Page 337 - Injection Molding Advanced Troubleshooting Guide

34.3 Jetting Troubleshooting 333

34.3.1„ Jetting Troubleshooting Molding Process Issues

Jetting is typically more related to the mold than the process; however, fill velocity
is a process variable that will impact jetting. Conducting a short shot study is valu-
able when trying to determine if jetting is occurring; there will be cases where the
actual defect shows as splay or blush because of how the jetting filled the mold.

34.3.1.1„ Molding Process: High Fill Velocity
High injection velocities during first-stage fill will impact how the plastic enters
the mold cavity. If the plastic is injected fast the melt flow front may not adhere to
the cavity wall but will instead jet across the cavity.
Reducing fill velocity can improve jetting problems. Even though slowing the fill
velocity can improve the jetting appearance on a part remember it will increase
cycle time and result in adding cost to the molded part. It is always better to address
a mold problem rather than process around the issue. Figure 34.3 shows an exam-
ple of a normal flow front developing from the gate as depicted in Moldflow. A
profiled first-stage fill velocity may allow for a slow initial fill with an increased
velocity after the melt front is established; this complicates the process, but if
nothing else works it may be the only choice.

Figure 34.3 Normal flow front developing from the gate (Moldflow)

Flow front development is important as the material enters the mold and continues
to fill. Short shots can help determine if the melt front is developing properly; look
for inconsistency and abnormal flow patterns. With the way jetting folds the melt