Abstract: When shear viscosity of polymer melts under micro-scale effect is studied, different shear viscosity variations are obtained by using various entrance correction methods, which is of significant importance for micro forming theory and technology. Shear viscosities of four polymers, including high density polyethylene(HDPE), polypropylene(PP), polymethylmethacrylate(PMMA) and polystyrene(PS), are investigated under the same experimental condition by both orifice correction and Bagley correction. Capillary dies with diameters of 1 000 μm, 500 μm and 350 μm are used. Experimental results show that the shear viscosity variations with die diameter alteration of PMMA and PS obtained by the two methods are opposite, which indicates that there are limitations when traditional entrance correction method is utilized in the measurement of micro-scale viscosity. Thus an improved testing approach is developed considering the pressure effect under micro-scale, based on the characteristics of the entrance converging flow. And the authentic variations of the shear viscosity with different die diameters are presented for the four polymers. The results show that the shear viscosities decrease with the decrease of die diameter for all the four materials in the test shear rate range, and the change amplitude is from 9.9% to 38.3%. The mechanism of different viscosity variation degree of the four materials is revealed from the perspective of molecular structures. Based on the phenomenological theory, a viscosity model, suitable for both macro and micro scales, is established. Good agreement between the model predictions and experimental data is obtained with an average error less than 3.7%, confirming the validity and efficiency of the proposed model.

Key words: Entrance correction, Microchannels, Polymer melts, Shear viscosity, Viscosity model