apparatus for injection molding plastic parts . the method includes the step of melting and pressurizing plastic material , directing the molten pressurized plastic material into passages leading to a mold , a selectively opening shut - off valve disposed along the passages , selectively closing the shut - off valve after a predetermined amount of molten pressurized plastic material has entered the mold and cooling the predetermined amount of molten pressurized plastic material in the mold to form a part . the apparatus includes an extruder providing pressurized molten plastic material , a mold adjacent the extruder , passages interconnecting the extruder with the mold , and a selectively operable shut - off valve disposed along the passage .