Structure-property Relationship of 3D Printed Polymers and Fibre-reinforced Composites

Abstract

Fused filament fabrication (FFF) has been utilised in various applications as it provides a cost-effective and flexible method to fabricate products in low volume. In carrying out both microstructural and mechanical characterisations, this study systematically elaborates the process-structure-property relationships of 3D printed thermoplastics, short fibre reinforced polymers and continuous fibre-reinforced polymer composites fabricated by the FFF process. The work is of great importance as it yields in-depth insight into the mechanisms of the novel manufacturing process, providing valuable guidance to improve the process so that high-performance parts can be fabricated and used in applications with critical load-bearing capacity requirements. This study starts from comprehensive experimental investigations into 3D printed bulk specimens of semi-crystalline polymers and short fibre reinforced polymer composites, respectively, with structural analyses performed to characterise the mechanisms of the FFF process. It is found that the mechanical properties of the printed materials suffer a dramatic loss in elastic modulus and strength compared with the benchmark materials with further compression moulding (CM) process. There is an urgent need for a method to accurately characterise the fusion bonding formed between individual filaments during the FFF process. Hence, a novel approach based on the essential work of fracture (EWF) concept was developed and examined in this study, to quantify fusion bonding by determining the value of specific essential work of fracture (w_e). The difference in fusion bonding between the in-plane deposition mode and the in-thickness deposition mode was identified. Additionally, compared with the results of experimental characterisation for printed films with further CM, the dramatically decreased values of w_e and its coefficient of determination (R2) of the 3D printed films indicated the formation of poor fusion bonding during the FFF process. The EWF method was then extended to characterise the fusion bonding between individual filaments of 3D printed short carbon fibre reinforced polyamide 6 (SCF/PA6) composite films fabricated in the two deposition modes. The fusion bonding in the in-thickness mode was found to be much weaker than that in the in-plane mode, a result not only of the narrower processing window but also of the formation of voids between the filaments. In-depth studies were performed to reveal the effects of short fibres on the specific essential work of fracture of the 3D printed composites, addressing the complex relationships between the matrix, fibre-matrix interfaces, fibre orientation, etc. Continuous carbon fibre reinforced polymer composites with significantly improved mechanical performance were fabricated using the FFF process followed by comprehensive characterisations. In comparisons with the benchmark with further CM, the tensile, flexural and Mode I interlaminar fracture properties of the 3D printed composites showed a considerable amount of reduction because of the formation of microscopic voids. Further investigations of the substantial impact of voids on failure mechanisms were undertaken by microstructural analyses, with factors contributing to the formation of voids identified. A discussion of the quality of fusion bonding for printed thin composite films was also presented to highlight the issues. A dynamometer was successfully integrated with a 3D printing platform to achieve in-process monitoring of three-dimensional forces applied during the deposition process, providing much deeper understanding of the process-structure-property relationships by analysing a mechanistic model of intimate contact and autohesion. It was found that the compaction force played a critical role in determining the printing quality of the printed parts. This part of the study also highlighted the possibility of achieving intelligent manufacturing by integrating smart sensors into the FFF process.