Sunday, June 17, 2012

Quality review...

Recent disappointing efforts to print some smaller delicate objects, specifically the competition piece "Dual Spiral Lightbulb" left me thinking long and hard about where the quality issues lay in my printer. Was it software settings? Were there mechanical issues? Up until this point I had growing confidence that my printer was capable of 'good' quality prints, but the challenges posed by spindly objects, caused me to question that assumption. What might be causing my print quality issues? What could I do about it?
Above is one of my many unsatisfactory attempts to print the "Dual Spiral Lightbulb". The example in the photo was probably my 'best' one, but suffered failure as it concluded, when the nozzle grabbed the top, then ripped the print off it's base as the nozzle parked up.

I struggled to reduce stringing, ooze, and 'plastic-starved print continuations'. No matter how I'd set the Slic3r retractions/restart lengths, extruder speed, etc., I couldn't find a balance, or even achieve consistency. I even tried out KISSlicer and it's equivalent settings (Suck/Prime and De-string Speed) but to no avail.

My extruder hot-end (1.75mm/.35mm) has a long melt-chamber. It's by now an old-style M6 threaded brass design, with a long 2mm hole from well above the heater block, to within < .5mm of the nozzle. This has to be partially to blame as it introduces an undesirable length of 'spongy' plastic between the firm retracting plastic filament (PLA) and the nozzle tip. It may also absorb the downward motion of the feeding filament as printing restarts. This, on it's own should have been possible to accommodate with software compensation, but I struggled to achieve good results after hours of software tweaks. I felt there were more issues at play.

I found there was quite a bit of backlash in the cold-end gears. I had tightened the gap between the two gears by moving in the motor but re-examining it, I don't think the motor held it's position. So I concluded the general quality of that gear set was too poor for my demands and it was time to print a new set!

The Herringbone Gear Set was my new choice. I printed them in PLA with the .35mm nozzle and .20mm layer height, and a careful speed of 30mm/sec, giving an excellent quality print. The quality of this print restored some of my confidence in the printer!

The smaller gear failed to slice in Slic3r, giving an "overlapping or self-intersecting facets" error. As an exercise I had a go with KISSlicer, learning how to match the layer, speed and other settings as I went along, and successfully sliced the smaller gear. KISSlicer has a very impressive interactive visual rendering of the object and slicing process, and allows close inspection of the model and print path in a three dimensional representation. It is a commercial software package, but even the free version is worth a look. 

Will all the test printing in the last week I found it very handy to add some additional buttons to Pronterface. It's easily done, and very convenient to have a pre-set buttons to hand to increase/decrease 'speed' or 'extrusion flow rate', with a simple line command of M220 SXXX, or M221 SXXX executed by the appropriate button. The M220 and M221 commands allow you to make live changes to current print and observe the effect, instead of having to quit out, adjust and regenerate the gcode. Custom buttons highlighted below.

The small gear, shown beside it's companion below, was printed at a slower speed of 15mm/sec perimeter, with .20mm layer height, again with the emphasis on print quality rather than speed. It was sliced with KISSlicer.

Next steps will be to fit the new gears and see how they work!

Thanks for viewing! As always, comments and questions welcome. I'm always contactable via the RepRap forum.

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