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Why the top surface on a heated bed?

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I hate to make yet another heated bed thread when there are quite a few up the top, however I can't find one where this question fits well.

Is there a reason why a piece of glass or aluminium is required on top of the PCB? Sure, it adds thermal mass to the system but the standard heater plates have a solid copper base which would spread heat out well right? The PCB should also be almost perfectly flat if it was made properly and should theoretically (at least in our simulations) maintain a more even heat across it's surface.

So, is there a reason which can be mathematically proven for needing the glass or aluminium, or is everyone doing it simply because other people have done it - and therefore thats the way it's done. Not being condescending here, I just see a lot of this happen in many things and am trying to understand if the same thing is occuring here.

Has anyone tried printing directly on the PCB (well, with polyamide tape over it) and had adverse results?

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I hate to make yet another heated bed thread when there are quite a few up the top, however I can't find one where this question fits well.

Is there a reason why a piece of glass or aluminium is required on top of the PCB? Sure, it adds thermal mass to the system but the standard heater plates have a solid copper base which would spread heat out well right? The PCB should also be almost perfectly flat if it was made properly and should theoretically (at least in our simulations) maintain a more even heat across it's surface.

So, is there a reason which can be mathematically proven for needing the glass or aluminium, or is everyone doing it simply because other people have done it - and therefore thats the way it's done. Not being condescending here, I just see a lot of this happen in many things and am trying to understand if the same thing is occuring here.

Has anyone tried printing directly on the PCB (well, with polyamide tape over it) and had adverse results?

A PCB is far from mechanically stable, and will bend, it will sag, and it will change dimensions when heated. it is just too floppy to be used as a printing surface all by itself, and it certainly will not stay perfectly flat.

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It seems to me to be not flat enough and at a guess would change some what even more with heat.

You wouldn't want a change in flatness of more than 0.1mm and I can see by eye that the PCB isn't that flat.

It's also too flexible I would imagine. I own one but have set it up yet so this is not known through experience, just a guess.

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I was thinking that flexability/sag wouldnt really matter as you would need an insulating or heat reflective surface underneath which would provide that support. If a high temperature epoxy was used to seat the PCB against the insulator, it should not move or flex. If the board was made of high temperature FR4 (which i doubt they are) it would be even more stable :)

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I was thinking that flexability/sag wouldnt really matter as you would need an insulating or heat reflective surface underneath which would provide that support. If a high temperature epoxy was used to seat the PCB against the insulator, it should not move or flex. If the board was made of high temperature FR4 (which i doubt they are) it would be even more stable :)

air is a pretty good insulator. but if you find all the necessary components to make a stable HBP from a single PCB, go for it. I think the reprap community has done a lot of testing&experimenting and came to theconclusion that it is not such a good idea, but that shouldn't hold you back to do your own research. using a bare PCB will give you the fastest heatup time, since it offers the lowest mass. I think it'll deform, getting unlevel, or getting scratched up, making it necessary to replace it.

I am not a fan of the PCB solution in the first place, and prefer silicon pad heaters under an alu plate, driven by mains voltage.

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