Do Cura's self-crossing infill patterns account for excess material from self-crossing?

As far as I can tell, Cura's infill patterns which are self-crossing, such as triangles, don't do anything special to account for extruding material over the crossing points multiple times. In theory this should produce either a blob around the crossing point, or excess pressure in the extruder due to blockage from the alread-deposited material that will result in over-extrusion after the point is crossed until the excess pressure is dissipated.

Is Cura doing anything to account for this? It doesn't seem to do travel moves across the intersection, but maybe it's adjusting the overall flow to compensate for the crossings "on average".

If not, it seems like crossing points near the walls would be a source of dimensional accuracy problems, which I've seen in tightly confined areas and which I'm looking for a root cause for.

1 Answer

With most filaments you don't need to do anything special to account for overfill caused by these crossing points. When the nozzle extrudes the second direction, it pushes the lines laid by the first direction out of the way. The same happens when the third direction crosses the first two. There's a blob, but it's a blob within the plane of the layer, so it shouldn't stick up out of the layer and upset subsequent layers. There are two complications which can occur though:

1. When filling each direction, it's usual to keep extruding when moving from one line to the next (not a travel move), which results in getting an extra half perimeter around your infill. This is fine for a single raster pattern, and for box infill, but with triangle, hexagram, or similar, you'd get three half-perimeters (one for each direction). The slicer needs to offset the third direction so that those extra perimeters don't overlap. I don't know how Cura handles this; it's possible that's the source of the dimensional problems you're seeing.

2. With abrasive filaments, or those that don't stretch (such as carbon fibre or metal fill filament), the "pushing the existing lines out of the way" doesn't really work. You just end up wearing down the outside of the nozzle and getting the kind of blobs you're afraid of, sticking up into the next layer. It's just best to avoid infill with crossings (box, triangle, hexagram, etc.) on such filaments.

• Regarding point 1, Cura calls this "zig-zaggify infill", and it's an option you can leave off. I think the way it orders infill line drawing, you get at most one along any perimeter. Slic3r seems to always do it. Aside from that, my concern is that the crossing (especially a 3-way one) that happens to fall very near the wall may displace the wall outward. There's nowhere else for the material to go since the crossing happens at the same place in each layer (and this is intentional for strength). – R.. GitHub STOP HELPING ICE Jul 3 '19 at 20:00
• Regarding When the nozzle extrudes the second direction, it pushes the lines laid by the first direction out of the way could you explain why it pushes it away (I never see this happen, nor ever heard it does), the OP hints to not pushing it away as the second pass it cannot extrude (as the top of the first pass line is on the same level as the bottom of the nozzle), causing a hick-up in flow and build-up in pressure. – 0scar Jul 4 '19 at 10:28