Abstract:
An extrusion diea plurality of die layers stacked one upon the other in a direction generally parallel to the central axis. Each of the layers has an annular flow distribution ring having an outer generally cylindrical surface and an annular feed ring surrounding and engaging the outer surface of the distribution ring. At the engaged surfaces of the feed and distribution ring, the flow from the feed ring is divided into a number of substantially equal flow portions. Seals between adjacent engaged surfaces confine the flow to between the circumferential surfaces of the feed and distribution ring. Half of the flow portions are directed to a recombination region on one side of the distribution ring, and half are directed to a recombination region on the other side of the ring. In recombining, weld lines are formed in high flow areas to minimize the delitarious effect of polimer degredation. Subsequently, the flows on opposite sides of the distribution ring are recombined such that the weld lines from one half are first modified to exhibit lower flow and then are layered with non-weld line portions modified to exhsibit higher flow from the other half. This further minimizes the negatie effect o weld-line areas. Flow from the two sides are regularly divided to yield even distribution of the combined flows.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to improving the quality of extruded annular products, particularly products produced by plastic resin extrusion lines and most particularly blown plastic film.  
         BACKGROUND OF THE INVENTION  
         [0002]    In making such cylindrical products, the material from which the product is formed is extruded from an annular extrusion die and pulled along the die axis. In the case of blown film, plastic resin is extruded from a heated extruder having an annular die and the molten polymer is pulled away along the die axis in the form of an expanded bubble. After the resin cools to a set diameter as a result of application of cooling air, the bubble is collapsed and passes into nip rolls for further manufacturing steps.  
           [0003]    As the film is extruded, thickness variations occur about the circumference of the bubble. The presence of thickness variations creates problems for downstream conversion equipment such as printing presses, laminators, or bag machines. In processes where the film is not converted in-line, but is wound onto a roll prior to converting, the thicker and thinner areas of many layers on the roll create hills and valleys on the roll surface which deform the film and magnify the subsequent converting problems especially with larger diameter rolls. It is therefore desirable to minimize such thickness variations, not only in blown film but in other extruded cylindrical products as well. To achieve this goal, processors use expensive equipment designed to randomize the position of these thick and thin areas over time or to automatically reduce the magnitude of these variations so that the finished roll is suitable for later converting steps.  
           [0004]    It is recognized that thickness variations are caused by a variety of factors such as circumferential nonuniformity in flow distribution channels (ports and spirals) within the die, melt viscosity nonuniformity, and inconsistent annular die gaps through which the polymer exits the die. Flow distribution problems inside the die are of particular concern because they typically take the form of relatively sharp, closely spaced high and low spots which are commonly referred to as “port lines”. Additionally, variability of the cooling air and non-uniformity of air aspirated into the cooling air stream from the atmosphere surrounding the extrusion line are major contributors to film thickness variation. Many film processors rely on conventional blown film equipment to determine the film thickness. This approach typically yields an average variation of +/−10 to 20% in film thickness overall, with the largest contributor typically being that of port lines.  
           [0005]    It is desired to make improvements in the die to obtain higher quality film and other products so that the downstream equipment can be run faster and longer and so that the end use products will have more consistent thickness.  
           [0006]    One major difficulty to overcome in designing a die is how to uniformly convert a typically non-uniform flow of molten polymer or other material that is conveyed to the die via a “melt” pipe into a relatively thin annular flow. Annular flow implies that there is an inner and outer forming wall as opposed to just an outer enclosing wall such as exists with the melt pipe. To introduce this inner forming wall into the molten stream requires that this new inner forming wall be rigidly fixed within the cavity of the outer enclosing wall of the die. To do this, connecting structures must be placed within the flow path of the molten material that temporarily disrupt the flow forming multiple, separate flows which then pass by the connecting structures and must be recombined in some way. Unfortunately, molten polymer exhibits non-uniform melt viscosity due mainly to variations in molecular level properties as well as local polymer temperature. These viscosity effects are collectively referred to as the rheology. One such property of major concern is that polymers exhibit “non-Newtonian” flow behavior. This means that the viscosity of the polymer changes depending on how fast it is moving through a given channel. The net effect when all viscosity effects are combined is that the polymer tends to segregate by viscosity making uniform recombination of multiple polymer flows very difficult. Additionally, molten polymer remembers its previous flow history and instead of seamlessly recombining, the multiple polymer flows tend to form unwanted “weld lines” where adjacent flows are recombined. The problem of weld lines intensifies when degredation of the polymer, occurs due to low polymer flow rates.  
           [0007]    Several approaches are presently employed to provide for connecting structure between the outer and inner forming walls of the die. One approach feeds from the centerline axis, a small distribution chamber in the die. This chamber separates and directs the polymer into several smaller, equally spaced pipes called ports, which diverge radially at some angle to the flow axis of the incoming melt. These ports convey the polymer out to a diameter appropriate for recombining into the annular flow which will exit the die. Another approach creates a mushroom shaped distribution chamber out of which relatively small, highly streamlined, spider-like connecting structures diverge radially at an angle to the flow axis that allow for quick recombination before forming the generally axial annular flow that exits the die. Yet another approach feeds the die radially from the side of the die and divides the flow one or more times through a network of flow channels similar to the branches of a tree which ultimately convey the separate polymer streams to a diameter appropriate for recombining into the annular flow which will exit the die. Generally, one or more of the methods of flow separation must be employed in a blown film die, but each causes problems with segregation and potential for weld lines to form. Special recombination techniques must be employed to limit these effects.  
           [0008]    Several techniques are used to recombine individual molten material flows into the annular flow that exits from the die. Some are designed to overlap the separate flows creating an onion-like layering effect while others simply butt opposed flows up against each other and allow time, temperature and pressure to force recombination to occur.  
           [0009]    In blown film production, the most common recombination technique commercially available employs channels which spiral around the axis of the die. These so-called spirals, overlap one another and allow molten polymer to gradually bleed out of the channel over a “land”, eventually to flow toward the annular exit of the die forming a layered, almost onion-like recombination flow. This annular flow of polymer exits the die at what is commonly referred to as the die lip. The major problem with this approach is that the flow channels and lands must be made non-uniform to compensate for Non-Newtonian flow and other non-uniformities exhibited by the polymer. Unfortunately, major differences exist in the flow characteristics of various polymer materials that are processed. For a given die design, it may be possible to obtain even distribution around the flow annulus for one material, however it will not be even for others. Instead, other materials tend to form somewhat sinusoidal high and low flow spots in locations which depend on the material properties being processed. Thus the spiral design approach is limited in its capability to process a broad range of materials while simultaneously holding thickness variations to a consistent, predictable minimum.  
           [0010]    A further problem is that the polymer or other material must necessarily take a long period of time to flow through the passages, i.e., a high residence time, which can lead to degradation of the material. Additionally, as the material flows through each passage, significant backpressure is created.  
           [0011]    In “pancake” designs which incorporate distribution channels and the spirals substantially into the face of a plate that is coaxial with the flow axis of the die, the wetted surface area is quite large so that, when combined with higher pressures, resulting separation forces between adjacent plates can grow to be so large that the die cannot be held together. This forces the designer of such dies to limit the pressure magnitude which tends to degrade even distribution. Further, in many cases, lower pressure is attained by enlarging the flow passages; however this leads to higher residence time causing degradation of polymer properties. In practice, pressure and distribution effectiveness must be balanced which can lead to limitations on how large the die can be.  
           [0012]    A less commonly used recombination approach does not overlap the flows but instead joins them at one or more discrete locations. In these locations where two opposed flows join together, the flow is very low causing the material to have very long residence times which degrades the polymer. This degraded polymer forms a distinct weld line that exhibits poor optical properties and reduced strength which have tended to limit the use of these designs. On the other hand, since there is no overlap, the flow channels are shorter than in overlap designs. This provides benefits in lower pressure and residence time which limits degradation and allows for larger designs. Non-overlapping designs also benefit from the clearly defined flow paths which force the polymer through the same geometry regardless of melt flow characteristics as opposed to the shifting around of the flow path associated with overlapping designs. This simplifies the die design process since non-Newtonian flow is well understood through defined geometries. Unfortunately, non-uniformities in distribution still occur as the melt flow characteristics change from those that were used to design the die. As a wider range of polymer choices are made available, this becomes more of a problem.  
           [0013]    Processors are presented with a growing number of choices of extrusion materials, each with their own special properties. For example, some polymers resist water vapor, others resist oxygen penetration, still others provide high strength or resist puncture. Increasingly, processors are finding innovative uses for these materials, oftentimes finding it desirable to combine different polymers together in a layered or “coextruded” structure to yield property benefits in several areas. To do this, dies are designed with multiple entry points which distribute the polymer flow into separate annular flows and subsequently layer these flows one inside the other while still inside the die. Although non-overlapping designs have been used, most prevalent are overlapping designs either in a concentric or pancake configuration. Pancake designs are better suited to larger number of layers because the individual layers can be stacked one on top of each other. Concentric designs are limited to about 5 to 7 layers simply because the die grows so large in diameter as to become impracticable.  
           [0014]    It has long been recognized that having multiple layers can provide a secondary benefit in that thickness variations present in each layer can somewhat offset one another. This has a drawback; since each layer&#39;s variation depends on associated melt flow properties, throughput rate, temperature etc., the variations typically will not always average out. In fact, they can even align one on top of each other yielding no thickness averaging whatever. This is especially true of overlapping designs since the melt variations shift significantly in position and magnitude with even subtle changes in a given layer. Commercial coextrusion dies are designed with adjacent layer spirals that typically wrap in opposed directions in an effort to capitalize on this averaging effect. In the case of concentric die designs, the spirals for each layer are necessarily different in design because they do not spiral around at the same distance from the flow axis of the die. Pancake designs can be designed with the same mechanical geometry, however the path length to the die lip is necessarily different for each layer because they are stacked one on top of each other. This causes differences in the flow behavior since each layer operates at a different pressure. It has been observed that commercially available dies designed to capitalize on averaging effects exhibit both very good and very bad variation in total thickness as the throughput rate is raised through its full operating range. This occurs as resultant layer variations first oppose (good) then align (bad) with one another. An additional problem with these designs is that even if thickness variations are opposed, yielding good overall variation, the individual layer distribution can still be bad. This has a negative effect, especially when each layer is designed to take advantage of different film properties—the layers responsible for providing a barrier to oxygen and separately to water vapor can individually be highly variable even though the total thickness is uniform. It is highly desirable to achieve uniform distribution for each individual layer as well as for the combination of multiple layers.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention features a regular division (RD) die which provides uniform distribution of molten extrusion material to each individual layer and exhibits a high degree of insensitivity to melt flow properties and a pressure resistive distribution system that does not limit the size of the die. This die design has particular application to the extrusion of polymeric blown film, but also applies to other forms of extrusion requiring an annular die. Blown film extrusion lines typically include a heated extruder for melting and pressurizing a flow of molten plastic resin, an annular die through which the molten resin extrudes and from which it is pulled away along an axis in the form of an expanding bubble, and an air cooling device constructed to direct cooling air into cooling contact with the bubble, to flow along the bubble and cause the molten resin to cool as the film expands until a substantially fixed maximum bubble diameter is achieved at a frost line spaced from the annular die.  
           [0016]    The RD design may be included as an integral part of one or more individual die layers within the complete die. According to one preferred embodiment, the RD design is integrated separately in each layer of a pancake style stackable die. Each layer includes a series of concentric rings one inside of the other that performs the functions of feeding, distribution, and recombination. These rings surround and contact one another to allow the polymer to pass between them unimpeded through passages cut into the surfaces of and/or through them. The rings are bolted together forming a single unitized layer that is stacked face to face with the other layers of the complete die, each layer with its central geometrical axis being coaxial with the flow axis of the die. Polymer is separately fed into the outside diameter of the outer feed ring of each layer, the polymer passing straight radially through the feed ring wall to the radially interior associated distribution ring. For purposes of the ensuing discussion, the location of the input through the feed ring is at location 0°.  
           [0017]    The distribution ring has flow channels machined into its radially outwardly-facing surface which act to divide the flow one or more times. Cutting the channels into the outside surface (or alternatively, the radially inwardly-facing surface or both) eliminates the detrimental effects of separation forces caused by polymer pressure; the forces produced by the polymer act against the surrounding feed ring instead of on the bolts which hold the layer(s) together.  
           [0018]    In the distribution ring, the polymer flow input from the feed ring is divided into an even number (2 n ) of separate and equal flows. In the preferred embodiment, the input flow is divided into eight (2 3 ) flows, in three stages. The first division of flow occurs at 0°, at which point the polymer flow is divided in two and each half is directed into one of two channels, each of which wraps 90 degrees around the circumference of the ring, one clockwise from 0 degrees to 90 degrees and the other counter-clockwise from 0 degrees to 270 degrees. At the 90 and 270 degree points, each flow (half of the original) turns and travels axially for a short distance prior to being divided a second time. The second divisions occur separately at the 90° and 270° points; at each of which the flow is again divided in half and the resulting portion of the flow (one quarter of the total input flow) directed into one of a pair of channels which wrap 45° in opposite directions from, respectively, the 90° and 270 . ° points, around the outside of the ring. These four flows end up at, 45°, 135°, 225° and 315°; at which points the flow is divided again, this time into opposite wrap angles of 22.5. The end result of these three divisions is eight separate flows which end at 45 degree intervals at, respectively, at 22.5°, 67.5°, 112.5°, . . . , 337.5. It will be noted that, after each division, equal opposite wrap angles ensure that there is equal path length and thus equal pressure drop for any path through which the polymer might flow.  
           [0019]    Each of these eight divided flows then passes radially inwardly through the first distribution ring, either directly to the recombination rings or, if further division is desired, to a second distribution ring. It will be recognized that, by using more than one distribution ring, a larger number “n” of divisions can be accomplished without pressure penalties. In any event, after the desired number of divisions are made in the distribution rings, the resulting flows are conveyed radially inwardly to the recombination rings through a divider plate that forms an integral part of the final (e.g., the most radially inward) distribution ring.  
           [0020]    The divider plate is relatively thin (measured axially of the die) compared to the main body of the distribution ring of which it is a part. The divider plate extends inwardly from the portion of the final radially inward distribution ring that forms the 2 n  polymer flows and tapers to a thin edge at its inner circumference. Within the divider plate, and generally prior to the taper, the 2 n  radial flows are alternately diverted to one side of the plate or the other. This provides two separate but identical flow patterns, each of which includes 2( n−1 ) recombination flows, issuing from ports located in either the upper or the lower face of the divider plate. These flows in turn are fed to a pair of recombination plates that abut the upper and lower faces of the divider plate.  
           [0021]    One recombination plate is mounted on either side of the tapered portion of the divider plate. The recombination flow ports on one side of the divider plate are offset in such a way as to be centered between ports on the opposite side of the divider plate. This allows for precise, mirror image recombination to take place, “split” on opposite sides of the divider plate. These split, mirror-imaged flows join together at the inner edge of the divider plate. The recombination flow channels on each side of the divider plate are designed to create a flow distribution that, when added to its mirror image, results in a flat flow profile.  
           [0022]    Insensitivity to melt rheology is attained by forcing the recombination plate flow to distribute in a non-overlapping manner, thus yielding predictable, non-shifting resultant polymer flow. Weld lines are avoided by placing an interceding land area directly in front of each port with the main flow channel passing on a diameter behind the land. Thus some of the flow from each port passes over the land and, of what remains, half flows down the channel one way and the other half flows in the opposite direction. Eventually the channel flow from one port meets opposite direction flow from the adjacent port. At this point, the main flow channel passes radially inward between the ends of adjacent lands. This creates a weld area, but because the weld area is in a high flow region the problem of polymer degradation is substantially eliminated. The main flow channel then splits again and passes on a diameter in front of each of the associated lands such that half flows down the channel one way and the other half goes the opposite direction. Thus the flow which originally was diverted around the land via the main flow channels is recombined with the land flow in a way which is predictably stable but yields a layered effect, similar to that produced in a spiral design but without shifts in position. The now annular and radially inwardly directed recombination flow passes over a final land to the tip of the divider plate where its mirror imaged split flow from the opposite side of the divider plate is added. The final channel and land are cut in such a way as to insure a smaller flow where the high flow weld line occurs and a larger flow centered on the interceding land. Upon addition of its mirror image, the deleterious effects of the weld area is minimized by the addition of the mirror images larger (non-weld) flow area.  
           [0023]    The shape of the flow issuing from the recombinatio area on each side of the divider plate prior to the flows being recombined is important to achieving a combined uniform flow from opposite sides of the divider plate. Although for a given material, the individual flows from each halve may also be uniform, they do not necessarily have to be. Rather, there is a wide diversity of curves which can be programmed into the design of the flow channels which after addition yield a uniformly flat combined profile. The mathematical study of “regular divisions of the plane” such as used in the study of crystallography or as can be found in graphical representations by M. C. Escher depict many suitable examples of both simple and complex profiles. A preferred profile for each split flow, is a straight line “triangle” profile which linearly increases from a minimum flow at the high flow weld to a maximum in line with the port. This profile repeats itself without discontinuity around the diameter of the layer. A second preferred split flow profile is a “sinusoidal” profile which also has its minimum at the high flow weld and maximum in line with the port.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic side view showing a blown film extrusion apparatus which includes a multi-layer regular division die according to the present invention.  
         [0025]    [0025]FIG. 2 is a schematic cross section (taken at A-A of FIG. 3) side view on an enlarged scale of the blown film extrusion regular division die of FIG. 1.  
         [0026]    [0026]FIG. 3 is a plan view of the general arrangement for a typical multi-layer blown film extrusion die.  
         [0027]    [0027]FIG. 4 is a partial cross sectional side view (taken at B-B of FIG. 4 a ) of one layer for the regular division die showing the general locations of the feed inlet, dividing channels, recombination ports and channels.  
         [0028]    [0028]FIG. 4 a  is a plan view of one layer of the regular division die of FIG. 1, showing the general locations of the feed inlet, dividing channels, recombination ports and channels.  
         [0029]    [0029]FIG. 4 b  is a schematic illustration, centered on the bore of the feed inlet of one layer of the regular division die of FIG. 1, showing the general locations of the feed inlet, dividing channels, recombination ports and channels on the exterior surface of the layer, as viewed looking radially inwardly.  
         [0030]    [0030]FIG. 5 is a schematic illustration of an upper recombination channel and associated land area, as viewed looking upwardly from the upper surface of the tapered portion of the distribution plate.  
         [0031]    [0031]FIG. 5 a  is a schematic illustration of a lower recombination channel and land area as positioned relative to FIG. 5, viewed looking downwardly from the lower surface of the tapered portion of the divider plate.  
         [0032]    [0032]FIGS. 6 and 6 a  are schematics cross sections of typically desirable flow proportions from upper and lower recombination rings.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    [0033]FIG. 1 illustrates a blown film extrusion system in which molten plastic resin is extruded to form blown film. Except for the die  10 , the system of FIG. 1 and its operation are generally conventional. In general, plastic pellets are fed into a feed hopper  2   a  and are transferred into an extruder  4   a  where they are melted, mixed and pressurized by the action of an extruder screw. The melt exits extruder  4   a  and is conveyed through melt pipe  6   a  where it is directed into blown film die  10 . Die  10  is designed to form the melt into an annular, cylindrical plastic melt flow  14  which is then extruded from an annular orifice die lip  16  at the top of die  10 . This annular melt flow is continually drawn away from the annular die lip  16  in a manner generally concentric with a process centerline  18 . The annular diameter of the melt flow enlarges as it progresses from the die until it reaches frost line  20  (indicated diagrammatically by a saw-tooth line) to form a cooled, solidified plastic tubular film bubble  22 .  
         [0034]    Primary cooling air for the process is supplied to external air ring  24  from a conventional air source (not shown). The air is applied to contact the extruding plastic melt adjacent the base portion of the bubble by air ring lips  26 . The air flows in annular air streams  28  along the outside expanding surface of the bubble. On some blown film processes, other forms of cooling are also employed. One such system (not shown) applies cooling air to the inside surface of the bubble, according to known techniques, and is commonly referred to as internal bubble cooling, or just “IBC”. The plastic melt is cooled sufficiently to solidify into tubular bubble  22  at frost line  20 .  
         [0035]    Also according to known techniques, tubular bubble  22  is continually drawn upward through collapsing frame  150 ,  150   a  where it is compressed into a flat sheet of film  22   a , also known as “layflat,” as it passes through a nipping point between nip rolls  152  and  152   a . These nip rolls are driven to continually pull the film through the extrusion process. Layflat film sheet  22   a  is then converted and/or wound into finished product by downstream processing equipment such as winder  156 .  
         [0036]    [0036]FIG. 2 shows a schematic cross section side view of the blown film extrusion die  10  of the regular division type with multiple die layers  30   a ,  30   b  and  30   c . Die layers  30   a ,  30   b  and  30   c  are essentially identical, and are rotated relative to each other as shown in FIG. 3. Each layer converts melt feeding in from a respective melt pipe  6  to cylindrical plastic melt flow  14  which is conveyed toward die lip  16  around a cylindrical inner mandrel  12 . Thus, layer  30   a  converts melt flow from melt pipe  6   a  to melt flow  14   a , layer  30   b  forms a second cylindrical plastic melt flow  14   b  which is conveyed toward die lip  16  around cylindrical plastic melt flow  14   a  and inner mandrel  12 , and layer  30   c  forms a third cylindrical plastic melt flow  14   c  which is conveyed toward die lip  16  around cylindrical plastic melt flows  14   b  and  14   a , and inner mandrel  12 . The three cylindrical plastic melt flows  14   a ,  14   b  and  14   c  layer adjacent to each other, and thus make up the total cylindrical plastic melt flow  14  which flows between inner mandrel  12  and outer mandrel  15  until it exits through annular die lip  16 . Layer  30   a  is held to die base  11  by a multiple bolts  34   a . Layer  30   b  is stacked on top of and held to layer  30   a  by multiple bolts  34   b . Layer  30   c  is stacked on top of and held to layer  30   b  by multiple bolts  34   c . At the top of the stack, outer mandrel  15  is stacked on top of and held to layer  30   c  by a multiple bolts  34   d . O-ring seals in annular seal areas  32 ,  32   a ,  32   b , and  32   c  prevent plastic melt from flowing outward between the respective flat, axially-facing, abutting surfaces formed by die base  11 , layers  30   a ,  30   b ,  30   c  and mandrel lip  15 .  
         [0037]    [0037]FIG. 3 shows a plan view of the general arrangement for a typical blown film extrusion die  10  of the regular division type with multiple layers such as  30   a ,  30   b  and  30   c  of FIG. 2. As shown in FIG. 3, layer  30   a  is fed from extruder  4   a  by melt pipe  6   a . Layer  30   b  and associated extruder  4   b  and melt pipe  6   b  are positioned at an angle to layer  30   a  and associated extruder  4   a  and melt pipe  6   a . Similarly, layer  30   c  and associated extruder  4   c  and melt pipe  6   c  are positioned at an angle to layer  30   b  and associated extruder  4   b  and melt pipe  6   b . This angle, e.g., about 60 degrees, is chosen to be large enough to provide clearance between adjacent extruders and melt pipes. Annular die lip  16  is formed by the outside surface of inner mandrel  12  and the inside surface of outer mandrel  15 . Multiple bolts  34   d  are arranged to hold outer mandrel in place. Multiple bolts  34   b , shown on FIG. 2, are directly beneath multiple bolts  34   d . Multiple bolts  34   a  and  34   c , also shown on FIG. 2, are one above each other and positioned in between stacked multiple bolts  34   b  and  34   d  so as not to interfere with one another. Any number of layers can be accommodated by this approach simply by stacking and bolting them in place as demonstrated in FIGS. 2 and 3.  
         [0038]    [0038]FIG. 4 is an enlarged cross-sectional view of a portion of the die  10  of FIG. 1 that incudes layer  30   a , and FIG. 4 a  is a top plan view. Layer  30   a  is composed of a series of concentric rings (feed ring  40 , distribution ring  42  and recombination rings  45 ,  46 ) one inside of the other, that perform the functions of feeding, distribution, and then recombining the flow of molten extruded material. In the illustrated embodiment, plastic and polymer flow passes radially through feed passage  50  to the outside diameter of distribution ring  42 .  
         [0039]    Feed ring  40 , as shown most clearly in FIGS. 4 and 4 a  is annular and has a generally vertical surface to which melt pipe  6   a  is attached, and a feed passage extending radially through it to a stepped inner surface that engages the outer radially directed surface of annular distribution ring  42 .  
         [0040]    Distribution ring  42 , in turn, defines a outer radially-facing surface that forms a series of annular steps  42   a ,  42   b ,  42   c , each of which has a generally vertical (but slightly sloped) radially-facing wall, and which in this embodiment are separated by flat, parallel (to each other and perpendicular to the axis of the die and layer) annular surfaces. The underside of the top, largest diameter wall portion  42   a  and the underside of the middle diameter wall portion  42   b , seal against corresponding surfaces formed at the inner radial diameter of feed ring  40 . The  0 -rings  43   a  and  43   b  provide seals at the abutting surfaces, and bolts  44  (see FIG. 2) hold the distribution ring and feed ring tightly together.  
         [0041]    At its interior side, distribution ring  42  includes an annular divider plate portion  42   d , centered on the overall height of the distribution ring but itself having a vertical height (measured along the axis of the distribution ring and die) that is not more than about 20% that of the overall distribution ring  42 . As shown most clearly in FIG. 4, in the illustrated embodiment, the top and bottom surfaces of divider plate portion  42   d  are flat and parallel to each other throughout most of the radial width of the divider plate portion, but taper towards each other adjacent the divider plate portion&#39;s inner edge.  
         [0042]    Recombination rings  45  and  46  overlie the top and bottom of divider plate portion  42   d , and are bolted together by bolts  34 . Adjacent their radially inner edges, recombination rings extend radially inwardly of the inner radial edge of divider plate portion, are closely adajcent to each other, and terminate close to the outer surface of inner mandrel  12 .  
         [0043]    The principal function of distribution ring  42  is to divide the single flow from feed ring  50  into a number (i.e., 2 2  in the preferred embodiment 2 3 , i.e., 8) identical flow portions. To accomplish this, a series of flow division channels  52 ,  54  and  58  are machined into the outer, generally vertical radially facing surface of step  42   b . The size and/or quantity of division channels (channels  52 ,  54  and  58  are shown in the illustrated embodiment) are limited only by the vertical dimension of the outside diameter of distribution ring  42 . Flow division channels  52 ,  54  and  58  divide the melt from feed passage  50  of feed ring  40  into eight separate radial port flows  59 . Because most of the flow is between the radially-facing surfaces of the feed ring  40  and distribution ring  42 , it will be evident that the forces  41   a  and  41   b , along the die axis, which tend to move the distribution ring  42  and feed ring  40  apart are relatively small since they act only on the projected area (from a plan view) between seals  43   a  and  43   b.    
         [0044]    The arrangement of the division channels is shown most clearly in FIG. 4 b , which is a fold out (or unwrapped) schematic illustrating the radially-outward facing surface of wall portion  42   b  of division ring  42 . As shown, division channels  52 ,  54  and  58  all extend circumferentially around the outward facing surface of the division ring, and lie generally perpendicular to the axis of the die. Flow from inlet feed passage  50  passes downwardly (through a short channel  51  extending parallel to the die axis and generally perpendicular to division channel  52 , into the center of division channel. Channel  52  wraps a total of 180 degrees around the exterior of distribution ring  42 , 90 degrees in opposite directions from the point at which the flow from inlet  50  is introduced into channel  52 , and separates the melt flow from inlet  50  into two oppositely directed flows. At each of the ends of channel  52 , a short vertical channel  53  directs the flow in the respective half of channel  52  (axially of the die layer) into the center of a respective one of flow channels  54 . Division channels  54  each wrap a total of 90 degrees (45 degrees in each direction from the point at which flow from a channel  53  is directed into the respective channel  54 ) around the exterior of distribution ring  42 , and divides the melt flow from channels  52  into a total of four flows. At each end of each division channel  54 , each respective flow portion is again directed vertically a short distance, through a short channel  55 , into the center of a respective one of division channels  58 . Division channels  58  each wrap 45 degrees (22.5 degrees in opposite directions from the point at which flow from channel  55  is directed into the respective division channel  58 ) around the outside of distribution ring  42 ) and again divide the flow, this time into a total of eight equal flow portions. At each end of each of distribution channels, the respective flow portion is directed into one of eight radial channels  57 ,  59 , which convey the flow portion radially through distribution ring  42  to (as shown in FIGS. 2 and 4) either the upper (in the case of channels  59   a, b, c, d ) or the lower (in the case of channel)  59   a′, b′, c′, d′ ) surface of divider plate portion of the distribution ring. As shown, each radial channl  59 ′, extends radially inwardly from a respective one of division channels  58  to the respective surface of divider plate portion  42   d , at a point just radially outwardly of the tapered portion of the divider plate portion. The polymer melt flow from division channels  58  is equally split to the top and bottom of the divider plate portion; half goes to upper ports  56   a ,  56   b ,  56   c  and  56   d  and the other half to lower ports  57   a ,  57   b ,  57   c  and  57   d.    
         [0045]    It will be noted that all of flow passages  50 ,  52 ,  54 ,  58 ,  59  of distribution plate  42  are symmetrical such that the path length that melt must travel to reach each port is equal, ensuring even distribution.  
         [0046]    At recombination rings  46  upper ports  56   a ,  56   b ,  56   c  and  56   d  on the upper side of divider plate  42   d  evenly distribute their associated melt flow to four equally spaced positions between the upper side of the divider plate and upper recombination ring  46 . At ring  45  lower ports  57   a ,  57   b ,  57   c  and  57   d  evenly distribute their melt flow to four equally spaced positions between the lower side of the divider plate and lower recombination ring  45 . The positions at the upper side of the divider plate are midway between those positions at the lower side of the divider plate  
         [0047]    As most clearly shown in FIGS. 4 and 5, a pair of radially-spaced circular channels  60 ,  64  are cut into the lower surface of recombination plate  46  and a similar pair of radially spaced circular channels  70 ,  74  are cut into the upper surface of recombination plate  45 . A plurality of arcuate recombination lands  62  are provided in the lower surface of recombination plate  46  between channels  60 ,  64 , and a similar plurality of arcuate recombination lands  72  are provided in the upper surface of recombination plate  45  between channels  70 ,  74 . Final lands  66 ,  76  are provided in, respectively, the lower surface of recombination plate  46  between channel  64  and the inner radial edge of divider plate of distribution ring  42 , and the upper surface of recombination plate  45  between channel  74  and the inner radial edge of the divider plate. In this embodiment each arcuate land subtends an area of slightly less than 90°.  
         [0048]    In general, melt flows from radial channels  59  either into channel  60  through ports  56  or into channel  70  through ports  57 . From the outer channels  60 ,  70  of the recombination rings, the melt flows inwardly, over respective recombination lands  62 ,  72  or through recombination channels  61 ,  71  between adjacent ends of portions of the lands, to inner recombination channels  64 ,  74 . The upper melt then flows out of inner recombination channel  64  between final land  66  and divider plate  42   d ; while the lower melt flows out of inner recombination channel  74  between final land  76  and divider plate  42   d . Recombination seals  47  and  49  prevent melt from leaking outward from outer recombination channels  60  and  70  respectively. The upper and lower melt flows join at the inner tip of divider plate  42   d  forming combined flow  68  that is conveyed inward to the outside wall of inner mandrel  12  where it forms cylindrical plastic melt flow  14   a.    
         [0049]    In the illustrated embodiment, the recombination channels, recombination lands, and final land are cut into the surfaces of recombination rings  45 ,  46  and the facing upper and lower surfaces of divider plate  42   d  of distribution ring  42  are generally flat. In other embodiments some or all of these may be cut into the divider plate.  
         [0050]    The arrangement of the recombination channels and lands at the lower surface on upper recombination ring  46  is shown most clearly in FIG. 5, which is a schematic, straightened out plan view of the recombination areas symmetrical about port  56   a , viewed from above. Flow enters outer recombination channel  60  through upper port  56   a ; as viewed in FIG. 4 a , one half flows clockwise down outer recombination channel  60  toward upper port  56   d  and the other half flows counterclockwise toward upper port  56   b . As the melt flows in opposite directions down (i.e., circumferentially of the die) the channel, some of the polymer melt flows radially inwardly across recombination land  62   a  to inner channel  64 . The rest of the melt flows circumferentially in channel  60  until it reaches the ends of recombination land  62   a  (which is centered on port  56   a  and subtends an arc of slightly less than 90 degrees), at which point it meets the similar but opposing melt flow originating from upper ports  56   d  and  56   b . Here the opposing flows join or “weld”, forming high flow weld lines  80   a  and  80   b  respectively. These joined flows turn and flow inward through the respective radial recombination channels  61   a  and  61   b  at the opposite ends of land  62   a  into inner recombination channel  64 .  
         [0051]    In inner recombination channel land  64 , the melt flows both radially inwardly across final land  66  as well as in opposite circumferential directions down inner recombination channel  64 . The flow down the inner recombination channel  64  is layered on top of flow coming across recombination land  62   a , and also flows radially inwardly across final land  66 . The profile (i.e., configuration) of the flow radially inwardly of final land  66  depends largely on the design of the final land, which as discussed hereinafter may be designed with variable lengths and/or gaps to program a desired melt flow profile.  
         [0052]    [0052]FIG. 5 a  is similar to FIG. 5, except that FIG. 5 a  shows the arrangement of the recombination channels and lands at the lower recombination area between the lower surface of divider plate portion and lower recombination ring  45 , viewed from above. Although the flow into the lower recombination area is from ports  57 , FIG. 5 a  illustrates the arrangement symmetrical about upper port  56   a  to the upper recombination area so that the relationship between the upper recombination area (of FIG. 5) and lower recombination area (of FIG. 5 a ) is most easily appreciated.  
         [0053]    In the lower recombination area, flow enters outer recombination channel  70  through lower ports  57   d  and  57   a  (shown, and also through lower ports  57   b  and  57   c  although not shown in FIG. 5 a ). As in the upper recombination area, the flow from each port flows down outer recombination channel, one half of the flow from each port flowing clockwise and the other half counterclockwise. As described in connection with FIG. 5, part of the flow in channel  70  flows radially inwardly over one of recombination lands  72   d  and  72   a , and the melt flow remaining at the ends of the lands welds together to form a high flow weld line  90   a , and flows inward through radial recombination channels  71   a  into inner recombination channel  74 . In the inner recombination the melt flows radially inwardly across final land  76 , as well as in opposite directions down inner recombination channel  74  where it is layered under flow coming across recombination lands  72   d  and  72   a . As in the lower recombination area, final land  76  is designed with variable lengths and/or gaps to program a desired melt flow profile  
         [0054]    It will be recognized that the recombination lands  62  and land channels  61  of the upper recombination area are offset at 45 degrees from the lands  72  and channels  71  in the lower recombination area. This arrangement places high flow weld lines from one recombination ring radially in line with ports from the opposing recombination ring.  
         [0055]    [0055]FIGS. 6 and 6 a  show two preferred melt flow profiles that exhibit regular division, i.e., the crosssections of the Flows from the upper and lower recombination areas are identical and fit together with no intervening space. High flow weld lines  80   a  and  80   b  (also  80   c  and  80   d ) occur in the low flow areas of final land  66 . High flow weld lines  90   a  (also  90   b ,  90   c  and  90   d ) occur in the low flow areas of final land  76 . When the upper and lower melt flows join at the inner tip of divider plate  42   d  forming combined flow  68 , the opposite recombination rings high final land flow area is added and washes the effects of the weld lines out. By choosing the shape of the flow profiles  82   a ,  82   b , ( 82   c ),  82   d ,  92   a , ( 92   b ,  92   c ) and  92   d  to be regularly divided, they all interlock to form a evenly distributed combined flow  68 .  
         [0056]    The present invention has been described in connection with certain structural embodiments and it will be understood that various modifications can be made to the above-described embodiments without departing from the spirit and scope of the invention as defined in the appended claims.