Surface Infill Testing

One of the options you can play with in Slic3r changes the infill pattern used on the bottom and top visible surfaces of a print. Different from the interior infill pattern, the surface pattern changes the look and texture of horizontal planes.

By default Slic3r will use a rectilinear surface pattern; simple parallel lines at a 45° angle. I decided it would be fun to compare the alternative surface fill patterns available in Slic3r (as of Slic3r Prusa Edition 1.37.1).

To make the comparison easier, I designed a simple set of small objects, each labeled with what would be the print resolution used to print them. For this set of tests I generated prints at four resolutions (0.05mm, 0.10mm, 0.20mm, and 0.35mm) using four different surface fills: Octagram Spiral, Archimedean Chords, Hilbert Curve, and Concentric fill.

The entire set of printed test tiles:

The quality and finish of the surface patterns seems to be highly dependent on the quantity of infill used on the interior of the print – some fill patterns have longer lines that benefit from denser infill for bridging support. In later sections I’ll pick out the best (in my non-quantitative opinion) patterns for the different print resolutions I tried out.

Octagram Spiral

The Octagram Spiral patterns looks like an eight pointed star, with repeated right angles filling out the sides and corners:

While this pattern looks nice and has an even and gap-less fill for 0.10mm and 0.20mm resolution, it does leave occasional pits and small gaps at 0.35mm. For this pattern, like the others, the 0.05mm resolution print is a loss.

Archimedean Chords

The chord pattern looks like a series of overlapping circular bands:

The chord pattern created a fantastic surface in the 0.10mm print. The 0.20mm and 0.35mm prints were gap-free and had consistent shape, but also suffered from slight puckering and raised edges along the chords.

Hilbert Curve

The Hilbert Curve pattern is a dense printed version of the Hilbert space-filling curve, and presents as a fractal pattern of right angles forming a continuous, uninterrupted line across the surface:

At 0.10mm resolution the Hilbert Curve surface fill is clear and distinct; you can trace the lines across the whole surface. At 0.20mm the features become indistinct, and while a pattern is evident in the noise, the surface appears as just that: noise.

At 0.35mm the space filling curve suffers from extreme pitting, with a grid like pattern of holes in the surface.

Concentric Fill

As far as I can discern, the concentric fill attempts to match the fill lines against the edges of the fill surface in parallel, with the different angles of lines meeting in the center. You can see the segments of lines attempting to parallel the curved and irregular angles of the quarter-circle cut out and extruded numbers:

The finish on the 0.10mm, 0.20mm, and 0.35mm were all quite good, with the tight intersections of different line sets on the 0.35mm print creating a slightly jagged joint.

0.35mm Resolution

Looking at just the 0.35mm resolution tiles, the Octagram Spiral and Archimedean Chords stand out as the cleanest surfaces.

If I had to choose, I would go with the Archimedean Chord surface fill: the angled joints between lines on the Octagram surface can sometimes create a rough bump, while the circles of the Chord pattern avoid such problems.

0.20mm Resolution

At 0.20mm resolution all of the surface fills work well, and which one to use would depend quite a bit on what is being printed.

The Hilbert curve, while still noisy at this resolution, creates a diffuse pattern when light is reflected in the surface. For prints using a sparse internal infill, I would probably choose the Octagram Spiral, as it minimizes, to some extent, long filament strands stretching across the entire print. For prints with denser infill and more complex edge geometry, I would choose the Archimedean Chord infill, which will minimize odd and irregular joints which might cause a rough or buckled surface.

0.10mm Resolution

The patterns and lines of the surface fills are really clear at 0.10mm:

At this resolution the Hilbert Curve stands out as a clean, diffuse, and consistent surface. The other surface fills have fine enough detail that they can distract from the print itself – it’s hard not to notice the lines and curves, humans are fantastic pattern matchers. On close inspection the Hilbert Curve also has a very distinct, and very noticeable pattern, but when viewed from more than a few inches away, the pattern fades away, and the surface just appears lightly, but regularly, textured.

0.05mm Resolution

The structural infill of the printed models played a large part in the print quality at 0.05mm resolution; significant gaps between structures and ultra-fine filament strands don’t mix well:

All of the surfaces infills exhibited the same flaws, in the same places: the gaps in infill. It’s impossible to make a useful decision about an optimal surface pattern with these prints, but some fared better than others. The long lines in the Concentric fill had the worst bubbling and holes, while the Octagram Spiral, with its shorter lines, minimized the problems (but didn’t completely avoid them).