solid model
A type of computer model (of a physical object) that is defined geometrically as consisting of virtual 3D solid shapes rather than as other geometric forms (such as lines and points alone, wireframes, pixels, or voxels).
solid model: geometry that occupies space, not just surface
A solid model is a complete three-dimensional computer representation of an object in which the interior volume is mathematically defined, not just the outer skin. Unlike a wireframe (which shows only edges) or a surface model (which shows only faces), a solid model contains information about what is inside. This distinction matters enormously in manufacturing because a solid model can be interrogated for properties like mass, center of gravity, moment of inertia, and volume without additional work.
Solid models are built from Boolean operations on primitive shapes: cubes, cylinders, spheres, cones, and tori. A part is constructed by adding (union), subtracting (difference), and intersecting these volumes. For example, a through hole in a block is made by subtracting a cylinder from a rectangular solid. The CAD software tracks all the geometry and topology, so every edge, face, and enclosed volume remains mathematically coherent. This coherence is what allows downstream operations to work reliably.
The two major kernel technologies are boundary representation (B-rep), which stores the object as a collection of faces, edges, and vertices; and constructive solid geometry (CSG), which preserves the sequence of Boolean operations used to build the model. Most modern CAD systems use B-rep internally because it converts more readily to manufacturing formats like STL (stereolithography) or STEP files. B-rep can also be edited by moving faces and edges directly, which CSG cannot.
In manufacturing workflows, solid models feed directly into CNC programming, finite element analysis, and 3D printing. A solid model can be sliced into layer files for additive manufacturing, or it can be processed by CAM software to generate tool paths. When models are poorly defined, with gaps between surfaces, inverted normals, or missing faces, the downstream software will reject them or produce defective output. Model validation is a critical step before manufacturing.
The term solid can mislead newcomers. It does not mean the object is physically solid; a hollow box is a valid solid model. It means the mathematical representation is complete and unambiguous about what is inside the boundary and what is outside. A surface model of the same hollow box would lack this distinction and would fail if you tried to calculate its weight or check whether a point lies inside or outside it.