Meaning
Periodic surface irregularities occur on extruded polymer products when high shear stress at the die wall exceeds the critical limit of the material. This phenomenon, known as sharkskin melt fracture, appears as a series of transverse ridges or small cracks that degrade the finish of the final part. The effect stops appearing when the extrusion speed decreases enough to keep the shear rate below the polymer chain disentanglement threshold.
Extrusion Boundary
Stability in the production flow depends on the relationship between shear stress and the elastic properties of the polymer melt. Operators identify this state by monitoring the die pressure and the uniformity of the extruded extrudate surface during continuous output cycles. If the line speed pushes the polymer beyond its inherent critical shear stress, the material fails to adhere to the die wall in a smooth fashion and the surface quality drops instantly.
Elastic Recoil
Polymers exhibit high levels of internal stress when forced through narrow orifices under high velocity conditions. These long chain molecules stretch under the intense shear force and subsequently snap back upon exiting the die because the material cannot dissipate the stored elastic energy in time. Rapid recovery of these chains creates the characteristic surface distortion seen on the finished profile.
Production Mitigation
Adjusting the geometry of the die entrance or heating the die surface reduces the likelihood of the defect during high volume manufacturing. Increasing the land length of the tool or applying chemical processing aids to the resin formulation also limits the extent of the periodic ridges. Manufacturers rely on these physical or chemical interventions to maintain high throughput rates without sacrificing the surface integrity of the polymer.
