Exceeding the recommended sintering temperature could lead to deterioration of the product chemical and mechanical characteristics. If a ceramic material is “over-fired”, e.g. 70/80°C higher than the recommended temperature, there would be growth in the crystals, with possible deterioration of the typical characteristics of the Zirconia.
Another important parameter is the time it is held at the maximum sintering temperature.
The sintering of the zirconia is in function of temperature and hold time: we say a product can be sintered at 1450°C with a hold time of 2 hours, or at 1500°C for 1 hour. The result is the same.
The HOLD time at maximum temperature allows complete sintering of the Zirconia products. Therefore, the greater the mass to be sintered, the longer will be the time at maximum temperature necessary for correct sintering. At least two hours are therefore necessary to sinter crowns or large bridges, whereas one hour is sufficient for a cap.
The “time” to reach the maximum temperature, on the other hand, depends on the size, shape and mass to be sintered, but does not influence the final quality of the Zirconia products.
Tooth with mass lower than 1g.
Sintering speed: up to 900°C, increase by 10°C/min. (600°C/h), then gradually reduce the heating speed to 1.5°C/min. in the section 1400-1450°C.
Tooth with mass up to 3g.
Heating speed: 5°C/min up to 900°C, then gradually reduce to 1°C/min in the section 1400-1450°C.
Tooth with mass higher than 3g.
Heating speed: 5°C/min up to 900°C, then reduce to 0.5°C/min in the section 1400-1450°C.
The maximum temperature time may be reduced to 1 hour, depending on the thickness of the prosthetic structure.
Certain sintering cycles may be completed in 8 /12 hours, but this depends on the geometry of the prosthetic structure to be sintered.
The laboratory must optimise the cycle on the basis of the product to be sintered to avoid distortions and cracks.
Sintering of Zirconia depends on the type of powder used. Some powders sinter at 1350°C, others at 1450°C, and others at 1500°C.
However, sintering temperatures have the following tolerance:
The sintering range for the ZYTTRIA Z401 WHITE and ZYTTRIA Z COLOR is between 1450°/1480°C and 1500°/1530°C for the ZYTTRIA Z402 EXTRA-TRASLUCENT.
Sintering of Zirconia depends on the type of powder used.
Some powders sinter at 1450°C (ZYTTRIA Z401 WHITE and ZYTTRIA Z COLOR), others at 1530°C (ZYTTRIA Z402 EXTRA-TRASLUCENT).
Due to the low thermal conductivity of Zirconia, the thermal gradient (or temperature increase and decrease speed) depends on the mass of the structure to be sintered. Besides mass, sintering speed also depends upon the shape of the cap, bridge or crown to be sintered.
The tooth with the highest mass determines the firing cycle speed.