Influence of printing constraints on residual stresses of FDM parts

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.