Extrusion & Post
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Bamboo-node pattern/melt fracture appears on the surface of heat-resistant polyethylene extruded products
Fenómeno do problema
Periodic coarse ripples or bamboo-node-like undulations appear on the extrudate surface, and in severe cases the extrudate shape is distorted.
Melt fracture is a serious defect in the extrusion process and directly leads to product scrapping.
Análise das causas
(1) Extrusion speed exceeds critical shear rate, melt flow unstable
(2) Die temperature too low, melt viscosity high
(3) Die sizing section length insufficient, melt not fully relaxed
(4) Resin molecular weight too high or molecular weight distribution too wide
(5) Improper die entrance angle, melt entrance flow unstable
Solução
(1) Reduce extrusion speed below the critical shear rate
(2) Increase die temperature to the higher value within 190-230°C to reduce melt viscosity
(3) Increase die sizing section length, generally 10-20 times the die diameter
(4) Select grades with moderate molecular weight distribution
(5) Optimize die entrance design, entrance angle 30-60°
(6) Add a small amount of processing aid (fluoroelastomer) to improve fluidity if necessary
Conclusão principal
Melt fracture is flow instability caused by extrusion speed exceeding the critical value, and the speed needs to be reduced, the temperature increased and the die design optimized.
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