Establishing a successful plastic injection production system requires effective machine maintenance to maintain the plastics machinery producing parts. Downtime caused by unplanned maintenance can cause huge scheduling, production and logistical problems adding to the cost to produce parts and taking away from an already slim profit margin. This article will outline some of the failure modes companies slip into, as well as define some basic requirements needed to establish an effective Preventative Maintenance System. It will also provide key process monitoring data that can help to identify problems as they develop to help plan maintenance events in advance to decrease down time.
Screw & Barrel:
Properly inspecting screws and barrels are paramount to controlling a validated process. Screws should be pulled every 6 months to measure flights, Barrel inside diameter and the screw shank, especially in the compression/ transition zone. This is especially important when running fiberglass/ metal-based materials that cause premature wear due to the nature of their structure. Key process monitoring variables that help identify changes in screw performance might be inconsistent cushion greater than .05 or fill time variability. Part weight fluctuations greater than +/- 1.5% can also point to screw and barrel wear.
Poorly maintained clamps can also be extremely detrimental to maintaining consistent processes. Clamp components such as bushings, pancake sensors, linear transducers and self-lube systems and lines should be inspected monthly for wear and proper operation. Clamp parallelism should be checked every 6 months. It should be checked sooner if abnormal flash events occur using a validated process. Signs that might show poor performance in clamp performance might be: inability to reach or maintain clamp force/ tonnage setpoints, inability to hold pressure during a tonnage event or mold open time inconsistencies
There are 3 primary contributors to poor temperature performance in a molding process: Barrel temperature, Mold temperature and hot runner. This section will outline the key inspection factors for each, as well as process monitoring data that be reviewed for troubleshooting purposes.
- Barrel Temperature- Temperatures that vary more than 2F show signs of poor calibration or failing heater bands. In addition, If at start up a zone heats much slower than the other zones around it this might point to heater band failures. In most cases, barrel temperature actuals should reflect no more than a 1F degree variance from setpoint.
- Mold Temperature- Mold Temperature fluctuations greater than 3F should be considered suspect. Mold temperatures and actuals should generally stay constant with no significant differences between actual vs. setpoint. Variations often point to a burnt heater, or a valve sticking open or closed. Regarding process monitoring, GPM and process to/ from pressures should also be monitored.
- Hot Runner- Hot runner set points should match actuals. Changing temperature controls from PID to manual should only occur when production needs require it for a short-term basis. Large swings in actual temperatures reveal potential calibration or failing heater issues. It is important to remember that in most cases, the hot runner is merely an extension of the nozzle into the mold cavity. These temperatures should generally be equal to each other.
It is important to note that the best approach to maintenance is planned maintenance events. Use maintenance log data to establish when breakdowns generally occur for each specific event. Then structure planned preventative approaches to eliminate unplanned maintenance work. Unplanned events generally lead to chaos planning, expedited parts and increased downtime. The most effective approach to a successful maintenance program is proper planning to eliminate poor performance.