For 3D blow molding models solved using shell elements, Ansys Polyflow can also compute the volume of the blown product. This is actually the volume defined between the parison (or sheet) and an oriented reference plane. The oriented reference plane is defined by

(17–21)

A point on the volume with coordinates () is considered if

(17–22)

For example, consider an oriented horizontal plane whose "positive side" is above. Typically, if the blown product is a sphere and the oriented plane is located below the sphere, the algorithm will evaluate the total volume of the sphere. The volume lying between the plane and the lower part of the sphere is actually evaluated twice, once with a positive sign and once with a negative sign, so that the two contributions cancel each other. If the plane cuts through the sphere, the volume of the part of the sphere above the plane will be evaluated. See Inputs for Computing the Volume of the Blown Product for details about setting up the calculation.

When solving a blow molding or thermoforming case with the shell model, the solver evaluates the actual volume of material at each time step. The result is written in the listing file, as well as in a probe file that can be displayed using Ansys Polycurve. This quantity is noteworthy, as it is an indicator of the solution quality. Overall variations of the volume of material around 1% are quite normal. Also, when using the default settings for the time marching scheme, the calculation is performed with an actual tolerance of 1% for each time step. Under certain circumstances, visible changes in the volume of material beyond the set tolerance can be observed after a large number of steps, even though the tolerance criterion is satisfied. When this happens, it is recommended that you rerun the calculation with a more severe tolerance (for example, 0.1% or less). For additional information about the tolerance, see the step about the Numerical Parameters in Setting Up a Time-Dependent Problem.