The fluidity of thermoplastics can usually be analyzed by a series of indicators, such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (processing length/plastic part wall thickness). Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, low apparent viscosity, high flow rate, and good fluidity. Plastics with the same product name must check their instructions to determine their flow whether it is suitable for injection molding.
According to mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:
- Good fluidity PA, PE, PS, PP, CA, poly(4) methylpentene:
- Medium fluidity polystyrene resin (such as ABS, AS), PMMA, POM, polyphenylene ether;
- Poorly fluid PC, rigid PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.
The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:
Higher material temperature will increase fluidity, but different plastics will be different, such as PS (especially plastic with high impact resistance and higher MFR value), PP, PA, PMMA, modified polystyrene ( Such as ABS, AS), the fluidity of PC, CA and other plastics will vary greatly with temperature. For PE and POM, the increase or decrease of temperature has little effect on their fluidity. Therefore, the former should adjust the temperature during the molding process to control fluidity.
As the injection molding pressure increases, the molten material bears greater shear force and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding.
The mold structure form, size, layout, cooling system design, flow resistance of the molten material (such as surface finish, channel cross-section thickness, cavity shape, exhaust system) and other direct influence factors affect the actual interior of the molten material in the cavity Fluidity, if the molten material is promoted to lower the temperature and increase the fluidity resistance, the fluidity will decrease. When designing the mold, a reasonable structure should be selected according to the fluidity of the plastic used. During the molding process, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust the filling conditions to meet the molding requirements.