Chuck Hull made a 3D object by curing a tank of resin one layer at a time

In 1983 Chuck Hull found that hardening tabletop coatings with a UV light source was too rough a method. He turned the reasoning around: you could build a solid object out of the cured layers themselves.

His device bathes a platform in a tank of liquid photopolymer and moves a focused UV beam to solidify each two-dimensional cross-section. After each layer the platform shifts down a little so the next layer forms on top. He filed US patent 4,575,330 on 8 August 1984.

Hull coined the term 'stereolithography' for the process, went on to found 3D Systems and, in 1986, received the patent that is now treated as the start of 3D printing.

Build the object by adding thin cured layers

Additive stacking handles internal geometry and undercuts a mill cannot reach.

Each layer is a flat 2D problem a computer can direct, so no tool-path is needed.

The prototype goes from file to part in hours instead of the weeks tooling would take.

Don't remove material to make a shape — grow the shape. A subtractive process is limited by what a tool can reach, while an additive one builds anything from its slices.

3D Systems commercialised the first stereolithography machines, and four decades later additive manufacturing spans metal, glass, food and human tissue. Hull's patent triggered the whole industry whose descendants now build parts that subtractive machines never could.