Underfeeding Issues in Extrusion Machines: Causes and Solutions

Underfeeding (material shortage or insufficient feeding) in extrusion machines can stem from multiple factors. Below are common causes and corresponding solutions:

Common Causes

1. Raw Material Issues
Moist material: Excessive moisture content causes bubble formation and incomplete filling when heated.
Irregular pellet size: Inconsistent particle size leads to uneven feeding.
Contamination: Foreign objects (e.g., metal scraps, paper) blocking the feed throat.
2. Feeding System Malfunctions
Poor hopper design: Inappropriate discharge angle or insufficient capacity.
Screw/barrel wear: Worn screw or scratched barrel interior reduces conveying efficiency.
Excessive feed zone temperature: Premature melting causes material to stick to the screw ("bridging").
3. Improper Process Parameters
Low backpressure: Melt backflow results in inaccurate metering.
Excessive screw speed: Incomplete melting leads to uneven density.
Incorrect temperature settings: Mismatched zone temperatures (e.g., too high in feed zone, too low in melting zone).
4. Mechanical/Electrical Failures
Drive system issues: Damaged gearbox or slipping belts cause unstable screw rotation.
Faulty heaters: Localized temperature failures impair melt flow.
Metering errors: Feeder calibration drift or control system signal interference.

Solutions

1. Material Handling
Pre-dry material: 80–100°C drying for 2–4 hours (material-dependent).
Install filters: Add magnetic separators or screens (20–40 mesh recommended).
Use uniform pellets: Ensure consistent size (3–5mm ideal).
2. Feeding System Checks
Optimize hopper: Add vibrators or switch to conical design to prevent bridging.
Repair screw/barrel: Measure screw clearance (<0.2mm normal); rechrome or replace if worn.
Adjust feed zone temp: Typically 20–30°C below melting point (e.g., 120–140°C for PE).
3. Process Adjustments
Increase backpressure: Set to 5–15bar (material-dependent) for denser melt.
Reduce screw speed: Operate at 60–80% of rated speed for thorough plastication.
Optimize temperatures: Peak temp in melting zone (e.g., PP: 160°C feed → 220°C melt → 200°C die).
4. Equipment Maintenance
Inspect drives: Check motor current fluctuations and belt tension.
Replace heaters: Test resistance (1–10Ω normal) with a multimeter.
Calibrate feeder: Verify accuracy via weighing (<±1% error tolerance).

Troubleshooting Flow

1. Symptom Analysis
Cyclic underfeeding → Check screw speed or drive system.
Continuous underfeeding → Investigate material, temps, or feeder.
2. Manual Tests
Pause auto-feeding; observe if manual feeding resolves the issue.
Measure actual vs. set temperatures (±5°C tolerance).

Preventive Measures

Scheduled maintenance: Clean hopper and inspect screw every 500 hours.
Process documentation: Log temperature, pressure, and speed data.
Operator training: Standardize preheating (≥30 mins to reach process temps).
If problems persist, inspect mold flow channels or verify extruder capacity matches production needs.