For the sake of future users, I have decided to periodically add tips and tricks in using COMSOL that I have figured out, most probably after hours of circuitous efforts. They will always be listed under the new COMSOL Tips category.
Today's topic: Intrusions
COMSOL has a very user-friendly interface for taking objects from 2D to 3D using the "extrusion" feature. But suppose one wants to design an object which contains screw holes or some other indentation. I've found that creating "punctures" in COMSOL is either impossible or very complicated.
Instead, COMSOL encourages users to always "add" to the object. In other words, one must form the lowest level first, then build layers sequentially on top using new work plane and boolean difference operators. This will probably be a bit clearer with an example:
1) First, create the planar projection in a work plane:
2) Extrude the first layer only in the regular fashion:
3) Add a new work plane which is offset in the z-direction to the deepest point of the intrusion.
4) Now, create the shape of the intrusion in this new work plane.
5) Use the Boolean "Difference" to let COMSOL know that, on this plane, the object has a hole.
6) Extrude once more from the second work plane to complete the intrusion.