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Bad Prints After Nozzle Diameter Change

Posted: Sun Aug 12, 2018 11:56 am
by greybeard
Printer: Prusa i3 MK8

Printing PLA

Been using this printer with 0.4 nozzle for 2 years and dialed in the process for excellent prints. Always get excellent prints. Always!

I just changed the nozzle to 0.3, and made appropriate changes in s3d.

Problem: First layer can be a mixture of: good, uneven, blobly and infill is light and mostly missing filament. Then, barely nothing comes from nozzle during print. But, I can Pause printing and manually move plastic via extruder axis move (or feeding by hand). A nice stream comes from the nozzle.

I checked and re-calibrated the bed height.
During the print, if I manually (lightly) force the filament, it prints evenly.
I can touch a piece of filament to the nozzle and it melts so, Temp seems good and constant.

Logic would suggest something to do with the filament gear feed but, since it feeds nicely via move axis, and I don’t see any damage or chips of plastic, I think the problem is elsewhere.

I did take it apart and verified the gear is fully engaged into the filament, no damaged parts or damaged/rough filament.
With machine off, I can manually move the filament in and out and it drives the gear/motor without slipping (filament is firmly held). So, hardware seems ok.

I pumped up the Temp to 235C (from usual 230 I print PLA).
Flow Multiplier to 1.09 (also tried 0.90)

Nothing seems to help… I even increased the nozzle/bed gap to much higher than needed... (photo of perimeter taken with larger gap).

Any suggestions?

Re: Bad Prints After Nozzle Diameter Change

Posted: Sun Aug 12, 2018 1:26 pm
by airscapes
greybeard wrote:
Sun Aug 12, 2018 11:56 am

During the print, if I manually (lightly) force the filament, it prints evenly.

Logic would suggest something to do with the filament gear feed but, since it feeds nicely via move axis, and I don’t see any damage or chips of plastic, I think the problem is elsewhere.

Is there a tension adjustment on that drive? If it prints when you push it then keep looking at the logical assumption. With the tension set correctly on my M2, no matter how hard I push on the filament it won't go in any faster than the extruder stepper will push it.. The teeth are embedded
in the filament squeegeeing it against the bearing.. Of course my printer is not the same as your.. However, if you can over ride a stepper motor with just a little force, something seems wrong to me.
Good luck!

Re: Bad Prints After Nozzle Diameter Change

Posted: Sun Aug 12, 2018 2:24 pm
by greybeard
Thanks.

SOLVED

Seeing the cyclic blobs got me thinking...

Although there were no indications, as all looked good (except the print), including steady temperature, I decided to see if perhaps it just didn't like the 0.3 nozzle.
So, I decided to put a 0.4 back on the machine. After replacing nozzle, it would not register temp. Bingo!

It was the Thermistor. I appear to have damaged it enough to permit temp readings that look constant but, the fast cycling did not not show circuit open/close until I man-handeled it.

I replaced it (fortunately, I have several). It's up and running (with 0.4 nozzle) and printing well. Guess I don't need the 0.3 on the machine anyway...

Thanks again for your reply