Extrusion & Post
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Bamboo pattern/melt fracture on the surface of low density polyethylene extruded products
Problembeschreibung
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.
Ursachenanalyse
(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
Lösung
(1) Reduce extrusion speed below the critical shear rate
(2) Increase die temperature to the higher value within the 160-220°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
Wichtigste Erkenntnis
Melt fracture is flow instability caused by extrusion speed exceeding the critical value; speed needs to be reduced, temperature increased, and die design optimized.
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