mwester wrote: ↑
Mon Nov 19, 2018 7:33 pm
I have to wonder if you're not suffering from overtemp.
I've tried reducing the temp to 200C. Right now I'm at 220C, and I've gone higher. No changes, same result regardless of temperature.
All of the above numbers are what Cura displays. See below for more accurate temp info.
So, I'm wondering if your filament is simply too hot, so that the retraction just stretches the filament, leaving some of it down in the hot-end, and the overly-hot hot-end melts what's there and drips it onto the bed.
Remember, the Mini's default sequence is:
1) Preheat the nozzle
2) Retract 30mm of filament
3) Wipe the nozzle
4) Finish heating the nozzle to 10C below the target temp
5) THEN push 30mm of filament into the printhead, to offset the 30mm of retraction in step 2
It is only at this step 5 that the dribbling occurs. So I don't necessarily think there's "leftover" filament in the nozzle... the machine is actively pushing filament into the head.
Yes, I do sometimes get some "pre-oozing" as the head heats up but before the 30mm push occurs, but the majority of the problem is after the 30mm starts.
One is that yu've replaced the fan that blows on the heat-sink, you stated above -- double-check that fan to make sure that it's pushing air through that head-sink.
Yep, definitely airflow through the nozzle and across the heaksink.
Another possibility is that you may simply have a bad thermister -- use an IR thermometer and check that the nozzle temp matches what the firmware is reading.
I have a calibrated Fluke contact thermometer. When Cura reports 220C, it reads 195C. This has been true since the Mini was brand new... the quality of the thermistors used by Lulzbot must be just awful. This machine has always had a roughly -25C offset from reality (the actual head temp is always ~25C lower than reported by the software).
I asked Lulzbot support where, in the firmware, the thermistor could be calibrated. I have 30+ years writing firmware and every time we work with sensors of any kind we always design in a way to enter a calibration value, even if it's just an offset to compensate for this sort of thing. This is just normal professional practice. Their response was "Nope, no way to do that." Seriously?!?
But in any case, I doubt we're looking at an overtemperature problem here. See below for more comments.
Regardless, the above doesn't address the lack of "stickiness" of the filament to the bed.
I had the same thought, which is why I started playing with the temperature. No matter what Lulzbot's firmware and Cura report as an absolute value, I just started making relative changes to see how things behaved. 220C displayed/195C actual seems to give me reasonable "squish" on the print bed without being so high that temperature further contributes to dribbling. Honestly, it's basically a balancing act trying to get this Mini to work at all.
Almost sounds as if there's something (like a layer of oil) on the PEI surface.
Remember, this is a BRAND NEW print bed straight from Lulzbot - their latest, most expensive version. And I clean the surface before every attempt with a bottle of alcohol that is dedicated to the printer and is sitting right next to it all of the time. No, I haven't roughed it up with ScotchBrite yet, but I would expect a factory-fresh print bed freshly cleaned with alcohol to work at least a little while.
BTW, I have managed to stop the printer before the entire first layer was ripped off, which allowed me to test the first layer adhesion to the bed. It feels fine - I think the blob on the nozzle just has a strong enough bond to overcome the normal adhesion force. There's supposed to be "interlayer adhesion" after all!
The root problem here is the blob of filament, the dribbling and oozing. It's done it since day one, even with the sample filament that Lulzbot included to print the Rocktopus.
EDIT: I've also tried flipping the print bed and printing PLA directly to the bare glass surface. No changes. The first layer adheres OK but the interlayer adhesion to the nozzle blob is still strong enough to rip the first layer away from the print bed.