Machining orthoses with desktop cryogenics, or using an open-source “3D printer” that not only replicates orthoses but can also replicate itself, may seem unattainably futuristic. But these technologies are on the verge of being ready for prime time, and attendees at the Custom Orthotic Insoles Technology Forum got a sneak peak.
Since 2004, mechanical engineers at the University of Bath in the U.K. have been developing and modifying a machine affectionately known as RepRap (short for replicating rapid-prototyper) that puts down layers of molten plastic to create 3D objects based on a digital design. Although other 3D printers have been available for nearly 30 years, the RepRap concept is unique in that it can essentially replicate itself by printing the majority of the parts needed to create a new machine. In addition, the software and instructions for assembly are open-source (available for free at reprap.org), and any parts that RepRap can’t print itself are readily available at hardware stores or online. The total cost of assembly is estimated to be about $500.
“It’s like MP3 music sharing, but for real solid stuff,” said RepRap inventor Adrian Bowyer, PhD, a senior lecturer in the mechanical engineering department.
Current RepRap models are not large enough for custom orthotic device fabrication, but University of Bath mechanical engineering student Chris Hanton has made this challenge the focus of his senior thesis project. The result is a larger machine that can accommodate a pair of foot orthoses, which will make its public debut after Hanton gets his degree, Bowyer said. The device takes two to three hours to make a complete polypropylene insole, but since the machine itself can be replicated cheaply, multiple machines working simultaneously can have the same output as a single faster machine.
Meanwhile, researchers in another corner of the same mechanical engineering department have developed a cryogenic process to facilitate the machining of soft elastomeric materials like EVA for such applications as customized athletic shoe outsoles.
The process involves freezing the material to below its glass transition temperature, or the point at which it becomes stiff enough for machining. In the case of EVA, that point is 120° K (-244° F). Calculations built into the software also account for the fact that the material contracts when frozen and will expand again once fabricated.
Although cryogenic milling machines are currently quite large, researchers envision the evolution of desktop cryo-mills that can machine an orthosis directly from a digital CAD-CAM system, according to Vimal Dhokia, PhD, a knowledge transfer research fellow in the Innovative Manufacturing Research Centre of the mechanical engineering department.
“We see that happening within a year,” Dhokia said.