Method for injection molding polyethylene terephthalate

A method of injection molding an undried polyethylene terephthalate employing a vent-type injection unit comprising a heating cylinder having an inside wall, a vent, a hopper and an injection screw rotatably and movably mounted in the cylinder, the length of said injection screw being divided into sequential first and second stages, each of said first and second stages being divided in sequence into a feed zone, a compression zone and a metering zone, said first stage also having a receiving zone connected to said feed zone of said first stage and receiving said undried polyethylene terephthalate from the hopper, said vent being located at a position corresponding to the feed zone of the second stage, said method comprising

• providing said undried polyethylene terephthalate in said hopper,
• causing said screw to rotate to thereby convey any polyethylene terephthalate present in the area between said injection screw and the inside wall of the heating cylinder,
• receiving said polyethylene terephthalate in the receiving zone of the first stage from the hopper,
• feeding and plasticizing said polyethylene terephthalate,
• wherein the feeding rate of the polyethylene terephthalate is limited by an arrangement such that the depth of the screw groove in the receiving zone is shallower than the depth of the screw groove in the feed zone, thus making an area available between two adjacent screw flights for molding the polyethylene terephthalate smaller in the receiving zone than in the feed zone of said first stage, and
• removing volatile substances from the polyethylene terephthalate through the vent.