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Author: MULAN -Plastic Molding Manufacturer
When the molten plastic flows, the frictional property between macromolecules is called the viscosity of the plastic, and the coefficient of this viscosity is called the viscosity, so the viscosity is a reflection of the fluidity of the molten plastic. The higher the viscosity, the stronger the melt viscosity, the worse the fluidity and the more difficult the processing. Mold injection molding manufacturers will bring you a detailed introduction below.
Comparing the fluidity of plastics is not based on its viscosity value, but on its melt flow index (MFI): the so-called MFI means that at a certain melting temperature, the melt is subjected to a certain pressure, and per unit time (usually 10 min) weight of the melt passing through a standard die. Expressed in g/10min, the viscosity of plastics is not static. Changes in plastic properties, external temperature, pressure, and other conditions can cause changes in viscosity.
The influence of molecular weight: the larger the molecular weight, the narrower the molecular weight distribution, and the greater the reflected viscosity. The role of low molecular weight additives: Low molecular weight additives can reduce the force between macromolecules. Therefore, the viscosity decreases.
When some plastics are molded, solvents or plasticizers are added to reduce the viscosity and make them easier to shape. Effect of Temperature and Viscosity: Temperature has a great influence on the viscosity of most molten plastics. Generally, the higher the temperature, the lower the reflected viscosity, but the viscosity of various plastic melts decreases in different degrees.
For PE/PP plastics, increasing the temperature has little effect on improving fluidity and reducing melt viscosity. If the temperature is too high, the consumption will increase, but the loss outweighs the gain. For plastics such as PMMA/PC/PA, the viscosity decreases significantly when the temperature rises, while the temperature rise of PSABS also has great advantages in reducing viscosity and molding.
The influence of shear rate: Effectively increasing the shear rate of plastics can reduce the viscosity of plastics, but there are some plastics, such as PC, whose viscosity is hardly affected by the screw speed. Effect of pressure: The effect of pressure on viscosity is more complicated. In general, PP&The viscosity of PE is not greatly affected by pressure, but the effect on PS is quite significant.
In actual production, on a machine with better equipment, attention should be paid to high-speed injection, that is, the effect of high shear speed, and the pressure should not be increased blindly.
