Extrusion & Post
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Bamboo grain/melt fracture appearing on the surface of linear low-density polyethylene extruded products
Problem Phenomenon
Periodic coarse ripples or bamboo-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, directly leading to product scrapping.
Cause Analysis
(1) Extrusion speed exceeds the critical shear rate, and melt flow becomes unstable
(2) Die temperature is too low and melt viscosity is high
(3) The die land length is insufficient, and the melt cannot fully relax
(4) Resin molecular weight is too high or molecular weight distribution is too wide
(5) The die inlet angle is improper, and melt inlet flow is unstable
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 the die land length, generally 10-20 times the die diameter
(4) Select grades with moderate molecular weight distribution
(5) Optimize die inlet design, with an inlet angle of 30-60°
(6) If necessary, add a small amount of processing aid (fluoroelastomer) to improve fluidity
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.
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