Breviary Technical Ceramics



 Process Flow Analysis

Control cards are interpreted with the help of statistical rules. The definitive statement is: this process is or is not statistically under control. A process which is under statistical control is stable and predictable. It is then possible to examine its capabilities, fulfil prescribed specifications and determine corresponding data. Such a parameter is called the capability index.

A statement about the spread of a process relative to the specified spread, independently of the position of the mean value of the corresponding product characteristics, is given by the cp value. In contrast, the cpk value permits the spread width and the position of the mean value of a process relative to the closest "critical" specification limit to be described

The minimum requirement is that the value of cp or cpk should be greater than one. This means that the range x = ±3s must lie within the specification limits.

Figure 234: Process capability

Figure 235: Reducing the spread of the process.


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