If an extruded profile, tube, or cable jacket comes off the line with a dull, ridged surface—or with a wavy, distorted cross-section that only gets worse as you speed up the line—you are almost certainly looking at melt fracture. It is one of the most common flow-instability defects in plastics extrusion, and it sets a hard ceiling on how fast a die can run before quality collapses. This guide covers what melt fracture actually is, the process and die-design variables that cause it, and the practical fixes extrusion engineers use to push output rates higher without losing surface quality.
What Is Melt Fracture?
Melt fracture is the general term for distortions that appear on or inside an extrudate when the polymer melt is deformed too quickly, or too abruptly, as it flows through the die. At low output rates, most thermoplastics extrude with a smooth, glossy surface. As shear rate rises, the flow eventually becomes unstable, and the defect usually shows up in one of two recognizable forms:
- Surface melt fracture (“sharkskin”) — fine, regular ridges running perpendicular to the flow direction, usually confined to the outer surface of the extrudate. It appears at a lower onset shear rate than gross melt fracture and, for many polymers, does not significantly change bulk mechanical properties—but it does hurt gloss, print quality, and lamination adhesion.
- Gross melt fracture — an irregular, helical, or fully chaotic distortion across the whole cross-section, sometimes periodic (“bambooing”). This is a bulk-flow instability rather than a surface effect, and it typically appears after a distinct jump or plateau in die pressure at a critical shear rate—the “stick-slip” or “spurt flow” behavior documented for linear polymers such as HDPE and LLDPE.
Between the two extremes there is often a transition zone where the surface looks matte or faintly wavy before the flow becomes fully chaotic. Where a given extrudate sits on that spectrum depends on the polymer, the die geometry, and the processing conditions—which is why the same extrusion tooling can run clean on one material and fracture on another at the same line speed.
What Causes Melt Fracture?
Shear Stress and the Critical Shear Rate
Melt fracture is fundamentally a stress-driven instability. Every polymer melt has a characteristic critical wall shear stress—typically on the order of roughly 0.1 MPa for many linear polyolefins, though the exact value depends heavily on molecular weight, molecular weight distribution (MWD), and melt temperature—beyond which the flow can no longer stay laminar and smooth. Because wall shear stress rises with output rate and falls as the die’s flow channel gets larger, the practical trigger is almost always some combination of running too fast through too small an opening.
Wall Slip and the “Stick-Slip” Transition
As shear stress approaches the critical value, the melt at the die wall can begin to slip intermittently rather than adhering smoothly (the no-slip condition breaking down). This stick-slip cycling is widely cited as the mechanism behind the pressure oscillations and periodic distortions seen in gross melt fracture of linear, narrow-MWD polymers. Broader molecular weight distributions and long-chain branching generally delay the onset of this transition, which is one reason branched materials (like typical LDPE grades) tolerate higher shear rates before fracturing than very linear, narrow-MWD HDPE or metallocene resins.
Die Geometry: Entrance Angle, Land Length, and Abrupt Transitions
Geometry concentrates or relieves stress independently of the polymer itself. A sharp, abrupt entrance from a wide flow channel into a narrow die land creates a strong extensional (stretching) flow component in addition to shear, and that extra extensional stress can push a melt into fracture well before the shear rate alone would predict it. A die land that is too short does not give the melt time to relax that entrance stress before exiting, so the distortion carries through to the finished extrudate. Sudden cross-section changes, sharp corners, weld lines, and dead spots inside the flow channel all add local stress concentrations that can trigger fracture even when the average shear rate across the die looks acceptable.
Material and Process Variables
Beyond geometry, several process and material factors shift the onset point:
- Melt temperature — higher temperature generally lowers melt viscosity and elasticity, which raises the shear rate at which fracture begins (though excessive temperature brings its own degradation risks).
- Molecular weight and MWD — higher molecular weight and narrower distribution generally increase melt elasticity and lower the critical shear rate; broader MWD blends typically tolerate higher rates.
- Contamination or degraded resin — gels, moisture, or thermally degraded material create localized flow irregularities that can look like or trigger melt fracture even at otherwise safe shear rates.
- Die surface condition — a worn, scratched, or corroded die land surface changes the wall slip behavior and can trigger fracture-like defects at rates that were previously stable.
How to Fix Melt Fracture
Because melt fracture has several independent drivers, fixes generally fall into three groups: reduce the stress the melt experiences, change the die geometry, or change the material’s flow behavior.
Process-Side Adjustments
- Reduce output rate (screw speed) to bring wall shear rate back under the critical threshold—often the fastest diagnostic step, since a clean surface at lower rate confirms a shear-driven cause.
- Raise melt temperature moderately to lower viscosity and elasticity, shifting the onset point higher, while watching for degradation or increased die swell as a trade-off.
- Smooth out upstream pressure or temperature fluctuations (screw wear, feed surging) that can push local shear rates above the threshold intermittently even when the average setpoint looks fine.
Die Design Changes
- Lengthen the die land so the melt has more residence time and channel wall contact to relax elastic stress before exiting—this is the single most common structural fix for gross melt fracture, though a longer land also raises pressure drop and can increase heat generation from shear.
- Streamline the entrance from the flow channel into the die land with a tapered or radiused transition instead of an abrupt step, reducing the extensional stress spike that triggers fracture at the entrance region.
- Eliminate dead spots and sharp internal transitions in the flow channel, including at the breaker plate and screen pack interface, since stagnant or degraded material re-entering the main flow is a frequent hidden trigger.
Material-Side Fixes
- Polymer processing aids (PPA), typically fluoropolymer-based, are widely used to coat the die wall and reduce wall shear stress, delaying both sharkskin and gross melt fracture onset without changing line speed.
- Blending in a broader-MWD or branched-grade resin can raise the critical shear rate at the cost of some other mechanical or optical property, so this is usually a formulation trade-off rather than a first response.
- Low-surface-energy die coatings on the land surface can promote controlled wall slip and reduce the stick-slip cycling associated with gross melt fracture, extending the usable output-rate window for a given die.
Quick Troubleshooting Reference
| Symptom | Likely Cause | First Fix to Try |
|---|---|---|
| Fine, regular ridges perpendicular to flow (matte surface) | Surface melt fracture (sharkskin) — wall shear near critical stress | Add PPA, or reduce output rate slightly |
| Irregular helical or chaotic distortion across whole cross-section | Gross melt fracture — stick-slip past critical shear rate | Lower screw speed; lengthen die land |
| Defect appears only at the die entrance region of a complex profile | Extensional stress spike at abrupt entrance geometry | Streamline/taper the die entrance transition |
| Defect worsened over time on a previously stable die | Worn or corroded die land surface, or degraded resin/gels | Inspect and refinish die land; check resin quality |
Frequently Asked Questions
What does melt fracture look like?
It ranges from fine, regular surface ridges (sharkskin) that dull the extrudate’s gloss to severe, irregular, helical, or chaotic distortion of the entire cross-section (gross melt fracture) at higher shear rates.
What is the difference between sharkskin and gross melt fracture?
Sharkskin is a surface-only defect that appears at a lower shear rate and often leaves bulk properties largely intact. Gross melt fracture is a bulk-flow instability that appears at higher shear rates, typically after a jump in die pressure, and distorts the whole cross-section.
What is the critical shear rate for melt fracture?
It varies by polymer, molecular weight distribution, and temperature—there is no single universal number. For many linear polyolefins, instability tends to begin once wall shear stress approaches roughly 0.1 MPa, but the practical onset for a given resin and die should be established by trial, not assumed from a general figure.
Can a polymer processing aid (PPA) fix melt fracture?
PPAs are one of the most common and effective material-side fixes. A fluoropolymer-based PPA coats the die wall, reduces wall shear stress, and typically delays both sharkskin and gross melt fracture onset without requiring a die redesign.
Does melt fracture affect the mechanical properties of the finished part?
Surface melt fracture (sharkskin) mainly affects appearance, gloss, and secondary processes like printing or lamination, with limited impact on bulk mechanical properties for many polymers. Gross melt fracture is a structural distortion of the part itself and typically does affect dimensional accuracy and downstream performance, so it should always be eliminated rather than tolerated.
Melt fracture is very often a die-geometry problem as much as a process problem—land length, entrance design, and internal flow-channel transitions all set the ceiling on how fast a given tool can run before quality breaks down. If you are pushing rate limits on an existing die or specifying a new one, our team can review the flow channel and land geometry for your application. Contact SPiDER EXTRUSION for a custom die/tooling consultation, or see our extrusion tooling and extrusion die vs. mold guide for related background.