Analysis of mixing efficiency in continuous polymer processing devices

Co-rotating twin screw extruders are widely used for compounding and blending in polymer processing. In a modular machine, conveying elements are used for pumping, whereas the dominant elements determining dispersive mixing efficiency are the kneading discs. In this study a flow field analysis was performed for the kneading discs region and the conveying elements of a Werner & Pfleiderer ZSK-53 co-rotating twin screw extruder. A fluid dynamics analysis package - FLDAP, using the finite element method was implemented to simulate the 3-D isothermal flow patterns.; The kneading discs simulated were three-lobe discs. The material used in the simulations was high density polyethylene characterized by a power law model. The flow field characteristics relevant for dispersive mixing are shear stresses generated and elongational flow components. We discuss the influence of processing parameters on the flow field mixing efficiency. We also compare the flow characteristics for the three-lobe kneading discs of the ZSK-53 with the two-lobe discs of a ZSK-30 machine.; Distributive mixing was studied in the conveying elements of the ZSK-53. The dominant distributive mixing mechanism in polymer processing is convection. Convection, involving movement of fluid elements/particles from one spatial location in the system to another, results in the creation of interfacial area or minor component distribution throughout the major component. Particle distributions were obtained by placing small particle clusters in the flow field and tracking particle motion.; Using the particle tracking algorithm, we defined and calculated indexes for distributive mixing efficiency. Length stretch distributions were used to quantify the capability of the equipment to spread minor component particles originally gathered in clusters. Strain distributions are reflective of material stretch history. We also introduced the calculation of interfacial area stretch. This index can give very interesting information about the material stretching and folding in the device.; Chaotic flow is inherently related to mixing. We looked at some features of chaotic flow in a single screw extruder and tangential twin screw extruder. We found a limited degree of chaotic motion in both these devices. The chaotic features of flow in these devices correlate well with the distributive mixing efficiency of the equipment quantified in terms of length stretch distributions and average length stretch values. However, the presence of some chaotic features of flow in a device is no guarantee for good distributive mixing.