Duet pressure advance experiments

This is a follow up to my post about exploring melt rates and printing at high speeds with a Diamond hot end. https://somei3deas.wordpress.com/2017/06/22/exploration-of-print-speeds-with-a-diamond-hot-end/

During those tests I noticed that the beginning and end of the moves were rough and raised. I also noticed that during a long corner to corner non-print move, the filament was oozing and being deposited in blobs. Both of these issues got worse as the print speed was increased. It seemed to me that pressure was building up between the extruder and hot end which was what was causing both of these issues and “normal” retraction settings were not enough to compensate. It was also apparent that using retraction alone, I would need to set it higher and higher as I pushed the speed up. So I decided to experiment with using the pressure advance setting that is in Duet firmware.

It should be noted that I had tried playing around with this setting some time ago. The wiki states that a value of 0.1 to 0.2 would likely be appropriate for Bowden tube setups. As my Bowden tubes are only 165 mm long, I thought that a low setting would be needed so I tried from  between 0.01 to 0.2, none of which made any unnoticeable difference, but then I didn’t really have a problem with print quality to start with. I thought at the time that maybe there was something about a mixing hot end that negated the effect of using pressure advance. It was only when I started playing around with higher speeds that I noticed an issue.

I started with everything as before – same filament still loaded. Print setting unchanged. Same gcode file (sliced at 100mm/sec). After laying down a few layers at slow speed to get a good foundation, I pushed the speed straight up to 150mm/sec which from my previous tests, was close to the maximum melt rate that I could extrude at (with a single filament), and more or less the worse case speed for showing up the rough ends of the moves and the non-print move blobs.

I started with a pressure advance setting of 0.2. That is to say, while the print was running I used M572 D0 S0.2 then did the same for the other extruders (D1 and D2). Even though they were only contributing 1% each and doing was doing 98% of the work, I thought it best to set them all the same. There did seem to be a slight improvement but not much.

So I went up to 0.3 and there was a marked improvement but the short front to back “Y” moves slowed noticeably. DJDemon on the Duet forum had reported this and DC42 (the writer of the firmware) said it was due to having a high pressure advance value combined with a low extruder jerk setting. So at this point I doubled the jerk setting from 600 to 1200 which resulted in an instant increase in speed. Note that only the short “Y” moves are noticeably affected as pressure advance only applies to the acceleration and deceleration phases of the extruder(s). The long “X” moves were still largely unaffected as the acceleration and deceleration phases are short relative to the constant speed portion.

Further improvements were noticed at 0.4 and 0.5 “S” values but going up to 0.6 made no further difference. That is to say that I couldn’t visually see any difference in print quality between using 0.5 and 0.6 so elected to use the lower value.

As before, I took some video footage and put together a short video which compares with and without pressure advance.

This one is much shorter – less than 8 minutes in total. When you look at what the extruders are doing, it looks and sounds absolutely crazy but prints beautifully.

 

As you can see, the roughness is almost gone and the non-print move blobs are history.

What is interesting is that the same pressure advance setting works for all speeds and both single extruder and three extruder configurations.

I will have to re-visit my retraction settings as it is highly likely that I’ll need to use far less.

As for why I need a much higher value than expected. I have a theory that maybe the Bowden tubes play less of a part than expected. Personally, I don’t buy into the theory that the filament itself can be compressed like a coil spring, but it could buckle somewhat inside the Bowden tube, but a 1.75 mm filament inside a 2mm tube isn’t going to buckle much. What I think happens is that it’s a more a function of the volume inside the hot end. We’ve seen that the Diamond has a large melt chamber (if we include the long 2mm diameter section). Also, it has three chambers which are connected together. So pushing filament into a high volume space, the pressure will build up more slowly (than a smaller volume space) but when we stop pushing the filament, the higher volume will mean that it will take longer for the pressure to normalise than it would with a smaller volume.

It’s just a theory and until we can get a pressure transducer inside a hot end, we’ll never know.

As ever, watch this space…………..