Linear advance, what to look for
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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...
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If i disable PA, i have to battle the same issues still, including retraction too... definately pa gives me perfect retracts
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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?
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@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.
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What do you want to know from.before i turned pa on?
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I had blobs at both start and stop, no matter what i did... 15 or 45mm/s and or 0mm or 7mm retract didnt matter...
If it didnt blob, it just stringed instead..But it generally keeps overextruding the first 20mm or so of a startpoint, after a retract, which makes awfull looking layers..
If i went too fast or too high in retraction, it underextrudes instead of overextrudes... there is no golden midpoint!
Though the best one i have found is 40mm/s retract and 20mm/s prime speed.. im using cura.. been using the same profile for over a year, on all printers ive ever had in my hands since i made this profile...
Always worked perfect..
This feels exactly like if it were an e3d all metal hotend that used too much and too fast retracts.. but its not... its an olsson block.. -
@johnjohn1990 I suspect that in the time it takes me to type this, you'll have posted twice more.
From what I can gather, you used to be able to print OK, but now you are having problems - yes? If that is the case, then what changed?
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@johnjohn1990 In your first photo showing the Z side of the cube I see a different problem- the infill lines are laying down in pairs with gaps between them. They should all be evenly spaced/touching, suggesting that there's some backlash in the XY motion.
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@deckingman said in Linear advance, what to look for:
@johnjohn1990 I suspect that in the time it takes me to type this, you'll have posted twice more.
From what I can gather, you used to be able to print OK, but now you are having problems - yes? If that is the case, then what changed?
Yes... i had a clog, and had to clean out my heaterblock and nozzle.. when i reassembled it again, it just wouldnt print nicely..
It then clogged again after about 20 prints...
Then i changed out all of the hotend parts twice, with 15-20 prints in between, all with the same results..If retracts looked good, layers were inconsistent, just as it is with the bad PA settings... and if i got layers to look good, retracts either blobbed or stringed, or had to be printed at temps i can print abs at which made the nozzles clog over and over... abs prints perfect btw, absolutely perfect..
Yes i did change out parts.. but the fact that it still were there, with the same parts it previously has been printing with, and have had issues ever since, is so strange to me...
PA fixes my retracts... right now it actually look extremely good... i am on the edge of saying that it has never delivered such a quality as this... but the print is only half done, so i wont judge that untill im sure...
Sorry if i tend to just throw a lot of replys.... its not my meaning to confuse...
Ive been chasing my tail for 3 days now, havent hardly slept cause ive used all nights on trying to solve this...
Then finally PA makes my retractions work perfect.. and seemingly also the layers, i just have to only use my trusted pla, for some reason.........
But damn.. it looks awesome so far! Maybe i should try disabling it and just see what happens, just to make myself feel even more dumb lol.. -
@mrehorstdmd said in Linear advance, what to look for:
@johnjohn1990 In your first photo showing the Z side of the cube I see a different problem- the infill lines are laying down in pairs with gaps between them. They should all be evenly spaced/touching, suggesting that there's some backlash in the XY motion.
Its an ultimaker, so yes there will always be a bit of backlash... but maybe it has gone out of allignment when i had to clean the hotend... i will check that when this print is done
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Just to be informative here...
Grey one is when it started messin up...
Green one is currently printing... looks good to me, buuut... time will tell
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It turned out beautifully
wow..If i were supposed to use this..
Should i write down the S value and temperature and speed of this, and then see what other types of pla and filament gives out, and do the same for that ?
Anything else to take into consideration?Sorry for all those questions...
I feel so dumb... but i just want to be on the safe side of things.. -
So now it prints better st 240mm/s rather than 40-60...
Throw me in the ocean and call be bertha....
What the..
Is it normal for PA to make one print 4-5 times faster?
My mate says it looks and sound exactly as if it were hes old prusa mk2
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Top and bottom layers are also extremely consistent now all over, even on big and small areas
its incredible to watch.. almost like getting your first printer again 
Its a shame i cant upload video.. i can believe it prints good and at 240mm/s, i even upped jerk from 10 to 20 and acc from 1500 to 2000 on x and y.. wow..
Do you have time for just 1 question from me? Before i say this i solved..
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@johnjohn1990 said in Linear advance, what to look for:
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...
I think you misunderstand what pressure advance is for. If you are getting blobs at retractions, the first thing you should do is increase retraction. I find that with no PA, the retraction needed is a little less than 1% of the Bowden tube length plus 1 to 2mm. So with a 750mm Bowden tube, I think you will need 8 to 9mm. With PA disabled, try increasing retraction 1mm at a time until the blobs disappear.
PA reduces under-extrusion at the start of a move and over extrusion at the end of a move. So its effect can be best seen at the edges of the test cube and in the top infill.
Large amounts of PA cause a certain amount of retraction to occur even before the end of a printing move. Therefore, when using PA you may need to reduce the retraction length, to avoid over-retraction which can lead to filament jams.
HTH David
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
@dc42 said in Linear advance, what to look for:
@johnjohn1990 said in Linear advance, what to look for:
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...
I think you misunderstand what pressure advance is for. If you are getting blobs at retractions, the first thing you should do is increase retraction. I find that with no PA, the retraction needed is a little less than 1% of the Bowden tube length plus 1 to 2mm. So with a 750mm Bowden tube, I think you will need 8 to 9mm. With PA disabled, try increasing retraction 1mm at a time until the blobs disappear.
PA reduces under-extrusion at the start of a move and over extrusion at the end of a move. So its effect can be best seen at the edges of the test cube and in the top infill.
Large amounts of PA cause a certain amount of retraction to occur even before the end of a printing move. Therefore, when using PA you may need to reduce the retraction length, to avoid over-retraction which can lead to filament jams.
HTH David
Thank you for your reply
how on earth did u come to that conclusion regarding retraction? Ive been trying to find a way to "calculate" retraction...
This is some of those golden grains ive been loking for, for years.. i hardly know what to say.. but.. thx for that input!!Ive never heard of anyone with a um2 using more than 6.5mm though.. but i will try higher than the 7mm max i have tried..
I used 3.4 mm at 25mms retracts before this..
Now i use 5mm at 40, which seems to work very good with pa on.. i even use 3.5mm on my 3mm um2 clone and there 3.5mm works perfect..But yes, i might have misunderstood it since i dont know how the motors are supposed to act in real life.. hence my post here in the first place... i just know it compensates up and down in flow and speeds to make the lines look more even and perfect, when adjusted correctly.. thats it.. thats what i know.. what goes into calibration, adjusting, "how to", i dont know....
if ppl ask me, i just say it works Kinda like a car ignition/retarding function...
But i dont know if that is wrong..But will give it a try with super high retract..
The grey one is with 60mm/s and literally unusable for a halfdecent printer..
But the green one looks so good, i cant even get my mind through it... i have never had such a good print from this exact printer ..
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@johnjohn1990 said in Linear advance, what to look for:
Thank you for your reply how on earth did u come to that conclusion regarding retraction? Ive been trying to find a way to "calculate" retraction...
Like you I built a printer with a long Bowden tube, and I was getting a small zit on the print at the start of every travel move. I struggled with this for hours before I eventually tried 8mm retraction, which solved it. The Bowden tube was about 650mm long.
Another of my printers has a 330mm Bowden tube and needs 4.5mm retraction. So my rule of thumb is based on experience of these 2 printers.
What I believe is happening is that when you extrude, the back pressure from the hot end causes the filament to compress by taking up a helical path inside the Bowden tube. When you retract, you have to pull the filament straight again before it is actually pulled out of the hot end. The longer the Bowden tube, the more the filament can shorten by taking up that heilcal path. So it stands to reason that retraction needed should increase linearly with the length of the Bowden tube. Then you need to add the retraction that you would need if there is no Bowden tube, which is normally reckoned to be in the range 0.5 to 2mm.
When I switched to Capricorn tuning, I was able to reduce retraction by 1mm, and with pressure advance enabled I reduced it by a further 1mm.
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
@dc42 said in Linear advance, what to look for:
@johnjohn1990 said in Linear advance, what to look for:
Thank you for your reply how on earth did u come to that conclusion regarding retraction? Ive been trying to find a way to "calculate" retraction...
Like you I built a printer with a long Bowden tube, and I was getting a small zit on the print at the start of every travel move. I struggled with this for hours before I eventually tried 8mm retraction, which solved it. The Bowden tube was about 650mm long.
Another of my printers has a 330mm Bowden tube and needs 4.5mm retraction. So my rule of thumb is based on experience of these 2 printers.
What I believe is happening is that when you extrude, the back pressure from the hot end causes the filament to compress by taking up a helical path inside the Bowden tube. When you retract, you have to pull the filament straight again before it is actually pulled out of the hot end. The longer the Bowden tube, the more the filament can shorten by taking up that heilcal path. So it stands to reason that retraction needed should increase linearly with the length of the Bowden tube. Then you need to add the retraction that you would need if there is no Bowden tube, which is normally reckoned to be in the range 0.5 to 2mm.
When I switched to Capricorn tuning, I was able to reduce retraction by 1mm, and with pressure advance enabled I reduced it by a further 1mm.
I went away from capricorn again cause my filament godt stuck in the tube once in a while, if i wasnt fast enought to pull the filament out when hot...
But i still use capricorn from the y splitter down to the hotend due to my i2k coupler and disc in the hotend is smaller than the diameter of a regular tube..I only get a blob on startpoints after retracts
endpoints are perfect, just need to be at 40-45mm/s retract speed..
Ive just always struggled with retraction.. always..
It was first when i got my 3mm olsson block i actually found a retract setting that worked perfect.. thats when i started to get back into 3d printing again.. i was thinking of selling all of it due to nothing would work just a tad as expected and i had more downtime than uptime..Thats one of the reasons i went away from e3d cause they are so super sensitive to retract...
But that seems very valid to me...
I believe youre having quite a high backpressure when printing? Cause i see no difference in 195 or 210c with my trusted pla, except the shine its got..
And the higher the temperature, better retracts.. which has always been the opposite for me.. but i guess that makes sense, in some weird way i cant explain..All the printers i have fixed for ppl i have based everything on own experiences just like yours with this..
Give me a stock cr10 and i will make it a mean machine thats comparable to a prusa..
We all have something were good and bad at.. and definately retraction has always been a pain for me...If pla was just as easy as abs i would be super happy
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I am so gratefull for your input with this! 8mm exactly, removes the points absolutely perfect!
Though unfortunately, it underextrudes after retracts on small areas now... for example on the furnace of a benchy or similar smaller areas, with retracts in between..
