Page 364 - Injection Molding Advanced Troubleshooting Guide

360 37 Pin Push

37.3.2.4„ Mold: Cavity Balance
If a multicavity mold is not balanced between all cavities there will be differences
in the molding process between each cavity. Nonuniform filling can lead to
over-packing in some cavities and under-packing in others, which can result in pin
push in some of the parts whereas others are fine.
Trying to accommodate for fill imbalance with the process will close the process
window for the mold severely. It is a much better decision to fix the mold and show
every cavity the same processing effects.
See Chapter 12 on cavity balance for more information.

37.3.3„ Pin Push Troubleshooting Machine Issues

Machine impact on pin push includes:
ƒ Machine performance
ƒ Ejector plate
ƒ Velocity to pressure (V to P) transition

37.3.3.1„ Machine: Machine Performance
The molding machine must be capable of repeatedly achieving the process that has
been set. If the machine is varying on any of a number of process parameters such
as pressures and ejector settings, pin push can be the outcome.
See Chapter 8 on machine performance for more details.

37.3.3.2„ Machine: Ejector Plate
Pin push can result from a nonuniform ejection force being applied to certain pins
in the mold. This issue is most likely to stem from using knock-out bars that are not
the same length. If the knock bars are uneven the ejector plate will push on the
ejector plate in the mold with uneven force and possibly cause pin push. Always
ensure that the knock-out bars are of the same length and screwed in equally.
Another potential cause of uneven ejection could be damage to the ejector plate or
ejection cylinder itself. If the ejector plate is becoming cocked as it moves forward,
the result will be uneven force on the ejector plate of the mold. Watch the ejector
plate of the machine for wobble or cocking as it travels forward. Excess wear in the
ejection system of the machine can lead to a variety of ejection problems with the
mold and part. In some cases the bushings for the ejection guides are worn out and
in others the ejector holes in the platen can become elongated if the knock bars are
not tied into the ejector plate.