Chapter18 Influence of Printing Constraints on Residual Stresses of FDM Parts C. Casavola, A. Cazzato, V. Moramarco, and G. Pappalettera Abstract The Fused Deposition Modelling (FDM) is nowadays one of the most widespread and employed processes to build complex 3D prototypes directly from a STL model. In this technique, the part is built as a layer-by-layer deposition of a feedstock wire. This typology of deposition has many advantages but produces rapid heating and cooling cycles of the feedstock material that introduces residual stresses in the part during the build-up. Consequently, warping, de-layering and distortion of the part during the print process are common issues in FDM parts and are related to residual stresses. The common techniques employed to obtain parts of correct shape and dimensions, such as depositing glue on the bed, have the aim to constrain the object on the printing bed, though this increases the residual stresses in the parts. The aim of the present work is to measure the residual stresses in several points of printed parts, both on top and bottom, in order to verify if the constrain conditions used during the printing produce substantial variation from a point to another. The residual stresses have been measured in ABS parts employing the hole-drilling method. In order to avoid the local reinforcement of the strain gage, an optical technique, i.e. ESPI (electronic speckle pattern interferometry), is employed to measure the displacement of the surface due to the stress relaxation and, consequently, calculate the residual stresses. Keywords 3D printing • Residual stress • Fused deposition modelling • Hole drilling • ESPI 18.1 Introduction The Fused Deposition Modelling (FDM), invented in the early 1990s by Stratasys, is one of the most employed 3D printing techniques in both consumer and enterprise business. This process has been employed to build complex 3D prototypes directly from a computerized solid model in many fields such as aerospace, medical, construction, and cultural [1, 2] but, nowadays, there are many other potential fields where it can be employed. Moreover, the diffusion of the low-cost desktop 3D printers such as RepRap, Ultimaker, Maker-Bot, Cube, etc., has made this technology widely accessible even at home and office. In this process, as for many others 3D printing technologies [3], the model is built as a layer-by-layer deposition of a feedstock material. Initially, the raw material is in the form of a filament that is partially melted, extruded and deposited by a numerically controlled heated nozzle onto the previously built model [1]. After the deposition, the material cools, solidifies and sticks with the surrounding material. Due to the layer-by-layer construction and the orientation of the material deposition, once the entire model has been deposited, the FDM part shows orthotropic material properties with a behaviour similar to a laminate orthotropic structure [4]. Initially, the FDM printers have been able to build parts only in acrylonitrilebutadiene-styrene (ABS) and polylactic acid (PLA). However, nowadays, many others materials have been employed and developed, e.g., metal [5], ceramics [6], bioresorbable polymer (PCL) [7], metal/polymers mixture materials [8], and short fibre composites [9]. The PLA, compared to ABS, have a stronger mechanical resistance and a lower coefficient of thermal expansion. The last property improves the printability of the material because reduces the de-layering problems and the warp effect during the printing phase. This distortion effect of the part during the print is one of the most important issues in the FDM process, because it could seriously affect the shape and the final dimensions of the parts or it could prevent the finalization of the objects due to unsticking of the object from the bed. The distortions are due to the continuous rapid heating and cooling cycles of the deposited material [10, 11]. A common technique in order to reduce this problem is to employ a heated bed with some type of adhesive on the surface. Although, such procedures help to reduce distortions, they can increase the residual stresses of the final part. C. Casavola • A. Cazzato • V. Moramarco ( ) • G. Pappalettera Dipartimento di Meccanica, Matematica e Management (DMMM), Politecnico di Bari, Viale Japigia 182, 70126, Bari, Italy e-mail: vincenzo.moramarco@poliba.it © The Society for Experimental Mechanics, Inc. 2018 A. Baldi et al. (eds.), Residual Stress, Thermomechanics & Infrared Imaging, Hybrid Techniques and Inverse Problems, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-62899-8_18 121
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