# What is solid modeling?

Modern mechanical CAD relies on solid modeling.

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A solid model is shown rendered. Image: SOLIDWORKS.

Solid modeling is a type of 3D computer modeling in which Euclidean math is used to represent objects with properties of solids, such as mass and moment of inertia. Unlike other 3D modeling techniques, such as surface modeling, in which models have only their exteriors defined, or wireframe models, which only define their edges, solid models have a definition of the interior of a part. Wireframe and surface models can be topologically ambiguous in terms of what is inside or outside. You might not be able to tell if a point is inside or outside the part with a wireframe model. A surface model, unless it is “watertight,” can also be topologically confused. But with a solid model, there is no such confusion.

Because of this topological clarity, a solid modeling program can be more informative. Not only can it instantly provide mass properties (weight, centroid, mass moment of inertia) but it can also detect the interference of one part with another, aka clashes.

All the popular mechanical design software programs are based on solid modeling.

For industries in which weight is critical, such as aerospace, and where large assemblies are common (think of the Boeing 777, the first passenger airplane created with solid modeling), getting the total weight of the whole aircraft with practically push-button ease makes solid modeling indispensable.

Here are the basic solid modeling operations. Image: Stefano Papa, University of Naples, ResearchGate.

Basic solid modeling is primarily done in two ways:

1. Creating primitive shapes, such as spheres, rectangular solids, and so forth, and combining them into more elaborate shapes using Boolean operations (union, subtraction or difference, and intersection)
2. Creating a 2D profile and extruding it along a straight line or path or revolving it about an axis.

These methods generate prismatic solids. More sophisticated programs can create irregular solids that can be formed from 3D meshes, creating faceted solids or smoother solids that are made from blending B-splines or faceted solids or surfaces.

Solid models can be created with parameters (parametric modeling).

Solid modeling is useful for product design, digital prototyping, simulation and manufacturing.

A digital prototype can be used in place of testing. For example, a solid model of a crash test dummy that accurately simulates the behavior of a human body because it has mass and centroids can replace much time-consuming and expensive testing.

When simulation needs to be used, a solid model with a clear representation of a solid mass can be broken down into finite elements—something that is impossible with surface models and wireframe models in which the interior and exterior are ambiguous. When performing a fluid simulation, a solid body can be extracted from the fluid volume to model the fluid path, such as the solid model of a vehicle on a road surface.

During manufacture, a solid model can easily convey to a computer-aided manufacturing (CAM) program what needs to remain from stock material after cutting tools have gone through their paths. Therefore, solid models are used to create CAM instructions for machine tools.

Solid models are also used less often in architecture, such as in building information modeling (BIM) models. The most widely used 3D architectural program, SketchUp, does not use solid models by default.

Solid models lend themselves to rendering, as their surfaces can be given visual properties, such as color and texture or material, to create photorealistic images.

Animation can make use of solid models as they have certain physical properties and therefore can be made to move realistically with less effort than with wireframe or surface models, where the physics would have to be programmed.

The disadvantages of solid modeling are cost and the complexity of programming in a solid modeling application. Consequently, solid modeling applications typically cost more than 2D or wireframe modeling programs.

Most solid modeling programs rely on geometry kernels, which most of the time are supplied by other vendors. The most popular geometry kernels are Parasolid by Siemens and ACIS, which are owned by Dassault SysteÌ€mes. PTC uses its own kernel (Granite). Autodesk’s solid modelers were based on ACIS kernel code, but the company bought the ACIS code and has adapted it for its own use.