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28.3 High Fill Pressure Troubleshooting 267
If the set points match the documented process then see Section 28.3.3 on ma-
chine troubleshooting for further information on possible causes for low melt tem-
peratures.
28.3.1.2 Molding Process: Fill Velocity
Verify that the fill time matches the documented process. If the mold is being filled
at a fill velocity that does not match the documented process the fill pressure will
be different. Keep in mind that as long as the process is not pressure limited the fill
velocity will be maintained by using whatever pressure is required. Fill time
should be repeatable shot after shot without fail because variation in fill time will
create other potential part defects.
28.3.1.3 Molding Process: Heavy Fill Only
Transferring from first stage to second stage should occur at a 95–98% full cavity.
If the machine is transferred late (transfer position set too low) and the cavity is
completely filled, the sudden resistance to flow will cause a very rapid spike in fill
pressure.
Fill only weight should always be checked when starting up a process and trouble-
shooting. An incorrect fill only weight will lead to a wide variety of potential de-
fects. A process that transfers late will be much more likely to flash the mold. Re-
member that fast fill velocities do not flash a mold but the sudden spike in cavity
pressure does if transferring too late.
28.3.2 High Fill Pressure Troubleshooting Mold Issues
Potential mold-related causes include:
Gate size and number of gates
Runner or sprue size
Hot runner
Plugged gates
Venting
Wall stock
28.3.2.1 Mold: Gate Size and Number of Gates
If a gate is too restrictive the pressure required to force the plastic through the gate
may increase. This is not as hard and fast of a rule as many people would prefer it
to be. There are cases when a smaller gate size has minimal impact on the actual
fill pressure required for the mold. It is important to understand that smaller gates
