Infill

When solid models are 3D printed they are not typically printed solid (except for very small objects). Slicer software generates infill. To be infilled, objects must be water-tight - that means that infill is not visible on already-printed objects, only while they are printing. Infilled objects can be 90% lighter than solid ones. Slicer software may be able to generate a dozen different patterns (e.g. honeycomb, rectilinear, triangles, grid). Along with the pattern the slicer expects a percentage of fill density (typically defaulting to 10-20%). Infills typically have walls of the same thickness as the nozzle, commonly 0.4mm - they can thus be quite delicate. Proper infill generation depends on the 3D printer being in good adjustment to assure that successive 0.4mm laydowns are exactly on top of the one below. Slicers generate solid layers top, bottom and on all sides of infills.

Experience is needed to learn when it is best for an object to be printed hollow, hollow with support or solid with infill. Infills generally print more trouble-free than supports. On larger objects, infills can drastically increase print time, even though fill density might be quite low. Infills of low density (e.g. grids where the lines are 5mm or more apart) can be quite practical - but there is a limit to how wide a space the top layer can span without sagging too much.

Related Information

Infill and support issues with 3D PLA prints

Top: The left one has a rectilinear fill, the right a square grid fill. Notice the fill on the left has buckled part way up - this piece is too tall for that type. While the grid fill on the right is far more stable, it is not as easily removed (although it can stay in place here).
Middle: The piece on the right printed in half the time because the only fill needed is at the bottom. The fill is rectilinear and easily removed - it affects the smoothness of the surfaces but they are not a finished ones so it does not matter. However that method is risky, notice the failed print on the lower right - upward pull of the flat section pulled it away. This happened twice more so I chalked up the one success to luck.
Bottom: Took about 16 hours. Like others, the grid support that printed out past the edge - that part has to be cut away with a sharp blade knife to enable mating with the other pieces.

3D printing case vs block molds for ceramics

Left are case molds, they are made by 3D printing the positive profile on a backplate (with holes for natches). These are secured into slotted rails. Right is a block mold, it is made by 3D printing the profile of a working mold with integrated rails. This one is printed vertically in four pieces. It is held together and straight with printed brackets. We pour rubber into these to make case molds. Each method has advantages and issues.
-Case: Faster to print. Easier to draw. Joins cast as easily removed bumps on the working molds. This is only suitable for prototyping, making one working mold.
-Block: Much more attention is needed in printing, there are more issues with orientation of print, infill, support, multi-piecing, fit and seam-filling. 3D drawing of these is more difficult. And block molds are bigger because they are molds of molds. They also need to be more precise to merit the cost of the rubber.

The importance of 3D printing the right way up

These are quarters of a block mold piece for a Medalta Potteries ball pitcher. The whole piece was far too big to print so I had to break it up into quarters. Quarter #1 (top right and bottom left) was made by shelling (hollowing) the whole thing in the CAD drawing process and quartering in the slicer. The other piece (top left and bottom right) was made by quartering and shelling each individually in the CAD software (Fusion 360). Quarter #1 has a dangling corner (front left of top right) so I had to print as shown so that infill would support it. Quarter #2 mates with a wall and that supports the whole curved mating edge (enabling upright printing). Notice also that the matting surface is not planar on quarter #1 (top right). And its inside surface as print artifacts. Quarter #2 was printed with the back-side down, thus only support was needed in the narrow channel where the rubber wall will pour. Surface quality is much better and it printed in 9 hours instead of 14 (these pieces are quite large).

The v2 ball pitcher is way too big to 3D print. What now?

This is a Medalta Potteries medium-sized ball pitcher block mold, version 2.0 - it has a more oval body shape. Upper left is the top section (actually, it is half of the top section, a base will fit on to create a three-piece working mold. There is to 3D print something this large in one piece in a consumer 3D printer. Even if it did it would require 50 hours of print time! Also, working surface quality is affected by the orientation of printing (by support impingement surface stair-stepping artifacts. Further, a large print would almost certainly warp and corner-lift during printing. Cutting it into four pieces and hollowing them individually (in the CAD software, lower right) solved all the problems. Each of the pieces is still quite large, taking 10+ hours to print. But they can each be hollowed individually and rotated to the optimum position for the best finished surface (and print speed). It is amazing how well these four pieces fitted together. This approach really paid off because I made a mistake - and I only needed to reprint one of the pieces!

By Tony Hansen
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