The fluidity of thermoplastics can be analyzed by a series of indices, such as molecular weight, melting index, flow length of Archimedes helix, apparent viscosity and flow ratio (flow length/wall thickness).
Low molecular weight, wide molecular weight distribution, poor regularity of molecular structure, high melting index, long spiral flow length, low apparent viscosity, high flow ratio, good fluidity. For plastics of the same name, it is necessary to check their specifications to determine whether their fluidity is suitable for extrusion moulding.
According to the requirements of die design, the fluidity of common plastics can be roughly divided into three categories:
The fluidity of PA, PE, PS, PP, CA and poly (4) methyl europiene is good.
Medium fluidity polystyrene resin series (such as ABS, AS), PMMA, POM, polyphenylene ether;
Low fluidity PC, rigid PVC, polyphenylene ether, polysulfone, polyaromatic sulfone, fluoroplastics.
The fluidity of various plastics also varies with the forming factors. The main influencing factors are as follows:
1. The fluidity of PS (especially impact resistant and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics varies greatly with temperature. The fluidity of PS (especially impact resistant and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics varies greatly with temperature. For PE, POM and temperature increase or decrease have little effect on their fluidity. Therefore, the former should adjust the temperature to control the fluidity when forming.
2. With the increase of pressure injection molding pressure, the melt material will be greatly sheared and its fluidity will increase, especially PE and POM are more sensitive. Therefore, the injection pressure should be adjusted to control the fluidity.
3. The form, size, layout, design of cooling system, flow resistance of molten material (such as surface finish, thickness of material section, shape of cavity, exhaust system) and other factors directly affect the actual fluidity of molten material in the cavity. The fluidity of molten material decreases when the temperature of molten material is lowered and the flow resistance is increased. The reasonable structure should be selected according to the fluidity of the plastics used in the die design.
The filling conditions can also be adjusted appropriately by controlling material temperature, mold temperature, injection pressure and injection speed to meet the forming requirements.