Meaning
High shear rates at the die exit threshold define the boundary where smooth extrudate surfaces degrade into visual sharkskin or severe gross melt fracture. Incorporating melt fracture additives into polyolefin formulations reduces wall slip resistance and eliminates surface turbulence during high-speed film extrusion or pipe profiling. The scope of these functional processing aids covers low-concentration fluoropolymer fluoroelastomers and structural processing polymers added to linear low-density polyethylene or polypropylene.
These additives do not alter the bulk mechanical properties or melt flow index of the finished plastic article, operating exclusively at the polymer-metal interface inside the die land.
Interfacial Coating
Fluoropolymer molecules deposit onto internal metal die surfaces to establish a low-energy dynamic coating layer. Fluoropolymers in melt fracture additives migrate rapidly to the die wall, converting stick-slip melt behavior into continuous fluid slip. This interfacial layer lowers backpressure in the extruder barrel and permits higher line speeds without surface tearing.
Disruption of this coating layer by abrasive pigments or inorganic fillers requires continuous additive metering during processing runs.
Extrusion Throughput
Processing lines running narrow-die blown film operate at higher output rates when interfacial shear stress remains low. Lowering extrusion head pressure reduces motor torque loads and energy consumption per kilogram of processed resin. Extruders processing linear low-density polyethylene eliminate sharkskin defects while maintaining lower melt temperatures.
Interference Boundary
Acid scavengers and anti-block particles interfere with additive accumulation on metal die walls. High concentrations of titanium dioxide scrub the functional fluoropolymer layer from the die land, reintroducing melt fracture defects during film blowing. Over-dosing these additives causes screw slip and surge in single-screw extruders, destabilizing bubble geometry.
Cleaning dies with purge compound removes the fluoropolymer layer, requiring a conditioning phase when restarting production.
