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Another approach to heating the print surfaceSunday, November 30. 2008In which the adventures of your narrator in his quest to create
a heated print surface are related...
The more I thought about the way that Metalab was heating their printing
surface, the more problems I saw with the approach. The most important one, in
my opinion, was the amperage requirement that heating a 12x12 inch working
surface with 12v electricity would entail. Roughly, we are looking at 150 Watts
to heat a 12x12 inch surface. That works out to about 12-13 amps of 12v
power.
Yesterday, it occurred to me that one might well achieve more or less the
same effect by using a conventional IR heating bulb instead of a nichrome heater
between two heat resistant layers with a copper printing surface. This morning I
acquired a 150 Watt IR bulb from my hardware stockist and decided to see what
was implied by using this approach.
 I bought a standard cage holder for the rather fragile IR bulb and used the
clamp to attach it to Tommelise's gantry with no problem at all.
My first experiment was to simply turn it on and focus it on a 6x6 inch single-sided printed surface board that I had in stock. I put down a piece of foamboard to keep the xy printing table from heating up and then laid the printed circuit board copper side up on top of it. As I expected, the emissivity of the copper was very low. While the air temperature in my flat was running 20-21 C I was unable to get the copper to reach much more than 27 C measured from the top. . The IR radiation was not only reflecting off of the copper, but it was doing so in a highly specular manner as well. If I aligned my IR thermometer so that it saw the reflection of the bulb itself, I saw temperatures of 100-110 C. If I changed the alignment a few degrees, the temperature dropped down to 27 C again. An interesting aspect which, in retrospect was obvious, was that what the copper side of the printed circuit board was seeing, vis a vis radiation, was the surface of the IR bulb, not the filament or anything like that. Thus, the surface of the bulb pretty much stabilized at about 110-120 and the center of the printed circuit board stabilized at about 100-105. It is worth noting that when I rapidly flipped the board and measured the temperature of the fiberglass-expoxy backside the temperature at no time exceeded 32 C. It seems that Metalab was able to achieve their temperatures pretty much as a result of the low emissivity of their copper-sided pcb's. At that point, I recalled that several developers had talked about printing onto HDPE sheets. I decided to try that in that I had a lot of quarter inch 12x12 inch sheets of black HDPE. That exercise gave me a result that made sense as well, in retrospect at least. Focussing the IR lamp on the HDPE sheet resulted in a slow warming of the center of the sheet. After about 20 minutes the center of the sheet was measuring 105 C while the edges were running about 40 C. The unexpected result was that the differential heating created a substantially raised dome of HDPE in the middle of the sheet. This photo was taken a few minutes after the sheet was removed from the xy print table. 
The degree of warping is quite clear in the photo. While the temperature on the top side ran 105 C the temperature of the bottom side read 80 C. Cooling off the sheet with tap water returned the sheet to its original state in a matter of moment. Several things can be taken from the exercises.
Another thing that might be useful is to not use IR bulbs but rather use a radiant coil, so that a much higher radiant temperature could be achieved. The glass face of the IR bulbs kept the temperature from getting much above 110 C for the wattage used. Trackbacks
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Stove or barbecue paint, maybe? I think at least some varieties are carbon black and/or graphite in a binder.
Smoking the surface with a candle would probably work for a while, but wouldn't be very durable.
Maybe you could spritz it with PAM cooking spray before printing? :-)
LOL! Keep in mind that we want the printed object to stick to this surface, not slide off. :-p
You could paint the underside of the PCB black, and use the IR bulb to heat it from underneath.
Heat rises. So I think heating from underneath makes more sense. Plus, you could use some sort of (fireproof) enclosure around the bulb to keep the heat from escaping.
Another idea is to use a space heater instead of an IR bulb.
This means that your xy printing stage is going to have a VERY tall apparatus sitting on it containing the IR bulb with the new print surface over that.
May be getting a bit fancy here, but if you want to heat the surface directly, while reducing the power requirement... thermal electric elements (TEC's) may be worth looking at. you would need a decent heat sink on the cold side, and the temperature difference across the TEC can only get to about 70 deg C, which may not be enough. Just a thought.
Sadly, if the Austrians have it right we need a delta T of about 100 degrees C. :-(
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