Extrusion apparatus and method

In accordance with the present invention an extruder screw having a helical conveying land is provided with a helical barrier land therewithin to form a pair of parallel helical flow channels. Each flow channel includes repeating wave cycles. Each wave cycle is of varying depth, having alternating wave crests and valleys. The wave cycles of the adjacent channels are helically offset so that a wave crest of one channel lies opposite a valley in the adjacent channel. The barrier land is undercut in a manner which facilitates the flow of melt thereacross from one channel to the other, while restricting the flow thereacross of unmelted solids. Solids thus tend to pass through a restriction formed by the wave crest while the melt travels across the barrier flight into the adjacent channel, thereby minimizing the formation of pressure pulses in the extrudate flow. The clearance above the barrier flight is small, and the width of the barrier flight is very narrow so as to impart little energy input i.e., heat, to the melt. Melting of the solids passing over the wave crest is maximized by dimensioning the wave crest restriction small enough to assure intensive working of the solids. The clearance between the wave crest and the wall of the surrounding barrel is in the range of from about 0.05 to about 0.10 times the square root of the diameter of the barrel wall. The clearance between a valley and the barrel wall is about 2.5 to about 5 times the clearance between a wave crest and the barrel wall.