Designing voxel models

Designing volumetric propertiea is quite a difficult task in the current software culture. None of the standard CAD programs work with volumetric specifications (in my limited experience).

Gladly there is Autodesk Monolith. You can import an STL, which is converted into a voxel field with densities of 100% inside the STL and 0% outside.

You can then model your properties by painting the voxel colors; you need to uncheck geometry so that you only affect the voxel colors, not the densities.

Set the visualization alpha really low so that you can easily see what’s going on inside the model.

When you’re ready go to the rasterization tab and select to only export the material ratio. You can then export the voxel model to a sequence of images. The image lightness encodes the material ratio and the alpha channel encodes the material density.

This is a really nice tool if you want to draw volumetric material property specifications. It can be used to specify an image sequence to be used in Cura as a density voxel model for 3d printing with FDM.

Support Brim

Last week I started a print in TPU with PLA support, but the support failed because it doesn’t adhere to the brim which I printed in TPU.

To fix my print I used hotglue to (melt the support and) glue the support to the brim. That hack worked!

In the meantime I decided that this behaviour should be improved. I wrote a feature which replaces the regions under support with a brim printed in the same material as the support. That way the adhesion of the support to the buildplate is improved and the brim region next to the object is still satisfied – albeit with 2 different materials.

Robust Connect Infill Polygons

The features to connect infill polygons ntogether in order to get continuous extrusion for all infill patterns was not very stable. It is quite hard to reliably find a location where you can cross to another polygon and it often happens that that crossing doesn’t permit a crossing back, so that the bridge between two polygons cannot be created. The implementation is now improved.

Whereas it used to connect 65% of all polygons before it now connects 99.5%.

Hopefully this improvement can be merged to Ultimaker Cura shortly.

Big gray scale project

Today I had a talk with Alessandro Sambini, Joris v Tubbergen and others about an art project which is centered around creating a life-size statue of a person using my gray-scale halftoning technique.

We need to do some initial experiments to see how well the techniques adapt to higher layer heights and bigger nozzle sizes to see whether the project will go through, but I have full confidence.

Permeable prints

In some cases you might want a 3D print with a very permeable structure. For example when using a 3D print as a mold for vacuforming, the permeability is used to transport the air out. Another example is when combining 3D printing materials with casting materials; in order to optimize the adhesion between the two materials, the casting materials permeates a bit into the 3D printed structure.

There are several ways in which to create such a structure. In this post I describe one which consists of a couple of simple Cura settings.

The idea is to print the whole object with infill only, so we set the Top/Bottom Thickness and the Wall Thickness to zero.
Then we set the Infill Pattern to Zig Zag and the Infill Density to 50%.

Now here’s the catch: we set the Infill Line Directions to [ 0,0,0,0,0,0,90,90,90,90,90,90 ]

We end up with a print which is rather porous and has little resistance to air flow.

I’ve printed one with TPU. Here’s the result.
Top:

Side:

Voxel models

Cura now supports voxel models, by loading in a sequence of images if the first image ends in a number.

The XY are stretched so that the object fits inside while the XY ratio is maintained.
The Z of the images is independently stretched so that all images cover the whole 3D model.

I’ve used it to create some prints which are based on DICOM files from CT-scans.

Note that the surface model is extracted from the DICOM files in a separate program (3D slicer).
I converted the DICOM files into standard png images using MATLAB.

Then I converted the images such that the highest density equaled the density I had set in Cura and a cropped them to where the surface model reached.

This is the result:

This is a sample MATLAB code:


files = dir('osseux/*.dcm');
for file = files'
path = strcat(file.folder, "/", file.name);
a = dicomread(path);
min_val = 300.0 / 32768.0 + .5;
max_val = 1800 / 32768.0 + .5;
minn = min(min(a));
maxx = max(max(a));
a_mapped = uint16(int32(a) + 32768);
adjusted = imadjust(a_mapped, [min_val max_val], [0.0 1.0], 1.2);
[filepath,name,ext] = fileparts(file.name);
outfile = strcat(file.folder, '/output/', name, ".png");
imwrite(65535 - adjusted, outfile{1});
end

Don’t blame me if this crappy hacked together code doesn’t work on your machine! 😉

Here’s a sample converted .cdm file:

Evaluation meeting

Today I had to present my Evaluation report, which covers the research topic, the progress and the planning (a.o.).

I got a provisional Go, which means that if I keep it up I can expect to continue my PhD research after the Go/NoGo meeting at 12 months into the research.