I meant to say Character Creator 3, not 3dxchange. There is an option to export as obj in T-pose or current pose. That's in the Pipeline version, not sure about others. Never tried it, but it should give you a single un-rigged  0bj file, which you might be able to print. 

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There are options in CC3 LOD to tie it all together into one mesh/material file.  But yea, I believe as Cary B says you then must bring it back in through 3Dexchange pipeline to get it rigged up.  I think....I got it that far but dont have pipeline so my attempts fell short when I wanted a single material file character to add popcorn fx fur to a character (https://forum.reallusion.com/445546/Fur-and-UV-Maps).

Here's a guess ... if you have the pipeline version you might try taking the character into 3dXchange and then exporting it as an OBJ file. Not sure if that would give you what you want, you would lose all your rigging. 

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Hi, I am new to Character Creator (and 3D modeling), and my question is - How do I merge all of my layers (not sure if this is proper term) of my character into 1 solid object? is this something i need to do in another application, or can i do this in Character Creator?

Once I am done with my avatar, accessories, posing, etc, is there a way to merge, join or close all open meshes to create 1 solid, "watertight" object? The end result an OBJ or STL that I can 3D print.

I don't know what the proper terminology of this action would be.  Any help is greatly appreciated.  Thank you!