Will Pure 90-Degree Sheet Metal Bend Produce Invisible Cracks?

on aluminum, make sure you bend perpendicular to the grain, or it will tear

"...would a simple coating of primer be the solution?"

No, it would make the film adhesion troubles worse.

Powdercoat is electostatic application. The paint film would inhibit the attraction required for the coating before heat. If properly applied and process is good, I am surprised adhesion is not good. My limited experience tells me that powdercoat can actually hide a little bit of 'sin'.

Are you doing this finishing in-house, or sub out?

[Edit] Is this rolled plate? If yes, as per PWSlack, the steel will have a 'grain direction' to it as well.

The finishing is done in a separate building here. We use cold rolled sheet metal. I've read that graining in sheet steel is not much of a problem anymore. The other responder was mentioning aluminum.

10 or 12 gauge is fairly thick sheet metal. It might need some treatment in an oven to normalize the metal a bit.

Let's look at what we can change.

Material type? Probably not.

Material orientation relative to bend? Yes. But, this may not help.

Coating process? Probably not.

Bend radius? Nope. I am having trouble with that 180° bend radius, though. That implies that you are left with two surfaces, inside the bend that are in contact. I don't know how you would coat those surfaces.

Heat prior to bend? Maybe.

Heat after bend? Maybe.

Prime? I think Doorman's aversion to primer may be misguided. I don't think that a thin, non-conductive layer of primer will inhibit the coating thickness or adhesion in an electrostatic process. It's different than plating.

If I were you, I'd run tests altering a single variable at a time and record the results.

Honestly, you have not given much real detail to work with.

On the bend radius, it's a two step procedure leaving a 1/16" gap with no trouble powder coating, except at the bend, as stated. I don't think priming would hinder the coating as well, but I'm going to check with that plant.

No one seems to be mentioning the elastic tension stress, which I think might be key. I was hoping the verbage would elicit some response in that area. If there is someone with metallurgical knowledge, having worked with some material's crystalline structure and the elastic effect on it, that could be interesting.

Thanks for your help thus far!

Well my experience with coatings says that the surface energy of steel isn't that upset with elongation. But there are people here with much more experience than I in metals.

Milo.?

www.finishing.com/Library/pennisi/powder.html, not real familiar with powder coating, but this website may help. It mentions some steps in steel preparation, prior to coating, that it didn't look like you're doing. Hope it helps.

I would prepare some test coupons with varying bend radii (stretch), coat them with your process, and then evaluate the coating adhesion. If you find a correlation between stretch and adhesion, I would approach the powder coating supplier or other finishing experts. You might even post your data on CR4 and provide us an opportunity to consider the problem with additional data.

A 180° bend is a seam used to reinforce an edge in thin sheet matal. For a 180° bend, I would form the edge over a minimum 1/4" dia wire.

Well I havebeen stamping out tons of the same guage steel material and this is really interesting.

There are what we call two types of grain side direction.....

one the same direction as of rolling (hot or cold) and other perpendicular direction....

NOw if bending is done in perpendicular direction chances of cracking is real high say 40%, and if done in same direction and keeping a bend radius when at right angle twice the thickness of sheet, no cracks are formed.

If a sharp angle is required use a softer option of steel say half hard or quarter hard steel.

Hope this helps