Abstract: A heat sink for use in electromagnetic welding of molded parts includes reinforcing fibers embedded in a matrix material, where substantially all of the reinforcing fibers are oriented unidirectionally in a fiber direction, where the reinforcing fibers have a thermal conductivity at room temperature from 100-1000 W/m.° K and an electrical resistivity at room temperature from 0.5-10??.m, and where the matrix material comprises a high temperature resistant material, optionally a thermosetting resin, having a glass transition temperature Tg above 350° C. The heat sink is used in a method of connecting surfaces of a first molded part and a second molded part by electromagnetic welding. Cooling of the outer surface of the first molded part is provided by the heat sink in direct contact with the outer surface.