Page 274 - Injection Molding Advanced Troubleshooting Guide
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266 28 High Fill Pressure
28.3 High Fill Pressure Troubleshooting
If a molding process suddenly is experiencing higher than normal fill pressures it
is critical to determine what has changed. If the fill pressure is much higher than
normal it will indicate that the pressure drop across the cavity is also greater than
normal, which means the plastic is not going to be packed like it was previously.
If the fill pressure reaches the maximum available for the molding machine the
process is considered to be pressure limited. When a molding process is pressure
limited the machine cannot control the fill velocity. Molding machines must have
more pressure available than is required to maintain control of fill velocity. Bear in
mind that processes that are within 10% of the maximum pressure of the machine
are on the edge and will likely have molding runs that are actually pressure limited
due to changes in material viscosity. When establishing a molding process make
sure that the required fill pressure is less than 90% of the maximum pressure of
the molding machine.
28.3.1 High Fill Pressure Troubleshooting Molding Process Issues
Possible molding process causes are:
Low melt temperature
Fill velocity
Heavy fill only
28.3.1.1 Molding Process: Low Melt Temperature
Fill pressure typically has an inverse relationship to melt temperature, especially
when dealing with amorphous materials. The lower the melt temperature the
higher the pressure required to fill a mold. There can even be conditions where the
melt temperature is so low that the mold will not fill completely before the melt
front freezes off.
If the process is experiencing a sudden increase in peak pressure a melt tempera-
ture check may provide the root cause. Verify that the actual melt temperature
corresponds to the documented melt temperature on the setup sheet. If there is no
documented baseline melt temperature for the process at least verify that the melt
temperature agrees with the supplier’s recommended values.
If the melt temperature is determined to be low the following items should be in-
vestigated:
Set points for barrel temperatures
Set points for back pressure and screw rotate speeds
