Funnels and Slic3r

At the beginning of the year, it’s often the time for projects, big or small. I decided that this was as good a time as any to make a few useful things for my car. I already have all the fluids I need to clean up my car. I just don’t have a funnel.

Rather than run across town to an auto parts store for a funnel to fill up the fluids in my car, I just made one, using this wonderful design I found on Thingiverse. Hat tip to bluesroq, for the great design.

You might also notice the tiny little funnel. Needed something to fill the flask from NYE. Hat tip to KKHausman for that design.

I think the more interesting thing I’ve observed, after extensively testing these funnels for their ability to be water-tight, is that it’s definitely a function of two factors, layer height (relative to the model size) and vertical/horizontal shell-count.

Bearing in mind that this is all slic3r specific, and so users of Skeinforge/Cura/Etc might not find this as handy. Shells are simply a representation of how many solid “walls” exist in the object when it’s being printed. In slic3r, these shells are represented as being either horizontal (perpendicular to the build-axis) or vertical.

If the object your printing is a cube, you could say, set your slic3r to have one horizontal shell and one vertical shell. So you would get a solid bottom layer, and a solid top layer, and 1 perimeter “wall” in which the gap between the edge of the walls is filled with your infill of choice.

When printing simple objects, that do not have lots of vertical angles, this is often an acceptable setting to reduce printing time, and increase printing speed. However, when printing a complex object like a funnel, you run into a difficult problem. Since each layer is set “back” slightly from the other to form the slope of the piece, with one vertical shell, you only get one point for each layer to bond to. Which means that if your printer has a half second hiccup, or your piece has too steep of an angle, there will be a hole. 

The solution is to add more vertical shells then. Because now, the interface of the two layers isn’t one outer wall offset from the outer wall below it, but instead 2 or 3. So far, I’ve found 3 shells to be the magic number for forming waterproof containers.

However, there is another complicating factor. Layer height.

If you haven’t noticed by now, there’s actually some nice overlap with calculus in this whole thing. At least a few people might be having flashbacks to Riemann Sums. Layer height has an immediate and noticeable impact on the quality of prints such as these. When I initially printed the tiny funnel, I printed it at .2 MM height I used with the flask.

The layers didn’t work. It’s such a small piece that the printer can only lay down 1 shell, with each layer barely touching the other. The solution then was to go down to .1 MM to improve the print quality. At .1MM the printer could put down multiple shells, and maintain the slope of the piece without issue.

On the other hand, the larger funnel? .4 MM. I decided to take a risk and it worked out well. At .4 MM the funnel prints quickly, and works well. It’s completely watertight.

So, to keep it simple. When setting up your prints in slic3r, think about the geometry of the piece you are making.

The best pieces are going to utilize a layer height and shell requirement that optimizes printing speed while still maintaining the integrity of the piece. Take some time while setting things up to look at how the piece is layed out. When slic3r completes, spend a few minutes checking out the preview in Repetier.

The last thing you want to see is a preview where it looks like single strands of filament have been carefully stacked on each other while barely touching. Those pieces rarely come out well.

Happy Printing in 2015!



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