Do these heatbeds stay flat in use?

I'm wondering whether a large aluminum heat bed like this stays flat when heated? If so, do you have to mount in some special fashion (slots on one end or something?)

Have you used a heat bed like this? What has been your experience?

[www.ebay.com]

I'm thinking of using a silicone heater with a simple aluminum plate this size, but wondering if I have to make special provisions for thermal expansion.

-Matt

Yes, that one will probably warp when heated. It probably isn't really flat at room temperature, either. If you put it on 4 leveling screws it is all but guaranteed to warp.

A 300 mm aluminum bed will expand about 0.5 mm when heated to 105C. If it moves in the Z axis it is relatively easy to mount the plate on three leveling screws without the expansion creating any lateral forces on the leveling screws. If the plate starts flat, it will stay flat. If you want a piece of aluminum that is flat enough to print on without having to add a piece of glass to flatten it, you want cast tooling plate (MIC6 , ALCA-5, eco-cast). If you mount it on 3 levelers and put a thin piece of PEI on it you'll have very few problems with prints letting go of the bed prematurely.

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

I would say that is NOT a build surface -- but just a heater.

You need some type of flat plate -- to be heated by that heater.
This heater should be mated to a aluminum tooling plate
or piece of glass to be a build surface.

Then a good straight edge (stainless steel rule) can show you the dips and hills