I received this e-mail: Subject: PPAP Preliminary Capability Studies
Date: Sat, 26 Sep 1998 19:08:35 -0400
From: Robert Raymond

We are a paint company with our final audit the last week of October. We are still having problems with Cpk's. We produce 10-12 batches (1000 gallons each) per year of a coating to our customer requiring QS certification. We are at a loss on how to do capabilities with such low amounts of data. Can we check a particular batch of paint, during the process, say 40-50 times for grind (particle distribution)? Can we spray out 40-50 panels on this batch and check gloss or viscosity to have some data?? I really don't know what else to do. Any help would be appreciated.

thank you

Bob Raymond

Do you have any critical characteristics as you only have to produce CPK's for these.
The point of CPK's is to show that the process is stable but most people use and are expected to use results from the product.
Does your customer demand CPK's or are you doing this because you believe that you must do it? Speak to your SQE and ask if he really needs this data.
How long does it take to produce 1000 gallons? Do you have a parameter in your process that you can measure every so often to get readings, does this parameter have distinct tolerances, otherwise there is a problem with CPK's.

Is anyone here involved in batch processes like this? Just wondering if anyone has any thoughts on batch production.

Being someone fairly new to my position, you wish to take this with a pinch of salt and probably some pepper:

We do Intial Capability studies on critical charactersitics for many of our production items.

In order to get an initial Cpk value we use just 50 samples from a batch of about 2000 products.

We then choose if we wish to conduct further studies dependant upon customer requirements and whether the process appears stable and within specification.