Effect of Fountain Flow on Fiber-Matrix Separation in Fiber Reinforced Injection Molded Parts

The purpose of this project is to investigate the occurrence of fiber-free regions at the outer surfaces of fiber reinforced thermoplastic resin during injection molding. Most people hypothesize that the fiber-free skin in injection molded parts is an effect caused by shear migration; however, since the material freezes at the instance of contact with the mold wall as the molten fluid continues to advance through the mold via fountain flow, hindering shear migration all together. Additionally, other researchers theorized that shear between fibers is what causes fiber-free occurrences on the surfaces of injection molded parts. This theory is disputed by experimental work presented here where it has been proven that fibers never actually come into contact with each other. In fact, fibers are observed to constantly be covered by a thin layer of fluid. Hence, the nature of the flow field at the flow front in conjunction with the fountain flow effect leads to fiber-free surfaces (upper and lower) on the final injected part. Furthermore, the skin-core issue is attributed to a combination of lubrication forces (hydrodynamic friction) between the fibers and Stokes forces (hydrodynamic drag).;This project utilizes analytical, experimental, and computer simulation approaches to better understand the behavior of fiber-filled polymer melts during the injection molding process. The analytical equations will provide a better understanding of the forces and parameters involved in this particular flow are developed first. This is achieved by performing dimensional analysis and applying Newton's equations of motion to analytically demonstrate the fiber interaction with the polymer mix and neighboring fibers. Furthermore, experimental tests visually revealed the behavioral movement of carbon fibers with different aspect rations mixed with the Newtonian thermoplastic resin (silicon oil) by mimicking the mold filling process of an injection molding machine. Also, microscopic images of cross-sectional samples were examined to highlight the filling pattern of fiber-filled resins, and the fiber distribution along the surfaces of the final part. Finally, the utilization of numerical simulations enabled the simulations of two- dimensional linear elastic thermoplastic behavior of fiber-filled polymer matrix with isotropic properties in an injection molding process.