Creality has developed what they call an "Infinite-Z 3D Printer." By tilting their nozzle at a 45-degree angle that prints onto a conveyor belt, rather than a fixed bed, their 3D Print Mill can theoretically print to any length.
Of course, in reality you'll be limited by outfeed, but the development team has successfully printed in lengths up to six meters (about 20 feet). "All in one piece," they write, "produced by a machine only 66cm (about two feet) long."
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The Kickstarter funding is bonkers; they were seeking just $13,000 to get production going, but at press time they had $947,760, with 24 days left to pledge.