Page 317 - Injection Molding Advanced Troubleshooting Guide

32.3 Glass Fibers on Surface Troubleshooting 311

machine pressure). If the machine is running close to the pressure limit the fill
velocity is likely to vary over time and the appearance of the part will also.
If the mold is being run in a machine other than the one it was sampled in make
sure to evaluate if the machine is capable of achieving the required fill velocity
throughout the shot. If the fill time cannot be obtained by increasing fill velocity
investigate if the machine can and does achieve the velocity required. See Chapter
8 on machine testing for more on evaluating velocity performance.
One key to filling fast enough with glass-filled materials is adequate venting. If
the mold is not vented well enough it will not be possible to achieve adequate fill
velocity without burning (see Section 32.3.2.1).

32.3.1.2„ Molding Process: Low Melt Temperature

High melt temperature will help get a resin-rich surface for the molded part. If the
melt temperature is too low the part will tend to look glassy. Also if melt tempera-
ture is low the fill pressure may increase and cause the process to become pres-
sure limited resulting in slowing fill velocity.
A melt temperature verification will determine if the process is being run at the
proper temperature. Do not evaluate melt temperature based on barrel tempera-
ture set points. The melt temperature must be measured either with a melt probe
or an IR thermal device. Verify against the documented setup sheet and the recom-
mended melt temperature of the material supplier.

Raising the melt temperature may allow the mold to fill with better surface appear-
ance. If the part surface is too glassy in appearance try to increase the melt
temperature toward the upper end of the recommended process window. Avoid
increasing the melt temperature too much or the material may degrade and cause
other defects.

Be conscious of using high back pressure when molding glass-filled material. As
the fibers travel down the screw, excess shear will cause the fibers to break reduc-
ing the length of the fibers. With long glass fibers this becomes even more critical
to the performance of the plastic in the molded part. The thing to beware of is that
the shorter glass fiber lengths will look better but will lose physical properties.

32.3.1.3„ Molding Process: Low Mold Temperature
Low mold temperatures are a leading cause of glass on the part surface. To keep
glass fibers buried, the mold temperature often needs to be run at the upper limits
of the recommended temperature.
In many cases to get optimized surface appearance a high-temperature thermo-
lator must be used to allow the mold to be run at temperatures above 212 °F. If
running high temperatures extra caution should be utilized to protect people from