Extrusion & Post
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Bamboo pattern/melt fracture appears on the surface of general-purpose polyethylene extruded products
Fenómeno do problema
Periodic coarse ripples or bamboo-like undulations appear on the surface of the extrudate, and in severe cases the shape of the extrudate is distorted.
Melt fracture is a serious defect in the extrusion process and directly leads to product scrapping.
Análise das causas
（1）The extrusion speed exceeds the critical shear rate, and the melt flow is unstable
（2）The die temperature is too low, and the melt viscosity is high
（3）The die land length is insufficient, and the melt cannot be fully relaxed
（4）The resin molecular weight is too high or the molecular weight distribution is too wide
（5）The die inlet angle is improper, and the melt inlet flow is unstable
Solução
（1）Reduce the extrusion speed to below the critical shear rate
（2）Raise the die temperature to the higher value within the range of 160-260°C to reduce the melt viscosity
（3）Increase the die land length, generally 10-20 times the die diameter
（4）Select a grade with a moderate molecular weight distribution
（5）Optimize the die inlet design, inlet angle 30-60°
（6）If necessary, add a small amount of processing aid (fluoroelastomer) to improve flowability
Conclusão principal
Melt fracture is flow instability caused by the extrusion speed exceeding the critical value, and it is necessary to reduce the speed, raise the temperature, and optimize the die design.
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