I’m trying to create a boolean union between two closed polysurfaces (for 3D printing) - they both intersect and are closed, so I don’t see why the boolean union is failing?

First of all, 1,467 intersections is a lot! Check for

Are there any co-planar faces? Move the walls slightly up or down

Is your tolerance accurate enough? Increase / add more zeroes.
If your part is small, then the intersections will be much smaller.

My boolean back up plan B is to to split each part to the other part:
Split A to B
Split B to A
Join A to B
Delete excess.

If you can only split the first (regardless of order selected) then my boolean back up plan C
Copy both A & B, move an exact distance away. You now have two sets.
On first set, split A to B (delete B)
On second set, split B to A (delete A)
Move one of the parts back so they are aligned (same exact distance)
Join A to B

It’s now at about 100 intersections. However, it’s not letting me do any booleans. Even though they intersect, it won’t let me split A or B or do anything.

Its such a complex intersection, you may need to move and scale the two parts before either a boolean or split will work.

Also, did you make 100% sure that the two pieces are closed AND there are no naked edges? Try these two:
► Analyze → Mass Properties → Volume, any number means it is closed
► Analyze → Edges → Show Edges, you should see 0 naked edges, no errors
If there is an issue with these two checks, then fix the geometry first before proceeding.

This is a good example on why a workflow or strategy is critical. Its just hard to know where the problems will be far down the road when you’re just getting started on the model.

Hi Vivi - if you look closely at the intersection curves, you’ll see that they are pretty messy - that is, there is no clear intersection among the parts - surfaces are nearly parallel between the two parts - while this theoretically maybe should work, it is not a good situation at the best of times. In addition the objects’ edge tolerances are larger than the file tolerance - you can see this in Wireframe views if you zoom in close to the area where the objects meet - this will lead to unexpected intersection curves as well.

Also - looking closely at the way the sides and bottom come together, I suspect this is not really what you intend anyway - see how poorly the elements fit together there? Little edges and corners sticking out etc.

I think you might try to make the cup shape first, as a simple revolve, then cut all the shapes out of it.

Pascal is absolutely right. The fact that you have 100 intersections and it refuses to join / trim / or boolean is telling you that you used the wrong strategy. Also, the two surfaces are too complex and could not be manufactured as currently designed… which is another reason to re-build.

Here is my visual analysis to help illustrate what I was saying. You have so many co-planar faces, there is no clear overlap to perform the operations discussed. When I said scale or move, I meant trying to avoid the co-planar situation. The second option is to create a transition geometry that is clean and closed – and boolean it with each part separately. Just make sure that every edge is inside your transition geometry.