What causes gaps in small region fills and how to prevent them?

Question

I saw this cool Nuka-Cola bottle opener. I made it:

As you can see, nasty gaps have appeared in the text. What could've caused them? I am sure I set the filament diameter correctly. So why did the Slic3er not generate enough paths here to fill the letters properly?

I used Slic3er and Repetier Host. My printer is Prusa i3 MK2 1.75mm. The material I used was PLA.

Answer 1

I had a similar issue as described in this question. Curiously, yours is almost opposite as my raised lettering was OK, but the surrounding areas were poorly filled.

I came to the conclusion that Slic3r was simply not able to properly handle the geometry in my part. I tried with Cura and had great results!

Answer 2

If you look closely at the slicer output, you will probably see that these regions just have outer shells, and no infill. What is happening here is that the part thickness is between 2 and 3 nozzle widths (or possibly between 4 and 5). If you ask for a 1 mm thickness, but only have a 0.4 mm nozzle, it is not easy for the slicer to split this into 2.5 passes.

One thing I found would sometimes help is to change the nozzle width setting in the slicer (to maybe 0.39 mm) and this sometimes helps (at the expense of needing to offset the extrusion rate to compensate).

Here are some rectangles of width 0.3 mm to 1.2 mm (increments of 0.1 mm) sliced in Cura 2.31. The first is 'normal', with wall count set to 2. Next is an inaccurate (small) nozzle setting of 0.38 mm, and finally the correct 0.4 mm nozzle, wall thickness of 1. The last setting shows infill (in yellow) rather than an inner wall (green).

Normal has a gap in anything over 0.8 mm wide.

If I reduce the nozzle width, I do get some inner walls, but only where there was already space for 2 walls.

When I only request one wall, the rest infill, (even though I have 20% infill), then the result is as expected. This seems to be a bug in Cura (or at least something which is improved in the version currently in beta).

There is a bit more detail in this question Missing top layers in Cura

Answer 3

The proper term here is Under Extrusion.

There are a lot of reasons this can happen. For my Ultimaker, over time it developed very bad underextrusion due to the build up of carbon. I had to do atomic pulls / increase the extrusion multiplier.

I would ...

1. verify that you move exactly the amount of plastic you think you are. Mark 200mm of material. Extrude 100. Fix the firmware steps per mm for Extruder if incorrect.

2. triple check the filament measurements. Try all along the length. See if the plastic has bad variance.

3. Verify that it is not printing as expected. Is your slicer generating the gaps? Letters are hard for slicing programs. If it cannot print a full line, it often will print a gap. You can adjust this by allowing greater line overlaps. This is probably what is happening.