Melt Fracture in Blown Film: Causes and How to Fix It
Rough, hazy film that looks like orange peel or sharkskin is one of the most common limits on blown film output. Here is what causes it and the options for removing it.
Melt fracture is a surface defect that forms as the polymer leaves the die. At low output the melt slides out smoothly. Above a critical shear stress at the die wall, the melt starts to stick and slip, and the film surface tears into fine ridges. Mild cases look like haze or a matte band; severe cases look like sharkskin or orange peel and weaken the film.
Linear low-density polyethylene (LLDPE) and metallocene LLDPE are especially prone to it. Their narrow molecular weight distribution and low long-chain branching give excellent film strength, but make the melt less forgiving at high shear rates than LDPE. Converters who move to mLLDPE for downgauging often meet melt fracture for the first time.
It is worth separating melt fracture from die build-up, sometimes called die drool or die lip build-up. Die build-up is material that collects at the die exit and breaks off as streaks, specks or lines in the film. The two often appear together and have a shared cause at the die wall, but die build-up is also driven by degraded polymer, fillers, pigments and low molecular weight material in the melt.
The process options all reduce shear stress at the die wall. Raising melt temperature lowers viscosity, but can make the bubble less stable. Widening the die gap lowers the shear rate, but changes drawdown and can affect film orientation and strength. Cutting output works, but costs money. Blending in LDPE helps, but can lower the toughness that made you choose LLDPE.
A polymer processing aid (PPA) removes melt fracture without those trade-offs. Dosed at a low let-down as a masterbatch, it coats the die wall over a conditioning period, so the melt slips through smoothly at full output. Most PPAs have been fluoropolymers, but non-fluorinated, PFAS-free processing aids are now available for converters supplying the UK, EU and global brands.
Getting the most from a processing aid.
A PPA needs time to coat the die. Judge results after the film has fully cleared, not in the first minutes.
Antiblock minerals and some pigments can scrub or absorb processing aid, so the dose may need adjusting.
Start a trial on a clean die so you can see what the PPA does, not what is left from the last run.
Record melt fracture clearance, die build-up and kg/h before and after, so the business case is clear.
If melt fracture appears only on part of the film circumference, check the die for damage, uneven temperature or deposits before changing the formulation. A defect that tracks the die is usually mechanical.
Aviura makes a conventional PPA, a PPA with antioxidant for film and recycled polymer, and PFF, a PFAS-free processing aid for PE and LLDPE film. Send us your resin, die gap, output and a photo of the defect, and we will recommend a grade and plan a trial with you.
Melt fracture is usually not a resin problem but a die-wall problem, and that is where the fix belongs.
See the PFAS-free processing aid guide