Dental ceramics

Zirconium dioxide (ZrO₂), also known as zirconia, has become the preferred choice for high-quality ceramic dental replacement materials due to its aesthetic appeal, biocompatibility, and durability. This high-performance ceramic is characterized by a natural tooth-like translucency and opalescence, resembling the natural tooth. It also stands out for its high resistance, low thermal conductivity, and absolute color stability.

To achieve an optimal microstructure for zirconia, a specific thermal energy input is required. This energy input significantly affects the light transmission and color effects of the material, depending on the angle of incident light, thus affecting the appearance of the final product. Therefore, controlling and monitoring the sintering process is crucial for reproducible aesthetic results. Process Temperature Control Rings (PTCR) can help you continuously monitor your sintering furnace and achieve uniform sintering results in a cost-effective manner.

What are Process Temperature Control Rings?

PTCRs are ceramic rings that shrink proportionally to the heat input. The degree of shrinkage depends on various factors, including the temperature profile of the furnace, heat conduction conditions, and the dwell time in the furnace. The PTCRs, which have a 20mm outer diameter, can be placed directly next to the zirconia material in the sintering tray or in any location within the furnace.

The Significance of Sintering in Dental Technology

The strength and durability of ceramic dental restorations against stress are achieved through the sintering process at temperatures between 1,300°C and 1,600°C. After sintering and post-processing, a gloss- or glaze-firing is typically performed. This procedure applies a special glaze to the restorations, providing them with an aesthetically pleasing surface and enhancing their durability and resistance to abrasive influences. The glaze-firing temperatures usually range from 700°C to 1,000°C, depending on the material's specific requirements.

In addition to ongoing material optimizations, treatment and work processes in dental laboratories are continuously becoming faster and more efficient. Consultation, model creation, computer-aided design, and manufacturing technology (CAD/CAM), as well as manual modeling techniques, material selection, shaping, sintering, and color design are made within a short timeframe. Fast firing cycles are gaining increasing significance in this context. The sintering process typically takes less than half an hour, which offers the significant advantage of completing the treatment quickly. However, this places complex requirements on the materials and sintering furnaces used.

Accurate temperature control is crucial for high-quality results during the sintering process and subsequent glaze-firing. Process Temperature Control Rings can assist with ongoing checks and documentation. These regular inspections can help fulfill challenges and minimize potential issues:

Challenges in the sintering process for producing ceramic dental restorations

1. Deformation or shrinkage:
Dental restorations may experience deformation or shrinkage, as they can be up to 20% larger in the green state than in the fired state. To ensure accurate fitting of the restoration, it is crucial to maintain precise temperature control and ensure correct and uniform shrinkage.

2. Cracks and breaks:
Rapid temperature changes, especially in the context of rapid firings, can lead to stress and cracks in the material. During rapid firings, the sintered material is heated to the target temperature at rates of up to 300°C per minute and then rapidly cooled. Defects in the material can compromise the integrity of both the tooth reconstructions and the glaze, negatively affecting their lifespan.

3. Porosity:
The mechanical properties of restorations are compromised if they are not homogeneously fired, and if any pores in the material are not closed.

4. Sintering temperature control:
The final temperature of the sintering process is crucial for achieving the desired translucency and opalescence of the product. Increasing the energy input can enhance translucency, while low energy input can reduce the light transmission.

5. Color Changes:
To achieve the accurate color, translucency of ceramic restorations and aesthetic quality, it is important to precisely maintain sintering parameters based on the material used. This is important for ensuring that the manufactured crown blends well with the patient's natural teeth and is aesthetically pleasing. Deviations in the sintering process can cause discoloration or opacity in the final product.

6. Surface texture / Bubble formation:
Variations in temperature control or heat distribution can result in rough surfaces on the sintered material. Air bubbles may form in the glaze during the glaze-firing process due to rapid heating or cooling. In both cases, restoring the aesthetics may compromise the cleaning process and reduce the lifespan of the item.

7. Temperature deviations / Thermocouple monitoring:
To ensure precise temperature control and homogeneous heat distribution in the furnace, PTCRs can be used as a monitoring medium for temperature deviations caused by locally placed thermocouples that age over time and lead to discrepancies between the real and measured temperature.

8. Furnace loading:
Furnace loading has a significant impact on heat distribution, which in turn affects color and sintering outcomes. In fully loaded furnaces, each sintered item has less energy available than in a lightly loaded furnace. Therefore, firing processes should be controlled at different rates, such as using varied heating rates, to achieve uniform sintering results.

Use of Process Temperature Control Rings

The most precise results can be achieved by calibrating PTCRs to your standard sintering process. Alternatively, you can use a custom test profile. We recommend using the PTCR Standard Process: heat up to the target temperature at a rate of 2°C / minute, maintain for 60 minutes, and cool down to room temperature at a rate of 2°C / minute. Many dental furnace manufacturers and zirconia disc producers also offer control programs specifically designed for the material or furnace.

After firing, the rings are removed, and their diameter is measured to determine the reference temperature using the supplied temperature tables. This temperature represents the heat input at the specific location in the furnace where the ring was placed. PTCRs assist in assessing heat input, furnace field homogeneity, and continuous monitoring of furnace operation.

Precise control and monitoring of the firing process are crucial to minimize deviations and optimize energy efficiency. In the dental field, there are two primary ring variants available: PTCR UTH (660°C – 900°C) for glaze-firings and PTCR HTH (1.450°C – 1.750°C) for the sintering process. Depending on the sintering material and specific parameters, PTCR ETH (850°C – 1.100°C) and PTCR MTH (1.340°C – 1.520°C) are also used. To select the most suitable ring type, the temperature of your firing/sintering process should be approximately in the middle of the temperature range covered by the ring type. If the maximum process temperature falls on the boundary between two ring types, consider other process conditions such as holding time and atmosphere, as these factors also strongly influence the ring's shrinkage. There is a total of eight PTCR ring types available, covering temperatures ranging from 560°C to 1.750°C.