Page 135 - Injection Molding Advanced Troubleshooting Guide

122 13 Cavity Instrumentation

„ „13.4„ Process Control

In-mold instrumentation can be used for process control. Both cavity pressure
transducers and cavity thermocouples can be used for process control. The four
typical means of process control are:
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1. Decoupled III processing
2. Valve gate control

3. Gas assist control
4. Mold temperature control unit (TCU)
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In Decoupled III processes the mold is filled with velocity control and still trans-
fers by position. There is a controlled velocity second-stage pack step that packs
the part until the cavity pressure reaches a set transfer pressure. After reaching
the cavity transfer pressure the machine will be transferred to a pressure-con-
trolled hold stage. The hold stage lasts for the established hold time usually until
gate seal. With this method of processing the packing of the part is controlled at a
given rate which provides a very controlled process, and the mold will always be
packed to the same pressure. An output signal from the monitoring equipment to
the machine will allow for the transfer to occur based on the cavity data. Decou-
pled III processing is a methodology that RJG Technologies specializes in provid-
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ing training and equipment.
For controlling valve gates the process monitoring system will use output signals
to fire valve gate controls. The process monitoring equipment can be set up to fire
the valve gate controls based on when the plastic reaches the transducer or ther-
mocouple. This allows firing of valve gate controls in a very repeatable manner
based on what the plastic is doing in-mold rather than on screw position as is nor-
mally done. When running a sequential valve gate sequence, the ability to control
firing based on plastic conditions will help to eliminate concerns with splay, flow
lines, and other process defects and inconsistencies.

With in-mold instrumentation overflows for gas assist can be actuated based on
plastic position in the mold. A thermocouple or cavity pressure transducer can
provide the direct feedback from inside the mold cavity to provide more accurate
timing for overflow actuation. Using an output device tied to the process monitor-
ing system the overflow can be actuated allowing for consistent processing.
In-cavity thermocouples (or thermistors) can be used to provide actual mold tem-
perature data to a mold temperature control unit (TCU). Most TCUs are set up to
control based on the output water temperature, however, for more accurate data
the mold temperature can be the control point on some TCUs. Using actual mold
temperature will provide for a more consistent control of the process over time by
reacting to changes that occur downstream of the TCU including: