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3D Printing of Ceramic Dental Prostheses
adjusted by shrinkage factor, although shrinkage is a sort the experiment resolution for holding temperature and
of volumetric distortion. holding time. The flexural strength was also affected
For specimen’s fabrication, we used a fused by heating rate, indicating that densification of material
deposition modeling process and filament filled by might have reduced the strength of material.
90% of ZrSiO . The main process parameters remained In parallel with the feasibility of the proposed concept,
4
constant, where extrusion temperature was 220°C, layer high temperature implies on the unfeasibility of concept
thickness was 0.1 mm, distance between filaments was regardless of time holding, cooling rate, and heating rate
0.2 mm, and nozzle diameter was 0.4 mm. In addition, we (Figure 4). In this study, the holding temperature was
have also considered no support material and no retract the most relevant parameter for feasibility, followed by
to build the specimens. In all the cases, the extrusion holding time. In contrast, heating and cooling rates were
temperature and chamber temperature were also kept found not to affect the feasibility. From the geometric
constant, while no bed temperature was established. The point of view, our analysis revealed that the shrinkage
fabrication environment was also controlled at 25°C varied from 13.4% to 27% into the feasible area. The
(room temperature) and 50% of relative humidity. lowest value of shrinkage was found in a condition when
The repeatability and error of the AM-generated the heating rate equals to 5°C/min, holding temperature
mold were also identified before performing the study. Ten 700°C, holding time 3.2 h, and cooling rate 30°C/min,
samples of the mold were fabricated, and their external and whereas the highest value was found in a condition when
internal geometry was measured. The internal geometry the heating rate equals to 2°C/min, holding temperature
was evaluated by transversal cuts, which helped to ensure 700°C, holding time 4 h, and cooling rate 2°C/min.
that the divergence between CAD- and AM-generated Figure 4 shows the main effect of control parameter
molds was up to 0.15 mm besides an error of 2% in the shrinkage. The holding temperature was the most relevant
small dimension of mold. factor for the shrinkage, while the heating rate showed
the smallest effect among the control parameters. With
3. Results and discussion respect to material mechanical strength, Figure 4 also
In general, the evaluation of concept feasibility was presents the main effect of control parameter on flexural
satisfactory, whereas a feasible process window was strength. The temperature and heating rate were the most
identified. From geometric point of view, the crown relevant factors for mechanical strength, while cooling
was obtained in low sintering temperatures because rate presented the smallest effect among the control
high levels of temperature distorted the geometry due to parameters. The mean values of flexural strength varied
excessive melting and high shrinkage. from 25 to 82 MPa, where the lowest values were found
With regard to the main effect of control factors in low holding temperatures (700°C) and short holding
on the feasibility, mechanical strength and shrinkage, time (1 h). On the other hand, the highest values were
holding temperature had the strongest effect on the obtained by long hold time (1 h) and 800°C of holding
feasibility and flexural strength in comparison with the temperature. It is also important to indicate that this
other control factors (Figure 3). On the other hand, process did not evaluate the effects of neither heating
holding time affects shrinkage the most. It shows that treatment nor material. Therefore, further studies are still
holding time and holding temperatures are the most needed for incorporating the use of stronger materials and
relevant factors for the augmented design. Therefore, heating treatments in this concept.
the second design of experiments screening increased In addition, Figure 5 shows a comparison diagram
of geometrical concept for feasibility as a function of
holding temperature and holding time. In this figure,
a feasibility line separates the results which were
considered feasible and unfeasible from the geometrical
point of view.
In addition, the feasibility was also separated in two
areas. High grain growth was obtained at low temperature
(700°C) during long holding time (4 h), producing
specimen with high densification. On the other hand, low
grain growth was obtained at low temperatures (700°C)
during short periods of time (1 h), producing specimen
with low densification. Of note, the central point of the
study indicated a limit of feasibility so that the sintering
Figure 3. Main effect of control parameter on feasibility, flexural process can be directly correlated to the absorbed energy
strength, and shrinkage. as a function of time and temperature. The unfeasible

94 International Journal of Bioprinting (2021)–Volume 7, Issue 1