Troubleshooting · Defect guide
Sharkskin and melt fracture
Why extruded film turns rough, and what to fix first
A film or extruded profile with a matte, finely ridged surface, or one that twists into waves along its whole length, almost always has one cause: shear stress at the die wall is higher than the melt surface can take. This guide covers how to tell the defects apart, the mechanism, four groups of causes, checks from cheapest to most expensive, and an honest look at when a polymer processing aid (PPA) helps and what strings come attached.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is sharkskin
This defect is most common in blown and cast film from LLDPE, and also appears in pipe, profile and wire coating. The key sign is that it gets worse when you raise output and eases when you slow down.
- Sharkskin: a matte, finely ridged surface with ridges running across the flow direction. It feels rough to the touch, makes film look hazy, and covers the whole width.
- Gross melt fracture: not just the surface; the whole extrudate twists into a helix, large waves or alternating thick and thin sections. It usually appears at higher output than sharkskin.
- Gels / fish eyes: scattered raised spots, not an overall rough surface. See Gels and fish eyes in film.
- Die lines: straight lines along the flow direction in a fixed position, caused by scratches or deposits on the die lip. They do not depend on output.
- Haze from cooling: uniformly cloudy film with a smooth surface, linked to cooling and crystallisation rather than surface tearing.
Mechanism: why the surface tears at the die exit
Inside the die, melt next to the wall barely moves while the centre flows fast. At the die exit, the surface layer that was nearly stationary must suddenly accelerate to the speed of the whole extrudate, so it is stretched hard right at the lip. When wall shear stress rises above a certain level, the surface tears in rapid, regular bursts: sharkskin. At even higher stress, the melt starts to stick and slip at the wall in turn (stick-slip), or the flow becomes unstable at the die entry, and the whole extrudate distorts: gross melt fracture.
The practical takeaway: sharkskin is a die-wall shear stress problem. There are three ways to fix it: lower the shear stress (less output, a wider gap, a hotter lip), change the material so it is less sensitive to shear (grade choice, LDPE blend), or let the melt slip at the wall (PPA).
LLDPE is most at risk, especially mLLDPE and other narrow-MWD grades, because their viscosity drops less under high shear. At the same output their wall shear stress is higher than LDPE, whose long-chain branching makes it flow much more easily when sheared. For background on MFI and molecular weight distribution, see Polymer basics.
Four groups of causes
1. Die and equipment
A narrow die gap makes the melt flow faster at the same output and raises shear stress. A die lip with scratches or build-up triggers the defect more easily and often leaves lines along the flow direction too.
2. Process
Output above what the die and resin can handle, and a die-lip or melt temperature that is too low, which raises viscosity. That is why the defect often appears when a line is pushed for more output, or when temperatures are cut to steady the bubble.
3. Material
Low-MFI, narrow-MWD grades such as mLLDPE, or a change in the LLDPE/LDPE blend ratio, change how the melt flows even with identical machine settings.
4. Formulation and additives
No PPA where the resin and output need one, a PPA that has not yet fully coated the die, or a PPA disturbed by other ingredients such as antiblock, fillers and some pigments.
Step-by-step checks, cheapest first
- Reduce output step by step. If the surface turns smooth, the cause is shear, not contamination. Note the output at which the defect starts; that is your benchmark.
- Raise die-lip temperature step by step, watching both surface and bubble stability. If the bubble becomes unstable before the surface clears, you will need another measure alongside.
- Clean the die lip and inspect it. Deposits or scratches at the lip make the surface tear more easily.
- Check the resin. Compare grade, lot and blend ratio with a period when the surface was good. If you recently moved to mLLDPE or cut the LDPE share, start there.
- Trial a small LDPE addition and watch the surface. Confirm the film's mechanical properties still meet specification.
- If you already use a PPA, check its conditions. Has the line run long enough for the PPA to coat the die, and have you recently added antiblock or filler, or changed a colour masterbatch?
- Widen the die gap. This is equipment work and the most expensive step, and it changes the draw-down ratio and molecular orientation in the film.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Output too high | Reduce output step by step; note where the surface turns smooth | Run below the onset point, or use other measures to raise it |
| Die lip or melt too cold | Raise die-lip temperature step by step | Set a higher die-lip temperature within the range where the bubble stays stable |
| Narrow die gap | Compare the gap with the resin and output you run | Widen the gap, or use a die designed for LLDPE |
| Deposits or damage on the die lip | Inspect the lip; look for lines along the flow too | Clean, polish or repair the die lip |
| Shear-sensitive resin grade | Compare grade and lot with a period of good surface | Choose a broader-MWD grade, or blend in LDPE |
| No PPA, or too little | Review the formulation; ask the resin supplier whether the grade already contains PPA | Trial a PPA and condition the line as the supplier recommends |
| PPA disturbed by other ingredients | Did the defect return after adding antiblock or filler, or changing a colour MB? | Adjust PPA level, or use grades their suppliers confirm are compatible |
| PPA not yet fully coating the die | The surface improves in patches as running time increases | Run continuously through the full conditioning period before judging |
How much can additives help?
This is one of the few defects where a processing additive genuinely works, with conditions. These are the material options that actually exist, including ones ACS does not sell:
- Polymer processing aid (PPA): usually a fluoropolymer added at a low level. It coats the die wall so the melt can slip, which lowers wall shear stress. The line has to run for a while before the coating is complete and the full effect appears. PPAs are not on ACS's product list.
- PPA watch-outs: mineral particles in antiblock, fillers and some pigments can adsorb PPA or scrub the coating off. If you also use slip and antiblock, ask both suppliers about compatibility and confirm it on your line.
- Regulation: fluoropolymer PPAs are classed as PFAS in some markets, where they are being regulated or considered for restriction. Fluorine-free PPAs are available, but their performance can differ. Check your customer's requirements and the destination market.
- LDPE blending and grade choice: not an additive but a resin-side change that often works, at the cost of possible changes in film mechanical properties.
What additives cannot fix: a damaged die lip, output far beyond what the die was designed for, and gels or fish eyes that come from the resin itself. For process background, see Plastics extrusion.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Sharkskin questions we hear often
We switched to mLLDPE and the film now shows sharkskin, with the same die and output. Why?
mLLDPE and other narrow-MWD LLDPE grades shear-thin less: their viscosity drops less as shear rises. At the same output, wall shear stress at the die exit is therefore higher than with the old resin, and it crosses the level where the surface starts to tear more easily. The usual fixes are a hotter die lip, lower output, a wider die gap, an LDPE blend or a PPA. Change one variable at a time to see which works on your line.
We added a PPA. Why is the film surface not smooth straight away?
A PPA works by gradually coating the die wall so the melt can slip. The line has to run continuously for a while until the die surface is fully coated (conditioning), and the film clears in patches before it is smooth all over. How long this takes depends on PPA level, die size and the other ingredients in the formulation; PPA suppliers usually give a recommended conditioning procedure. If nothing improves after a long run, check whether something in the formulation is interfering with the coating.
Sharkskin came back after we increased antiblock. Is that related?
It can be. Mineral particles such as silica or talc in antiblock, and some pigments, can adsorb PPA or scrub the coating off the die wall, leaving too little active PPA. It typically shows up after a masterbatch change or a higher antiblock level. Try adjusting the PPA level, or choose antiblock and PPA grades that their suppliers confirm work together, then verify on the actual line.
Can we still use fluorinated PPAs? How do PFAS rules affect them?
Most PPAs on the market are fluoropolymers, which some markets classify as PFAS substances that are being regulated or considered for restriction, and some brand owners now ask for fluorine-free formulations. Fluorine-free PPAs are available, but their efficiency and required dosage can differ. Check your customer's requirements and the rules in the destination market before choosing a formulation, and compare options on your own line.
Film surface still rough after tuning the line?
Send a close-up photo of the film surface, your resin grade, the masterbatches you use and your output on LINE. Our technical team will help you separate die, process and formulation causes.