The nature of friction between large molecules when mol […]
The nature of friction between large molecules when molten plastic flows is called the viscosity of plastics, and the coefficient of viscosity is called viscosity, so the viscosity is a reflection of the flowability of molten plastics. The higher the viscosity, the stronger the melt viscosity. The worse the fluidity, the more difficult it is to process.
Comparing the fluidity of a plastic does not depend on its viscosity value, but on the size of its melt flow index (called MFI): the so-called MFI, that is, at a certain melting temperature, the melt under the action of the rated pressure, per unit time (Normally 10 minutes) The melt weight through a standard die. Expressed in g/10min, the viscosity of the plastic is not static, the characteristics of the plastic itself, external temperature, pressure and other conditions, can contribute to changes in viscosity.
First, the influence of molecular weight
The larger the molecular weight, the narrower the molecular weight distribution and the greater the reflected viscosity.
Second, the impact of low-molecular additive economy
Low-molecular-weight additives can reduce the interaction between macromolecules, thus reducing the viscosity. Some plastic moulding time adding solvent or plasticizer is to reduce the viscosity and make it easy to mould.
Third, the effect of temperature viscosity
The effect of temperature on the viscosity of most molten plastics is very large. Generally, the temperature rises, and the lower the reflected viscosity, the lower the viscosity of various plastic melts.
In PE/PP plastics, raising the temperature has little effect on improving the fluidity, reducing the melt viscosity, and the temperature is too high and the consumption is increased.
For PMMA/PC/PA and other plastics, the temperature rise viscosity decreases significantly, and the PSABS rise temperature has greater benefits for lowering the viscosity in moulding.