Preparing models for 3D printing

Main stages: optimization, orientation, scaling, export to STL. Each stage can have its own challenges.
Technology: SLS

1. Optimization

Zbrush models typically contain millions of polygons, therefore they are too dense to be printed. A printing shop will either reject a model which is too heavy or they will try to reduce the density on their own. Luckily Zbrush has its own optimization tools which are quite good.

Zplugin menu → Decimation Master

First you should carry out preprocessing of the object (Decimation Master → Pre-process Current), and then, after setting the desirable parameters, you can start the optimization process itself (Decimate Current button). It should produce a mesh which looks exactly the same but is much lighter. (The trade-off is that Decimated mesh becomes uneditable for the most part).

When setting up Decimation I prefer to use the percentage parameter. Percentage indicates what density the optimized model will have in comparison to the original mesh (10% works fine for most models). Keep in mind that this procedure can be slow, and in some cases Zbrush may crash (if it runs out of memory). For that reason I divide the parts into groups of 3-4 million polygons. On the picture below you can see the same object before and after optimization. After the optimization the two meshes remain separate and they can easily be split into different elements by using Auto Groups operation.

2. Orientation

3D objects are printed by adding layers of material. As a result horizontal surfaces may have visible defects of this sort. If impossible to avoid, you can at least try to minimize them. Here’s an example:

Defects on the left are more distinctive than those on the right.

The most reasonable orientation in this case would be this:

3. Scaling and packing

Obviously before printing anything you have to set the correct scale of the model. For this particular doll I judged the overall scale by the size of the head. In other words I didn’t really care what was the precise length of the snake as long as the head was the way I wanted it. It had to be big enough so that I could comfortably string the doll and position the tongue inside the mouth. That’s why for this doll the size of the head was my main reference, and all the other parts were scaled in relation to the head.

Unfortunately there are no rulers in Zbrush and all parameters are relative. I’d made my own ruler out of small spheres and the size of the ruler is my main reference.

Now the packing. All the parts should be packed into cages. That way small parts don’t get lost or mixed up with parts from a different doll. I prefer to pack the tiniest parts into smaller box inside a big one.

The thickness of the bars should be at least 0.6-0.7 mm (you can use Deformation → Inflate to increase the thickness).

Then we assemble everything inside a box (not unlike playing Tetris). Spaces between elements should be at least 2 mm.

This box goes inside a bigger box.

Notice that now everything is in place and it’s almost ready for printing. But we still have to export the model, and the ruler is still in place, because we still need it.

But before we move on there’s still a step you can’t skip. It has to do with axis. Zbrush is an artistic software and it uses Y as it vertical axis. CAD-related apps (which regulate 3D printing) on the other hand use Z as it vertical axis. And since orientation is important you have to make sure that your entire set uses Z as it’s up axis. If not, you’ll have to rotate everything.

The model has to be exported as an STL file, because STL is the format used by 3D printers. Zbrush has a special plugin for it (Plugins → 3D Print Exporter).

As you can see, it has three parameters: X, Y and Z. Each defines the length of the model alone a specific axis. Since our ruler is in Z axis and we know it’s length, we can do the following.

First choose mm (we need metric system) and then press the ‘Update Size Ratio’ button. The numbers will change and now we can type in the length of the ruler into Z field. X and Y will again update with proportionally correct numbers.

Now we can hide the ruler (the ration will still be correct) and press STL button to export the model.

Final step in NetFabb. NetFabb Basic is a free software very useful for making sure you model has exported correctly.

Import your STL and first make sure the orientation is right. Then check the scale. It’s enough to see that length, hight and width are roughly correct, because if they are off they are not off by a fraction. Instead you’ll get crazy numbers, which is a good sign that you have to fix the model.
If the scale is way to big it probably means that you have a hole somewhere in your model. That sometimes can be fixed within NetFab (Repair function). If it doesn’t help, you’ll have to export your file with different parameters. But fixing models in extreme situations deserves it’s own tutorial.

Until next time!