Extrusion & Post
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Sharkskin on the surface of linear low density polyethylene extruded products (Variant: strengthened measures)
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.
In particular, additional attention is needed on the original basis; this problem is more prominent and requires key attention.
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 linear 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
(6) On the original basis, additional attention is needed as process parameter fluctuations or equipment status changes have aggravated this problem
Solution
(1) Appropriately increase die temperature to the higher value within the 190-250°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
(6) And strengthen process monitoring; on the original basis, additional attention is needed, with special attention to parameter stability
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.
Targeted preventive measures need to be taken in different production scenarios.
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