Design of an orifice die to measure entrance pressure drop

The entrance pressure drop (DeltaP(ent)) associated with the flow from a cy lindrical reservoir into a capillary is of great importance in melt rheolog y. First, it is required in order to determine the true wall shear stress i n a capillary rheometer when only the overall pressure drop is measured. In addition, it is used to calculate an average extensional viscosity at stra in rates that are not accessible using a true extensional rheometer. We rec ommend the use of a specially designed orifice die, in place of a Bagley pl ot, for the determination of this entrance pressure drop. This orifice has the following characteristics: its length to diameter (L/D) ratio is no mor e 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) 200 1 The Society of Rheology.