In additive manufacturing processes, 3D parts with complex shapes are made by depositing melted resins and by laser-heated metallic and ceramics powders, layer upon layer, in precise geometric shapes using CAD or scanning data. These processes are to manufacture the 3D parts without dies and are mainly utilised for small production runs. The price of fused deposition modelling machines of plastic parts is considerably cheaper than that of selective laser meting and direct metal laser sintering machines of the metallic parts, and the application is largely expanding. In fused deposition modelling, a plastic filament is fed into a heated nozzle, then extruded through the nozzle, and deposited on a heated platform. The strength of the 3D-printed plastic parts is generally lower than that of injection moulding due to the pores included in the parts [1]. Sood et al. [2] investigated the effects of conditions of fused deposition modelling such as the layer thickness, orientation, etc. on the static strength of the parts. Dawoud et al. [3] evaluated the effect of the raster angle on the tensile, impact and bending strengths of 3D-printed parts. Turner et al. [4] reviewed the bonding behaviour of extruded plastic and its modelling in 3D printing. Although the strength of the 3D-printed parts is not very high, Nakamura et al. [5] exhibited the applicability of 3D-printed plastic dies to sheet metal forming processes such as bending and deep drawing.
To reduce the weight of automobiles, the use of CFRP (carbon fibre reinforced plastic) components increases. In resin transfer moulding for producing CFRP components, thermosetting resin is injected into a carbon fibre fabric set in a heated mould and is cured by holding for several minutes. Although the produced components have high strength, the long holding time and low resin penetration are drawbacks. These drawbacks are improved by hot stamping of composite sheets consisting of alternately laminated thermoplastic sheets and carbon fibre fabric. The composite sheets are heated, then press-formed, and solidified by cooling with dies at the bottom dead centre for several tens of seconds. Tatsuno et al. [6] investigated the effects of hot deep drawing conditions of composite sheets on the strength and quality of the formed parts. Okayasu and Sato [7] employed resistance heating for hot stamping of a composite sheet to improve the productivity, whereas the strength of the formed parts was comparatively low. Since the carbon fibres are not plastically deformed, it is not easy to hot-stamp parts with a complex shape due to the fibre fracture and fabric distortion.
Although fibres are manually placed to produce CFRP parts with a complex shape by autoclave moulding and resin transfer moulding, the automated fibre placement approaches have been developed [8], and the placement orientation and path for fabrication are optimised by numerical simulation. The tailored fibre placement is to optimise the mechanical properties of CRRP parts by partial control of fibre quantities and orientations [9]. Although thermosetting resin is generally employed for the automated fibre placement approaches, El-Dessouky et al. [10] applied thermoplastic resin to the reinforcement around machined holes.
CFRP parts begin to be produced by 3D printing. The CFRP parts are produced by simultaneously extruding bundled carbon fibres and plastic filament fed from different feeders through a nozzle and by moving the nozzle and platform in the planar and height directions, respectively [11]. Kabir et al. [12] reviewed recent development of 3D printing of CFRP parts. Blok et al. [13] indicated that the tensile strength of 3D-printed CFRP parts having the continuous fibres was about 9 times higher than that having short fibres. Yang et al. [14] showed that the bending and tensile strengths of CFRP parts having continuous fibres are about 2 and 5 times higher than those without the fibres, respectively. Since the strength of 3D-printed CFRP parts is generally lower than injection-moulded parts, Oztan et al. [15] observed microstructures of 3D-printed CFRP parts such as imperfect interfaces between printed layers, microvoids, incomplete filling, etc. Caminero et al. [16] evaluated the effects of the layer thickness and fibre volume content on the interlaminar bond strength of 3D-printed continuous carbon, glass and Kevlar® fibre reinforced nylon composites. Akhoundi et al. [17] indicated that the strength of 3D-printed CFRP parts with a 50% volume content of carbon fibres was almost in agreement with that calculated from the rule of mixture. Since the fibre orientation is aligned with the direction of the nozzle movement, it is not easy to control the direction requiring high strength for parts with a complex shape [18].
As another 3D printing process of CFRP parts, Mori et al. [19] sandwiched bundled carbon fibres between plastic layers without extruding fibres from a nozzle, and Nakagawa et al. [20] heated the printed part with microwave to increase the bond strength of the fibres to the plastic layers. In this process, it is easy to locally control the number and orientation of fibres, whereas microwave heating is not suitable for controlling the heating temperature. Laser heating is widely employed for welding, machining, forming, etc., and the temperature control is comparatively easy. Katayama et al. [21] developed a rapid lap joining process of metal and plastic plates without adhesives. Jaeschke et al. [22] bonded CFRP and GFRP (glass fibre reinforced plastic) sheets with laser transmission welding. Meng et al. [23] employed fibre laser for 3D printing of CFRP parts to heat an extruded composite. A laser power of several watts is sufficient to heat plastics, and the laser machines having such a power are cheap.
In this paper, laser-assisted 3D printing of CFRP parts with sandwiching fibres between plastic layers was developed to improve the strength of the parts. The bond strength of carbon fibres to the plastic layers was increased by laser heating. The effect of plastic colour in laser heating on bonding was examined. In addition, tailor CFRP parts with a closed cross-section were 3D-printed, and the deformation behaviour and bending strength were evaluated from the three-point bending test.