How to keep the size of cosmetic packaging injection products stable?

August 28,2021

Injection molding process is the primary process in the cosmetic packaging material, in the process of injection molding products often encounter the size of the unstable situation, the size of cosmetic 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 cosmetic container product size is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design and changes in material performance.

1. Improper selection of molding materials
The shrinkage rate of cosmetic packaging 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 cosmetic packaging 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.

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2. 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.

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.
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.

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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.

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