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Dimensional instability refers to the change in the size of plastic parts between each batch of molded products or between each molded product of each mold under the same injection molding machine and molding process conditions.The change in product size is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design and changes in material properties.

1) Inconsistent molding conditions or improper operation

During injection molding, various process parameters such as temperature, pressure and time must be strictly controlled in accordance with process requirements, especially the molding cycle of each plastic part must be consistent and cannot be changed at will. If the injection pressure is too low, the holding time is too short, the mold temperature is too low or uneven, the temperature at the barrel and nozzle is too high, and the plastic part is not cooled enough, the shape and size of the plastic part will be unstable.

2) Improper selection of molding raw materials

The shrinkage rate of the molding raw materials has a great influence on the dimensional accuracy of the plastic parts. If the molding equipment and mold have high precision, but the shrinkage rate of the molding raw materials is large, it is difficult to ensure the dimensional accuracy of the plastic parts. In general, the greater the shrinkage rate of the molding raw materials, the more difficult it is to ensure the dimensional accuracy of the plastic parts. Therefore, when selecting molding resins, the influence of the shrinkage rate of the raw materials after molding on the dimensional accuracy of the plastic parts must be fully considered. For the selected raw materials, the range of shrinkage rate changes cannot be greater than the requirements for the dimensional accuracy of the plastic parts.

3) Mold failure

The structural design and manufacturing accuracy of the mold directly affect the dimensional accuracy of the plastic parts. During the molding process, if the rigidity of the mold is insufficient or the molding pressure in the mold cavity is too high, the mold will be deformed, which will cause the molding size of the plastic parts to be unstable. If the matching clearance between the guide pin and the guide sleeve of the mold exceeds the tolerance due to poor manufacturing accuracy or excessive wear, the molding size accuracy of the plastic parts will also

Therefore, when designing the mold, sufficient mold strength and rigidity should be designed, and the processing accuracy should be strictly controlled. The mold cavity material should use wear-resistant materials, and the cavity surface should preferably be heat treated and cold hardened. When the dimensional accuracy of the plastic part is very high, it is best not to use a one-mold-multiple-cavity structure. Otherwise, in order to ensure the molding accuracy of the plastic part, a series of auxiliary devices to ensure the mold accuracy must be set on the mold, resulting in a high mold production cost.

4) Equipment failure

If the plasticizing capacity of the molding equipment is insufficient, the feeding system is unstable, the screw speed is unstable, the stop function is abnormal, the check valve of the hydraulic system fails, the thermocouple of the temperature control system is burned out, the heater is short-circuited, etc., the molding size of the plastic part will be unstable. Once these faults are found, targeted measures can be taken to eliminate them.

5) Inconsistent test methods or conditions

If the method, time, and temperature of measuring the size of the plastic part are different, the measured size will be very different. Among them, the temperature condition has the greatest impact on the test, because the thermal expansion coefficient of plastic is 10 times greater than that of metal. Therefore, the standard method and temperature conditions must be used to measure the structural dimensions of the plastic part, and the plastic part must be fully cooled and fixed before measurement. Generally, the size of the plastic part changes greatly within 10 hours of the demolding mode, and it is basically fixed after 24 hours.