Thixotropic flow of toothpaste through extrusion dies

A commercial toothpaste is investigated in this work as a model paste system to study its processing characteristics in capillary flow using various dies. Its rheological behaviour has been determined as that of a yield-stress, thixotropic material with a time-dependent behaviour, and severe slip at the wall. The rheological data obtained from a parallel-plate rheometer were used to formulate a constitutive equation with a structural parameter which obeys a kinetic equation, typically used to model thixotropy. The predictive capabilities of this model are tested against capillary data for a variety of capillary dies having different length-to-diameter ratios (L/D), contraction angles (2 alpha), and contraction ratios (D-b/D)(2), where D-b is the diameter of the barrel of the capillary rheometer. The major trends are well captured by the thixotropic model and show that slip is the essential parameter in predicting the flow behaviour of toothpaste. (C) 2011 Elsevier B.V. All rights reserved.