An extrusion is a 2D geometric description of a model, which can be extruded in the third dimension, specified by an extrusion vector, to create FLAC3D zones. Multiple sets of extrusion data can be created, each under a different name. At any time there may only be one currently active set. Commands on set objects (blocks, points, edges, segments) will apply to the objects in the currently active set only (see extrude set select), while mesh commands will apply to all sets.

Points are created in space, connected by edges, which are themselves connected into blocks. Edges are either linear or curved, in which case they follow a path given by control points. Blocks of two types can be created in the Extruder: blocks with structured mesh referred as “regular blocks” (these are typically 3- or 4-sided blocks) and blocks with unstructured mesh - “irregular blocks” (these blocks may have convex or concave boundary, may be multiply-connected and/or contain nested blocks). Both structured and unstructured meshes consist of triangular and/or quadrilateral elements. Upon extrusion, a single zone (brick or wedge) will be created from each element of the mesh.

Number of zones and their ratio can be assigned for every edge and they propagate automatically across all regular blocks attached to the edge. However, these parameters are unique for every edge in an irregular block; upon any changes of the edge parameters, including vertex locations, the unstructured mesh has to be re-created. A multiplier factor can be assigned to a regular block to increase (densify) the number of zones in it. Irregular blocks do not use multipliers.

Groups can be assigned to points, edges and blocks, which are then assigned to the gridpoints, faces and zones derived from them.

The information in this section is a reference on the extrusion commands and FISH functions. See The Extrusion Pane for instructions on constructing extrusions graphically in the interface.