Extrusion & Post
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Oversized Dimension Exceeding the Upper Limit of High-Density Polyethylene Injection-Molded Products
Fenómeno del problema
The actual dimensions of the product exceed the upper tolerance limit of the drawing, and the gap during assembly is too large or tight fitting is impossible.
Oversized dimensions are usually related to insufficient shrinkage, and are prone to occur after changing the material batch number or adjusting the process.
Análisis de la causa
(1) The holding pressure is too high or the holding time is too long, and the product is over-compensated for shrinkage
(2) The mold temperature is too low, and the fast cooling rate and low crystallinity lead to reduced shrinkage
(3) The barrel temperature is too low, the melt density is high, and the shrinkage is small
(4) The cooling time is too long; after the product fully shrinks in the mold, the dimension is small, and the shrinkage allowance after demolding is small
(5) The shrinkage value taken during mold dimension design is too large
Solución
(1) Appropriately reduce the holding pressure or shorten the holding time
(2) Appropriately increase the mold temperature to the higher value within the range of 30-80°C to increase shrinkage
(3) Appropriately increase the barrel temperature to the higher value within the range of 180-260°C
(4) Verify the mold dimensions according to the actual shrinkage of 1.5-3.0, and modify the mold if necessary
(5) Establish a process parameter recording system, and check the first-piece dimensions at each shift change
Conclusión clave
Oversized dimensions are mainly caused by insufficient shrinkage; the actual shrinkage is controlled within the tolerance range by adjusting the holding and temperature parameters.
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