Injection mold processing What are the reasons for the size change?
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 of the temperature control system, the heater is broken, etc., the molding size will be unstable. If these faults are detected, targeted measures can be taken to eliminate them.
2. Improper selection of molding materials
The shrinkage of molding materials has little impact on the dimensional accuracy of plastic parts. If the precision of molding equipment and mold is very low, but the shrinkage of molding materials is very small, it is also easy to ensure the dimensional accuracy of plastic parts. In general, the smaller the shrinkage of molding materials, the more difficult it is to ensure the dimensional accuracy of plastic parts. Therefore, when selecting molding resin, it is necessary to fully consider the influence of material molding shrinkage on the dimensional accuracy of plastic parts. For the selected raw materials, the variation range of shrinkage shall not be greater than the requirements for dimensional accuracy of plastic parts. It should be noted that the shrinkage difference of various resins is relatively small according to the crystallinity of the resin used for analysis. Generally, the shrinkage of crystalline and semi crystalline resins is smaller than that of amorphous resins, so the range of shrinkage change is small, and the corresponding plastic parts formed after shrinkage fluctuation are also large; For crystalline resin, the crystallinity is low, the molecular volume decreases, plastic shrinkage, and the size of resin spherulite also affects the shrinkage rate. The spherulite is large, the molecular block gap is large, and the plastic shrinkage rate is large, so the impact strength on the plastic is low. In addition, if the particle size of the raw material is complete and the water content is inappropriate, and then the raw material and new raw material are mixed evenly, the performance of each batch of raw materials will be different, which will also cause fluctuations in the model. Formal dimension.
3. Mold failure
The design and manufacturing accuracy of the mold structure indirectly affect the dimensional accuracy of plastic parts. During the molding process, if the mold is not rigid enough or the mold cavity is very low under the molding pressure, the mold will deform, and the dimensional molding of plastic parts will be unstable. If the fit clearance between the mold guide pillar and the guide sleeve block is disjointed due to low manufacturing precision or excessive wear, the molding dimension accuracy of plastic parts will also be reduced. If the mold cavity is slightly worn due to hard filler or glass fiber reinforced material inside the molding material, or if the mold is molded with multiple mold cavities, there are errors between the mold cavity and the gate, runner errors, and improper balance between the feed port and other materials, such as the reasons for different filling molds, which will also cause dimensional fluctuations. Therefore, in the design of the mold, it is necessary to design sufficient mold strength and stiffness, strictly control the machining accuracy, use wear-resistant materials for the mold cavity, and the cavity surface is good for cold treatment and cold hardening treatment. If the requirements for the dimensional accuracy of plastic parts are very low, do not use the mold multi cavity structure to ensure the molding accuracy of plastic parts. A series of auxiliary devices must be set on the mold accuracy, so that the production cost of the mold is very low.
4. The forming conditions are never the same or the operation is inappropriate
During injection molding, the temperature, pressure, time and other technical parameters must be strictly controlled according to the process requirements, especially the molding cycle of each plastic part must be the same and cannot be changed at will. If the injection pressure is very high, the pressure holding time is very long, the mold temperature is very high or even, the temperature at the barrel and nozzle is very low, and the cooling of the model is insufficient, the size of the model body will be smooth. In general, lower injection pressure and injection speed are used to properly extend the filling and pressure holding time of the mold, increase the mold temperature and material temperature, and overcome the defect that the size is always unstable. Remind everyone that the change of ambient temperature also has a certain impact on the fluctuation of forming size. The processing temperature of equipment and mold should be adjusted immediately according to the change of external environment.
5. Test methods or conditions are not the same
If the method, time and temperature of measuring the size of plastic parts are different, the measured size will have very small differences. The temperature condition has little influence on the test, because the cold expansion coefficient of plastic is 10 times higher than that of small metal industry. Therefore, it is necessary to use the method and temperature conditions specified in the standard to measure the structural dimensions of the plastic parts, and the plastic parts must be fully cooled and shaped before measurement. Generally, the volume change of the plastic part is very small within 10 hours of demoulding, and it is basically shaped within 24 hours.
