Issue 50

N. Chatzidai et alii, Frattura ed Integrità Strutturale, 50 (2019) 407-413; DOI: 10.3221/IGF-ESIS.50.34 408 extruded as thin rasters through a nozzle onto the build platform to form the first layer [6,7]. The next material layer is deposited upon the previous one, while the process is continued to form the final desired model [8]. After its deposition, the extruded material rapidly cools, solidifies, and bonds with the surrounding material [9]. As long as the material is hot (above T g ) it can bond with the previous layer or the adjacent rasters. The bonding mechanism no longer takes place once the material cools below its T g . Therefore, the longer the material is kept above its T g , the better the bond between layers and rastres [10]. On the other hand, the deposited material should solidify as quickly as possible to avoid deformation due to gravity, or weight of the above deposited material [11]. Moreover, the non-uniform thermal gradients that exhibit during the FDM process, develop residual stresses which may induce warping and delamination [12,13] or even part distortions, dimension inaccuracy or part fabrication failure [9]. As a result, the knowledge of temperature evolution during the com- ponent's fabrication process is one of the primary concerns in FDM process. The quality, accuracy and properties of the final printed parts depend also on other user control process parameters, such as build orientation, layer thickness, infill density, raster angle, extruder temperature and air gap between adjacent rasters. A lot of research has done the last two decades on the influence of the printing process parameters on the mechanical properties of the printed parts [3,7,14-24] and/or the bonding degree between the rasters [3,20,22,25-28]. Sood et al. [22, 27] examined five important process parameters (layer thickness, orientation, raster angle, raster width and air gap) both experimentally and numerically, and showed that they do not work alone, but interact during the FDM process and influence the tensile and compressive strength of the final part. Besides the effect of the process parameters on the mechanical properties of the final object, particular interest has shown by researchers on the temperature field and the related temperature gradient as a result of different process conditions. Sun et al. [20,29] tried to correlate important process parameters with the quality of bond formation via temperature measurements in the bottom layer of an FDM fabricated component. Zhang and Chou [30] considered the influence of tool-path on temperature variation to explain thermo-mechanical distortion through residual stress distribution. More recently, Costa et al. [31] examined the contribution of various thermal phenomena developing during fused deposition techniques to the overall heat transfer and to the mechanical deformation of the fabricated parts. Later, the same authors [11], presented an analytical solution for the transient heat transfer during filament deposition and cooling considering four main process parameters (extrusion velocity, filament dimensions, sequence of deposition and environmental temperature). Zhang et al. [32] used the boundary-adjusting finite difference method to adapt a three-dimensional transient mathematical model to describe the influence of various process parameters on the temperature variation of any FDM printed cuboid specimen. Li et al. [33], studied experimentally the effect of major process parameters, such as layer thickness, deposition velocity and infill rate, on the bonding degree between the rasters, on the temperature profiles during the FDM process and on product's mechanical properties. In that work, the experimental process was conducted for a component made from PLA using an open-source MakerBot FDM printer. In the present study, the real-time temperature profiles of rectangular specimens were investigated, under different printing velocities and raster orientations. The recording of the temperature values was achieved through the integration of temperature sensors in various layers of the printed specimens. The experimental results were compared to those derived by Finite Element Analysis. E XPERIMENTAL PROCEDURE ectangular specimens of commercial thermoplastic Acrylonitrile Butadiene Styrene (ABS) were built on the Maker- Bot Replicator 2X FDM Printer. The dimensions of the specimens were 40 mm x 20 mm and consisted of 41 layers. The thickness of each layer was 0.254 mm. The advantage of a such an opensource FDM printer is the ability of changing a large number of building parameters, such as printing speed, layer thickness, density of the rasters (infill density), building orientation and temperature of the printing heads. In the present study, all the above parameters remained constant, besides the building orientation and the printing speed. Temperature sensors (K-type thermocouples, ±1.5°C) were embedded at the center of the 1 st and/or the 21 st building layer, for the recording of the temperature variations throughout the building process and until the completion of the specimens' fabrication. This type of sensors has a sensing tip of 0.25 mm thickness. The temperature data recordings were obtained and analyzed through a data acquisition instrument. The embedment of the thermocouples was carried out man- ually. Special retainers were designed and built simultaneously with the specimen to assure the integration of the sensors at the desired positions. The building process of the 3D printed rectangular specimen started with deposition of a raft of small thickness, made of polystyrene to increase the contact surface between the specimen and the platform. This polystyrene raft was considered R