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One of the final steps for all ceramic products is the firing or sintering process. Using raw materials with rigid specifications, preparing body formulations to exacting standards and employing closely controlled forming processes will only produce a quality finished product if the firing process delivers the correct amount of heat work to your products. Heat work is the actual measurement of the combined effect of heat and time. For meaningful quality control/assurance, you must be able to measure the uniformity and consistency of heat work delivered during the firing process.
Both tunnel and periodic kilns are equipped with several thermocouples to monitor temperatures and provide the electronic temperature controllers with the necessary information for controlling the firing process. The thermocouples are usually mounted in the crown or sidewall of the kiln and do a great job of measuring temperature at a given point in time and space. However, they are not actually measuring the temperature in the ware setting, and they cannot confirm the level of heat work being delivered to the ware throughout the cross-section of the kiln. As a result, quality-conscious ceramic manufacturers also run traveling thermocouples and/or place pyrometric devices within the setting. Although traveling thermocouples can sometimes be difficult to use, pyrometric devices are a simple and cost-effective way to monitor your firing process on a daily basis.
To maximize the use of pyrometric devices, you should first establish a system for monitoring the heat-work delivered by the firing process, then develop a database to track heat work measurements and display them in a user-friendly format. Finally, you'll need to correlate the heat work measurements to the fired properties of the ware. By using this simple three-step procedure, you can improve your control of the firing process-and your overall product quality.
Monitoring Heat WorkFirst, design a placement diagram for the location of pyrometric devices within the setting of the ware on your kiln(s), and establish a schedule for placing and retrieving the devices (see Figure 1). Next, convert the pyrometric device measurements to temperatures and enter these figures into a database. Using a digital measuring device connected directly to a computer makes the conversion of shrinkage to the measured temperature simple, and the data can be automatically transmitted to a custom database (see Figure 2).
Analyzing the DataUse a simple statistical software program to display the data in X-bar and R-bar control charts (see Figures 3 and 4). These charts will indicate any trending of the firing process before it becomes a major problem, and they also serve as a benchmark for the firing process. When problems do occur, it is essential to know where the firing has been historically before you eliminate or indict your kiln as the source of product defects.
Correlating the Data to Fired PropertiesCeramic products have a window of firing range that produces quality ware. Quality and required properties are defined differently for different products, and may include fired absorption, strength, shrinkage, density, color, etc. If you are not sure of the ideal firing range of your products, it is probably worth your time and effort to conduct a series of designed experiments to determine the firing range of the critical parameters.
Once the correlation of product properties to heat work is established, the heat work analysis serves as a sound, nondestructive quality control tool.
Enhancing Quality ControlGaining an understanding of your product's firing range and monitoring your kiln's efficiency in delivering heat work uniformly to your products can be easily accomplished by using pyrometric devices such as standard pyrometric cones, pyrometric shrinkage keys or process temperature control rings. Plotting the data provided by the pyrometric devices in statistical process control charts makes the information easily accessible and user-friendly. Your quality assurance program will be enhanced through your knowledge of how well the firing process is delivering the proper heat work to your product.
For more information about measuring heat work during firing, contact Orton Ceramic Foundation, 6991 S. Old 3C Hwy., Westerville, OH 43082-9026; (614) 895-2663; fax (614) 895-5610; e-mail firstname.lastname@example.org ; or visit http://www.ortonceramic.com .