Aluminum building platform with or without glass?

So, as some of you may know from my other question about heated beds, I am about to upgrade my MK2B PCB bed heater with a much larger 24V silicone heater. Today I finally ordered a custom 250x320mm heater with a total power of 400W (0,5W/cm^2).

I would like to attach it to an aluminum spreader plate in order to distribute the heat evenly. Now the bigger question - which is the best alternative to set it up? I am thinking of the following options:

1. Thin aluminum sheet (1mm) over the silicone heater and a 4mm glass over it - the aluminum spreads the heat evenly and the glass ensured a nive and even build surface - maybe the best of both worlds
2. 4mm machined aluminum sheet with no glass - my biggest fear here is warping when heating and cooling as well as flatness across such a large build platform
3. 3mm machined aluminum sheet with 4mm glass over it - same issue here - if the aluminum plate warps, there will be no good contact between the glass and the aluminum sheet, so the glass plate may end up being unevenly heated. I furthermore think that the heating time would be much longer here. (3+4mm would be the biggest package attachable to the Y axis)

I am no big fan of option 2 because the glass has some obvious advantages when cleaning and is also super flat. I am currently using option 1 with the PCB heater and I regularly use a razor to clean the build platform, which will definitely scratch an aluminum sheet, even if anodized.

I did research a lot and read quite a lot of forum threads about heated beds. Which got me more confused than I originally was. Since warping depends on the way the aluminum sheet is attached to the bed, here are some pictures to get a general idea what my setup looks like:

- I have a classic movable Mendel-style Y-axis. Regarding the heated bed concept, my setup is very similar to this one (really big JPG, just look at the way the heated bed is fixed to the carrier plate):
[makerstoolworks.com]

- The heated bed is attached to the Y axis via three brackets:


- Here are all three of them:


So.. what is your expert advice? I would normally go for option 1 but got cold feet after a lot of reading. On the one hand, glass has worked fine for me with my current setup. On the other had, Thomas Sanladerer suggests using pure aluminum with no glass. However, many users complain that the aluminum sheet warps a lot which leads us back to the idea of a combination of aluminum and glass.

Of course there is a fourth option - pure glass, no aluminum, but I wouldn't risk letting the bed heat up unevenly.

Any help is appreciated

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

I don't think you need worry about heating times because you have a powerful heater. My delta uses 4mm glass on top of 5mm aluminium with 0.4W/cm^2 and it heats up very fast.

As you have a moving bed, the total mass is an issue. The lowest mass option is probably 4mm machined aluminium. However, I don't know whether 4mm is thick enough for a bed of that size, and my gut feeling is that 5mm or 6mm would be more appropriate.

I favour removable glass beds for several reasons:

- they are cheap to replace if you damage them
- you can experiment more easily with different print surfaces
- at the end of a print you can remove the bed plate and stack it vertically for faster cooling
- if the print is sticking to the bed too well, you can put it in the fridge or freezer to help it separate
- you can put another glass plate on your printer and start the next print right away.

The disadvantages of glass are the extra mass, and you need to increase the indicated bed temperature to allow for the temperature drop across the glass. Also you need a little extra bed area to allow you clip the glass to the bed plate without the clips getting in the way of the print head. On my delta printer I made the bed plate and glass 15mm larger all round than the heater and the printable area.

1mm sounds rather thin for a heat spreader to me, so I suggest at least 1.5mm.

Edited 1 time(s). Last edit at 06/28/2016 05:54PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

If you can get an appropriately sized piece of borosilicate glass, sounds like option 1 is good. No real downsides I can think of. I have a pure aluminum bed on my new build, but that's due to sheer size. It's thick enough that warping shouldn't be an issue (1/4") but I would have opted for glass had I had the option, due to the nearly atomic flatness of the latter.

I agree that when you have a really large bed, glass loses its advantages because it's not practical to remove it after each print.

There is no need to use borosilicate glass when using a heat spreader. Ordinary float glass is adequate, and easy to obtain cut to size from your local glazier.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

My twopence worth. If you use aluminium as build surface i.e. no glass, then it needs to be perfectly flat. That really means machined tooling plate which in turn means it'll have to be pretty thick. That adds a lot to the weight. It also takes a long time to cool down so you'll have to wait quite a while before you can remove the object and start another print. Also, machined tooling plate isn't particularly cheap and it's easily damaged especially if like me, you have to get a bit violent to remove the printed object (I regularly have to resort to a gasket scraper and hammer). So my personal preference is to use glass as the print surface mainly because a) it is (usually) cheap to replace and b) as soon as a print has finished, you can remove the glass with the printed object still attached, clip on another piece of glass and start another print while the first is cooling.

I'm reliably informed by a man who knows nothing about 3d printers but everything about glass, that toughened glass is good for up to 200 deg C and in his opinion, it would be a better choice than Borosilicate glass for this application.

None of what follows is advice because it is untested but. My planned print bed will be static in X&Y, but moving up and down in Z so I don't much care about weight. It's 400mm x 400mm so for that I'll be using an 800W AC silicone heater. This will be fixed to a cast, machined aluminium tooling plate, 10mm thick (thinner would have done the job but the price for 10mm was very similar so why not?). Under the tooling plate are 2 layers of 6mm "Thermoboard" (used as insulation for under floor heating). On top of the tooling plate will be 4mm thick glass, polished edges, dubbed corners, toughened, sandblasted and Brite-Guard coated. This may or may not mean that I can print directly on to the glass without any tape/glue/paste/exotic materials. I'll report back whether it works or not.

Thank you all for your input so far!

As far as borosilicate glass is concerned, I am convinced that normal window glass from the friendly local glazier works just as well. First of all, normal glass can withstand pretty high temperatures, provided that the glass plate is heated evenly. Glass would only shatter if big temperature differences occur. A 4mm glass plate would be easy and cheap to acquire whereas borosilicate glass with this exact dimensions may be more difficult to find and definitely more expensive.

So I guess that the 4mm glass plate has already won a place on my build platform. What remains now is to decide how thick the heat spreader plate should be. I plan to attach the silicone heater to the aluminum heat spreader via 3M doublesided adhesive and then just to put the glass on top of the aluminum plate. However, assuming a 2mm heat spreader, if the heat spreader warps, this would probably influence the heat exchange between the aluminum and glass plates because there will no longer be good contact across the whole surface. Does it make sense to apply a thin layer of thermal paste between the aluminum and glass plates? Or stick them together with 3M thermal conductive tape? The glass will no longer be removable but so be it. Or maybe I am overestimating the effects of warping...

I currently have a 2mm heat spreader and 4mm normal window glass and this combination seems to work fine, which doesn't mean that I wouldn't improve it if possible....

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

You could put something rigid under the heat spreader which would help to prevent it warping and would also act as insulator. Something like 6mm mdf or ply might work. I'd caution against using window glass just because if it does break, you'll have nasty sharp shards to deal with. In that respect, toughened glass fractures in a much more benign way (but it's your skin)

The problem with using toughened glass is that it can't be cut to size, so you have to design the bed around a size you can get. I've seen kitchen chopping boards made from toughened glass (typically black, which should work very well with my IR sensor), however they usually have a textured surface.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
The problem with using toughened glass is that it can't be cut to size, so you have to design the bed around a size you can get. I've seen kitchen chopping boards made from toughened glass (typically black, which should work very well with my IR sensor), however they usually have a textured surface.

Sorry DC, have to disagree. It starts off as standard float glass which any glass merchant should be able to cut it to whatever size you want and from pretty well any thickness. Then they either send it away to be toughened or do it in house. It can't be cut once it has been toughened so you need to give them the correct size to start with, but you can have any size of shape you want. I've had smoke glass panels made and then toughened for an AV unit I made. Bespoke etched glass door inserts for a dining room unit. Even a bespoke glass hearth. All cut, polished then toughened.

By toughened glass, do you mean tempered glass or do you mean something like Gorilla glass?

Guys, you are drifting off topic. I am not using tempered or otherwise hardened glass if the price difference to normal glass is significant.. 4 mm glass is not that easy to break if the heat is evenly distributed.

I was thinking if a 2mm heat spreader underneath the glass is enough and whether aluminum warping is going to influence the heat exchange with the glass bed.

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

Price difference - from my glass merchant it adds about 15%. Personally I think that's a small price to pay for the safety it offers but as I said, it's your skin / arteries.

I'm talking toughened as in tempered glass - nothing exotic like Gorilla glass.

Don't know if 2mm aluminium will warp which is why I suggested something rigid underneath it.

Toughened glass works great, hardly off topic I'd say cost is not significant, and it deals better with temps we use. I ordered my custom piece locally, cost £8. They had to send it off for tempering, but it only took 2 days. I even cool the glass with a fan after using it to get the prints off quicker

How about a removable aluminum plate clipped to the non-removable one just like you would with the glass? I wouldn't want to not have something that can be removed because it will need to be cleaned and if a part is hard to get off I don't want to tweak the printer. I have found you can't count on the glass being flat. The flattest I have found is a 4mm aluminum plate white glass on top. The aluminum plate has been staying flat and would be fine to print on directly but I want something I can take off so I keep the glass on there.

Edited 1 time(s). Last edit at 07/01/2016 03:48PM by msaeger.

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Newbie with Folgertech 2020 i3.

This is an interesting idea - using a second aluminum plate instead of glass on top of the 2mm heat spreader. This is btw the solution implemented in Mendelmax 3.0, however, they are using a steel plate.

A thick aluminum plate would be option 2, but I think that printing directly on it would eventually lead to scratches... Which is why I am thinking of glass...

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

You can easily protect the bed from scratches with some kind of tape. Whether Kapton, blue painters or some other kind (PET tape, etc.) depending on what you're printing with to get better adhesion.