PCB heated bed mounting

So I was lucky enough to receive two sheets of 2mm thick aluminum plate and was thinking about rebuilding my PCB (prusa's) heated bed.

I've also got some 4000F degree capable glass sheets that I'd like to use (since I did buy them and would hate for them to go to waste).

Here's the idea, from the top, down:

glass
2mm aluminum
heated PCB
3mm corrugated cardboard
3mm corrugated cardboard
2mm aluminum

I think it'll be a shame to cover the nice red/white artwork of the PCB (as I have it simply under glass at the moment), but I think the aluminum would spread the heat much better.

I'm also thinking the PCB would work better if the tracing side is facing up (artwork down). Maybe also use some heat sink compound between the top aluminum sheet and the PCB?

Comments, ideas, thoughts, suggestions welcome.

Hey Nelson,

Aluminum already has great thermal conductivity and a low specific heat capacity, which means its great at evenly spreading heat. Adding a thermal compound, like a heatsink grease will create a thermal bridge and help the "flow" of heat from the pcb to the aluminum plate. I would also suggest to make sure that the thermal compound is not electrically conductive, just as a precaution.

I don't think you're going to see much of difference between placing the etching up (towards your build platform), but that does mean you don't have an added layer of insulation, being the pcb itself, so I don't think it would hurt.

good Luck Nelson

[bdc]

The PCB does have a layer mask, so I'd hope that its electrically isolated... sounds like I could probably get away without the heat sink compound though - might be overkill. Heat does rise, after all.

Oh, and I'm not Nelson

Anyone else have thoughts on the glass? I'd figure it makes for a flat-as-flat-can-be platform - but I do have my worries about how well it will hold the heat compared to the aluminum alone.

Glass on top works fine because the heat does not need spreading when you use the PCB. It already has the copper top surface to spread heat which is produced fairly evenly anyway.

I think which way up you mount it depends on which way you want it to curl. I think the solid copper side expands more than the zigzag side, so it domes upwards when heated. When you put glass on top that works as it presses against the bottom of the glass in the middle and is held against it at the edges..

I think if you mount it upside down it will dome downwards so would need supporting in the middle underneath.

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[www.hydraraptor.blogspot.com]

I've used glass (3mm) and aluminum (slightly thicker) plates on top of the Prusa PCB. Although glass is flatter, I found that it takes longer to heat up compared to the aluminum plate. Now that I have my Mendel in a box, perhaps I'll go back to glass.

TheCase Wrote:
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> Heat does rise, after all.


Actually, heat does not rise. It moves from high temperature to low temperature via path of least resistance.

Low density air (because it is higher temperature) rises above high density air (low temperature). This is heat transfer by natural convection.

The heat transfer from hot PCB traces to top of bed should be by conduction (parts touching each other). On paper, when two flat objects touch each other they contact over the entire surface. In the real world, surfaces have local bumps and pits on top of regional peaks and valleys. As a result, mating surfaces have three point contact unless there is sufficient force to deform the surface irregularities. Smoother and flatter surfaces require less force to overcome the surface irregularities. Thermal grease is used to fill the gaps that, inevitably, remain. This is why computer chips and heat sinks are very flat and shiny (smooth), and use thermal grease in between.


Some thermally conductive, dielectric grease is likely to help. (This may be difficult to find since thermal and electrical conductivity tend to go together.)

Tyro

dump the cardboard try Oatey® Soldering Flame Protector
I got mine at Lowe's [www.lowes.com]
easy cut to size tape to Alloy bed put the heated plate on it then glass on plate tape corners

I use something similar from Home Depot but I'm not sure if it's better than carboard for this purpose.

Edited 1 time(s). Last edit at 09/27/2011 03:16PM by brnrd.

The bottom of the two layers of cardboard that I currently use are barely warm to the touch - not quite sure its worth dropping close to $20 on something else.