Fiber damage mechanisms in multi-die pultrusion of thermoplastic CF prepreg filaments

During the multi-die pultrusion process of continuous carbon fiber reinforced thermoplastic prepreg filaments, fiber buckling damage tends to occur with increasing pultrusion speed. To investigate the underlying mechanisms, the phase-change flow behavior during the pultrusion process was analyzed using a coupled Carreau non-Newtonian fluid model and a porous-medium flow model. An error-correction factor for the equivalent fiber bundle was introduced to establish a numerical framework for resin flow and fiber deformation in a multi-die pultrusion process. Based on this framework, resin viscosity, shear rate, and axial shear stress acting on the fibers under different processing conditions were quantitatively analyzed, revealing the mechanisms governing fiber deformation. Experimental validations from heat-transfer and fiber-tension measurements showed quantitative agreement with the simulation results, with a deviation within 14–16%. In addition, a robustness analysis of the heat transfer coefficient was conducted, showing a maximum deviation of only 5.03%, which confirms the reliability of the proposed model. A fiber-strain-based damage criterion was proposed by combining experimental results with the simulations. At a die temperature of 250°C, prepreg filaments were fabricated at pultrusion speeds of 4, 6, 9, and 13 mm/s. The average ultimate tensile force gradually decreased from 132.47 N at 4 mm/s to 120.40 N at 13 mm/s, corresponding to a cumulative reduction of about 9%, with each speed increment contributing 2–4% drops. By comparing experiments and simulations, a pultrusion speed of 6 mm/s was identified as the critical threshold for fiber damage, corresponding to an elastic strain of 8.8 × 10 −4 , and this strain-based criterion provides a quantitative basis for optimizing process parameters.