How can I evenly split a cube? I am trying to make a mold to model the airway.  Is there a way to split a cube all the way through in multiple parts. I am trying to cut the cube in this fashion.  Is there way to cut it like this? I tried using the knife tool, but that only cut the surface, as opposed to cutting all the way through the object.
If you say the knife does not work for you, as it only cuts the surface, then you may want to use the option Z to Cut Through, for a snapping on angles to ensure a clean cut, you can also use C, so a cut from the front or any orthogonal view can be made in a straight line.

Here the optional functions of the knife tool:


A cut through cube:


Another option would be to use bisect, where you can also find some optional functionality mentioned while the tool is active.

Here a bisect example, bisect has the advantage that you can select the fill option and it will generate a surface that closes the cut:


I believe bisect might be a good idea as you can adjust not only the direction but also the position interactively.

Happy Blending.