The wonders of heat

As winter begins to descend upon us, not only do we suffer but so does the printing process. I am not yet sure exactly what causes the deformation of larger pieces as they are printed (my preferred understanding is that as the layers cool, they contract, thus pulling the warm top together more than the bottom which has already cooled, but I have also been led to believe that heating makes Kapton/Koptan tape adhere better and other explanations are known) but it appears that heating the platform upon which we print would make these problems go away. Bre from Makerbot was visiting the Ars Electronica festival this year and after he came to our (apparently infamous) Ars Party, he mentioned that the two most important things in making stuff work are: the heated pad and more LEDs (blinking lights make everything better!). We have inherited some LEDs from our friend Peter at the Arts University Industrial Design class, so that is on the to-do list, but for now we are concentrating on the heated pad.

After a series of middling success stories with heated pads running on something between 12V and 30V, we have taking nophead’s advice from the Hydraraptor to build a solid mains 240V AC powered heater. Chris M obtained some thrown out heating wire from a veneer press construction and we did some experiments with passing 240V through it. Heat. Smoke. Movement. A slight amount of fear as we wondered what the movement was about, but no danger and heat was present.This seems to be enough to get us going!

The heating wire, embedded in UHU chimney caulking compound that is drying.

Turning this wire into a heated pad was going to be harder. A piece of aluminium, some material from UHU stuff that is used to fill the gaps in chimneys and suchlike, and we assembled the base. Getting the coils set up halfway right, then holding them in place as we surrounded thrm in the UHU chimney caulking goop, was a bit of an adventure.

But they held!

Wires in place, with the magnets in their positions, the platform  seems solid. Attaching wires generated some heat and (scary!) some smoke. A momentary panic but no real danger.

Today I have been fighting with the electronics and the mounting. Getting all the parts to fit together in safe ways, balancing the stability of the platform, the mobility of the wires, getting earth connections set up properly in case something goes wrong. This is all important stuff but somehow full of contradictions.

The heating system. At the bottom the arduino plus interface board, in the middle the relay board (with an LED because that always helps) and the heating platform itself at the top.

The older temperature control circuit and Ardiuno patch that controls an LCD with three push buttons so we can set up a desired platform temperature, monitor that temperature and have the control work was re-worked for the 220V system: the transistors now drive a relay and the PID system that so cleverly does proportional control of temperature using all sorts of mathematical cleverness was replaced with a simple bang-bang system with hysteresis: if the temperature is more that 5% too low, turn on the heater until it is 5% too high, turn it off then let it cool to 5% too low, repeat. There is a problem that the 5% should probably be 1% because there is a rather large latency in the heating, but I am sure it can be worked out.

So now the problem is to get the platform on the actual build platform. This means that the zero point for the Z axis needs to be re-set and the cables need to be routed so that the whole build surface can be moved safely. The one annoying thing right now is that the surface is not as flat as the build platform surface (or vice versa) so the whole heated bed wobbles around the middle magnet. Solution: remove it! This might end up making the traction of the building surface worse but…less wobble is certainly more important.

Okay, let’s test this thing….

Update: First print. I stuffed up the zero point so the extruder head scratched the surface, digging up the Koptan tape and twisting the whole printing platform (held only with magnets) about 20 degrees. BUT I slid the whole thing up a twist on the Z axis and it printed through and: BINGO! perfect bottom surface, a mirror finish, perfectly sticking. Wowza. So this does help.

Second print: changed the Carve parameter to be a bit higher (0.35 -> 0.45) and notice that the infill seems also to have changed. This might be a side effect of the carve change, as there is less grip. The heated pad makes the Koptan stickier for the object: then once they are removed, they cool down and pop off perfectly. Mirror finish. Full points for the heated pad. Putting the spring steel printing surface back on, it bows up with the effect of the heat: as it finally gets warmed through the bowing disappears.

Third Print: The infill really is different (must have changed automagically when I changed the carve parameter) and with a higher extruder feed rate and temperature the sucking in seems to be a lot better. Th final top surface is not closed: I do not know whether it is meant to be. More like a dense net. Next try: the thin-walled test structure.

Five more prints. Things are getting better. There are problems because at each Zaxis change, a small gap seems to be produced. Trying a large jitter value (50.0 instead of 10.0) I see that the extruder motor actually turns off for each jitter motion. Seeing t there seems to be a certain lag with the extruder, this might tell us why there are gaps in the walls when we go up. So I try Jitter equal to 0.0 and we will see…

Yes. End of layer gives a blob, the start of the next layer has a gap: there is a lag in the extruder so turning it off lets it run for another second or three, giving the blob, and the start is delayed meaning that there is a gap.

Summary of the day (and night: it is coming up to the witching hour!):

  • the heated platform ROCKS! in making prints better. I re-did a TUBA nameplate and it came out dead flat, just like a bought one!No need to rough up the print surface or clean with acetone or anything, it seems to simply work, as promosed in the adverts
  • the extruder lag is a pain the the ar*e: this seems to give nasty messes. I hope we can fix this with the geared extruder that we have started building. I will re-do the big gear because with any luck the heated bed will keep it flat like it should be.
  • The allocation of variables and other parameters in the skeinforge sections is a complete mess. I am sure someone understands it, but they are certainly a bit special, with an amazing ability to remember all sorts of details! Why are the main temperatures stuck in the raft parameters: I do not use a raft, why do I change my main temperature there?? And why does it not work!?!? I presume because I do not use a raft so I have to define it in my start.txt – but I have no idea how I should know this…..
  • Going through the calibration steps (like on the thingiverse objects or this) is a good thing to do from time to time, even if it is just to remind you that there are a lot of details to play with and to re-work your preconceptions.
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