Thermoplastic TPO have different levels of fluidity, which can be analyzed based on a series of indices such as molecular weight, melt flow index, Archimedes spiral flow length, apparent viscosity, and flow ratio (flow length/wall thickness of the plastic part). Plastics with small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt flow index, long spiral flow length, small apparent viscosity, and large flow ratio have better fluidity. For plastics of the same brand, their data sheets must be checked to determine if their fluidity is suitable for injection molding. According to the requirements of mold design, the fluidity of commonly used plastics can be roughly divided into three categories:
Good fluidity: nylon, polyethylene, polystyrene, polypropylene, cellulose acetate, poly(4-methylpentene);
Moderate fluidity: polystyrene resin series (such as ABS, AS), organic glass, polyoxymethylene, and polyphenylene ether;
Poor fluidity: polycarbonate, rigid polyvinyl chloride, polyphenyl sulfone, polysulfone, polyarylsulfone, and fluoroplastics.
Temperature. Higher temperature increases fluidity, but different plastics have different characteristics. The fluidity of polystyrene (especially impact-resistant type and with higher MFR value), polypropylene, nylon, organic glass, modified polystyrene (such as ABS, AS), polycarbonate, cellulose acetate, and other plastics varies greatly with temperature. The fluidity of polyethylene and polyoxymethylene is less affected by temperature changes. Therefore, the temperature should be adjusted during molding to control fluidity for the former.
Pressure. With increasing injection pressure, the melt is subjected to a greater shear force, increasing its fluidity, especially for polyethylene and polyoxymethylene, which are more sensitive. Therefore, the injection pressure should be adjusted to control fluidity during thermoplastic TPO molding.
Mold structure. The form, size, layout of the gating system, design of cooling system, flow resistance of melt materials (such as surface smoothness, thickness of runner section, cavity shape, and exhaust system), directly affect the actual fluidity of the melt in the cavity. Anything that reduces the temperature and increases the flow resistance of the melt will reduce its fluidity.
During mold design, a reasonable structure should be chosen based on the fluidity of the plastic used. During molding, the filling situation can be controlled by adjusting the material temperature, mold temperature, injection pressure, and injection speed to meet the requirements of molding.