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Understanding creep behavior provides insights for design improvements and quality assurance.
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Controlling the temperature when sintering dental zirconia crowns, veneers, and implants is critical in order to achieve the highest quality.
The color, size and strength of dental parts made from zirconia are directly related to the temperature they are sintered. The target temperature required to properly sinter Zirconia does not allow for much error. Without proper control of the temperature, all of the detail and workmanship put into the parts can be ruined if placed into a poorly controlled furnace. Control is achieved through the use of electronic temperature controllers, thermocouples and calibrated shrinkage products (TempTAB®).
Electronic controllers are basically volt meters. They convert the millivolt signal generated from the thermocouple into a temperature. Extra computer logic is added along with some hardware to make the controller do some additional tasks to heat and cool the furnace on a programmed heating cycle. To perform properly, the electronic controller has to be calibrated. This is not a one-time thing. Electronics degrade over time and require service to compensate for the degradation. If not accounted for, a controller can easily misreport the actual temperature by as much as 50 degrees C. Check your owner's manual and contact your furnace supplier to find out how to calibrate the furnace according to their recommendations.
Thermocouples are made from two dissimilar metallic wires that are welded together to form a bead. When the bead is heated, a very small electrical signal is generated in the wires. The electrical signal is on the order of millivolts and can be detected by connecting the ends of the wires to a volt meter (electronic controller). Since the wire used to form a thermocouple degrades with each thermal cycle, the small electrical signal it generates will change as well. Over time the signal will degrade to the point where the temperature read by the electronic controller will no longer be accurate. The furnace readout will continue to display a temperature value, but the actual temperature can be off by over 100 degrees.
To help the operator monitor the accuracy and repeatability or the furnace, they can place a calibrated shrinkage product into the furnace. The TempTAB 700, produced by Orton, is just such a device. The TempTAB 700 is made to be sintered along with the Zirconia parts. The operator will measure the diameter of the TempTAB after the thermal cycle is complete and input the measured value into the TempTracker software program. Orton engineered the TempTAB 700 so that the shrinkage can be converted to a representative equivalent sintering temperature. By using the TempTAB on a regular basis,( daily or weekly) the operator can determine when the thermocouple and electronic controller begin to degrade and insure that the zirconia reached the proper sintering temperature.
Since the TempTAB do not rely on an electrical signal that can change with each cycle, they can be trusted to provide a real measurement of the temperature seen within the furnace. By using the control chart generated by the free TempTracker software, the operator can see when it is time to either replace a thermocouple, or call for calibration service. The best time to begin using the TempTAB is immediately after a calibration or at the start of using a new furnace to establish a benchmark for future comparison.