Transient capillary rheometry: Compressibility effects

The “rise time” required to achieve a steady pressure reading in a capillary rheometer operated at constant piston speed can be very short or very long depending on the amount of the material in the barrel, its isothermal compressibility, the flow rate and the geometrical characteristics of the dies used [S.G. Hatzikiriakos, J.M. Dealy, Start-up pressure transients in a capillary rheometer, Polym. Eng. Sci. 34 (1994) 493-499]. When a short die having a large diameter is used, a maximum can also be obtained in the pressure transient, which is solely due to the compressibility of the material. This phenomenon is explained by means of a viscous mathematical model including compressibility, whose predictions are compared with experiments for a molten linear low-density polyethylene (LLDPE).