Correct way to separate mesh into two subdivided parts

Question

New here, hope this is the proper board for this kind of question.

I'm relatively new to Blender, and I've had cases where I'd like to separate an object that's becoming a bit cumbersome to handle into separate meshes. For instance this, I made a helmet with a rim (or whatever you call the extrusion near the edges) like this:

Turns out the rims almost needed more vertices than the rest of the helmet, and handling their material etc. would be easier if they were a separate object. I tried keeping the whole thing relatively low poly and achieve the curves etc. by using subdivision surface, like so:

My question is, what's the "correct" or most common way to go about this; I could just select the faces near the edge and separate them & fill the missing faces to make them watertight, but then the subdivisions wouldn't make the helmet match the rim's lines etc. exactly anymore and there would be some ugly gaps between the two. I could approximate this by fiddling with edge creases & careful positioning of the vertices that "meet" the other object, but I'm not sure this is the way to go. Is there any way to "match" or attach vertices in an object into ones in another object, so the two would line up perfectly, while still keeping the objects themselves separate?

Or am I just doing it wrong, and keeping it as a single, bigger mesh would be preferred?

This might be a very basic thing, sorry about that - it's just something that's been bugging me and I'd like to learn the correct method instead of having to adjust something I came up with later on. Thanks in advance!

Answer 1

Based on my understanding of your problem, you could separate by material slots.

Assign different material slots for the fragments of your mesh you want to separate:

It doesn't matter what materials are assigned to the slots, e.g. a single material can be assigned everywhere and the mesh will still be separated - I use different materials to visualize how I want to split the mesh. The point is to use different Material Slots, not necessarily different materials.

Subdivide:

Add a simple geometry nodes setup:

For the 0 input of the geonodes modifier only the geometry with first (0 index) material slot will be visible:

If you duplicate it, and increase the input to 1, you will get seemingly the old shape, but separated into two objects:

Now you can for example apply the modifiers of one of the parts, subdivide it more, and sculpt it. Just keep in mind to Keep Corners:

Answer 2

a.Make your selection. Then you can create a vertex group out of your selection. If you miss anything it is easy to edit or reload.

b. In edit mode, You can either Press shift+d to duplicate your selection, to leave your model untouched, or skip this step to do a cut and copy

c. Press P and choose selection

You now have a separated part of your mesh. Other more advanced users may have different advice, but to me it is looking fine when you subdivided, so, consider keeping everything the way it is, and saving a selection group so that you can easily change the material of that detail.