Why Melt Fracture Is the Hidden Enemy of High-Speed Extrusion

As cable factories push their extrusion lines harder—higher RPM, tighter OD control, and thinner insulation—an old problem is reappearing with new force: extruder melt fracture defects.
What seems like random surface roughness on the insulation layer is actually a complex interaction of polymer rheology, die pressure, shear stress distribution, and temperature uniformity inside the extruder head.

Recent field reports from XLPE, PVC, TPE, and LSZH lines show that melt fracture begins long before operators notice “sharkskin” or “orange-peel” textures. The root cause lies in polymer flow instability, usually triggered by excessive shear rates at the die exit.

This technical report dissects the mechanism and provides engineering-level solutions for stabilizing extrusion quality.

1. Understanding the Physics Behind Extruder Melt Fracture

1.1 Why Shear Rate Is the Primary Trigger

Melt fracture happens when the polymer melt experiences shear stress that exceeds its elastic limit.
When the melt can’t relax quickly enough, the surface ruptures as it exits the die—creating:

• sharkskin marks

• longitudinal cracks

• periodic die lines

• matte or “grainy” appearance

Shear rate increases when:

• screw speed is too high

• melt temperature is too low

• die land length is too short

• polymer viscosity is too high

In high-output cable lines, increasing speed without adjusting die design is the most common cause.

2. Mechanical and Rheological Causes Inside the Extruder

2.1 Die Entry Geometry and Flow Distortion

A sharp die entry angle creates stress concentration zones.
This leads to:

• boundary layer disruption

• elastic recoil of the melt

• turbulence near the die exit

Even a 0.1–0.2 mm change in die gap can raise shear stress by 10–20%.

2.2 Temperature Non-Uniformity Across the Melt

If the melt temperature varies by more than ±3–5°C, the polymer’s relaxation time changes across the cross-section, causing asymmetric flow and surface tearing.

Sources:

• dead zones in the barrel

• uneven heater band output

• non-uniform screw compression

2.3 Polymer Molecular Weight Distribution (MWD)

Narrow-MWD polymers are more prone to melt fracture because they have limited ability to dissipate stress.
This is common in:

• low-grade PVC

• recycled PE blends

• low-cost LSZH compounds

3. Methods to Fix Extruder Melt Fracture Defects

3.1 Increase Melt Temperature by 5–15°C

Higher temperature lowers viscosity, reducing shear stress.
But control is critical—too high leads to:

• scorch

• yellowing

• insulation thinning

Recommended approach:

• Increase gradually by 2°C per zone

• Prioritize die head and adapter sections

3.2 Reduce Screw Speed or Output Rate

A 5–10% reduction in RPM often eliminates melt fracture instantly.
This reduces:

• shear rate

• die pressure

• surface tearing

But this also reduces productivity—so combine with die optimization for long-term stability.

3.3 Polish or Modify Die Land Surface

A smoother die reduces friction and drag.

Engineering approaches:

• mirror-polish to Ra ≤ 0.2 μm

• increase die land length

• introduce tapered entry geometry

• apply hard chrome or DLC coating

3.4 Add Fluoropolymer-Based Processing Aids

Processing aids create a lubricating micro-layer between the melt and die wall.
This is the most effective solution for:

• XLPE

• high-density PE

• low-shear LSZH

Typical dosage: 200–800 ppm, depending on polymer type.

3.5 Improve Temperature Uniformity Across Barrel Zones

Solutions:

• recalibrate heater bands

• check thermocouple response time

• reduce dead spots via screw redesign

• ensure stable cooling around hopper

Uniform melt equals uniform relaxation time.

3.6 Install a Melt Pressure Transducer at the Die

If pressure fluctuation exceeds ±3–5%, melt fracture risk increases.
Closed-loop control helps stabilize:

• output

• temperature

• viscosity

3.7 Use a Low-Shear, High-Compression Screw

A screw with gentle shear and extended mixing zones prevents polymer overstress.

Recommended upgrades:

• barrier screw

• Maddock mixer

• spiral groove feed section

4. Case Study: 24% Faster Line Speed After Eliminating Melt Fracture

A mid-size power cable plant reported severe sharkskin defects at speeds above 380 m/min.
After implementing:

• die surface polishing

• 10°C melt temperature increase

• 400 ppm processing aid

• screw RPM reduction by 6%

The results:

• melt fracture eliminated

• surface roughness reduced by 60%

• stable OD achieved

• final production speed increased from 380 to 470 m/min

This proves that melt fracture is not a permanent limitation—it's a solvable instability.

Conclusion: A Stable Extrusion Process Requires Mastering Melt Rheology

Fixing extruder melt fracture defects demands a precise understanding of polymer flow behavior, mechanical die design, and thermal uniformity.
Plants that integrate high-stability die geometry, controlled shear rates, advanced processing aids, and real-time monitoring achieve higher speeds and consistent insulation quality.

In modern cable manufacturing, controlling melt stability is not just a defect solution—it is a strategic advantage that drives line efficiency and reduces scrap.