Sharkskin Instability

Surface Roughness

Cracks form on the outer layer because the core of the melt is moving faster than the surface. If sharkskin instability remains present, the visual appeal and dimensional accuracy of the extrudate are compromised. The defect repeats at high frequencies across the entire product.

Shear Threshold

Molecules lose their grip on each other when they are accelerated from zero velocity at the wall to a high speed at the exit. By reducing the extrusion speed, manufacturers can often eliminate sharkskin instability without changing the material formulation. Using a die with a smoother exit profile also helps maintain clarity.

Extrudate Quality

Smooth surfaces are essential for secondary bonding or clean aesthetic requirements. Because sharkskin instability limits production throughput, companies search for additives that act as lubricants inside the die. Success is measured by the return of a glossy surface.