A job with ultraviolet coatings
Charles “Chuck” Hull was an American engineer working for a company that made ultraviolet lamps and used ultraviolet-cured coatings, in California in the early 1980s. These coatings are liquid photopolymers that harden quickly when exposed to ultraviolet light, and they were used to put durable finishes on tabletops and other surfaces. Hull’s work involved experimenting with how thin layers cured.
According to his own accounts and most histories, the key thought came in 1983: if a thin layer of photopolymer can be hardened by light, then stacking one cured layer on another, each shaped by where the light falls, could build a solid object. He spent his spare time developing this idea in the lab, and he is said to have produced a small test object, a cup-like part, as an early demonstration. Details such as exact dates and the first object vary slightly between accounts.
The patent
Hull filed a U.S. patent application in 1984, and it was granted in 1986 as patent 4,575,330, titled as an apparatus for production of three-dimensional objects by stereolithography. He is credited with coining the name stereolithography for the process. The patent set out the idea of forming an object by curing successive layers of a liquid under computer control.
In 1986, Hull co-founded 3D Systems, based in California, to commercialize the invention. The company introduced its first machine, the SLA-1, in the late 1980s, and early customers were industrial design and engineering groups who wanted models for testing and review. Early units were expensive and needed a separate oven or light chamber to finish curing the parts.
The STL file format
Hull is also widely credited with the file format that became the common language between design software and printers. Called STL, which is generally explained as short for stereolithography, it describes a surface as a mesh of triangles. The format is simple, which helped it spread, and it remains in use today even though it lacks features such as color and units in its basic form.
The Japanese researchers
Hull was not the only person working in this area. In Japan, Hideo Kodama of the Nagoya Municipal Industrial Research Institute published a paper in 1981 describing how a photo-hardening resin could be exposed in layers to produce a plastic model. This is generally regarded as one of the earliest published descriptions of the layered-photopolymer method. Reports say he also filed a patent application but did not complete the process in time, though sources differ on the details.
Other Japanese researchers and companies explored related methods in the 1980s, and Japan was an active center for early work, including machines that used light to cure resin in various configurations. Because the surviving documentation is uneven and often in Japanese-language technical journals, English-language histories sometimes give it less space than it deserves.
A French team filed a patent in 1984 only weeks before Hull’s application, and it was reportedly abandoned by the sponsoring companies. That episode is a reminder that Hull’s priority is not as clean as popular summaries suggest.
How to describe Hull’s role
A balanced statement is that Hull was not the first person to think of layered building, but he was the one who patented and commercialized a working stereolithography system and gave the field its early vocabulary. The title of “inventor of 3D printing” is often given to him in press coverage, and recognition such as induction into inventors’ halls of fame reflects that. It is a fair summary of his influence and a looser one of historical priority.
In some respects, the difference between those two things is the point. Many technologies have a first idea, a first demonstration and a first successful business, and these are often different people.
What came next
3D Systems grew into a large company and later acquired many others. Competing processes also emerged. Carl Deckard’s selective laser sintering at the University of Texas, Scott Crump’s fused deposition modeling at Stratasys, and later binder-based powder processes created a diverse field. Hull himself stayed with 3D Systems for many years in senior technical roles.
By the 2010s, printing based on his original idea of curing resin with light had spread into dental labs, jewelry casting, and desktop machines for hobbyists, many of them using lasers or projectors. The first machines look primitive by comparison, but the central idea has remained unchanged: a computer decides where light hardens a liquid, one layer at a time.