Page 272 - Injection Molding Advanced Troubleshooting Guide

264 27 Excessive Cycle Time

27.3.4.1„ Material: Regrind
Sometimes when running regrind the pickup rate of the screw will vary. This
occurs sometimes because of regrind bridging the feed throat or not feeding well
through the feed lines. When the material is lacking the screw will be starved of
material and will continue to run until a full shot is built. Another impact of this
extra screw run time can be a change in melt temperature of the plastic in the bar-
rel during the increased rotation time of the screw. Watch how material is feeding
the machine and verify that the clamp opening is not waiting for the screw to
recover.

27.3.4.2„ Material: Additive Package
There are a variety of factors in the formulation that can impact the cycle time
capability of a material. One example is a variation in nucleating agents in
semi-crystalline materials. The higher the level of nucleating agents the faster the
cycle time can be. Another additive impact could come from variation in lubricants
and/or release agents because these compounds can influence how the screw is
feeding material during plastication.
If the material is the root cause of needing to increase cycle time there will nor-
mally be an effect when changing into a new lot of material. To determine if the
material is contributing to the problem try swapping to another lot of material or
even try sampling another similar material to determine impact.