Keith's Electronics Blog

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.

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.

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.

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.

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.

Great.

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.

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.

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.

I had written about jamming my CupCake nozzle trying to extrude PLA and having to disassemble the whole nozzle to clean and rebuild it. I got new PTFE thermal barriers (the white Teflon® tube) from MakerBot, put things back together, and quickly jammed the heater with ABS, which I’d never done before.

I conversed with Nop Head about the increased force required to push filament through the heater and he pointed me to the junction between the brass heater barrel and PTFE thermal barrier. It needs to be absolutely closed, and when reassembling I’d left a tiny gap into which the ABS expanded (covering the end of the brass in the photo), interfering with smooth flow of more ABS into the brass. He gave me this video link to a vivid demonstration of the phenomenon, in which he is attempting to push filament into a heater assembly that’s wide open at both ends:

He gave me the tip to reassemble the brass heater and PTFE barrier with a drill bit of the nominal hole diameter inserted. When the two are turned together as tightly as they should be, the PTFE will begin to deform and just barely begin to grip the drill bit.

Believing the filament hole through the PTFE is metric and not owning metric drill bits, I figured the next best thing would be to use filament itself (which would be slightly undersized; but I could tighten until friction increased and then back off slightly). To my surprise, I couldn’t push filament through just the PTFE tube by itself.

Upon inspection, I discovered swarf where the little filament hole from the top meets the larger heater hole from the bottom, and this swarf was interfering so much with the motion of the filament that even after forcing the filament end past it, I still had difficulty moving the ABS filament through the tube. No wonder the heater was jamming!

I reached into the large end of the hole with a rat-tail file and used its tip to push the swarf into the small hole, then filed it free by push-filing upwards into the hole. After cleaning the PTFE thoroughly, I reassembled the heater and fired it up again.

In the past, I’ve been able to hear my stock DC gearmotor slog down when the filament reaches a certain point inside the heater, and I’ve always assumed that was due to the constriction of the nozzle. Not so, as my motor no longer slogs, and instead merrily pushes filament through like chocolate sauce dribbling out of the corner of a topping pouch! I have to assume that my original PTFE barrier had the same problem (to a lesser extent) all along, and I wonder how many others out there do too.

Thermal Gradient and Glass Transition Phase

While rebuilding the heater, I initially thought that part of my original problem must have been the top of the brass getting hot enough to melt the plastic filament that close to the PTFE, as I had always pictured the filament melting somewhere inside the brass. I’ve also read about hot-end designs with a heater near the tip and a heatsink higher up to keep the feed area cool and the plastic solid while inside it.

I thus rewound all of the nichrome near the nozzle end and thermally insulated only the tip, thinking the cooler area above would help keep the filament from melting within the PTFE.

Even though after deswarfing the PTFE, the extruder initially worked far better than it had before, it proved not to be as reliable as desired, and I did notice the filament feed beginning to bog down. I had an ongoing email conversation with Nop Head, and he gave me this fantastic explanation of the hot end, which I reprint here with his blessing:

Most of the temperature is dropped across the PTFE and the point where the plastic melts (when stationary) will be about half way between the end of the barrel and the top of the insulator. This is where it is most viscous, so it is is good that it is inside something very slippery.

Brass is about 1000 times more thermally conductive that PTFE, but the barrel has a much smaller cross sectional area, so the thermal resistances are not as far apart as that, but still considerably different.

When the filament is moving the melt point will be much closer to the top of the brass due to the time it takes to melt because ABS is also a very poor conductor of heat and has a high specific heat capacity.

It’s a shame PTFE is not transparent, as it would be a lot more obvious when things were not working.

I think when at rest the filament melts in the PTFE. When moving I think it only gets above the glass transition, so when there is a gap it expands into it and jams.

Well, heck! If that’s the way it works, it sounds like I want the brass heated evenly (at least in this Plastruder MK3 design) to keep the filament as soft as possible at the brass-PTFE junction.

I disassembled the heater yet again, this time rewinding the nichrome as evenly along the length of the barrel as possible and once again insulating the whole barrel. After reassembling, I haven’t noticed the feed motor bogging down as it was before.