A Decision That Can Make or Break Your Case

Zirconia has grown from its humble beginnings as a “metal alternative” to the high-strength, highly esthetic solution that we see today. During this evolution, the material size has been standardized, but the formula and manufacturing of zirconia are still far from consistent, as sintering temperatures and ramp/cool rates vary greatly. As the final step in the zirconia production process, the importance of sintering is often overlooked. Every laboratory has its own unique needs—ranging from materials to milling machines to indications they would like to add in the future. To be as successful as possible with producing zirconia restorations in-house, it's critical to choose a sintering furnace that is available at a price that works best for each laboratory, has the flexibility to accommodate all the materials on the market, and also produces consistent results. The following are the most important considerations for selecting the right sintering furnace for your laboratory.

The majority of furnaces use one of two types of heating elements—molybdenum disilicide (MoSi₂) or silicon carbide (SiC). Each has its own pros and cons.

Furnaces with MoSi₂ heating elements boast two differentiating factors: a longer life and higher top heating temperatures. These elements last roughly twice as long as SiC heating elements, but are twice the price when replacement occurs, meaning that the cost is the same in the long term. The top temperature reached with MoSi₂ elements can be as high as 1800°C, depending on the program; however, most top out around 1650°C, which is more than enough for any zirconia available on the market today.

Importantly, there is a bi-weekly or monthly cleaning cycle that is needed to purge pollutants from the firing chamber. Neglecting the periodic cleaning cycle can cause degradation of restorations, as the zirconia will absorb leftover elements, which can have an impact on shade.

SiC elements typically reach top temperatures ranging from 1500°C to 1600°C, depending on the make and model. For this reason, it's very important to make sure your favorite zirconia products don't need to reach a higher temperature than this if you choose a furnace with this type of heating element. There is no maintenance or cleaning cycle required for SiC elements, as they do not contaminate, but like mentioned previously, they do have a fairly short life so having extra elements on the shelf is a good idea just in case.

Speed sintering typically works best with SiC-type elements due to their inherent ability to heat and cool very quickly. Some furnaces with MoSi₂ elements are capable of heating very quickly but cannot cool with equal speed.

It is important to note that rapid cooling of restorations in furnaces with MoSi₂ elements may result in element fractures due to thermal shock. The material is actually the most critical part of speed sintering. It's very important that the manufacturer of the zirconia has tested speed sintering to make sure there is not a huge reduction in material strength during these cycles. With the right material and furnace, speed sintering can be game-changing for production environments and rush cases.

Another variable that's important to remember: when speed sintering, the furnace capacity is approximately 25%, so be sure to adjust the firing volume accordingly.

If you read any technical data sheets on sintering furnaces, you'll note that manufacturers typically list how many units can be accommodated per tray and how many trays can be stacked, yielding “firing volume”—a total number of units that can be sintered in a single cycle. When sintering zirconia, it's very important that the heat permeates both the thinnest and thickest parts of the zirconia in the furnace to produce a structurally sound restoration. Therefore the firing volume, size of the chamber, and number of heating elements are all factors that should also be considered. For example, some furnaces offer six heating elements and a large firing chamber, while others only have two heating elements and a much smaller chamber.

Here are some helpful sintering tips for laboratories:

•  For a furnace with only three heating elements, don't fire more than one full-arch bridge at a time.

•  Cases that are larger and more complex, such as a full arch, can take more than 10 hours to ensure all parts are heated and cooled at the same speed to prevent warping.

•  Never pack a furnace full. Always leave enough space between units, as it's not worth the risk of having an entire batch of units come out only partially sintered.

•  Make sure any furnace under consideration can meet production needs while only being filled to 50% capacity.

•  Consider owning two sintering furnaces, as redundancy reduces downtime and prevents a pinch when a spike in production occurs.

Another option to consider is whether a front-loading or bottom-loading sintering furnace is the right choice. This choice can have a significant impact on pricing.

Due to a consistent thickness of the muffle and the thermal characteristics that are inherent to the design of bottom-loading furnaces, it is the author's opinion that this option typically delivers better thermal uniformity. It is, however, typically costlier.

In the past, having a furnace with programming functionality was rare. But now almost all furnaces are fully programmable, although some have simpler user interfaces than others. It's very common for manufacturers to post videos online that show detailed step-by-step tutorials on how to program and operate their products. Reviewing such support documentation offers good insight on what to expect and can help make the purchasing decision easier.

Always consult the manufacturers' resources online and contact your reseller for best practices for maintaining your sintering furnace. Make sure that if you consider buying cleaning powders and other products for your sintering furnace, that those products are fully supported by your specific furnace.

If you are getting less than ideal translucency, your furnace might not be hitting top temperatures accurately. In that case, a simple recommendation would be to raise the top temperature by 10°C to 20°C and make changes from there. For a more scientific process, there are temperature calibration rings. (More information on temperature calibration rings can be found at insidedentaltech.com/idt994).

Sintering is likely the most important part of the zirconia manufacturing process and can make or break a beautiful restoration. Make sure the company you are buying from stands behind their products by providing great support, stocking replacement parts, and maintaining loaner programs in the event of a return for repair. Having a reliable sintering furnace that consistently produces beautiful restorations is well worth the long-term investment.

Sam Wainwright, BS, is Digital Equipment Sales Manager at CAP/Zahn Dental in Stoneham, Massachusetts.