Extrusion & Post
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Longitudinal retraction rate of random copolymer polypropylene pipes exceeding standard
Problem Phenomenon
After the pipe is heated under specified temperature and time conditions, the longitudinal retraction rate exceeds the standard limit.
Excessive retraction rate reflects large internal stress or imperfect crystallization in the pipe, affecting pipe reliability in use.
Cause Analysis
(1) Random copolymer polypropylene has large residual internal stress after extrusion
(2) Cooling rate is too fast, and molecular chain orientation has not relaxed
(3) Haul-off speed is too large, and the pipe is overstretched longitudinally
(4) Barrel temperature is on the low side, and melt plasticization is insufficient
(5) Sizing cooling is insufficient
Solution
(1) Appropriately increase barrel temperature to a reasonable value within the 190-260°C range to improve plasticization
(2) Appropriately reduce the cooling rate to allow molecular chains time to relax
(3) Adjust haul-off speed and reduce the draw ratio
(4) Increase the cooling water tank length to ensure full cooling
(5) Perform water bath annealing after pipe demolding to eliminate internal stress
Key Takeaway
Excessive retraction rate is caused by large internal stress or excessive orientation and needs to be solved from temperature control, cooling rate and haul-off speed.
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