Page 238 - Injection Molding Advanced Troubleshooting Guide

24.3 Cracking Troubleshooting 229

24.3.2.4„ Mold: Lifter or Slide Concerns
Cracking can occur if the part is sticking on mold details such as slides or lifters.
Always verify that there is adequate draft on the mold component to allow release.
In many cases the surface finish of the slide or lifter must be correct to allow re-
lease; this may mean a draw polish in many applications.
Check that the lifter or slide detail has enough travel to allow complete release of
the part. There are cases where the slide or lifter does not have the required travel
and causes the part to hang up on the detail which can lead to cracking or other
damage to the molded part. This is another key item to verify during mold design
reviews.

24.3.3„ Cracking Troubleshooting Machine Issues

When it comes to cracking, the major machine issues will relate to the potential to
degrade the material, including:
ƒ Overheating
ƒ Lack of control

24.3.3.1„ Machine: Overheating
Anything that degrades a material can lead to cracking or breakage of molded
parts. The degradation on the plastic will lead to a reduction in the material’s phys-
ical properties, which can lead to this problem. Also, if material is degraded it may
have a greater tendency to stick in the mold leading to higher force required for
ejection and potentially causing damage to the part.
If all barrel zones are set to the correct temperature (per documented process)
check that all zones are actually at the set point. Verify the percent amp draw on
the zones, because if one zone is calling all of the time it may be overheating. A
zone that calls for constant power may be reading an incorrect thermocouple.
Usually if zone thermocouples are swapped one zone will be calling for power at all
times but another zone will never call for power. When heat zone thermocouples
are swapped there will be very inconsistent heat on the barrel. Newer controllers
have thermocouple burnout detection that will help eliminate some issues with
thermocouples.

24.3.3.2„ Machine: Lack of Control
As mentioned above, lack of thermal control is a key potential to degrade material.
In addition to the above points regarding temperature issues remember that shear
input is the largest contributor to melt energy in processing plastic. If the back
pressure or screw speed is not well controlled for the machine the excess shear can