Tuesday, February 23, 2010

Extruders Pt. 1: Backflow and Bench Experiments

Our work on the SpoolHead has been put on hold as we troubleshoot our plastic extruder. What would happen is that the extruder would work great on a fresh piece of PLA, but when we left it to cool and ran it again, it would be unable to extrude (even with Wade's geared extruder providing massive amounts of force). What could explain the extruder working perfectly at the beginning, and grinding to a halt the second time?

We're hoping to fix this as soon as possible, because no doubt there will be much troubleshooting of the SpoolHead as well. To understand the problem, we've been conducting some benchtop tests, observing how the PLA behaves inside the extruder nozzle.

Bing conducted experiments to determine why, after five minutes of successful extrusion, subsequent cooling and re-heating of the extruder caused it to fail. From our experiment, the resistance to flow due to viscosity through a narrow aperture is actually quite small; however, with the current Reprap-standard extruder design, a far more significant problem occurs. Because the nozzle is drilled wide, there is a void between the filament and the metal which fills with molten plastic. Even with a steep temperature drop at the front of the nozzle, and heat sinks, the plastic retains enough of its thermal energy to flow back up the nozzle and cool in problematic places. Indeed, steepening the temperature gradient with heat sinks seems to exacerbate this issue, because it becomes impossible for heat to reach this cooled plastic.

The problem is that the process is not quasistatic; because of the hot plastic's ability to flow rapidly up the nozzle before it can cool to the same temperature as its surroundings, the temperature distribution inside the extruder is not the same on the first run as it is on the second.

Here's how I visualize the process:
(This figure represents the extruder setup we were using, which had heat sinks to provide a steeper temperature distribution).

What to do about it?
The best solution would probably be Nophead's, which is to have a cone opening outwards, so that pressure on the fluid forces the plastic further outward, rather than back up the nozzle. However, I believe it is also important to maintain as small a void space between the barrel and the filament as possible. That means maintaining tight tolerances, both on the drill diameter of the extruder nozzle, and the filament itself. (Not all PLA filament is created equal.)

Using a PEEK insulator, which bonds relatively well to molten PLA, is probably not the best idea also. Having, if possible, a seal at the transition to the insulator would be of use as well. PTFE's tendency to flow is useful in this regard. We have made a PTFE insulator that is slightly under-drilled right at the end, so that it seals firmly against the plastic. Hopefully this will prevent backflow.

For now, we are also planning to have a wider nozzle at the tip, which will reduce the pressure that causes backflow. This will reduce our part-printing precision, but that's not really a concern for the SpoolHead project, because the plastic extruder merely needs to work.


  1. how is the contact (thermal) from the heatsink to the nozzle?

    if it was cool enough, it should harden and act as a self sealing plunger.

  2. I applied thermally conductive paste, so it should be pretty good contact. We also ran a fan. I did expect that if the cooling is strong enough it should harden and act as a seal as you say.

    But the problem is (I think) that the hot backflow has enough thermal 'mass' that it can reach high up, even if it's being cooled...

  3. This comment has been removed by the author.

  4. I'd suggest that there is possibly also another issue at play.... Swell. The heat transfer from the metal to the PEEK will cause the PEEK to warm up, and at some point in the cooling gradient up the nozzle at a point where the plastic is not yet molten but is absorbing heat from the PEEK, it will "swell" a tiny bit inside the PEEK, and the swollen filament will fail to fit down through a tight-fitting hole if it's positioned BELOW that swell point inside the PEEK.
    The solution to this is to make most of the PEEK a tight fit, except the very bottom, and to make the "seal" section you described in the metal, not the peek. Also, putting a little taper in the inner top of the metal will help too.

  5. Hm... but it can only swell in the PEEK if there is clearance between the PLA and the PEEK (or else there's no room for it to swell). Of course, that's a strong argument for using PTFE instead of PEEK, because if it is warm and there's no clearance, then the PLA will bond quite strongly to the PEEK. But with PTFE that's not really a problem, I hope.

  6. Few different problems. Few different solution.

    The plastic itself will be conductive. Problem is after setting more heat is required to melt it than to maintain it liquid once it is liquid.

    Next problem is that you are thinking the closer in the better. You are forgetting something critical the caplillery effect. The closer the walls are the higher the molten plastic will rise so more likely to jam the works. Unless you can get excact fit as you have said in impossible due to filement varation. Closer is not going to work.

    So the solution most likely is a bigger head not smaller. 3 holes drilled instead of 2. 1 for the size nossile. 1 large for the melt chamber(about size of filement. 1 larger again to stop caplillary effect on filment. Finally a guide to direct filment in center.

    ie -=< = kind of setup.

  7. There will be some filament deformation due to the drive system (on some systems a LOT of deformation). This should be taken into account when designing a system that requires close tolerances.

  8. wait after reheating. you need to let the tip but also the whole manifold warm up to get good extrusion again.

    with a cooled filament inside we heat the tip up for a solid 7 min before attempting to extrude again.

  9. Hi

    I had a similar problem using Peek insulator. In the end I bushed the Peek with a 6mm PTFE liner right up to the pinch wheel. I then drilled 3.5mm right through everything right to the back of the nozzle. So far this seems to have cured my problem.

  10. Why do you drill a large hole in the PEEK to start with?

    My extruder is made with a brass barrel that's screwed into a tabbed piece of PEEK.

    First, I make the barrel, with M6 thread in the insulator end, M4 thread in the nozzle end. Neether end has any thread for the outermost 2mm - they are 4mm without thread in the nozzle end, and 6mm in the insulator end. I drill a 2.5mm hole through it.

    I then drill the M5.6 hole in the peek - tab it, and screw the nozzle into it.

    Then I drill the barrel to my target hole size - usually 2.8mm for PLA or 3.1mm for ABS. I drill all the way through the barrel AND the PEEK. This guarentiees perfect match between the two parts, which keeps filament from creating a clog with material that's exiting the barrel in the PEEK end.

    Well, at least that works for me - you should try it.

    I use a 17mm PEEK rod.


  11. Just as a follow-up:

    Further experiments showed that our tighter tolerances solved the backflow problem. However, the friction between our stainless steel extruder nozzle and the glass-transition-softened plastic alone was high enough to halt extrusion, even with no plug.

    Adding a small amount of machine oil to the inner surface of the stainless steel appears to have resolved this friction problem.

  12. I've seen exactly this problem with ABS and loose tolerances - some of my earlier extruders had larger bores, and a clear 'lip' would form where molten plastic had pushed back up - not just die swell.

  13. Hi,

    I got the same problem with the PLA sticking to the PEEK. I tried it in different ways, but everytime when I take things apart, it appears that the PLA has been molten until about a centimeter above the copper heater barrel. When the heater stops, the heat is transported upward through the PLA, so it melts and forms itself exactly along the PEEK, whatever the diameter is (due to the pressure). When it cools down, the nozzle is still leaking, and the PLA drizzles out. The PLA inside the PEEK solidifies. When I reheat my heater barrel, it will never melt the PLA inside the PEEK again, since there is no heat conducting PLA anymore. The same situation happens with ABS.

    I tried different solutions to overcome this, also changing the diameters, creating a tight fit, but nothing worked. The one I am testing right now works pretty well so far, and is a small piece of teflon tube inside the peek. The PLA doesn't touch the PEEK anymore. My extruder always manages to push it out.

    A drawing I made is at http://www.flickr.com/photos/protospace/4388243118/

    And the first reliable print with the extruder: http://www.flickr.com/photos/protospace/4384467837/

    Best regards,
    Siert Wijnia

  14. Yes, this sounds like exactly the same problem we had. I'm glad you solved it =)

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