SPC tools for wall thickness and weight - Suggestions?

[Thipan M]

spc tools

hi,
This is the first time posting.

what is the good spc tool to monitor "wall thickness" for plastic parts ? and " weight" for the plastic parts?


thanks and take care

 

[Mike S.]

Welcome, Thipan. Glad to have you aboard.

Your question is a bit too open ended for me to give you an answer. The answer will probably be control charts of some type. I would suggest you get a good book on SPC -- you can probably find some recommendations in this forum. You can also find some good info at these sites:

(broken link removed)

www.freequality.org

Or, if you can provide some more detail and be a bit more specific you can probably find people here who can help you.

 

[Proud Liberal]

Do not attempt to use Xbar and R charts. The molding process is autocorrelated and your control limit will be skewed (and not in your favor). X/MR charts work better.

Also, do even bother with weight. The sensitity of the measurement isn't capable of making meaningful use of the data. If your intent is to capture short shot information, an attribute chart may be useful. The area that a tool will short shot is fixed so only evaluate the cavity that fills last (prior to packing the tool). You can easily identify that by intentionally decreasing the shot size until you determine which cavity that is. Focus your efforts on that cavity.

 

[Darius]

Hi Thipan,

If Zeno is right about the Xbar and R charts, and autocorrelation is a problem, you may need the autocorrelation coefficient applied to the sigma estimate (1/(1-r^2)^0.5) to inflate the control limits a little bit.

Sigma(x) = MRaverage/1.128*(1/(1-r^2)^0.5)

X/MR charts are the best tool for continuos process (even as a replacement for attribute charts), that are very common on the chemical industry.

About the sensitivity of the measurement, you know your variables and the capabilities of the equipment used to weight, you could also try X/MR charts as weight or as a percentage out of specs per unit of time.

Good luck