Page 396 - Injection Molding Advanced Troubleshooting Guide

394 40 Short Shot

All materials will have some viscosity variation over time. If using wide-specifica-
tion materials the variation will be much larger. Viscosity variation is one of the
critical reasons a process needs abundant pressure for repeatable fill times. If the
process is close to being pressure limited and the viscosity increases the process
may become pressure limited and have a short shot.
If short shots start immediately after changing to another lot of material, a vis-
cosity shift is most likely the cause. Keep in mind that a robust process should be
capable of absorbing incoming viscosity changes. One way of validating viscosity
impact on a process is to make sure that process development is conducted with
multiple lots of material. If a process cannot produce quality parts through the
®
range of viscosity variation, instrumenting the mold to run a Decoupled III pro-
cess may be the best decision. An RJG Decoupled III process uses a cavity pres-
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sure transducer to transfer from the pack stage to the hold stage; this will help
ensure that the mold has been packed to the same level. Also, through use of end
of fill cavity pressure transducers the process can easily be monitored for short
shots and set up to automatically reject suspect shots.

40.3.4.2„ Material: Moisture Content
Moisture will decrease the viscosity of nylon. If process development is conducted
using wetter nylon the mold will fill easier than with dry nylon. Dry nylon may
impact the viscosity enough to cause short shots and even create a pressure-lim-
ited condition. Try to ensure consistent levels of moisture when molding nylon.

For details on material drying see Chapter 9.

40.3.4.3„ Material: Inconsistent Material Feed
If the material is not being fed consistently to the machine short shots can result.
As the screw is recovering a lack of feed can starve the screw and lead to inconsis-
tent shots and possibly short shots.
If material is being fed from a container such as a gaylord it is important to ensure
that the feed hose is consistently pulling material. A gaylord tipper may help with
material feed.
Some materials such as long glass-filled materials and some regrind will be diffi-
cult to feed. In some cases a vibrator may need to be added to ensure that the feed
throat does not bridge over with pellets.

When using central material feed systems make sure that there is adequate timing
for all hoppers to load consistently. In some cases the feed system may be timed
incorrectly, which can result in machines being starved of material between load
cycles. Check the hoppers to ensure that material is being loaded in time to keep
up with machines recovery rate. Feed times to the machines may need to be ad-
justed to allow adequate feed to all machines in the system.