Abstract: An improved method for creating cooling ducts in cast resin coils uses uniform cross section thermoplastic tooling segments. These segments are wound into windings between predetermined layers or radial sections, oriented parallel to the coil axis. They extend through the height of the coil. After the coil is cast, impregnated, or saturated with resin, and the resin is cured, the segments are withdrawn. The removal of a segment results in a continuous cooling duct. The thermoplastic segments are either solid machined from sheet or extruded with thin outer walls. Extrusion allows for uniform cross section relatively long somewhat flexible segments, which will cause the radius of curvature of the segment to conform to the shape of the coil due to the tension from the conductor and reinforcing material during the winding process. This will have the result of reducing the number of different segment designs required to support production of a variety of different diameter coils.