Extrusion & Post
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LDPE Blown Film Spiral Instability
Problem Phenomenon
The film bubble rotates and swings in a spiral shape, film thickness fluctuates periodically, and winding is uneven
Cause Analysis
Melt strength too weak is the main cause of spiral instability: molecular weight is too low or branching is insufficient; the spiral airflow of the air ring also disturbs the film bubble; shear overheating at the die lip aggravates the instability
Solution
① Switch to high molecular weight low melt index grades or blend in LDPE to increase branching; ② open the air vent wider to reduce air speed; ③ enlarge the die gap to reduce shear heat; after adjustment, take continuous samples to verify the effect, and resume mass production only after confirming stability
Key Takeaway
For spiral instability the formulation route is the first choice; high molecular weight plus branched structure is the molecular basis of film bubble stability; after confirming effectiveness, solidify the parameters and practices into the work instruction to prevent recurrence
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