Friday, 26 December 2008

New Materials

HydraRaptor's extruder suddenly stopped working in the middle of a build a few weeks ago. I tried upping the temperature and pushing the filament with pliers but it would not budge. All that happened was the heater barrel slipped a few threads in the PTFE insulator.

It was a bit difficult to find out what was wrong because it was full of solidified plastic when cold. I unscrewed the nozzle and placed it in some acetone to dissolve the ABS. It appears that the hole in the nozzle was blocked by burnt plastic. It probably formed when I had some high temperature accidents and experiments recently.

I should have realised the nozzle was blocked, but it has never happened before. If I had then I could have just unscrewed it, cleaned it out with acetone and put it back on again. In the event pushing the heater out of the PTFE pretty much wrote it off.

Not for the first time, I decided to rob parts from the extruder I was making for my Darwin. These are all made from different materials in order to see if small improvements could be made.

The barrel is made from aluminium. It is a better thermal conductor than brass, is easier to machine being a lot softer, and is cheaper.

To make the thermistor more easily removable I mounted it in ring of aluminium with a tapped hole.

The thermistor was glued in with Cerastil H-115 and the ring was screwed onto the barrel with some heatsink compound in the thread. By adjusting the beta I was able to get the reading to agree with a thermocouple inside the barrel to within a couple of degrees. I don't know if that means the ring was at the same temperature as the middle of the barrel or if it was lower and I compensated with a beta value that is not actually the beta of the thermistor. Either way it produces the desired result.

I also made an aluminium nozzle with a 0.3mm aperture. I broke the drill bit as it went through. I am not sure if that was due to the aluminium snatching more than brass does, or me being careless. I have broken loads of small drills recently and blunted some bigger ones by accidentally drilling with my lathe in reverse!

The picture also shows where the thermistor ring mounts.

Another modification I made was to put a PTFE cap over the nozzle.

This has two benefits: -
  1. It is a good insulator so it helps to keep the nozzle warm.
  2. Being non-stick, and also cooler than the nozzle surface, it stops filament from sticking to it. I use a brush to wipe the nozzle. This works well with HDPE but ABS tends to curl upwards and stick. Since I added this cap the nozzle wipe has worked 100%. It remains to be seen if it works with PCL and PLA.
It is a snug fit but when it gets hot PTFE expands a lot so it slips off. I held it in place with a tiny screw and an indentation in the nozzle made with a drill point.

Another new material I used was Polyetheretherketone (PEEK) instead of PTFE for the thermal break. This has similar insulating properties to PTFE and a slightly better working temperature range. It machines well but forms burs very readily.

I found it much sturdier at working temperature, I don't need a pipe clip to stop the barrel popping out now, but I think it may be a bit harder to push molten plastic through, being less slippery.

The other thing I changed was I used insulated nichrome. When using bare nichrome I have to put down a thin layer of Cerastil to insulate the barrel, leave it to set, then wind the heater and cover it with more Cerastil. That makes it a two day job. By using insulated nichrome I can just wind it straight on the barrel and then cover. But what I didn't think about was that I normally make the soldered connections under the Cerastil, which I could not do this way. All in all I think bare nichrome is best as it makes a much neater job. Here is the previous heater that I made way back in March :-

So after all these "improvements" how did the new extruder perform?

Not very well! I tried it with green ABS first but could not get it to extrude reliably. I swapped the nozzle for my previous 0.5mm brass one and that got it working.

I then switched to some plain ABS that I bought a while ago but have not been able to use because it is very oval. It was too wide for my previous extruder. This extruder has a 3.5mm bore so it should easily fit but I could not get it to work reliably. It takes an enormous force to push it into the extruder. I am not entirely sure why. If I pull it out and push some green in I can extrude the plain that is left in the barrel easily so it isn't any harder to push it through the nozzle but it is to push it into the heater.

Since I foolishly changed every material at the same time it is hard to evaluate which things are better and which are worse. I have recently formed the opinion that the extruder design is far from optimum. I think we need a much sharper thermal gradient and a shorter heater barrel. I think a lot of force is wasted pushing slightly softened plastic down the thermal break.

My next attempt will have a very short thermal break with a heatsink at the cold side. I will also make it easier to strip down and reassemble. A problem with the current design is that once the heater barrel is screwed in and full of plastic it is hard to remove it.


  1. hey nophead,

    i had written some quick thoughts on some extruder improvements on the forum and like your post here they concentrate on the thermal gradient present during operation as well as during "standby".

    you mentioned that the barrel should be kept short, but that introduces the material breaks that may cause trouble for some people when they make their own. how about instead causing the thermal gradient occur within the length of the barrel by stacking alternating large and small washers (to form classic heat sink fins) on the upper half of the barrel and localizing the heating element in a large thermal mass (stack of large washers with no spaces) at the bottom 1/3rd of the barrel length. (just note the 1/6th gap that is left). obviously there are some details left out here, but thats to come later.

    my hope is that if properly insulated the thermal mass at the bottom would hold a more steady temperature whether or not you were actively extruding cool material through it, and the heat sink created towards the top will expel any heat that transfers through the barrel towards the supply.

    when i get back home from holiday i'll be making some prototypes and posting the images and results somewhere that the rest of the community can critique.

    Im definitely awed by all of the great work that has happened already in this project and i am looking forward to contributing to it myself.

    andres, chicago, usa.

  2. If you look at my previous post: a long thermal gradient down the barrel does not work. What is needed is a sharp change from hot to cold. That needs a short section with low thermal conductivity. That section also needs to be low friction. PTFE is ideal for that.

  3. The thermal gradient is key. It must be very very short. As short as possible. Someone mentioned that the commercial machines go through an air gap like I've been trying but they insure that the plastic stays cool outside--and thus won't mushroom up around the plastic--by blowing a cooling jet of air across it. That shouldn't be that hard to do and deserves a little bit of testing to see if it a) works and b) is home-feasible.

    Anyway, nice work, Nop. I do that bit of changing too many things at one time all the time. Nice to see the more experienced still occasionally do that too!


  4. Yes I thought all the changes would be improvements but actually the extruder doesn't work reliably at all.

    There is far too much resistance to pushing the filament. I think possibly increasing the bore from 3.3 to 3.5 allows more back flow and that the PEEK is not as slippery as PTFE.

  5. Nop,

    You wrote:
    > ... I normally make the soldered connections under the
    > Cerastil, which I could not do this way.

    If you're successful at soldering nichrome, can you *please* let me know what solder/flux/temperature iron you're using (and any special prepping of the nichrome.) I've (barely) been able to torch braze to nichrome wire, and would love to learn how to make good (e.g. durable, low R) connections between nichrome and regular electrical wire -- typically bare or tinned copper.

  6. I twist the nichrome and tinned copper wires together to get a good mechanical connection. Then I solder it with high temp (300C) solder (tin 5%/lead 93%/silver 2% 5 core Ersin flux) with my soldering iron set to max power.

    I don't think the solder particularly sticks to the nichrome but it does cover the joint to keep it from oxidizing. I bury the joints in Cerastil.

    It seems to be durable. I still have the first heater I made in March working.

  7. It could be that the oval filament is hard to push through the barrel because when it expands on heating just before melting, it might have a widest diameter of say 3.6 mm and so scrape against the 3.5 mm extruder. You could try boring out the extruder a bit, or just abandon the oval filament until it can be ground up and turned into new filament in a future granule extruder.

  8. This extruder does not work reliably with green ABS either. When I first push a new piece of filament in it extrudes easily but after a while it gets almost impossible.

    When the filament is stationary there is always going to a soft section in the insulator where it is past glass transition but not fluid. When you push on that it expands to fill the tube. Also when extruding there is some backflow that fills the tube and flows back to the point where it freezes. So there is always going to be some almost solid plastic filling the tube. In order to minimise the resistance I think we need to keep the hot cold transition very short and slippery.

  9. FWIW, you're not alone breaking small drill bits :-(
    One thing I've found (re-recalled) is that small bits do better when you spin them *fast* and feed them into the workpiece extra slowly. The bearings on my (old, rather beat up) mill get pretty noisy, so I don't like to run it at max speed, but for this application, top speed works best for me.

    -- Larry

  10. You can calculate your cutting speeds and feeds a little more accurately in the case of these smaller bits to save yourself from breaking as many. This generally isn't worth it with the larger bits but is well worth it for the smaller.

    Your cutting speed RPM is calculated by:

    RPM = (CS x 4)/D

    Where CS is an assigned surface speed for a given material and D is the diameter of the drill. CS for various materials:

    Low-Carbon Steel CS = 90
    Aluminum CS = 300
    Cast Iron CS = 70
    Alloy Steel CS = 50
    Brass or Bronze CS = 120

    For your feeds you can either go by feel of the cutting action or by observing the chip but this only works for larger bits. For smaller than 1/8 th bits you should only feed .001" per RPM. That's .02854 mm per RPM. Pretty slow as Larry_P said.


  11. We also had to increase the diameter of the hole from 3mm to 3.2mm because the ABS filament we got from the rrrf is a little bit thicker than 3mm at some points. With the ptfe this was no problem.

    The diameter of the hole in the brass piece is still 3mm which works for us. I think it already melts at the beginning of the brass.