###### Abstract  
Three-dimensional (3D) printing is a simple and versatile process for producing parts of complex geometries. Although the process possesses several manufacturing advantages, such as rapid prototyping, customization, and complexity, the optimization of the 3D printing procedure remains a challenge. Here we explore the influences of various processing conditions on the mechanical properties of melt extrusion- and solution extrusion-printed polylactide (PLA) products by adopting a robust experimental design model. In addition to the commercially available melt extrusion 3D printer, a novel solution-type 3D printer has been exploited especially for this study, which consists of a solution-type plunger-actuated feeding system, stepper motors and motion components, a power supply unit, a print bed, a user interface, and connectivity. The effects of various parameters were investigated by adopting a robust experimental design. We compared the parts printed using the melt extrusion and solution extrusion methods and found that, in the melt extrusion printing, the print speed and fill density were the principal parameters affecting product quality, while in the solution extrusion printing, oven temperature, fill density, and PLA/dichloromethane (DCM) ratio were the key parameters. By scanning electron microscopy, we found that the melt extrusion-printed parts exhibit a strip-like microstructure and the solution extrusion-printed parts show a fused surface morphology. Due to the addition of solvent, the solution-printed PLA material show a different thermal profile in the differential scanning calorimeter analysis, which in turn affects the mechanical behaviour of printed parts. 3D printing; melt vs. solution extrusion; processing parameter; polylactide; process optimization +
Footnote †: journal: AppliedThe advantages of 3D printing, including manufacturing options, rapid prototyping, customisation, and complexity making it one of the most promising technologies. Through creating layers, 3D printing opens a whole new way in which products are created, and it offers many advantages compared to the usual traditional manufacturing methods. The most widely used 3D printing process is the fused-deposition modelling (FDM) procedure [3], which extrudes hot polymer melt of thermoplastics from a large coil and deposits the materials on the growing work. The printing head shifts in two horizontal directions to form one layer at a time. Then, the head migrates vertically in small sequential steps to create a new layer. A microprocessor controls the speed of the extruder head to constitute a three-dimensional product. Despite its popularity, one major limitation that confounds the overall success of melt-extruded FDM is the restricted selection of commercially-available polymeric filaments for printing, mostly acrylonitrile butadiene styrene (ABS) and polylactide (PLA). If other materials are needed, including polyethylene, poly(ethylene terephthalalateco-1,4-cyclodextrininedmethylene terephthalalate), polyamide, thermoplastic elastomers etc., they must be made into filament by thermal extrusion before being printed by FDM [4]. However, this not only increases the overall cost, but may also pose some limitations, especially when the printed device is used for drug delivery applications [5; 6]. Mixing the input polymeric materials with drugs at elevated temperatures during filament preparation or a 3D printing process can inactivate and/or degrade the drugs [7]. To overcome this limitation, we developed a novel solution extrusion-type 3D printer in our lab, which utilises the layer-by-layer method to deposit materials to create desired structures. A solution that dissolves the polymeric materials for printing is extruded out from a syringe by an actuator driven by a step motor. The printing head is migrated under computer control to obtain the desired geometry. Once the solvent evaporates, the liquid form material ejected from the syringe solidifies and deposits itself in successive layers within the 3D printing volume.