# Photopolymerization of Ceramic Resins by Stereolithography Process: A Review
## 1 Introduction  
Additive Manufacturing (AM) encompasses a series of processes that enable the making of three-dimensional (3D) parts by adding material, layer by layer, directly from computer-based 3D model data. This alternative approach to conventional training processes is attracting the interest of the ceramic industry in order to create geometrically complex parts with an almost net shape without the use of expensive tools. Ceramic materials are used in different fields, such as automotive, aerospace, biomedical, luxury, electronics, casting molds and cores, thanks to their particular properties that make them indicated for several applications: hardness, high strength, high fracture toughness, high temperature and good thermal shock resistance. Implementation of AM technologies with ceramic materials has been much slower than that for polymeric and metallic materials due to the difficulty in making components with good resolution and mechanical properties comparable to conventional ceramic manufacturing processes [1]. Considering the economic aspect, unlike conventional manufacturing processes used for ceramics such as injection molding [2], the overall cost of manufacturing per piece with AM technologies is independent of the geometric complexity of the component. This has brought the attention of industries also in consideration of the fact that many applications of ceramic components require low production volumes and therefore AM can become a particularly interesting and economical solution to replace injection molding. A lot of studies report investigations about different AM technologies application for ceramic parts manufacturing, such as fused deposition modelling (FDM) [3], layer object manufacturing (LOM) [4], Stereolithography (SLA) [5; 6], and selective laser sintering (SLS) [7]. SLA process is the most interesting and attractive in terms of geometrical and dimensional accuracy as well as surface quality, especially compared to all others AM technologies [8]. SLA process (Figure 1) converts liquid materials into solid parts, layer by layer, by selectively curing them using a light source in a process called photopolymerization. In the SLA process, light-reactive thermosettingmaterials called \"resin\" are used. When resins are exposed to certain wavelengths of light, short molecular chains come together, polymerizing monomers and oligomers into rigid or flexible solidified geometries. The parts produced, called green part, undergo a subsequent UV curing cycle to completely solidify the outer surface of the part. New applications to SLA processes have been investigated from 1990s, when several studies on combination between ceramic powders and photo-sensitive resin leaded to new characterizations of specific slurry whose features depend on chemical and physical parameters of singular elements. The first limit in the production of ceramic slurry concerns the rheological behavior. Rheological characterization allows a measurement of the viscosity of the solution as a function of the shear rate. The ceramic solution for the SLA process is closely related to the solid load of the composition. Ceramic resins are often non-Newtonian fluids, so they are not characterized by a specific viscosity value, but there is a non-linear relationship between the viscosity of the fluid and the shear rate (Figure 2). [PERSON] et al. [9] dispersed ceramic powders into an ultraviolet-curable solution with a solid loading value around 45-55 vol.%. Rheological behavior needs to be investigated in relation to parameters such as solid loading, dispersant, dye or other chemical components  
Figure 1: Stereolithography process: (**a**) top-down and (**b**) bottom-up apparatus. Figure 2: Flow curve. In rheology, the force is expressed as shear stress and the speed is expressed as shear rate. percentage. Low solid-loading ceramic slurry have been studied in relation to the high shrinkage of the final part [10].