For 2D blow molding or thermoforming problems, standard volume elements (that is, quadrilaterals and triangles) are always used, because CPU time is not prohibitive. For 3D cases, however, the use of shell elements is strongly recommended. Shell elements are either defined on a boundary of a 3D volume region, or simply generated as 2D elements in 3D space.

The basic assumption of the shell element is that the thickness is small compared to other dimensions of the parison. In this case, the thickness is taken as an independent variable that is defined over the calculation domain (that is, the shell). The mass conservation equation, when derived for the shell elements, leads to a transport equation for the thickness of the parison. The thickness appears as a separate variable that replaces the pressure and is computed together with velocities and node positions.

Calculations involving shell elements are not only much faster than those for volume elements, they are also much more robust in 3D problems because there is no need to maintain good mesh quality in the direction of the thickness. The shell element’s major assumption is that shear effects are neglected.

See Generating a Mesh with Shell Elements for information about creating a mesh with shell elements in it.