Extrusion & Post
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Bamboo Marks/Melt Fracture Appearing on the Surface of Polypropylene Extruded Products
Problembeschreibung
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.
Ursachenanalyse
(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 cannot fully relax
(4) The molecular weight of the resin 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
Lösung
(1) Reduce the extrusion speed to below the critical shear rate
(2) Increase the die temperature to the higher value within the range of 200-280°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 entrance design, with an entrance angle of 30-60°
(6) Add a small amount of processing aid (fluoroelastomer) to improve fluidity when necessary
Wichtigste Erkenntnis
Melt fracture is flow instability caused by the extrusion speed exceeding the critical value; the speed needs to be reduced, the temperature increased, and the die design optimized.
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