The method of an embodiment of the present invention comprises the following steps: (A) filling a melted thermoplastic material for a first time; (B) closing the forming mold and blowing air into the forming mold; (C) opening the first forming mold and removing the chair legs; (D) placing at least two of the chair legs into a second forming mold; (E) filling the melted thermoplastic material for a second time; and (F) closing the second forming mold and blowing air into the second forming mold.