Archive for January, 2012

Rebuilding the CupCake Extruder Heater (When It Wasn’t Very Broken)

Sunday, January 29th, 2012

After some success back in June using fans to cool extruded layers on my CupCake — in fact, right after that success — it slowed extruding and eventually stopped extruding altogether. This is the story of my life with the CupCake — a very brief success from time to time, but never persistent nor replicable.

When I say stopped extruding, I mean the motor actually ground to a halt. Usually it chews a divot into the filament, but this time it stopped. And I was pretty sure — don’t remember whether I actually checked the on-screen display or not — that the nozzle temperature had dropped and the filament wasn’t melting any more.

I know people talk about extruding ABS at temperatures as low as 200°C, and I don’t find that to be the case in my CupCake. Mine is calibrated, and mine doesn’t like temperatures that it thinks are lower than 220°C, and mine doesn’t really like temperatures that it thinks are below 225°C. So it really doesn’t take much to make it unhappy.

Nichrome wire crimped to teflon-coated wire

I had just rebuilt and rewound my heater at the time, and I knew that I had crimped the nichrome to the teflon-coated lead wires with silver crimp beads. I had a suspicion that the joints had become oxidized under the crimps, and the 7.3Ω resistance across my heater wires seemed high for my CupCake.

A couple of weeks ago I disassembled the heater and found that one of the two connections was indeed quite scorched and oxidized.

Nichrome wire crimped to teflon-coated wire

After cutting away the crimp tubes, cleaning the end of the nichrome wire with fine sandpaper, cutting back the lead wire, and recrimping, I tinned both lead wires with solder. Solder doesn’t stick to nichrome; but being coated with solder, the joint (which already had a solid mechanical connection from crimping) should be much less prone to oxidation.

After the rebuild, the heater measured 6.8Ω. Half an ohm difference doesn’t sound like that much until you’re trying to get to 225°C. Since power P = V2 / R, at 7.3Ω, P = (12V)2 / 7.3Ω ≈ 19.7W; and at 6.8Ω, P = (12V)2 / 6.8Ω ≈ 21.2W; so maybe that could be enough to make the difference at the high end of the extruder’s temperature range.

ReplicatorG control panel with temperature failing to reach set point

Aaaand … after reconnecting things, I still couldn’t get the temperature above 222-223°C, even though it now had some 7% more power. That doesn’t seem quite right.

Pulse-width-modulated heater signal on oscilloscope

When in doubt, scope it out. Yeah, after almost a full minute of failing to hold the temperature at the set point, the software PWM in the extruder controller was still running the heater at about a 50% duty cycle. That definitely doesn’t seem quite right.

And isn’t something I can easily fix, either. The ReplicatorG version I was running didn’t have a control panel for the heater PID settings, so (even assuming I was smart enough to fiddle them into shape) I would have had to recompile the code each time I wanted to make a tweak, which wasn’t palatable.

Firmware Upgrade

But I thought I’d heard that newer ReplicatorG versions did bring the PID coefficients into the machine control panel, so I upgraded ReplicatorG from 0024 to 0029r2, and let it upgrade my firmware from v2.4 (I think) to v3.0, and lo! lost communication between ReplicatorG and the CupCake. It said it had a connection but all the menu options to talk to the CupCake were greyed out.


This is apparently a known problem claimed to have something to do with the Mac’s localization settings for the string representation of “,” and “.” in numbers. Srsly? And the suggested tweaks didn’t fix it for me, so I’ll just wait for the next ReplicatorG release. And since the Mac package of ReplicatorG continues to be a DMG file of all the pieces you have to drag into /Applications/ReplicatorG, rather than a ReplicatorG folder that one could conveniently drag into /Applications like everyone else provides … I guess I should feel lucky to have a Mac version at all, and I’m not holding my breath for a fix on this problem.

Anyway, downgrading ReplicatorG to 0026 restored my connectivity and got me a look at those sweet, sweet PID coefficients.

ReplicatorG heater temperature graph

Which I no longer need, ’cause with the upgraded CupCake firmware, the PID algorithm seems to work right. Reaching for the knobs was obviously an attempt at a workaround, and the real fix is oh so much better.

Plastic-extruded filter holder assembly

The stringing on this diffusing filter holder is my fault, not my machine’s — I have a 0° (or 90°) overhang on a concave curve, so there’s no way it was going to come out clean. I still wanted to see what it would do, and it performed admirably under the circumstances of an impossible model.

And then stopped working again.

Filament drive motor locked up against the filament. A-gain. (Yay, great grip on the drive pulley, and nozzle retaining washer not breaking!) Temperature claims to be steady where set.

My utility is fairly cold these days. I’ll try enclosing the build chamber again in hopes that although the nozzle is hot enough, the teflon tube is too cold — but I bet I end up disassembling and drilling out the inside of the tube and nozzle again.

Should have been designed with a quick-release.