Plastic fluidity
In the molding process, the ability of the plastic melt to fill the mold cavity at a certain temperature and pressure becomes the fluidity of the plastic. The quality of plastic fluidity directly affects the molding process parameters to a great extent, such as molding temperature, molding pressure, molding cycle, mold pouring system size and other structural parameters. When determining the size and wall thickness of plastic parts, the influence of fluidity must also be considered. Plastic processing requires a certain temperature to melt plastic powder or particles, and then processed into plastic products of a certain shape through extrusion, injection molding, blow molding, blown film, etc. The fluidity of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length / plastic wall thickness). Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small apparent viscosity, and large flow ratio, good fluidity; according to mold design requirements, the fluidity of commonly used plastics can be roughly divided There are three categories.
Good fluidity: PA, PE, PS, PP, CA, nylon, polyethylene, polystyrene, polypropylene, cellulose acetate, poly (4) methylpentene.
Medium fluidity: polystyrene series resin (such as ABS, AS), organic glass, polyoxymethylene, polyphenylene ether;
Poor fluidity: polycarbonate, rigid PVC, polyphenylene oxide, polysulfone, polyarylsulfone, fluoroplastic.
Fluidity generally refers to fluid, so fluidity in plastic refers to fluidity in the molten state during processing. If the fluidity is poor, the shape of the processed plastic products is irregular, the internal pressure of the processing equipment is too large, the production continuity is poor, and the working efficiency is low. Generally, this problem can be solved by changing the processing temperature and adding appropriate additives. The flowability of various plastics also changes due to various molding factors. The main factors affecting the following are as follows:
①The fluidity increases when the temperature is high, but different plastics also have their own differences, PS (especially impact resistance type and MFR value is higher), PP, PA, PMMA, modified polystyrene (such as ABS, AS) , PC, CA and other plastic fluidity changes greatly with temperature. For PE and POM, the increase or decrease of temperature has little effect on its fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.
②Pressure injection molding pressure increases, the molten material is subject to shearing effect, and the fluidity also increases, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding.
③The form, size, layout, cooling system design, flow resistance of molten material (such as cavity surface finish, channel thickness, cavity shape, exhaust system) and other factors directly affect the molten material in the mold Actual fluidity in the cavity.
The mold design should be based on the fluidity of the plastic used to design a reasonable structure. During molding, the injection molding process can also be adjusted by controlling the molding temperature, injection pressure and injection rate to meet the requirements for the quality of plastic parts.
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