Experimental Studies On The Rheological Characteristics Of Polymer Melts

Since the molding for most polymers is in the molten state,the study of rheological characteristics of polymer melts is important to establish the constitutive equations, determine the molding process parameters and improve the quality of molding plastic parts. Capillary rheometer is widely used in the study of the rheological characteristics of polymer melts, but most researchers use a unitary die diameter and usually more than 1 mm. In recent years, micro-injection molding technology has developed rapidly. The flowing behaviour in the filling process for micro-injection molding is different with traditional injection molding owing to the small size of cavity and channel, and the relatively large surface area. By a double capillary rheometer, the viscocity properties and elastic properties of four polymer melts (PS, PMMA, PP and HDPE) are investigated under the die diameter from 1.5 mm to 0.5 mm, in addition, wall slip during the flow of polymer melts is studied.First, the effects of the die diameter on the shear viscosity of polymer melts are analyzed. The effects of temperature on the shear viscosity and the effects of temperature and shear rate on the non-Newton exponent are discussed at the die diameter of 0.5 mm. The results show that the shear viscosity of PS and PMMA increases with the decrease of die diameter, while the shear viscosity of PP and HDPE decreases with the decrease of die diameter. When the die diameter is 0.5 mm,Arrhenius equation is found to be suitable to describe the relations between shear viscosity and temperature for four polymer melts. The non-Newton exponent of four polymer melts increases with the increase of temperature and decreases with the increase of shear rate.Second, the elastic behavior of polymer melts is studied by the entrance pressure drop and the stress relaxation time. The effects of the die diameters on the entrance pressure drop and on the stress relaxation time are analyzed. The effects of temperature on the entrance pressure drop and on the stress relaxation time are discussed at the die diameter of 0.5 mm. The results show that with the decrease of die diameter, the elastic deformation of polymer melts increases,and with the temperature rise, the elastic deformation of polymer melts decreases.Finally, the wall slip of polymer melts is researched. The effects of shear stress and temperature on the wall slip velocity are discussed. The effects of wall slip on the flow of melts are analyzed. The Garreau model parameters are obtained by fitting test data eliminating wall slip. The results show that the slip velocity increases as an exponential function with the increase of shear stress. The critical shear stress of wall slip decreases with the rise oftemperature. The wall slip will reduce flow friction for PP and HDPE, while will increaseflow friction for PS and PMMA, and the smaller the die diameter is, the more obvious effectis.