The union, they say, is when you blend two or more 3D models together to create a new one.
It needs to be done the right way. Most of the time, the combination alone is not enough.
With the button Combine, we gather the items up, creating a group of objects. The items do not intersect with each other.
When you import 2 3D models inside MeshMixer, for example, two spheres, the Object Browser immediately opens up.
If it doesn’t, you can always click on the horizontal menu:
View > Show Object Browser and it’ll pop up.
Hold Shift and left-click on the elements to select them. A dropdown menu will appear. Click on Combine.
After this, you’ll notice the presence of two 3D models, inside the program.
Select both of them, one by one. Then, click on Combine; this way you’ll create only one group, objects included.
Now, your two objects are combined with each other. However, they aren’t intersected.
As you can see in the following picture, there is an error in the combination of their insides.