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Faster Sintering Ovens Make Zirconia Production Easier
Shorter cycles, expanded capabilities, ergonomic designs help laboratories
By Keith Miolen, CDT
Dental laboratories utilize a variety of processes and machines necessary to carry out their production processes. Some of the equipment and processes remain relevant to today’s modern production methods; others have become decreasingly utilized and face certain extinction. But most equipment and processes have evolved to keep pace with change. We have seen centrifugal casting machines evolve into induction casting systems, porcelain firing ovens transform into pressing ovens, and in recent years the advent of a process known as sintering, which requires a sintering oven. Sintering is the process of heating partially sintered zirconia, a material that can only be fabricated by using CAD/CAM technology, to its final hardened state and strength.
Over the past decade, the laboratory industry’s dependency on zirconia has increased dramatically due to the demand from dentists and patients. This demand stems from the awareness of zirconia’s more esthetic capabilities compared with those of metal-based restorations, which occasionally leave an opacious appearance when the ceramics applied are contoured too thinly. The sintering process of zirconia can be likened to the casting process of an alloy in that both depend on an interim expansion or shrinkage procedure in order to achieve the final fit of a framework or full-contour design. Just as the liquid expansion ratio of an investment dictates the final fit of a cast alloy, sintering zirconia dictates fit by downsizing the milled zirconia via heat to fit upon the scanned and designed die.
Thus, based upon the demand for zirconia, sintering ovens are now a central component of our production cycle and just as important as milling machines. And, like milling technology they have continued to evolve and progress since the introduction of the first furnace more than 10 years ago.
Traditionally, sintering ovens have been known to be non-ergonomic, bulky, and requiring cycles up to 8-12 hours to sinter zirconia restorations, depending upon the width and the span of the designed prosthesis. Despite those factors, the results have been acceptable fits. Due to the rapid adoption of zirconia-based restorations, many laboratories have had no choice but to alter their production cycles and employee schedules to accommodate the sintering process. One popular solution is to design and mill during the day, and sinter overnight. However, when mills need repair or burs break, sintering is often delayed in order to catch up on cases that need to be re-milled. The end result can be cases falling 16 to 24 hours behind schedule. For these reasons, sintering has presented a challenge for the laboratory industry.
When sintering was first introduced to the market, a small number of furnace options existed. Now, however, a wide range of choices are available from various manufacturers and distributors. Also, other materials can be sintered in these ovens. In addition to zirconia, pre-programmed cycles now include programs for the sintering of CrCo (chrome cobalt), which allows the metal to be milled in a soft state and sintered rather than cast.
Another advancement has been the expansion of the platforms that hold the sintering trays, which allows homogenous heat distribution and increases the number of units that can be sintered in a single cycle.
After firing, today’s sintering ovens continue to boost production with the addition of new and enhanced cooling rates. These new features increase production capacity by offering reduced cooling cycle times that bring temperatures down at a rate that is a fraction of what once was required, thanks to the addition of multiple fans.
Sintering oven manufacturers are now offering a variety of firing cycles as well as options to customize program cycles, allowing the sintering process to be reduced potentially by hours. These custom cycles are known as "speed" or "short" cycles. They have been developed after vigorous testing of sintering temperatures and the amount of time the material must be exposed to heat in the sintering process. The testing revealed that full sintering can take place in a shorter amount of time while still maintaining an accurate fit. These speed cycles are standard in most of today’s sintering ovens and vary from 10 minutes to one hour, depending on the thickness and the span of the unit being sintered.
The newer sintering furnaces also accommodate the increasing demand for esthetic zirconia. Exposure to extremely high temperatures (today’s sintering furnaces can be fired up to 3000°F) can transform a previously opaque material into one with a high degree of translucency.
Several other new advancements are making sintering ovens easier for laboratories to incorporate into their workflow. Redesigned interiors with higher-grade heating elements, as well improved outer frames, result in more ergonomic ovens that require less benchtop space than older models. Also, touchscreen capabilities and remote control have made operating a sintering oven a very user-friendly.
These widespread technological advancements bring today’s sintering ovens to the forefront of our production processes. Increasingly productive and efficient, the sintering oven is now a necessary piece of equipment for most laboratories to remain profitable.
Keith Miolen, CDT, is the Chief Operating Officer of Aurora Dental Arts, a 43-person, full-service laboratory in Auburn, New York. He is an NADL Pillar Scholarship Recipient and an active IDT Editorial Advisory Board member.