Bed flattening option

I am quite sure that flatness of the glass bed on my prusa style printer is a problem which led me to an idea which is to adhere a sheet of polyproplyene to my glass bed and using the hot nozzle to shape the plasticised bed into a flat surface (as flat as the gantry will produce anyway). Has this been done before? I know it would be slow and consume a lot of energy. Interested in considered opinions.

For very large format printers a somewhat related practice is used since there is pretty much no chance of a large bed being flat. A sacrificial piece of polystyrene foam is placed on the build plate and the z axis is set with zero being a few millimeters below the surface of the sheet. The print head just melts into the foam and makes its own flat surface as it gos. The foam is single use of course and has to be removed from the part after.

Hi John,

Very interesting.

I have since solved the problem with my printer, it works so well now, I am very happy.

I suspected the bed was bowl shaped (low in the middle) so I:

1) made a design like a naughts and crosses grid, one layer high, with 20 mm between lines, in X and Y directions
2) printed the grid very slowly and it confirmed my suspicions
3) flipped the glass bed over, cleaned it and coated it with PVA glue.

Works so well, the first layer is so reliable, it does not seem worth adding ABL with this situation. I just keep adding PVA glue where it is lost after removing the printed objects.

Quote
John Meacham
For very large format printers a somewhat related practice is used since there is pretty much no chance of a large bed being flat. A sacrificial piece of polystyrene foam is placed on the build plate and the z axis is set with zero being a few millimeters below the surface of the sheet. The print head just melts into the foam and makes its own flat surface as it gos. The foam is single use of course and has to be removed from the part after.

I've never seen polystyrene used that way- when the nozzle hits it, it will melt and produce toxic fumes.

You may be thinking of the technique that Stratasys used to use in some of their older printers that used a sacrificial foam bed. The nozzle would bury itself about 1 mm into the foam and physically displace it while the plastic from the nozzle would stick to the foam. It would create a raft that is in the XY plane of the printer and that raft is what was printed on. I don't know the chemical composition of the original foam, but it didn't decompose at nozzle temperature. Several years ago I looked into using that technique when I became frustrated with a glass bed in my printer. I found that polyisocyanurate (PIR) foam works well and also does not decompose at nozzle temperatures. PIR foam is available in 4' x 8' sheets at home depot for $15. One piece of foam can be used many times so it's a pretty cheap way to deal with a large, unflat bed.

Prints stick.

Don't use polystyrene!

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