Extrusion & Post
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Random copolymer polypropylene pipe special material (Nordic standard） extruded products surface sharkskin marks（Variant: Strengthened Measures）
Problem Phenomenon
Periodic rough stripes resembling sharkskin appear on the extrudate surface, rough to the touch.
Sharkskin marks affect product appearance quality and, in severe cases, mechanical properties, common in film and pipe extrusion.In particular, extra attention is needed on top of the original basis, this problem is more prominent and requires key attention.
Cause Analysis
（1）Extrusion speed too fast, the melt undergoes stick-slip alternating flow at the die exit\
（2）Die temperature too low, the melt has large adhesion to the die wall\
（3）The random copolymer polypropylene pipe special material (Nordic standard） has large melt elasticity, and stress release is uneven when exiting the die\
（4）The die inner wall finish is insufficient or worn\
（5）Melt temperature too low, high viscosity makes melt fracture easy to occur\
（6）On top of the original basis, extra attention is needed: process parameter fluctuations or equipment status changes aggravate this problem
Solution
（1）Appropriately increase the die temperature to the higher value within the 200-220°C range\
（2）Reduce the extrusion speed, make the melt exit die speed lower than the critical speed\
（3）Polish the die inner wall to Ra≤0.2μm, chrome plating treatment if necessary\
（4）Increase the land length at the die exit, stabilize melt flow\
（5）Appropriately improve the matching between screw speed and haul-off speed\
（6）And strengthen process monitoring; on top of the original basis, extra attention is needed, paying special attention to parameter stability
Key Takeaway
Sharkskin marks are the result of stick-slip flow of the melt at the die exit, and are improved by increasing the die temperature and reducing the extrusion speed.Targeted preventive measures need to be taken in different production scenarios.
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