Abstract: In the injection molding of undercut and continuously slotted duct sections from a plastic material where a multi-part mold structure including a core is used, the core of the mold structure is removed, after the injection molding operation, by cooling the plastic material used to a temperature below its softening temperature, releasing the bearing contact of the mold supporting the lateral exterior of the duct section and the exterior base of the duct section, providing a bearing contact with the surface of the duct section upwardly from its base with the bearing contact affording a force restraining the duct section from movement in the direction outwardly from the slotted opening of the duct, and pulling the core outwardly through the continuous slot while the bearing contact with the duct section holds it against displacement due to the pulling action.