Process Capability Study for Electrostatic Powder Coating

[sridharafep]

Please give your views / suggestion. We have a painting process (electrostatic powder painting_semi automatic line_manual touchup and auto spray powder) for sheet metal components and the specification minimum coat thickness 70 microns and maximum 110 microns. We monitor this specification random inspection in a day and process is going on. Some days the coat thickness goes up or down and further feedback instruction given to the powder sprayer to control.

Now i would like to know is it necessary to check the process capability for this process? Is it worth spending time, in that case how can i proceed? As you know in this process the paint thickness at various area various and can't maintain say only 70 microns throughout area of the sheet.
In that case how sampling / inspection data can be collected for calculating the process capability. For your information we have more variants in components run in a batch itself (More than 50's of items having different profile)

Please give your views regarding. If any one have sample excel data sheet done - similar please let me have.

Appreciate your feedback.

Regards,
Sridhar

 

[gleclair]

Since you know that there is veriation in coating, all I could say is test each sheet in 5 places (from high to low density areas) and post the average on your chart. Then run your process capability off of that.
I don't know if this would be the right way but this is what I would try.

Glen

 

[mhozturk]

Please give your views / suggestion. We have a painting process (electrostatic powder painting_semi automatic line_manual touchup and auto spray powder) for sheet metal components and the specification minimum coat thickness 70 microns and maximum 110 microns. We monitor this specification random inspection in a day and process is going on. Some days the coat thickness goes up or down and further feedback instruction given to the powder sprayer to control.

Now i would like to know is it necessary to check the process capability for this process? Is it worth spending time, in that case how can i proceed? As you know in this process the paint thickness at various area various and can't maintain say only 70 microns throughout area of the sheet.
In that case how sampling / inspection data can be collected for calculating the process capability. For your information we have more variants in components run in a batch itself (More than 50's of items having different profile)

Please give your views regarding. If any one have sample excel data sheet done - similar please let me have.

Appreciate your feedback.

Regards,
Sridhar

Hello Sridhar,
Saying that the paint thickness goes up and down day to day, and also from area to area (I guess UNPREDICTABLY and UNINHERENTLY), I can easily say that your process is not mature yet (and also afraid that out of specification characteristics do exist in your final products, at least locally). First you have to identify the causes of these variations for elimination. These factors may be the pre-treatment processes' parameters prior to painting, the temperature of the baking furnace, the powder paint (material itself), position of the job pieces/paint guns' nozzles/line speed, etc. These of course are your profession to identify.

Anyhow, I can at this moment easily say that "to check the capability for this process" is definitely do NOT worth spending time. Don't forget: CAPABILITY MAKES NO SENSE FOR A PROCESS WHICH IS NOT IN A STATE OF STATISTICAL CONTROL.

Regards
Murat

 

[Jim Wynne]

Anyhow, I can at this moment easily say that "to check the capability for this process" is definitely do NOT worth spending time. Don't forget: CAPABILITY MAKES NO SENSE FOR A PROCESS WHICH IS NOT IN A STATE OF STATISTICAL CONTROL.

Just a note of clarification: "Process capability" is not exclusively synonymous with any particular capability index, such as Cpk, where the statistical control requirement applies. In its generic sense of the term "process capability," all processes should be proven capable, with "capable" perhaps being defined differently for different processes. What the OP needs to do is define "capable," and then figure out a way to prove it.

 

[mhozturk]

Hi Jim,
I replied the thread thinking that our colleague were to evaluate capability for the purpose of SPC charts (maybe initiating the application). When I rechecked the threat, I found no info about such a purpose. Therefore my reply should be taken into consideration when we are talking about SPC chart initiating decision for such a process.

Regards
Murat

 

[Bev D]

Since you know that there is veriation in coating, all I could say is test each sheet in 5 places (from high to low density areas) and post the average on your chart. Then run your process capability off of that.
I don't know if this would be the right way but this is what I would try.

Glen

actually that would not be the right way. The average suppreses the actual amount of variation seen.

The real questions are: what is the part used for? what function does the paint perform (in other words how critical is the thickness across the part??)

Jim is also correct, the stability of the process is a critical first assessment - you must plot the variation in time sequence on a control chart first. Then if you have a stable process (and you can only tell if you plot the data) you can assess capability.

 

[Darius]

I think that the actual data could clearify the problem. Going Up or down is no description at all (you may expect to see it on normal variation of a stable process).

In that case how sampling / inspection data can be collected for calculating the process capability. For your information we have more variants in components run in a batch itself (More than 50's of items having different profile)

The data could be sampled on different parts and chart it as one single chart, but if a difference on different areas is expected, to chart it as one chart goes against "rational sampling". You have to use your knoledge to try charting parts that may behabe alike on the same chart.