Screw Requirements for PP Sheet Extrusion (Plasticization Focus)

1. Length-to-Diameter Ratio (L/D)

• Requirement: 28:1 ~ 32:1
• Too low (<24:1): insufficient melting and mixing, leading to unmelted particles.
• Too high (>36:1): excessive residence time, risk of thermal degradation and yellowing.
• Optimal range ensures adequate time for solid conveying, melting and homogenization for PP sheet.

2. Compression Ratio (CR)

• Requirement: 3.0:1 ~ 3.5:1
• PP is a crystalline polymer with large volume shrinkage during melting; a moderate-high CR ensures efficient compaction, air removal and complete melting.
• Too low: poor compaction, voids, unstable melt flow.
• Too high: excessive shear heat, melt overheating and degradation.

3. Three-Zone Geometry (Feed–Compression–Metering)

• Feed Zone: Deep, constant-depth flights (h ≈ 0.15–0.20D) for high solid conveying efficiency; prevents bridging.
• Compression Zone: Gradually decreasing channel depth (25%–30% of screw length); gradual compression avoids abrupt shear spikes.
• Metering Zone: Shallow, constant-depth flights (h ≈ 0.05–0.10D); builds stable pressure, ensures uniform output and temperature.

4. Screw Type & Mixing Capability

• Standard: Single-flight, square-pitch screw is common for general PP sheet.
• Upgraded: Barrier screw (BM type) preferred for high-quality sheet.
  • Separates solid bed from melt pool; improves melting efficiency, lowers melt temperature, reduces crystal points.
• Mixing Section: Add distributive/dispersive mixing elements (e.g., pins, Dulmage, barrier rings) in metering zone for:
  • Uniform melt temperature (ΔT < ±1.5°C)
  • Homogeneous color and additive dispersion
  • Elimination of flow marks and gel defects

5. Shear & Thermal Control

• Moderate Shear: Avoid excessive shear rate; PP is shear-sensitive at high temperatures.
• Melt Temperature Target: 200–230°C (depends on PP grade; homopolymer ~210°C, copolymer ~220°C).
• Low Shear Heat Design: Proper flight clearance (0.10–0.15 mm for D=65–90 mm); prevents local overheating.

6. Material & Surface Treatment

• Base Material: 38CrMoAl nitrided steel (hardness ≥1000 HV); good wear resistance for PP with additives.
• Surface: Hard chrome plating or bimetallic alloy coating; improves corrosion resistance and reduces material adhesion.

7. Key Performance Targets (Plasticization Quality)

• Complete Melting: No unmelted PP particles or crystal points.
• Temperature Uniformity: Melt temp variation < ±1.5°C across the melt stream.
• Pressure Stability: Die pressure fluctuation < ±2% for consistent sheet thickness.
• Output Stability: Consistent mass flow with minimal surging.