Extrusion & Post
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Sharkskin appears on the surface of heat-resistant polyethylene extruded products (Variant: mass production scenario)
Problembeschreibung
Periodic rough stripes appear on the surface of the extrudate, resembling sharkskin, with a rough feel.
Sharkskin affects the appearance quality of the product, and in severe cases affects the mechanical properties, and is common in film and pipe extrusion.
In particular, in continuous mass production, this problem is more prominent and needs key attention.
Ursachenanalyse
（1）The extrusion speed is too fast, and the melt undergoes stick-slip alternating flow at the die exit
（2）The die temperature is low, and the melt adhesion on the die wall is large
（3）The heat-resistant polyethylene melt has large elasticity, and the stress release is uneven when exiting the die
（4）The finish of the die inner wall is insufficient or worn
（5）The melt temperature is low, and the viscosity is high, making melt fracture easy to occur
（6）In continuous mass production, process parameter fluctuations or equipment status changes have aggravated this problem
Lösung
（1）Appropriately raise the die temperature to the higher value within the range of 190-230°C
（2）Reduce the extrusion speed so that the melt exit speed is below the critical speed
（3）Polish the die inner wall to Ra≤0.2μm, and chrome plate if necessary
（4）Increase the land length at the die exit to stabilize the melt flow
（5）Appropriately improve the matching of the screw speed and the haul-off speed
（6）And establish standardized work instructions, with special attention to parameter stability in continuous mass production
Wichtigste Erkenntnis
Sharkskin is the result of stick-slip flow of the melt at the die exit, and is improved by raising the die temperature and reducing the extrusion speed.
In different production scenarios, targeted preventive measures need to be taken.
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