"We have been asked to powder coat steel parts to match an aluminum anodized hinge. So far we have not had much success. Do you think we’re spinning our wheels trying to match a powder to a plating finish? I know that the “chrome” powders that are available really don’t achieve a chrome plating finish. Would I be better off asking the customer to revise the specification to allow us to powder coat both the steel parts and the hinge if they want everything to match?"
We have been asked to powder coat steel parts to match an aluminum anodized hinge. So far we have not had much success. Do you think we’re spinning our wheels trying to match a powder to a plating finish? I know that the “chrome” powders that are available really don’t achieve a chrome plating finish. Would I be better off asking the customer to revise the specification to allow us to powder coat both the steel parts and the hinge if they want everything to match?
Powders can be formulated to approximate the appearance of anodized silver. It won’t be an exact match, but it will come pretty close. The powder coating will provide a low-gloss metallic effect and probably will have to be formulated specifically for your application, rather than being off-the-shelf from a color card. Depending on the environment to which the coating will be exposed, a low-gloss clearcoat might be required for extra durability. You might want to consult a powder coatings supplier for assistance.
We are experiencing a problem with powder coating coverage on the inside corners of a light reflector with a depth of about 6 in., similar to the inside of a bowl. I think the correct term is Faraday cage penetration. We currently apply our powder coating to the inside of the reflector using two flat blades that are mounted in slots to form a crisscross pattern. These blades separate the inside of the reflector into four sections, like a pie. We have tried applying two to three coats but have had little success getting coverage where the blades meet the inside reflector walls. We have tried reducing our kV setting to 40-60 kVs, still with little or no success. Would the gun design play a role in this area, if the gun nozzle were mounted on an extended arm to enable the gun to reach deeper inside the cavity of each section? We are using a bullet-type nozzle tip as recommended. Also, what size powder do you recommend?
This is a common problem. You have approached it correctly by reducing your kVs (actually your current) and trying different nozzle types. I suggest that you go even lower in kVs (down to 30-35). If this doesn't improve the coverage, then I would definitely try another powder. Excess fines (<10 microns) can exaggerate a Faraday penetration condition. Less than 5% of the particles should be below 10 microns in size. Many off-the-shelf powders have up to 12% of particles less than 10 microns.
One last thing - ensure that you have a good ground on your parts. Poor grounding will make Faraday coverage impossible.
How important is TiO2 in color matching? If we use rutile 828 (Tronox®) instead of Ajantox® while making powders, will we experience a difficulty in color matching between the two grades? Also, is there a test that can allow us to determine a product’s lifespan without corrosion - i.e., how much corrosion resistance can be expected if a coating passes 500 hours of salt spray testing?
TiO2will make a difference in color. There are inherent differences in TiO2, depending on the process, treatment and particle size distribution. However, adjusting the color of your powder normally requires only a slight change in the tint concentration.
As for the salt spray comparison to actual corrosion resistance, no clear-cut correlation exists. There are far too many variables in the real world. Nonetheless, salt spray testing (ASTM B-117) is a good comparative tool for measuring the differences between various formulas, substrates and metal preparation processes.
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FINISHING ANSWERS: Ask Joe Powder
April 1, 2008