Hot to calculate the emty cavity volume in the assembly model ?

Unless there is a better way that I am not aware of, I suggest making a volume that encompasses your empty volume (a "block").

You assembly the part for analysis and the "block" you just created.

Assemble them to where the empty volume, or level is filled.

Here is the trick: Activate the "block" part in the model tree.

Now find the merge command.  There are several ways to achieve this.  It is in essence the way Creo does Boolean operations.

Once the merge is complete, you should have your part removed from the "block".

Now trim away the areas outside the inner volume with a series of revolve-cuts or extrude-cuts... and whallah, you now have a analysis part that has the mass properties including ... volume.

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finding volume of voids