Improving 3D print quality, by doing the obvious

Some time ago I bought and assembled a RepRap Prusa with the intention of using it to prototype things.  It has saved me countless hours in prototyping my CNC but getting good quality prints out of it has been challenging at times.  Granted my standards for good have gone up even as I've developed a hunger to print as fast as possible.

Printing fast has worked for the CNC, which has large, fairly simple geometries.  It's nice to be able to print an axis end in two hours, rather than eight.  But when I try to print small, accurate objects I tend to run into issues, especially with PLA.  The biggest one has been that smooth surfaces end up ridgy, see for example the print in the image below:

That print is supposed to be a linear bushing with circular sections, but the print is garbage due to the ridges.  I tried a lot of things to reduce them, most focused on the mechanical rigidity of the printer.  I tightened everything up and added diagonal braces in the X-direction. I even 'floated' the entire printer by sitting it on soft foam blocks so that the entire printer could move as a rigid body when the bed and extruder moved rather than cushioning jerk with elastic deformation.  Amazingly enough, this simple idea CAN actually improve print quality for wobbly printers quite a bit, but did not fix my problem.

The focus on mechanics and dynamics is probably not surprising given my background, but it turned out to be misplaced.  What should have clued me in that the mechanics were not to blame is that everything got a lot worse with PLA, even when using the same travel/speed/acceleration settings that I had been using with ABS.

Eventually I did realize this and started looking for extruder/heat problems, measuring and adjusting temperatures and filament diameters, changing nominal nozzle diameters and so on.  None of these seemed to have any effect.

Finally I ran across a post (I don't remember where I found it), that said to just print perimeters slower.  This seemed crazy since i) the mechanics were sound ii) the extruder is perfectly happy running quite quickly with the beast of a stepper I have on it.  But I tried it and my prints instantly got better.

Here is a print with perimeter speed of 40 mm/s.  Although it's difficult to tell in the photo, the surface is actually quite a bit better than the bushing printed above using 80 mm/s perimeters. Finally here is the same print, but with 20 mm/s perimeters:

The difference in the photos is much more evident here and even more so in the real world.  Note that all prints were made with aggressive, active cooling.  So what gives? Why does the print quality improve when printing slower in PLA, even though the extruder and printer stiffness are the same that was used for ABS at much higher speeds?

I have a theory, taken from the post that recommended printing slower and combined with (my own) conjecture about the print material.  The theory is that communication lag between the host software and firmware causes the motion to stutter a bit, particularly on smooth curves that are discretized with many small segments.  With limited lookahead, the printer does a few segments very quickly and empties its buffer before the host can refill it.  This causes the printer to periodically, but regularly, stall.  Since the temperature of the extruder has to be kept high to print at high speeds, the PLA in the nozzle is runny and gobs out during these stalls, creating the surface ridges.  I suspect the surface imperfections are less noticeable with ABS because it generally is not as liquid as the PLA, allowing you to get away with higher feeds without noticing the buffer starvation.

I should be able to test this by changing the baudrate on the printer.  Doubling the send-rate should allow me to increase (ideally double) the feedrate, but will require recompiling the firmware to test.  When I get a chance to test the idea out, I'll post the results.

In hindsight, slowing things down was an obvious thing to try.  My Prusa bobs like a cork on the soft-feet at 80 mm/s and all of my experience on other 3D printers suggests that slower printing gives improved results.  But I wanted to have it all and kept increasing the feedrate.  However, if I am right about this, perhaps I can go back to printing at high feeds while keeping the high print quality.