Page 23 - Plastics News June - 2020

Fundamentals of Proper Press Shutdown
Garrett Mac Kenzie
Press shutdown procedures have a significant impact on The nature of some materials, such as nylon, acetal and
achieving smooth machine startups and restarts to PVC, may require purging the screw with an inert
improve uptime. This article will outline potential material like PP, PS or a purge compound to prevent
downtime events that have a direct relationship with degradation. Over time, that degraded material can
poorly performed press stops. cause screw and/or tip breakage, and even lock up the
screw and barrel. That could ultimately necessitate
In the fast-paced world of injection molding, the key
replacement of both. It goes without saying that the
drivers of a plant's profits are downtime, efficiencies and costs and downtime associated with these types of
scrap. A company's success relies heavily upon quick and repairs can be astronomical.
efficient startups. Elimination and avoidance of
unplanned downtime events also plays a critical part in Hot-Runner Approach
profitability .Press shutdown procedures have a
Hot-runner care is crucial for mold-damage prevention.
significant effect on achieving smooth machine startups
and restarts. Moreover, presses shut down poorly quite Hot runners that are left on for extended periods of time
often lead to process failures and downtime. can perform poorly due to overbaked and/or degraded
materials. In the case of polycarbonate, long heat
exposure can cause contamination issues that lead to
huge scrap events. High fiberglass content left in the hot
runner can lead to plugged drops. Any one of these
situations can lead to extra mold changes and unplanned
mold maintenance. Hot runners that are left on for
extended periods of time can perform poorly due to
overbaked and/or degraded materials. Mold Hot-runner
controllers should be turned off during long shutdown
periods. This will help to prevent degradation in the hot-
runner manifold.

Shutdown
Any time mold activity goes idle, it is time for mold
inspection and care. Slide slots and ejector/ guide pins
should be inspected for damage, galling, and
lubrication. In-press mold storage is vital if the press is to
be down for the longer term. The mold should be left
with mold halves touching but not under pressure.

Water Shortfalls
Water shutdown is crucial to downtime and damage
prevention. Long-term shutdowns can lead to unnoticed
Screw Failures leaks. When a mold isn't moving, water performance
changes. This type of leak can lead to rust, which forces
A poor approach to screws is frequently the cause of
unneeded downtime and failed startups. Shutting down a mold to be pulled for service. Rust on a textured
without running the screw dry can lead to feed-throat surface can cost thousands in tool repair and downtime.
bridging, which can require hours of servicing to remove Three-Stage Shutdown Procedure
melted plastic from the throat. Presses shut down poorly
quite often lead to process failures and downtime. The following procedures outline proper purging and
Leaving the carriage forward for long periods of time can shutdown of presses. There are three basic time frames
lead to nozzle drool and material blowback on restart. that should determine the technician's actions. These
Any time a press is shut down, it is wise to back the procedures have been developed based on using
carriage off, remove material from the barrel, and materials such as nylon. The purpose here is to reduce
retract the screw 1 in. to release pressure and help the number of purge approaches, while also instilling
prevent drooling. At this point, the sprue bushing should good shutdown habits. The cost of extra purge puddles is
be inspected and cleaned in preparation for restart. much cheaper than the cost of machine or mold repair
and downtime associated with poor shutdown practices.

June 2020 23 Plastics News