Introduction: Why Melt Fracture Is a Serious Extrusion Problem
A smooth pipe surface isn’t just about appearance — it directly affects:
- Pipe strength & pressure rating
- Customer acceptance
- Compliance with IS / ASTM standards
- Rejection & reprocessing costs
Melt fracture often appears suddenly and operators try quick fixes like lowering speed —
which kills output but doesn’t solve the root cause.
This guide explains why melt fracture occurs and how to eliminate it correctly.
What Is Melt Fracture in PVC Extrusion?
Melt fracture is a surface distortion defect that appears as:
- Rough or matte pipe surface
- “Sharkskin” texture
- Wavy or cracked appearance at die exit
It occurs when melt stress exceeds the material’s elastic limit as it exits the die.
Common Causes of Melt Fracture in PVC & uPVC Pipes
Excessive Shear Stress at the Die
High shear = polymer chains stretch too much and snap back unevenly.
Why it happens:
- High screw RPM
- Small die gap
- Sharp die land edges
Improper Melt Temperature Profile
Too cold = high viscosity
Too hot = material degradation
Both conditions increase melt instability.
Poor Screw Design or Wear
Worn screws create:
- Inconsistent melt flow
- Pressure fluctuation
- Localized overheating
Inconsistent Material Formulation
Variations in:
- K-value
- Lubricant balance
- Filler dispersion
cause unstable melt elasticity.
Die Surface Roughness or Damage
Scratched or worn dies increase friction at exit — a very common overlooked reason.
Step-by-Step Troubleshooting Guide (Plant-Friendly)
Step 1: Check Die & Land Geometry
Polish die land Increase die land length slightly Remove sharp edges
This alone solves ~30% of melt fracture cases.
Step 2: Optimize Barrel Temperature Zones
| Zone | Recommendation |
|---|---|
| Feed zone | Slightly cooler |
| Compression zone | Gradual rise |
| Metering zone | Stable, not excessive |
| Die zone | Uniform & controlled |
Avoid sudden temperature jumps.
Step 3: Reduce Shear Without Reducing Output
Instead of slowing production:
Increase die diameter marginally Use low-shear screw configuration Optimize compression ratio
Step 4: Improve Material Homogeneity
Pre-mix additives thoroughly Ensure uniform feeder flow Maintain consistent moisture levels
Uneven dispersion = uneven melt elasticity.
Step 5: Inspect Screw & Barrel Wear
Check:
- Flight edge sharpness
- Clearance between screw & barrel
- Build-up zones
Replace or refurbish if wear exceeds tolerance.
Advanced Solutions for Persistent Melt Fracture
Use fluoropolymer processing aids Apply chrome or mirror finish on die Switch to optimized PVC grade for extrusion Install melt pressure sensor near die
These solutions are ideal for high-output extrusion lines.
Quick Diagnostic Table
| Symptom | Likely Cause | Solution |
|---|---|---|
| Rough surface | High shear | Die optimization |
| Wavy texture | Temp imbalance | Barrel tuning |
| Cracks at exit | Die damage | Polish/replace |
| Intermittent defect | Feeding issue | Feeder calibration |
Conclusion
Melt fracture is not a material problem — it’s a process control issue.
When you optimize:
- ✔ Die design
- ✔ Temperature profile
- ✔ Shear control
- ✔ Screw condition
You get:
- ✅ Smooth pipe finish
- ✅ Higher output
- ✅ Lower rejection
- ✅ Happier customers
Want Expert Help With Extrusion Defects?
We help PVC & uPVC pipe manufacturers with:
-
Extrusion line audits
-
Die & screw optimization
-
Output enhancement without quality loss
-
Troubleshooting recurring surface defects
