Calendering thermoplastic materials



A general equation for the true shear rate encountered in calendering non-Newtonian fluids is derived. Based on several constitutive equations (for a power-law, a three-constant Oldroyd, and a modified second-order Rivlin-Ericksen fluid), calendering is analyzed from the hydrodynamic point of view. The significance of dimensionless groups (the Deborah number, the Weisenberg numbers, and the viscoelastic ratio number), consisting of rheological and kinematic parameters, is discussed for scaling from prototype to production calendering. Correlation of experimental data obtained by using laboratory and production calenders is presented, and the scaling criteria obtained from the theory are examined. The onset of unstable flow, which causes non-uniform internal strain patterns (nerve) in calendered sheeting, is discussed in terms of the Weisenberg number. Good and poor calendering regions for a polymer are discussed qualitatively by using a dynamic response diagram, and the importance of the overall calendering conditions on the final sheeting quality is discussed.