Dental inventors at T.O.P. Service fur Lingual-technick came up with a great idea: put dental braces on the tongue side of the teeth and they become almost invisible. Each orthodontic appliance requires up to 16 brackets and a connecting archwire. Then they realized the classic engineering problem: the precision the tiny devices require makes them hard to manufacture.

The solution, casting the tiny brackets, is made possible by the Solidscape ModelMaker II rapid prototype machine from Solidscape, Merrimack, NH (solid-scape.com). It makes investment casting patterns. T.O.P.'s head of research and development Ralf Paehl says the key evaluation criteria were resolution, surface finish, castability, and precision. T.O.P. designers scan a plaster model of a patient's teeth to create digital versions that are loaded into T.O.P. Service's design software. Technicians can then position brackets and adjust their features for best results. The bracket design is then output as STL files for building investment-casting patterns.

The Solidscape T66 Benchtop, capable of 13-micron layers, constructs bracket patterns with proprietary thermoplastic ink-jet technology. A cutter mills the horizontal plane for precise layer thickness and flatness. Support material is dissolved off the finished patterns.

A next step attaches runners to patterns and assembles them to make a casting tree. The tree is then dipped into a “speed plaster” to create the investment-casting shell and heated to 690°C to burn out the patterns. Dental gold is cast into the investment shell which is broken away after cooling to yield the metal brackets. The runners are removed, and the brackets tumbled in a polishing compound to smooth the surfaces. Lastly, brackets are mounted to the malocclusion model. The whole process takes 10 to 15 days.