Sunday, 4 January 2009

New year, new extruder?

The RepRap design has always aimed to be cheap and easy to make from readily available materials. What I desire though is good performance and reliability, and put those priorities ahead of the others. To me they are absolute requirements and the others are things to be optimised afterwards. With that in mind I set about trying to design a reliable extruder that I can make with the tools and materials I have available.

As it is experimental I wanted it to be modular so I can swap out things that don't work. I started with the heater. It takes me two days to make one so I wanted it to be removable and reusable. I made an aluminium bobbin with an M6 thread through the middle of it so it can be fitted to different barrel designs. The outside diameter is 12mm and the inner diameter is 8mm. It is also 12mm long. The flanges are 2mm and 3mm with a 7mm gap for the nichrome and Cerastil.

The surface is roughed up to make the Cerastil adhere well. It has a hole to accept the thermistor to make it a self contained closed loop.

I put down a layer of Cerastil about 0.5mm thick using a plastic jig and left it to cure over night.

I used two strands of 0.1mm nichrome in parallel to make the heater. That only needs 90mm to make about 6Ω. I normally use 8Ω but I anticipated more heat loss in this design.

To make connections to the heater I used two strands of 0.2mm tinned copper wire and attached them with reef knots.

I then covered the knots in high melting point solder.

Using such fine copper wire may be a mistake as Bert pointed out on my previous post. Time will tell.

I made a jig to keep the wire taught while winding it on the bobbin.

At this diameter it is only about three turns of nichrome.

Finally I covered the windings in Cerastil H-115 and also used it to glue in the thermistor.

I made the barrel as short as possible. That turned out to be 25mm to have room for the heater and the nozzle and a mounting flange. The standard design uses a 45mm heater barrel.

The vaned section is a heatsink to keep the rest of the filament path cool. Sandwiched between the hot and cold sections is a 12mm length of 10mm diameter PTFE tube.

The idea is to keep the thermal transition as short and slippery as possible to make it easy to push the slightly molten plastic through. The PTFE extends 5mm into the heatsink to give a good contact area for cooling. It extends 2mm into the hot barrel and 5mm is in the air gap. It is an interference fit and is under compression. When it gets hot and expands the seal should only get tighter.

The metal parts were drilled to 3.3mm on the lathe and once assembled it was all drilled out to 3.5mm. As the PTFE was drilled in situ the hole is perfectly aligned and there are no gaps.

The thermal loss through PTFE, which has a conductivity of 0.25 W/m°C, will be: -

220 × 0.25 × π (0.0052 - 0.001752) / 0.005 =0.76W, assuming the heatsink is at 20°C and the barrel is at 240°C.

The barrel is held on by three M3×25 stainless steel bolts. The holes are counter bored so only the last 5mm of thread is in contact with the heatsink. Assuming the mean diameter of the thread is 2.75mm the heat loss through the bolts is: -

3 × 220 × 17 × π × 0.0013752 / 0.02 = 3.3W

Longer bolts could reduce this by about half.

Here it is with the heater, nozzle and PTFE cover installed. There is heatsink compound between the heater and the barrel, and the nozzle thread is sealed with PTFE tape.

The wires are insulated with PTFE sleeving and terminated to a 0.1" header mounted on a scrap of Vero board. This mates with an old floppy drive power connector. I put the thermistor in the middle and the heater on the outer contacts so it doesn't matter which way round the connector goes.

The clamp seems to grip aluminium a lot better than it does PTFE but I also put an M3 bolt into a blind tapped hole to ensure it cannot slip. A good move as it turned out.

I powered it up without the pump and calibrated the thermistor. With the nozzle at 240°C the "cold" section reached 100°C and softened the ABS clamp. Obviously my home made heatsink is woefully inadequate.

To keep it cool I added a small fan. That keeps the cold section at 30°C, much better.

The black sheet is Teflon baking parchment that I used to stop the fan blowing on the hot section.

I haven't attached the motor yet but I have tested hand feeding white, green and black ABS as well as HDPE. The ABS feeds easily through the 0.3mm nozzle and the HDPE with moderate force. I think they will all work well with the motor drive.

When the filament is pulled back out only a few millimetres has expanded at the end. In contrast, without the fan the filament swelled most of the way to the top and jammed. You can see the difference here: -

Keeping the melted section short is the key to making the filament easy to feed. The other improvement is that the PTFE is no longer a structural element. It is held in compression and appears to make a good seal with simply a push fit.

I am sure I can both improve the thermal separation and make it easier to make with a couple of design iterations before redesigning the other half of the extruder.


  1. this is looking great! i'm glad to see everything happening with the extruder these days.

    what kind of duty cycle are you seeing compared to the mark 2 extruder? is the heat sink drawing out a huge amount of heat from the nozzle?

    great work.


  2. It does draw significant power but not huge. I have upped the heater rating from 18W to 24W and the duty cycle has gone from about 70% to 80% so perhaps 6W. I calculated about 4W through the bolts, the rest is probably due to the surface area of the flange.

    Longer bolts should improve that somewhat. Some insulation round the heater and flange would help. Perhaps replacing the flange with a stainless steel washer would be an improvement as well.

  3. Hi!

    Im always really excited to read your experiments. Just two minor points which should ease the understanding:
    - when you write down the equation could you also write with symbols also? (and not just the numbers) Like: H = k*A*dT/x (k =thermal conductivity[W/(m*K)]; A=surface[m2]; x=thickness between cold and hot[m]; dT = delta T[C], the temperature difference in celsius)
    Would be easier to identify the numbers. (hmm, maybe it is not that important, anybody can google it up)

    - a photo with all the parts in one place. I have difficulty to see how is it mounted the heatsink and the ptfe block. (Have those threads inside?)

    But you can safely ignore those points, because your posts are already well written with big images(!), and Im not just reading it as a pleasure, but it educates me also. Im always learning new stuffs.


  4. Hi Khiraly,
    There is no PTFE block. There is just the short 10mm diameter tube which you can see across the 5mm air gap in one of the photos. It is simply pressed into counterbores made with a 10mm endmill. There are no threads on the PTFE.

    The only thing not shown in the photos is the standard polymer pump which just bolts on top of the clamp. It isn't shown because I haven't fitted it yet but I will this evening as I need to make something.

  5. Thanks again for sharing your experiments, nop! I especially like the modularity to make parts reusable.

    What do you think about making the design even more modular and seperating the flange with the embedded thermistor from the heating section? Do you think the new connection (alu-alu or alu-copper) would cause a big temperature loss and a wrong measurement at the thermistor?

    I was thinking about it because I would prefer a removable thermistor (exchangable with a thermocouple, possibly), but access to a lathe takes me days and I do not want to waste a thermistor.

  6. Hi Daniel,
    If you look at the second picture in New Materials you can see that I mounted the thermistor in an aluminium ring and it worked very well.

  7. Yes, but I wondered if there was a reason you decided against doing it the same way again. Anyway, there seems to be no reason against it then ;)

  8. No major reason. It makes the combined length a little shorter and because I terminate the wires in a four pin connector it is more convenient and robust to keep them together but less modular as you point out.

  9. Daniel, a tip.
    When holding screwthreads in the lathe, you can hacksaw through the side of a couple of nuts and then screw those onto the screwthread. Then when you tighten the lathe jaws they will hold the screwthread tightly without risk of damage.
    Happy New Year!

  10. oops, sorry Nophead that's a tip for you not Daniel.

  11. Hi Andy,
    Thanks for the tip. I will no doubt find it useful but did I mention thread damage somewhere?

  12. Now that you have a narrow melt zone, is it possible to reverse the feed screw and stop the extrusion process?

  13. I think that has always been possible since I got rid of the flexible drive (it unwinds) but I designed my electronics to be one direction only because I knew the flexible drive was not reversable.

    I will make a new extruder controller once I have got reliable mechanics sorted.

    Even if you stop the flow completely it will not be as good as a valve because the molten plastic is still attached and gets dragged out of the nozzle when the head moves.

    Very fast head movements (120mm/s) are on my list of things to try when I get my Darwin working. That way any ooze is spread very thinly.

  14. I woke in the early AM with another wierd idea, what if it's possible to fully retract the fiber up into a channel pathway and feed another material/color?

    Dump the change over into the nearest infill gap.

    Back to stoping the flow, maybe retract the Z axis and move towards a infill if one is available?

  15. It takes several feet of filament to flush through a colour change. Probably easier to just have an extruder for each colour and mount them close or use head swapping.

    It would be nice to be able to inject dye into the nozzle but it probably would not mix enough. That way you could have any colour with three or four dyes.

    Perhaps we could have a manifold type nozzle that takes three flows and mixes them.

    Enrique's excellent Skienforge suite can hide the ooze in the infill, I just haven't made my software compatible with that aspect of it yet.

  16. Hi Nophead the tip was in response to your photo of the screwthread held in the lathe. It does not get a good grip because the only points of contact are on the tips of the thread.

  17. Ah yes. It is only a light grip for coil winding so not a problem in this case. A good tip for drilling threaded rod though, as is required for the standard heater barrel.

  18. Have you tried putting PTFE washers between the flange and the holding bolts? It should reduce thermal losses in them, I think.

  19. Ooops, looks like someone suggested that elsewhere already - well, anyway, the PTFE rod should be pretty easy to turn down a bit in diameter, drill down the center and slice into a set of nice, heat-resistant washers...