Extrusion & Post
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Bamboo pattern/melt fracture on the surface of linear low density polyethylene extruded products (Variant: strengthened measures)
Problem Phenomenon
Periodic coarse ripples or bamboo-like undulations appear on the extrudate surface; in severe cases the extrudate shape is distorted.
Melt fracture is a serious defect in the extrusion process, directly causing product scrap.
In particular, additional attention is needed on the original basis; this problem is more prominent and requires key attention.
Cause Analysis
(1) Extrusion speed exceeds the critical shear rate, melt flow becomes unstable
(2) Die temperature too low, high melt viscosity
(3) Insufficient die land length; the melt is not fully relaxed
(4) Resin molecular weight too high or molecular weight distribution too wide
(5) Improper die entrance angle, unstable melt entrance flow
(6) On the original basis, additional attention is needed as process parameter fluctuations or equipment status changes have aggravated this problem
Solution
(1) Reduce extrusion speed below the critical shear rate
(2) Increase die temperature to the higher value within the 190-250°C range to reduce melt viscosity
(3) Increase die land length, generally 10-20 times the die diameter
(4) Select a grade with moderate molecular weight distribution
(5) Optimize die entrance design, entrance angle 30-60°
(6) Add a small amount of processing aid (fluoroelastomer) when necessary to improve flowability
(6) And strengthen process monitoring; on the original basis, additional attention is needed, with special attention to parameter stability
Key Takeaway
Melt fracture is flow instability caused by extrusion speed exceeding the critical value; speed needs to be reduced, temperature increased, and die design optimized.
Targeted preventive measures need to be taken in different production scenarios.
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