Page 158 - Injection Molding Advanced Troubleshooting Guide

146 16 Blush

Whenever running with a hot runner system be sure to check that the orifice of the
hot tip is not causing a restriction that in turn is leading to the problem. People
often overlook the hot runner tip on the mold.
In rare cases runner geometry may contribute to gate blush (see case study below).

Case Study: Runner Geometry

This was a case of a polypropylene part where a blush defect was occurring
after gate seal during screw recovery. The theory was that the large runner
size was allowing the plastic pressure to break through the sealed gate
causing the blush appearance. The runner was 0.375 in diameter and
approximately 3 in long. The full-round runner was welded up to produce a
half-round runner, which helped the blush appearance some without
increasing the fill pressure or causing any other problems. A standing rib
was welded in the mold to core out the existing half-round runner. When the
mold was run again the blush was gone and there was only a slight increase
in fill pressure. The smaller runner prevented the plastic pressure during
screw recovery from breaking through the sealed gate. This is not a com-
mon case of gate blush but demonstrates that sometimes the total picture
must be examined via the STOP process.
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Another thing to consider trying when working on blush problems is to break the
sharp edge of the gate as it enters the cavity. Breaking these gate corners can help
reduce the shear rate at the corners of the gate. Reduction of the shear rate may
decrease the blush effect as the plastic travels through the gate.
Gate land length is the other area that must be addressed. The land length is the
distance where the gate narrows to its final thickness between the cavity and the
runner. Long gate land length can also contribute to blush issues. The land length
of the gate should be minimized to avoid premature restriction of the melt flow.
Long land lengths will also lead to increased pressure drop through the gate. Nor-
mal industry standards will recommend gate lands of approximately 0.030 in.
Avoid long land lengths to minimize the opportunity for blush.

16.3.2.2„ Mold: Hot Runner Tip Temperature
If a hot runner tip and drop have different temperatures a blush-like appearance
may occur just inside the gate. Try to minimize differences between actual melt
temperature and the set point of the hot runner system.
If running a hot runner system verify that the hot runner temperatures are set
correctly. Try adjusting the tip temperature to determine impact on the blush.
Again remember that hot runner temperatures should be set equal to the actual
melt temperature.