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CHARACTERISTICS OF SHEAR VISCOSITY DURING EXTRUSION OF RUBBER BLENDS

During rubber profile extrusion unvulcanized rubber blends are pressed through a jet with specified geometry. For that purpose the raw material is melted and homogenized in a screw extrusion press due to heating and internal friction. Before the extrusion tool is reached the melt is heated within an injection head onto the desired temperature. The production speed of this continuous process is controlled by increasing the revolutions of the screw press. After the heated rubber material leaves the extrusion tool the shape of the produced profile is guaranteed by an assembly line whose speed is insignificantly higher as the speed of the extruder. The vulcanization process is completed in a salt bath. More details about the different parts of an extrusion line can be found in MICHAELI [2]. Due to the viscoelastic behavior heated rubber melts expand upon exiting from capillary tubes and extrusion tools, respectively. This swelling of the extrudate on emerging from a die is typical for such materials. Thus, the shape of extrusion tools for the fabrication of rubber profiles has to consider the respective swelling phenomenon inversely. Up to now extrusion tools are designed on the basis of empirical knowledge of the non-linear viscoelastic flow behavior. Therefore, the injection heads were designed for every specific rubber blend. Thus, the geometry of the sealing profile is obtained by empirical adaptation of the extrusion die. Thus, in rubber profile extrusion there is an increasing interest in predicting the deformation of profile shapes after leaving the tool of the extrusion line by means of numerical simulations. Usage of finite element software allows drastic reduction of the time-consuming adaptation process for the design of extrusion tools. For this task, the influence of an extrusion process on the shear strain rate dependent viscosity ( ) γ η & has to be determined. This knowledge cannot be identified adequately by means of standard experiments, such as capillary and torsional rheometry.