Sharkskin is usually a flow instability at the die exit. It is rarely proof of contamination or a bad resin lot. Denn's review places typical onset at roughly 0.1 MPa of wall shear stress, though the real figure shifts with grade and die.
This article follows one sequence: identify the defect, isolate the cause, apply controlled corrections, and validate a stable process window.
Key Takeaways
- Sharkskin is the mild surface form of melt fracture, tied to high shear stress at the die exit.
- Cut output first: if the defect fades at lower throughput, it is likely shear-driven.
- Main levers are temperature, speed, die geometry, resin rheology, wall slip and processing aids.
- Rule out die lines, gels, contamination and gross melt fracture before changing materials or tooling.
What Is Sharkskin in Polymer Extrusion?
Sharkskin is a surface defect where the extrudate shows fine, repetitive roughness or ridges across the flow direction, like shark skin. It appears as the melt leaves the die.
The die-exit mechanism
Cogswell's cohesive-failure model explains it well. As the melt leaves the die, the velocity profile rearranges: material at the wall accelerates from near-rest toward plug flow. That creates high local tensile stress. When the stress exceeds the melt's cohesive strength, the surface cracks.
NIST experiments on polyethylene saw the melt stick or weakly slip upstream of the exit, then split into a slow surface layer and a faster core.
Thresholds are not universal. Reported figures include about 0.1 MPa (Denn, 2001) and 0.138 MPa for one LLDPE at a 160 °C exit (Rothstein, 2004). Treat them as context, not limits for your grade.
Appearance and severity
Sharkskin is the first stage in a progression:
- Sharkskin: fine, small-amplitude ridges across the flow.
- Stick-slip ("bamboo"): alternating rough and smooth bands at higher stress, around 0.3 MPa in Denn's review.
- Gross melt fracture: helical or chaotic distortion at higher rates.
Not every polymer shows every regime.
| Defect | What you see |
|---|---|
| Sharkskin | Fine ridges across the extrusion direction |
| Die lines | Continuous grooves along the flow direction |
| Gels / fish eyes | Hard, clear, round or oblong embedded spots |
| Gross melt fracture | Large, irregular distortion of the whole extrudate |
What Causes Sharkskin in Polymer Extrusion?
Process conditions
Output rate, screw speed, drawdown and die gap set the shear rate. Melt and die-land temperature set viscosity. Together they decide whether you cross the stable-flow limit.
A cold die land raises viscosity and wall shear stress. Locally heating the exit delayed onset in a 2004 LLDPE study. A 2006 study found that lower wall temperature reduced the amplitude of roughness that had already formed. Onset and severity respond differently, so assess both.
Change temperature cautiously. Too much heat risks degradation, sagging and loss of dimensional control. Davis-Standard's blown-film guidance lists low melt temperature and a narrow gap as typical sharkskin entry points.
Material and rheology
Melt viscosity, molecular weight, its distribution and melt elasticity all matter. NIST identifies narrow-distribution linear PE as particularly susceptible. Yet the literature is contradictory: broadening the distribution can raise, lower or not change critical stress. No universal rule holds.
Examples worth knowing:
- Development-grade metallocene HDPE showed worse sharkskin than a tested Dowlex LLDPE (Arda and Mackley, 2005).
- Blending branched LDPE into LLDPE or HDPE, or adding a thin LDPE outer layer, can reduce surface distortion, but small branching additions give conflicting results.
- Syndiotactic PP showed sharkskin and wall slip in Indian research (IIT Kanpur, 2000), while isotactic PP of similar molecular weight did not. Do not generalise to all PP.
Resin-lot variation, recycled content, moisture and contamination can all shift onset. No quantitative lot-specific effect or recycled-content limit is established, so compare supplier melt-flow data against actual line behaviour instead of assuming grades behave alike.
Die and tooling design
Die-lip condition, surface finish, entry streamlining, gap and land length all contribute. Curved exits delay separation and reduce stress concentration, and smoother walls reduce severity.
In a 2012 PE pipe study, increasing the ratio of head land length (L) to die-to-mandrel gap (T) avoided sharkskin. Tested L/T ratios are not listed in the public abstract, and the right geometry depends on rheology, dimensions, throughput and pressure drop. Do not copy one study’s geometry blindly.
Polymer–die interface and additives
Strong adhesion to the die wall raises resistance to flow. A fluoropolymer processing aid (PPA) builds a low-energy coating on the metal and promotes slip. NIST saw strong slip and postponed onset after PPA treatment, although sharkskin eventually returned at higher rates. It extends the window; it is not a guaranteed cure.
PPAs differ from internal lubricants. Their demonstrated action is at the wall boundary, so do not treat all lubricants as interchangeable. Check compatibility, dispersion, dose, downstream effects and food-contact or customer requirements.
How to Troubleshoot and Fix Sharkskin
Confirm the defect first
Check four things:
- Are the ridges fine and transverse, not lengthwise grooves?
- Is the pattern repeatable?
- Does it begin at a consistent output or shear condition?
- Is it free of embedded particles?
Also rule out die lines, gels, contamination, die damage, output surging and dimensional variation. Producer guidance links surface lines to buildup, contaminants, nicks and burrs, which are not automatically sharkskin.
Run controlled, reversible tests
- Reduce output or take-off speed. If the surface smooths, the defect is rate-dependent. This is both a diagnostic and a temporary fix.
- Adjust die-land or die-lip temperature, one variable at a time. Watch melt temperature, pressure, dimensions and signs of degradation.
- Raise melt temperature or widen the gap where the product allows.
Inspect tooling and history
Clean the die lips and look for scratches, deposits, abrupt entry geometry, uneven gap and pressure instability. Ask what changed recently in tooling or maintenance.
Then compare a known-good recipe with the current one. Resin lot, screw speed, throughput, temperature profile, die configuration and cooling are the usual suspects.
Evaluate material and processing-aid options
Review grade, melt-flow data, molecular-weight characteristics, recycled fraction and lot consistency before switching resin.
If you trial a PPA, set a controlled baseline first. Supplier dosing varies widely:
- 3M's FX 5920A: typically 400-1000 ppm
- Kynar guidance: 100-1500 ppm
- Ampacet's PFAS-free grade: about 1-3% masterbatch
Masterbatch let-down is not active ppm, so never compare those figures directly.
Plan the trial around:
- Conditioning time: coating can take 15-60 minutes (3M guidance). Recycled LLDPE trials showed instability easing in about 10 minutes and surface improving by about 30.
- Interactions: abrasive antiblocks strip the coating, and some HALS reduce PPA efficiency at low shear or temperature.
- Downstream checks: gloss, haze, adhesion, sealing, printing, mechanical properties and recycling.
Escalate to lab testing
Capillary rheometry (ASTM D3835, ISO 11443) and pressure monitoring map the stable window and compare resins or additives before costly trials. One caution: ISO 11443 viscosity methods are invalid for materials that do not adhere to the wall, which matters when studying slip-promoting PPAs.
Validate at commercial throughput over a long run. Document the operating limits, not just the first acceptable sample.
How to Prevent Sharkskin and Control the Process Window
Define the process window in writing and set alarms for early drift. Document at least:
- Output and screw speed
- Melt and die temperatures
- Pressure and die configuration
- Cooling and finished dimensions
Research has evaluated in-line detection through dynamic pressure micro-fluctuations in LLDPE extrusion, though no universal alarm threshold has been established.
Keep the routine simple:
- Inspect die lips on a schedule and clean deposits.
- Control resin handling and document changeovers.
- Check surface quality routinely so wear and material variation show up early.
- If PPA is used, log additive level, conditioning time, defect response and pressure.
For processors evaluating a new formulation, processing aid or resin grade, Distil supplies engineering plastics, polypropylene compounds and functional masterbatches with material-selection support. That support will not guarantee defect removal, but it helps teams match materials to a documented process baseline.
Frequently Asked Questions
What is sharkskin material?
Sharkskin is not a material. It is a surface defect, a mild form of melt fracture, appearing as fine ridges when the melt turns unstable at the die exit, usually at high shear.
What is a common defect in extruded products?
Common defects include sharkskin, die lines, melt fracture, gels, bubbles, surging and dimensional variation. Sharkskin is distinguished by fine ridges running across the flow.
What is a piping defect in extrusion?
The term varies. In metal extrusion it means a central funnel from billet-surface oxides; in polymer pipe work it may refer to voids or surface flaws. It is not automatically sharkskin.
What are the four main types of extrusion?
For metals, the four basic types are direct, indirect, impact and hydrostatic. Polymer extrusion is usually grouped by product, such as film, pipe or profile, not a universal four.
What is the L/D ratio in extrusion?
L/D is screw length over diameter. Longer screws give more melting and mixing but also longer residence time. LyondellBasell suggests at least 24:1 for film, preferably 28:1 to 30:1. It differs from die L/T, which is land length over gap.
