3D Systems originally sued Formlabs over eight patents it was granted in the 1990s. All of them covered different parts of the stereolithographic 3D printing process, which uses a laser to cure liquid plastic. Formlabs almost singlehandedly established the desktop SLA market, so it’s not a surprise it became a target for 3D Systems, the founder of which invented SLA printing in 1984. 3D Systems also named Kickstarter in its original suit for the role it played in selling Formlabs’ first machines, but that has since been dropped.
Patents have been a tricky subject for 3D printing startups, many of which started out as open source projects more interested in providing makers with hackable machines than making money (MakerBot, the best known desktop 3D printer maker, is one example).
MakerBot has since been bought by Stratasys, an international corporation that holds many of the patents that once held MakerBot back. 3D Systems and Stratasys have both stepped up their aggressiveness and taken startups to court in the past few years over patents. Keep abreast of legal developments at http://3dprintinglawfirm.com