Page 403 - Injection Molding Advanced Troubleshooting Guide

402 41 Sink

If a non-return valve is leaking there will be some things to check:
ƒ Make sure that the decompression is set correctly.
ƒ Watch the screw during injection because if the non-return valve is leaking the
screw will rotate during injection.
ƒ Inject forward against the mold with a part and sprue in it. If the screw travels
forward even though the plastic has nowhere to go the non-return valve could be
leaking.
ƒ Try moving the shot size and transfer position back 1 in and running a shot. If
the shot holds at the larger transfer position it may indicate a worn spot in the
barrel rather than a worn non-return valve.

41.3.1.5„ Molding Process: Slow Fill Velocity

When a mold is filled with slow velocity during first stage the viscosity of the
material will be higher leading to a larger pressure drop across the cavity. This
increase in viscosity may reduce the ability to pack out the part.
The fill time should match the documented fill time. If the fill time is not matching
the setup sheet the first-stage velocity should be adjusted accordingly. A repeat-
able fill time is one of the critical process requirements. If the first-stage velocity
has been reduced due to a tooling issue such as burning, fix the mold.

41.3.1.6„ Molding Process: High Mold Temperature
A high mold temperature will impact the cooling rate of the plastic. Hotter molds
have a tendency to produce more shrink in the plastic which in turn can lead to
additional sink.
Ensure that the water temperature and flow rate are at the desired levels. If flow
rate has decreased it will have a major impact on the cooling potential, especially if
the flow is no longer turbulent. Use a flow meter to evaluate the actual gallons per
minute (GPM), but if a flow mater is not available disconnect the water return line
and allow it to run into a bucket while timing it. Using the bucket method can give
an idea of the GPM of flow, but remember that the water is free-flowing and does
not have any of the closed-circuit back pressure that it can experience when every-
thing is connected. Sometimes the poppet valve on the thermolator will not open to
allow fresh cooling water into the circuit, which will cause the water temperature
to rise over time.
Look for water lines that are not connected properly, for example a circuit that is
looped without a water-in line. Check for any ball valves that are closed and block-
ing water flow. Beware of “vapor locks” or water circuits that have air trapped in
them. All lines should be fully purged with water prior to starting up the mold.
Verify water flow through all circuits.