 Weld line on a glass-filled material

Unfilled materials will give the best weld lines; adding fillers will result in more
obvious weld lines. Sometimes the solution to objectionable weld lines is to turn
them into a meld line, which can be achieved by adding an overflow to let the
material flow together after welding together.

Case Study: Metallic-Filled Materials
In this case an ABS was being molded with a metallic flaked colorant. The
metallic flakes had a tendency to orient with flow which created variation in
appearance depending on the flow direction. Another major issue was that
whenever a weld line formed the metallic flakes would go from aligned with
flow to aligned perpendicular to flow. The flakes at the weld line with a
change in orientation led to a very obvious weld line and the very resin-rich
plastic right at the weld line also contributed to a visual difference. See Fig-
ure 47.6 for a magnified view of a weld line on a silver metallic material.
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