Plastic caking and feeding failure is mainly caused by the hopper not being removed after machine shutdown. Residual heat from the rear barrel continuously bakes the raw material at the feeding port and leads to hopper bridging and material caking. This fault frequently occurs with low-melting-point materials such as transparent PP/PE materials and soft TPR transparent materials.
1. Fault Principle & Root Cause
Heat Source Analysis: After shutdown, residual heat radiates continuously from the No.1 heater zone at the rear of the barrel. Since the hopper base stays closely attached to the feeding throat, the raw material is kept at a critical melting temperature for a long time. The outer layer of plastic pellets becomes semi-melted and adhered while the inner core remains granular, eventually forming honeycomb-shaped agglomerates with melted outer layers and clustered inner particles.
Feeding Blocking Mechanism: The semi-melted material adheres firmly to the flight of the screw feeding section. When the machine restarts, the caked material wraps around the screw. Plastic pellets can no longer be conveyed along the screw grooves, resulting in screw idling and feeding failure, namely hopper bridging blockage.
2. On-site Rectification Solutions
2.1 Emergency Treatment (For Caking & No-feeding Failure)
① Cooling and embrittling: Turn off the heating of the rear barrel and turn on the cooling water circulation for the feeding throat. Low temperature will embrittle the caked material for easy cleaning.
② Manual material cleaning: Remove the hopper and thoroughly clean all caked residue attached to the screw flights and feeding throat. Do not scrape or pry the screw with hard tools to avoid damaging the screw coating and precision.
2.2 Long-term Prevention (Avoid Recurrence)
Standard Shutdown Operation: For shutdown longer than 30 minutes, move the hopper away from the feeding throat to isolate residual barrel heat. For long-term shutdown, completely empty residual materials from the feeding throat and barrel.
Equipment Optimization: Regularly inspect the cooling water circuit of the feeding throat to ensure the cooling ring is unblocked and maintains stable temperature control. Reduce the temperature of the rear No.1 heating zone by 5~10℃.
Process Adjustment: For transparent materials with multiple additives and low melting points, perform empty injection before shutdown to discharge residual materials at the barrel front end.