Abstract: A method of injection molding or extruding a polymer composition having a predetermined bulk density associated with particles, granules or pellets of the polymer composition and a predetermined melt density when the polymer composition is fully melted and compressed. The method of the invention employs a screw having a volume compression ratio that is greater than or equal to the ratio of the predetermined melt density to the predetermined bulk density of the polymer composition and up to 1.25 × the ratio of the predetermined melt density to the predetermined bulk density of the polymer composition.

Abstract: A screw comprising a screw shaft having a thread spirally positioned about the screw shaft so as to form a plurality of flights, said screw having a feeding zone, a compression zone and a metering zone, and means in said screw for providing a mass of material in a flight in the feeding zone that is substantially the same as the mass of material in a flight in the metering zone, wherein said means includes flights formed in the metering zone having a pitch and depth based upon the volume of the material in a molten state and flights formed in the feeding zone having a pitch and depth based upon the volume of the material in a bulk state. This design results in a screw which has a balanced mass flow, and thus a constant pressure gain along the screw without pressure peaks.