Printing trouble with 1.75mm black Botmill PLA

Sadly, I bought 5lb of black Botmill PLA and can't print it worth anything. This is my first use of PLA after only printing ABS, of which I got great results. Here are my issues, maybe you can help.

At "normal" 4043D (I assume) temp of 180C, it doesn't extrude. Gradually tweaking the temp up until it DOES extrude smoothly gives a filament that sags and can't hold a thin wall. Non heated kapton on glass. So far, I am only printing the 0.5mm thin wall calibration STL with a 0.5mm nozzle, 0.4mm layer height, 0.6mm width, and have dropped the feed down from my normal 35mm/sec to 20mm. Still no good. I have my choice of nothing or extrusion that can't hold it's shape.

So.. Anyone using 1.75mm black from Botmill with good results? Anything in my description jump out as a common symptom? Any reason to suspect something wonky with the filament, or is this just a tuning issue?

I'm suspicious if it doesn't extrude at 180º. In my machine w/ a MG hot end, 3mm 4043D extrudes tolerably well as low as 160º. What may be happening is that the PLA is expanding inside the hot end and causing a blockage toward the top due to heat from the heater part leaking upwards. Try removing all the filament from you hot end, cooling the tube with a fan, and trying again.

You are right. After a hot end teardown and rebuild I suspected as much. I spent the last few days rigging a ducted fan for the top of the hot end and the spotty, stringy "looks like temp too low" behavior is gone. It must have been PLA-lock due to the filament expanding in the PTFE. So thank you!

My extrusion still isn't ideal - I'm getting walls that don't quite adhere and have little voids in places where the PLA seems to have bent up mid stream and not stuck to the layer below. The corner definition is also somewhat rounded, at least it's not as sharp as what ABS used to extrude like. I'm running about 170C now - do these new symptoms sound like it's too cold? I was hesitant to go higher as it seemed the last few layers were still soft and moved just barely as the new layer was put down. But I know the layer-layer adhesion is not great as my latest parts can be split by a strong twist.

That was 0.5mm height on a 0.5mm nozzle, BTW. Dropping it down will be my next experiment, but I'm certainly interested in your feedback as well.

Yes, that layer height was bad - lowering it, and I'm getting reasonable prints. For temp, I slowed down to 20mm but it seems reasonable that if I would eventually be able to go faster. Perhaps with a fan.

Wow yeah, 0.5 mm layer height is really high. I print between 0.3 and 0.05mm layer heights with no problem and I would definitely say lower is better with PLA. It also sounds like you may have to adjust your filament packing density or extrusion multiplier (depending on if you're using Skeinforge or Slic3r) up a bit, as PLA has different physical properties than ABS. I'd say 185º is a good temp for PLA. I print at 185º with very high speeds (≥140 mm/sec) but a lower temperature is probably good for slower speeds. Also, unlike ABS, a fan blowing on the in-progress print is highly beneficial to PLA, because it tends to harden more slowly.