Chapter 8 Rheology of long fiber-reinforced composites in sheetforming
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This chapter introduces the reader to sheet forming philosophy, processing methods, and the materials applied to produce thermoformed components. Rheological properties that must be understood are introduced first for unfilled polymer metls by discussing viscosity and relaxation phenomena in shear and elongational flows. With this foundation we present the same properties for filled viscous fluids and we show the sometimes surprising results such as shear thickening. Filler aspect ratio affects shear and elongation phenomena; this presentation shows these effects and lists the theories developed to account for the filler/matrix interactions. Following the background material, the section on rheological measurement techniques discusses standard techniques for unfilled materials (rotational and capillary flow viscometers) and their limited use with filled polymers. The presented nonconventional apparatus (linear, squeeze flow and elongational viscometers) reveal the scale issues involved in measuring rheological properties of thermoforming systems. The chapter concludes by considering why the rheological properties are important and how to use them in sheet forming. Mechanisms that affect forming flows (resin percolation, transverse squeezing flow, axial intra-ply shear deformation, inter-ply shear deformation extensional flow) show the complexity of the process. Finally, we deliberate the outlook for application of thermoforming and improvement of the knowledge base of system behavior. 1997 Elsevier B.V. All rights reserved.
