Extrusion & Post
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Bamboo Stripes/Melt Fracture Appearing on the Surface of High-Density Polyethylene Extruded Products
Problem Phenomenon
Periodic coarse ripples or bamboo-like undulations appear on the surface of the extrudate; 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.
Cause Analysis
（1）The extrusion speed exceeds the critical shear rate, and the melt flow becomes unstable
（2）The die temperature is too low, and the melt viscosity is high
（3）The die land length is insufficient, and the melt is not fully relaxed
（4）The resin molecular weight is too high or the molecular weight distribution is too wide
（5）The die entrance angle is improper, and the melt entrance flow is unstable
Solution
（1）Reduce the extrusion speed to below the critical shear rate
（2）Increase the die temperature to a higher value within the 180-260°C range 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 entrance design, with an entrance angle of 30-60°
（6）Add a small amount of processing aid (fluoroelastomer) when necessary to improve flowability
Key Takeaway
Melt fracture is flow instability caused by the extrusion speed exceeding the critical value, and it is necessary to reduce the speed, increase the temperature, and optimize the die design.
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