Drahtbeschichtung - Development of flow optimized die geometries for the coating of enamelled magnet wires

The efficient and highly controlled coating of wires represents a key process step in the production of enamelled magnet wires. After each coating step, the deposited fresh enamel is dried and cured, as the wire further passes the corresponding sections in the wire enamelling machines. The coating procedure basically begins with drawing the wire through a bath of fresh enamel, where it obtains its primary deposition. Immediately afterwards the wire passes an enamelling die, whose gap height at the exit determines the desired deposition height. The flow field inside the die, which is driven by the fast moving wire, is associated with extremely high shear rates and high pressure peaks, similar to the flow inside journal bearings. The present work essentially attempts to develop a reliable and computationally efficient model for the description of this highly sheared flow. The model predictions shall finally help to derive a flow optimized die contour, which leads to minimum shear forces. The resulting reduction of the total drag force on the wire can strongly contribute to increase the productivity of the enamelling wire machines, as it allows for higher production velocities as well as the application of very viscous enamels, which follows from the trend to use enamels with higher solid content to deposit more solid mass per pass and save expensive diluent solution. Both possibilities would be otherwise strongly limited by the hazard of unacceptably strong deformation or even breakage of the wire. The computational model has further to account for non-Newtonian flow behaviour typically met in wire coating enamels, and for heat transfer due to the intense local generation of viscous heat. The proposed flow optimized die geometries are going to be realized in prototype dies and tested under realistic production conditions on a demonstrator machine at MAG. The present project is jointly funded by the industrial partner MAG and the Austrian Research Promotion Agency (FFG) within the general program framework.