Extrusion & Post
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Excessive shrinkage rate of low density polyethylene extruded products
Problem Phenomenon
Extruded products shrink significantly after cooling, exceeding the tolerance range.
Excessive shrinkage rate causes undersized product dimensions, affecting fit and use.
Cause Analysis
(1) High crystallinity of low density polyethylene; cooling crystallization causes volume shrinkage
(2) Melt temperature too high; large volume expansion, large shrinkage after cooling
(3) Slow cooling rate; high crystallinity, large shrinkage
(4) Haul-off speed too slow; product swells at the die
(5) Improper raw material grade selection, excessive shrinkage rate
Solution
(1) Appropriately reduce barrel temperature to the lower value within the 160-220°C range
(2) Appropriately speed up cooling rate, reduce crystallinity to reduce shrinkage
(3) Adjust haul-off speed, appropriately increase draw ratio
(4) Select grades with small shrinkage rate, shrinkage rate 1.5-3.0
(5) Reserve shrinkage allowance in mold design
Key Takeaway
Excessive shrinkage rate is caused by high crystallinity or high temperature; it needs to be solved by controlling temperature and cooling rate.
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