Advanced Ceramics / Firing and Drying

CASE STUDY: Consistent Firing Results

April 1, 2011
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Ross Ceramics uses Bullers Rings as an essential part of its statistical process control procedure.

Ross Ceramics' ceramic cores are used in the investment casting of high-performance, directionally solidified and single-crystal gas turbine engine components. (Photograph reproduced with the permission of Rolls Royce plc, © Rolls Royce plc 2010.)


Bullers Rings are pyrometric devices that are used to accurately measure the effects of heatwork within a kiln firing environment.

With 40 years of experience in the manufacture of ceramic cores, Ross Ceramics Ltd. (RCL) is continually at the forefront of advanced ceramic core technology. As a wholly owned Rolls Royce subsidiary, RCL has particular expertise in the manufacture of the complex geometry ceramic cores that are used in the investment casting of high-performance, directionally solidified and single-crystal gas turbine engine components for aerospace and industrial applications.

RCL's materials are developed to give stability at high casting temperatures, no adverse core to alloy reactions, and dimensional consistency. The use of Bullers Rings is a critical part of RCL's routine quality assurance procedures.

What Are Bullers Rings?

Bullers Rings are pyrometric devices that are used to accurately measure the effects of heatwork within a kiln firing environment. (Heatwork is the action and effect of temperature over time on a ceramic product. If the product receives too much or too little time at the correct thermocouple temperature, it may be under- or over-fired.)

UK-based Taylor Tunnicliff Ltd. specializes in the manufacture of Bullers Rings, which are used by many of the world's leading tableware, sanitaryware, brick, refractories and technical ceramics manufacturers to measure the amount of heatwork that has taken place in the kiln. Bullers Rings provide an independent, accurate, reliable and essential indication that the firing of the product is correct and unaffected by any variations in temperature and kiln loading.

Figure 1. Bullers Ring readings are inputted directly to SPC charts.

How are Bullers Rings Used?

The physical properties of the ceramic core are of paramount importance for RCL and, indeed, Rolls Royce. To help ensure that the finished parts meet quality requirements, RCL bases its statistical process control (SPC) charts directly on Bullers Ring readings.

Bullers Rings are first placed on and around the setter blocks and saggars on the kiln cars. Readings are taken after the firing, and these readings are inputted directly to the SPC chart. As can be seen in Figure 1, the readings are plotted against an average and a range, per kiln side. Over time, Ross determined the appropriate Bullers Ring range for its firing parameters. Close monitoring of the SPC charts ensures that corrective action can be taken immediately, should the Bullers Rings readings fall outside the acceptable parameters. For a Type 55 Bullers Ring on a firing of 960-980°C, a typical range of 10 to 14 can be expected.

In addition to providing essential SPC data, RCL has found Bullers Rings particularly useful when products need to be fired on an alternative kiln. For example, a new kiln may have a more powerful or efficient burner. The use of Bullers Rings enables the firing profile for the new kiln to be set up accurately and efficiently.

Summary

Bullers Rings are used to verify consistent heat distribution between firing cycles without the expense of extensive testing. The use of Bullers Rings provides a clear indication that the correct properties have been achieved, resulting in RCL's ceramic cores performing as expected in the casting process. This cannot be achieved using thermocouples alone, since they only measure temperature. If Bullers Rings were not used, extensive, costly and destructive thermal expansion testing would be required to ensure that the ceramic cores are fit for their purpose.

For additional information, contact Taylor Tunnicliff Ltd., Normacot Rd., Longton, Stoke-on-Trent, England, ST3 1PA; call (44) 1782-501174; fax (44) 1782-328807; e-mail bullers@taylortunnicliff.co.uk; or visit www.taylortunnicliff.com.

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