Epoxy/Steel Fiber Composites—A Simple Model to Predict the Fiber Sedimentation J.L. Leite, 1 M.G. Rasteiro, 2 G.V. Salmoria, 3 C.H. Ahrens, 3 A.S. Pouzada 1 1 IPC/Institute for Polymers and Composites, Department of Polymer Engineering, University of Minho, Guimara ˜ es 4800-058, Portugal 2 Chemical Engineering, Coimbra University, Po ´ lo II, Pinhal de Marrocos, Coimbra 3030-290, Portugal 3 CIMJECT, Departamento de Engenharia Meca ˆ nica, Universidade Federal de Santa Catarina (UFSC), Floriano ´ polis 88040-900, SC, Brazil Sedimentation of short steel fibers (SSFs) is an impor- tant phenomenon observed in the manufacture by casting of polymer/metal composites. Modeling of the fiber sedimentation has been a subject of research but hardly applied in the context of injection molds. In this study, the sedimentation velocity of the SSF suspen- sion in nonreactive epoxy resin was evaluated theoreti- cally and experimentally. The sedimentation behavior of single and concentrated SSF in epoxy resin was fol- lowed experimentally to obtain the terminal and sedi- mentation velocities. These data were interpreted using theoretical models that take into account the hindered settling factor and the shape factor. The ex- perimental data can be correlated with the theoretical analyses. The findings were used to propose a simple model to predict the sedimentation of steel fibers in viscous resins that are used in composites for making molding blocks. POLYM. COMPOS., 31:1378–1386, 2010. ª 2009 Society of Plastics Engineers INTRODUCTION Composites based on epoxy resin and metal fillers are being increasingly used for molding blocks of hybrid injection molds. Sabino-Netto et al. used short steel fibers (SSFs) to improve mechanical and thermal properties that are relevant for the performance of molding blocks [1]. These authors and Brito and Sa ´nchez [2] in their studies with epoxy composites filled with SSF pointed out that the incorporation of SSF increases the tensile strength, the elastic modulus, and the impact strength. The sedimentation process, in which the dispersed par- ticles settle out in a fluid as a consequence of gravita- tional force, is of great practical importance when compo- sites with high density fibers are cast and before the resin polymerizes. Coe and Clavenger [3], and Kynch [4] developed the first studies to describe theoretically the process. According to Mackaplow and Shaqfeh [5], the sedimentation of isolated particles can be described theo- retically or observed experimentally, but the effect of the interaction of particles and the sedimentation velocity should be given more attention. In a concentrated suspen- sion, the settling velocity is influenced by the interparticle forces. This is known as hindered settling, which reduces the particle settling velocities due to the interactions with surrounding particles and with the fluid up flow [6, 7]. According to Coe and Clavenger [3], the sedimentation of concentrated suspensions consisting of particles of equal densities develops into four phases during the sedimenta- tion process. Figure 1 shows a scheme of the sedimentation process that consists from the top downwards: (A) Clear liquid, (B) suspension of smaller particles and discrete par- ticle settling, (C) hindered settling (aggregated particles and suspension of particles of both sizes with concentration equal to the initial value), (D) transition zone (particles resulting from aggregation of individuals particles), and (E) the sediment (compression zone). Mirza and Richard- son [8] extended this concept to the case of a suspension of particles of two or more sizes. More recently, Flotats [9] developed a mathematical model for these situations. The manufacture of composites reinforced with par- ticles of density higher than the fluid (e.g., metal or ceramics in a polymeric fluid) is influenced by the grav- ity. This phenomenon, as reported by various authors [10–12], prevents the uniform distribution of the par- ticles that are incorporated as reinforcement in the final product. Correspondence to: A.S. Pouzada; e-mail: asp@dep.uminho.pt Contract grant sponsor: Portuguese Foundation for Science and Technol- ogy, Brazilian Ministry of Education (Covenant FCT-CAPES). DOI 10.1002/pc.20923 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMERCOMPOSITES—-2010