Investigation into surface-coated continuous flax fiber-reinforced natural sandwich composites via vacuum-assisted material extrusion

In this study, vacuum-assisted material extrusion (similar to 1 Pa) is performed to eliminate matrix voids and improve the bonding quality of deposited layers by reducing heat loss and entrapped air during manufacturing. Continuous flax fibers with short polyester fibers/polylactic acid matrix sandwich composites (skin/core thicknesses of 2 and 25.4 mm, respectively) are fabricated via vacuum-assisted material extrusion. Compared with material extrusion, vacuum-assisted material extrusion yields higher bending and short-beam shear strengths by 36.6% and 27.2%, respectively; higher bending and short-beam shear stiffnesses by 12.3% and 9.5%, respectively; and higher averaged loading cycles at the failure stage by 47.3% and 101.5% in the three-point bending and short-beam shear loads, respectively. Vacuum-assisted material extrusion with surface coating is convenient, low cost, high in dimensional accuracy, and does not require reduced printing speed compared with conventional methods, thus satisfying the industrial targets of carbon emission reduction and high-volume production cleaning.