Extrusion & Post
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Sharkskin appears on the surface of heat-resistant polyethylene extruded products
Fenómeno do problema
Periodic rough streaks resembling sharkskin appear on the extrudate surface, rough to the touch.
Sharkskin affects product appearance quality and, in severe cases, affects mechanical properties; it is common in film and pipe extrusion.
Análise das causas
(1) Extrusion speed too fast, melt undergoes stick-slip alternating flow at the die exit
(2) Die temperature too low, melt adhesion to die wall large
(3) Heat-resistant polyethylene melt elasticity large, stress release uneven when exiting the die
(4) Die inner wall finish insufficient or worn
(5) Melt temperature too low, high viscosity prone to melt fracture
Solução
(1) Appropriately increase die temperature to the higher value within 190-230°C
(2) Reduce extrusion speed so that the melt exit speed is lower than the critical speed
(3) Polish die inner wall to Ra≤0.2μm, chrome plating treatment if necessary
(4) Increase sizing section length at the die exit to stabilize melt flow
(5) Appropriately improve the matching of screw rotation speed and haul-off speed
Conclusão principal
Sharkskin is the result of stick-slip flow of the melt at the die exit, and is improved by increasing the die temperature and reducing the extrusion speed.
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