Plastic Pail Mould Manufacturers introduces the change […]
Plastic Pail Mould Manufacturers introduces the change process of large injection molding processing dimensions:
1. 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 is faulty, the thermocouple and the heater of the temperature control system are broken, etc. All of these will lead to dimensional instability of large-scale injection molding. If these faults are detected, targeted measures can be taken to eliminate them.
2. Improper selection of molding materials. The shrinkage of large injection molding materials has little effect on the dimensional accuracy of plastic parts. If the precision of large-scale injection molding processing equipment and molds is low, but the shrinkage rate of the molding material is small, it is easy to ensure the dimensional accuracy of the plastic parts. Generally speaking, the smaller the shrinkage of the molding material, the more difficult it is to ensure the dimensional accuracy of the plastic part. Therefore, when choosing a molding resin, it is necessary to fully consider the impact of molding shrinkage on the dimensional accuracy of plastic parts. For the selected raw materials, the change range of shrinkage should not exceed the requirements of the dimensional accuracy of the plastic parts. It should be noted that, depending on the crystallinity of the resin used for analysis, the difference in shrinkage of various resins is relatively small.
3. Mold failure. Structural design and manufacturing accuracy indirectly affect the dimensional accuracy of plastic parts. In the large-scale injection molding process, if the rigidity of the mold is insufficient or the cavity of the mold is very low under the molding pressure, the mold will be deformed and the dimensional molding of the plastic part will be unstable. If the fitting gap between the guidepost and the mold guide bushing block is not in a straight line due to low manufacturing accuracy or excessive wear, the molding dimensional accuracy of the plastic part will also be reduced. If the mold cavity is slightly worn due to the presence of hard fillers or glass fiber reinforced materials in the large-scale injection molding process, or the mold is formed with multiple mold cavities, there will be errors between the mold cavity and the gate, and the runner will also Error, an improper balance between the feed port and other materials (such as different filling molds) can also cause dimensional fluctuations. Therefore, when designing a mold, a mold with sufficient strength and rigidity should be designed, and the processing accuracy should be strictly controlled. The mold cavity uses wear-resistant materials to make the surface of the cavity suitable for cold treatment and cold hardening.
4. The forming conditions are always different or the operation is improper. When large-scale injection molding is processed, technical parameters such as temperature, pressure, and time must be strictly controlled according to the process requirements. In particular, the molding cycle of each plastic part must be the same and cannot be changed at will. If the injection pressure is high, the holding time is long, the mold temperature is high or uniform, the temperature of the barrel and nozzle is low, and the mold is not cooled enough, the size of the mold body will be smooth. Generally use lower injection pressure and injection speed, appropriately extend the filling and holding time of the mold, and increase the temperature of the mold and the material to overcome the shortcomings of dimensional instability.
5. The test methods or conditions are not the same. If the method, time, and temperature of measuring the size of the plastic part are different, there will be a small difference in the measured size. Among them, the temperature condition has little effect on the test, this is because the cold expansion coefficient of plastic is 10 times higher than that of the small metal industry. Therefore, it is necessary to use the methods and temperature conditions specified by the standard to measure the structural dimensions of plastic parts. Plastic parts must be sufficiently cooled and formed before they can be measured. In the large-scale injection molding process, the volume change of the plastic part is usually small within 10 hours after demolding, and the shape is basically fixed within 24 hours.