Monday, 23 February 2009

Three times better

I made a plastic mounting plate to allow me to test the new stainless steel extruder in my test rig.



Here it is under test: -



I felt that this design was working well. Now I have the figures to back it up. It is three times better! I.e. with the same weight the extrusion rate is three times faster through the same nozzle and at the same temperature. With 8.27Kg I am now extruding HDPE at 9.43 mm3/s.

This is a dramatic improvement, especially considering it did not work at all until I added the taper to the end of the transition zone. It shows that the design of the entrance to the extruder is critical and at least as important as the exit.

It is really good news because using stainless steel as the insulator really simplifies the extruder and at the same time extends its temperature range and makes it strong and reliable.

By replacing the heater block with one made with a vitreous enamel resistor and a screw-in welding nozzle, I should have a design that can be made with a drill press, a couple of taps and a die.



I haven't tried the welding tip yet, but now I have a means of comparing it against the standard nozzle.

24 comments:

  1. Looks Great so far, Chris. Question - How is the weight on this design compared to the Mk II?

    ReplyDelete
  2. The last one with the aluminium plate was quite heavy but this one with a commercial heatsink is quite light.

    The big weight issue will be the drive. Moving to a stepper will be a lot heavier than a GM3 unless I can get it to work with a tiny stepper and gears.

    ReplyDelete
  3. I did a back-of-the-envelope calculation using your data and come out with about 1700 psi in your extruder barrel. Does that number make sense to you?

    Also, are you seriously proposing that the extruder driver ought to be able to apply over 8 kg of force on the filament? Have I got that right?

    ReplyDelete
  4. I think I calculated ~100 Bar so about the same.

    However, this design is giving three times the rate, so I can cut back the weight.

    The pressure is not as high as we calculated. The previous design was producing 1/3 of the flow rate with the same nozzle, so the nozzle pressure must have been about 1/3 of this one for the same weight. I.e. a lot of the resistance was friction into the extruder, not out of the nozzle. Only a fraction of the force goes to create the pressure in the liquid plastic. The rest is pushing a soft plastic plug through the transition zone.

    Hopefully this design is pretty efficient so most of the weight does form nozzle pressure, so perhaps 30 Bar. I will try ABS with it tonight. I expect that will go through much more easily and PCL and PLA will be somewhere in between.

    We are till talking a few kilograms for HDPE, which explains why we need proper bearings, brass bushings just don't cut it.

    ReplyDelete
  5. as well as being faster, is this design more consistent than the 90-163 second variation with the PEEK version?

    ReplyDelete
  6. Yes with 4.6Kg I got the following: -

    75, 67, 86, 81, 88, 78 giving an average flow rate of 3.81mm^3/s.

    I think the flow of liquid plastic though the nozzle will be pretty consistent but the friction in the transition zone is more random. A sort of stick - slip scenario like plate tectonics.

    ReplyDelete
  7. Thinking about it, this variability shows why closed loop control is essential and even a force feedback system is not going to be good enough to predict volume dispensed.

    ReplyDelete
  8. I think that you may well have something there with that last photo. That is VERY attractive.

    ReplyDelete
  9. I have made a very similar nozzle to yours - see my blog- made out of brass. I found that with the drilled out welding tip it worked better if I inserted it further into the bar as it transfered heat to the welding tip more easily. I have not done the measurements that you did but just on feeling the pressure when pushing HDPE into the nozzle it was much lower with the inseted nozzle.

    Great work Nophead !
    Andy

    ReplyDelete
  10. Hehe, tried to post and then realize I was logged into the wrong account...Pam and Stan now becomes...DEMENTED CHIHUAHUA!

    That's fantastic work, Nop! I'm gonna make myself one of your new designs and see if I can get it working as well as you did. Really hope this works out and we can nail down exactly what configuration of entry allows for the most efficient extrusion.
    Demented

    ReplyDelete
  11. Andy,
    Thanks, I will try that if I find it too cool or perhaps not widen the hole as much. I intend to cover it with PTFE as that helps to keep it warm and stops filament sticking to it. Looks like you are using PEEK for that?

    The SS barrel is good for high temperatures but using PEEK the way that you have looks easier to make and probably doesn't need such a big heatsink as I am using. I might try that way as well.

    Demented,
    Thanks for the encouragement. I knew it was you as I follow your blog.

    ReplyDelete
  12. nophead, try a box Intel heatsink - they're circular, with a short aluminium cylinder in the center and lots of fins extending radially. Something like this: http://tbn3.google.com/images?q=tbn:JzSbWmnfh-NTUM:http://www.egielda.com.pl/images/art/cache/41ddcf7ddd432371567e8e26e23cb06a.jpg

    They come in tall and short varieties depending on the processor (the short one should be enough, it's pretty light even though it seems to have a huge surface area) and should be very easy to come by - the fan included with them is damn loud and inefficient, enough so that most people who buy a boxed Intel processor just get a real cooler and throw the stock one away or sell it for $2 on eBay. It will certainly look a lot better than a normal CPU heatsink and will be a lot easier to drill and assemble.

    ReplyDelete
  13. Yes but it way too big and heavy to be throwing around on a Darwin. The heat that travels up the stainless steel is only a few watts. The little North/South bridge (I forget which) heatsink pictured works fine for HDPE and ABS. It is a bit too hot for PLA though, which has a very low Tg and high Tm. Some insulation round the heater may fix that, or a slightly bigger heatsink.

    ReplyDelete
  14. If you can manage to modify the attachment a little (without sacrificing stiffness), that might help as well -- the heatsink appears to be right underneath a substantial plastic 'ceiling' that might well be trapping some heat rather than letting it escape.

    ReplyDelete
  15. I'm just getting started with the whole RepRap thing, so I read through the history of your machine. It seems to be the current state of the art, a very impressive device.

    I have to say that I find it inspiring, especially since I have an X-Y table like the one you used sitting on a shelf. Maybe additive methods aren't so far away after all.

    Be careful with those aluminum clad resistors! They have a reputation in the power conversion field; Some of my colleagues call them, especially the larger sizes, "hand grenades". They are hermetically sealed, and when they heat up, they expand and release gasses internally. If you run them too far past their ratings, they explode violently. At least, so I've been told.

    Have you considered using fired, glazed porcelain for your heat shield layers instead of plastics? It's strong, readily available (although you will have to borrow some kiln time), cheap, easily shaped, and survives temperature well. And the thermal conductivity, while higher than the plastics, is still relatively low.

    ReplyDelete
  16. Anon,
    Yes it could do with some holes to let the air rise. As I write this I am making a cowling to mount a fan so I can see the effect of more cooling.

    ReplyDelete
  17. Hi CaladanJen,
    Glad to be inspiring, hope to see you make one.

    Yes somebody else warned me that AL clad resistors can explode. I must admit I find it hard to believe due to simply raising the temperature of the gas inside. I could imagine an explosion due to flash over and vaporisation if the resistor breaks down on a high energy circuit.

    I maybe wrong, but these are the smallest ones, so if they do go it will be more like a bullet than a grenade!

    Early on I did play with a ceramic insulator taken from a spark plug. At the time it did seem to conduct rather a lot compared to PTFE, but it was probably less than the stainless steel I am playing with at the moment, so it might work with a heatsink.

    ReplyDelete
  18. Thanks, I hope to get the chance to build one.

    As for the resistors, I have never personally seen one of them explode, but I've heard enough credible stories to make me nervous. Cram enough expanding gases into a sealed container, and bad things can happen. With that said, the people that I've heard this from have plenty of other spectacular stories about things rapidly disassembling when confronted with a large enough stored energy reserve (electrolytic capacitors, steel chassis, a concrete wall, someone's arm), so I may just not be getting the whole story.

    I have to confess that I am not much of a car person, and I'm not sure which ceramic is used for spark plugs. Not all ceramics are created equal; some conduct heat, and even electricity, exceptionally well, and some quite poorly. In this case, the numbers look like this:

    Stainless Steel: 16 W/mK
    Porcelain: 1.5 W/mK
    PTFE: 0.25W/mK

    Not as good an insulator as the plastic, but much better than the stainless steel.

    ReplyDelete
  19. My understanding is that for a fixed volume of gas the pressure is directly proportional to absolute temperature. By raising the temperature from 20C to 300C I would approximately double the pressure of any gas inside. Assuming it is initially at atmospheric pressure, that means the net pressure is only about 1 Bar. I can't imagine a few cc of gas at 1 Bar even rupturing the case, let alone causing a grenade type explosion.

    On the other hand I can imagine if I connected the resistor directly across the mains it would explode. The wire inside could vaporise and the conductive vapour could act like a short across the mains. For the few milliseconds it would take the fuse to blow the current would be hundreds of amps, even on a domestic supply. That sort of energy could produce thousands of degrees causing the contents to expand rapidly in a similar way to a chemical explosion.

    I haven't seen a resistor do it but I have seen an overloaded 400W mains transformer explode. It made a hell of a bang and the guy testing it ended up with a black face and a surprised expression just like cartoon characters do!

    When comparing thermal conductivities you have to keep the strength in mind. Stainless is very strong so I can get away with a wall thickness that is only a fraction of a millimeter. That means the cross sectional area is much smaller so the actual the conduction is comparable to PTFE.

    Ceramics are one of the things RepRap aims to be able to print in the future. The exciting thing is that it is possible to print a tube that starts off as metal at one end and transitions to ceramic at the other with composite blends in between. I expect that would have interesting thermal properties for this sort of application.

    ReplyDelete
  20. Hello,

    I'm one of the invited speakers at the Oekonux 4 conference in Manchester (March 27-29, 2009), and I wanted to see if I could stop in and visit you. Our work is http://openfarmtech.org. We'll be building a RepRap for printing metal casting molds. Please amail me at opensourceecology at gmail dot com. We're doing this within the context of building the world's first, replicable, open source, global village.

    Sincerely,
    Marcin Jakubowski
    Open Source Ecology
    http//openfarmtech.org/weblog/

    ReplyDelete
  21. Instead of drilling down the length of the welding tip, would it make scene to just cut it shorter, almost down to a nub?

    If it were shorter you might be able to forgo the insulator that is over the tip all together, and might be faster for those of us who do not have a drill press

    ReplyDelete
  22. I drill to within about 1mm (or less) of the end so it could not really be cut that short.

    Somebody reported that they could drill the welding tip without a drill press as the original hole acts as a pilot hole for the bigger drill.

    ReplyDelete