Investigation of additive manufacturing technology for in-situ reinforcement of polyetherimide with continuous carbon fibres

The need for lightweight structures in engineering applications is continuously rising and their benefits, such as economic and environmental, become increasingly desirable. Polymer matrix composite materials provide a feasible alternative to metallic parts reducing weight, due to their lower densities, without compromising mechanical strength [1]. In addition to materials, the manufacturing processes have also been under the microscope in an effort to reduce lead times and manufacturing steps from design conception to final product. Additive manufacturing is a still emerging technology that allows for the fabrication of prototypes as well as final products directly through a computer aided design model. Unlike traditional manufacturing processes that are subtractive, additive manufacturing reduces waste while producing near net shape parts [2]. The fabrication of polymer and polymer matrix composite components through additive manufacturing technologies has been repetitively attempted. The most popular technologies for those materials are selective laser sintering and fused filament fabrication. Ultimately, this research aims at the fabrication of continuous fibre reinforced high-performance polymer components and, hence, the modification of existing technologies will be required. This project focuses on fused filament fabrication due to the availability of open-source configuration firmware and system components. High-performance polymers such as poly(ether ether ketone), polyether ketone, polyphenylene sulphide, polyamide-imide, and polyetherimide were taken into consideration. Certain grades of polyetherimide when reinforced show great potential in replacing metallic parts for high temperature and high strength applications. ULTEM 1000 has extensively been used for injection moulding and extrusion as it is available only in pellet form. It is an unfilled general purpose grade material suited for the fabrication of interior automotive and aerospace vehicles where chemical resistance is required [3]. ULTEM 9085, which is a polyetherimide and polycarbonate compound, is an aerospace grade readily available in filament form and, hence, has attracted a lot of attention in additive manufacturing. It is commonly used for the low-volume production of highly-customised jigs and fixtures [4]. ULTEM 1010 is a recently developed pure grade of polyetherimide available in filament form, which makes it a great candidate for fused filament fabrication processes. It has a range of applications in different industry sectors from automotive and aerospace to medical and railway due to its superior performance. It has relatively high tensile and flexural strength [5] and it is an excellent candidate for this project due to limited research available on its manufacturability and performance. Its reinforcement with carbon fibre through additive manufacturing technologies is yet to be explored.  To understand the state of the art in fused filament fabrication a comprehensive literature review has been conducted. Through this review, the inherent limitations of this process, such as poor interlayer bonding and void formation, and proposed solutions have been identified. Additionally, state of the art technologies in the field of continuous fibre reinforcement of thermoplastic polymers have been reviewed. Different techniques employed for the improvement of the fibre/ matrix interface quality are, also, being investigated along with techniques for the improvement of interlayer bonding, such as the application of heat during and post processing. It is recognised that the existing technologies cannot successfully process high-performance polymers and therefore a custom system was developed, and a novel heating block was designed. The polymer matrix material, the carbon fibre tows, as well as their products are characterised. The development of a custom system and the challenges associated with such a task are also being discussed. The thermal, rheological, and mechanical properties of ULTEM 1010 were investigated to understand the processing conditions most appropriate and set the baseline for mechanical performance. The degradation and glass transition temperature of ULTEM 1010 were measured to be 514 and 212°C, respectively and steady shear as well as oscillatory rheology revealed an expected shear thinning behaviour with decreasing viscosity as temperature increased. The tensile and flexural properties of FFF-manufactured ULTEM 1010 specimens showed a dependence on process parameters, such as air gap, build direction, raster angle, and nozzle temperature and their interactions. Tensile strength and Young's Modulus were found to be between 60 and 94 MPa and 0.7 and 1.6 GPa respectively, and flexural strength and modulus between 62 and 151 MPa and 2.2 and 3.1 GPa respectively. The carbon fibres used were custom developed for the purposes for this project and therefore, the effectiveness of the different sizing methods used was evaluated through Fourier-transform infrared spectroscopy and thermogravimetric analysis. Surface treatment was imperative for the successful sizing of the fibres with polyetherimide resin, and the increased presence of resin adhered onto the carbon fibre tows was found to be desirable for better processability. A novel heating block design was conceptualised for the in-situ impregnation of those carbon fibres with ULTEM 1010 and the fabrication of specimens. The extrusion and deposition of reinforced filaments was achieved, however challenges, such as thermal insulation, material overflow, and backflow had to be overcome. The reinforced filaments achieved tensile strength up to 276 MPa, approximately three times that of the unreinforced filaments and the composite specimens yielded tensile strength of 240 MPa and Young's Modulus of 6.3 GPa, also notably greater than those of their unreinforced counterparts.