Saturday, 7 March 2009

Simply better

I find it very satisfying when making something simpler also makes it better. I tested the simplified heater / nozzle design using the same stainless steel insulator and heatsink arrangement, so I could get a direct comparison of the results.

The heater warms up a lot faster than the one made with two AL clad resistors. It also extrudes faster and the times are more consistent. ABS pushed with 2.32Kg went from 3.7 mm3 to 4.6 mm3, an increase of 24%. HDPE pushed with 4.6Kg went from 3.8 mm3 to 9.3 mm3!

The nozzle is 0.6mm rather than 0.5mm, which reduces its contribution to the pressure by a factor of 2, but all my other tests have shown that what happens at the other end of the heater dominates the force requirement. As I improve things though, the nozzle hole becomes more significant.

Here are the drawings :-

Although it looks complex it isn't difficult to make with a drill press, drill vice, and some taps and dies.

I glued the thermistor in with Cerastil, but I expect it could just be wrapped in tin foil and jammed in like the ceramic resistor, taking care to insulate the wires of course. I use PTFE sleeving.

I didn't need to seal the threads with PTFE tape. I just screwed them up tight and there was no sign of any leakage.

The next thing to try is putting a taper in my PEEK version to see if that can be made to perform as well as this one.

Of course I haven't built anything yet with any of these designs, so caveat emptor.


  1. Could you repeat the 4 dimensions for the primary hole along the vertical view? The way they are now is, I think, possible to figure out (From bottom to top, is it 5.0, then 4.2, then 5.5 tapped to 6.40 (1/4" threads?)), but it would be clearer with those 4 numbers separated out somewhere, instead of just all bunched up as specs for a single hole.

  2. Yes sorry it is not clear, I will do detailed instructions once I settle on a final design.

    A 4.2mm hole is first drilled all the way through. The top 6mm is drilled out to 5.5mm and then tapped with a bottoming tap to 1/4" UNF. That is because the SS pipe I used was 1/4" OD.

    The bottom of the hole is then tapped M5 x 0.8, deep enough to take the nozzle thread. That can be an ordinary tapered tap.

  3. Kevin O'Connor7 March 2009 at 23:02

    Very nice!

    Have you considered using a sports ball inflating needle instead of the welding tip?

    It looks like brass needles are not uncommon (which should conduct heat well). The needle could be cut down to a shorter length and (if the orifice is too large) plugged with solder and then drilled out.


  4. No, do you see an advantage to using an inflating needle over a welding tip?

    The ones I have seen are long thin-walled tubes with a small bore. I imagine they would be poor in this application for two reasons. You want the tube to be short with a bore several times bigger than the hole at the end, so that it does not constrict the flow. You also want a fairly thick wall to conduct heat to the end. Copper is a much better conductor than brass.

    I suppose if you cut it so it was only a few mm long it may work.

    Welding nozzles are cheap and easily available. They do have to be drilled out, but that is not hard. The end looks close to the ideal shape for extrusion but I have yet to see how well it works.

    Ideally they would be shorter, Andy Hall suggested we can counter-bore so that less sticks out below the heater block.

  5. Kevin O'Connor8 March 2009 at 01:16

    Thanks for the feedback. I was envisioning cutting the needle to a length of 2mm or so - just enough to prevent the extruded plastic from sticking to the heater.

    The advantage would be not needing to drill the welding bit and reducing the distance between the heater and extrusion point.

    One downside I found is that they don't use a standard thread size.

    I'll pick up a couple and play with them.

  6. How about using two nozzles with the inclusion of a shutter?

    That way one could be fine detail and the other course for rapid structure building of solid areas.

  7. Yes two heads would be nice. The software would be interesting though. If say the infill diameter was twice the outline diameter then you would do two layers of outline and then a thick layer of infill.

    Where it would get complicated is when the outline changes between layers. The infill would then meet a stepped edge.

    Also, where there are narrow bits the wide infill would not be able to get in, so you would need two layers of the thin infill.

    All perfectly feasible but adding to the complexity.

    The other problem is that with two nozzles side by side at the same height, one may mar the surface created by the other.

    I would be happy to get a single nozzle extruder working reliably for 1000's of hours. By making use of die swell and changing the head speed it is possible to change the filament width by a factor of 2:1 with the same nozzle. Given a powerful enough extruder that could allow infill at four times the outline rate if the limiting speed was the head movement.

  8. Great design. I have made one and it only took me 90 minutes from starting to warming up.Thanks nady

  9. Pardon me for using this method to contact you, but I couldn't find a email address anywhere public. Your expertise with this particular style of hot ends and comparatively slow machines is unparalleled as far as I'm aware, so you're the person to ask this question.

    But first, some back story.

    For the past month or so we were struggling with building a hot end of a design very similar to this for a repstrap, which, due to local availability of belts and gears, or rather, the lack of it, was made with leadscrews in a most primitive fashion possible. (We don't expect it to last longer than we need to produce a Mendel-alike.) As a result, it's extremely slow (on the scale of F300 in millimeters, because we suck at mechanical design) and we were having problems galore due to very slow extrusion speeds.

    We initially started with glass nozzles, and while these, due to low thermal conductivity of glass, produced no barrel or nozzle jams at all, eventually we decided to abandon them as our local sources making them could not make the exit holes to spec, and we were unhappy with the print quality. We switched to experimenting with metal nozzles. Since we can't find PTFE anywhere (we're avoiding parts that require ordering from overseas as much as we can due to how ridiculously unlucky we are with ordering anything, and we would rather settle on something consistently repeatable than a one-off solution) we are using a heater-thermal barrier-radiator design very similar to the one you describe here.

    Hot ends made from a single piece of brass produce barrel jams after around 5-6 hours of uninterrupted work due to the plastic plug expanding too far. The problem went away when we switched to a stainless steel barrel made according to the directions you gave above, with an aluminium heater block connecting it to a brass nozzle. (We couldn't make a suitable hole in aluminium, but it comes out nicely in brass.)

    Unfortunately, that's where we ran into another problem which we so far couldn't solve. Now, it's the brass nozzle itself that's getting clogged, after about ten hours of uninterrupted work. Pushing a wire inside while it's hot unclogs it for a while, but soon it gets clogged again. Disassembling the hot end and cleaning it makes the problem go away, but we can't tell what is causing the jam even after cleaning it out -- whether it's some kind of impurity in the ABS we're using accumulating, some kind of contaminant introduced in the mechanism, or something else.

    My current theory is that the tapered zone in the barrel and various joints between parts produce areas where the plastic has a chance to be stay for a long time, which eventually results in thermal decomposition and production of some kind of tar, which has a much higher melting point and viscosity, and that's what's clogging the nozzle. But if that theory is correct, this will inevitably happen with any kind of hot end assembled from multiple parts.

    So what I wanted to ask is, does this happen to your hot end, and if it does, what is the typical time until it happens? What is the temperature and feedrate you typically extrude at with it?

  10. No that doesn't happen normally with my extruders. After a few months the nozzle aperture starts to constrict due to burnt plastic but all I do is ream it with a small drill.

    I extrude ABS between 240 and 255C depending on the type at between 8mm/s and 32mm/s.

    I did once have something like you described It never happened again, so I assume the plastic got too hot for some reason.

    If it gets too hot, or if it is contact with the air for a long time, it will decompose to a harder substance. I don't get that in normal operation though. I do extrude as slow as 5mm/s when making small objects, but not for long periods of time. I wonder if very slow extrusion allows the plastic to remain in the extruder too long, whereas it would normally be flushed through before it can decompose.

    Have you tried running it a little cooler? Is it coloured ABS or natural?

  11. Thank you for a prompt answer.

    It's black ABS, and while I don't know the specific grade, it's originally meant for welding. (On the plus side, it's about five times cheaper than importing it from Europe, and, well, we did get quite a lot of smaller objects printed with it by now...) Dissolving it in acetone produces what looks like very fine black powder suspended in the solution, only clearly visible at something like 1:10 plastic:acetone, and we don't know what the pigment is.

    6 mm/s is the constant speed for this machine, and while we did manage to run it at 12mm/s with acceleration, it would still take most of a day to make a major Mendel-like part, and we could not get acceleration completely skip-free, so right now we're stuck at this speed.

    Running it cooler is first on the list of things to try, (It's been 230C, calibrated against a multimeter thermocouple inserted straight into the molten ABS next to the exit hole. Next attempt is going to be at 220C.) but right now we're also trying to make the brass nozzle that is more like a straight cone on the inside, so that there is less chance to have a stationary zone near the exit hole.

  12. Sounds like a reasonable plan. I expect a slow flow is less turbulent, so less likely to flush out plastic from small gaps.

  13. Well, it looks like our original theory might be wrong, so I'm going to post the observations as a comment, if you don't mind, just in case anyone stumbles upon this page later having a similar problem.

    While the decomposition products do appear to be slowly generated and condense wherever they please, and might eventually obstruct the nozzle, the real cause for clogging in our case seems to be a contaminant.

    That is, the common silicone-zinc oxide thermal grease we have been using to improve thermal conductivity between various components of the hot end. Since disassembling the extruder for cleaning involves unscrewing the barrel from inside the radiator, and we have been ditifully replacing the thermal grease every time, some of it would get on the upper end of the barrel and then spread along the channel by the incoming filament.

    The thermal grease mixes throughly with the plastic when it is liquid, but unlike the plastic itself, it decomposes quickly around 200C, leaking the silicone oil, which exits the nozzle, and leaves zinc oxide mixed in with ABS, which retains the color and most other properties of plastic, except that it won't melt and is much harder than raw plastic. This eventually accumulates down in the nozzle and clogs it shut.

    We have discovered it when a freshly cleaned hot end almost immediately clogged and leaked transparent oily liquid instead of plastic -- turned out, a sufficiently large dollop of thermal grease ended up in the nozzle. After heating the remaining black residue to about 250C, it turned into grey ash, which didn't cling to the walls so hard, but still would not melt or flow.

    We've reassembled it without any thermal grease at all. The next long print run produced another jam after ten hours, but this time, it was a jam resulting from the plastic plug travelling too far up the barrel, and the nozzle was still open. Apparently, running such an extruder at too low a temperature produces a thicker plug -- it mostly melts on the brass nozzle itself, but the lower end of the steel barrel is sufficiently hot to gel it. The gel doesn't liquefy fast enough, and there's no way to go for it but up, as the filament presses down, so eventually it goes up past the taper. Raising the temperature to 245C and easing up on the filament retraction in Skeinforge allowed us to finish a 20-hour print, it's printing the next 2-hour part with no obvious snags, and it looks like we finally have a design that won't fail again... though I'm still knocking on wood here. :)

    I do wonder if there's any reasonably common thermal grease replacement that won't result in such clogging.

    As a side note, what's the usual nozzle hole diameter you use at the speeds and temperatures you cited above?

  14. I think the key is to not get thermal grease inside the extruder. On some designs I just use it between the heater block and the barrel thread. More recently I haven't bothered with it.

    I have a 0.4mm nozzle on HydraRaptor and 0.5mm on my Mendel at the moment.

  15. Alas, I spoke too soon (hardly surprising when it takes many hours to ensure I am correct). It still clogged on the very next part, so 20 hours is still our record with metal nozzles. :(

    Upon further experiments and some extensive arguments, it appears to me that there's two distinctly separate problems involved:

    1. Plug crawling too far up for the motor to push it through. This can be alleviated by tapering the barrel (or lining it with PTFE, which we cannot do at the moment), but...
    2. Tapering, or otherwise increasing the barrel diameter, will result in creating an expansion zone. An expansion zone buffers the molten plastic and prevents the plug from crawling much higher, but can produce zones where semi-molten plastic will remain floating inside molten plastic. This can then produce random flow obstruction, or keep a piece of plastic inside for long enough that it will eventually stop melting altogether due to deterioration.

    Small snags on the edges do produce a tar-like substance, but that accumulates much slower than a blob that eventually cakes up in an expansion zone. Naturally, any contaminant like the zinc oxide accelerates it happening even if introduced in quite minor quantities, but it still can happen if there's no contaminant.

    There should be a sweet spot between the two problems somewhere, that will result in a stable extrusion with a certain grade of plastic in a specific temperature range -- that sweet spot expands as extrusion speed goes up, because turbulence increases and heat spread inside the expansion zone improves, so it's all evenly molten. Apparently, at 6mm/s this sweet spot is very narrow, so we've never been able to hit it closely enough.

    This balancing act is a bit too much for us at this stage, and we still haven't procured any PTFE, so we're back to glass nozzles for now -- but this time we got a buthane torch to fix the exit hole to 0.4.

  16. I am not sure I get your point about the expansion zone. The taper does increase the diameter a little and then it tapers down to the nozzle. All that area is at about twice the melting point of the plastic so it should all be fully molten and not exposed to air. The plastic I use does not turn to tar in those conditions. I haven't tried long periods below 16mm/s of 0.3mm filament, but I would be surprised if a slightly lower flow rate would be enough to cause the problem.

    I had an extruder motor lead come loose the other day so the machine ran with no filament flow for about 8 hours. When I restarted it the first few meters of filament that came out were brown but nothing hard enough to block the nozzle.

    Perhaps your plastic is the problem. Or perhaps you have you have voids that can trap plastic that I don't have.

  17. It may well be that my plastic is the problem. Unfortunately, there's nothing I can compare with right now, and won't be for quite a while. It does appear to start caking up well before it actually turns to tar and produces the brown residue, much more readily than anything I've seen reports of.

    I'm not sure myself what's exactly going on in there, but that's one more reason to go back to glass for the moment, at least this way it's a little more visible. :)