Hideo Kodama of Nagoya Municipal Industrial Research Institute made the first published account of a 3D printed solid model in 1981. Kodama used photopolymer technology in which a vat of photopolymer material is exposed to a UV light that hardens the part.
How Photopolymer 3D Printing Works
First, the user creates a 3D model and sends the file to the 3D printer. A vat of photopolymer liquid is exposed to UV light, which projects an image of the part onto the material, layer by layer. As each layer is exposed to the UV light, the layer hardens and the vat is drained of photopolymer liquid another layer. The process is repeated until the part is complete.
See Digital Light Processing, Photopolymerization, Ultraviolet Curing, and Vat Polymerization