Page 282 - Injection Molding Advanced Troubleshooting Guide
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274 28 High Fill Pressure
28.3.4 High Fill Pressure Troubleshooting Material Issues
Material causes can include:
Viscosity increase
Moisture content
28.3.4.1 Material: Viscosity Increase
Materials will have natural variation over time. As the viscosity of a material in-
creases, the pressure to fill a mold will increase. This increase in viscosity may
come from a change in average molecular weight of material, a change in filler
content, or a change in additives. Every material will vary, but some materials will
tend to have a wider range of variation than others. Melt index measurements are
the typical measure used in industry to evaluate a material; however, it is import-
ant to understand that the melt index is determined at extremely low shear rates
that do not simulate molding shear rates.
Changes in peak pressure on the press can be a good indicator that the viscosity of
a material has changed. Higher viscosity means harder flowing, which means
higher pressures. Viscosity is a measure of the resistance of a material to flow;
higher viscosity means that the material flows with more resistance.
Case Study: Fill Pressure Increase with Barrel Size Change
This particular case was molding a PC/ABS part in a 170-ton press with a
small barrel. The mold was moved into a 330-ton press to evaluate flash.
Even though all the process documentation was followed for barrel tempera-
tures, recovery time, etc., the fill pressure was substantially higher in the
larger machine barrel. All intensification ratios were taken into account but
the fill pressure was still higher in the larger machine. Further investigation
showed that due to the shear rate during plastication the material was not
achieving the same viscosity from barrel to barrel. The larger screw was
turning slower and not generating as much shear heating during recovery.
Remember that it is important to account for shear differences between
different screw sizes.
28.3.4.2 Material: Moisture Content
In many materials moisture content during processing will cause hydrolysis of the
material. Hydrolysis will cause a reduction in molecular weight of the material,
leading to shorter molecule chains. With shorter molecule chains the viscosity of
the material will drop and it will flow easier. This can create situations where there
are large changes in peak fill pressure due the variation in viscosity that the mois-
ture content is causing.
