Page 446 - Injection Molding Advanced Troubleshooting Guide

446 44 Stringers

Check the actual melt temperature of the material. If the melt temperature is too
high per the documented setup sheet investigate why. Evaluate the barrel tempera-
ture set points, the actual back pressure, and the screw recovery speed. If any of
these are not running to the documented process the result can be overheating
of the material. Determine that all actual machine variables meet the specified set
points.

44.3.2„ Stringers Troubleshooting Mold Issues

Potential mold-related problems include:
ƒ Hot tip temperature
ƒ Tip-bearing surface
ƒ Lack of cooling
ƒ Valve gates
ƒ Tip orifice size

44.3.2.1„ Mold: Hot Tip Temperature
On hot runner molds without valve gates the tip temperature must control the
freeze off of the plastic at the tip. If the tip temperature is set too high or the tip is
overheating the result is likely stringing. Hot drop temperatures can be verified
with a long thin probe that can be inserted into the drop to measure temperatures
within the drop (see Figure 44.3). Always ensure proper personal protection equip-
ment is worn and use appropriate safety precautions when checking temperatures.
Check that the hot runner temperatures are set correctly based on the documented
process. Verify that the actual hot runner temperatures are reading correctly.
Avoid running hot runners without thermocouples, because simple percent cur-
rent output settings will result in inconsistent heating of the zone.
Positioning of hot runner thermocouples can lead to overheating of sections of a
hot runner. A well-designed hot runner system will provide uniform heating across
the entire manifold. Areas of heat sink can cause overheating due to inconsistency
in the manifold. If there is too much contact area on a drop the mold steel will act
as a heat sink and the zone may have to run hot to avoid freezing off.