Photopolymerization is the process of turning a liquid into a solid. It works with liquid or viscous photosensitive resins consisting of monomers, oligomers and a photoinitiator. When exposed to a specific wavelength (often in the UV range), the photoinitatior triggers a chemical reaction (crosslinking) that leads to the formation of a hard material.
Certain 3D-printing techniques rely on photopolymerization to create intricate objects. The most common techniques are known as SLA and DLP 3D-printing.
In SLA and DLP, a liquid or very soft UV-sensitive resin is stored in a vat with a partially submerged flat platform that the 3D object will be built upon. The 3D object is built layer by layer, and each layer is created by exposing the liquid resin to the UV light according to the design specifications. The photopolymerization reaction happens in the light-exposed areas leading to the formation of a hard layer of cured resin. The process is repeated until the 3D "print" is complete.
After the entire product is exposed to light in the right amount and in the right place, it is raised out of the vat for additional finishing. This usually involves a trimming away of rough parts and any support structures, as well as a final UV cure in a special UV oven for maximum hardness. It must also be rinsed of any chemical residue before handling, as the vat liquids and curing materials may be toxic.
The advantages of photopolymerization make it an ideal choice for applications where an accurate level of fine detail and smooth surfaces are desired. It also allows for large volume parts, as the DLP light engines can project in a wider area than some laser prints; as long as the appropriate light engine is used, resolution can be maintained even in these larger print areas. Complex objects of various sizes are possible through this method.
The projects created through photopolymerization have advanced many industries, from the mechanical fields through the use of intricate parts and prototypes to health technology, where items such as hearing aids and jaw implants have helped patients live fuller lives.
Photopolymerization lets designers work with a variety of materials, too. Resin mix-ins include ceramics, metals, and other composites that can achieve a range of characteristics when exposed to light. Toughness, ductility, flexibility, stretchability, and even optical properties can vary, depending on the materials used. The wavelength and energy density of the light source will also play a role in the final product’s features.