Photopolymerization – Vat Technology

Stereolithography is an additive manufacturing process using a layer-by-layer fabrication procedure in which the selective curing of a polymer resin by a computer-controlled UV laser beam enables the fabrication of a three-dimensional part. Initially used in the plastics industry, stereolithography has expanded in the field of ceramics only from the mid-1990s with the development of photopolymerizable ceramic pastes/suspensions. These photosensible ceramic systems make it possible the manufacturing of ceramic parts with complex shape, high dimensional accuracy, excellent surface finish and good mechanical properties. The main applications targeted are biomedical, luxury, electronics, casting molds and cores.

As additive manufacturing technology, stereolithography provides the advantage of not requiring the use of molds during the production. Parts are produced directly from CAD files containing the desired designs and dimensions. Moreover, stereolithography is capable of producing complex and functional parts with good quality surface within a day. This represents significant savings in term of time but this manufacturing process remains more appropriate for high-value parts due to high cost of polymerizable ceramic material and the need of cleaning, debinding and sintering parts.

Stereolithography is based on photo-polymerization of a reactive system consisting of a high concentration of ceramic particles (50-65 % Vol) in a curable resin. This resin consists essentially in one or more monomer(s)/ oligomer(s) and a photoinitiator, controlling the polymerization reaction.

 

Stereolithography combines numerical tools for conception and manufacturing equipment. As a first step, a computer-aided-design (CAD) file of the object is converted into a compatible format, called STL format. Then, the object of the STL file is virtually sliced in the transverse direction according to the desired layer thickness and vertically depending on the object’s shape. In a second phase, STL file information is provided to the stereolithography device in order to build the part. The photopolymerizable ceramic paste/suspension is firstly deposed using a scraper on a support table (Figure 1(a)). Then the UV laser beam, directly controlled from a program describing the cross-sectional pattern in each layer, interacts with the reactive system thereby selectively curing the ceramic paste/suspension in the described pattern (Figure 1(b)). Afterwards, the support table moves down to the desired layer thickness in order to manufacture the next layer. The process is repeated for all layers. After completion of the part, the green part is removed from the non-polymerized paste/suspension and a suitable solvent is used to remove the uncured ceramic paste/suspension from the part. Finally, the green part needs to be debinded and sintered in order to remove the organic phase and to obtain a dense ceramic part.


 

                                                Figure 1(a)                                                                                                   Figure 1(b)