Squeezing flaw of viscoplastic fluids subject to wall slip

Polymer processing operations such as compression molding, sheet forming an d injection molding can be modeled by squeezing flows between two approachi ng parallel surfaces in relative motion. Squeezing flows also find applicat ions in the modeling of lubrication systems, and in the determination of rh eological properties. Here, analytical solutions are developed for the cons tant-speed squeezing flow of viscoplastic fluids. It is assumed that the fl uid is purely viscous, and hence viscoelastic effects unimportant. The rheo logical behavior of the viscoplastic fluids is represented by the Herschel- Bulkley viscosity function. The deformation behavior of commonly encountere d viscoplastic fluids is generally complicated by the presence of wall slip at solid walls, which is a function of the wall shear stress. The slip coe fficient that relates the slip velocity to the shear stress is affected by the material of construction and also the roughness of the solid surfaces, leading to the possibility of different slip coefficients at various solid surfaces. The model developed in this study accommodates the use of differe nt slip coefficients at different solid surfaces. The accuracy of the solut ions is established, and the effects of various parameters such as slip coe fficient and apparent yield stress are examined. The solutions provide usef ul design expressions that can be utilized for squeezing flows of viscoplas tic fluids, with or without wall slip at the solid boundaries.