Pay Attention to Your Ovens

Sintering remains a key part of zirconia workflows

Daniel Alter, MSc, MDT, CDT

When everything is working well, a sintering oven can be an easily forgotten piece of equipment in the dental laboratory. However, for those encountering challenges with their zirconia outcomes, temperature could be the issue, so it is critical to purchase the proper sintering oven and use it correctly. When the temperature is too low, zirconia exhibits as more opacified and dull; when it is too high, zirconia becomes more translucent but there is diminishing of chroma and value, so the restoration is grayer. When manufacturers' instructions for use (IFU) are followed and the sintering oven is performing properly, restorations should be consistent and predictable.

What factors are important to consider when purchasing?

First is the heating element, which is categorized as either molybdenum disilicide or silicon carbide. Some heating elements last longer than others, and replacing them can entail different challenges and costs. Make sure to research these factors before purchasing. Another consideration is the furnace's ability to hold temperature; some are better insulated in the chamber to maintain that high caloric temperature, and others permit fluctuation, which absolutely affects the quality of the zirconia. The better and more stable the temperature maintenance is throughout the cycle, the better your outcomes will be. Additionally, consider electrical needs, as many sintering ovens require 220 volts.

What is important regarding maintenance?

Temperature, again, is the key, so proper calibration is critical. Some furnaces need to be calibrated manually, while others are automatic. There are calibration rings available that can help. Regardless, laboratories need to ensure that their furnaces are calibrated. Otherwise, if your actual temperature is different from what you intend, the outcome of the restoration will be affected. Even if you have multiple furnaces from one manufacturer, do not assume that because one is calibrated correctly, they all are. Another important note regarding maintenance: Pre-sintering stains tend to leave residue in the chamber, so if you use them, I suggest purging the oven regularly so that those colorings do not travel into the heating filaments and impact functionality. Purging can be accomplished by leaving a piece of white, raw zirconia in for one sintering cycle to absorb the leftover colors.

How effective is so-called speed sintering?

Speed sintering is a term that is used loosely, as some companies use it to describe 3- or 4-hour cycles, but some cycles require as little as 23 to 40 minutes from the green state all the way through. Some manufacturers have developed zirconias that do very well with speed sintering, but keep in mind that it can only be accomplished on specific types of ovens. The process requires a greater electrical capability and a quicker heating ramp-up, so if you are interested in speed sintering, do not try it on a standard oven, but rather look into ones specifically designed for it.

Key Takeaway

While pre-sintered zirconia is available on the market, the manufacturing of zirconia in its green state is significantly kinder on our milling equipment, so understanding how to properly sinter zirconia remains important for the dental laboratory.

About the Author

Daniel Alter, MSc, MDT, CDT, is a professor of restorative dentistry at New York City College of Technology and executive editor of Inside Dental Technology.