Archive for May 21st, 2011

Thing-O-Matic: Polyholes Print Quality at 25, 50, 75 mm/s

Nophead’s polyholes test piece (search for it here to find other attempts) seems to be a particularly difficult-to-print object: it’s 1 mm thick and has narrow openings between the holes.

Here are three variations, printed in quick succession, on three different plates, at 0.33 mm layer thickness, 2.0 w/t = 0.66 mm width, and speed variations to maintain those numbers:

  • 25 mm/s, 50 mm/s, 1.65 rev/min
  • 50 mm/s, 80 mm/s, 3.3 rev/min
  • 75 mm/s, 100 mm/s, 5.0 rev/min
Polyholes at 25 50 75 mm per sec

Polyholes at 25 50 75 mm per sec

Polyholes - large hole detail

Polyholes - large hole detail

Those are as-printed, with no cleanup other than breaking off the final filament leading to the extruder.

I’ve turned off Comb, because the extruder really isn’t dribbling very much and high-speed travel stretches what’s left into very fine hairs. Comb tends to run the nozzle along the path of the infill threads, which can be jarring at 80 mm/s.

A higher-resolution slice (clicky for more dots) down the right side shows the largest filled area; I stretched the contrast and added a little unsharp mask to highlight the threads. The 25 mm/s version has much straighter fill lines, with the other two exhibiting side-to-side jitter; the platform seems unstable  at those speeds.

The upper hole is nominally 9.5 mm and measures 9.1 mm on the 25 mm/s plate and 9.2 mm on the other two. This is with my HoleWindage tweak set to zero, so the larger holes now print 3% smaller than nominal. That’s a considerable improvement over my previous attempts, with the new Z-min height measurement making all the difference.

Polyholes - small hole detail

Polyholes - small hole detail

This slice down the left side shows that the small holes come out rather ugly, but they’re actually pretty close to the right size amid all the chaff: around 0.3 mm too small. Of course, that’s a huge percentage of the nominal 1 to 4 mm size, but so it goes.

The fill around the small holes looks much worse on the higher-speed plates.

The longer threads parallel to the X axis have an odd stippled pattern that might be coming from the extruder stepper motor. Given that it’s driving an 8:51 gear reduction, I find that hard to believe, too. It’s only present on the fill threads, so most likely it’s something weird.

One of Skeinforge’s myriad settings very predictably chops off the initial or final (or both?) section of the inner extrusion of some circles. It’s probably not Clip, because that’s set to 0.34 √ó width = 0.24 mm and the gaps can be just shy of 1 mm long. Ditto for early Reversal, which has threshold of 3 mm that’s much smaller than the distance to the next perimeter thread. More tinkering seems inevitable.

So a super stock TOM can print upwards of 50 mm/s on non-critical parts, but something around 30 mm/s would work better should you care about surface finish. I think the results will be much better on thicker parts that can absorb some of the excess plastic around the holes; this is likely a lower limit on print quality.

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