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#### yerasij

Background: We work with contract manufacturers (CM's) who make Injection molding parts for us. Historically, we used to collect 32Pcs. (not sure where that came from) data and calculate CpK (without understanding if the process is stable or if the distribution meets normality assumption). In the last year or two, we have been working with all our CM's to establish proper control charts and understand the stability and reduce "special causes". We overcame many challenges (trying to understand the root cause and eliminating them or adding the "causes" if it helps reduce variation etc.,) and now we are at a step where we want to improve how we understand and report out capability.

There are many CTQ's identified on the drawing (some are safety critical and some are critical to cosmetics). Now were are trying to understand the "real" capability of the process and understand # of defects shipped out.

The questions I have are:

**Question 1**: Lets assume an injection molded part (PC) has 5 characteristics (SPC A,B,C,D and E), and 4 of them (SPC A,B,C and D) are "non-normal". When I try to do best fit method (JMP or Minitab)- all the 4 curves have different distributions. Is it okay to pick different distributions for every characteristic ? (same parts coming from same cavities) or do I need to pick one distribution that has an "Okay" fit for all the characteristics.

Please note that I am only trying to understand the process capability (DPPM) and not trying to compare the characteristics.

The same SPC's follow a different distribution on the next set of tools (which has the same tool structured and similar process parameters). I did not see any statistical difference between the tools or cavities in means or variation.

**Question 2**: Why aren't the dimensions of a simple injection molded part always follow normal distribution ? I looked at the tool structure and wasn't able to find a "physical limit". Why are they not following normal distribution as expected ? Is that bad ?

Please let me know if you need any more information to help answer these questions.