Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing 1989-2024
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L.a.b. CIELAB color issues: gloss variation, matching different finishes, etc.
Tip: Readers can learn from your situation
but not from an abstract question, so
most readers just skip past them.
Q. Is it possible for mil thickness to effect L.A.B. readings?
Greg Oswald- Fremont California
January 11, 2024
adv (affil link): Color matching spectrophotometers on eBay and Amazon
A. YES it can. My previous employer was a coil coater and I routinely used film thickness to control colour.
Ron Zeeman- Brampton, ON Canada
February 3, 2024
⇩ Related postings, oldest first ⇩
Q. I'm responsible for the Quality Control Department in my company. In the last few days, we started to have problems of colour and gloss deviations in painted parts, delivered by different suppliers. We want to start inspecting the painted parts that came from different suppliers, so I need to know if there is any standard that establishes colour and gloss tolerances for painted parts?
VÌtor Soares- Torres Vedras, Lisboa, Portugal
2001
A. Colour: in general CIE Lab ± 0.5 units for the L, a, and b CMC ratio of 2:1 with tolerance of 1 are generally used. Gloss: 60°, reading ± 5 units is pretty typical.
Ron ZeemanContinuous Colour Coat - Brampton, ON, CANADA
Tolerancing based on delta E
Q. What is a typical tolerance based on delta E (CIE Lab) for an electrostatic paint line within the aluminum architectural industry using 70% Kynar paint.
Danny Wozniak- Dublin, Georgia, USA
2006
A. A delta E of 0.5 would be typically used. Delta E is a flawed system as it relies on the volume of a box. A long thin box can have the same volume as a square box. So your colour can be off even though the delta E is identical. This can be constrained by limiting the individual delta L, a, and b readings to know more than 0.3. The second problem is that the human eye is not as sensitive to saturated colours as pastel colours resulting in an unnecessary tight tolerance for the saturated colours. Using a tolerancing system such as Delta E CMC with a commercial factor of 0.5 would be a better alternative to overcome both problems.
Ronald ZeemanCoil Coating - Brampton, ON., Canada
A. Mr. Wozniak, additional possible answers could depend on why you need the information, are you...
Investigating a vender quality problem?
Looking for a paint vender?
Setting up a quality plan?
Investigating an existing quality spec.?
Something else? (just trying to be helpful).
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
Color matching hardcoat aluminum with Teflon
Q. I work for a medical manufacturer, as a quality tech.
I am often asked why the appearance of anodized parts varies so much.
We deal mostly with hardcoat w/ Teflon products.
Sometimes the finish is dark, sometimes more like a bronze.
Can the color be controlled from batch to batch, supplier to supplier, lot of material, etc.
Thank you,
manufacturer - Rochester, New York
April 20, 2010
A. Batch to batch - no excuse for not controlling. supplier to supplier - well if you go out to 5 different anodizers you will get five different colors as a function of time, concentration, temp, current density; some are artists, some are scientists, some are psuedo-scientists. Lot of material - the Mexico stuff and the Chinese stuff will be a different color from the three big USA quality companies. That said, now alloy to alloy WILL vary scientifically. Heat treat to Heat treat will vary scientifically. Surface finish to surface finish will vary scientifically, etc.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
A. 1) Many different hardcoat processes meet the minimum requirements for MIL-A-8625 / MIL-PRF-8625
[on DLA] Type III. Due to bath temperatures, acid concentrations, power applications variations; these processes can have dramatically differing appearances.
2) All anodic coatings are formed from the base aluminum. Therefore, variations in alloy composition, temper, and manufacturing methods can result in different appearances.
3) All other things being equal, even small variations in coating thickness can create differing appearances. Good process design and control will help minimize these differences.
Luke Engineering & Mfg. Co. Inc.
Wadsworth, Ohio
A. MIL-A-8625 Type III hardcoat requires more on hardness control but less on color, however, if the color is so important to you, as long as you specify the color in terms of CIELAB (value a, b and c or values of delta a, delta b or delta c), there should be platers who know how to achieve it (material selection, current density, bath temperature, electrolyte concentration etc)but bear in mind that the price would be much more higher than normal Type III.
John Hu- Singapore
CIE Lab coordinates for a paint spec
Q. Hello, I have a customer print that is calling out a powder coat finish, and their color that they call out is CIE Lab L:78.27, a:-1.93, b: -10.93. Texture is medium to fine.
Is there a to convert this to a RAL number or other color scale that would be usable by my powder coater to obtain a match from a powder supplier?
Thanks for your help.
Jason
Process Engineer in Metal stampings and fabrication - MOUNDS VIEW, Minnesota USA
April 16, 2015
A. This is a pretty standard description for colour measured on a spectrophotometer, which any supplier worth their salt would have. They will need to know illuminant required, probably D65 and whether gloss included or excluded and observer 1931 or 1964.
I've worked with colour for close to 40 years never used RAL. Some customers ask for Pantone, used DIN once.
I found a converter on the web giving these numbers; 182,195,214. It should be a light greenish blue, pastel blue.
Coil Coating - Brampton Ont. Canada
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