Plastics extrusion machinery systems comprised of single screw or twin screw extruders, pipe heads, co-extrusion manifolds, pelletizing heads, sheet dies, electrical controls, temperature controllers, melt pumps, water troughs, vacuum sizing tanks, static mixers, screen changers, sheet cooling/polishing units, feed blocks, conveyors, pull-roll units, winders, thermoformers, power shears or traversing saws, "flying" knife cutters, sheet stackers, and replacement or structural parts thereof