A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 1 7 Compared to other additive manufacturing (AM) processes, vat polymerization features the highest manufacturing resolution and is considered the best technique for bioprinting, in addition to being well suited to prosthetics production and other medical applications. The equipment required for vat polymerization is relatively large with raw material provided as needed, resulting in an overall cost-effective process. Other advantages include high precision, the ability to manufacture large products in a relatively short time frame, and the capacity to produce transparent items. The materials used for vat polymerization should be photopolymers. According to the mechanism of photo- polymerization, this particular AM method canbuildproductswithhigh accuracy at a relatively fast speed, but the raw materials are restricted to photo- crosslinkable resins. Vat polymerization employs a phenomenon where liquid photosensitive resin is hardened by irradiation with UV light in a layer-by- layer process. As shown schematically in Fig. 1, a laser beam of UV light is focused on the polymer based on the cross-sectional profile of the model. According to the direction of laser irradiation, the process is classified into two types: irradiation from the upper side, and irradiation from the lower side. For relatively large objects, upper irradiation is more appropriate. This method has been used to build prototypes of cars and other large objects. Conversely, lower irradiation has been used for relatively small items. Historically, development and application of the upper irradiation method has been the mainstream process. However, the lower irradiation technique has recently emerged and its applications are expanding. When UV light is irradiated onto the polymers, the irradiated parts must harden for the 3D printing process to continue. Certain types of polymers can be hardened through crosslinking by the irradiation of electromagnetic waves. These electromagnetic waves belong to the UV part of the spectrum. Figure 2 shows the process for crosslinking by irradiation of UV light. Photopolymers are composed of monomers, oligomers, and photoinitiators (PIs) in ADDITIVE MANUFACTURING: VAT POLYMERIZATION FOR MEDICAL APPLICATIONS Vat polymerization uses ultraviolet light to harden liquid photosensitive resins into medical products such as surgical models and artificial limbs. Hideyuki Kanematsu, FASM,* National Institute of Technology (KOSEN), Suzuka College, Japan Dana M. Barry, Clarkson University, Potsdam, New York, and The State University of New York at Canton Rafiqul Noorani, Loyola Marymount University, Los Angeles Paul McGrath, Clarkson University *Member of ASM International Fig. 1 — Schematic of vat polymerization for upper and lower irradiation methods.
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