Low-cost method to reduce interlayer voids in material extrusion: in situ layer-by-layer solvent treatment

Material extrusion (MEX) is worldwide known as one of the most flexible additive manufacturing (AM) technologies for the fabrication of complex polymeric structures. However, the extremely high geometrical freedom has a price to pay: the presence of interlayer voids between consecutive extruded layers is the main backwards of MEX technology. Interlayer voids make 3D printed parts weaker (poor mechanical properties) compared to polymeric components fabricated by means of counterpart processes (i.e., injection molding). The present research work introduces a novel approach for the reduction of voids based on the layer-by-layer application of solvent vapor during the fabrication process, to smooth every single deposited layer. In this way, the new extruded layer has a greater area to bond with the previously extruded layer, resulting in an overall reduction of the porosity. The proposed approach is cost-effective, and it is based on the stop and go method enabled by MEX technologies: the fabrication process is paused after every layer, and when the solvent treatment is performed, the 3D printing process is resumed. The effectiveness of the layer-by-layer solvent vapor approach was evaluated, thereby resulting in a great reduction of the void density and average void area of 96%, and 79% respectively, and an increase of the wetting factor of 34%. Such findings pave the way for the exploitation of the proposed approach for the fabrication of complex structures with a reduced number of voids to be employed as structural components.