In the fast-paced world of plastic injection, the key drivers of a plant’s profits are downtime, efficiencies and scrap. A company’s success relies heavily upon quick and efficient start-ups. The elimination and avoidance of unplanned downtime events also plays a critical part in profitability.

Procedures used for press shutdown have a significant effect on achieving accomplished start-ups and press restarts. Presses shut down poorly quite often lead to process failures and downtime, This article will outline potential downtime events that have a direct relationship with poorly performed press stops. It will also provide a solid approach based on time that will help to reduce, and in most cases eliminate scrap and downtime associated with press shutdown.

Screw Failures

Screw approach is frequently the cause of unneeded downtime and failed start-ups. Shutting down without running the screw dry can lead to feed throat bridging, which can literally require hours of servicing to remove melted plastic from the throat.

Leaving the carriage forward for long periods of time can lead to nozzle drool, and material blowback on restart. Any time a press is shut down, it is wise to back the carriage off, remove material from the barrel and retract the screw 1.00” to release pressure, and help prevent drooling. The sprue bushing should be inspected and cleaned in preparation for restart.

The nature of some materials such as nylon, acetyl and PVC may require purging the screw with an inert material such as polypro, styrene or a purge compound to prevent degradation. Material degradation can cause screw/ tip breakage, even screw/ barrel lock up requiring screw barrel replacement. The cost of repairs and downtime associated with these types of service can be astronomical!

Hot Runner Approach

Hot runner care is crucial for mold damage prevention. Hot runners that are left on for extended periods of time can perform poorly due to overbaked and/or degraded materials. In the case of clear polycarbonates, 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.

Mold Shut Down

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

Water Shortfalls

Water shut down is crucial to downtime and damage prevention. Long term shutdowns can lead to unnoticed leaks. When a mold isn’t moving, water performance changes! This type of leak can lead to rust, which can result in pulling a mold for service. Rust on a textured surface can cost thousands in tool repair and downtime.

3 Stage Shut Down Procedure

The following procedures outline proper purging and shut down of presses. There are 3 basic time frames that distinguish the actions of the technician. These procedures have been developed based on using materials such as nylon. The purpose of this is to reduce the number of purge approaches, and to instill good shut down habits. The cost of extra purge puddles is much cheaper than the cost of machine/ mold repair and downtime associated with poor shutdown practices.

If press will be down for 30 minutes or less:

• Screw is to be ran forward to remove shot from barrel
• With screw empty, screw is to be sucked back one inch
• Purge removed from press bed

If down time length is unknown, or longer than 30 minutes but less than 4 hours:

• Carriage is to be retracted to separate tip from bushing
• Tip is to be inspected for blow back
• Throat and barrel are to be purged completely dry with polypropylene
• With screw empty, suck screw back 1 inch
• Hot runner is to be shut off
• Purge removed from press bed

If down time will be 4 hours or full shift:

• Carriage is to be retracted to separate tip from bushing

• Tip is to be inspected for blow back
• Throat and barrel are to be purged completely dry with polypropylene
• With screw empty, suck screw back 1 inch
• Hot runner is to be shut off
• Mold is to be sprayed lightly with rust preventative
• Mold is to be closed, not under tonnage with mold faces touching
• Water is to be shut off at Main valves
• Purge removed from press bed

Weekend Shut Down:

• Carriage is to be retracted to separate tip from bushing
• Tip is to be inspected for blow back
• Throat and barrel are to be purged completely dry with polypropylene
• With screw empty, suck screw back 1 inch
• Hot runner is to be shut off
• Mold is to be sprayed with rust preventative
• Mold is to be closed, not under tonnage with mold faces touching
• Water is to be shut off at Main valves
• Dryer is to be banked at 100F

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