# Additive Manufacturing of Ceramic Reference Spheres by Stereolithography (SLA)
## 5 Summary and Conclusions  
This work analyses the influence of different design parameters on alumina parts obtained through additive manufacturing. The technology used was stereolithography (SLA) with an ultraviolet laser, and the fabricated parts were spheres with different supports and structures. These fabricated and analyzed artifacts could be used as reference elements or targets in metrological applications. The analysis also considered the position of the parts on the print tray. The influence of parameters (wall thickness, shape, infill, and orientation) was determined for the dimensional analysis of the spheres through the diameter and for the geometric analysis through the form error. The values of both characteristics were obtained by contact measurement of the spheres using a high-precision coordinate measuring machine (CMM). The entire manufacturing process (printing, cleaning, debinding, and sintering) was carried out according to the parameters specified by 3D Ceram, the supplier of the printing machine, the raw material, and the cleaning material. The maximum deviations found in the diameter range between +33 \(\upmu\)m and \(-\)84 \(\upmu\)m, while the form error deviations range from 69 \(\upmu\)m to 178 \(\upmu\)m. Although the precision obtained in the diameter is on the order of hundredths and below two-tenths for the form error, these values are considered, a priori, too high for the spheres to be considered as reference elements in low-range metrology. It should be noted that the qualification/calibration spheres used in this range typically have much lower form errors, even submicrometric. While all parts exhibit slight ovalization in the horizontal plane (OXY) and some flattening along the vertical axis (Z), with errors in the range of tens of micrometers, those positioned in positions 1 and 3 of the print tray (outer positions) show better results concerning the dimension (diameter). These parts not only exhibit less deviation from the nominal value but also show less dispersion in the values of the different spheres. Among these spheres, those fabricated with a 2 mm wall are the best quantitatively. On the other hand, the results indicate that the highest geometric quality of the spheres is achieved in those fabricated with a 2 mm wall, with an empty interior (no internal support) and, again, in position 2 of the print tray. It is clear that a thin wall (1 mm) is not sufficient to prevent deformation during the manufacturing process of alumina spheres via ultraviolet light stereolithography. In any case, these observed deformations in the spheres could originate from two sources: machine errors and/or process errors. Probably, the deformations result from a combination of both, as the ovalization in the horizontal plane seems to be due to a squaring error of the X and Y axes, while the flattening in the vertical direction appears to be a linear error of the machine's Z axis, or a deformation caused in the material during the sintering process (flattening due to its own weight). One of the most evident disadvantages observed during the experimentation is the high failure rate in manufacturing (25%) as well as the overall time required for production with this technology. However, these drawbacks are inherent to the process given the current state of development of this technology (ceramic parts manufactured via SLA). As a summary, and based on the dimensional and geometric error results, it can be concluded that, as of now, the SLA process is not suitable for calibration artifacts in low-range and/or high-precision sensors. However, for medium- and long-range sensors and in the verification of master parts, this technology is viable, as deviations of less than 84 \(\upmu\)m in diameter and 178 \(\upmu\)m in form errors have been observed.