Solving Surface Pit Issues in PVC Rigid Granule Extrusion Profiles

🔍 Common Causes of Surface Pits

PVC Resin Issues - Uneven molecular weight distribution creates melt flow inconsistencies - "Fish eyes" or gel particles (undissolved polymer clusters) disrupt surface uniformity
Additive Challenges - Poor dispersion: Additives fail to blend evenly with PVC matrix - Compatibility issues: Aggregates form due to low additive-PVC miscibility

Mixing Problems - Insufficient mixing time → heterogeneous blends with additive clumps - Incorrect mixing temperature → incomplete plasticizer absorption
Extrusion Parameter Missteps
- Temperature Imbalance - Too low: High melt viscosity causes friction-induced surface damage - Too high: PVC degradation releases volatiles forming bubble pits
- Screw Speed Issues - Too fast: Inadequate residence time for proper melting - Too slow: Overheating from prolonged shear stress

Select Premium Resins - Source PVC with narrow molecular weight distribution (MWD < 5) - Insist on <0.1% fish eye content and strict impurity testing
Optimize Additive Systems - Choose additives with ≥95% PVC compatibility (test via FTIR spectroscopy) - Pre-disperse masterbatches using high-speed mixers (600-1000 RPM for 15+ mins)

Perfect the Mixing Process

1. 50-70℃ for 10 mins (dry blending)  
2. 110-130℃ for 20 mins (plasticizer absorption)

- Use twin-shaft mixers for shear-controlled dispersion

Fine-Tune Extrusion Parameters
```
Hopper: 50-70℃  
Barrel zones: 160-185℃ (gradient increase)  
Die: 180-190℃ (uniform for smooth flow)  
                
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- Screw Speed: 30-50 RPM (balance between throughput and melt quality) - Maintain die pressure at 80-120 bar for stable melt extrusion