Causes and Solutions for Irregular Bubbles in Extrusion Process

What are the reasons for irregular bubbles in extrusion? The reasons and solutions for irregular bubbles during the extrusion process are as follows:

Cause Analysis

1. Material Issues
Excessive moisture content (>0.05%): Damp materials vaporize at high temperatures;
High volatile content: Decomposition of low-MW additives (e.g., lubricant >1.5%);
Poor filler dispersion: Agglomeration of hygroscopic fillers like CaCO₃.
2. Process Control Failures
Incorrect temperature settings:
Typical cases:
Feed zone >120℃ (PVC) → Premature melting traps gas;
Die head <160℃ (PE) → High melt viscosity impedes degassing;
Excessive screw speed (>80% rated): Shear-induced localized overheating;
Vacuum degassing failure (<-0.06MPa).
3. Equipment Factors
Screw wear (clearance >0.3mm): Unstable melt conveying;
Screen pack clogging (>120 mesh): Abnormal backpressure;
Poor die design: Melt stagnation zones.

Solutions

1. Material Treatment
Pre-drying:
PE/PP: 80-90℃×4h;
PVC: 60-70℃×6h;
Add 0.2-0.5% desiccant masterbatch (e.g., CaO-based);
Optimize mixing: 110-130℃ for 15-20min.
2. Process Optimization
Temperature profile:
Vacuum system upgrade:;
3-stage vacuum (-0.095MPa);
Vent length >5D (D=screw diameter).
3. Equipment Improvements
Screw repair:
Adjust compression ratio to 2.5:1-3:1;
Add mixing elements in barrier section;
Die flow path polishing (Ra<0.2μm);
Install melt pressure sensors (alarm >25MPa).

Detection Methods

1. Bubble Analysis
Microscopy (100X): Distinguish steam bubbles (round) vs. decomposition bubbles (irregular);
DSC test: Decomposition peak > process temp +20℃.
2. Online Monitoring
IR moisture analyzer (material inlet, ±0.005% accuracy);
Ultrasonic bubble detector (Φ0.1mm sensitivity).

Preventive Measures

1. Establish material moisture database (per-batch testing);
2. Monthly screw clearance checks (feeler gauge);
3. Adopt transparent dies (visualize melt flow).