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print issue

Posted by simono71 
print issue
December 16, 2012 04:50PM
I have a recurring issue that I am having trouble diagnosing and fixing. I have a mendel max with a greg's extruder and j-head, ramps 1.4 and marlin firmware.

What happens is this, the first layer will go down absolutely fine. Then the second layer and those that after that will come out weird. The extrusion breaks up and sometimes look like little dots, and the perimteter shells don't connect, but seem too thin, and the layers don't seem to connect together very will either. I have read about people having this issue when the heat is not high enough. I use 185 and 230 with pla and abs respectively. I have been using these settings to get decent prints for a while, and then all of sudden this happens. I have tried increasing the temp with no effect.

After a while of a series of lousy prints, it will just start to print fine for a while, and then after a few more prints, back to the issue above. It is annoying as I don't know what is causing it and how to fix it. Sometimes it will happen in the middle of a decent print too.

Does this sound like some kind of blockage? Plastic continues to come out so I don't know if this would be considered a blockage (I have tried pushing down on the plastic and forcing extra out just in case), and as I said before heat doesn't seem to fix it. Occurs with both ABS and PLA.

I attached an image of a print I was doing. The first layer went down great. I left it printing to see if it would correct it self, but it just got fuzzier and fuzzier. As you hopefully can see the layers aren't sticking and it is blobby. This was orange PLA. Tried increasing temp up to 205 but no luck. When this is not happening my prints come out fine at 185. I have a .35 nozzle and usually print .2 mm layers.
Any help greatly appreciated.

Edited 1 time(s). Last edit at 12/16/2012 05:06PM by simono71.
open | download - SDC13636.JPG (407.5 KB)
Re: print issue
December 16, 2012 05:25PM
try halving all your feed rates. this will allow you to troubleshoot if it is an issue with feedstock/extruder. have you just switched batches of feedstock?

also reverse your feedstock out of extruder, is it still round or is it square in shape? if it is square then you have applied too much force.
Re: print issue
December 16, 2012 07:09PM
Could you explain what I am looking for by halving the feed rate? I am afraid I don't have a ton of experience here, and I am not sure what I am looking for. If halving the feed rate produces a good print then what can I conclude?

I have not switched batches.
Re: print issue
December 16, 2012 09:16PM
when my printing starts to go bad i:
- slow the printing down really slow ie. 10mm/min - if the print then works then for me it means my extruder is slipping under the more work it has to do to extrude at higher speed.
- plus at a low speed i can SEE a bit better what is going on. ie. is the dot dot stuff happening at the same point on the gear of your extruder?
- if it something to do with the first and second layers then i try to keep all the parameters the same and then vary them later
ie. start with keeping the first and subsequent layer temp and speed the same.
- i print my pla at 190-195
- 90% of my problems are the bed to nozzle height (but u said the first layer is ok).

Prusa 'Explorer' (3dStuffMaker), GEN6, J-head Mk III-B, Bowden Extruder, Marlin 1.0.0 RC2, Repitier-Host V0.84 and Slic3r 0.9.8, PLA. Live at Victoria, Australia.
Re: print issue
December 16, 2012 09:31PM
I slowed down and no improvement. I have seen this happen a lot with my pla, but this happened with abs (which I have just started to play with) after several successful prints. I switched back to pla as trying to trouble shot with the smell was not good.

With the pla in there I had already cranked the temp up (about 205), and just for kicks I booted it to 225 (for pla), and then my prints started to work. Guessing maybe some crud in the nozzle that the high temp burnt off?

Not sure how I would fix with ABS? If I crank much higher past 240 things could get too hot.
Re: print issue
December 17, 2012 12:55PM
I have a different extruder than you (MK8 clone) so this may not be any help.

I have had a lot of jamming issues. One issue that took me a while to figure out was that I was over extruding just a little bit. That cause high back presure to cause plastic to build up in the nozzle and fail the print. Most often after a 100% fill layer. This would happen a lot on the first couple of layers too from being too close to the bed at the start.

I would suggest running a test with the filament dia set too high, like 1.76mm if it is really 1.75mm, or set your flow rate less than your feed rate. This will cause your extruder to use too little plastic so there is plenty of space for the plastic to go and reduce the pressure it takes to extrude.

I was surprised to find that this made a bigger difference than changing the temperature around.
Re: print issue
December 17, 2012 03:25PM
Which version of j-head do you have? The PTFE lining on the Mk-IV tapers in where it meets the hot end and tends to tighten over time if there's a little too much pressure from the locking hex nut. It's happened to me a couple of times. I think since the mass and heat capacity of the j-head is less than most, the PID parameters on Marlin and Sprinter can result in the hot end overheating due to overshoot which makes this worse. I just recently loaded the latest Marlin and used the autotune feature to try to avoid this in the future.
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