Extrusion & Post
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Sharkskin pattern appearing on the surface of polyolefin plastomer extruded products
Problem Phenomenon
Periodic rough stripes appear on the surface of the extrudate, resembling sharkskin, with a rough feel.
Sharkskin pattern affects the appearance quality of the product and, in severe cases, affects the mechanical properties, and is common in film and pipe extrusion.
Cause Analysis
（1）The extrusion speed is too fast, and the melt undergoes stick-slip alternating flow at the die exit
（2）The die temperature is too low and the melt has large adhesion to the die wall
（3）The polyolefin plastomer melt has high elasticity and the stress release is uneven when exiting the die
（4）The die inner wall finish is insufficient or worn
（5）The melt temperature is too low, the viscosity is high, and melt fracture easily occurs
Solution
（1）Appropriately raise the die temperature to the higher value within the range of 180-230°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, with chrome plating treatment when necessary
（4）Increase the land length at the die exit to stabilize the melt flow
（5）Appropriately improve the matching of screw speed and hauling speed
Key Takeaway
Sharkskin pattern 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.
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