Page 169 - Injection Molding Advanced Troubleshooting Guide
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158 17 Brown Streaks
17.3.2.4 Mold: Hot Runner Insulator Gap
Many hot runner tips have an insulator gap that helps to isolate the hot runner
heat from the cavity. If these gaps are not sealed (most hot runner manufacturers
supply seals to fill these gaps) material will become trapped and can potentially
degrade leading to brown streaks. These gaps can also create major issues when
conducting color changes.
17.3.2.5 Mold: Sprue Bushing Seat
If the sprue bushing seat on the mold is damaged or does not match the radius on
the nozzle tip, leakage may occur, which can lead to brown streaks. Inspecting the
sprue bushing seat will show if there is any damage that could cause leaks. If dam-
age is found the seat can easily be remachined to eliminate the problem. In Figure
17.4 an example of a leak between the nozzle tip and the sprue is seen; this type of
leak can lead to black specks and brown streak contamination. Please note that
cardboard between the nozzle tip and sprue bushing does not seal a leak as much
as some processors wish it would.
Figure 17.4 Contamination leakage between nozzle and sprue bushing
