3D printing : behind the scenes

As mentionned in A solid foundation , we printed our first full-scale sculpture last friday, and it was a success.

We were relieved because there were multiple factors that were not 3D printing-friendly :

  • Our scultpure needs to be hollow, because we need to be able to put all the PCBs and LEDs in it
  • Not only should it be hollow, it needs to be translucent, and thus very thin
  • The thickness should also be relatively constant in order to have uniform lighting
  • The sculpture has several parts which are almost horizontal

Why were these problematic ? Well, let’s see how 3D printing works

Slicing

The first step in 3D printing is to create a .stl file, which is the list of 2D polygons that make up the 3D model. Up to that point, there is no notion of volume or thickness, it only encodes a 2D surface embedded in a 3D euclidean space.

Then, the .stl file needs to be imported in what is called a slicer, a software specific to the 3D printer to be used. We used the Zortrax M200 from the Fablab in Telecom Paris (it proved to give better details than the Ultimakers 2 also avaible in the Fablab), and therefore used the slicer Z-suite.

A slicer slices a 3D .stl model in thin horizontal layers: the 3D printer will then print each layer at a step, building up from bottom to top by placing thin filament on each of the 2D layers. There are two importants factors for a slicer: the support structures and the infill.

The support structures are structures that the slicer will add so that the model won’t fall apart during the printing process. Each layers needs to be build upon the previous layer, so if a surface has a low angle it won’t have anything to support it and will fall apart. Support structures can be more or less removed after printing, and are more or less customizable in the slicer.

To convert a 2D surface to a 3D volume, the slicer will first give a thickness to your polygons, and then it will look for closed surfaces and will fill them depending on how much infill you ask for.

In our case, our slicer gave us a thickness of around 0.5mm, and we didn’t want anymore thickness so we used an infill of 0%

Near perfect result

Luckily for us, the model did not fall apart even though it was really thin, had very low angles and almost no support structure. We believe the dome shape of our sculpture is what made it hold together. There was however some imperfections inside the petals, which were visible when lighting it from under, but we should be able to remove them by milling or filing.

An unfortunate software limitation

The problem is we also need to worry about attaching the sculpture to our PVC plate to spin it at 30 rotations a second.

The best option we came up with was to have a couple of filled petals at the bottom (there are a few we won’t light anyway because they are more than half buried) to put threaded brass inserts. The problem is Z-suite doesn’t allow you to have both a shell with 0% infill, and petals with 100% infill, even though it should be technically possible to print… After trying many options, we figured the best option would be to print individual filled petals, and to then insert and glue them in our phyllo with epoxy glue.

The filled petals, with a hole in them to put the insert in.
To help visualize: the shell is in blue, and the filled petal we will insert is in red

I also printed a blueprint of the position of the holes which will be needed to drill the holes in the PVC

Feel free to comment if you have another suggestion for attaching our sculpture, we’re not 100% sure the epoxy glue will hold well during the rotation…

We will soon receive a new, more translucent 3D filament and will test everything with it, we will keep you updated !

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