Using Boolean to Break Apart a Larger Model - Cutting Pieces from a Puzzle

Hello all! I’m new to Blender as of this week and I am learning it for a very cool project. As one of my final classes at university I will be casting in bronze a 30" x 24" model of the island of Ireland complete with natural wonders and ancient monuments.

To make the mold for the foundry I took mapping data of Ireland I am painstakingly drawing out the county lines. From there I’m using the boolean tool to break the larger model into individual counties for 3D printing.

Here’s what the current progress looks like:

To create the above I deleted the vertices off a cube and extruded it to form thousands of points tracing along the county lines. It’s slow but it works.

After that I delete the unnecessary lines to isolate (and enclose) a county as such:

Then use the boolean (intersect) tool to delete everything I don’t need:

From there it’s exported as a .OBJ, cleaned up, the exported as a .STL for Makerbot Print

The process works and I’m super happy with the results! However, sometimes the boolean tool behaves in a way I don’t expect and I cannot isolate a section. Here’s an example - this county of Ireland is isolated and closed off as I’ve done for all the others:

But, when using the boolean (intersect) tool it just leave the data in tandem with the cube:

Whereas boolean (union or difference) deletes the part I need:

What can I do to isolate selected sections for exporting?

This is a rather long first post so thank you for taking the time to read it. When the casting is finished I’ll post pictures of the final project!

Booleans can be unpredictable at times and you may have to mess around with its settings until something works.

What you can try to do is make the cutter a solid object instead (fill the top and bottom edge loops with faces). With intersect mode, you should be able to keep the inside portion more reliably. (I haven’t checked but making sure the normals of the cutter is correct might matter too, with faces selected, press Alt N and then recalculate outside).

Thank you for the help! Using two faces from a cube, I joined them to the cutter to make a single object:

Boolean (intersect) just yielded a ribbon of land:

Enabling Hole Tolerant did something really funky:

By normals I’m not sure what you mean. Pressing Alt+N didn’t do anything noticeable or pull up any menus. Could you explain that more? Also, is there a difference between join and merge in this application?

With capping the top and bottom of the cutter object, you can use the fill function rather than joining faces from a cube. In edit mode, select one of the edges all the way around (if you hold alt whilst selecting, the whole edge loop will be selected). Press F to generate a face that connects between what you selected.


There is a difference between geometry that is joined (and not connected) and geometry that has been merged together (connected). You’d preferrably want to use connected geometry for using booleans as it will ensure that Blender knows what part of an object is its inside and what part is the outside. With holes, you may not get what you expect as there is no inside.

The normals menu only opens in edit mode.

Both those tips have help immensely!

About half of the problematic sections have been fixed - I’m going to keep playing around with your suggestion and see if I can figure out a solution for the other sections

Now, all files can reliably be cut into their sections. With some files the parts of the walls and bottom are missing like in the example below:

The most import part (terrain detail) is present. With the other boolean options I can isolate the walls and using the face command put the bottom back on. Is there a way to merge the walls to a section where they are missing? If I can rejoin them as a single object it might solve the final issue. Thanks again!

Yes. Merge by distance. Overlapping vertices can be merged together to form a single mesh, and overlapping faces will get removed in the process.

First join the edges and terrain into 1 object (ctrl J).
In edit mode, select everything, press M to open up the merge menu, and click merge by distance.

Extra tip: pressing L when hovering over part of a mesh in edit mode will select everything that is linked (connected together). I use it in the video above to show what’s connected and what’s not.