Extrusion & Post
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LLDPE Blown Film Sharkskin and Melt Fracture
Fenómeno do problema
Periodic fine roughness (sharkskin) appears on the LLDPE film surface, in severe cases overall orange peel and thread-like patterns, more obvious during high-speed production.
Análise das causas
LLDPE has linear molecular chains, high melt viscosity and lacks long-chain branch lubrication, so the die lip exit shear stress easily exceeds the critical value causing surface-level or even overall melt fracture; the higher the line speed and the lower the die temperature, the more severe it is.
Solução
①Increase the die temperature by 3~5°C; ②appropriately reduce the line speed/output; ③add fluoropolymer processing aid (PPA) to form a lubricating layer on the die wall; ④polish the die lip to reduce resistance; ⑤blend with LDPE or select wide molecular weight distribution grades to improve the processability if necessary.
Conclusão principal
LLDPE sharkskin = 'high viscosity meeting high shear'; PPA is the standard solution, and raising the temperature and reducing the speed is the foundation; when pursuing output, the sharkskin critical line speed is the capacity ceiling, which must be systematically solved rather than forcibly raising the speed.
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