The first work on slit rheometry was conducted by the group at Central Laboratory TNO, Delft, The Netherlands.1-3 In their work, they reported on velocity profiles and pressure measurements as well as the influence of slit aspect ratio. They experimentally determined that an aspect ratio of 10:1 or greater was sufficient to provide good results. Flush mounted pressure transducers were inserted into the die to measure the axial pressure drop. These mechanical measurements were complemented by optical measurements of melt flow birefringence in the 1–3 plane of the usual coordinate system of slit rheometry. Han and co-workers4-9 also constructed a slit type apparatus using flush mounted pressure transducers. Measurements were presented of wall normal stress distribution in the axial direction with the presence of an “exit pressure.” This pressure is the extrapolated value of the wall normal stress to the die exit, but is not equal to the primary normal stress difference as shown by Walter’s10 analysis of slit die rheometry. Slit rheometry has two conflicting practical limits to producing good results. Walter’s analysis assumes that the flow in the slit, where pressure measurements are being made, is two-dimensional and is fully developed. Thus, a long die is favored so that all pressure transducers are positioned to produce a constant axial pressure gradient. But as was presented by Robens and Winters, the temperature of the melt increases towards the end of the die due to viscous dissipation. Thus, a short die is favored for isothermal tests.
KeywordsIsothermal Test Exit Pressure Steady Shear Viscosity Silicone Fluid Engineering Thermoplastic
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