Sintering Furnaces

Most zirconia being milled today is referred to as “green-state” or “pre-sintered” zirconia. It starts out as a slurry of zirconium oxide, yttrium oxide, hafnium oxide, aluminum oxide, and a number of other trace compounds that are pressed at room temperature into a block, cylinder, or disk. In this state, the material is relatively soft and malleable, not yet suitable for milling. It must be placed into a furnace in the processing facility and fired at a precise temperature, eliminating binder compounds while it hardens and turns into the chalk-like state that milling centers and milling laboratories would recognize. Material density in the pre-sintered state is approximately 40% to 50% of the theoretical maximum density.

Hot isostatically pressed (HIP) zirconia starts out with essentially the same slurry of materials as green-state or pre-sintered zirconia. It is pressed at extremely high pressures and high temperatures, simultaneously forming and sintering the zirconia in a single step, achieving 99% of the theoretical maximum density.

Ceramic sintering is the process where heat, and sometimes pressure, transforms a ceramic material, reducing porosity and increasing particle density. Other properties, such as strength and translucency, are often enhanced. Pre-sintered zirconia starts out in a monoclinic crystalline structure, with a chalk-like appearance and feel that is easy to mill or contour with a slow-speed handpiece. At around 1,100°C to 1,200°C, zirconia transforms from the monoclinic structure to a polytetragonal crystalline state, increasing particle density, strength, and translucency. The chalk-like appearance and texture is replaced with an extremely hard, dense, and incredibly strong ceramic material that even a high-speed handpiece with diamond burs would struggle to cut. Sintering also causes zirconia to shrink approximately 25%.

While zirconia transforms from one structure to another at around 1,100°C to 1,200°C, most sintering furnaces fire at temperatures closer to 1,500°C. Final sintering temperatures can have a profound effect on the zirconia. Typically, the higher the temperature, the denser the zirconia b comes, usually close to 99% of the theoretical maximum density.

Prior to being placed in the sintering furnace, green-state zirconia is usually positioned in a crucible filled with zirconia beads. The beads allow for movement of the zirconia as it shrinks (Figure 1).

Zirconia manufacturers provide a r commended sintering temperature profile for their material. This profile typically includes a temperature ramp rate, final temperature, hold time, and sometimes a cool down ramp rate (Figure 2). Varying from this profile may cause deviations from the published specifications for density, strength, and translucency. Different zirconia blends, such as High Strength for bridge frameworks or Ultra Translucency for full-contour restorations, even from the same manufacturer, may have different sintering profiles.

A typical sintering cycle may run from 6 to 8 hours depending on ramp rates, final temperature, and hold time. Some zirconia manufacturers offer r commendations on high-speed sintering temperature profiles and indications. Others specifically do not endorse high-speed sintering, or are silent on the subject.

Most of today’s porcelain furnaces are totally programmable. Many different porcelains, stains, and materials have forced manufacturers to make furnaces programmable and be able to store different programs for easy retrieval.

Early sintering furnaces typically only offered a single, fixed sintering profile. Some furnaces had options for several pre-programmed cycles. Initially, there were not many zirconia options available, and most manufacturers r commended very similar sintering profiles.

Today, with so many different zirconia suppliers and formulations, most furnaces are programmable. Ramp rates, final temperature, hold time, and sometimes a cooling rate can all be manually programmed into the furnace, usually by pressing a series of buttons on the front. Some of the more advanced furnaces are not only programmable, but have built-in memory so multiple sintering profiles may be stored and recalled for a specific production run, just like today’s porcelain furnaces.

There are three main size categories of furnaces, each having their benefits and focus toward certain segments of the market. The largest ones are suitable for general production in a milling center. Features include a large volume-heating chamber, with a capacity of 150 to 200 units and are typically fired in the longer 6- to 8-hour cycles. Heating a larger chamber uniformly with a large volume of zirconia inside takes time to do properly. These are typically the most expensive, yet most productive, furnaces.

There are medium-sized furnaces suitable for laboratories and milling centers that may handle 60 to 100 units of production per 6- to 8-hour sintering cycle. These furnaces are probably the most common in use today and are generally the lowest-priced furnaces available.

Some of the smallest furnaces offer a rather unique benefit. While most only handle 20 to 50 units of production per cycle, many of them are capable of running sintering cycles in as little as 90 minutes. Smaller heating chambers can heat up and cool down quickly. These sintering furnaces can be very helpful in same-day milling and sintering of cases. However, even though high-speed sintering has been shown in some studies to provide slightly increased density and strength, not all zirconia suppliers are endorsing their use.

The two common heating methods used by sintering furnaces are ceramic elements and microwaves. Figure 3 shows the glow coming from the elements during a typical sintering cycle. Note that the platform is normally fully raised during sintering.

Molybdenum disilicide (MoSi2) is the most common material used for heating elements. Some furnaces with these elements are capable of achieving temperatures as high as 1,800°C. While they are more expensive than other materials, MoSi2 elements are generally very stable, maintaining a constant electrical resistance over time.

Silicon carbide (SiC) elements are gaining popularity. They are self-supporting and lend themselves to span distances that MoSi2 typically cannot cover. SiC elements are limited to temperatures around 1,600°C. They are less expensive than MoSi2 elements but their electrical resistance tends to increase over time, which will prompt more frequent replacement.

Microwave sintering is one of the newest sintering options. Monoclinic zirconia does not respond to microwave energy directly. Microwave sintering ovens require a susceptor plate and often susceptor material in the sintering trays that absorb microwave energy converting it to heat. This technique is used in residential microwave ovens to help cook or crisp certain frozen food products. Microwave ovens can be used for high-speed sintering and can be scaled for larger volume capacity. Cooling of the operating equipment b comes the challenge, but can be ove come with auxiliary cooling systems.

Most of the zirconia in use today is shaded. Sometimes the zirconia is pre-shaded from the supplier, other times it is submerged in shading liquid or the liquid is painted on before sintering. Often the zirconia still remains bleach white prior to sintering, but shading pigment b comes very apparent after sintering (Figure 1). The temperature as well as temperature profile can affect how the shading liquids work. It is advisable to test shading materials, techniques, and zirconia when sintering profiles are changed.

Sintering furnaces are necessities for milling centers and laboratories milling their own zirconia. New options exist in terms of heating method, programming, and capacity. Laboratory owners need to carefully consider which type best fits with their operation. Sometimes the best fit may end up being a combination of sintering technologies and capacities.