Another CupCake Nozzle Jam, PTFE Thermal Barrier Swarf, Heater Winding, and Glass Transition

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.

MakerBot CupCake Plastruder MK3 heater barrel with ABS leaked around barrel/barrier junction

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.

MakerBot CupCake Plastruder MK3 heater with nichrome wound near tip (doesn't work well)

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.

MakerBot CupCake Plastruder MK3 heater insulated only at tip

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.

MakerBot CupCake Plastruder MK3 heater barrel with nichrome wound around entire length

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.

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