Extrusion & Post
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LLDPE bubble spiral instability
Problem Phenomenon
The bubble rotates and sways in a spiral, film thickness fluctuates periodically, and winding is uneven
Cause Analysis
LLDPE's extensional softness gives the bubble low tensile stress at low strain rates, prone to spiral instability; spiral airflow disturbance from the air ring; shear overheating at the die lip aggravates instability
Solution
①Equip dual-lip air ring and IBC to enhance stability; ②open the air outlet wider to reduce air speed; ③increase die gap to reduce shear heat; after adjustment, continuously sample to verify the effect, and resume mass production only after confirming stability
Key Takeaway
LLDPE spiral instability is a direct consequence of extensional softness; dual-lip air ring plus low air speed is the standard countermeasure; after confirming effectiveness, fix the parameters and practices into the work instruction to prevent recurrence
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