PSI - Issue 25

S. Valvez et al. / Procedia Structural Integrity 25 (2020) 394–399 S.Valvez et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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3. 3D printed continuous carbon fibre reinforced PLA composites

An alternative to conventional plastics and steel is the use of continuous fibre reinforced thermoplastic composites. These materials have excellent mechanical performance, recyclability and potential use on light-weight structures (Chen et al. 2011; Fujihara et al. 2004). The most common composites combine, for example, continuous carbon fibres and PLA matrix, in filamentary shape, that can be used in the fusion deposition modelling technique (Tian et al. 2016). However, regarding mechanical properties, it is well documented that they are greatly influenced by deposition parameters, FDM machine setting-up parameters, environmental factors, material properties, etc. Li et al. (2016) produced continuous carbon fibre reinforced polylactic acid composites by the rapid prototyping approach of three-dimensional (3D) printing, and they analysed the benefits achieved by comparing the mechanical properties between samples printed with and without carbon fibres. The pre-processing of carbon fibres, involving methylene dichloride solution added to PLA particles partially dissolved, was found to improve the interfacial strength due to the weak bonding interface between carbon fibre and PLA. In this context, they reported tensile and flexural strengths 13.8% and 164.4% higher than neat carbon fibre reinforced samples, respectively. Compared to neat PLA samples, these values are 225% and 194.3%, respectively. The storage modulus of the modified carbon fibre reinforced samples was about 166% and 351% higher than the PLA and PLA with neat fibres, respectively. This significant benefit was attributed to the higher fibre/matrix interface strength demonstrated by the SEM (Scanning Electron Microscope) micrographs of the different samples. The homogeneous distribution of PLA between fibres and nearly void-free microstructure found in modified carbon fibre samples indicated a better wetting and stronger interface of the modified carbon fibre samples. Tian et al. (2016) investigated the influence of deposition parameters on the mechanical performance of 3D printed continuous carbon fibre reinforced PLA composites, and parameters such as temperature of liquefier, layer thickness, feed rate of filament, hatch spacing, transverse movement speed were optimized to maximize the mechanical properties. According to the open literature, temperature significantly influences the impregnation of fibres into matrix, therefore, in the same context, the temperature of the liquefactor in the printhead is also very important to obtain higher bond strength between deposited lines and layers. Different printing temperatures from 180 to 240ºC were analysed. Temperatures lower than 180°C were excluded because it is very difficult to extrude the melted plastic with the fibre, due to the poor flowability, but they were also excluded higher than 240ºC because the filament is melted almost into liquid and can flow naturally from the nozzle with the action of the gravity. They concluded that the appropriate printhead temperature should be between 200 - 230°C and, for 230ºC, reaches the maximum flexural strength and modulus with values around 145 MPa and 8.6 GPa, respectively. At 240ºC, the surface accuracy is loosened due to the overflow of melt PLA. In terms of layer thickness, various values were analysed between 0.3 and 0.8 mm, considering a printing head temperature of 210°C, feed rate of 100 mm/min, printing speed of 100 mm/min and hatch spacing of 1.2 mm. Both the maximum flexural strength and modulus have their maximum values for a layer thickness of 0.3 mm (about 240 MPa and 20 GPa, respectively), but a significant decrease is observed for higher thicknesses, reaching values around 58.3% and 65% lower for 8 mm, respectively. Considering the nozzle tip diameter used by the authors, the maximum hatch spacing value was fixed at 1.8 mm and a minimum of 0.4 mm. Hatch spacing lower than 0.4 mm promotes excessive overlaps and interrupts the printing process. These authors observed that, when the hatch spacing decreases from 1.8 to 0.4 mm, the flexural strength increased from 130 MPa to 335 MPa and the flexural modulus from 6.26 GPa to 30 GPa. The feed rate of the filament is related to the unit volume of material fed into the printing head. Therefore, with the same tip diameter of extrusion nozzle, this parameter determines the inner pressure and extrusion speed of melt material in the printing head. These authors considered values of 60, 80, 100, 120, 140 and 160 mm/min, and from 60 to 80 mm/min the flexural strength increases from 95 MPa to 182 MPa. However, increasing the feed rate does not lead to higher flexural strengths. The observed increase may be caused by the increase of inner pressure in liquefier and overlapping contact pressure between adjacent deposited lines due to large unit volume of extruded materials. On the other hand, the short impregnation period for fibre with matrix may explain the absence of benefits for higher feed rates. Finally, the printing speed was also studied, and values of 100, 200, 300, 400, 500 and 600 mm/min were considered by the authors. They observed that high printing speed would also decreases impregnation period and pressure but increases the fibre content on the specimens. These are two contradictory factors, which caused the printing speed as an insignificant influence on the flexural strength. These authors also concluded that the carbon fibre content directly depends on the layer thickness, hatch spacing, and feed