## 4 Conclusions  
In this work, the potential of the iro3d SPD process to produce glass-ceramic parts using glass microspheres obtained from recovered glass waste was investigated through the production of cuboid specimens and two geometrically complex test parts. A preliminary study was performed to have a clearer vision of adequate sintering conditions, process requirements, and experimental procedure intricacies. These observations led the authors to try to control the distortions of samples by mixing the glass build powders with sand powders (that will not shrink or melt during sintering). Consequently, in this research, the influence of sintering temperature hold time, the sand percentage in the build powder, and specimen position in the crucible were tested. The assessment was complemented with the production of test parts with geometrically complex details, which allowed us to infer process limitations regarding minimum thicknesses, hole sizes, or overhang production capabilities. Overall, specimens were evaluated in terms of geometrical deviation from the theoretical dimensions and subjected to three-point bending and microhardness tests. The authors conclude that the positioning of parts inside the crucible and, more specifically, the material amount and weight above it significantly influenced the final geometrical accuracy of parts. The higher the horizontal specimens were in the crucibles, the bigger the warpage observed. Neither the temperature nor the hold time had a clear correlation with shrinkage. However, increasing the sand percentage up to 60% reduced the specimen's shrinkage to a minimum. Horizontal specimens with higher sand percentages did not shrink, and no warping was observed. Specimens produced with greater sand percentages were slightly lighter, larger, and less dense due to the lower mass and apparent density of the sand. Additionally, vertical specimens were slightly larger and heavier when compared with horizontal specimens. This could be justified by the fact that in the deposition stage, the build powder was poured much more frequently in the vertical specimens, even though in smaller areas, which resulted in an excess of powder being poured. Possibly due to an increase in humidity or due to the size and morphology of the powders, their flowing inside the pourers could be interrupted due to clogging, both in the deposition and in the refilling stage. This effect could also be more pronounced in vertical specimens. In the cylindrical crucibles, it could be seen that almost all the build powder used was fine-sized, due to the section area of the specimens. On the other hand, when printing horizontal specimens in the rectangular crucibles, around 25% of the build powder use was fine, and the remaining 75% was rough. This fact had an unknown influence on the comparisons between build orientations, and for this reason, the authors suggest studying the influence of single-size usage of powders. Overall, increasing sintering hold times resulted in improved mechanical properties, which was corroborated by the smaller number of porosities in the fracture surface of speci  
Figure 14: Produced test parts and their comparison with the theoretical CAD model. (**A**) Test part 1. (**B**) Test part 2.  
mens. With a value of 36 MPa, the maximum transverse rupture strength was registered in horizontally built specimens with a sintering time of 8 h. All the specimens of this batch revealed greater strain at rupture when compared with batches sintered for lower hold times. Naturally, vertically printed specimens were generally weaker when compared with horizontally built specimens under the same conditions. When the sand was mixed with glass as build powder, only percentages up to 20% resulted in specimens presenting brittle behavior as all the other specimens printed with higher percentages of sand were too weak and simply tore apart. The fracture surfaces of specimens revealed that higher hold times reduced the number of porosities inside the specimens, corroborating the flexural strength results.