[MAS.865](../../) > [Additive Manufacturing](../) > SLA
#### [the first 3D printing technique (the importance of filing a patent)](https://www.sculpteo.com/blog/2016/12/14/the-history-of-3d-printing-3d-printing-technologies-from-the-80s-to-today/)
#### Right side up/ Top Down Photopolymerisation
The build platform is lowered from the top of the resin vat downwards by the layer thickness.
A UV light cures the resin layer by layer. The platform continues to move downwards and additional layers are built on top of the previous.
Some machines use a blade which moves between layers in order to provide a smooth resin base to build the next layer on.
After completion, the vat is drained of resin and the object removed.
Monomersand Oligomers The core of the resin; carbon chains that will make up the solid parts.(Decides mechanical and thermal properties: Since photoreactice resins undergo a chemical change on the point of hardening, they can be tuned to produce different result )
Photoinitiator Molecules that react when exposed to UV light, initiating the reaction.
Additives Visual and functional supplements, such as pigments or dyes. (apearance)
While they’ve reached their final shape and form, the polymerization reaction is not yet driven to completion, so are not fully set.
During the SLA 3D printing process, the components of the resin form covalent bonds. This provides high degrees of lateral strength, but the polymerization reaction is not driven to completion. Rather, the print process is modulated in a way that keeps the layer in a semi-reacted state called the “green state.” This green state differs from the completely cured state in one very important way: there are still polymerizable groups on the surface that subsequent layers can form covalent bonds with.
single-line lithopaneblack resin 1 layer on acrylic board
Expose pre sensitize pcb(if you want to live longer don't try it: Chemical Lamination that will resolve in water after UV exposure, rest of the lamination act like protective film during etching process. Soak it in Ferric Chloride and uncovered part will be etched. Ingestion of ferric chloridecan result in serious morbidity and mortality. )
Inverted SLA comes with its own set of limitations. Due to the peel forces affecting the print when it’s separated from the surface of the tank, the build volume is limited, and larger support structures are required to keep the part attached to the build platform. Peel forces also limit the use of more flexible materials—Shore hardness below ~70A, because the support structures become flexible as well.
Why Multi-Photon: Overcome the limit of Rayleigh criterion.Based on the limitation of optical diffraction, generation of Airy Pattern. IR have larger wavelength so less diffraction, Multi-photon technique require much higher photon density to triger TPA(two-photon absorption) to achieve polymerization