Systematic Compounding of Ceramic Pastes in Stereolithographic Additive Manufacturing

Abstract

In this paper, stereolithographic additive manufacturing of ceramic dental crowns is discussed and reviewed. The accuracy of parts in ceramic processing were optimized through smart computer-aided design, manufacturing, and evaluation. Then, viscous acrylic resin, including alumina particles, were successfully compounded. The closed packing of alumina particles in acrylic pastes was virtually simulated using the distinct element method. Multimodal distributions of particle diameters were systematically optimized at an 80% volume fraction, and an ultraviolet laser beam was scanned sterically. Fine spots were continuously joined by photochemical polymerization. The optical intensity distributions from focal spots were spatially simulated using the ray tracing method. Consequently, the lithographic conditions of the curing depths and dimensional tolerances were experimentally measured and effectively improved, where solid objects were freely processed by layer stacking and interlayer bonding. The composite precursors were dewaxed and sintered along effective heat treatment patterns. The results show that linear shrinkages were reduced as the particle volume fractions were increased. Anisotropic deformations in the horizontal and vertical directions were recursively resolved along numerical feedback for graphical design. Accordingly, dense microstructures without microcracks or pores were obtained. The mechanical properties were measured as practical levels for dental applications.

Keywords: additive manufacturing; ceramic component; dental crown; nanoparticle paste; stereolithography.

Figure 1

Schematic illustration of stereolithographic additive manufacturing (STL-AM). Cross-sectional layers are formed on paste materials by laser scanning. Solid objects are fabricated via layer laminating and interlayer bonding.

Figure 2

Drawing pattern of film specimens to optimize lithographic conditions. Hole diameters and layer thicknesses were measured by a digital optical microscope. Dimensional tolerances and curing depth were estimated.

Figure 3

Volume fractions at compounding ratios. Binary particle sizes of 170 nm and 1.7 μm in diameters were adjusted. Closed packs of spherical fillers were simulated by a distinct element method (DEM).

Figure 4

Dispersion profiles of bimodal powders in resin matrixes. Maximum contents were optimized at 80% of the total volume fraction. Computer graphics were plotted according to the DEM simulations.

Figure 5

Intensity profile of optical irradiation at the laser spot. Vertical sections of polymerized regions were visualized. Ultraviolet (UV) light propagation in the particles gaps was simulated by the ray tracing (RT) method.

Figure 6

Alumina particles dispersed in acrylic resin. Paste containers were rotated and revolved for processing times of 300, 600, and 900 s. Cross sections of solid films are shown in (a–c).

Figure 7

Dynamic profiles of shearing stresses and speeds. Rheological profiles were analyzed by a kinematic viscometer (KV). Viscose pastes processed for 300, 600, and 900 s are shown in (a–c), respectively.

Figure 8

Corresponding values of the curing depth and dimensional tolerances for laser irradiation powers. Layer thickness and hole diameters were measured in the film specimens, as shown in Figure 2.

Figure 9

Dental crowns fabricated by STL-AM. Top and side views of the graphic model (a); composite precursor (b), and ceramic component (c) arranged in the same magnification.

Figure 10

Ceramic microstructures in alumina crowns. Crystal grains observed by the scanning electron microscopy (SEM). Cross sections of the (a) horizontal and (b) vertical planes are defined as the parallel and vertical views of the laminated layers.