Diamond hot end potential print speeds and pressure advance testing
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If you were only using retraction on layer change, then without pressure advance you are likely to get a blob at the start of each travel move tat follows a printing move anyway. Is that where the blobs are?
I hadn't forgotten that you were using a Diamond, but I understand it can use the E3DV6 or E3D Lite6 heatsink and other parts. My concern was that if you were using the V6 heatsinks then you need to be careful not to over-retract. The Lite 6 with its PTFE liner is much more tolerant of retraction and probably a better choice for printing PLA.
Hi David,
Without pressure advance but with a lot of (firmware) retraction (about 4mm with 165mm long Bowden tubes), the blobs started to appear at above 70mm/sec or so print speed. It was as if the filament was oozing and got deposited in blobs along the length of the long (300mm plus) non-print move. In fact, that's exactly how I visually observed it happening. As the print speed was increased, the blobs got worse (more pressure build up before the long non-print move). Enabling pressure advance has completely eliminated them. i.e there is no oozing during the long non-print move. Trying to control that by only using retraction would mean having to have more and more retraction as the print speed is increased.
Thanks for the heads up ref the heatsinks. RepRap.me recommend using the lite version rather than the V6 so that's what I've always used. It could be I end with zero or near zero retraction - don't care as long it prints well.
The video is being rendered as I write this. I'll post a link as soon as I get it uploaded to YouTube. I think you'll find it interesting if not amazing to watch. Yes, the extruders run backwards at high speeds but just give a bit of jerk at "normal speeds". At high speeds it looks (and sounds) crazy, but prints beautifully.
Ian
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…..............................would still like to know what the baseline is for just the hotend and extruder - mark a point on your filament tight against the frame before the extruder, extrude 50mm into air, and find the maximum speed where that mark still moves 50mm.
Have you tried your results with 'Autospeed' in Slic3r? (Print settings -> Speed -> Autospeed (advaned))
Hi Rob,
Extruding in air is on my list (everyone want me to try something).
Haven't tried the slic3r autospeed feature. It's experimental which has always put me off. I prefer to set my own speeds for perimeters (large and small), infill, solid layers etc etc. but I might give it a go one day.
Ian
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Blog is done - link here https://somei3deas.wordpress.com/2017/06/25/duet-pressure-advance-experiments/
If you don't want to read the words, the (crazy) video is here https://www.youtube.com/watch?v=lnYYNfVoxmQ&t=304s
This one is much shorter - less than 8 minutes.
Ian
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Ian, I finally found time to watch your videos. I'm very impressed by the printing speeds you have been able to achieve! I'm glad that pressure advance helped you. Your video is an excellent demonstration of what pressure advance does and how the extruder movement appears.
Is the carriage that carries the Titans driven, or towed by the main carriage?
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Hi David,
Thanks for the feedback.
The extruder carriage is just dragged around by the main carriage. If you look closely at the video you'll see one of the 7/02 guide wires had snapped - need to replace it with 16/02 or some such. Not surprising given that I've been chucking 1.6kg around at up to 300mm/sec. (Y is worse at over 3kg). The wires are to keep the Bowden tubes in compression during large or fast moves. There is a bit of an explanation of it all here https://somei3deas.wordpress.com/my-corexy-printer-build/ and a video here https://www.youtube.com/watch?v=5RptHCX7z0o&t=184s
I initially thought about putting motors, belts and pulleys on the extruder carriage and mirroring the main XY motors but dismissed it due to complexity. It's also complicated by the fact that the extruder Y carriage and Y axis are wider than the main carriage.
Effectively there is some decoupling between the axes so when the main carriage is doing short moves, the extruder carriage remains more or less stationary (see video). The downside to that is that when doing big fast moves, the extruder carriage lags behind the main carriage slightly so gets jerked at the start, then at the end of the move, the inertia tries to carriy it on while the main carriage is decelerating, resulting in another jerk at the end. However, the extruder carriage is light compared to the main carriage and even at those silly speeds, I didn't notice anything particularly horrible happening. The wires prevent the Bowden tubes from pulling the hot end mount up on it's sliding dowels.
Time will tell. If I end up having to drive the extruder carriage as well as the main carriage, I'd be inclined to use a shaft with pulleys top and bottom connected to each set of belts and a central pulley connected to the motor, probably with 1:2 gearing (1 on the motor, 2 on the shaft).
It continues to evolve….............
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I have given this some thought, and come to the conclusion that the Diamond needs 3 times as much pressure advance as a similar single-filament hot end would. Here's why.
Let's suppose that you are extruding with only 1 filament. When you add extra steps to pressurise the single extruder, the extra pressure in the mixing chamber will make filament flow back through the two inactive heat breaks, compressing the filaments in those Bowden tubes until the pressure is equialised. Ideally we should pressurise both of them too. But since we don't, we will need 3 times the pressure advance movement on the main extruder drive to allow for the back flow and get the same hot end pressure.
If you extrude with all 3 filaments equally, this is a better situation. However, because each extruder is only extruding 1/3 of the total filament, the amount of pressure advance applied to each extruder is only 1/3 of what is required. So again you need 3x the pressure advance.
Ideally, we should calculate the pressure advance based on the total speed of extrusion of all 3 filaments, and apply that amount of pressure advance to all extruder drives regardless of the mix. I may implement this in a future firmware release.
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Hi David
Good thought but I have to disagree. What you probably missed is that when using a single filament, I actually set the mixing ratio to 0.98:0.01:0:01. That is to say, I always keep the other two filaments moving, albeit slowly. So I find it hard to imagine that the pressure on one input is imparting a significant back pressure on the other two inputs, given that they are being (slightly) driven as well.
My personal gut feel is that it's simply a function of the size of the melt zone (which is big on a Diamond). If you have a large volume containing a fluid at a given pressure, then poke a 0.4mm hole in it, it will take longer for the pressure to normalise than a if you had a container of much smaller dimensions. Similarly, if you push fluid into a large volume container at a given pressure, it will take more time to pressurise that container than it would a smaller container. I'm not explaining it well but it's just how hydraulics works.
Or another analogy but in reverse. When a fluid is heated it expands. If the system containing the fluid isn't vented, then an expansion vessel is needed (a kind of diaphragm and spring arrangement). Typically on a domestic heating system, the expansion vessel needs to be about 10% of the total volume. So the bigger the system, the bigger the expansion vessel needs to be otherwise pressure will build up to an unacceptable level.
I personally find it hard to believe that it's possible to compress filament when it's in a solid state. It might buckle slightly inside the tube but that's about it.
What we need is someone to test pressure advance with an E3D volcano which has similar internal dimensions to one input of a Diamond. I'm willing to bet that would need similar pressure advance values to what I ended up with.
Please don't mess with the algorithm. The same pressure advance value works fine for both single filament and all three filaments, at high print speeds and at low print speeds.
Ian
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If you have an extrusion multiplier of .01, there could be many times when those extruders never produce a single step. Look at a typical gcode file you print, see the distance the E axis is driven, and calculate how many steps that is at your e steps/mm. If it is less than 100, your other filaments are likely not being driven often.
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Ian, it is certainly possible to compress filament in a Bowden tube. With filament loaded but the hot end cold, turn the extruder motor current off and turn the gear wheel by hand. You will find that from the point where it starts resisting, you can compress it a lot.
There is a lot of spare space around 1.75mm filament in a 2mm ID tube, The filament has a cross sectional area of 2.41mm^2 while the tube has an internal area of 3.14mm^2. That's 30% more space than the filament occupies. The filament can take up some of this space by assuming a helical shape. The tighter the pitch of the helix, the more filament you can fit in a given length of tube, and the more the filament will resist this. If you use 1.8mm ID tubing instead (which might work for your short Bowden tubes), you may find that you need less pressure advance.
When you are using just one extruder, whether the other 2 extruders are stationary or they move a little is irrelevant. The pressure in the melt chamber will be varying a lot, and the filament in the unused tubes will compress accordingly. That pressure needs to be either resisted by applying full pressure advance to the other 2 extruders, or compensated for by using 3 times the pressure advance on the main extruder.
I don't think the volume of the melt zone is relevant. Fluids are generally regarded as incompressible (the exception would be filament with steam bubbles in it), and pressure waves are transmitted in fluids very quickly.
Your point about the expansion vessel might be relevant if the temperature in the melt zone was varying a lot; but it should be more or less constant at the hot end temperature you set. There will be a small variation due to the varying melt rate, but I think the amount of expansion will be small. Whereas it's clear from your video that pressure advance is applying several mm of additional movement to maintain pressure when you are extruding quickly. That's several cubic mm of extra filament going into the Bowden tube and coming back out again. I can't believe that it's all disappearing into the melt chamber and not coming out somewhere else.
Trying a Volcano would be interesting. My expectation is that at a given extrusion temperature and filament diameter, and using the same Bowden tube length and ID, a larger nozzle will need less pressure advance, because the back pressure per unit extruder drive speed is less. So a Volcano with a 0.5mm nozzle should I think need 1/3 the pressure advance of your Diamond, and if you fit a larger nozzle then it should need less. All this assumes that the same nozzle temperature (not indicated temperature) for both hot ends.
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@bot:
If you have an extrusion multiplier of .01, there could be many times when those extruders never produce a single step. Look at a typical gcode file you print, see the distance the E axis is driven, and calculate how many steps that is at your e steps/mm. If it is less than 100, your other filaments are likely not being driven often.
I was talking mixing ratios, not extrusion multiplier bit I think I see where you are coming from. I guess for very small moves, the 1 % won't actually be acted on as it would be less that 1 micro step, which would potentially lead to maybe a maximum of 2% under extrusion on very small moves. In practical terms, I can't say that this is visible in the final printed object.
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David,
Assuming you are right, and most of the pressure build up is due to the filament buckling and taking up space in the Bowden tubes, then I'd have thought that as my Bowden tubes are only 160mm long then I'd need a very low number for pressure advance. You say that when it's melted the filament is a fluid and can't be compressed. So the compression happens in the unused tubes (but only because the filament is solid and assuming a helical shape). However, the length of these unused tubes is only 21mm. As there are two of them, that would equate to an increase in Bowden tube length of 42mm. If we add that to the 160mm, we still only have an effective Bowden tube length of 202 mm. Even if we take all 3 Bowden tubes and add 21mm to all three, we still only get a total length of 543mm which is by no means unusual. I'm having a job believing that this is all down to the filament acting like some sort of coil spring (or 3)
Ian
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I take your point that with short Bowden tubes, you should need less pressure advance than with long ones.
Are you saying that you have one Bowden tube that is 160mm long, and the other two are only 21mm long? I thought they were all about the same length?
You can work out what additional movement pressure advance is causing at the extruder drives. To do this, first work out the extrusion speed. For example, for a move that prints parallel to the X axis, take the E parameter (if using relative extrusion) and divide by the amount of X movement, then multiply by the feed rate for that move and by your speed factor. Convert that extrusion speed from mm/min to mm/sec. Then multiply by the pressure advance. This will give the additional extruder movement in mm.
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@bot:
If you have an extrusion multiplier of .01, there could be many times when those extruders never produce a single step. Look at a typical gcode file you print, see the distance the E axis is driven, and calculate how many steps that is at your e steps/mm. If it is less than 100, your other filaments are likely not being driven often.
I was talking mixing ratios, not extrusion multiplier bit I think I see where you are coming from. I guess for very small moves, the 1 % won't actually be acted on as it would be less that 1 micro step, which would potentially lead to maybe a maximum of 2% under extrusion on very small moves. In practical terms, I can't say that this is visible in the final printed object.
Oh, no doubt it wouldn't have a visible effect. I was just commenting because I guessed the missed microsteps may prevent any pressure-equalization that driving the other extruders may have. It seems irrelevant now.
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There's a simple test to see filament compression when cold. Extrude slowly until you hear the motor is about to missatep (on duet this might be harder since is much more silent). Then mark the filament on the feeder and then release the lock mechanism of the feeder. Then you can see how much mm compression you had while cold inside the bowden.
I did this test long ago before moving to directdrive and indeed the filament compress and flex like a spring.
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There's a simple test to see filament compression when cold. Extrude slowly until you hear the motor is about to missatep (on duet this might be harder since is much more silent). Then mark the filament on the feeder and then release the lock mechanism of the feeder. Then you can see how much mm compression you had while cold inside the bowden.
I did this test long ago before moving to directdrive and indeed the filament compress and flex like a spring.
Did you mean "Extrude slowly and gradually increase the extrusion rate until you hear the motor is about to miss a step"?
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David,
No the Bowden tubes themselves are all the same length - i.e 160 mm. The extra 21mm is the 2mm diameter hole inside the Diamond nozzle from the heat break down. My point is that, if pressure build up is purely due to the 1.75 mm diameter filament being compressed by being pushed into a helix inside inside a 2mm tube, then the only thing that would affect the pressure build up would be the length of the tube(s). So adding 21mm to length of the Bowden tube makes it 181 mm and if we assume that, even with a single filament, pressure is building up in all three tubes, then the combined length is (3x181) 543mm. The Wiki states that with Bowden tube set ups, a value of 0.1 to 0.2 is usually about right. I'd have thought that would relate to typical Bowden lengths of say 300 to 600mm? In my case, the combines length of all three tubes fits within that range yet I have found that a pressure advance value of 0.5 is required. Therefore, my conclusion is that compression of the filament within the tube is not the only factor that affects pressure build up and possibly has less of an impact than the theory might suggest.
It's not all about compression. There can be a build up of pressure without any compression taking place. If we ignore the fact that the hot end has a hole in the end for now, as we force filament in at say 1N/m^2, it will take some time for the pressure inside the hot end to build up, depending on the volume of the chamber into which we are forcing it. Because pressure is force per unit area, the bigger the surface area, the more force is required to attain a given pressure. So a large volume has more force stored up than a small volume if they are both at the same pressure. So when we force filament into a large chamber, it takes much more to reach the same pressure then if the chamber was smaller. Hence the need for more compensation (in terms of length of filament). The hole in the end of the nozzle simply slows down the rate of pressure build up.
Added to the above, we have the fact that we are heating the cold filament. As the volume of the chamber is fixed, the filament cannot expand so the pressure builds up. The higher the melt rate, the higher the pressure build up.
Personally, I think that these factors have more of an impact on pressure build up than current theory would have me believe. I think it's a better explanation of why the Diamond hot end needs more compensation than "normal". Compression of the filament within the Bowden tubes cannot be the overriding factor because the combined length of all three, isn't enough to explain the fact that the Diamond hot end need 2 to 3 times more pressure compensation than is deemed to be normal (for that length of Bowden tubing).
I guess we could spend time quantifying the effects of all the various parameters but I'm too old. It would be a good thesis for some youngster to explore.
I'm happy to just accept that the Diamond, on my machine, with my extruders, my Bowden tube lengths, my heater, etc etc needs a 0.5 pressure compensation value.
Ian
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There's a simple test to see filament compression when cold. Extrude slowly until you hear the motor is about to missatep (on duet this might be harder since is much more silent). Then mark the filament on the feeder and then release the lock mechanism of the feeder. Then you can see how much mm compression you had while cold inside the bowden.
I did this test long ago before moving to directdrive and indeed the filament compress and flex like a spring.
Did you mean "Extrude slowly and gradually increase the extrusion rate until you hear the motor is about to miss a step"?
No, he said do it cold. So essentially just push filament into a blocked Bowden and see how much you can force in. Interesting concept - might give it a try but with my Titans, I'd have to choose the point where it starts to grind away the filament.
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David,
…The Wiki states that with Bowden tube set ups, a value of 0.1 to 0.2 is usually about right. I'd have thought that would relate to typical Bowden lengths of say 300 to 600mm? In my case, the combines length of all three tubes fits within that range yet I have found that a pressure advance value of 0.5 is required...I'm not saying there isn't something about the internals of the Diamond that needs more pressure advance, but I am saying that a 3-in mixing extruder will need 3 times as much pressure advance as a single-filament extruder, other things being equal.
The figure of 0.1-0.2 sec in the wiki was based on feedback from other users. Maybe I should do some more tests on my delta with its 600mm Bowden tube, including high speed tests like yours. I was wary of trying higher pressure advance than 0.2 because my E3DV6 is prone to jamming. If I reduce the retraction distance enough then it may be OK.
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Hi David,
Please don't take my comments the wrong way - I wasn't being critical of you or the Wiki. I think we are both agreed that the Diamond probably needs three times more pressure advance. Where I amicably agree to disagree is the mechanism behind that assertion. Maybe for now, it might be prudent to change the text of the Wiki to something like "Typical values are 0.1 to 0.2 but can be in the range 0.01 to 0.5 depending on characteristics of the extruder, hot end, and filament path and can also be affected by extruder acceleration and jerk settings", or sum such loosely worded statement.
When I get chance, I'll do some more testing to see if the amount of pressure advance can be reduced at lower (normal) printing speeds. I did a quick test to make sure that nothing was horribly wrong using 0.5 pressure advance at low speeds but didn't do any extensive testing. It is possible that I may need less pressure advance at lower speeds although my initial findings didn't indicate that would be the case.
The effect of pressure advance is not an easy thing to quantify as it relies on somewhat subjective examination of the print quality. I guess also, it will be affected by the extruder itself as well as the acceleration and jerk settings so could vary wildly from user to user.
Ian
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Since the filament doesn't fit exactly in the bowden tube, there is not only the filament compressing, but the filament MOVING inside the tube. This is probably much greater than the filament compressing, but once the slack is taken up, only the filament compression adds further. This could possibly explain why 3 bowden tubes needs more pressure advance than 1 of the same length: you;re getting 3 times the "backlash" as well as the combined "compression."
