Page 319 - Injection Molding Advanced Troubleshooting Guide

32.3 Glass Fibers on Surface Troubleshooting 313

blocked off and the part filled fine; however, the end of fill knit line was still
visually unacceptable. Because the new end of fill location ended up at the
location of the second original gate a decision was made to try blocking the
runner to the second gate but leaving the gate and a small section of the
runner open. This open gate and runner section allowed the material at the
end of fill to overflow from the part and created a visually acceptable knit
line. The wide-open gate allowed plenty of venting at this knit line and
allowed the material to flow in a meld line fashion.
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32.3.2.2„ Mold: Hot Runner Temperatures
If hot runner temperatures are run too low the material might be cooling off as it
sits in the hot runner manifold. Hot runner temperatures should be set at the melt
temperature of the material to avoid allowing the material to heat or cool when it is
in the manifold.
Temperatures in a hot runner system should be controlled with a thermocouple
rather than with an amperage percent setting. Good thermal control of the hot
runner system will allow for maintaining an optimized melt all the way through
the melt delivery system. Remember that a hot runner mold is just an extension of
the melt delivery system.

32.3.2.3„ Mold: Flow Length
Long flow lengths with glass-filled materials can often lead to situations where the
machine is either pressure limited or close to it. When the machine is pressure
limited it will not maintain volumetric flow rate, which gets worse the further the
mold fills. As the flow rate decreases glass on the surface will tend to be more
evident. The main task concerning flow length is to evaluate number and location
of gates during design. Verify with the material supplier recommended flow length
to wall stock ratios.

32.3.2.4„ Mold: Gating
As mentioned in Section 32.3.2.3 the number and location of gates are critical to
allowing adequate filling of a mold. It is also important to watch for excessive re-
strictions in gates or hot drops that lead to large increases in fill pressure. If the
gate is too restrictive the process may become pressure limited, which will in turn
lead to a decreased fill velocity capability. Watch machine pressure curves during
fill to see if there is a very rapid rise at start of fill because this may indicate that
there is a restriction that is driving the pressure high and potentially closing the
process window.