Extrusion apparatus and method

In plasticating apparatus of the type in which a rotating screw, including helical conveying flight means, is disposed within and cooperates with the inner wall of a barrel provided with inlet and outlet openings and wherein particles of synthetic resinous material are introduced through said inlet opening, plasticated by said rotating screw and advanced toward said outlet opening, the improvement comprising a screw having a section for assuring complete melting and mixing of said resinous material, said screw section being provided with a pair of channels extending side by side helically along said screw section with an outwardly protruding barrier flight being located between said channels, each of said channels having a base surface whose radius from the screw axis of rotation varies cyclically through at least four cycles along the helical length of such channel to provide a channel of varying depth, with the variations of said cycles being arranged so that the combined cross-sectional area of said channel is not constant entirely throughout a cycle, the maximum base surface radius portions of the channels on opposite sides of said barrier flight adjacent each area of increasing radius being undercut to provide sufficient clearance between its outermost surface and the inner barrel wall to permit the flow thereover of molten resinous material from the channel containing area of increasing radius to an axially adjacent area of the other channel to the pair.

In plasticating apparatus of the type in which a rotating screw is disposed within and cooperates with the inner wall of a barrel provided with inlet and outlet openings and wherein particles of synthetic resinous material are introduced through said inlet opening and into a helical passage formed by conveying flight means of said screw, plasticated by said rotating screw and advanced toward said outlet opening, the improvement comprising a screw having a section for assuring complete melting and mixing of said resinous material, said screw section including outwardly extending barrier flight means disposed intermediate said conveying flight means to divide said passage into channels extending side-by-side helically along said screw section, each of said channels having a depth which varies cyclically through at least four cycles along the helical length of such channel, the minimum depth portions of the channels on opposite sides of said barrier flight means defining wave crests which are displaced helically from each other, a substantial portion of each channel situated between successive wave crests in the channel being of substantially constant depth, the barrier flight means adjacent each wave crest being undercut to provide sufficient clearance between its outermost surface and the inner barrel wall to permit the flow thereover of molten resinous material from the channel containing an area of decreasing depth to an adjacent area of the other channel, the outer width of said barrier flight means being shorter than that of said conveying flight means so that the shear energy imparted to molten material flowing over said barrier flight means is low.

In an extruder of the type comprising a barrel provided with a cylindrical bore communicating with a lateral inlet opening for receiving particulate synthetic resinous material and communicating with axial outlet means through which molten synthetic resinous material is forced, an improved screw rotatable within said bore for plasticating the synthetic resinous material and delivering high quality molten material through said outlet means at steady but high throughput rates without overheating the material, said screw comprising a metering section near the discharge end of the screw for receiving the resinous material after a substantial amount of melting has occurred but while some incompletely melted material may be present in the material stream, said metering section being provided with a helical conveying land protruding into such close proximity to the wall of said bore to substantially preclude the flow of quantities of molten resinous material thereover and having a pitch such that the axial advance of a conveying flight along the screw in each 360 degrees about the screw axis is greater than the diameter of said bore, said section also being provided with a helical barrier land running parallel to said conveying land in the space between axially adjacent portions of said conveying land to divide into first and second channels the resinous material passage defined by axially adjacent portions of said conveying land and by channel bottom surfaces disposed closer to the screw axis than the outermost edges of said conveying land, said passage including portions of varying cross-sectional area along the length thereof, the width of the outermost edge of said barrier land being less than that of the outermost edge of said conveying flight and being less than eight percent of the conveying screw pitch, said barrier flight being undercut to provide sufficient clearance between said bore and said barrier flight outermost edge to permit flow of molten resinous material over said barrier flight outermost edge with low shear energy input to the material, said channel bottom surface in said first channel having a radius which increases and decreases cyclically over at least four cycles to give multiple crests and valleys in said first channel, said channel bottom surface in said second channel having a radius which increases and decreases cyclically over at least four cycles which are out of phase with those in said first channel to give multiple crests and valleys in said second channel, said crests having clearances with respect to said barrel bore in the range of from about 0.05 to about 0.10 times the square root of the diameter of the barrel bore and said valleys having clearances with respect to said barrel bore in the range of from about 2.5 to about 5 times the clearance between the barrel bore and the adjacent crests.

In plasticating apparatus of the type in which a rotating screw is disposed within and cooperates with the inner wall of a barrel provided with inlet and outlet openings and wherein particles of synthetic resinous material are introduced through said inlet opening and into a helical passage formed by conveying flight means of said screw, plasticated by said rotating screw and advanced toward said outlet opening, the improvement comprising a screw having a section for assuring complete melting and mixing of said resinous material, said screw section having a diameter of at least about three and one-half inches and including outwardly extending barrier flight means disposed intermediate said conveying flight means to divide said passage into channels extending side by side helically along said screw section, each of said channels having a depth which varies so that in each channel there are at least four spaced apart high shear regions with lower shear regions therebetween, each high shear region including a wave crest and portions on opposite sides of said wave crest having clearances with respect to the inner barrel wall not greater than 1.5 times the wave crest clearance, the wave crest clearances being small enough so that the resinous material passing thereover is worked but said high shear regions being not longer than twenty percent of the screw diameter to so limit the total quantity of energy imparted to the material that overheating is prevented, the wave crests being displaced helically from each other, and the barrier flight means adjacent each wave crest being undercut to provide sufficient clearance between its outermost surface and the inner barrel wall to permit the flow thereover of molten resinous material from the channel containing an area of decreasing depth to an adjacent area of the other channel, the outer width of said barrier flight means being shorter than that of said conveying flight means, and being less than eight percent of the screw pitch, so that the shear energy imparted to molten material flowing over said barrier flight means is low.