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ISO/ASTM definition: “vat photopolymerization, —an additive manufacturing process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization.”
Vat photopolymerization can also be known as (in alphabetical order):
➢ Continuous Liquid Interface Production or CLIP
➢ Scan, Spin and Selectively Photocure Technology or 3SP
➢ Solid Ground Curing or SGC
➢ Stereolithography or SL
➢ Stereolithography Apparatus or SLA® (3D Systems Corporation)
➢ Two-Photon Polymerization or 2PP
Stereolithography was the first AM process to be invented. The first patent was filed in 1975 which described a two-laser 2PP process. The first parts were made by Dr. Hideo Kodama of Japan using SL in 1981. Additional patents followed in 1984 when in three different parts of the world, people patented the SL processes. First on May 23 in Japan by Yoji Marutani, then on July 16 in France by Jean Claude André, Alain Le Méhauté and Olivier de Witte, and lastly on August 8 in the United States by Charles W. Hull. Chuck Hull was the first to commercialize the technology when he founded 3D Systems in 1986. In 1988, 3D Systems commissioned Alberts Consulting Group to create a file format that could be sliced, resulting in the STL file format. In 1991, Cubital introduced the Solid Ground Curing process but later ceased operations in 1999. In 2015 Carbon3D introduced a novel concept named CLIP using an oxygen-permeable bottom plate to help speed up the printing process. As the original patents surrounding this technology have lapsed, many startups have emerged taking advantage of this original AM process.
Figure 1: Vat Polymerization example setup
This process involves using a liquid resin as the main type of material. Specifically, this liquid resin has the special property of being able to become solid once it is exposed to light. This light can be ultraviolet as in SL processes, or for 2PP, with two photons of near-infrared (NIR) light hit within a very short period of time (several femtoseconds). This liquid resin is held in a container or vat, in which a flat build platform is partially submerged. This platform starts near the surface of the liquid and then gets exposed to light. This light can be a UV laser(SL), a digital light processing (DLP) projector, a UV light bulb filtered through a printed mask(SGC), an LCD screen similar to home theater projectors(CLIP), or even from very quick pulses (femtoseconds in length) of near infrared (NIR) laser light tightly focused to a very small area(2PP). Once the resin is cured and made solid, the build platform either moves further into the vat, or partially comes out of the vat leaving the solid cured portion just under the surface and then the process is repeated. In the case of SGC, the uncured resin is removed and then replaced with a liquid wax that solidifies, and then both the cured resin and wax is machined flat using a cutter and prepared for the next layer. If the process involves the platform coming out of the vat the resin needs to be transparent or have the solidification process occur at the very bottom of a vat with a clear window or bottom. However, this can cause the resin to solidify to the bottom which would prevent the platform from moving, or cause it to solidify so closely to the bottom that when the build platform moves up, significant suction is created which results in very slow movements. Recent developments by Carbon3D and the creation of the CLIP process have resulted in very quick builds due to the clear bottom acting as an oxygen permeable membrane which inhibits solidification of the resin within a certain zone around the clear bottom of the vat, which eliminates this suction force. This has shown to increase build speeds from 25-100 times compared to other AM processes, including SL.
Advantages to this type of AM are that it is capable of very high detail surface finish, even down to the nano-scale level, it can be very fast compared to other processes in terms of pure volume, and it’s also able to be scaled up to build desk-sized objects in very large vats.
Disadvantages include a limited number of material properties found in UV curable resins, which are not the most robust materials in terms of durability, strength, or stability. These resins can change shape over time, potentially change colour, and usually need a post-curing UV light oven to cure the material fully in order to get the most strength out of them. Some resins are also toxic and special gloves need to be used to handle parts until they are fully cured. Depending on the geometry of the part, support structures are required and can be very complex and require manual removal afterwards.
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