Extrusion & Post
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Sharkskin Appearing on the Surface of Polypropylene Extruded Products
Problem Phenomenon
Periodic rough streaks resembling sharkskin appear on the surface of the extrudate, with a rough feel.
Sharkskin 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 too low, and the melt adhesion to the die wall is large
(3) The polypropylene melt has high 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 too low, and the high viscosity makes melt fracture easy to occur
Solution
(1) Appropriately increase the die temperature to the higher value within the range of 200-280°C
(2) Reduce the extrusion speed so that the melt speed at the die exit is below the critical speed
(3) Polish the inner wall of the die to Ra≤0.2μm, and apply chrome plating when necessary
(4) Increase the land length at the die exit to stabilize the melt flow
(5) Appropriately improve the matching between the screw speed and the haul-off speed
Key Takeaway
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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