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How to set up Continuous CpK monitoring of an injection mold process

#1
I need to establish continuous CpK monitoring for an injection molded part with three critical features and I’m wondering how to set this up.

Is there a reliable excel template that I could download then use to cumulatively record the data and monitor the Cpk?

How many parts should be pulled from each production run? Any info about how to set this up would be appreciated.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#2
why Cpk? why not just a straight forward control chart?
Cpk values will be very variable with relatively small sample sizes
what is your sampling size and frequency?
will you be sampling every cavity? if not how many cavities - how will you select them?
 
#3
The idea of using Cpk is coming from management. Sample sizes and frequency haven't been established and are part of my initial question. There is only one cavity. Typically, a production run is about 300 pieces.
 
#4
So here is what we do for customers who require a specific CPK to be maintained. We setup and approve the start of the run. We then take 20 plus samples out of the first 100 or so pcs. to establish the initial CPK -- so we don't get to the end of the run and find out the cpk isn't there. We then monitor on a continuious basis, adding samples to the calculation as we go.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
Sample size will matter.

but why does your management want Cpk? there are ways of ensuring high capability without calculating the flawed Cpk index.
 
#7
The idea of using Cpk is coming from management. Sample sizes and frequency haven't been established and are part of my initial question. There is only one cavity. Typically, a production run is about 300 pieces.
Only one cavity makes matters much simpler but "three critical features" leads to some interesting questions. Short run SPC methods are available for handling multiple product features but things get more complicated if they are correlated (multivariate normal). The bottom line is however that SPC techniques are available for multiple characteristics.

Cpk should not be recalculated for every sample, because this is as bad as allowing a control chart's control limits to adjust themselves when each subsequent sample goes into the process statistics. The idea is to fix the limits based on past experience, which includes an estimate of the process mean and variation which in turn give you Cpk and Ppk. (As this is apparently single-unit flow, Cpk and Ppk will HOPEFULLY be equal unless there are changes in process conditions during the run that cause the process mean to change.) 20 data (I am assuming one measurement per sample as it is a single-cavity mold) are not really adequate to estimate Cpk anyway. This could be an application of short-run SPC in which target means and standard deviations are assumed from past experience.

The only real reason to recalculate Cpk and Ppk is if a process improvement reduces the variation, in which case the control limits will need to be tightened to reflect this.
 
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