Extrusion & Post
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Dimensions of random copolymer polypropylene injection molded products too large, exceeding the upper limit
Fenómeno del problema
Actual product dimensions exceed the drawing tolerance upper limit, with excessive clearance during assembly or inability to achieve a tight fit.
Oversized dimensions are usually related to insufficient shrinkage and easily appear after changing material batches or adjusting the process.
Análisis de la causa
(1) Holding pressure is too high or holding time is too long; the product is over-compensated for shrinkage
(2) Mold temperature is on the low side; fast cooling rate and low crystallinity lead to reduced shrinkage rate
(3) Barrel temperature is on the low side; melt density is large and shrinkage rate is small
(4) Cooling time is too long; the product fully shrinks in the mold and the shrinkage allowance after demolding is small
(5) The shrinkage rate value used in mold dimension design is too large
Solución
(1) Appropriately reduce holding pressure or shorten holding time
(2) Appropriately increase mold temperature to the higher value within the 40-80°C range to increase shrinkage
(3) Appropriately increase barrel temperature to the higher value within the 190-260°C range
(4) Verify mold dimensions according to the actual shrinkage rate of 1.0-2.0, and repair the mold if necessary
(5) Establish a process parameter recording system, testing first-piece dimensions at each shift change
Conclusión clave
Oversized dimensions are mainly caused by insufficient shrinkage; the actual shrinkage rate is controlled within the tolerance range by adjusting holding pressure and temperature parameters.
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