Injection molding process is the primary process in the packaging material, in the process of injection molding products often encounter the size of the unstable situation, the size of plastic injection molding products with the molding cycle to extend and change or with environmental changes and fluctuations.

Dimensional instability

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

1. Improper operation

During injection molding, various process parameters such as temperature, pressure and time must be strictly controlled in accordance with the process requirements. In particular, 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 cooling of the plastic parts is insufficient, the plastic parts will be unstable.

Under normal circumstances, higher injection pressure and injection speed can be used to appropriately extend the filling time and holding time, improve the mold temperature and material temperature, which is beneficial to overcome the size instability fault.

If the size of the plastic parts is larger than the required size after molding, the injection pressure and melt temperature should be reduced appropriately, improve the mold temperature, shorten the filling time, reduce the gate section area, so as to improve the shrinkage rate of the plastic parts.

If the size of the molded part is smaller than the required size, the forming condition to the contrary shall be adopted.

It is worth noting that the change of environmental temperature also has a certain influence on the fluctuation of the molding size of the plastic parts. The process temperature of the equipment and the mold should be adjusted timely according to the change of the external environment.

2. Improper selection of molding materials

The shrinkage rate of forming material has great influence on the dimension precision of plastic parts. If the molding equipment and mold precision is very high, but the shrinkage of forming raw materials is very large, it is difficult to ensure the dimensional precision of the plastic parts. In general, the larger the shrinkage of forming raw materials, the more difficult to ensure the dimensional accuracy of plastic parts. Therefore, in the selection of molding resin, we must fully consider the impact of the shrinkage of raw materials after molding on the size accuracy of plastic parts. For the selection of raw materials, the shrinkage of the range of change cannot be greater than the size of the plastic parts precision requirements.

It should be noted that the shrinkage of various resins vary greatly, according to the degree of crystallization of the resin analysis. Generally, the shrinkage of crystalline and semi-crystalline resins is larger than that of non-crystalline resins, and the shrinkage variation range is also larger, and the shrinkage fluctuation after the corresponding plastic molding is also larger; For crystalline resin, high crystallinity, molecular volume reduction, plastic shrinkage, resin spherulite size also has an impact on shrinkage rate, small spherulite, small intermolecular void, plastic shrinkage, and plastic impact strength is relatively high.

In addition, if the size of molding raw materials particles are uneven, poor drying, reraw materials and new materials mixed unevenly, the performance of each batch of raw materials is different, will also cause the plastic molding size fluctuation.

3. Mold failure

The structural design and manufacturing precision of the mold directly affect the dimensional precision of the plastic parts, in the forming process, if the rigidity of the mold is insufficient or the mold cavity bear the forming pressure is too high, the mold deformation, it will cause the plastic parts forming size instability.

If the clearance between the guide column and the guide sleeve is out of tolerance due to poor manufacturing accuracy or too much wear, it will also reduce the precision of the molding dimension.

If there are hard fillers or glass fiber reinforced materials in the molding raw materials leading to severe wear of the mold cavity, or when adopting multi-cavity molding of a mold, the mold filling is inconsistent due to the error between each cavity, the error of gate, runner and other reasons, and the poor balance of the feed inlet, etc., which will also cause the size fluctuation.

Therefore, in the design of the mold, the mold should be designed enough strength and rigidity, strict control of machining accuracy, mold cavity material should use wear resistant materials, cavity surface had better heat treatment and cold hardening treatment. When the dimensional precision of plastic parts is very high, it is best not to use a mold multi-cavity structure, otherwise in order to ensure the molding precision of plastic parts, must be set up on the mold to ensure the accuracy of a series of auxiliary equipment, resulting in the mold production cost is very high.

When the plastic parts appear partial thickness error, is often caused by mold fault. If it is under the condition of one mold and one cavity wall thickness deviation, generally due to the mold installation error and poor positioning leading to the mold cavity and core relative position deviation. At this point, for those whose wall thickness and size requirements are very accurate plastic parts, can not only rely on guide column and guide sleeve to locate, must add other positioning devices.

If it is the thickening error under the condition of multiple cavities of a mold, in general, the forming error is small at the beginning, but the error gradually increases after continuous operation, which is mainly caused by the error between the mold cavity and the mold core, especially when hot runner mold is used to form this phenomenon is the most likely. For this purpose, a double cooling loop with little temperature difference can be set in the mold. If it is a shaped thin-walled round container, a floating core may be used, but the core and cavity must be concentric.

In addition, when making mold, in order to facilitate the repair mold, generally always used to do cavity smaller than the required size, the core do larger than the required size, set aside a certain allowance for repair mold. When the plastic molding hole diameter is much smaller than the outer diameter, the core pin should be done a little bigger, this is because of the molding hole plastic shrinkage is always greater than other parts, and to the direction of the hole contraction. On the contrary, if the inner diameter of the plastic forming hole is close to the outer diameter, the core can be made smaller.

4. Equipment failure

If the plasticizing capacity of molding equipment is insufficient, the feeding system is unstable, the rotating speed of the screw is unstable, the stopping function is abnormal, the check valve of the hydraulic system is out of order, the temperature control system appears the thermocouple is burnt out, the heater is broken, etc., which will lead to the molding dimension of the plastic parts is unstable. These faults can only be detected after taking targeted measures to eliminate.

5. The test methods and conditions are inconsistent

If the measurement method, time and temperature of plastic parts are different, the measurement size will be very different. Temperature has the greatest influence on the test, because the coefficient of thermal expansion of plastics is 10 times greater than that of metals. Therefore, must use the standard specified method and temperature conditions to determine the size of the structure of plastic parts, and plastic parts must be fully cooled to finalize the design before measurement. General plastic pieces in the mode of 10 hours of size change is very large, 24 hours to finalize the design.