Injection molding, temperature, pressure and time and other process parameters must be controlled in strict accordance with the 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 pressure holding time is too short, the mold temperature is too low or uneven, the temperature of the cylinder and nozzle is too high, and the cooling of the plastic parts is insufficient, it will lead to the instability of the size of the plastic parts. Under normal circumstances, the use of higher injection pressure and injection speed, the appropriate extension of the filling mold and pressure holding time, improve the mold temperature and material temperature, is conducive to overcoming the size of the unstable fault.

If the shape size of the plastic parts is greater than the required size, the injection pressure and melt temperature should be properly reduced, the mold temperature should be increased, the mold filling time should be shortened, the gate cross-sectional area should be reduced, so as to improve the shrinkage rate of the plastic parts.

If the size of the plastic parts after forming is less than the required size, the opposite molding conditions should be taken.

It is worth noting that the change of the ambient temperature also has a certain impact on the fluctuation of the molding size of the plastic parts, and the process temperature of the equipment and mold should be adjusted in time according to the change of the external environment.

The shrinkage rate of molding raw materials has a great impact on the dimensional accuracy of plastic parts. If the accuracy of the molding equipment and mold is very high, but the shrinkage of the molding raw materials is very large, it is difficult to ensure the dimensional accuracy of the plastic parts. Under normal circumstances, the greater the shrinkage rate of forming raw materials, the more difficult it is to ensure the dimensional accuracy of plastic parts. Therefore, in the selection of molding resin, we must fully consider the effect of the shrinkage of raw materials after molding on the dimensional accuracy of plastic parts. For the selected raw materials, the change range of the shrinkage rate can not be greater than the requirements of the dimensional accuracy of plastic parts.

should be noted that the shrinkage rate of various resins is different, according to the degree of crystallization of the resin analysis. Usually, the shrinkage rate of crystalline and semi-crystalline resin is larger than that of amorphous resin, and the shrinkage change range is also relatively large, and the corresponding shrinkage rate fluctuation after molding of plastic parts is also relatively large; For crystalline resin, high crystallinity, molecular volume reduction, the shrinkage of plastic parts is large, the size of the resin spherulite also has an impact on the shrinkage rate, spherulite is small, the gap between molecules is small, the shrinkage of plastic parts is small, and the impact strength of plastic parts is relatively high.

In addition, if the particle size of the molding raw materials is uneven, the drying is poor, the recycled material is mixed with the new material is uneven, the performance of each batch of raw materials is different, it will also cause the fluctuation of the molding size of the plastic parts.

Mold structure design and manufacturing accuracy directly affect the dimensional accuracy of plastic parts, in the molding process, if the rigidity of the mold is insufficient or the molding pressure in the mold cavity is too high, so that the mold deformation, The liver causes the molding size of the plastic parts to be unstable.

If the gap between the guide column and the guide sleeve of the mold is abnormal due to poor manufacturing accuracy or too much wear, it will also reduce the molding dimensional accuracy of the plastic parts.

If there are hard fillers or glass fiber reinforced materials in the molding raw materials leading to serious wear of the mold cavity, or the use of a mold multi-cavity molding, there are errors between the cavity and gate, runner and other errors and poor balance of the feed port and other reasons to fill the mold is inconsistent, will also cause size fluctuations.

Therefore, in the design of the mold, should design enough mold strength and rigidity, strict control of processing accuracy, mold cavity material should be used wear-resistant materials, the surface of the cavity is best for heat treatment and cold hardening treatment. When the dimensional accuracy of the plastic parts is very high, it is best not to use the structural form of a mold with multiple cavities, otherwise in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices must be set up on the mold to ensure the accuracy of the mold, resulting in high production costs of the mold.

When the thickness error occurs in the plastic parts, it is often caused by mold failure. If it is under the condition of a mold and a cavity, the thickness error of the wall thickness of the plastic part is generally due to the installation error and poor positioning of the mold, resulting in the relative position deviation of the mold cavity and the core. At this time, for those plastic parts whose wall thickness size requirements are very accurate, they can not be located only by guide posts and guide bushings, and other positioning devices must be added.

If the thickness error is generated under the condition of multiple cavities in a mold, under normal circumstances, the error is small at the beginning of the molding, but the error gradually becomes larger after continuous operation, which is mainly due to the error between the mold cavity and the core, especially when the hot runner mold molding is most likely to produce this phenomenon. In this regard, a double cooling loop with very small temperature difference can be set in the mold. If the thin-walled circular container is formed, the floating core can be used, but the core and the mold cavity must be concentric.

In addition, when making a mold, in order to facilitate repair, it is generally always used to make the cavity smaller than the required size, and the core is larger than the required size, leaving a certain repair margin. When the inner diameter of the plastic molding hole is very small than the outer diameter, the core pin should be larger, this is because the shrinkage of the plastic parts at the molding hole is always greater than other parts, and it shrinks in the direction of the hole center. On the contrary, if the inner diameter of the molding hole is close to the outside diameter, the core pin can be made smaller.

If the plasticizing capacity of the molding equipment is insufficient, the feeding system is unstable, the speed of the screw is unstable, the stopping effect is abnormal, the check valve of the hydraulic system is faulty, the thermocouple is burned out in the temperature control system, the heater is broken, etc. Will lead to the molding size of the plastic parts is unstable. As long as these faults are found out, targeted measures can be taken to eliminate them.

If the measurement of the size of plastic parts, time, temperature is different, the measured size will have a great difference. Among them, the temperature conditions have the greatest impact on the test, because the coefficient of thermal expansion of plastic is 10 times larger than that of metal. Therefore, the method and temperature conditions specified in the standard must be used to determine the structural size of the plastic parts, and the plastic parts must be fully cooled before they can be measured. General plastic parts in the mode of 10 hours within the size change is very large, 24 hours to the basic shape.

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