Composite upcycling: An experimental study on mechanical behaviour of injection moulded parts prepared from recycled material extrusion printed parts, previously prepared using glass fibre polypropylene composite industry waste

Upcycling pre-existing material extrusion (MEX) fabricated products is nascent as the industry's broader adaptation of MEX printers can be observed. This manuscript elucidates the post-process injection moulding manufacturing technique applied to MEX-printed thermoplastic composite waste materials composed of industrial glass fibre-reinforced polypropylene (GFRPP) composite. It evaluates the impact of glass fibre weight fractions (15%, 30%, and 40%) on the injection moulded specimens' flexural and ultimate tensile strength. The mechanical performance of the injection moulded parts was compared with MEX additive manufactured parts derived from identical material. Homogeneous glass fibre distribution within the thermoplastic matrix was discerned in the injection moulded components utilising an optical light microscope. The mean ultimate tensile strength of the injection moulded parts was 29 MPa for the samples exhibiting a 40% fibre weight fraction. It was demonstrated that the increase in glass fibre content from 15% to 40% improved the tensile strength, flexural strength, and elastic modulus of injection moulded specimens by 52%, 135%, and 52%, respectively. Scanning electron microscopy (SEM) of the fracture surface of the injection moulded specimens revealed that the glass fibres predominantly displayed fibre pull-out rather than fibre breakage. Subsequent SEM imaging indicated that in contrast to the MEX printing manufacturing technique, the injection moulding fabrication process of GFRPP generally reduced porosity and minimised internal defects. The findings indicated that injection moulding post-process manufacturing possesses superior mechanical attributes compared to specimens fabricated through MEX printing. The results have substantiated the successful transformation of waste industrial GFRPP material into standardised specimens through the injection moulding fabrication technique. This study shows the potential of upcycling printed engineering-grade components via injection moulding, thereby converting them into functional composites within a circular economy framework.