Micro-Machining DENTAL CERAMICS

Evidence from India suggests that people were drilling, filing and shaping living teeth as early as about 9,000 BCE. Despite this 11,000-year history, modern dentists and laboratory machinists concede that the inside of a human mouth is a particularly poor place to attempt precision work.

Obviously, dentists still fashion cavity fillings and temporary restorative repairs inside their patients' mouths. But permanent dental restorations, like crowns and bridges, are now invariably manufactured in specialized dental laboratories. Though "manufactured" is the accepted term within the big labs, it may be a bit misleading. Although they regularly turn out thousands of pieces each day, each one is custom-made.

THIS IS NOW

Glidwell Laboratories of Newport Beach, CA is the largest and perhaps the most modern dental restoration company in North America. This dental lab, among others, consider their technicians to be artists, even though CAD/CAM systems were introduced into tooth manufacturing more than a decade ago. But, new systems for digitally scanning a prepared tooth model and then micro-machining the crown material have changed things dramatically. This fresh approach has expanded productivity and improved the quality of the product. Now, stronger and more aesthetically pleasing materials can be used for dental restorations, by using ceramic materials, which now can be machined economically, taking the place of molded and hand-finished malleable metals, like gold or chrome alloys.

Glidewell's Vice President of Research and Development for Digital Manufacturing Technology, Mervyn Rudgley, is one of those striving to change the way dental crowns and bridges are made. "I don't view this facility as a laboratory," he explains, "I view this as a factory." Since joining Glidewell three years ago, his primary task has been to develop the company's own branded line of CNC-machined ceramic crowns. After an intensive period of development, Glidewell's zirconia-based Prismatik CZ crowns are now in full production.

HOW WE GOT HERE

The traditional starting point for crowns and bridges is a plaster casting that labs create from a mouth impression. A resilient inner sleeve, called the coping, is designed to precisely fit the prepared model tooth. It is investment-cast from chrome-cobalt alloys, or is formed by wrapping a metal-impregnated wax sheet directly over the plaster casting. This strong yet flexible coping is then covered with a hard, natural-looking porcelain outer shell. The liquid porcelain is first poured or injected into a wax mold surrounding the coping, and then fired and fused to the metal.

In contrast, a one-piece crown is cast from solid gold using the lost-wax molding process to shape the inner and outer surfaces at the same time. Gold crowns are often used to restore rear teeth, but are less aesthetically desirable for front teeth. Ceramic restorations that are more natural looking clearly are the trend of the future, and the demand for them is increasing.

WHY SO COMPLICATED?

If ceramic materials can be molded, why is machining necessary? The answer lies in the incredible power of the human bite. Posterior teeth clamp down with a force of up to 200 lbs. If the object of that bite is a small caraway seed, with 0.1mm2 of contact areas, the compression on the teeth can exceed 1million lbs./ in. While standard molded, hand-colored porce lain matches natural tooth enamel very well, it isn't strong enough in the thin marginal area near the gum line to withstand the force of a strong bite. That's why metal copings were originally devel oped – as thin, flexible understructures for the porcelain crowns. Recent man-made ce ramics like zirconia are much stronger, and since they're white, they promise to be a near-perfect material for crown copings. But, zirconia is essentially a sintered material that cannot be molded precisely; it must be machined.

If zirconia is already white, why make a sleeve, then cover it with porcelain? Why not simply machine a solid crown from it? The ironic answer is, the material is too white.

CUTTING THEIR TEETH

Glidewell Lab's new open-technology approach to ceramic machining contrasts starkly with existing closed-system solutions available in the industry. Rudgley's team developed CAD/CAM software interfaces and production techniques that work openly with white-light or laser 3-D scanners and high-speed CNC mills. Although designed specifically to cut zirconia, the open-system elements can be readily adapted to the next generation of ceramics.

Though some other systems hard-mill zirconia, Glidewell chooses to machine its 0.5mm thick copings from a less dense, bisque-fired block, and then sinter the cut pieces at 1,500o C for 12 hours to harden them. The technique eliminates the need for costly diamond cutting tools that are rapidly consumed during hard milling. This is one reason Glidewell can produce all-ceramic crowns for about the same cost as conventional porcelain-fused-tometal units.

Haas Automation OM-2A Office Mills are the CNC machines of choice for Glidewell's open system. A metal-frame fixture mounted to the machine's table holds a 4 in. x 6 in. x ¾ in. block of clinical zirconia for machining. Pressurized air flowing across the fixture carries the abrasive machining debris into a 3 in. diameter vacuum line that exits through the back of the OM-2A Office Mill, and eventually connects to the lab's central vacuum system.

Up to 16 individual copings can be machined from each block. Rough cutting is performed with a 3mm ball endmill, and finish passes are done with a similar 1mm tool — all at 30,000rpm. Total cycle time for each coping is about 16 minutes; the entire block of 16 copings runs for about four hours.

RELIABLE MANUFACTURING

The Haas OM-2A Office Mills that Glidewell uses are considerably more powerful, and constructed much more solidly than the lightweight machines. Equipped with 30,000rpm, 20-taper spindles and automatic tool changers, the compact Office Mills are well suited for continuous production.

"In a previous endeavor," stresses Rudgley, "I learned the importance of good machine reliability, and how closely that's linked to a good service network. That's why I ultimately chose Haas machines for this one. We run 24/7 here, producing about 350 ceramic copings per machine per week. We have exactly five days to turn out a crown. Since the porcelain firing takes two, that means we must scan the model, design the nesting and cutting paths, mill both sides and sinter the coping in less than three days. Extended machine downtime is unthinkable."

"All told, we're doing very well with this system," Rudgley concludes. "I think our zirconia product is going to continue to grow 10% to 15% each month for the foreseeable future."