Unsteady mass transfer by water vapor diffusion is described with the same mathematical formalism as unsteady heat transfer by thermal conduction. In this context, concentration fields are calculated in the same way as temperature fields, with the material parameter thermal diffusivity being replaced by the diffusion coefficient. At the surface of the part, a saturated water vapor layer is always assumed, so that the mass transfer resistance can be neglected.


The calculation approaches of the software assume a diffusion-controlled mass transport. Capillary transport processes due to material porosity are not taken into account. Plastics with high proportions of solid additives (e.g. above 50 vol%) may, due to insufficient bonding between matrix and additive, possibly not satisfy the condition of pure diffusion.


The solution for water diffusion in plastics was already published in 1959: Jouwersma, C.: Die Diffusion von Wasser in Kunststoffe. Chemie-Ing.-Techn. (31), 1959/10, pp. 652–658. In this article, the author also critically discusses the approximation formulas commonly used up to that time (Eulitz; Blank).