Page 127 - Injection Molding Advanced Troubleshooting Guide

114 13 Cavity Instrumentation

ƒ Displacement along the vertical axis is based on the type of transducer. These
values may be pressure, linear displacement, temperature, or any other data be-
ing collected. Again the lower left is the zero point for this data.
ƒ Color code in upper left-hand corner details what the data curve colors corre-
spond to.
ƒ Machine pressure will climb from zero to the transfer pressure; second-stage
pressure should appear as the flat horizontal line shown in Figure 13.2 for the
duration of set second-stage time.
ƒ Screw recovery is represented by the lower horizontal line on the machine pres-
sure curve that is on the lower right of the screen.
ƒ Notice how the machine pressure and screw displacement start to increase at the
start of the cycle but the cavity pressure only starts to rise at the transfer posi-
tion. This indicates that this cavity pressure transducer is located at the end of
fill; if it was closer to the gate there would have been a pressure rise during first
stage.
Cavity pressure transducers are often located either just inside the gate (post-gate
transducer) or as close to the end of fill (end-of-fill transducer) as possible. For
process monitoring the end-of-fill transducer will provide data on what occurred
through the process and will give accurate information on whether the mold filled
or not. Process control usually relies on a post-gate transducer to signal when to
transfer from pack to hold. With both post-gate and end-of-fill transducers an accu-
rate picture of the plastic pressure across the cavity is provided. Below is some key
information that transducers can provide:
ƒ Post-gate transducers will show a distinct hitch that indicates the change be-
tween dynamic filling pressure and static hold pressure (see Figure 13.3). The
two cavity pressure curves with higher pressure are post-gate transducers (see
labels). Notice how they start to rise early in the cycle and change slope at trans-
fer. This change in slope indicates where the cavity is full and building pressure
to the cavity pressure peak.
ƒ Post-gate transducers provide useful data on gate seal. When a gate is sealed
the plastic that is packed against the transducer will be maintained after the
machine completes second-stage time. If the gate is not sealed plastic will dis-
charge through the gate resulting in a rapid reduction in cavity pressure when
second-stage pressure is removed. In Figure 13.3 the post-gate curves drop im-
mediately when second-stage time ends. This indicates that the gates were not
sealed on this mold. Determining gate seal time with a post-gate transducer is
simply a matter of adding second-stage time until the post-gate cavity pressure
maintains a consistent slope. This is a quicker method than the typical gate seal
study conducted by weighing parts molded with different second-stage times.