Back pressure Problem???

Hi. I have a prusa I3. My hotend is cooled with a 40mm fan. It is an all metal.
From the very beginning I started printing with ABS. With good results. I learnt to use ABS. I have never had a jam with ABS
After a couple of months I decided to try PLA and B-PET (a recicled version of pet). PLA temp is suposed to be something around 180-210 and B-pet is around 240-260. So I can see that my loved ABS was being printed in the middle of this range (I use 235 degrees).
First try with PLA: failure The hotend jammed. I though that there ware leftovers of ABS in the extruder... I re cleaned, I Im sure it was perfectly clean of ABS. Tried again, same failure, hotend jammed. After a lot of reading, I thought that may be my all metal hotend was not suitable for PLA (anyway, a lot of people was printing without problems with all metal hotends and PLA). I bought a new hotend, all metal too. Teh vendor showed me that he was printing PLA with the exact same model I was buying. A new failure for me, my hotend still was jamming. I definitly didnt want to ry a ptfe hotend since I wanted to print B-pet and at those temperatures I neew that ptfe breaks down.
I decided to quit the PLA. I tried B-Pet... a new failure. The hotend jammed too, same sympthoms!!!!
I tried ABS... perfect print.

After trying averytjing: agressive cooling on the hotend, on the heatbreak, canola oil, W40 on the filament, faster printing, slower printing, you name it, I tried it (I think, suggetions are welcome), I decided to stay with ABS

Yesterday I did a discovery: Y decided to try again. I started with PLA. Bed calibration, PLA loaded, start the print, and after a couple of line the filament was being eaten by the rod, a jamm in progress. But this time I was looking closely and I saw that the noozle was too close to the bed (aluminium heated bed), and I remembered an article about back pressure. So, without stoping the print, I used the z babystepping option of my lcd and rised the Z axis like 0.1mm and the extrusion started to come out again!!!!

So I guess that the back pressure created for being too clese to the bed was my problem.
I tried with a better bed level calibration and finally could print a PLA piece!!!!! I though "problem solved"

So I moved to B-Pet. And finally I almost culd print a pice too!!!!!! The only problem I found was that this time, printing at 265 degrees, extrussion was ok, but from time to time the noozle clogs like before, like for 1 layer and then it resumes extruding.


Does anybody has any idea of what is happening here? Why I have these problems with PLA and B-pet and no with ABS?
It looks like I solved the PLA printing but no entirely the problem with B-pet. I would like to try naylon, but if I have problems with b-pet, and naylon prints even a little hotter, I think I will have problems too?

I moticed that both PLA and B-pet start to soften at "low temperatures" something that does not happend with ABS, so could this be also a problem with temperature in the heatbreak? But I have a 40mm fan cooling the hotend.... People printing naylon, what kind of cooling do you use in the hotend for high temperatures (+260) because I have the feeling a 40o mm fan is not enough (If I tough the aluminium art of the hotend I can feel that is is a little too hot, everybody says it shuld feel cold)

Thanks for your help!!!!

Hi, yes, changing filaments is fun, eh? PLA is probably the most difficult filament for the hotend to handle, because it softens, swells and deforms at relatively low temperatures. It also seems to have some of the highest friction against stainless steel of any of the plastics which makes matters worse if you are using an all metal hotend.
If I understand correctly, when your PLA prints fail the drive gear (or hobbed bolt) chews through the filament, leaving a semi-circular chunk missing from the filament, but the extruder motor keeps turning smoothly? PLA is a much harder filament than ABS and requires much higher tension on the idler, so I'd start there. The tension I use for pla is enough to crush abs to about 1/2 it's normal diameter. It's a mess if I forget to adjust the tension between filaments, and the pla will slip and grind at abs tensions.

Don't be afraid of nylon, it's actually one of the easier plastics to print with, and gives very strong parts. The key is to print slowly with it. Controlling stringing is difficult, but for functional parts (where you want the strength) that's often not an issue. Also, I've used ptfe liners at an indicated (but uncalibrated) 265C for nylon with no signs of problems yet, so a ptfe lined hotend isn't out of the question if you want to print pla as well as higher temp plastics.

Back pressure can certainly cause problems for an extruder that's close to its limits, but a well tuned extruder can push filament out of a head that's hard up against the print bed.

As far as fans go, I'm using a 40mm fan on my hotend and getting reliable results with pla over multi-hour prints, but there is a lot of variation in the air moved by different fans of the same size.

Hi. Thanks for your feed back. Yes, when PLA prints fails, the hobbed bolt chews through the filament if I dont stopp it. And yes, the steeper all the time keeps moveing smoothly.

Just wondering what your temperature profile looks like. Is the PID calibrated for 265? Is the firmware max temp limit set above 265?

Yes, the pid was calibrated for 250 but temperature is extremely stable (variable between 265 and 263.5)
Max temp is 290 in the formware

I think I solved the problem, this ight be useful for someone else too so I will make a sumary and the solution I found (is not new).
I can perfectly print in ABS, but no with PLA and B-PET. PLA use lower temps than ABS, and B-PET use higher temps (around 250/260). The sympthom was hotend clogged after some minutes sometimes just seconds with both materials.
I can manually extrude for ever if I want, but I get a jamming when printing.
Both PLA and B_PET seems to be highly higroscopic , they really like water... and all my filament rolls had been exposed to the ambient may be for 2 months (in the case of my pla, for 4 months befores I finally decided to use it, I always use ABS...)
As I said, if I manually extrude, I can do it for ever, slowly or fast, without any jamm. The problem shows when printing. I rised the hotend and I started to extrude 500mm. First thing I noticed while looking closely: at the very beginning I could see steam coming out with the first mms of extruded filament!. The the extrusion continued without problem. At 250mm of extruded filament I used a metal balde and put it in the noozle tip like if it was the printing plate... the hotend clogged after some seconds!
So conclusion: the problem is not temperature, is not the all metal hotend being unable to handle the material (actually the same hotend model on the same prusa I3 priter was being used to print with PLA and B-Pet without problems), was not printing speed. The problem is humidity, water in the filament. Steam is acumulateing inside the melting chamber somehow and creating enough pressure to cause a "clogged hotend" that actually is not clogged , it is that the force neeeded to push against this pressure is so big that eventually the filament is eaten by the hobbed bolt.

I put it the filament in the oven for about 1,5 hs and tried a print.... and this time everything went ok, so I guess this is the solution to my problem.

I have read in the forum similar cases, al related to PLA and petg clogging hot ends.... guys, take a look to this humidity problem, it might be your problem too!

Thanks everybody for the help. Now I will look info on how to dry a hole rol of filament

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Now I will look info on how to dry a hole rol of filament

Very, very carefully I had an accident with a roll of nylon I was trying to dry out. I had to take it off the 'spool' as I didn't know what temperature that would cope with. As it got drier it got more springy, and rose up in the oven until it touched the heating element and melted. What a mess that was to clean up! Fortunately now I keep the spools in plastic boxes with desiccant they seem to be staying dry enough without resorting to the oven.

Did you try repeating your back pressure test with the dried filament? While dry filament will generally print more easily than wet, I doubt if you could hold back the extrusion by hand unless the extruder was right on the edge of failing anyway, so I suspect the real problem is with the lack of grip that your extruder is getting on the filament.

Lack of grip was one of the things I also checked, but no, the hobbed bolt was doucle check, and its tension too, the problem was there despite of testing low tension, highes tension and all the posible range of tensions i the middle.
And yes, once i finished the drying, I did the test again: 500 mm extrussion. The first 250 mm came on with no problems as before. then I pushes the piece of metal against the noozle as it it was the printing plate. This time extruded filament started to flow in the borders and I actually could felt the force of the extruded material comeing out of the noozle, and after all the material run out (250mm of extrusion), no jamming/clogging happened.

After that, I did the test printing as commented before and it was ok

Hi, Thanks for leading me to this post....
My problem is the other way round....
I'm getting perfect, larger-ish prints from PLA but I have found that the "PLA" I ordered is actually ABS....

I reset my printer for ABS and I thought I had it with about a meter extruded in the air and when I went for a print, all looked fine 'til maybe an hour in...... BLOCKED hot end!

Well for me. I guess it wasnt water in the filament. No steam came out. Turns out my nozzle was loose. The overtemp weakened the aluminum block. I decided to tighten it with a 6mm socket. I think I overtightened it and stripped the threads. So the nozzle was wobbly and oozing filament. This made the z height unstable.

Well at least you found the cause and can figure out how to avoid it next time.
Part of the learning experience IMHO, thankfull in your case not a too costly experience.

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Downunder35m
Well at least you found the cause and can figure out how to avoid it next time.
Part of the learning experience IMHO, thankfull in your case not a too costly experience.

True. I just bought a new hotened. Waiting for delivery. Until then my system is out of commission