Multi Cavity Sampling Plan - MSPs (Multi-Stream Processes)

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S

Scott McDonald

#12
:bigwave: Thanks alot everyone for your threads. Alot of good info. I'm going to try and put some of this to use now and if any questions come up, I'll be getting back.
 
A

Atul Khandekar

#13
Glad to do what we can, Scott.

In fact a question like this one made me do some research and study - in the process I learn a few things myself!
 
S

Stan Hilliard

#14
Scott,

If fixed differences exist between cavities, these differences would enlarge the standard deviation of a random sample of 20, assuming that the sample is across all cavities.

The distribution of a random sample across all cavities would be bimodal or multimodal.

The ANSI/ASQC Z1.9 standard is based on the assumption that the random sample across cavities follows a normal distribution.

It is possible for the product from each cavity to meet requirements individually -- and still for a random sample across cavities to be rejected by Z1.9.

A sampling plan design that I have used successfully for product from multi-cavity molds is a double one-sided plan for the mean. It could use the same 20 part random sample across cavities. It has two AQL-means, with a spread between the AQL-means equal to the maximum difference between the means of cavities.

The known standard deviation used with such a plan is the pooled within-cavity standard deviation. This value will not suffer any inflation due to between cavity differences.

The RQL-mean for such a plan is calculated with the normal distribution in such a way as to match the Z1.9 RQL percent nonconforming.

When a known-sigma plan like this is used, a secondary decision rule is needed to check the "known" value. This can be done with a range test.

The calculations for the mean plan and the range check can be done by (my) software program TP414.

http://samplingplans.com/programtp414.htm

Examples of this type of mean plan are here:

http://samplingplans.com/outputvariablesmean.htm

Sincerely, Stan Hilliard
 
C

Chemman

#17
Multi Cavity Sampling Plan

Scott,

First of all, I am not sure why your customer is doing the analysis on just 20 pcs. 30 pcs is the very minimum, without using students-t. The estimate of capability increases with the sample size. This is unless there is something I am not aware of. If so, someone please explain.

Secondly, the process capability can not be evaluated by sampling parts produced off of different manufacturing processes and considering them as one. Each cavity, set when it was cut and limited only to tool wear, produces an unique mean and distribution. Individually, this cavity may produce a capable product over and over again for the long-term. Thats what the customer is interested in - statistical evidence that the process is stable and not outputting nonconforming material. If the capability indices suggest that each cavity individually meets this requirement, then there should be no problem. Data generated from randomly sampled product mixed with different cavities could result in a standard deviation calculation that suggests a process that is not capable. See attached. The resulting distribution could be non-normal for which the statistics are invalid. Using the averages from subgroups may smooth it out and appear to produce a more normal distribution, but I don't think this is correct to do either. It is my understanding that statistics can only be applied to data from the same population or process. Each cavity will produce its own unique distribution or population. Unless I am wrong here, it is my opinion that your customer needs to be shown the data and convinced he is receiving good product. I would not back down from this especially if you believe the parts are good and a number such as 1.33/1.67 is being misunderstood. Looks like an opportunity for an annual visit to the customer.

Regards,
Chemman
 

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R

rui12524

#18
Hi,

It seems to me that you are taking the hard way to solve your problem... most plastics materials WILL absorb water and grow, WILL shrink after ejection, and may experience this two conditions well afer ejection due to ambient conditions, buit in stress, etc. Bottom line, :bonk: , although you do the correct procedure in determining the Cpk, you have no control after the measurements are taken. A couple of questions arise:
1.) How do you do the inicial study? (nº pieces, time in production)
2.) how do you take the samples at production (remenber that some materials wil have odd behaviour in a couple of hours interval)
3.) how do you determine the SC limits for production from the initial study?

RMC:D
 
D

David Mullins

#19
I found many years back that sampling more than one cavity at a time can get you into trouble with the other cavities.
 
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