Injection molding method and apparatus with reduced piston leakage

A metal injection molding apparatus is provided with features which reduce the amount of metal which enters the drive mechanism of the apparatus. The apparatus contains a piston having the head and the shaft, the shaft having a diameter smaller than the diameter of the head. At least one piston ring circumscribes the piston shaft. The apparatus contains an injection chamber having an accumulation portion and a shaft housing portion. The shaft housing portion has openings in the sidewalls and a polygonal internal cross section. The accumulation portion of the injection chamber is maintained at a higher temperature than the shaft housing portion.