Extrusion & Post
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Sharkskin on the surface of low density polyethylene extruded products
Problem Phenomenon
Periodic rough stripes appear on the extrudate surface, resembling sharkskin, with a rough feel.
Sharkskin affects product appearance quality and, in severe cases, mechanical properties; it is common in film and pipe extrusion.
Cause Analysis
(1) Extrusion speed too fast; stick-slip alternating flow of the melt occurs at the die exit
(2) Die temperature too low; large melt adhesion to the die wall
(3) High melt elasticity of low density polyethylene; uneven stress release when exiting the die
(4) Insufficient inner wall finish of the die or wear
(5) Melt temperature too low; high viscosity makes melt fracture likely
Solution
(1) Appropriately increase die temperature to the higher value within the 160-220°C range
(2) Reduce extrusion speed so that the melt die exit speed is below the critical speed
(3) Polish die inner wall to Ra≤0.2μm, chrome plating when necessary
(4) Increase land length at the die exit to stabilize melt flow
(5) Appropriately improve the matching of screw rotation speed and haul-off speed
Key Takeaway
Sharkskin is the result of stick-slip flow of the melt at the die exit; it is improved by increasing die temperature and reducing extrusion speed.
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