Recently I have had some problems with my prints releasing from the heated build plate. After thinking about it for a little while I remembered that these two failed prints of my Filter Media Support (http://www.thingiverse.com/thing:1367288) occurred only after I switched from using binder clips to hold my glass plate to the heated bed to Bed Clips (http://www.thingiverse.com/make:205466). I thought, “maybe these plastic bed clips are simply not holding the glass plate tight enough to the heated bed?” Sure enough when I went back to the binder clips all was fine.
I could not stop there as I had to do a little heat transfer calculation (I have been a chemical engineer for 30 years after all) to see what the actual temperature at the surface of the glass build plate is. I found that with the bed temperature set at 110C (measured from the thermistor taped to the bottom of the heated bed of course) if there is assumed to be an absolute zero gap between the glass plate and the heated bed and the ambient temperature is say 80F (I have my MakerFarm Prusa i3v in an enclosure I built) then the actual temperature at the print surface, the top of the glass plate is about 105C. If there is a small gap between the heated bed and the glass build plate, say 0.5mm like if plastic clips are used to hold these two together instead of binder clips, the temperature at the top of the glass plate falls all the way to 90C!
Of course I had to write a simple web page using JavaScript so that you and make these calculations for a variety of bed temperature settings and gaps, see http://www.63alfred.com/3dprinter/heated_bed.htm
I have never seen any of this discussed before as it comes to 3D Printing. I am wondering, could a lot of the problem of getting prints to stick in many cases simply being because they are under the impression that the bed temperature they set on their printer is simply not that actually present at the contact point of the first layer of printed plastic itself?
