SINGLE-SCREW EXTRUSION OF VISCOPLASTIC FLUIDS SUBJECT TO DIFFERENT SLIP COEFFICIENTS AT SCREW AND BARREL SURFACES

Commonly encountered viscoplastic fluids including concentrated suspen sions of polymeric and ceramic composites, foams, gels, concrete, food products, and energetic compounds exhibit wall slip during their flow and processing. For some viscoplastics fluids, especially highly fill ed suspensions, wall slip may dominate the flow and deformation and he nce the processing behavior of the suspension The wall slip velocity i s generally a function of the wall shear stress and temperature. Vario us factors including the materials of construction i.e., chemical natu re and the roughness of the wall surface affect the wall slip behavior of viscoplastic fluids. In this study an analytical model of the extr usion of viscoplastic fluids under isothermal and fully-developed cond itions in shallow channels is developed. The model accommodates the us e of different slip coefficients at barrel and screw surfaces. It thus permits the investigation of effects of introducing different materia ls of construction for the barrel and screw surfaces and development o f design expressions.