Typical time scales for blow molding and thermoforming are rather short: from on the order of a tenth of a second for a milk bottle, to on the order of a few seconds for a gas tank. Hence, starting from a reasonable assumed initial rest state, stresses develop within that time interval. Although the deformation speed (and therefore the strain rate) is usually unknown, deformations remain moderate. Consequently, dynamic or linear measurements alone provide a good characterization of the melt for blow molding and thermoforming.

The selected spectrum of the rheological model may, of course, involve very short times and very long times. Actually, very short time scales (that is, those smaller than the typical process time) can be replaced by a purely Newtonian contribution. Similarly, long times (that is, much longer than the process time) probably do not have the opportunity to develop their own contribution to the stress, and can be omitted, if necessary.