Components made from thermoplastic composites (TPC) by additive manufacturing (AM) and automated fiber placement (AFP) are gaining popularity because of the material system’s inherent advantages over thermosets, but high material costs and limited production flexibility are hindering more widespread use. This paper discusses prototyping, fluid flow modeling, and testing of a novel in situ preimpregnator (prepregger) for lower-cost TPC feedstock used in AM and AFP. Prior attempts by researchers to address this technical need have generally resulted in low-quality prepreg with low fiber volume fraction (V_f) and high void content, which can be explained by Darcy’s law applied to the highly viscous polymer melts. Uniformly spreading the fiber tow thin before impregnation is the key to rapid and complete impregnation. Based on initial experiments with a flexible experimental setup to quickly test concepts, a prototype prepregger consisting of dry fiber payout reel with slight torsional resistance, fixed pin fiber spreader, double slot die coater fed with polymer melt from a microextruder, consolidator with pultrusion exit die, and prepreg downstream drive mechanism was designed and built. Predictions from a steady-state flow model of the coating die/microextruder system assuming creeping flow and Newtonian fluid behavior agreed well with experimental flow measurements for a variety of polymer melts and operating conditions. Once ideal input parameters were determined, prepreg tape specimens exhibited complete impregnation but low V_f and excessive encapsulation due to insufficient consolidation and removal of excess resin. Experiments where pultrusion dies were used for filament manufacturing showed significantly improved V_f and prepreg quality. Future work is suggested to facilitate commercialization of this promising process technology.