Thursday, 9 April 2009

More weight lifting

I have had a couple of extruder jams recently when doing the first layer infill. I do that at 195°C to avoid it sticking to the raft. It seemed that ABS was much harder to extrude at that temperature, so I did a range of tests to find out how flow rate and force vary with temperature.

I used my lead kebab test rig with this extruder, which has a 0.5mm nozzle: -



Most measurements are averaged over 8 tests of extruding 40mm of filament, so it took a long time to get these results.

These are the basic measurements: -

Flow rate for a given force seems to increase fairly linearly with temperature. The single points are the weight that I found gave about my normal extrusion rate of π mm3/s. Below are the same points plotted against weight: -



So force does increase rapidly below 220°C.
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7 comments:

  1. Anonymous10 April 2009 at 09:10

    What is the conclusion?
    The "4mm splined shaft" and the "13mm timing pulley" never be good for ABS extrusion? (judging from your comparison table)

    Why the jamming happened? Your extruder motor didnt have enough torque?


    Khiraly

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  2. Unknown10 April 2009 at 09:32

    Nice work.

    Excellent results. My extruder also works best at about 220C (good flow, no brown colour), but that's based on feelings and hand force, not hard data.

    You're really helping everyone by providing the figures!

    Thanks!

    Renoir

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  3. nophead10 April 2009 at 10:19

    Khiraly,
    No the motor did not stop, the filament just slipped and got ground away. The first time it happened I tightened the springs. The second time I removed the springs and just used a fixed distance between the guide and the thread. I have run that extruder a lot so I don't really know why it decided to jam now, but the graph shows it will happen at low temperatures if it is going to happen.

    I don't think the thread is as sharp on the extruder I am using compared to the one I made the measurements with. The one I use is a zinc steel thread rod with a nut soldered on. I did sharpen the thread. The one I tested was stainless steel that I tapped myself and is very sharp.

    My conclusion is that I need as much drive as I can get. I don't think other people use such low temperatures to control the raft bonding. Maybe I can increase the temperature and increase the speed to compensate. The other way of reducing bonding is to raise the height of the first layer and / or use a more open weave on the raft. I prefer not to do that because it reduces the quality of the bottom surface.

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  4. nophead10 April 2009 at 10:30

    Renoir,
    I find that ABS does not start to change colour until at least 260C, provided the air does not get to it. However, it does depend on where you measure the temperature. All my recent figures are measured at the heater barrel rather than the nozzle, which can be 20C cooler.

    It will go completely black at lower temperatures if you leave it exposed to air long enough. E.g. any plastic that gets stuck to the outside of the nozzle get reduced to a hard black substance, presumably mainly carbon and the more volatile components boil off. That does not seem to happen if you leave it stewing inside the heater barrel for a long time but I expect it would if air could get in or the volatiles get out.

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  5. nophead10 April 2009 at 13:17

    Khiraly,
    Actually the reason it stopped working is that the bearings have worn down too much. It just stopped at the same stage again.

    I will replace it with my second extruder drive that I was testing for pull. Yet again I find it impossible to have two working extruders at the same time! I will use it to make a new one with ball bearings.

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  6. Anonymous10 April 2009 at 13:41

    >Actually the reason it stopped working is that the bearings have worn down too much.

    The same story again:-(

    You blogged about it a year ago if I remember correctly. The bronze bearings does not hold for a long time...

    Khiraly

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  7. nophead10 April 2009 at 13:54

    Actually it was brass ones that didn't last long. These are stainless steel and I have been using them from day one. The zinc steel shaft wears down and that is half the problem. I have gone through two or three shafts but only ones set of steel bearings.

    The replacement has phosphor bronze bearings against stainless steel. I have increased the diameter from 3mm to 3.5mm. I don't expect them to last long. When I made them I didn't realise that proper bronze bearings are sintered so they are porous and contain lubricant.

    I have some deep groove ball bearings which should last better. The main problem with the half bearings is that you can't keep them lubricated because the plastic dust soaks it up.

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