Extrusion & Post
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Excessive Shrinkage of Polypropylene Extruded Products
Problem Phenomenon
The dimensions of the extruded product shrink significantly after cooling, exceeding the tolerance range.
Excessive shrinkage causes the product dimensions to be too small, affecting fitting and use.
Cause Analysis
(1) Polypropylene has high crystallinity, and cooling crystallization causes volume shrinkage
(2) The melt temperature is too high, the volume expansion is large, and the shrinkage after cooling is large
(3) The cooling rate is slow, the crystallinity is high, and the shrinkage is large
(4) The haul-off speed is too slow, and the product expands at the die
(5) The raw material grade is selected improperly, and the shrinkage is too large
Solution
(1) Appropriately reduce the barrel temperature to the lower value within the range of 200-280°C
(2) Appropriately accelerate the cooling rate to reduce crystallinity and reduce shrinkage
(3) Adjust the haul-off speed and appropriately increase the stretch ratio
(4) Select a grade with small shrinkage, with a shrinkage of 1.0-2.5
(5) Reserve a shrinkage allowance during mold design
Key Takeaway
Excessive shrinkage is caused by high crystallinity or high temperature, and needs to be solved by controlling the temperature and cooling rate.
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