A new method for manufacturing by molding a component, composed partly of a hard thermoplastic material and partly of a soft elastomer material, is proposed. According to the method the first part (53) of thermoplastic material is first formed, then this first part (53) is placed in a mold capable to receive an elastomer material and consequently the second part (51) is formed by injection molding. Simultaneously the surface (58) of the first part (53) is engaged with the surface (54) of the second part (51) and said surface (58) of the first part is subjected to the injection temperature and pressure of the soft elastomer and causes to soften it and to fuse it to said surface (54) of the second part. The method is illustrated on the hand of a valve.