High output twin screw extruder process
High output twin screw extruder process of rigid PVC profile other than pipe is disclosed in a twin conical single screw extruder, equipped with high shear screws. The process includes extruding a compound specially adapted for the process, the compound contains a polyvinyl chloride resin with an average particle size of from 130 to about 200 microns, a lubricant system containing no more than about 3.5 parts of one or more lubricants, an organo metal salt, from 0 to less than 0.75 parts of a polymeric processing aid, at least one filler/pigment, and a stabilizer for PVC. The process achieves extrudate surfaces characterized by a smooth, dull finish. The process advantageously operates at high extrusion output rates under the conditions specified.
High shear sheet production extruder provide compression of the powder compound. Compression is provided either by decreasing the screw flight depth or the lead or a combination of both. The degree of compression can be either single or double compression. The term "high shear" also includes what is referred to now in the industry as the term "ultra high shear". Ultra high shear screws have preferably double compression. Referring now to the feed section, there are two stages, the first stage and the second stage. A high shear twin screw design is exemplified by single compression in the first stage and slotted flights (or back cut flights) toward the front end of the feed section. The depth of the slots can be from a few millimeters up to the entire depth of the flight down to the root member. Slots which are cut in the flights are either excavations in the side of the flight or slots which are cut through the flight. Slots allow back flow of the masticating melt, increase the shear action and turbulence of mixing. An "ultra" high shear screw is exemplified by double compression and deep slots cut in the flights in both the first and second stages of the feed section.
The preferred high shear and "ultra" high shear twin screws are now commercially available from Davis Standard®, a division of Crompton and Knowles Corp., under the part numbers 350754 and 350734, respectively for use with a model GC-8, a 61 mm twin screw extruder. Part numbers 350750 and 350731 refer to high shear and "ultra" high shear twin screws, respectively, for use with a model GC-6 a 41 mm twin screw extruder available from Davis Standard®.
The process of the present single screw extruder is readily adapted for co-extrusion. Co-extrusion of grooved clear-edge louvers is disclosed in U.S. Pat. No. 4,628,980. The grooved or clear edged louver is typically embodied as flanges and provide for a decorative panel insert. The body of the louver inward of the clear edges is formed using the compounds adapted herein for the conical twin high shear screw process. The co-extrusion is provided by passing two melt streams through a slotted die. Slotted dies can be fabricated by skilled practitioners and utilized in a conventional manner with the exception that in the process of extruding the slat or flat portion between the edges, a conical twin screw extruder set must contain the high shear screws and the processing conditions noted herein below should be followed.