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SPC and Injection Moulding

P

Palt88

#51
Two comments:

1. Each cavity must be evaluated separately. Mixing streams of production will only confound the data. The delta Xbars between cavities will impact the evaluation of sigma.

2. Since injection molding is an auto-correlated process, the use of Xbar/R charts are invalid as the premise of independence has been violated (again sigma will not be accurately estimated). The more appropriate tool should be IX/MR.
1. Ok but following each cavities separately is almost impossible to manage...
I can see that in case of variations, all cavities separately , or the mean of a "shot", is going have the same trends. Is this match with the subject you are thinking of?

2 thanks for the point. Do you mean it's better to follow moving range of standard and/or mean of subgroups?

Thanks
 
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Howard Atkins

Forum Administrator
Staff member
Admin
#52
Two comments:

1. Each cavity must be evaluated separately. Mixing streams of production will only confound the data. The delta Xbars between cavities will impact the evaluation of sigma.
This is open to debate: Why do we want a good CPK?
To prevent possibilities of bad parts, BUT when you have fit and function then it is required that the CPK is such to allow assembly, especially if it is automatic. Cavities do not arrive sorted so that the customer needs the CPK for the mixed cavities!
 

bobdoering

Stop X-bar/R Madness!!
Trusted
#54
Re: SPC and Injection Molding

Cavities do not arrive sorted so that the customer needs the CPK for the mixed cavities!
The correct answer is you need both - total and individual cavities. If the total is low, the individual will determine what needs addressed.

Chances are with multiple cavities the output will be multi-modal, therefore non-normal and Cpk calculations will not be applicable...but might be good for some backyard-finger-in-the-wind estimates.
 
K

krishnau1

#55
Your article and this discussion is very helpful to me and my team as we have a real good methodology to explain to customers who harp on spc study for sc dimensions. May of the customer representatives are from other process background and would know little about the injection moulding process capapbility, especially when tolrances band is around 0.6mm. In many cases mean is ntentionally shifted to any one side to achieve desired fitment and feel, and dimension in drawing is left unchanged. While we easily achieve the cp , cpk always goes much below 1. Thanks again for the insights, team.
 
A

Amy_060209

#56
Thank you for your analysis.Though I am not so familar with the procedure of mould injection,it expend my field of vision.
 
F

fredoch66

#57
After the subject arose in the cove in the last few weeks (Measuring Injection Molding Quality using Weight of final assembly) I found this article that I wrote in 1992 as part of a project that I did for a practical plastic engineering course that I took.
Hello Howard,
I read the papper you wrote and I have to say that I'm really interested on it.
Now let see if I am lucky...
Do you think that you still have the results of the study you made on those times?
Actually, I would like to see exactly what you have done and try to understand every correlations that can exist between weight, dimensional aspect, and process parameters.

Thanks!
 

Howard Atkins

Forum Administrator
Staff member
Admin
#59
Hello Howard,
I read the papper you wrote and I have to say that I'm really interested on it.
Now let see if I am lucky...
Do you think that you still have the results of the study you made on those times?
Actually, I would like to see exactly what you have done and try to understand every correlations that can exist between weight, dimensional aspect, and process parameters.

Thanks!
I might have but I am traveling at the moment

When I get home I will look
 
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