Teacher Dan Dan from Dongguan Hengli Xiaosa Vocational Training School shared to avoid uneven shrinkage of products due to poor mold design
When designing precision injection molds, if the mold structure or temperature control system is not properly designed, it is easy to cause uneven shrinkage of plastic parts. This phenomenon will have a negative impact on the accuracy of plastic parts and the stability of plastic parts. In order to avoid such problems, the gating system and temperature control system design need to pay attention to the following points.
① Although the pouring system should keep the material flow balanced in the multi-cavity injection mold, if the plastic melt flowing through the pouring system cannot reach and fill the cavity at the same time, that is, an unbalanced pouring system is used, the melt is in each The pressure received in the cavity will be different, so the shrinkage rate will often vary greatly between the plastic parts formed in each cavity of the same mold. Therefore, the pouring system of the multi-cavity precision injection mold should adopt a balanced layout . The arrangement of the cavity not only affects the balance of the gating system, but also has a great relationship with the thermal balance of the mold temperature field. Obviously, if the temperature field of the mold cannot maintain thermal equilibrium, the shrinkage rate of the plastic part cannot be kept uniform and stable, so precision injection molding is impossible. Practice has proved that in order to ensure the thermal balance of the temperature field and the material flow balance of the gating system, when designing a multi-cavity precision injection mold, the number of cavities ffl: should not exceed 4 in the world.
② The temperature control system should be able to individually adjust the temperature of each cavity. When using multiple cavities for precision injection molding, in order to keep the temperature of each cavity consistent, try to prevent the shrinkage of the plastic part caused by the temperature difference of the cavity. The temperature control system in the mold should be able to adjust each cavity individually, that is, a separate cooling water channel is provided for each cavity, and a flow control device should be set at the outlet of the cooling water channel of each cavity to enable Keep the flow of each water channel consistent. Generally speaking, the cooling water temperature adjustment accuracy in the precision injection mold should be below 2#C, and the temperature difference between the water inlet and the water outlet should be controlled within 2°C. In addition, in precision injection molds, it is best to design cooling channels according to the shape and structure of the product. Otherwise, the shrinkage rate of the plastic parts may vary greatly, and the accuracy of the plastic parts cannot be guaranteed. The molding temperature of precision injection molding includes not only the melt temperature and mold temperature, but also the ambient temperature. During precision injection molding, the exhaust fan is prohibited in the workshop and the indoor temperature needs to be controlled at about 27°C.
19.5.3 Avoid product deformation due to mold problems
The size of precision injection plastic parts is generally not too large, the wall thickness is relatively thin, and some have many thin ribs, so it is easy to deform during demolding, which will inevitably cause the accuracy of the plastic parts to decrease. In order to avoid demoulding deformation of plastic parts, the design of demoulding ejection mechanism should pay attention to the following aspects.
①Precision injection plastic parts should preferably be demoulded using pusher plates, which helps prevent the plastic parts from deforming during demolding. However, if the ejector plate cannot be used for demolding, other suitable demolding and ejecting mechanisms must be considered. For example, for rectangular plastic parts with thin ribs, in order to demould the plastic parts smoothly and prevent deformation, a circular push rod with a small diameter or a rectangular push rod with a small width can be used in the ribs, and at the same time, balance Configuration.
②The demolding angle of precision injection molded parts is generally relatively small, and it is not easy to demold. In order to reduce the demolding resistance and prevent
Plastic parts are deformed during demolding. Appropriate technical requirements must be put forward for the processing method of the demolding part, and the roughness of the molded parts of the envelope part of the plastic parts must be appropriately reduced. The mode direction is the same.
19.5.4 For products with complex shapes, an insert structure should be adopted
High-precision parts generally need to be ground, but due to the shape limitation, it is best to use the insert structure, which is not only beneficial to grinding processing, but also beneficial to exhaust structure design and heat treatment. The structure design of the inserts should pay attention to the following points: It is convenient for grinding and processing, and the positioning structure of each insert is reliable, which is convenient for assembly, maintenance and replacement. The insert design should ensure that the mold has sufficient rigidity and strength, and the frame of the fixed insert should be designed. The insert design should preferably adopt a general structure or a standard structure.