Sidebar: Stereolithography Cuts Its Teeth

sidebar: stereolithography cuts its teeth

A stereolithography application that has made the transition to rapid manufacturing is the Invisalign process, developed by Align Technology of Santa Clara, Calif., to manufacture teeth aligners—a clear plastic replacement for wire braces.

The process is an example of stereolithography used for mass customization. The company worked with 3D Systems of Valencia, Calif., which supplied the high-end SLA-7000 solid imaging machines to create the thermoforming tools on which the plastic aligners are formed.

Because each patient's teeth are unique, the process starts with a set of dental impressions, explained Len Hedge, vice president of manufacturing at Align Technology. Plastic is poured into the impressions to create a representation of the patient's teeth. That physical model is scanned and converted into a digital file. Then a suite of software tools, developed in-house, calculates the orthodontic treatment, which consists of the tooth movements that a series of aligners will produce over time.

Once the digital representation of the treatment is done, stereolithography takes it back to the physical world. Hedge said that the process required a rapid prototyping technique that was capable of high throughput and high accuracy, and selected the SLA-7000 machine, which had just been introduced. The SLA-7000 has dual beam capability: a 10-mildiameter laser beam for detailed components and, to speed the process, a 30-mil-diameter beam for cross sections that do not require as much accuracy, Hedge said.

The machine has a large platform that can hold 90 aligner patterns—about two and a half patients' worth. The standard for accuracy of a build is within 1.5 thousandths of an inch.

After the teeth reproductions are formed, they are brought to a thermoforming machine and used as tools to form the plastic aligners. The aligners are pressure-formed in the thermoforming machine, which uses air pressure to slide the heated plastic over the mold. The aligners are clear, made from a blend of polycarbonate and polyurethane to impart the desired mechanical properties and tooth movements. The thermoforming mold of stereolithography resin has to withstand the heat, temperatures, and pressures of the thermoforming process. The plastic used for the aligners, which are 30 to 40 thousandths of an inch thick, has a melting point of 425°F. The stereolithography resin also has small shrinkage and a fast build time, Hedge said.

Depending on the length of an individual's treatment, the patient is supplied with a series of 12 to 48 aligners. Each aligner is worn for about six weeks, correcting the teeth in progressive stages.

Align Technology has ordered 39 SLA-7000 systems from 3D Systems, and currently operates 16 at its Santa Clara location. Last year, the company produced 1.1 million molds, and expects to manufacture 4 million molds this year, said Hedge.