Non-isothermal non-Newtonian flow and interlayer adhesion in Large Area Additive Manufacturing polymer composite deposition.
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Heller, Blake P., 1989-
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Large Area Additive Manufacturing (LAAM) has seen substantial improvement in recent years due in large part to the addition of short carbon fiber reinforcement to the polymer feedstock. The addition of short carbon fibers improves the anisotropic mechanical and thermomechanical properties of printed beads which are dependent on the spatially varying orientation of carbon fibers. In this research, the finite element method is used to evaluate Stokes flow of a temperature dependent Carreau-Yasuda fluid for a two-dimensional planar flow field within a Strangpresse Model 19 LAAM extrusion nozzle. A shape optimization method is developed to zero the surface normal velocity to compute the polymer melt flow field geometry immediately after the nozzle exit during deposition onto a moving platform. Fiber orientation tensors are calculated within the extrudate using a custom software program which applies a rotary diffusion function to account for fiber interaction and the orthotropic fitted closure. The bulk mechanical and thermomechanical properties are calculated using the orientation homogenization method in the deposited bead. The computed thermal conductivity of the carbon fiber polymer composite that composes the deposited bead is then used to evaluate bead cooling for single and multi-bead printed layers which provides temperature along the interlayer bond line with respect to time. Infrared heating of printed beads, an addition to the current deposition process, is also modeled to determine the increase in temperature along the bond line for LAAM printing. Post deposition smooth and textured rolling are used to improve polymer bonding and increase bond area between interlayer surfaces. Interlayer bonding of printed beads with and without processing improvements are evaluated with shear testing of lap joint test specimens. Results show high fiber alignment in the longitudinal direction and increased thermal conductivity in all three dimensions of the fiber filled polymer beads which decreases the bonding time between layers and decreases bond strength. In addition, these deposition processing modifications increase bond strength by increasing temperature between layers, intimate contact between layers, and bonding surface area. Improved bonding between layers increases overall part strength and industrial viability of large area additively manufactured parts.