Page 254 - Injection Molding Advanced Troubleshooting Guide

26.3 Dimensions Troubleshooting 245

Due to the cost of cycle time increases, adding cooling time is typically an un-
attractive option for changing dimensions. If cooling time improves the dimensions
it would be well worth investigating ways to improve the cooling of the mold to
minimize the cycle time.

26.3.1.8„ Molding Process: Decompression

If a process is running without adequate decompression the non-return valve will
not set consistently, which in turn can lead to issues with dimensions. Verify that
the machine is running with decompression and that the cushion is not showing
large variations and/or bottoming out. Adding a small amount to decompression
and verifying impact is a simple item to validate during troubleshooting. Different
non-return valves will react differently to decompression and some experimenta-
tion may be required.

26.3.1.9„ Molding Process: Cushion
Normally the key to watch for on cushion is verifying that the machine is running
with a cushion. If the screw bottoms out, the machine will not be able to transfer
pressure through the mold, which will lead to a part that would be small dimen-
sionally.
Lack of cushion could be from the shot size being set too small, plastic leaking at
the nozzle or hot runner, lack of decompression, or a machine issue (see below).
Before adding material to the shot size verify where the plastic is going. Automati-
cally adding shot when the press is leaking at the nozzle will create damage to the
heaters and thermocouples on the barrel as the plastic starts to encapsulate the
barrel. If the press has been running fine, first verify where the plastic is going
before adding shot!

26.3.2„ Dimensions Troubleshooting Mold Issues

Possible mold related concerns are:
ƒ Gate size
ƒ Sprue and runner size
ƒ Cooling
ƒ Hot runner control
ƒ Component deflection

26.3.2.1„ Mold: Gate Size and Location
If the gate size of a molded part is too small the part will not be effectively packed
and may experience excessive shrink. If a gate is small it can freeze prematurely,