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Cutting Edge Technology Doesn’t Just Cut … It Presses
A glimpse into pressing ovens and their futuristic capabilities.
Since the introduction of the first pressed-ceramic material in 1991, dental technicians have relied on press technology for the fabrication of all-ceramic products. Prior to this date, fabricating an all-ceramic restoration required applying feldspathic porcelain to refractory models or using the foil technique. These methods were time-consuming and considered by many to be tedious and too labor intensive. Therefore, the vast majority of laboratories that provided all-ceramic products were perceived to be of higher quality and charged sufficient fees to offset the difficulty of fabrication. The volume of all-ceramic restorations that these high-quality laboratories were supplying 2 decades ago are but a small fraction of the all-ceramic pressing technology market share of today.
Eventually, pressing technology opened the door of opportunity to the average laboratory. Through pressing, contour is established in wax, a process that was not done prior to 1991 other than for full-cast crowns. The process allowed for an evaluation of the final contour result before the layering of ceramics began (Figure 1). Press technology not only allowed all-ceramic restorations to be produced with ease, but it also offered more predictable results. It allowed all laboratories to offer a high quality, all-ceramic product that could be consistently produced, which was a milestone moment for our industry.
During the early years of pressing, only a few manufacturers offered pressing furnaces or materials. Pressed restorations were considered to be a high-end esthetic product; in addition, the longevity factor of all-ceramic restorations remained in question. Rumors of micro-fractures, breakage, and de-bonding created clinical uncertainty for those who dared prescribe them. Therefore, many manufacturers and distributors hesitated to enter the pressing market. Those select few companies who did come to market were faced with the task of troubleshooting shade selections for their ceramic pellets and dealing with ceramics cracking during fabrication, mis-presses, and split rings. These issues posed technical uncertainty for those laboratories that ventured into adopting the technology. However, through research and development, new and improved technologies paved a path of confidence and overcame these clinical and technical issues. Further research and development in pressing ovens and materials was demanded by the industry, primarily driven by the lifelike vitality of the restorations it produced.
Evolution of Pressing Ovens
In order to appreciate the technological advancements taking place in modern pressing oven technology, the evolution must be understood. Initially, there were only a few ovens on the market from which to choose. And when originally introduced, pressing ovens had only a single function, which was to press a ceramic pellet by means of air pressure into a wax-fabricated mold within the investment. They served no other purpose and were considered a costly investment by most laboratory owners. It was also perceived that a specific manufacturer’s pressing oven was only capable of pressing that specific manufacturer’s ceramic pellets. Beyond the technology’s limited capabilities, another disadvantage was the method of pressing in those early furnaces. They used heat and air pressure to achieve the final product. Often the furnaces would apply too much pressure, which either cracked or split the ring. If too little pressure were applied, a miss-press or short-press would result, leaving a void around the margin areas. Regardless of whether too little or too much pressure was applied, the case would need to be redesigned in wax. Operators would then increase the temperature of the pressing oven to offset the split ring or miss-press issues, but occasionally the increased heat would alter the value of the ceramic pellet due to increased heat or the longevity of a press.
The pressing mechanism was a single plunger coming down from the top of the oven and making contact with another plunger placed within the invested ring. A drawback to this method was that the pressing oven could only press one ring at a time and, for many years, the ring could not be larger than 200 grams. But as the years passed, more material options became available and investment ring gram sizes expanded. However, laboratories that pressed the newer materials, such as lithium disilicate, in older style ovens discovered that an extensive reaction layer bonded to the surface of the restoration. This reaction layer was created by the lengthy press time within the pressing cycle of the older ovens. Even though the mechanics of early-generation ovens had always been dependable, some of the newer materials demanded the use of new pressing technology.
Today’s pressing ovens are a much better balance between mechanics and technology. This fusion allows them to serve a multitude of purposes. Not only do these ovens offer diversity, they are more productive, time efficient, ergonomically designed, interactive, and have the ability to troubleshoot themselves. One might call the modern pressing oven the “smartphone” of today’s laboratory. Equipped with a desktop layout and functions that operate from touchscreen digital displays, these laboratory marvels are easier to use while providing more services. So how did these technological advances actually affect the revolutionization of the pressing oven?
Pressing Oven Diversity
Many new pressing ovens offer firing programs for porcelain, crystallization programs for CAD-milled restorations, and sintering programs for zirconia structures. Dual- and triple-purpose technology now enables one oven to provide services that once took two or three. By adding custom-firing programs, many press departments are now self-sufficient. They can produce a finished product of higher quality because firing and glaze outcomes can be evaluated within the department consistently. This new attribute contributes to a more productive workflow while providing a backup firing oven in the event a separate firing oven needs troubleshooting. Crystallization and sintering programs allow CAD-fabricated materials to be introduced within a pressing oven for the first time rather than firing and pressing to them. Crystallization cycles for milled lithium disilicate previously required a separate firing oven, as did zirconia substructures for final sintering. These materials may all be categorized as all-ceramics, but they are very different from one another. The simple fact that a pressing oven can process such diverse materials is a testimony to the fusion of their technology. It is obvious—today’s pressing technology is paramount when producing an all-ceramic, whether it is glass, lithium disilicate, or zirconia. What about porcelain-fused-to-metals (PFMs)? The answer is absolutely. The “bread and butter” of our industry can be produced by pressing porcelain to metal (Figure 2) or by applying feldspathic porcelain to metal, then firing in your “pressing oven.”
The new generations of pressing ovens have greatly improved laboratory production capabilities. Typically, the press process is dictated by shade. Once the shade and pressing pellet have been determined and selected, the volume to be pressed then determines whether one or multiple rings must be pressed. The problem with this scenario is if multiple rings are to be pressed, they must be pressed one ring at a time, which is less than productive (Figure 3). This technique is still common among new pressing ovens. However, there are several ovens that possess the technical means to press more than one ring at a time, even if the shade or pellet selections vary. This is accomplished by two different techniques. One technique is an oven that has twin tables that can be fired simultaneously, thus allowing the pressing of two rings at once. The second technique alters the way wax-ups are sprued. This technique is supported by a Trix Press (Jensen Dental, www.jensendental.com) muffle system that allows the pressing of five different shades (up to 20 grams) at once (Figure 4). These two technologies offer increased production in less time and have the potential to be ground-breaking new techniques.
Pressing ovens have become more like computers, much as “smart technology” has evolved our cell phones. Equipped with digital technology, today’s pressing ovens even have Internet capabilities as an option, USB ports, and come pre-programmed with as many as 200 programs left open for customization. These programs are activated through a desktop setup when the desired icon/function is touched. Scrolling through the various programs is done much the same way—either by a touch or knob control that scrolls. These ovens also feature faster logic boards that perform in real-time, allowing the flow of ceramics to be monitored and viewed on a screen display. This display provides a wealth of information, such as program cycle progress, which includes press time and posting of any errors that may occur. Press time has become a prominent factor in pressing, because it is during this phase that reaction layers are developed. The slower the press, the greater the increase in reaction layer. Today’s ovens press faster than before to compensate for this. Technology has also changed the means by which pressing takes place. Modern ovens no longer rely on air pressure to press but rather on resistance based on volume, which is digitally measured. This allows the pressing of split rings to occur with a degree of success that could not be done previously. Many new capabilities can be downloaded into existing ovens that upgrade these logic boards, bringing a new dimension to today’s pressing technology.
Manufacturers have stepped up and resolved many technical uncertainties that faced those who first invested in pressing technology. The market has expanded, offering us more options when selecting an oven to press. These ovens now offer increased production, fewer pressing errors, ease of use, troubleshooting capabilities, diversified uses, and consistency through cutting-edge pressing technology.
About the Author
Keith Miolen is the director of technical services and education at Gibson Dental Designs in Gainesville, Georgia.