1. Why Wear Tolerances Matter

• The screw flights and barrel inner wall must maintain precise clearance.
• Excessive wear increases the gap, leading to:
  • Leakage or backflow of molten plastic
  • Inconsistent plasticizing
  • Lower shot weight accuracy
  • Burn marks or unmelted resin

📏 2. Typical Wear Tolerances

Tolerances depend on screw diameter (D) and resin type, but here are common references:

• Screw-to-barrel clearance (radial gap):
  • New: 0.001–0.002 × D (e.g., 0.2–0.3 mm for a Ø100 mm screw)
  • Maximum allowable wear: 0.003–0.005 × D
• Flight land wear:
  • Should not exceed 0.1–0.2 mm before efficiency drops significantly.
• Barrel bore wear (measured with air gauge or bore gauge):
  • Acceptable ovality: ≤0.1 mm
  • Taper not more than 0.2 mm per 100 mm length
• Screw root wear:
  • Up to 0.5 mm may still be usable, but check against clearance.

🧪 3. Factors Affecting Wear

• Material type: Glass-fiber, flame-retardant, and filled plastics accelerate wear.
• Surface treatment: Nitrided, bimetallic, or coated barrels extend tolerance life.
• Process conditions: High temperatures, long residence times, or poor lubrication increase erosion.

📋 4. Inspection & Maintenance

• Measuring tools: Air plug gauge, bore gauge, micrometer, feeler gauge.
• Check frequency: Every 6–12 months, depending on resin abrasiveness.
• Rule of thumb: If clearance exceeds 2–3× original design, replacement or repair is needed.

✅ Summary:
For most injection molding machines, the recommended screw-to-barrel clearance is 0.001–0.002 × D (new), with a wear limit up to 0.005 × D. Beyond this, plasticizing efficiency, melt quality, and shot consistency decline, and the screw & barrel should be refurbished or replaced.