Need help reducing warpage or twist in sheet metal tray induced by heat

[Engineeringtech]

Hi everyone. First of all I have to say I am not a mechanical engineer. I am a technician with training in electronics and machining. Since my employer has no mechanical engineers on staff, the job falls on me to do mechanical design to the best of my ability.

My employer makes electronics modules. One system we are working on right now fits in a 20" x 11" x 1/2" aluminum housing. Basically, the housing looks like two interlocking cookie trays. I was asked to design the housing from .025" thick aluminum, and built prototypes from 3003 annealed alloy. (The required bend radii are VERY tight - .005 - .010", and I was uncertain I could do this on my old equipment with a harder alloy.) When I completed the prototypes, they were straight and true, and dimensionally accurate. I wanted to weld the corners of the folded up flanges, but since we don't have any welding equipment, I could not do this.

Our electrical engineer advises me that when he installed our module and other components inside the housing, and ran them up to full power (100 Watts), the assembly twisted and warped. When he removed the heat, the assembly recovered almost fully to it's original shape. I'm not surprised, but I don't know what to do about it. He thinks that making the part out of the same thickness galvanized aluminum might stop the part from twisting. I've ordered material, but I doubt it will be that simple.

I don't know much about metallurgy. Should I be using a different alloy? Would it be helpful if we brought the material up to temperature BEFORE we formed the flanges? Should I try to incorporate some sort of rolled bead into the design to stiffen it? I'm not allowed to change the material thickness.

Thanks for taking the time to read this...

[JAlberts]

The type of distortion you are describing can occur on large flat thin sheets (with or without flanges) when there is a localized heat source that creates an temperature hot spot and metal expansion in a localized region of the sheet's surface.
Changing the sheet material to galvanized aluminum will have no effect in reducing the warping. Also, there will be no real benefits from rolling the flange edges or flange corners.
The only effective way to minimize the warping is to minimize the the localized hot spot source component in the electrical assembly.
If possible, clamping the pan assembly corners to a more substantial base plate could reduce the the degree of overall warping of the assembly.

[jcs003]

maybe adding a heat sink will cure what ails ya.

John

[Engineeringtech]

The type of distortion you are describing can occur on large flat thin sheets (with or without flanges) when there is a localized heat source that creates an temperature hot spot and metal expansion in a localized region of the sheet's surface.....

Thanks for your reply. The heat source is concentrated along the longest bend lines of the rectangular housing. Does that suggest anything I can do? ( I can't move the heat sources.)

[Engineeringtech]

maybe adding a heat sink will cure what ails ya.

John

Thanks. I will check into that...

[CCR5600Design]

Thinking outside the "box" for a minute...

If you were to roll some stiffening beads into the tray, would the beads interfere with the installation of the electronics? Something like this?

tray.jpg

[Engineeringtech]

Sorry for the delay in response. Actually the stiffening beads are something we are considering. Diagonal lines running from corner to corner.... But I have no experience with this. The metal is .024" thick galvanized low carbon steel. The "cookie tray" is 20" x 12" We have to buy a press brake to bend the side flanges up at 90 deg. Could we use the same machine to put in the beading BEFORE we bend the sides? Or would the metal wrinkle and pucker too much? I've seen videos of people using an english wheel to do a form of beading. But I don't know if that is feasible on low carbon steel, or whether we could make the straight lines we want.

[CCR5600Design]

No, for that thin gauge material, you simply need an inexpensive bead roller (use your favorite internet search engine) available from several manufacturers/distributors. It would be best to roll the beads in the material before bending the flanges.