Extrusion & Post
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Sharkskin Texture Appearing on the Surface of High-Density Polyethylene Extruded Products
Problem Phenomenon
Periodic rough stripes appear on the surface of the extrudate, resembling sharkskin, with a rough hand feel.
Sharkskin texture affects the appearance quality of the product and, in severe cases, affects the mechanical properties; it is common in film and pipe extrusion.
Cause Analysis
（1）The extrusion speed is too fast, and the melt undergoes alternating stick-slip flow at the die exit
（2）The die temperature is low, and the melt has a large adhesion force on the die wall
（3）The high-density polyethylene melt has a large elasticity, and the stress release is uneven when exiting the die
（4）The finish of the inner wall of the die is insufficient or worn
（5）The melt temperature is low, and the viscosity is high, making melt fracture likely to occur
Solution
（1）Appropriately increase the die temperature to a higher value within the 180-260°C range
（2）Reduce the extrusion speed so that the melt exit speed from the die is lower than the critical speed
（3）Polish the inner wall of the die to Ra≤0.2μm, and perform chrome plating when necessary
（4）Increase the land length at the die exit to stabilize the melt flow
（5）Appropriately improve the matching of the screw rotation speed and the haul-off speed
Key Takeaway
Sharkskin texture 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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