Aluminum heated bed thickness ?

I need help on a new Aluminum heated bed, I need it to be 450mm x 4500mm i have ordered a Silicone Rubber Heater -Heating Pads 240V 400*400MM.
But need to no how thick i need the aluminum bed to be or 5mm or 6mm ??. I made a 4mm x 450x450mm aluminum bed but it bens with the heat. I need it to be metal as i have a inductive sensors for z stop.
Can i use stainless steel ?? if so what size do i need.
The new printer i made is 600 x 600 x 750mm CoreXY.
Thanks
Paul

I have a little smaller bed in my printer- 317 x 305mm and used 1/4" (6.35 mm) cast aluminum tooling plate. It is supported by three leveling screws. You can get an idea of flatness here:

Unheated: [vimeo.com]

Heated: [vimeo.com]

Don't try to use plain aluminum sheet- it is extruded and isn't very flat. Get cast aluminum tooling plate. It comes milled flat.

Steel probably won't be very flat, unless you get it milled, which will be more expensive than aluminum. Steel is very heavy and isn't a good thermal conductor, so, like glass, you'll have hot and cool spots which isn't very good for keeping prints stuck to the bed.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Hey there is a SEARCH function in this forum!

see

Aluminum Sheet Between Heater and Glass

[forums.reprap.org]

quote the_digital_dentist
"Ebay is chock full of cast tooling plate cut-offs for very reasonable prices. Here's a shopping tip: if it isn't called "cast tooling plate" or "jig plate", or "MIC-6" or similar descriptive term, you can't count on it being flat.
Here's a 12 x 12" plate for $40 shipped: [www.ebay.com]
Here's a 24 x 24 x 3/8" plate for $160 shipped. [www.ebay.com]
If you're going to build a quality, custom sized printer, you're going to have to put in a little effort (and spend a little money). It took me all of 30 seconds to find those listings."

I have a 3mm thick Aluminum bed on AO-10x. It's on top of the circuit board based warmed beds. It appears to function admirably and is a lot less mass than the glass one.