Linear advance, what to look for



  • Ive tried running a few xyz calibrqtion cubes with long distance travels and retracts in between to check if LA could remove the uneven extruded layers(good retract with no blobs=bad layer quality, vice versa)

    But ive tried all values from 0.1-0.8 now, with 0.05 increments in between each adjustment...

    I see no difference qt all in quality, other than my layers do look better now..

    Which value am i supposed to go with...
    The extruder behaves differently at every value, but i get perfect extrusion on corners no matter what value i set it at..

    But it seems to generally underextrude the higher value i set it at.. (kinda like flowrate decrease)..

    I also believe that i need to adjust LA depending on temperature, actual flowrate, filament type and color or maybe even a new spool?

    I cant find a heck of a lot when googling, that is relevant on newer firmwares...



  • My bowden tube is 750mm from the bondtech gears, down to the nozzle tip..



  • Ofc i did mean pressure advance... 🙈


  • administrators

    It would help to see some photos of your calibration cubes, and for you to point out exactly what it is that you want to improve.



  • @johnjohn1990 Linear advance is a feature that is designed to compensate for extruder slippage. So if you don't have any extruder slippage, then you likely won't see any difference or it might even make matters worse. Some people calibrate linear advance by extruding 100mm of filament with the hot end connected and heated, then measure the actual amount of filament that was extruded. Personally, I think this is not a valid approach because unless you have a monstrously large printer with a bed length of around 5 metres you are never going to extrude 100mm of filament in a single print move. Having said that, I did once try it and found no measurable difference between the demand and actual. One possible reason for this is that I was using a 0.5mm nozzle and the other two people who were testing it at that time, and who did see a difference, were using 0.4mm nozzles so they likely had high back pressure in the nozzle which was causing the extruder teeth to slip. Which leads me to another point in that if you do find you need to use linear advance to compensate for extruder slippage, then you will need to revisit the pressure advance settings because increasing the extruder rate mid move, will increase nozzle pressure towards the end of that move.

    Oh cr*p. Just finished typing when I saw that you meant pressure advance, not linear advance. Ignore all of the above.



  • @deckingman as i write, i cant find any golden midpoint for my retracts and layer quality...
    And i meant pressure advance, not linear advance, made a bugger there....

    I am using a .4 nozzle and everything is setup correctly...

    But i dont see any visual difference to my print no matter what i set it at...
    except retraction is absolutely perfect now(which is the main course i want to try this again)

    But the layer quality is still crap, sometimes... its not consistent as i would expect..

    Also i get an extremely minor under extrusion on start points that happen on outside 90 degree corners.. nowhere else...

    I dont know if my extruder is slipping... its a bondtech, so it shouldnt really be able to...

    I have had a few clogs in my hotend at when it happens, it shoots plastic out everywhere and decouples the ptfe and metal coupler on my um2 hotend lol...

    This only happens with pressure advance on.. as soon as i turn it off it doesnt say it...

    But i cant see inside the bondtech and watch if it slips...
    I can just hear it saying a ton of funky sounds that varies depending on the value 🙂

    I basically just want to know if it matters anything what my flowrate etc is.. 90 or 100 or 110%...

    Last time i played around with it i simply couldnt get it to do ANYTHING good at all... it messed up the print completely... but that printer also ran at 1800 isc and 3000 acc, so it was crazy fast and super sharp corners already... so i quickly abandoned it...
    But this printer is more like a stock um2, but just 1.75 rather than 3mm.. 🙂



  • @dc42 yes i will see to that in a few minutes, gonna walk the dogs first though 😁

    Hope i can do it on mobile version.. 🙂



  • Ehm.. how do i post pictures? I dont have an url link for it... if it is the directory on my phones storage, then i dont know that either.. 😂



  • @deckingman Did you mean non-linear extrusion? Linear advance is Marlin's version of pressure advance. I'm a novice with the duet and want to make sure I'm not missing something.



  • there is an upload file button during the reply. on android pictures are in the dcim folder



  • Im having huge layer inconsistensies... im used to the smooth and even layer lines as on the backside..
    But its very uneven in the layers.. this happens on all prints, with 98% of my pla... this seems to become more prominent the higher i go in temperature..

    I have been messing with this retraction stuff for days, but when k got dialed kn retraction, layers started to go bad... hoped this PA could fix it...

    Batch 1..
    Cube 1
    0_1555589140408_20190418_140143.jpg 0_1555589148987_20190418_140130.jpg 0_1555589160172_20190418_140104.jpg ! 0_1555589477241_20190418_140054.jpg

    Cube 2
    0_1555589524031_20190418_140043.jpg 0_1555589565675_20190418_140036.jpg 0_1555589604166_20190418_140025.jpg 0_1555589648289_20190418_140015.jpg

    This is batch 2...

    Cube 1

    0_1555588000045_20190418_133314.jpg
    0_1555588011363_20190418_133308.jpg
    0_1555588019757_20190418_133258.jpg
    0_1555588030446_20190418_133252.jpg

    Cube 2
    0_1555589689256_20190418_133245.jpg 0_1555589728518_20190418_133223.jpg 0_1555589773339_20190418_133209.jpg 0_1555589785743_20190418_133202.jpg

    Batch 1 is ranging from 0.1 to 0.3..
    Batch 2 is made with higher values, ranging from 0.6 to 0.8, so i expect that to be worse....
    but the difference in both of them are crazy.. same gcode and everything just different values..
    these are printed diagonally on the bed, with about 150mm spacing between, just to see how retraction performs on long travel retracts.. so dont mind the small blobs from that... can be removed by poking to them 🙂

    And i dont know what to adjust, i wont adjust settings without any thought, cause suddenly everything goes haywire and i have to start all over from scratch and waste several kg of filament for everything 😕



  • @jamesm said in Linear advance, what to look for:

    @deckingman Did you mean non-linear extrusion? Linear advance is Marlin's version of pressure advance. I'm a novice with the duet and want to make sure I'm not missing something.

    Yes I did - I wasn't aware that Marline referred to Pressure Advance as Linear Advance so I assumed you meant non-linear extrusion which was what all my waffle was aimed at -
    ignore it all.

    To post picture, look at the buttons across the top when you compose a post - to upload a picture is the middle button - looks a bit like a boulder falling down a mountain.

    Ref pressure advance. What happens in a hot end is this. As you push filament into a hot end, pressure can build up, mostly because you are trying to push the molten filament through a small orifice, and because it takes time for the solid filament to melt. Obviously this doesn't happen if you print really slowly but the faster you print and the longer an extrusion move, then the more pressure can build up. When you do a print move, the carriage responds almost instantly to the changes in acceleration at the start and end of the move but the filament does not. There is a lag at the start of the move because the filament is viscous and acts like a sponge, hence an initial pressure build up without much change in the amount of filament coming out of the nozzle. So the carriage accelerates up to speed but the rate of change of flow rate for the extruded filament is much slower resulting in under extrusion at the start of the move. At the end of the move, the opposite happens. That is to say, the carriage decelerates but because of the latent pressure that has built up inside the hot end, the filament is still being forced out at a relatively high rate despite the fact that the extruder is slowing down. It therefore follows that this leads to over extrusion at the end of a move.

    It therefore also follows that the longer the move, and the faster a move the more pressure will build up. So what works well for me is to print a large cuboid with 100% infill and observe the start and end of each line to look for under or over extrusion. Then I change pressure advanec "on the fly" until I can detect no difference in the extrusion between the start, end and middle of the bead that is being laid down. Once this value has been obtained, I find that it works well for all speeds. BUT, anything that can change the viscosity of the filament will affect the pressure compensation requirement. So that's the filament itself, as well as the print temperature. I haven't found any need to change the PA value for different reels of filament - one value seems to work well for all PLA although a different value is needed for PETG.

    There is also a strong interaction between instantaneous speed change setting (jerk) and pressure advance. Essentially, more pressure advance compensation is needed for lower "jerk" settings and vice versa". With a high jerk setting, the carriage velocity starts and ends at a higher value so the acceleration and deceleration phases are shorter.

    HTH

    Edit. I was typing all the above waffle before I saw your pictures. PA isn't going to fix that. Disable it for now until you get everything else dialled. I'd say in general that because of the gas between layers and lines, you have some under extrusion issues.



  • @deckingman
    Yeah i figured it out, you couæd slide the topbar to the side... this design just looked so oldschool that it didnt cross my mind 😂

    Well looks like it should be so much of a hassle then...

    See my pics, specially with the top and bottom, since i cant get the perfectly evenly look you mention, which i would LOVE to have..

    But my main focus were the layer lines though...
    But if the top is supposed to be better, i have absolutely no idea what is going on.. since the walls go completely bad above the 0.3 margin...



  • @deckingman

    Under extrusion happens due to the pa... if i turn if off it both under and overextrudes.... tried 3 different all new hotends, rqnging temps from 190 to 260c with pla... yes 260c!!!! And it can print with that, but doesnt fix this...

    I saw no difference at all from 90 or 100 percent flowrate, so i do t need to get my bondtech dialed in... it has always worked perfectly...

    I have always had perfect looking layer lines on this printer... but i cant get it now... at all.. in any way...

    And it happend along the way lf my dialing in retraction on new, with the new heaterblocks... wasted over 2kg of filament of freaking bemchies man... i hate this pile of shet



  • I read everywhere that pa shoukd make blobs at retracts go away and make nice even layers...

    Which is exactly what im doing this for...



  • If i disable PA, i have to battle the same issues still, including retraction too... definately pa gives me perfect retracts



  • I hqve now switched over to some of my more trusted pla...

    Does much better now... looks almost perfect as what i can see..

    But the extruder is "jerking" like mad... is that a good or bad thing?
    Pa is currebtly at 0.15



  • @johnjohn1990 Lots of posts, lots of questions, lots of everything..........

    I think you should take a step back and try not to change too many things. As I said before, pressure advance isn't the answer. You need to get everything else dialed in first. Print nice and slow. What speed are you using? What layer height? If you print slow (say 40mm/sec), then you are unlikely to get a pressure build up so you are unlikely to need to compensate for it. Likewise retraction. It's only applied at the end of moves to prevent blobs forming due to the filament oozing during non-print moves. It won't have any effect on the gaps you are seeing.

    Have you done all the basic stuff? Have you got your steps per calibrated - especially the extruder. Have got the correct values for your thermistor? Have you tuned the heater?



  • @johnjohn1990 I can't keep up with all your posts. I don't know what question I'm trying to answer. Disable PA for now - it;s most likely masking something more fundamental which is what you need to fix first.



  • What do you want to know from.before i turned pa on?


 

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