Distortion problems in perforated shelves

I was just apprised of a lab problem where all of the shelves in four cell incubators, which look like large bar fridges, are warped. This turns out to be a problem for certain cell growth protocols where they have 150mm petri dishes which need to be dead level so the growth medium is same depth across the dish.

The shelves are about 18" x 18" and (in two incubator models) are 18 and 20ga stainless with three 3/8" up flanges on the sides and back and the front folded down. So the folded edges are straight. To promote air circulation the shelves are perforated, maybe 25% open, but only to up an inch or so from the edge, which obviously is much neater as the flanges are all solid and smooth. It all looks CNC punched as the perforation holes looked like they were cleaned up on the back side a bit. At any rate, the shelves all either twist diagonally or bow up in the middle. I know my cookie sheets do this when cooking pizza at high temperature, so it makes me think the perforations have expanded the central plane so they can't get flat. I've folded plenty of 18 and 20ga metal and even if a regular sheet is warped when you fold up the edges they tend to flatten out pretty well.

I'm thinking of trying to fold up a shelf out of pre-perforated sheet, trading nasty edges (which don't really matter) for flatness. I've worked with 16ga 60% open stainless in the past don't recall it being all that distorted, possibly because the larger open factor gives more ability for the metal to relax. Or perhaps it's annealed or perhaps I'm just dreaming. At any rate I have a piece and will try. I'd like to know if anyone has seen this warping though and otherwise has any thoughts about how to make a really flat shelf that's perforated.

If they have to be dead level, make a deeper pan to hold water, or suitably inert goo, and float the petri dishes in it.

Otherwise, maybe make the shelves out of welded wire rod, like a real fridge shelf, or a pallet rack shelf. Leave a bit of room at the edges.

In HVAC, they put corner-to-corner creases in large panels. Wouldn't be truly level, but perhaps more predictably distorted, as the direction of distortion would already be established. Little stick-on plastic feet of the right thickness could level individual dishes.

Chip

How about making 3 point support shelves. Check out "kinematic mount principal? Like granite tables that sit on an uneven floor. You set a plate on 3 rubber pads and the frame legs sit on leveling plates and screws.
The frame can be out of level, but the table is level and the 3 points that eliminates twists.

stress relief the sheets in a furnace, that's about the only thing you can do short of learning how to flatten sheet metal with a hammer and a punch.

once the metal gets stretched even slightly, its going to go somewhere.


you may be dealing with a situation where because of the perforations, the center of the sheet has expanded and contracted faster than the outer perimeter and it has fatigued and stretched.

Would it be feasible to make a perimeter frame from square steel tubing with diagonals corner to corner and pop rivet the shelves to the frame ?

What temperature are the shelves seeing ?

Doing perf panels can be super frustrating especially trying to keep them flat.

We make flat perf panels out of .032 aluminum customer request them to be flat within +/- .010 over the panel. After trying everything under the moon our only option was to have them flattened. Found a shop that runs them through a machine that looks sorta like a timesaver sander machine. Came out perfectly flat. Customer is happy all is right.

This has to happen before the forming of course.

I wonder if laser cut holes would be different?

Thanks guys, you are largely supporting my plans. I'm pretty sure the distortion is due to the perforations and based on other experiences I've had with laser cutting so think laser would be the same or worse. Now, we actually have a waterjet cutter and we could stack a bunch of sheets and waterjet the holes but I'll try a couple of easier things first.

Plan A is to try with pre-perforated sheet which is often warped over a long length but not distorted like these. I suspect they must punch it hot and/or flatten and strain relieve it. The edges will be sharp and ugly but we can spot weld a strip of 20ga solid to the front to protect peoples hands.

Plan B would be to waterjet the shelf from 16ga solid with a big hole in the middle and then spot weld a flat sheet of perforated into it.

Plan C would be to take flat aluminum thick enough to support the trays without any folding (say 3/16") or maybe 12Ga stainless, and cut vents in it with no folding. Waterjet is great for not introducing stress but it can expose latent stress so this approach isn't guaranteed either.

Welded wire rod is also an option as they are straight forward to spot weld, but I'm feeling they'd be hard to make without setting up some good fixtures, which would be a project in itself.

Any of these can be shimmed up level in the 37 Celsius incubators as they aren't going to move once installed, but we need them to be flat to start with.

N.B. Water leveling would be cool but will never fly with the biologists due to contamination concerns.

This needs hopefully to be done by early next week so I'll post pictures!

Laser cutting won't cause that problem its down to punching and leaving a solid rim, every punched hole has a stretched rim, do enough and that adds upto distortion. Punch all the way across a sheet and it self relives.

laser cutting will also do it to a degree if theres too many holes - rim, but its a lot less of a issue compared to punching, you only see it if theres zillions of narrow slots! Water jet probably would never be a issue on that.