Page 445 - Injection Molding Advanced Troubleshooting Guide

44.3 Stringers Troubleshooting 445

Sometimes the only way to eliminate stringing is to add an insulator between the
nozzle tip and the sprue bushing. This insulator is most frequently a piece of card-
board; however, there are molded insulator tips and Kevlar fiber pads that can also
be used. The insulator will frequently solve the stringing problem.

Figure 44.2 IR image of extended nozzle and adapter

44.3.1.2„ Molding Process: Low Decompression
If the decompression setting on a machine is set too low the shot will have a higher
residual pressure acting on it. This pressure builds during screw recovery and is
relative to the amount of back pressure applied during recovery. This pressure on
the melted shot will continue to act as a force against the plastic in the nozzle and
nozzle tip. Excessive pressure on the shot will push the plastic at the front of the
nozzle tip and may allow the material to string and also drool when the mold opens
and the sprue leaves contact with the nozzle tip.
Increasing the decompression set point can help keep the plastic away from the
end of the tip and in turn reduce stringing. Watch for excess decompression be-
cause it can cause splay problems.

44.3.1.3„ Molding Process: High Melt Temperature
Even if the nozzle temperature is set appropriately the actual melt temperature of
the material may be too high, which results in stringing. Melt temperature that is
running high will make it more likely for stringing to occur.