(UPDATE: See the next post for the fix that’s been sent out by Printrbot, which works a treat.) A lot of people have been complaining about the heated bed on the Metal Plus. The original design didn’t heat the bed very fast, couldn’t get seriously hot, and had to have bed temperatures fiercely tweaked because the actual surface temperature of the bed was about 15 C below the temp indicated by the thermocouple (which would mean that even if you could heat the bed to 100C-plus, you might not be able to crank the settings that high without modifying a bunch of profiles and software).
I did a bunch of things that have made heating much faster and brought bed temp much closer to the thermocouple number. Now Printrbot has announced that they’re shipping a fix kit, which is great — at least one of the things they’re doing is something I didn’t think to implement, and I wonder if I did anything they decided against.
The first thing I did was add heat sink compound to the heater plate — it was pretty clear when I disassembled the bed that even with a machined and smoothed recess, the plate wasn’t making good, uniform contact. (I thought about sanding off the flashy logo, which leaves air gaps where it isn’t, but didn’t bother.)
Next, a layer of thin corrugated cardboard on the underside of the bed, followed by a layer of reflective bubble wrap insulation. You can’t put the bubble wrap directly against the heater because it starts melting at about 85 C. I didn’t put insulation over the thermistor, so that it would be exposed to an environment a little more like the top of the build plate. And I also had to leave the bubble wrap off a strip on one side of the bed because the Y limit switch sticks up there. If I opened up the board and replaced the regular limit switch with a nano-sized version, bed temperature would be a bit more uniform.
Then I started in on the non-build-plate parts. The two “wings” on either side of the build plate were hitting about 30 and 50 C respectively, so I threw some more bubble wrap on them (just on top because the underside has clearance issues like the bed). And I took some ceramic insulating tape left over from when I was going to build new hot ends for my Cupcake, and used it as a spacer between the linear rails and the heated bed. The rails had been reaching 40-50 C as well. I think that thermal break not only improves heated bed performance but also makes the rails happier and slightly more precise. According to the email I got, the Printrbot fix will involve a thermal break for the “wings”.
Finally, because my basement is at about 15 C these days, I bent up a couple of pieces of aluminum strip and taped some 7-mil mylar to them to make an enclosed build chamber. Don’t know how much it really helps, but I’m sure it doesn’t hurt.
Next up: diddling the fan shrouds so that I can print ABS reliably without waiting for the all-metal extruders.