REALISTIC ANALYSIS OF FLOW IN WIRE-COATING DIES

We have constructed a rigorous numerical solution of the equations of flow and energy within a general wire‐coating die that provides a far more accurate and detailed description of the flow than any previously available. The program* can accommodate uni‐ or multi‐sectional dies, in which the sections have conical or cylindrical shapes. Melt rheology was satisfactorily represented over the full range of shear rate at processing temperatures by a model with three regions for each isotherm, requiring six constants evaluated from rheometric data. Thirty‐three cases have been run, all of them in the domain of moderate‐to‐high wire speeds, with the two most important wire‐coating resins, low‐density polyethylene (LDPE) and plasticized polyvinyl chloride (PVC). Although the radialconvection term was omitted from the energy equation to overcome calculational instability, we believe the accuracy of our calculated results is chiefly limited by knowledge of melt properties. Temperature profiles typically exhibit two maxima, the more pronounced one being found close to the die wall. In coating a No. 22 AWG wire with LDPE at 2000 ft/min in one cylindrical die, the maximum rise, occurring at the die exit, was 100 K. Profiles of axial velocity across the stream exhibited maxima in some cases, not in others. Shear rates ranged from zero to about 106 s−1 and viscosity profiles were highly variable. Because the melt passes through the die so quickly, the calculated results for isothermal and adiabatic walls were not very different, even though the melt streams were only about 0.3 mm thick. While the investigation was focused on high‐speed wire coating, in which viscous dissipation is extreme, the program can also be used for analysis and design of dies for making pipe and blown film. Copyright © 1979 Society of Plastics Engineers, Inc.