Cpk or Ppk in packaging closure manufacturing?



I am a Quality Manager in a closure/end manufacturing facility. Our products are used on products such as composite cans for powdered beverage and peanuts. The Cpk/Ppk is a vital measurement that our company lives and dies by. We currently use DataMyte collection systems and run Turbo SPC, a dos based program. Here recently, I pulled raw data out of Turbo and transfered it over to Minitab. After doing this, I discovered that Turbo's Cpk was the Ppk in Minitab. Many of our customers want Cpk reports for the products they are receiving. But our company wants long term capability, Ppk. As I understand, Ppk makes an assumption on what you will do long term if you make no changes. Is this correct? Our process involves metal stamping, we change dies and die components on a certain number of strokes. Therefore, we are consistently makes changes from month to month. What would be the best measurement to use in this type of manfacuring?

Al Dyer

Cpk is used when historical data shows the process to be stable. It is based on using a sample standard deviation.

Ppk is used when a process is chronically unstable but showing a predictable pattern. It is based on the standard deviation of not the sample size, but of all the individual samples.

Your company needs to base its decisions on the above criteria and as always give the customer what he wants. Most software programs today have an option to report either or both of these measurements. One can be used internally and one can be used for the customer.

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