Paint Color Control

[Richard]

Hello,
I am a supplier quality engineer working for a medical device company. We are currently very close to launch some new products. The design team intends to assign a tolerance on product color to as per customer?s requirements. A CIE color spec with a 0.5 delta E will be used instead of the current subjective visual inspection with color chips. I appreciate if some one can share some experience in painting process. My questions are:
1. Is 0.5 delta E a tight tolerance for a normal painting process? What are the typical process variations which can affect the color process stability?
2. How is painting process controlled? Is Spectrophotometer used and how often should color be checked? I understand the color measurement can be tricky based on some other relevant threads on this board. Can anyone share process control plan or documents? Is SPC used?
Hope you could help.
Thanks.
Richard

 

[ScottK]

Hello Richard.
I've dealt with color both in pharmaceutical labeling and in silk screening in medical device manufacturing.

There are books of information on the questions you have... In order to relate what you are doing to my experience can you give a little more detail on what you are painting and what process you are using? is it paint or printing? That sort of thing.

 

[Richard]

Thanks for your quick response.
My company defines the product paint spec as per customer's requirements. The contract manufacturer sprays the paint on our plastic enclosures which are provided by us. I believe the powder coating will be used in supplier?s painting process.
I am also thinking car industry has tight paint color control and would like to hear some advice.

 

[allan-M]

We run a paint line where liquid paint is applied to coils of sheet metal, usually steel. Clearly the color is a pretty important customer requirement, so our tolerances might be tight.
Each paint we run has a physical color standard. We put the color standard in the instrument then read the produced product. Our industry uses the Lab color space, and the specification we use is +/- 0.5 units deviation from standard on any of the 3 Lab scales.
Color in our process, apart from the paint itself, can be especially affected by film thickness, cure temperature and cure dwell time
I don't know much about CIE - Lab conversion
We don't have long runs so drift and calibration is not a superbig concern: pretty soon we will put in another color standard for the next run

 

[ScottK]

I think with powder coating you will see little color variation within lots. You may see it lot to lot though. Coating thickness would be the concern here.
How many colors are you doing?

If it's only a few I wouldn't spend the money on a spectrophotometer, but come up with light/target/dark standards that the customer approves. I'd only worry about color measurement if there are lots of color changes and colors are being mixed for every lot. If the lot of paint/powder is going to last for months then it's not add much value to measure Lab and delta E.

Even more so - if your subcontractor doesn't use a color measurement devices I would stay away and lean toward visual standards.

 

[Ron Rompen]

Hello,
I am a supplier quality engineer working for a medical device company. We are currently very close to launch some new products. The design team intends to assign a tolerance on product color to as per customer?s requirements. A CIE color spec with a 0.5 delta E will be used instead of the current subjective visual inspection with color chips. I appreciate if some one can share some experience in painting process. My questions are:
1. Is 0.5 delta E a tight tolerance for a normal painting process? What are the typical process variations which can affect the color process stability?
2. How is painting process controlled? Is Spectrophotometer used and how often should color be checked? I understand the color measurement can be tricky based on some other relevant threads on this board. Can anyone share process control plan or documents? Is SPC used?
Hope you could help.
Thanks.
Richard

I am the quality manager for a company which does painting for automotive, so perhaps I can give some guidance.

Delta E of 0.5 is EXTREMELY tight - the human eye can perceive (at best) ? 1 unit of color shift, so making a tolerance tighter than that is not really worthwhile.
Process parameters for painting vary depending on the type of painting being done (spray deposition, dip coating, electrostatic, etc). Without knowing how it is being applied, I can't really give you any guidelines on what the normal controls are.
SPC is seldom used in the traditional sense. We monitor our process parameters (temperature and humidity of the spray booths, viscosity of the paint, line speed, cure oven temperature, etc) but many of these are inter-related, so a change in one (ambient temp or humidity) can drive a change in another (often on the fly).
A spectrophotometer is a useful tool, but the final determination of a color match always seems to come down to the human eye. I have seen cases where the Xrite values show a deltaE of less than 1.0, but the customer just doesnt like the color......it looks 'wrong' or 'muddy' or 'sparkly' or (fill in adverb here).
The biggest problem you will run into is defining what is an acceptable level of visual contamination (dirt, lint, runs, etc). Work with your supplier, and ask THEM to explain to you how they would evaluate the suitability of a painted part.

 

[Richard]

Thanks to everyone for your excellent information.

The paint specification with 0.5 Delta E is specifically required by one of our customers. It may apply to both enclosure and bezel. I believe powder coating process will be on magnesium enclosure and wet paint spray on plastic bezel. We haven't finalized the design yet, but I believe there may be 3 -5 varieties of white on enclosure and 15+ different colors on bezel.

Hope these information can clarify my original questions.

I have some questions of detail process controls such as when and how often spectrophotometer is used? What are the critical process characteristics and test/inspection items? But I am afraid they are too detail for you to help with on this message board. Anyone can share process control plan?

Thanks again for your generosity in providing very helpful information.