1. Types of Wear in Screw & Barrel

1. Abrasive wear
   • Caused by glass fiber, mineral fillers, or pigments.
   • Results in erosion of screw flights and barrel wall.
2. Adhesive wear (metal-to-metal contact)
   • Happens if lubrication is poor or clearance is too small.
   • Leads to galling and scratches on the screw/barrel surface.
3. Corrosive wear
   • From flame-retardant plastics, PVC, fluoropolymers, or halogen additives.
   • Chemical attack on steel leads to pitting and rough surfaces.
4. Erosive wear
   • High shear speeds + hard fillers = “sandblasting” effect on metal surface.

📏 2. Signs of Wear

• Increased screw-to-barrel clearance
• Drop in shot weight consistency
• Poor melt homogeneity (black specs, unmelted pellets)
• Longer cycle times, higher back pressure
• Frequent need to adjust processing parameters

📊 3. Acceptable Wear & Tolerances

• New screw-to-barrel clearance: 0.001–0.002 × screw diameter (D)
• Maximum clearance before replacement: 0.005 × D
• Flight land wear: ≥0.1–0.2 mm affects performance noticeably
• Barrel bore ovality: should be ≤0.1 mm

Example (Ø60 mm screw):

• New clearance ≈ 0.06–0.12 mm
• Max wear limit ≈ 0.30 mm

🛠 4. Prevention & Solutions

• Material choice:
  • Use bimetallic barrels and wear-resistant screw coatings for glass-filled resins.
• Operating practices:
  • Avoid running the barrel empty at high temps.
  • Keep back pressure moderate.
  • Purge properly before shutdown.
• Maintenance:
  • Regularly check screw/barrel with gauges.
  • Schedule inspections every 6–12 months (shorter for abrasive resins).
• Repair/Replacement:
  • Nitriding, hard surfacing, or plasma spray coatings can restore parts.
  • If wear exceeds tolerance, replace to avoid production losses.

✅ In summary:
Injection molding screw and barrel wear mainly comes from abrasive, corrosive, or adhesive factors. Once the clearance grows beyond 0.005 × screw diameter, you’ll see serious performance issues — at that point, refurbishment or replacement is recommended. Using wear-resistant materials and proper maintenance extends service life significantly.