A single model engine for receiving and processing a 3D surface model representing the surface of a 3D object, the 3D surface model comprising at least two distinct surface regions associated with at least two different materials. The single model engine automatically produce a set of interior sheets inside the 3D surface model, the set of interior sheets defining interior boundaries and interior volumes of the different materials for the 3D object. The single model engine combines the 3D surface model with the set of interior sheets to produce a single unified model that represents the surface and interior volumes of the 3D object that comprise a single solid object having at least two different materials. At print time, the single model engine performs an export technique to produce an exportable form of the single unified model that can be received and printed by a 3D printer.