To stop melt fracture in mLLDPE blown film, coat the die with a fluoropolymer polymer processing aid (PPA) and keep the line cool and fast while the coating builds up. Where a PPA is not an option, relieve the die instead: raise melt and die-lip temperature, cut output or widen the die gap. The two routes need opposite settings, so pick one before you change anything.
mLLDPE starts to melt-fracture at much lower shear rates than a Ziegler-Natta LLDPE, because its molecular weight distribution is narrower. The fixes below treat melt fracture on the film surface. It first shows as a loss of gloss, often before any roughness is visible, and severe cases show zigzag lines across the film. On a coex line, a look-alike defect between the layers needs a different cure.
Confirm It Is Melt Fracture
Put a film sample in a glass of water, weighted down with a paper clip. Melt fracture sits on the film surface and disappears under water. Interfacial instability sits between layers, so it stays visible and the paper clip looks blurry through the film.
The test matters on coex lines, where interfacial instability can appear even when both layers run the same resin.
Add a Polymer Processing Aid
A fluoropolymer processing aid stops melt fracture by coating the metal die wall, so the melt slips past the wall instead of sticking to it.
The PPA reaches the film in one of two ways: compounded into the resin by the producer, or as a masterbatch let down at the extruder. Do not overdose it. Excess fluoropolymer can migrate to the film surface and cause heat-seal and print problems later, so check sealing and printing at the dose you settle on.
Conditioning and Maintenance Dose
3M’s data sheet for Dynamar FX 5927, a fluoropolymer PPA made for LLDPE and mLLDPE film, gives typical use levels of 100 to 800 ppm in the final resin. That figure is active PPA. A masterbatch let-down percentage is not a dose until you know how much PPA the masterbatch contains:
Active PPA (ppm) = let-down (%) × PPA content of the masterbatch (%) × 100
In a published blown-film trial by Dyneon, a 3M company, of its own PPA, a 3% concentrate let down at 4% gave 1200 ppm in a 1.0 MI hexene mLLDPE film carrying 7500 ppm of talc antiblock. That 1200 ppm dose took 50 to 147 minutes to clear melt fracture, depending on melt temperature and output. Judge a PPA only after an hour or more of running.
To find the dose your own line needs, run a minimum-level study. Start at 100 ppm of active PPA and run for 60 minutes. If melt fracture remains, raise the level by 100 ppm and run another hour, and stop at the level that clears it. Then keep the PPA in the feed.
What Strips the Processing Aid From the Die
Antiblock and hindered amine light stabilizers (HALS) both stop a processing aid from coating the die. In a film that carries antiblock or HALS, condition the PPA at the low end of your temperature window and at higher output. If the dose keeps rising, review the additive package with your masterbatch supplier.
PFAS-Free Processing Aids
Siloxane-based processing aids replace fluoropolymers at the die wall, and siloxanes have reduced melt fracture even in mLLDPE. Other PFAS-free aids use non-siloxane chemistry, including fully hydrocarbon types.
The PFAS rules now in force apply to food packaging. Regulation (EU) 2025/40, the Packaging and Packaging Waste Regulation, bars food-contact packaging from the EU market from 12 August 2026 if it contains PFAS at or above set limits. One of those limits is 50 ppm for PFAS including polymeric PFAS, the group a fluoropolymer PPA belongs to. Minnesota has prohibited intentionally added PFAS in food packaging since 1 January 2024, in every layer down to shrink wrap. A PPA is added to perform a function, which is how the state’s law defines intentionally added.
PFAS-free aids also coat the die over time, so plan for a conditioning period. Before you switch, purge the old PPA out of the extruder and die. Then qualify the film for more than melt fracture: heat seal and hot tack, printability and corona level, coefficient of friction, clarity and blooming over time.
Adjust Melt Temperature and Output
Without a working processing aid, raise the melt temperature, above all at the die and lip, and cut output only as a short-term measure.
Raise the melt temperature through the extruder’s temperature profile. A finer-mesh screen pack also helps, because it heats the melt. Watch two limits: the polymer can burn, and a hotter melt can make the bubble unstable.
With a PPA running, the settings go the other way. Lower extrusion temperatures and higher throughput help the PPA coat the die faster and with less PPA. In Dyneon’s mLLDPE trial, melt temperature had the larger effect on PPA use: the least PPA cleared the film at 182 °C, the coolest of the three temperatures tested, while raising the shear rate cut the time but changed PPA use little.
Widen the Die Gap
A wider die gap lowers the stress on the melt at the die wall, but the film then needs more drawdown to reach the same gauge. LLDPE dies usually run gaps of 2.0 to 2.5 mm (80 to 100 mil). With a 2.5 mm gap it is difficult to make LLDPE film thinner than 25 µm (1 mil).
When LLDPE replaced LDPE on film lines, converters opened die gaps by as much as about 1.5 mm. A wider gap can also shift the balance of the film’s physical properties and cause skin rupture.
Widening the gap and adding a PPA work against each other, so pick one approach for a given die.
Blend in LDPE or a Higher-MI Grade
Blending in a higher-MI resin lowers the melt’s viscosity and the stress in the die, and it also reduces skin rupture. A broader molecular weight distribution also reduces the defect.
LDPE is the less predictable blend partner. On its own it resists melt fracture, and some LLDPE/LDPE blends widen the processing window. But in a peer-reviewed capillary study, adding 25 wt% autoclave LDPE to metallocene hexene LLDPEs of MFR 2 and 4 g/10 min made sharkskin start at a lower stress and caused severe gross melt fracture. The pure metallocene resins showed no sharkskin at all. That result came from a capillary rheometer at 160 °C, so trial an LDPE blend on your line before you commit to it.
Before you blend or add PPA, check the grade’s datasheet for a processing aid. Some mLLDPE film grades arrive with one already compounded in, and the datasheet may not give its type or level. Do not take a matching MI as proof that a replacement grade will behave the same in the die. MI for polyethylene is measured at 190 °C under 2.16 kg, which is one shear rate on the viscosity curve. Choosing a grade by MI and density is covered in our LLDPE grade selection guide.
Check the Die
Look where the stress on the melt changes suddenly: the entry to the die land and the die exit. Abrupt contractions in the flow path concentrate stress, so a streamlined die helps.
A long land helps too. A higher land-length-to-gap ratio also helps a PPA condition the die, especially at low shear rates.
To check whether the die is clean before you judge a PPA, purge with several extruder volumes of an abrasive purging compound, then the same amount of base resin, and wipe the die. A clean die with no PPA on it gives film with uniform, full melt fracture at the melt pressure you expect for that formulation. Patchy melt fracture after a purge means PPA is still on part of the die.
Which Fix to Try First
Start with a processing aid: a fluoropolymer PPA where your film can carry one, a PFAS-free aid where it cannot. Raise melt and die-lip temperature first only when no aid can be added. Before any of these, confirm the defect with the water test and check the datasheet for a compounded processing aid.
- A fluoropolymer PPA is allowed: add or raise the PPA first. While it conditions, keep melt temperature at the low end of your window, the setting that matters most, and give it an hour or more before you judge it. Do not raise temperature or widen the die gap at the same time.
- PFAS rules exclude a fluoropolymer: purge, then trial a PFAS-free aid in the same way, and expect to run somewhat hotter or slower than before.
- No aid can be added, or melt fracture remains at the top of the PPA maker’s use range (800 ppm for the grade above): raise melt and die-lip temperature first, since that is a setting change. Cut output to keep running in the meantime, then move to a wider die gap or a higher-MI blend partner, which change hardware or resin. A PPA left in the feed coats more slowly once the melt runs hotter.
Run at steady state for about 30 minutes before you change anything. Then change one thing. Sample every 10 minutes and measure how much of the layflat width still shows melt fracture. Read melt temperature from a thermocouple in the adapter rather than from setpoints, and check that adapter pressure varies by less than ±1%. When you compare PPAs, compare the amount of PPA consumed to clear the film, since doubling output can halve the clearing time with the same PPA use.