Extrusion & Post
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LDPE Blown Film Melt Fracture Sharkskin
Problem Phenomenon
The film surface shows wavy periodic patterns, commonly called sharkskin, with reduced transparency and gloss.
Cause Analysis
The melt viscosity is high, forming a velocity gradient inside the die that forcibly drags the surface layer, showing wavy patterns after freezing; low die gap temperature, small gap, and low melt index all aggravate it.
Solution
①Raise temperature settings to improve flowability; ②Raise die lip temperature to reduce viscosity; ③Widen the die gap to weaken the velocity gradient; ④Use PPA processing aid or blend in an appropriate amount of LDPE to improve rheology.
Key Takeaway
Sharkskin is the surface manifestation of shear instability; raising temperature, widening the gap, and adding additives are quick countermeasures, while adjusting resin rheological properties is the fundamental solution.; After confirming effectiveness, solidify the parameters and practices into the work instruction to prevent recurrence
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