Design of an orifice die to measure entrance pressure drop

The entrance pressure drop (DeltaP(ent)) associated with the flow from a cylindrical reservoir into a capillary is of great importance in melt rheology. First, it is required in order to determine the true wall shear stress in a capillary rheometer when only the overall pressure drop is measured. In addition, it is used to calculate an average extensional viscosity at strain rates that are not accessible using a true extensional rheometer. We recommend the use of a specially designed orifice die, in place of a Bagley plot, for the determination of this entrance pressure drop. This orifice has the following characteristics: its length to diameter (L/D) ratio is no more than 0.5, its entrance angle is greater than 90 degrees, and the exit of the orifice is designed to prevent melt from touching the die wall. (C) 2001 The Society of Rheology.