The Role of Rheology in Polymer Extrusion （在聚合物挤出流变学的作用）.pdf

The Role of Rheology in Polymer Extrusion John Vlachopoulos Department of Chemical Engineering McMaster University Hamilton, Ontario, Canada E-mail: vlachopj@mcmaster.ca David Strutt Polydynamics, Inc. Hamilton, Ontario, Canada 1.0 Rheology Rheology is the science of deformation and flow of materials [1]. The Society of Rheologys Greek motto Panta Rei translates as All things flow. Actually, all materials do flow, given sufficient time. What makes polymeric materials interesting in this context is the fact that their time constants for flow are of the same order of magnitude as their processing times for extrusion, injection molding and blow molding. In very short processing times, the polymer may behave as a solid, while in long processing times the material may behave as a fluid. This dual nature (fluid-solid) is referred to as viscoelastic behavior. 1.1 Viscosity and Melt Flow Index Viscosity is the most important flow property. It represents the resistance to flow. Strictly speaking, it is the resistance to shearing, i.e., flow of imaginary slices of a fluid like the motion of a deck of cards. Referring to Figure 1.1, we can define viscosity as the ratio of the imposed shear stress (force F, applied tangentially, divided by the area A), and the shear rate (velocity V, divided by the gap h) SHEAR STRESS F / A   (1.1) = = =  SHEAR RATE V / h 