Rheological Quality Control Limits in Polymer Extrusion Manufacturing
Polymer extrusion quality control limits require dynamic multi-point shear viscosity bounds at process shear rates to eliminate scrap and protect converting margins.
Sharkskin Defect
Meaning
Surface irregularities characterized by a matte or rough texture appear on extruded plastic products when the polymer melt experiences high exit velocities. The emergence of a sharkskin defect occurs due to the rapid stretching of the fluid layer as it leaves the die lip. It is a common limitation in high-speed polymer extrusion lines.
Material Flow
Polymer molecules are highly stretched when they exit the confinement of the extrusion die. If the flow rate is too high, this transition creates the sharkskin defect as the surface of the melt ruptures to relieve the built-up tensile stress. This spoils the cosmetic finish of the product.
Processing Window
Temperature adjustments can expand the operational limits of the extrusion line. To eliminate the sharkskin defect, technicians can increase the temperature of the die region, which reduces the viscosity of the polymer and allows it to flow more smoothly. This adjustment allows for higher production speeds.
Additive Solution
Processing aids can be added to the polymer blend to reduce friction at the metal-polymer interface. These fluoropolymer additives prevent the sharkskin defect by creating a lubricating layer on the inner surfaces of the die. This lubrication allows the melt to slide out without tearing.
Even a tiny concentration of these additives, often measured in parts per million, can increase the critical shear rate at which surface fracture occurs, allowing the extrusion line to run at faster line speeds without compromising product quality.