Copper print bed surface? (Page 1 of 2)

hmm very interesting id like to hear what the big wigs  say.

Yeah, but I'm not using it as a heater, rather as a replacement to the aluminum bed.

I have this idea in my head that since copper is such a great heat conductor (High quality CPU heatsinks are made of copper), it should be able to take the heat generated by the heat pad, and distribute it throughout it's surface quickly, unlike the aluminum surface which takes a while to come up to the same temps as the heat pad and it usually has hotspots, and uneven heat distribution.

After replacing the original Solidoodle heat pad with a QU-BD and running some tests, I found that while the QU-BD pad heats up very fast (about 3 minutes), the aluminum surface (the top of the bed) take at least twice that long to get up there.  It also seems to cool off relatively fast, so the heating system has to work pretty hard to maintain an average temperature.

Take a look at this temperature curve.  After it reaches the desired temps, it goes into a 15/15 second (ON/OFF) cycle to maintain the average temperature.

I'm thinking that the copper bed with attain something like 10/30 second (ON/OFF) cycle, as it should be able to heat faster and cool off slower.  That should be quite a power saver, considering right now the heated pad draws about 120W by itself when it's on.

The amount of time(on/off) ratio shouldn't change that much with copper as it will still have a similar radiant loss... more or less.... the biggest thing I find that changes heat loss when printing is the amount of abs on the bed as it actually acts as an insulator, with that said using a couple pounds of copper will make the temp swings less drastic which should in theory make things stick better

Another thing to think about is you should be able to find copper plate/buss bar a lot cheaper at a recycle/scrap yard as they only pay $3/pound for it ...tops and you should be able to get it for a few bucks over that... depending on the yard.

True... I really didn't factor that in

I found rods, wire, and thin sheets, but not plates.

Anyway, the temperature across the bed will be more uniform, power consumption will go up a bit, and if you want to get rid of the cycling, change to PID bed control.

By having a higher temp it would radiate more heat=more heat loss and in turn take more power to keep up the temps.

That really doesn't make sense.

The thermistor that is sandwiched between the heater pad and the print bed.  The thermal control module reads the thermistor only and acts according to those readings, it knows nothing about heat lost through surface radiation, and that is what accounts for the difference between your thermistor readings and actual surface readings.

Increasing the Repetier preset to make up for that differential will not increase the amount of power required except during initial heatup, and only minimally, because it only takes a few seconds more to go from 95°C to 105°C (or whatever those settings may be).  After the preset is reached it takes the same amount of power to maintain it whether it's 95°C, or 125°C

What I'm beginning to think that Elmoret and yourself are saying is that copper is actually inefficient at retaining heat, that it radiates heat faster than it builds it.  I'm not a metallurgist, but I think that presumption is wrong.

Maybe someone that knows their metals could chime in.

It shouldn't. Thermodynamics is hard.

Yes.

It's an ADC on a uC, yes.

Agreed. It is controlling the temperature at thermistor. This is correlated with the surface temperature.

Incorrect.

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 other materials with lower specific heat capacity.

I'm 3 months away from a PhD in Mechanical Engineering, and I have BS degrees in Mechanical Engineering and in Aerospace Engineering. Does that count?

Sorry... I tried to keep it short and simple    I have been around metals and many other industries for quite a bit of time, currently run a small metal casting shop with our own foundry and machine shop and although I know how things work I am also not a very good teacher... so I'll let Tim explain it ... He is much better at it than I could ever be

Then...

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).

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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.