Newbie's 1.75mm hotend/extruder design

I'm working with a group of high school students on a 3D printer design as an engineering exercise, the only problem being that I have no experience with 3D printing. (I'm and engineer that's done lots of machine design, though.) The hotend/extruder seems like the trickiest part, and I wanted to get a handle on that first. We're on a budget so I wanted to see if we could make the extruder (we have a CNC mill, and a crappy lathe) rather than buying one. After a couple of prototypes, I seem to have stumbled across a design which seems to work and is pretty easy to make:

The hotend is just an aluminum heater block/nozzle (0.4mm orifice) with a PTFE liner going most of the way thru:

The heater block is not supported at all by the PTFE - it is held in place by the two outboard screws. The key to the design is the way the PTFE liner is supported. Just above the heater block there is a torlon (could be PEEK) collar slipped over the liner to make sure is doesn't expand, as the melt zone can creep a little above the heater block. Above the torlon collar is a 8-32 nut, which conveniently, is almost the perfect size for threading onto the 4mm dia liner tube. This nut keeps the liner from creeping upward during operation. During assembly the two outboard screws are tightened just enough to keep the liner snug, but then locknuts are used at the top and bottom of each screw hold the heater block at a fixed distance.

For the drive mechanism, I'm using the standard sort of shaft hobbed with an 8-32 tap. I start with an 8mm shaft, but the business and is turned down to 7mm. The gear is running against an 8mm bore bearing with an eccentric hub. The hub is spring-loaded against the drive gear with some notches for different compression settings. On the actual prototype, I grooved the bearing's outer race to keep the filament from deforming so much.

So far, it all seems to work pretty well. I've extruded a few dozen feet of 1.75mm ABS at just over 5mm of filiment per second with no hit of jamming or leakage. So finally, here are my questions, for those with actual experience:
1. I don't want to add a fan if I can avoid it. The PTFE provides a much better thermal break than all metal designs by about a factor of 20. Can I punt on the fan or am I whistling through the graveyard? Will the cool end get too hot just from the radiant heating?
2. What's going to happen to the PTFE liner over time? When I thermal cycle, the PTFE shrinks noticeably when cool, but expands back out and forms a tight fit when re-heated. Will this continue to be well behaved?
3. Uninsulated, the heater block runs at about 12w to keep it heated to 220 - 230c. Is this about normal? I tried adding some modest fiberglass insulation, but that only cut the power by about 0.5w.
4. What am I overlooking?
Thanks for any comments or suggestions.

The feedstock will eventually heat up at where the ptfe is exposed, and when it does the plastic will liquefy, and then swell and burst the ptfe tubing. There needs to be something stronger around the ptfe liner, or something to keep the feedstock from ever heating up before the ptfe tubing. i would still go for active cooling, as your cool region is still short.


you could wrap duck tape around the ptfe , you just want it to keep its shape. also you will want to assure heat stays below. probably attach a heat sink on the torlon material above the hot end.

a lot of heat is still carried away by the bolts supporting the hot end. try moving them further out towards the end of the heater blocks, and insulating them with nylon collars (not sure of correct term at the moment)

it is a good idea to wrap the heater block with at least 1 wrap of kapton tape. this will reduce heat loss from air while head is in motion.

Edited 3 time(s). Last edit at 04/29/2013 11:16PM by jamesdanielv.

Thanks for the suggestions. I'll do some more long-term testing and see how bad the heating is of the upper section of the PTFE liner.

By my calculations the thermal conduction up the PTFE tube (if it were completely insulated) would be less than 0.1w. The conduction up the 4-40 screws is about 1w but they screw directly into a fair amount of aluminum. My gut feeling is that small amount of heat up the tube will be radiated away before the upper section gets hot enough to soften the ABS. I'm more worried about direct radiation from the hot section, and also convection, neither of which I have a good feel for.

After a bit more testing, it does look like if I let the extruder sit idle for 5 min or so, the melt zone does creep up above the torlon collar. As long as I'm pushing filament, though, the melt zone stays restricted to just above heater block. With extended use (say, 45 min), and with more or less continuous feeding, everything is well behaved. It seems like this would be OK for normal operation. Is this an issue with other types of hot ends - that the melt zone (and more importantly, the semi-molten zone) get longer when the extruder is idle?

In any event, it seems advisable to extend the torlon collar all the way up to the 8-32 nut. It'll add a little more conduction, but it will keep the PTFE liner secure.

Contact me through eBay I sell the SG2 hot end, if you would like, I will donate one to your cause. I will change my listing to accept "best offer" and you will pay the lowest eBay allows + shipping.

knowbusiness Wrote:
-------------------------------------------------------
> Contact me through eBay I sell the SG2 hot end, if
> you would like, I will donate one to your cause.
> I will change my listing to accept "best offer"
> and you will pay the lowest eBay allows +
> shipping.

Thanks for the kind offer. So far, our home-brew hot end seems to be working OK, but if it starts to give us grief, I'll certainly take you up on your offer.

you really want some support above the torlon, ptfe gets soft when it gets warm, i had a gap about 0.5mm high between a ceramic, and the cold end. and a print that took 2hrs. the print job failed because the ptfe stretched like a balloon into that little gap, and eventually tore, and started leaking and oozing pla material down the hot end.

for short print jobs ptfe is ok with no support, but if a long print jobs is required and a low flow high resolution print is in process, the heat will go up the ptfe, unless you have active cooling, or a support structure all the way up. but you need a structure that allows convection, so it stays cool. even 3 wraps around with aluminum tape used for heater stove pipe will work well with this. just make sure there is absolutely no gap petween the alluminum tape and the torlon. the aluminum tape will prevent the tubing from expanding when it softens.

I think I am starting to understand better what you are getting at with the overall heatflow issue. How about this:

I've replaced the nut at the top of the exposed PTFE with an aluminum tube with a short threaded section to grip the PTFE at the cold end. The PTFE will be fully supported and the cold section heatsinking will be brought all the way down to the torlon collar. With this design, I can also easily add more aluminum surface area to the upper structure for additional heatsinking.

Your point about slow feedrates causing more melting up the tube has me wondering - do you typically vary the temperature dynamically during a print? It seems like you'd want to actually be regulating the power to the heater, something like:

P_total = P_losses + K x Feedrate

where P_losses is the amount of power required to just match the radiant/convective/conductive losses at the target temp with zero feedrate. You'd then add a proportional term for the rate of new material to be melted. Is this what gets generated by the slicer software?