Investigating twist level control for modulating the impregnation process in high-quality 3D printed continuous flax fiber/PLA composites

Abstract

High-quality 3D printing of continuous flax fiber reinforced composites (CFFRCs) is a key priority for sustainable manufacturing. To enable low-void printing, a pre-impregnation process with tunable yarn twist level was introduced. Based on this method, pre-impregnated continuous flax filaments (PICFFs) with controlled twist level were fabricated. CFFRC specimens printed from PICFFs spanning 30–430 turns/m were evaluated and analyzed by microscopic images to quantify fiber orientation and void content, with their mechanical behavior being assessed by tensile testing with fractography. The effects of twist level control on fiber alignment, impregnation quality, and tensile properties were then investigated. The results show that as the yarn twist level decreases, voids in the yarn and between filaments are both reduced, and distributions of fiber orientation angles converge toward the print path. The internal void content and mean fiber orientation angle for the printed CFFRC specimen at the yarn twist level of 30 turns/m decrease by 69.7 % and 25° compared to the specimen fabricated from non-detwisted yarn, respectively. In addition, the dominant failure mode shifts from interfacial debonding and pullout to fiber fracture with matrix tearing. As the yarn twist level decreases from 430 turns/m to 30 turns/m, mechanical properties of the CFFRC specimen are improved accordingly, with the tensile strength and elastic modulus increasing by 21.6 % and 53.3 %, respectively. The method proposed in this work provides a novel and effective way of improving the performance of CFFRCs through controlling key process parameters.