Page 384 - Injection Molding Advanced Troubleshooting Guide

382 40 Short Shot

Avoid processing around mold problems. In some cases, to limit flash, the fill veloc-
ity or transfer position will be modified, which in turn leads to short shots. Many
times processing around one problem creates another defect.

40.3.1.4„ Molding Process: Second-Stage Pressure Low
If second-stage pressure is too low the mold will not finish filling after transferring
from first stage. Running an RJG Decoupled process means that the mold will be
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only 95–98% full with no second-stage pressure applied. As pressure is applied to
the plastic the remaining 2–5% of the cavity will fill and the part will be packed
out. Some details in the mold such as thin ribs will require enough cavity pressure
to force the plastic into the detail, but if cavity pressure is not high enough the part
will be a short shot.
Compare the actual second-stage pressure to the documented process. If cavity
pressure transducers are available verify that the cavity pressure is meeting the
template value. Account for intensification ratio when molding on a hydraulic ma-
chine. If the pressure is low determine why; it may be a simple incorrect setting or
may be a machine problem.

40.3.1.5„ Molding Process: Velocity to Pressure Transfer
The transition between first-stage velocity and second-stage pressure can result in
short shots. Depending on how fast the machine reacts and reaches set pressure
the part may not fill out. Figure 40.4 shows process monitoring data for a bad ve-
locity to pressure (V to P) response. Notice how all the three cavity pressure curves
follow the dip in pressure that the machine experiences during transfer. For more
details on velocity to pressure response see Chapter 8 on machine performance.

Figure 40.4 Process monitoring curves showing poor velocity to pressure response