Extrusion & Post
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Sharkskin appearing on the surface of linear low-density polyethylene extruded products
Fenómeno del problema
Periodic rough streaks appear on the extrudate surface, resembling sharkskin, with a rough hand feel.
Sharkskin affects product appearance quality and, in severe cases, mechanical properties; it is common in film and pipe extrusion.
Análisis de la causa
(1) Extrusion speed is too fast, and the melt undergoes stick-slip alternating flow at the die exit
(2) Die temperature is on the low side, and melt adhesion to the die wall is large
(3) Linear low-density polyethylene melt has high elasticity, and stress release is uneven when exiting the die
(4) The die inner wall finish is insufficient or worn
(5) Melt temperature is on the low side; high viscosity makes melt fracture likely
Solución
(1) Appropriately increase die temperature to the higher value within the 190-250°C range
(2) Reduce extrusion speed so that the melt exit speed from the die is below the critical speed
(3) Polish the die inner wall to Ra≤0.2μm, with chrome plating if necessary
(4) Increase the land length at the die exit to stabilize melt flow
(5) Appropriately improve the matching of screw speed and haul-off speed
Conclusión clave
Sharkskin is the result of stick-slip flow of the melt at the die exit and is improved by increasing die temperature and reducing extrusion speed.
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