Melt fracture is a rough or matte surface that forms when the melt is pushed through the die at a higher stress than it can take. Sharkskin is the mild form: fine ridges across the direction of flow, on the surface only. Gross melt fracture is the severe form, where the whole extrudate comes out wavy or distorted. Look first at output rate, die gap, and melt and die lip temperature. It is most common with linear, narrow molecular weight distribution polyethylene such as LLDPE.
Also described as: rough extrudate, matte surface, orange peel, rippled surface on film or pipe.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A fine matte haze or small ridges running across the machine direction, over the whole surface. It appears above a certain output and clears when the rate is cut. | Sharkskin. The stress on the melt surface at the die exit is too high. | Output rate; die gap; die lip temperature |
| Bands of smooth, glossy surface alternating with rough surface, with head pressure swinging in step | Stick-slip flow. The melt alternately grips and slips on the die wall. Seen mostly with linear polyethylene. | Head pressure trace; the output rate at which the bands start |
| Severe wavy, twisted or lumpy distortion of the whole extrudate, not only the skin | Gross melt fracture. The melt is being stretched too fast where it enters the die. | Output rate; melt temperature; die entry and land geometry |
| Rough on one side only, or only at certain positions around or across the die | A cold or uneven die lip, an uneven die gap, or deposits on one lip. | Lip temperature at several positions; die gap; lip condition |
| Roughness started when the blend changed: less LDPE, or more LLDPE or metallocene LLDPE | The blend now has a narrower molecular weight distribution and thins less under shear, so die stress is higher at the same rate. | Blend ratio and feeder settings; melt index of each component |
| A surface that ran smooth turned rough some time after a change of resin, antiblock or color concentrate | The processing aid layer on the die metal has been lost or is no longer being fed. Mineral antiblock and some pigments wear it away or tie it up. | Whether the resin or a concentrate contains a processing aid, and whether it is still being dosed |
| Roughness started with a new lot at the same settings | A lower melt index, or a different additive package. | Melt index on the COAs of the good lot and the bad lot |
Checks in order, cheapest first
- Cut the output and watch the surface. If the roughness clears at a lower screw speed and returns when you go back up, it is melt fracture. Note the rate and head pressure where it starts, because every later change is judged against that point.
- Measure the melt temperature and raise it in small steps. A hotter melt is thinner and puts less stress on the die. Stay inside the supplier’s range. The cost is a weaker, slower-cooling melt, and more risk of gels and build-up from overheated resin.
- Check die and die lip temperature all the way around. A cold lip or a failed heater band gives roughness on one side or at one position. Raising the lip temperature a little often helps sharkskin more than raising the whole barrel.
- Check the blend and the feeders. Confirm the LDPE and LLDPE ratio is what the recipe says and that every feeder is running. If you rely on a processing aid concentrate, confirm it is being dosed. A clean die takes time to coat before the surface clears, and antiblock slows this down.
- Clean the die lips. Deposits and oxidized resin on the lips roughen the surface locally and stop a processing aid from coating the metal. Use brass or copper tools only. The die is hot, so wear gloves and a face shield.
- Compare lots. If the roughness arrived with a new lot, compare melt index on the COAs and run a retained sample of the good lot on the same line. If the good lot is also rough, the cause is the line or the settings and not the resin.
- Look at the die. A wider die gap lowers the stress at the same output and is the standard tooling fix for LLDPE-rich film. It also means more draw-down, which changes orientation and film properties and makes gauge harder to hold. Entry angle and land length matter for gross melt fracture. These are tool changes, so confirm the cheap checks first.
- Change the resin last. Melt fracture is a mismatch between the resin, the die and the rate. If the die and settings cannot be changed and the rate is needed, a different melt index, a broader molecular weight distribution or a processing aid is the next place to look.
The numbers
At a given output and die, a lower melt index means a stiffer melt and more stress at the die. Molecular weight distribution matters as much, but it is not on a COA, so ask for it. Know the melt index and density of every resin in the blend.
| Resin | Melt index (g/10 min) | Density (g/cm³) | Melt range on our processing chart |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | Not listed |
| LLDPE, all grades | ~0.5–50 (rotomolding ~2–7) | ~0.915–0.940 | Not listed |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | 320–450°F / 160–232°C (LDPE) |
| HMW-HDPE film, pipe and sheet grades | Very low, often <0.1 at 190°C/2.16 kg. High-load melt index ~5–15 at 190°C/21.6 kg for film grades | ~0.945–0.960 | 380–525°F / 193–275°C (HDPE) |
| A typical bimodal HMW-HDPE bag-film grade | ~0.06, with a high-load melt index of ~9.5 | 0.950 | As above |
| Polypropylene homopolymer | See the data sheet | See the data sheet | 374–525°F / 190–274°C |
Typical published ranges from our LLDPE, LDPE film and HMW-HDPE pages. Melt index is measured at 190°C/2.16 kg unless stated. The melt ranges come from the processing chart, which is written for injection molding; use them only as an outer check and set an extrusion line from the data sheet for your grade. Polypropylene melt flow is measured at a different temperature (230°C), so it cannot be compared with a polyethylene melt index. See melt flow index explained.
What happens at the die
- Sharkskin starts at the die exit. Inside the die the melt at the wall is hardly moving. As it leaves the lip, that surface layer has to speed up to the speed of the rest of the extrudate almost at once. If the stretch is more than the melt can take, the surface tears in fine, regular ridges.
- Gross melt fracture starts at the die entry. Where the melt is squeezed from a wide channel into a narrow one, it is stretched hard. Above a critical rate the flow there becomes unstable, and the whole stream comes out distorted.
- Stress is what matters. It rises with output and with melt viscosity. It falls with a wider die gap and a hotter melt. Any of these can move a line across the threshold, which is why a problem can appear with nothing more than a speed increase.
- Molecular weight distribution sets how much room you have. LDPE and broad-distribution resins thin out a lot under shear, so they run through a narrow gap at low stress. LLDPE, and metallocene LLDPE most of all, thins less and reaches the sharkskin threshold at a lower rate.
- Polymer processing aids work on the metal, not the melt. They are additives used at low levels that coat the die surface so the melt slips over it at lower stress. Many are fluoropolymer-based. They need time to coat a clean die, and abrasive or competing additives can strip the coating.
- It is more than a cosmetic defect. Sharkskin raises haze and lowers gloss, and a fractured surface can cost film strength and make printing and sealing less even.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Add a polymer processing aid, in the resin or as a concentrate | It coats the die metal so the melt exits at lower stress. This often clears sharkskin with no change of rate or die. | Added cost and a conditioning period at every start. Antiblock, some pigments and some light stabilizers interfere with it. Many are fluoropolymer-based, and some customers and regulations now restrict fluorinated additives, so ask before you rely on one. Check food-contact status. |
| A higher melt index grade | Lower melt viscosity, so lower die stress at the same output. | Lower melt strength and bubble stability, and usually lower toughness and stress-crack resistance. |
| A broader molecular weight distribution, or more LDPE in the blend | The melt thins more under shear, so it passes the die at lower stress. | LDPE lowers puncture and tear strength and limits how thin the film will draw. Seal and optical properties move as well. |
| Conventional LLDPE in place of metallocene LLDPE | A somewhat broader distribution that is easier to extrude. | Less toughness and a weaker seal than the metallocene grade gave you. The reason the metallocene grade was chosen may be lost. |
| A grade supplied with processing aid already in it | No separate concentrate to dose, and the level is consistent. | A new grade to approve, with the same additive questions as above. |
Any blend change moves film and part properties. Seal temperature, slip, clarity, strength and dimensions all move with it. Run the new material through the customer’s converting or assembly step before you approve a swap.
Often confused with
- Gels. Gels are separate lumps with smooth film between them. Melt fracture covers the surface evenly. See gels and fisheyes in film.
- Die lines. A die line runs in the machine direction and stays at one position. Sharkskin ridges run across the flow and cover the whole width. See die lines and die build-up.
- Surging. Bands that repeat along the product with a swinging head pressure can be stick-slip at the die or a feeding problem in the extruder. Stick-slip bands differ in surface finish and start at a set rate. Surging changes thickness and usually traces to the feed. See extruder surging.
- Moisture or volatiles. A surface roughened by moisture has pits, bubbles or streaks and does not clear when you cut the rate. Check filled grades and resin that has collected condensation. See resin storage and moisture.
Quick answers
What causes sharkskin in extrusion?
Sharkskin forms when the stress on the melt surface at the die exit is too high. The surface layer has to speed up at once as it leaves the lip, and it tears in fine ridges. Stress rises with output and melt viscosity, and falls with a wider die gap and a hotter melt. It is most common with LLDPE.
How do I fix melt fracture?
Cut the output first. If the roughness clears at a lower screw speed, it is melt fracture. Then raise melt temperature in small steps inside the supplier’s range, check die lip temperature, confirm the blend and any processing aid feed, and clean the die lips. A wider die gap is the standard tooling fix for LLDPE-rich film.
Why is my extruded surface rough?
A rough or matte surface that appears above a certain output and clears when the rate is cut is melt fracture: the melt is pushed through the die at a higher stress than it can take. Roughness on one side only points to a cold or uneven die lip, an uneven die gap or deposits on one lip.
Is melt fracture caused by the resin or the die?
Melt fracture is a mismatch between the resin, the die and the rate. Run a retained good lot on the same line. If it is also rough, the cause is the line or the settings and not the resin. Change the resin last: a different melt index, a broader molecular weight distribution or a processing aid.
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