PURPOSE: To make denser the peripheral structure of a part for driving dog spikes, by a method wherein a metallic mold for forming a lower holes in a sleeper at positions designated for driving dog spikes to fasten a rail is separately provided in the interior of a metallic mold for molding a sleeper from waste plastics, and the former mold can be moved transversely as the molded article is cooled.

CONSTITUTION: A metallic mold frame 10 is provided with a pouring port 12 at the center of the upper surface of the frame, and fitting holes 15 for the metallic mold 13 for forming the lower holes are provided on the left and right of the port 12. The fitting holes 13 are in the shape of slots so that the mold 13 can be moved transversely, and the mold 13 is fixed to the fitting holes 15 by thumb screws 14. A molten waste plastic is poured into the metallic mold and is thrown into water together with the mold, but immediately before that, the screws 14 are untightened so that the mold 13 can be freely moved transversely in response to the shrinkage of the molded article to occur by cooling. Accordingly, the peripheral parts of the holes 7 are also rapidly cooled by the mold 13 and become a dense structure.