thermal conductivity(k)       400      200          .8           .4

               radiation (ϵ*k)      12        20          .64         .36                   

                       density         9           3            2

Hazer is right.  The above technobabble (that just happens to keep the lights on in your house and your car and computer running - but whatever) says that the Cu plate will move heat twice as fast as the Al, and be 3X heavier.  Heavier may not be a big deal (~doubles the total weight if you are using a glass plate).

Since the heat distributes through the glass so much more slowly (0.4% as fast as Al, 0.2% as fast as Cu) and radiates from the surface slower (3% as fast as Al, 5% as fast as Cu), you would not notice the difference using a Cu plate over an Al plate.

That is the reason.

Hope this helps.

Forget the technobabble from earlier, lets make it simple: Copper conducts. We know this. It is why it is preffered as a thermal conductor for PC heatsinks/water-coolers. It dissipates heat quickly. For us, this is bad. You say copper would heat up quicker but cool down slower. How does that work? It would cool down just as quickly as it heats up, and there my friends is the trick.

The reason why glass works good, and tile works better is becuase they are thermal insulators. They dont want to change temperature, either to heat up or cool down. And that is why they work, the cooling down takes much longer allowing the plastic to cool down evenly.

So the trick is to heat up our insulator faster. Our problem lies in the placement of the thermistor, and that our bed temperature control is defaulted to bang-bang. If our thermistor was placed as close to the surface of the print bed as possible with an adequate PID value set, then you could quickly heat up the bed with an over-powered heater.

This would require drilling through the bed material almost to the point of breaking the surface (aluminum easy, ceramic tough, glass near impossible).

Or instead of inductive heating you can try convection heating (heated AIR blown directly at the surface just like Stratsys).

What-ever. Your copper idea wont work the way you think it will, but dont trust me. Have fun experimenting.

You can drill plate glass, you only need a diamond or tungsten carbide bit. Maybe go to a local glass shop and give them the dimensions along with seeing if they can drill the holes for you also. The glass shop would even bevel the sides, or sand them down for you so you wouldn't cut yourself. You can always adhere the heating element to the bottom with kapton tape. I would also see about making it a 4 hole leveling system. instead of a 3 hole leveling system. That would actually make the bed level out without any type of leaning, especially if your measurement come from the print head.

You can do all those things to regular glass, but not tempered glass.  Tempered glass must be cut, drilled and shaped to whatever you need it to be, before tempering.  Once it's undergone the tempering process it can't be altered without shattering.

Additionally, because you can't drill it, we'd have to make some kind of holder for it that will have the spring loaded adjustment screws.  Not quite as easy as the copper plate.

To be honest though, I've read through your posts and at this point am not 100% certain what the goal is.

It's not a definitive goal, but I was thinking of replacing the 8"x8"x1/8" aluminum plate with an identically sized, solid copper plate.

I have already replaced the original heater pad with a QU-BD pad that heats up quite fast (about 4 minutes compared to the original 12-15minutes), then put back the insulation cloth to trap the heat at the bottom so it doesn't radiate outward. 

It was when I ran some temperature readings with an IR gun that I found that while the heater pad itself comes up to temp quickly, the top surface of the aluminum lags behind by about 4 minutes.  Adding the glass on top adds another minute or two.  So the ultimate "goal" is to speed up the process.  Copper being a much better conductor than aluminum, should transfer the heat from the bottom of the plate (where the heater pad is), to the top in less time.

As a side benefit I would have thought that copper might have a better heat retention than aluminum, but after reading a bunch papers on the thermal properties of copper, I now realize that the same superior conductivity which makes it heat up fast, will also make it cool down just as fast, so that final benefit is just wishful thinking.

My personal favorite setup, that I hope to eventually setup, is the following:

A rigid frame, probably something like a 6mm by 3mm frame of aluminum (those are alu "strips" in a rectangle). Then there needs to be a flat layer, and it needs some way of adjusting. So maybe it could be an insulative lightweight wood. Like balsa wood.  And then a layer of kapton-heater. It's kapton-tape with copper etched to it. the copper etching pattern could be made with a very mild bias towards the edges, so that it counters the effects of edge heat loss. and a magnetic attachment system adheres a 2mm glass layer on top of this kapton.

Some caveats: You probably want to limit how quickly the heater responds. Something like this could put some heat stresses on the glass if you just blast it on. But other than that, it's the best I have for a heater system.

In terms of your idea to add copper, I wasn't clear- will you be adding this *in addition* to the aluminum? Or are you adding it to replace the aluminum? The copper will draw temp from the heater to the surface of the copper faster. And it freely conduct not much differently than aluminum. But if you air flow on the copper the difference in conductivity and the greater importance that has in forced convection, might make it more difficult to reach a given max temp.

To be honest though, I've read through your posts and at this point am not 100% certain what the goal is.

I'm just here to brag that I avoided this thread at first because I saw it as a potential train wreck from the start .

Only benefit of pure copper might be a thinner plate (It's something like 3x the conductivity, so for equal edge spread you could be a third as thin, though it's about twice the specific weight, so it's hardly a gain.). Plus, the alu plate is already so thin that it can warp easily from internal stresses and transient heat stresses.

Hi, so I couldn't resist adding my 2 useless cents to the well covered topic .

Congrats, and thanks for your donation.  Your $0.02 will be well spent.

Only benefit of pure copper might be a thinner plate (It's something like 3x the conductivity, so for equal edge spread you could be a third as thin, though it's about twice the specific weight, so it's hardly a gain.). Plus, the alu plate is already so thin that it can warp easily from internal stresses and transient heat stresses.

Hi, so I couldn't resist adding my 2 useless cents to the well covered topic .

You said you don't know much of thermodynamics - wikipedia goes a long way.

So I just spent some time reading some white papers and trying to educate myself on the subject, and it appears that we're talking about 2 different things that, while related, have in fact no direct correlation to each other.

Thermal Capacity/Specific Heat and Thermal Conductivity.

Thermal Capacity is the amount of energy required to change the temperature of a specific material. 

Thermal Conductivity is how quickly a specific material can absorb and transfer thermal energy across a specific area.

For the specifics of this project (Solidoodle Printer bed upgrade), thermal capacity is not all that relevant in my opinion, as the amount of energy required to heat up the printbed is constant, and is determined by the amount of power required by the heater pad to reach a preset temperature (in my case about 150W for about 4 minutes to reach 100°C)

Thermal Conductivity is more to the heart of what I'm looking for, because it directly affects how quickly the printbed heats up.

Because there's a 4 minute difference between the heater pad/thermistor reaching the preset temps and the top of the print surface getting there, using a material with a higher conductivity coefficient will give me faster heating of the printing surface, and more even distribution.

Will it require more power to keep it at that temperature?  I don't think so, if the heat dissipation is about the same as the heat absorption, then the power consumption should be the same as what I have today (50/50 ON/OFF ratio).

=======================================================================

When I started this thread, I was looking for some feedback from, hopefully, someone who had done this and had some direct experience.  Little did I know I would get a lesson in thermodynamics (brain hurts...). 

So for now I think I'll just bookmark the eBay listing for the copper plate, and if I decide to try this sometime in the future I'll post my findings here.

elmoret wrote:

Nothing is "efficient" or "inefficient" at retaining heat. Efficiency is production/input work or energy. Copper conducts heat well and stores heat well. What this means:

1.) if you heat the center of a copper plate, the edges will be warmer than if the plate was made out of aluminum or another material with lower thermal conductivity.
2.) more energy is required to raise the temperature of a mass of copper relative to aluminum or other materials with lower specific heat capacity.

Then...

elmoret wrote:

Copper stores heat energy less effectively - about half as well per kg of material. So given that the heater's output on warmup is constant, copper will reach operating temperature in roughly half the time (neglecting the effects of ambient cooling, which are roughly equal, though copper loses here slightly since it does a better job spreading heat, and thus the average temperature across its surface is closer to the setpoint temperature)

Absolutely correct, it stores heat well relative to most materials, but not as well as aluminum. I was operating from memory in the first post and looked up the actual values in the second. I'll edit the post. My mistake.

Nothing is "efficient" or "inefficient" at retaining heat. Efficiency is production/input work or energy. Copper conducts heat well and stores heat well. What this means:

1.) if you heat the center of a copper plate, the edges will be warmer than if the plate was made out of aluminum or another material with lower thermal conductivity.
2.) more energy is required to raise the temperature of a mass of copper relative to aluminum or other materials with lower specific heat capacity.

Then...

Ceramic store heat, I did get a bathroom tile and used on the bed.
did get a bit longer to heat up, but once it was hot, it kept the heat very long time after switching the bed off,

I finally switched to glass cause the bathroom tile had a surface that did not stick as well.

but provided you find one bare. without the white stuff on it , I'm pretty sure it will be as your first assumption about copper.

ps: come to think of it, I didn't use hairspray at the time either, I will try again and maybe take a screen shot of the temp reading too just for "educational purpose" ;°