Support for Quality Control Programs

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Using Pyrometric Cones

In use, Standard Pyrometric Cones are placed alongside the product when the kiln is loaded. If the objective is to measure temperature differences top-to-bottom and side-to-side, then cones are placed throughout the kiln car. If temperature uniformity, car-to-car or firing-to-firing is desired, then cones are placed in the same general location of each car. After firing, the angle of cones is measured. Good practice is to only use cones bent more than 20° and less than 100°. Measurements to the nearest 5°C are sufficient for most quality programs. Tables available from Orton are used to convert cone angle into temperature.

Pyrometric Shrinkage Products

Shrinkage products are also used extensively to monitor kiln performance. These products are made from compositions that have a uniform shrinkage over a temperature range of several hundred degrees. They are used in a manner similar to that described above for Pyrometric cones.

Orton's shrinkage product is called TempChek. It is a high quality Pyrometric product that is used to determine temperature differences within kilns. Orton testing has shown shrinkage products to have greater standard deviations when compared with Standard Pyrometric Cones. This is because the sensitivity of shrinkage products (% shrinkage/°C) is much less than that of Pyrometric Cones (angular degrees/°C). Also, measurement of a dimension is subject to more error than measurement of the bending angle of a cone.

Testing of commercially available shrinkage products shows standard deviations in measured widths equivalent to as much as + 30°C. The Orton TempChek will normally provide repeatable measurements of less than + 5°C.

Use of TempChek

Use of the Orton TempChek is simple. TempCheks are placed in various locations where temperature measurement is desired. Dimensions of the green piece are controlled during pressing so that no measurements are needed on the unfired piece. After firing, the product's width is measured to the nearest 0.01 mm using a Measuring Gauge available from Orton or suitable vernier caliper.

The fired width is converted to an equivalent temperature using a chart provided with the product.

Problem Solving with Cones and TempCheks

Temperature profiles from within the kiln can be charted manually or with a personal computer. Many companies with tunnel kilns place Pyrometric products (Cones or TempCheks) throughout a single kiln car each 24 hours to provide an overall map of temperature variations within the product load and then place Pyrometric products in one or more key locations on all other cars to monitor car-to-car kiln performance. For periodic kilns, the Pyrometric products are placed throughout each firing.

It is common for Pyrometric cones to show temperature or atmosphere problems even before control and measuring thermocouples. Orton engineers regularly work with users of its cones to resolve firing problems and to map the firing behavior of a kiln. When cones behave or look differently, it usually indicates a change in firing conditions, such as atmosphere. See Troubleshooting Cones.

An example of how cones can solve firing problems. A ceramic manufacturer was experiencing a high reject rate of a technical ceramic product fired in a periodic kiln. They noted small cracks in the fired cone that were not evident in firings from other kilns and contacted Orton to determine if we could help them. These cracks represented a potential clue. After examining these cones, we determined they had a high residual cristobalite content that could only have existed by rapid cooling of the cones. After being advised that fast cooling was the most likely cause, the company investigated and found that a night fireman was leaving the air on through the burners to cool the kiln faster. Changing this practice reduced the high reject rate.

Testing And Standardization

Orton Standard Pyrometric Cones and TempChek products are dry pressed instead of plastic formed. This produces a more consistent product, gives higher green strength and a more consistent shape.

Materials used to make Standard Pyrometric cones and TempCheks are mixed, sampled and test pieces made and fired on controlled heating rates. Cone batches are tested against Standard Cones traceable to those evaluated by the National Bureau of Standards in 1956. Temperature Equivalent tables for TempChek are developed using measuring gauges and thermocouples traceable to NIST Standards. This assures accurate, reproducible and traceable behavior for all Pyrometric Products from Orton.

Orton supports manufacturers with their efforts to obtain their ISO 9001 Registration. This typically includes assisting in development of their firing practices and procedures (calibration of Measuring and Testing Mediums) for ISO 9001 Registration.

Control Procedures

Control tests made by Orton include the following:

• Testing of incoming raw materials using screen analysis, percent moisture and PCE value, where applicable.

• Testing of each production batch by sampling mixed materials, forming test cones and making comparison firings at three or more heating rates with Standard Cones. The bending angle of the test and Standard Cones are measured to within 2 angular degrees. When a difference of more than 1°C exists, small corrections are made in the mix and the procedure repeated. An average of one correction is common.

• Powder preparation for pressing consists of blending standardized powder with binder, pressing aides and sometimes colorants (to aid in Cone identification by users). Materials are dry blended, water is added and small granules are formed that are suitable for dry pressing. Moisture content and particle size are control variables.

• Dry pressing controls include pressing pressure, parting thickness of each cone pair and pressed density. Samples of each production run are randomly sampled and tested for strength, density, physical appearance and fired behavior. These typically must deform to within 10 angular degrees of Standard Cones.

• Inspection and packaging involves checking for obvious defects and randomly checking packed cones for quality and quantity of the cones in the package.

Written procedures and record keeping exist for all production and quality control operations. A unique lot number is assigned to all material and product. This can be used to identify when the powder was standardized and when it was made into products. Manufacturing records are kept for 3 years and quality control records for 10 years.

Variables Affecting Performance

Over 20 variables have been identified that can affect the bending behavior of Pyrometric Cones. These include cone composition, particle size of raw materials, type of forming process, moisture during forming, density of the part, geometry of the part, setting height and angle and the heating rate. Atmosphere also affects bending behavior.

The manufacturing process used by Orton is designed to minimize the influence of those variables that can be controlled at the factory. Investigative work showed the importance of proper mounting height and angle. As much as + 3°C variation can result from improper mounting of Large Cones. Temperature Equivalent data supplied by Orton is for a mounting height of 1.75 inches (4.45 cm) and an 8° mounting angle. Cones mounted higher will bend at an earlier temperature. At 2 inches (5.08 cm), the cone will bend 2°C earlier.

As a result of studies on cone behavior, Orton developed and manufactures Self-Supporting Cones that fix the mounting height and angle. Comparisons were made of the bending angle between dry-pressed Self-supporting Cones and plastic-formed Large Cones fired in a uniform temperature kiln. The dry pressed cones show a much smaller standard deviation in bending angle. Comparisons with cones made by other manufacturers have shown even larger variations in bending angle when fired under the same conditions.

Batch Certification

Orton measures and compares the performance of its Pyrometric products against Standards traceable to the United States National Institute of Standards and Technology (NIST).

Orton provides Certification for its Pyrometric Products. This Certification states that proper manufacturing procedures were followed and that all testing was conducted against Standards traceable to NIST. For Pyrometric Cones, the maximum deviation in °C from Standard Cones is provided for at least two heating rates. For TempChek, the standard deviation over several temperature ranges is provided.