In blow molding and thermoforming, extension is the dominating component of the deformation. Extension develops in two main directions, and is accompanied by a reduction in thickness. The magnitudes of the extensions in the main directions may differ, and they depend on the geometry. For example, inflating a cylinder involves an azimuthal extension field, while inflating a sphere involves an isotropic extension field.

Since extension is the main component in this process, knowledge of the extensional response of the material is usually necessary. However, although the extension rates can be high, the overall deformation usually remains moderate. The Hencky strains are typically on the order of 1 or 2, which correspond to Cauchy strains up to 7.