Page 386 - Injection Molding Advanced Troubleshooting Guide

384 40 Short Shot

The process technician had reduced the back pressure while purging to al-
low the screw to recover a full shot. After building a shot and starting the
machine on cycle the process technician forgot to reset the back pressure.
This case was an excellent reminder to check the process while on cycle
and always STOP to think through the potential root causes.


40.3.1.8„ Molding Process: Low Melt Temperature
Low melt temperatures often raise the viscosity of the melt resulting in material
that flows less easily. High viscosity material will not fill part details as easily and
will lead to an increased pressure drop that may cause short shots.
Verify that the melt temperature matches the documented process and is appropri-
ate for the material. Increasing melt temperature is often done to help a diffi-
cult-to-fill mold. Keep in mind that added melt temperature increases cooling time
and may lead to material degradation. Use a melt temperature probe or a thermal
imaging camera to determine melt temperature, and raise it if it is low.

40.3.1.9„ Molding Process: Mold Temperature

Low mold temperatures can impact the ability of the plastic to fill the mold. Cooler
mold temperatures will cause a thicker frozen layer and potentially restrict flow in
the mold. As plastic flows through a mold the mold surface temperature will rise
(see Figure 40.5). If a mold has areas that are difficult to fill initial startup shots
may not fill due to a low mold surface temperature.

Figure 40.5 Mold surface temperature rise over 50 consecutive shots