1. Inconsistent molding conditions or improper operation
Process parameters such as temperature, pressure and time have to be strictly in line with process requirements. Particularly, the molding cycle of each plastic part must be consistent and no changes can be made arbitrarily. Unstable gauge can be aroused from low injection pressure, short holdup time, low mold temperature, high temperature of barrel and nozzle and insufficient cooling of plastic part.
Higher injection pressure and speed, longer molding and holdup time as well as higher mold and material temperature is helpful to overcoming unstable gauge. If the size after molding is larger than the required, it's appropriate to lower the temperature of injection pressure and molten material, raise mold temperature, shorten molding time and reduce sectional area of the gate, to improve the shrinkage of plastic part. But if the size is smaller than the required, the molding conditions shall be opposite. It's worth noting that changes in ambient temperature influences unstable gauge of the injection molded parts. So, temperature of device and mold should be adjusted timely with ambient temperature.
2. Improper raw material
The shrinkage of raw plastic injection materials exerts great influence on dimension accuracy. It's difficult to ensure the dimension accuracy for those high-precision molding devices and molds with great shrinkage. In general, the bigger the shrinkage of raw material, the more difficult the dimension accuracy of plastic parts. Therefore, it's a must to fully take the effect of shrinkage on dimension accuracy into consideration in selecting raw material. The shrinkage of the chosen material can't be larger than the demanded dimension accuracy.
It should be noticed that shrinkages of various resins have a big difference analyzed by crystallinity. The shrinkages of crystalline and semi crystalline resins are usually bigger than non-crystalline resins and also have larger variation. The corresponded plastic parts have larger shrinkage fluctuation. Crystalline resins of high crystallinity and narrow molecular volume make their plastic parts big shrinkage. The size of resin spherulites affect shrinkage as well. The smaller the resin spherulites, the narrower the space between molecular and the less shrinkage of plastic parts. But it causes higher impact strength.
In addition, raw material with uneven grain sizes, weak dryness and heterogeneous mixture of reworked material and new material make for dimension fluctuation of plastic parts, as well as different performance of each batch of raw material.
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