WO2007033328A2

WO2007033328A2 - Method and apparatus for forming a synthetic extrudate

Info

Publication number: WO2007033328A2
Application number: PCT/US2006/035855
Authority: WO
Inventors: Shashank Gulabchand Kasliwal; Jerry William Jones
Original assignee: B & P Process Equipment And Systems, L.L.C.
Priority date: 2005-09-14
Filing date: 2006-09-14
Publication date: 2007-03-22
Also published as: WO2007033328A3

Abstract

A method for forming a synthetic extrudate in a mixing apparatus (10) that includes a pair of screw shafts (16) having intermeshing co-wiping screw sections (18) within a mixing chamber (14) and having conveying portions (57) rotatably cantilevered for self-journaled support within respective conveying chambers (22) downstream from the mixing chamber. Polymeric and organic material is fed into the mixing chamber, the polymeric material is heated to a molten state, mixed with the organic material and degassed while the polymeric material is in a molten state, and the mixture is conveyed downstream through the respective conveying chambers and discharged.

Description

METHOD AND APPARATUS FOR FORMING A SYNTHETIC EXTRUDATE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[001] This invention relates generally to an apparatus and processes for producing synthetic composite materials, and more particularly to co-wiping, co- rotating, twin screw extruders, and processes of mixing ingredients therein to form a synthetic extrudate.

2. Related Art

[002] To meet an increased demand for wood products, and to decrease the burden on diminishing natural resources, simulated wood products have become increasingly popular. In developing manufacturing processes for simulated wood products, it has proven difficult to achieve a process for producing an end product of sufficient strength and appearance in a reasonably cost effective manner. Some efforts have been made to manufacture simulated wood products formulated from a composition of polymeric materials with ground wood material and/or fibrous cellulosic material obtained from recycled waste paper products. Unfortunately, these efforts have typically been met with cost issues, and issues related to poor quality that can result from non-uniform distribution of the composite materials as well as extruder-based degradation of the composite materials caused by overheating that results from non-uniform flow of the materials while they're in a molten state.

SUMMARY

A method is provided for forming a synthetic extrudate. The method includes providing a mixing apparatus (10) that includes a pair of screw shafts (16), intermeshing screw sections (18) of which are supported for intermeshing co-wiping rotation in the same sense about respective generally parallel axes (53) within a mixing chamber (14), and conveying portions (57) of which are rotatably cantilevered for self-journaled support within respective conveying chambers (22) disposed downstream from the mixing chamber, the screw shafts being operably attached to a drive mecham^'sm (55). Polymeric and organic material is fed into the mixing chamber and the polymeric material is heated to a molten state. The molten polymeric material is mixed with the organic material within the mixing chamber to form a generally homogeneous mixture and the mixture is conveyed downstream along the mixing chamber from the downstream inlet. The mixture is degassed while the polymeric material is in a molten state and is conveyed downstream from the mixing chamber along the respective conveying chambers and discharged. hi addition, a mixing apparatus is provided that comprises a mixing chamber (14), an inlet (41) configured and positioned to receive material into the mixing chamber, and a pair of co-wiping screw shafts (16) supported for rotation about respective generally parallel axes (53). Each screw shaft is operably attached to a drive mechanism (55) configured to rotate the screw shafts at the same speed and in the same sense. The screw shafts include respective midsections (68) disposed within the mixing chamber and having screw sections (18) adapted to intermesh with one another. The screw shafts also include respective conveying portions (57) rotatably cantilevered for self-journaled support within respective conveying chambers (22) arranged generally parallel to one another downstream of the mixing chamber. The conveying portion of each screw shaft has an outer diameter smaller than an outer diameter of the midsection of each screw shaft. The conveying portions are configured to convey mixed material axially downstream from the screw shaft midsections toward an outlet (59) disposed axially downstream from the conveying portions of the screw shafts.

BRIEF DESCRIPTION OF THE DRAWINGS

[003] Some of the objects, features and advantages of the invention will become readily apparent in view of the following detailed description of the presently preferred embodiments and best mode, appended claims, and accompanying drawings, in which:

[004] Figure 1 is a schematic partial cross-sectional top view of a portion of a mixing apparatus according to one embodiment and showing a pair of screw shafts used in a cooperative manner to produce a synthetic composite wood material;

[005] Figure 2 is a view similar to Figure 1 of a portion of a mixing apparatus according to another embodiment;

[006] Figure 3 is a top view of the screw shafts of the embodiments of

Figures 1 and 2;

[007] Figure 4 is a schematic partial cross-sectional top view of a mixing apparatus according to one embodiment showing separate extrudates being discharged through separate profile dies, the material forming the extrudate having been partially removed for clarity;

[008] Figure 5 is a view similar to Figure 4 according to another embodiment and showing a single extrudate being discharged through a single profile die;

[009] Figure 6 is a view similar to Figures 4 and 5 according to yet another embodiment and showing material being passed through a pelletizer;

[0010] Figure 7 is a perspective view of a pair of screw tips adapted and positioned for attachment to downstream ends of screw shafts of each mixing apparatus of the above-described embodiments;

[0011] Figure 8 is an end view of the screw tips of Figure 7 with the rotational sweep of each screw tip shown in phantom;

[0012] Figure 9 is a side view of the right hand screw tip of Figure 8 taken from direction 9;

[0013] Figure 10 is a side view of the right hand screw tip of Figure 8 taken from direction 10;

[0014] Figure 11 is a cross-sectional view of the extruded synthetic wood material taken generally along line 8-8 of Figure 4; and

[0015] Figure 12 is a partial orthogonal view of screw sections of mid portions of the screw shafts of the mixing apparatti of Figures 1 and 2; and

[0016] Figure 13 is an axial cross-sectional view of the screw sections of

Figure 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Referring in more detail to the drawings, Figure 1 illustrates a portion of a twin screw mixing apparatus according to one embodiment, and referred to hereafter as an extruder 10. Extruders 10 of this type are readily available from B&P Process Equipment, Inc. in Saginaw, Michigan, and are known to be useful for producing a synthetic extrudate, such as, by way of example and without limitations, a synthetic wood or lumber material 12. Ingredients may be conveyed, mixed, and devolatilized (dried by application of heat and vacuum) to form a generally homogeneous mixture in a common mixing chamber 14 of the extruder 10. The mixing chamber 14 may have a generally figure 8-shaped continuous inner wall 15, as viewed in lateral cross section, and may carry a pair of generally parallel screw shafts 16 that are rotatable in the same direction or "sense" within the mixing chamber 14. The screw shafts 16 maybe arranged for co-wiping along intermeshing screw sections 18 of the screw shafts 16 and may also be arranged for co-wiping along mating mixing elements 20. As shown in Figures 12 and 13 the screw configurations of the screw sections 18 of the screw shafts 16 may also include undercut conveying elements 19 to improve material flow through the mixing chamber 14 despite low bulk density and obviate the need for the use of a stuffing device to keep such material moving through the mixing chamber.

[0013] Downstream of the mixing chamber 14, the mixture diverges into a pair of individual radially spaced conveying chambers 22 as is best shown in Figure 4. The basic structure is disclosed in prior U.S. Patents 3,195,868; 4,752,135; and 5,439,286 which are incorporated herein by reference and have been assigned to the present applicant.

[0014] The conveying portions 57 of the screw shafts 16 extend through the conveying chambers 22 to convey separate flows of the mixture under relatively high pressures of up to 25.86 MPa. In other words, rotation of the screw shafts 16 causes the conveying portions 57 of the screw shafts 16 to convey the mixture downstream from the mixing chamber 14 along the respective conveying chambers 22. The separate flows may then be discharged axially downstream and may be merged into a single mixture within a transition manifold or single discharge cavity 24 that is disposed downstream from the conveying chambers 22 and is in fluid communication with the conveying chambers 22 as shown in Figures 4-6. A combined flow may then be discharged axially from the discharge cavity 24 either through multiple profile extrusion dies 26, as shown in Figure 4, to facilitate forming separate extrudates 12, merged through a single profile extrusion die 28, as shown in Figure 5, to facilitate forming a single extrudate 12, or through a pelletizer 31 as shown in Figure 6 to produce compounded pellets 33 comprising organic and polymeric components. In either case, the conveying portions 57 of the screw shafts 16 rotating in the conveying chambers 22 provide sufficient propulsive force to the mixture to force the mixture through the die 26, 28 without further propulsive forces being applied to the mixture between the screw shafts 16 and the extrusion die 26, 28 other than screw tips 58 as described below. The die 26, 28 may be provided as disclosed in U.S. Patent No. 5,516,472, by way of example and without limitations, which is incorporated herein by reference. [0015] As shown in Figure 1, the extruder 10 may include an elongate processing chamber 29 formed within an extruder body 30, extending between an inlet end 32 and a discharge end 34 of the extruder 10, and including both the mixing chamber 14 and the conveying chambers 22. The extruder body 30 may comprise a series of abutting successive barrel sections 35, 36, 37 coaxially arranged in an end- to-end fashion. The extruder body 30 is represented here, by way of example and without limitations, as including seven such barrel sections 35, 36, 37, though the extruder body 30 could include more or fewer barrel sections. The number of barrel sections 35, 36, 37 included in the extruder body 30 typically depends on a variety of factors, including, by way of example and without limitations, the type of finished material to be formed, the type and size of extruder being used, and space limitations. For premixed, dry ingredients, the ratio of total mixing chamber length to barrel bore diameter (L/D) is about 33, which includes an L/D of about 3 for a single screw conveying chamber barrel section 35, 36, 37. In the embodiments of Figures 1 and 2, the first six barrel sections 35, 36 define the mixing chamber 14, while the last barrel section 37 defines the separate conveying pressure-building chambers 22. A partition wall 38 is located adjacent and axially downstream from the mixing chamber 14 to define, in part, a pair of separate, generally cylindrical inner walls 40, which in turn define the separate conveying chambers 22.

[0016] As shown in Figure 1 the ingredients may be introduced through a main upstream feed inlet 41 disposed adjacent an upstream end 32 of the mixing apparatus 10. The ingredients enter into a first barrel section 35 of the barrel sections 35, 36, 37 through the main upstream feed inlet 41 via an inlet hopper 42, such as through the use of gravimetric or volumetric feeders. The ingredients may include a polymeric material in the form of a thermoplastic resin, such as, high density polyethylene (HDPE), low density polyethylene (LDPE), polyvinyl chloride (PVC), or polypropylene (PP), for example, and/or an organic material such as wood flour, pellets, or fibers, and/or wastepaper, for example. Other organic ingredients may include natural fibers from kenaf, flax, rice hulls, jute, sisal, coconut, and hemp, for example. Additionally, additives such as, antioxidants, UV stabilizers, colorants, impact modifiers, and lubricants can be included to facilitate manufacturing efficiencies and to enhance the material properties of the finished product. The ingredients may be in powder or pellet form and may be pre-dried to a moisture content of about 2 percent or less. [0017] The first barrel section 35 may have an inlet portion 44 arranged to receive the ingredients from the inlet hopper 42 through the upstream feed inlet 41. The inlet portion 44 of the first barrel section 35 may have an L/D of about 2.5 and may be cooled to about 25 degrees Celsius (⁰C) to prevent premature melting of the ingredients. Downstream from the inlet portion 44, the first barrel section 35 may have an exit portion 46 with an L/D ratio of about 2.5 and may be heated and cooled to facilitate regulating the temperature of the ingredients at about 160 ⁰C. The barrel sections 36 of the mixing chamber 14 downstream from the first barrel section 35 may be heated and cooled to maintain the mixture at a temperature in the range of about 150-175 ⁰C. The temperatures of the barrel sections 35, 36, 37 may be regulated independently from one another to assist in controlling the temperature of the mixture as it flows from one barrel section 35, 36, 37 to another. To facilitate controlling the temperature of the ingredients as they progress along through the respective barrel sections 35, 36, 37 the extruder barrel chamber 30 may be jacketed or cored, as disclosed in the previously referenced '135 patent.

[0018] Upon being introduced into the apparatus 10, the ingredients are mixed and conveyed along the barrel sections 35, 36 by the intermeshing co-wiping screw sections 18 and mating mixing elements 20. The screw shafts may be driven at a rotational speed in the range of between about 150-250 rpm, by way of example and without limitations, to provide a throughput rate of about 181 kg/h for a 50 mm extruder, for example, and up to about 3700 kg/h for a 160 mm extruder, for example. [0019] To degas and regulate the moisture content of the mixture at least one mixing barrel may include one or more degassing vents 48 that may be in the form of lateral vacuum vent sniffers as shown in Figures 1 and 2. In the embodiment of Figures 1 and 2, two of the mixing barrels include vacuum vent stuffers dedicated for use as degassing vents 48. The vents 48 may be equipped with single helix twin screws 50 and vacuum pumps 49 that cooperate to keep the barrel ingredients under a vacuum or relative negative pressure to assist in devolatilizing water and other volatiles from the mixture, while also preventing the inadvertent escape of composite mixture therefrom. The use of two of these vents 48 for this purpose allows high moisture content material, e.g., material having greater than 6 weight percent moisture content overall, to be introduced into the device 10 without pre-drying. In other words, the mixture may be devolitalized through the use of stuffing devices and the application of staged incremental vacuum to the stuffing devices to degas moisture and volatiles in the mixture.

[0020] A second embodiment of an extruder 110 is shown in Figure 2. Parts of the second extruder embodiment 110 that correspond to identical or similar parts of the first extruder embodiment 10 are indicated by the same reference numerals that are used to describe those parts in the first embodiment above. The second extruder embodiment 110 may incorporate a downstream side inlet 51, which may be a vent stuffer as shown in Figure 2, fed by a side inlet feeder 52 arranged between an inlet end 32 and a discharge end 34 to allow at least one of the ingredients to be introduced downstream from the upstream inlet 41 and main inlet hopper 42 at the inlet end 32 of the apparatus 110. As such, one or more of the ingredients, such as the polymeric materials or a mix of polymeric and organic materials, can be introduced into the mixing chamber 14 through the upstream inlet 41 via the main inlet hopper 42 and then heated by the combined application of external barrel heating and mechanical shear agitation, preferably until polymeric components reach a molten state, before the introduction of other ingredients through the downstream inlet 51 via the downstream side inlet feeder 52. This allows any of the more heat-sensitive and/or shear-sensitive organic materials, such as wood fibers or paper based materials, for example, to be introduced downstream from the less temperature sensitive materials, thereby preventing or at least minimizing shear and heat damage to these thermally sensitive organic components. As such, polymeric materials, or other ingredients having relatively high moisture content, such as about 3% by weight or more, can be introduced into the mixing chamber 14 through the upstream inlet 41 via the main inlet hopper 42. The polymeric components can then be brought to a molten state via heating elements and/or friction imparted by shearing caused by mixing elements 20 upstream of the downstream inlet 51, shown here, for example, as being arranged in the second barrel 36. Upon being brought to a molten state, materials including, for example, heat or shear sensitive organic materials and, optionally, polymeric materials, can then be introduced into the mixing chamber 14 via the downstream inlet 51 to mix with the molten material. Otherwise, the extruder embodiment 110 of Figure 2 is generally the same as the extruder embodiment 10 shown in Figure 1 and described above.

[0021] The screw shafts may be or may comprise elongate members extending along respective generally parallel axes 53 between opposite shaft upstream and downstream ends 54, 56. The upstream end 54 of each screw shaft 16 may be adapted for operable attachment to a drive mechanism 55 of the character, for example, shown in the patents referenced above, so that the screw shafts 16 can be driven in the same direction of rotation and at the same speed of rotation. Conveying portions 57 of the screw shafts 16 adjacent the downstream ends 56 of the screw shafts 16 may be rotatably cantilevered for free floating receipt and self-journaled support within the respective conveying chambers 22. hi other words, the conveying portions 57 of the screw shafts 16 extend through the conveying chambers 22 for rotation within those chambers 22. The adjacent downstream ends 56 of the screw shafts 16 may be adapted to support screw tips 58 for co-rotation with the screw shafts 16. The downstream ends 56 of the screw shafts 16 are represented here, by way of example and without limitations, as including threaded counterbores 60 (Figures 4-6) to receive the screw tips 58 in threaded engagement.

[0022] Each screw shaft 16 may include a mid-portion 62 upstream from its conveying portion 57 and received in the mixing chamber 14. Because the diameters of the conveying chambers 22 may be less than the diameters of the mid-portions 62 of the screw shafts 16, the conveying portion 57 of each screw shaft 16 may have a diameter that is correspondingly smaller than that of the mid-portion 62 of each screw shaft 16. Because the mid-portions 62 of the screw shafts 16 are in intermeshing contact with one another, the reduced diameter conveying portions 57 of the screw shafts 16 define a gap 66 as shown in Figures 4-6. The gap 66 is sized to receive the partition wall 38. The reduced diameter conveying portions 57 of the screw shafts 16 may be sized for close wiping of the inner walls 40 of the conveying chambers 22 and may be self-journaled within the conveying chambers 22 by a generally uniform radial pressure imparted by the mixture as the mixture is being conveyed about the circumference of the conveying portions 57 of the screw shafts 16. As such, the screw shafts 16 may be supported for rotation without the need for support bearings adjacent their downstream ends 56 and the discharge end 34 of the extruder 10. As such, the conveying portions 57 convey mixed material axially downstream from the screw shaft midsections 68 toward an outlet 59 disposed axially downstream from the conveying portions 57 of the screw shafts 16, minimizing flow obstructions by maintaining generally axial flow of the material and imparting sufficient motive force to the mixture to propel the mixture through an extrusion die or pelletizer 31 without additional intervening pumping devices such as single screw extruders or gear pumps. The rotational support provided by the conveying portions 57 allows for a greater outside diameter (OD) to inside diameter (ID) ratio, e.g., of 1.76 or greater, for the screw configurations of the midsections 68 of the screw shafts 16 since the midsections need not support themselves for rotation within the respective mixing chambers 14.

[0023] The screw sections 18 and mixing elements 20 may be splined on the screw shafts to cause the screw sections 18 and mixing elements 20 to rotate with the screw shafts, thus providing the desired flow of the ingredients along the barrel sections 35, 36, 37 of the extruder 10. However, the screw sections 18 and mixing elements 20 could alternatively be keyed to the screw shafts, if desired as is disclosed in U.S. Patent No. 5,439,286, which is incorporated herein by reference and assigned to the present applicant. The screw sections 18 may be fabricated to include varying degrees of pitch to provide a corresponding increase or decrease in the forward velocity of the mixture, as desired, depending on the specific mixture formulation and extrudate being produced. The screw sections 18 in the mixing chamber 14 may be arranged to closely intermesh in co-wiping relation with one another and for close wiping of the inner wall 15. In other words, the screw sections 18 are arranged to cooperatively wipe one another as well while at the same time wiping the inner wall 15 of the mixing chamber 14. The mixing elements 20 may be arranged adjacent one another in circumferentially offset relation to mate with and wipe one another and the inner wall 15, as disclosed in the aforementioned patents.

[0024] As shown in Figures 7, 9, and 10, each of the screw tips 58 may have an enlarged midsection 68 with a threaded stud portion 70 extending in one direction from the midsection 68 along a longitudinal rotational axis 71 of each stud portion 70. As best shown in Figure 8, each screw tip 58 has an eccentric axial profile as viewed in the direction of its rotation axis, to provide enhanced perturbation or agitation of passing mixture than would a screw tip having a generally circular axial profile. Each of the screw tips 58 may also have an eccentric extension or radially offset helical portion in the form of a helical finger 72 extending generally axially from its midsection 68 opposite its stud portion 70. The helical finger 72 of each screw tip 58 has a distal end surface 78 radially offset from the rotational axis 71 of the screw tip 58. The screw tips 58 are positioned such that rotational sweeps of their respective helical fingers 72 overlap one another as is again best shown in Figure 8. This overlap agitates material flowing between the two screw tips 58 that would otherwise have been undisturbed or only indirectly disturbed. The screw tips 58 may additionally be positioned and rotated such that the rotation of the helical finger portion 72 of each screw tip propels fluid into a cavity or concave portion 75 of the other screw tip 58 eliminating any undisturbed material inventory or dead spots, while enhancing the compound's homogeneity.

[0025] The threaded stud portion 70 of each screw tip 58 is arranged for mating engagement with the threaded bore 60 in one of the screw shafts 16, the axis of the stud portion 70 of each screw tip 58 being generally coaxial with the axis 53 of the respective screw shaft. Preferably, the hand of the thread on the stud portion 70 is configured to cause the respective screw tip 58 to tend to tighten itself on the screw shaft 16 as the screw shaft 16 is rotating in a normal operating direction. To facilitate the tightening of the screw tips 58 onto the respective screw shafts and to facilitate the removal of the screw tips 58 from the screw shafts, diametrically opposite flats 74 may be formed on the midsection 68. This enables a tool such as a wrench having complementary flat engagement surfaces to grip or otherwise engage the flats 74 formed on the midsection 68.

[0026] The helical finger 72 of each screw tip 58 may be radially offset from the stud axis 71 of each screw tip 58 and may extend to define a helix angle of about 65 degrees. The helical fingers 72 of the screw tips 58 can eliminate or reduce stagnant or so called "dead spots" in the mixture and tend to homogenize the mixture as the screw tips 58 rotate in the discharge cavity 24. As such, the mixture within the discharge cavity 24 may be brought closer to an equilibrium temperature. The helical finger 72 of each screw tip 58 may have rounded or arcuate rotationally leading and trailing edges 73, 77 so as to define a twisted or helical, generally- cylindrical outer surface 76 extending between the midsection 68 and the distal end surface 78. In other words, the helical finger portion 72 of each screw tip has an arcuate profile as viewed from any direction, i.e., a smooth, curvilinear outer surface devoid of sharp edges, to eliminate dead spots that tend to form adjacent the relatively sharp edges of helical screw tips.

[0027] The distal end surface 78 of each screw tip 58 may be inclined to its rotational axis 71 to define an angle (A) of about 105-120 degrees between the distal end surface 78 and the axis 71. During rotation of the screw tips 58 the inclination of the distal end surfaces 78 of the screw tips 58 facilitates movement of the mixture in a forward direction (F) toward the profile dies 26 as shown in Figure 4, a distribution manifold or single profile die 28 as shown in Figure 5, or a pelletizer 31 as shown in Figure 6.

[0028] Upon flowing through the dies 26 of Figure 4 or die 28 of Figure 5, one or more extrudates of synthetic wood 12 may be formed side-by-side without the need for a secondary pump or propulsion mechanism downstream from the screw shafts 16. This may be facilitated by the build-up of pressure and minimal pressure losses along the lengths of the separate conveying chambers 22. Pressure losses may be minimized by providing a smooth transition from the larger diameter intermeshing screw portions 18 of the respective screw shafts 16 to the smaller diameter non- intermeshing conveying portions 57 of the respective screw shafts 16, while providing a corresponding transition in the processing chamber 29 from the mixing chamber 14 to the smaller diameter conveying chambers 22. This allows the non-intermeshing screw shaft conveying portions 57 to be brought into a circumferential wiping relation with the inner walls 40 of the separate conveying chambers 22. Once extruded, the synthetic wood 12 may then be cooled downstream from the dies, such as through the use of a water and/or air curtain, for example.

[0029] The embodiments of the manufacturing processes for the synthetic extrudate 12 discussed above are intended to be illustrative of some presently preferred embodiments, and are not limiting. Various modifications within the spirit and scope of the invention will be readily apparent to those skilled in the art. For example, the screw configurations can be adjusted to achieve the desired back pressures, as necessary, and the barrel sections can be fewer or greater in number, depending on the size and type of machine, space constraints, and ingredients being used. The invention is defined by any claims stemming from this disclosure, including, but not limited to, the claims that follow.

Claims

We claim:

1. A method of forming a synthetic extradate, the method comprising the steps of: providing a mixing apparatus (10) including a pair of screw shafts (16), intermeshing screw sections (18) of which are supported for intermeshing co-wiping rotation in the same sense about respective generally parallel axes (53) within a mixing chamber (14), and conveying portions (57) of which are rotatably cantilevered for self-journaled support within respective conveying chambers (22) disposed downstream from the mixing chamber, each of the screw shafts being operably attached to a drive mechanism (55); feeding polymeric and organic material into the mixing chamber; heating the polymeric material to a molten state; and mixing the molten polymeric material with the organic material within the mixing chamber to form a generally homogeneous mixture; conveying the mixture downstream along the mixing chamber; degassing the mixture while the polymeric material is in a molten state; conveying the mixture downstream from the mixing chamber along the respective conveying chambers; and discharging the mixture.

2. The method of claim 1 in which: the step of providing a mixing apparatus includes supporting a pair of screw tips (58) on downstream ends (56) of the respective screw shafts (16) such that the screw tips are supported in a discharge cavity (24) disposed downstream from the conveying chambers (22) for rotation with the respective screw shafts within the discharge cavity (24); and after the step of conveying the mixture into the separate conveying chambers, there is included the additional step of conveying and merging the mixture into the discharge cavity (24).

3.

The method of claim 2 in which the step of discharging the mixture includes discharging the mixture axially from the discharge cavity (24) through an extrusion die (26, 28) without further propulsive forces being applied to the mixture between the screw tips (58) and the extrusion die.

4.

The method of claim 2 in which the step of discharging the mixture includes discharging the mixture axially from the discharge cavity (24) through a pelletizer without further propulsive forces being applied to the mixture between the screw tips (58) and the pelletizer.

5.

The method of claim 2 in which the step of discharging the mixture includes extruding separate extrudates (12) downstream from the discharge cavity (24).

6.

The method of claim 1 in which the step of feeding polymeric and organic material into the mixing chamber includes: feeding polymeric material into the mixing chamber through an upstream inlet (41) of the mixing apparatus; and feeding organic material into the mixing chamber through a downstream inlet (51) of the mixing apparatus disposed downstream from the upstream inlet.

7.

The method of claim 6 in which the step of feeding polymeric and organic material into the mixing chamber includes feeding organic material into the mixing chamber through the upstream inlet (41) along with polymeric material.

8.

The method of claim 6 in which the step of feeding polymeric and organic material into the mixing chamber includes feeding polymeric material into the mixing chamber through the downstream inlet (51) along with organic material.

9.

The method of claim 6 in which the step of melting the polymeric material includes melting the polymeric material within the mixing chamber (14) between the upstream and downstream inlets (41, 51).

10.

The method of claim 1 in which the step of feeding polymeric material includes feeding one or more of the thermoplastic resins selected from the group of thermoplastic resins consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, polyvinyl chloride, and polypropylene.

11.

The method of claim 1 in which the step of feeding organic material includes feeding one or more organic materials selected from the group consisting of wood flour, wood pellets, wood fibers, wastepaper, kenaf, flax, rice hulls, jute, sisal, coconut, and hemp.

12.

The method of claim 1 including the additional step of feeding one or more additives selected from the group consisting of antioxidants, UV stabilizers, colorants, impact modifiers, and lubricants.

13.

The method of claim 1 in which the degassing step includes degassing the mixture through a first degassing vent (48) downstream from the downstream inlet (51).

14.

The method of claim 13 in which the degassing step includes degassing the mixture through a second degassing port (48) downstream from the first degassing port.

15.

The method of claim 14 in which the degassing ports comprise stuffing devices (48) and the degassing steps include applying a staged incremental vacuum to the stuffing devices.

16.

A mixing apparatus, comprising: a mixing chamber (14); an inlet (41) configured and positioned to receive material into the mixing chamber; a pair of co-wiping screw shafts (16) supported for rotation about respective generally parallel axes (53), each screw shaft operably attached to a drive mechanism (55) configured to rotate the screw shafts at the same speed and in the same sense, the screw shafts including: respective midsections (68) disposed within the mixing chamber and having screw sections (18) adapted to intermesh with one another, and respective conveying portions (57) rotatably cantilevered for self- journaled support within respective conveying chambers (22) arranged generally parallel to one another downstream of the mixing chamber, the conveying portion of each screw shaft having an outer diameter smaller than an outer diameter of the midsection of each screw shaft, the conveying portions being configured to convey mixed material axially downstream from the screw shaft midsections toward an outlet (59) disposed axially downstream from the conveying portions of the screw shafts.

17.

The mixing apparatus of claim 16 further comprising a pair of profile dies supported downstream from the conveying portions (57) of the screw shafts (16), each of the profile dies being arranged to receive mixed material discharged from the conveying chambers (22) and to simultaneously form a pair of composite extrudates.

18.

The mixing apparatus of claim 16 further comprising a common discharge cavity (24) disposed downstream from the conveying chambers (22), the discharge cavity being in fluid communication with the conveying chambers.

19.

The mixing apparatus of claim 16 further comprising two screw tips (58) carried by the respective screw shafts (16) for coaxial rotation with the respective screw shafts.

20.

The mixing apparatus of claim 19 in which the two screw tips (58) have respective rotational axes (71) and eccentric axial profiles.

21.

The mixing apparatus of claim 20 wherein the screw tips (58) are positioned such that rotational sweeps of respective eccentric extensions (72) of each screw tip overlap one another.

22.

The mixing apparatus of claim 20 wherein each of the screw tips (58) includes a generally axially extending helical finger portion (72) having a distal end surface (78) radially offset from the rotational axis (71) of the screw tip (58).

23.

The mixing apparatus of claim 22 wherein the screw tips (58) are positioned and rotated such that the rotation of the helical finger portion (72) of each screw tip propels fluid into a cavity portion (75) of the other screw tip.

24.

The mixing apparatus of claim 22 wherein the helical finger portion (72) of each screw tip (58) has an arcuate rotationally leading edge (73).

25.

The mixing apparatus of claim 22 wherein the helical finger portion (72) of each screw tip (58) has an arcuate rotationally trailing edge (77).

26.

The mixing apparatus of claim 22 wherein the helical finger portion (72) of each screw tip (58) has an arcuate profile as viewed from any direction.

27.

The mixing apparatus of claim 20 in which a distal end surface (78) of each screw tip (58) is inclined to the rotational axis (71) of the screw tip.

28.

The mixing apparatus of claim 16 in which the screw configurations of the midsections (68) of the screw shafts (16) include undercuts.