Abstract:
An extrusion die for extruding biodegradable material, the extrusion die including: a mandrel; an outer member positioned near the mandrel; an extrusion orifice between the mandrel and the outer member; a member in communication with at least one defining member of the extrusion orifice, wherein the member is capable of producing relative movement between outer member and the mandrel, wherein the relative movement has a component transverse to an extrusion direction of biodegradable material through the extrusion orifice; a flow control device which controls flow of biodegradable material through the extrusion die; and a positioning device which positions the outer member and the mandrel relative to each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application that claims the benefit of prior U.S. Continuation application Ser. No. 09/638,934 filed Aug. 15, 2000 by Hans G. Franke et al. entitled “Extrusion Die for Biodegradable Material with Die Orifice Modifying Device and Flow Control Device”, now U.S. Pat. No. 6,533,973 which is a continuation and claims benefit of U.S. application Ser. No. 09/035,200 filed Mar. 5, 1998 by Hans G. Franke et al. entitled “Extrusion Die for Biodegradable Material with Die Orifice Modifying Device and Flow Control Device” now U.S. Pat. No. 6,183,672B1. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the formation of shaped objects from expanded biodegradable materials, and, in particular, to an extrusion die for ultimately forming sheets of biodegradable material. 
     Biodegradable materials are presently in high demand for applications in packaging materials. Commonly used polystyrene (“Styrofoam” (Trademark)), polypropylene, polyethylene, and other non-biodegradable plastic-containing packaging materials are considered detrimental to the environment and may present health hazards. The use of such non-biodegradable materials will decrease as government restrictions discourage their use in packaging applications. Indeed, in some countries in the world, the use of styrofoam (trademark) is already extremely limited by legislation. Biodegradable materials that are flexible, pliable and non-brittle are needed in a variety of packaging applications, particularly for the manufacture of shaped biodegradable containers for food packaging. For such applications, the biodegradable material must have mechanical properties that allow it to be formed into and hold the desired container shape, and be resistant to collapsing, tearing or breaking. 
     Starch is an abundant, inexpensive biodegradable polymer. A variety of biodegradable based materials have been proposed for use in packaging applications. Conventional extrusion of these materials produces expanded products that are brittle, sensitive to water and unsuitable for preparation of packaging materials. Attempts to prepare biodegradable products with flexibility, pliability, resiliency, or other mechanical properties acceptable for various biodegradable packaging applications have generally focused on chemical or physio-chemical modification of starch, the use of expensive high amylose starch or mixing starch with synthetic polymers to achieve the desired properties while retaining a degree of biodegradability. A number of references relate to extrusion and to injection molding of starch-containing compositions. 
     U.S. Pat. No. 5,397,834 provides biodegradable, thermoplastic compositions made of the reaction product of a starch aldehyde with protein. According to the disclosure, the resulting products formed with the compositions possess a smooth, shiny texture, and a high level of tensile strength, elongation, and water resistance compared to articles made from native starch and protein. Suitable starches which may be modified and used according to the invention include those derived, for example, from corn including maize, waxy maize and high amylose corn; wheat including hard wheat, soft wheat and durum wheat; rice including waxy rice; and potato, rye, oat, barley, sorghum, millet, triticale, amaranth, and the like. The starch may be a normal starch (about 20-30 wt-% amylose), a waxy starch (about 0-8 wt-% amylose), or a high-amylose starch (greater than about 50 wt-% amylose). 
     U.S. Pat. Nos. 4,133,784, 4,337,181, 4,454,268, 5,322,866, 5,362,778, and 5,384,170 relate to starch-based films that are made by extrusion of destructurized or gelatinized starch combined with synthetic polymeric materials. U.S. Pat. No. 5,322,866 specifically concerns a method of manufacture of biodegradable starch-containing blown films that includes a step of extrusion of a mixture of raw unprocessed starch, copolymers including polyvinyl alcohol, a nucleating agent and a plasticizer. The process is said to eliminate the need of pre-processing the starch. U.S. Pat. No. 5,409,973 reports biodegradable compositions made by extrusion from destructurized starch and an ethylene-vinyl acetate copolymer. 
     U.S. Pat. No. 5,087,650 relates to injection-molding of mixtures of graft polymers and starch to produce partially biodegradable products with acceptable elasticity and water stability. 
     U.S. Pat. No. 5,258,430 relates to the production of biodegradable articles from destructurized starch and chemically-modified polymers, including chemically-modified polyvinyl alcohol. The articles are said to have improved biodegradability, but retain the mechanical properties of articles made from the polymer alone. 
     U.S. Pat. No. 5,292,782 relates to extruded or molded biodegradable articles prepared from mixtures of starch, a thermoplastic polymer and certain plasticizers. 
     U.S. Pat. No. 5,095,054 concerns methods of manufacturing shaped articles from a mixture of destructurized starch and a polymer. 
     U.S. Pat. No. 4,125,495 relates to a process for manufacture of meat trays from biodegradable starch compositions. Starch granules are chemically modified, for example with a silicone reagent, blended with polymer or copolymer and shaped to form a biodegradable shallow tray. 
     U.S. Pat. No. 4,673,438 relates to extrusion and injection molding of starch for the manufacture of capsules. 
     U.S. Pat. No. 5,427,614 also relates to a method of injection molding in which a non-modified starch is combined with a lubricant, texturing agent and a melt-flow accelerator. 
     U.S. Pat. No. 5,314,754 reports the production of shaped articles from high amylose starch. 
     EP published application No. 712883 (published May 22, 1996) relates to biodegradable, structured shaped products with good flexibility made by extruding starch having a defined large particle size (e.g., 400 to 1500 microns). The application exemplifies the use of high amylose starch and chemically-modified high amylose starch. 
     U.S. Pat. No. 5,512,090 refers to an extrusion process for the manufacture of resilient, low density biodegradable packaging materials, including loose-fill materials, by extrusion of starch mixtures comprising polyvinyl alcohol (PVA) and other ingredients. The patent refers to a minimum amount of about 5% by weight of PVA. 
     U.S. Pat. No. 5,186,990 reports a lightweight biodegradable packaging material produced by extrusion of corn grit mixed with a binding agent (guar gum) and water. Corn grit is said to contain among other components starch (76-80%), water (12.5-14%), protein (6.5-8%) and fat (0.5-1%). The patent teaches the use of generally known food extruders of a screw-type that force product through an orifice or extension opening. As the mixture exits the extruder via the flow plate or die, the super heated moisture in the mixture vaporizes forcing the material to expand to its final shape and density. 
     U.S. Pat. No. 5,208,267 reports biodegradable, compressible and resilient starch-based packaging fillers with high volumes and low weights. The products are formed by extrusion of a blend of non-modified starch with polyalkylene glycol or certain derivatives thereof and a bubble-nucleating agent, such as silicon dioxide. 
     U.S. Pat. No. 5,252,271 reports a biodegradable closed cell light weight loose-fill packaging material formed by extrusion of a modified starch. Non-modified starch is reacted in an extruder with certain mild acids in the presence of water and a carbonate compound to generate CO 2 . Resiliency of the product is said to be 60% to 85%, with density less than 0.032 g/cm 3 . 
     U.S. Pat. No. 3,137,592 relates to gelatinized starch products useful for coating applications produced by intense mechanical working of starch/plasticizer mixtures in an extruder. Related coating mixtures are reported in U.S. Pat. No. 5,032,337 which are manufactured by the extrusion of a mixture of starch and polyvinyl alcohol. Application of thermomechanical treatment in an extruder is said to modify the solubility properties of the resultant mixture which can then be used as a binding agent for coating paper. 
     Biodegradable material research has largely focused on particular compositions in an attempt to achieve products that are flexible, pliable and non-brittle. The processes used to produce products from these compositions have in some instances, used extruders. For example, U.S. Pat. No. 5,660,900 discloses several extruder apparatuses for processing inorganically filled, starch-bound compositions. The extruder is used to prepare a moldable mixture which is then formed into a desired configuration by heated molds. 
     U.S. Pat. No. 3,734,672 discloses an extrusion die for extruding a cup shaped shell made from a dough. In particular, the die comprises an outer base having an extrusion orifice or slot which has a substantial horizontal section and two upwardly extending sections which are slanted from the vertical. Further, a plurality of passage ways extend from the rear of the die to the slot in the face of the die. The passage way channels dough from the extruder through the extrusion orifice or slot. 
     Previously, in order to form clam shells, trays and other food product containers, biodegradable material was extruded as a flat sheet through a horizontal slit or linear extrusion orifice. The flat sheet of biodegradable material was then pressed between molds to form the clam shell, tray or other food package. However, these die configurations produced flat sheets of biodegradable material which were not uniformly thick, flexible, pliable and non-brittle. The packaging products molded from the flat sheets also had these negative characteristics. 
     As the biodegradable material exited the extrusion orifice, the biodegradable material typically had greater structural stability in a direction parallel to the extrusion flow direction compared to a direction transverse to the extrusion flow direction. In fact, fracture planes or lines along which the sheet of biodegradable material was easily broken, tended to form in the biodegradable sheet as it exited from the extrusion orifice. Food packages which were molded from the extruded sheet, also tended to break or fracture along these planes. 
     Therefore, there is a need for a process which produces a flexible, pliable and non-brittle biodegradable material which has structural stability in both the longitudinal and transverse directions 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a extrusion die through which biodegradable material can be extruded which has structural stability in both the longitudinal and transverse directions of the material, which has a flow control device which controls flow of biodegradable material through the extrusion die, and which allows the inner and outer walls of the extrusion orifice to be adjusted relative to each other to modify the circumferencial wall thickness of the cylindrical extrudate. 
     According to one embodiment of the invention, the die extrudes a tubular shaped structure which has its greatest structural stability in a direction which winds helically around the tubular structure. Thus, at the top of the tubular structure, the direction of greatest stability twists in one direction while at the bottom the direction of greatest stability twists in the opposite direction. This tubular structure is then pressed into a sheet comprised of two layers having their directions of greater stability approximately normal to each other. This 2-ply sheet is a flexible, pliable and non-brittle sheet with strength in all directions. 
     According to another embodiment of the present invention, the flow rate of the biodegradable material is regulated at a location upstream from the orifice and at the orifice itself to provide complete control of extrusion parameters. In particular, the head pressure of the biodegradable material behind the extrusion orifice is controlled to produce an extrudate having desired characteristics. 
     According to a further embodiment of the invention, an annular extrusion die allows the inner and outer walls of the extrusion orifice to be adjusted relative to each other to modify the circumferencial wall thickness of the cylindrical extrudate. 
     According to one aspect of the present invention, there is provided an extrusion die for extruding biodegradable material, the extrusion die comprising: a mandrel; an outer member positioned near the mandrel; an extrusion orifice between the mandrel and the outer member; a member in communication with at least one defining member of the extrusion orifice, wherein the member is capable of producing relative movement between the outer member and the mandrel, wherein the relative movement has a component transverse to an extrusion direction of biodegradable material through the extrusion orifice; a flow control device which controls flow of biodegradable material through the extrusion die; and a positioning device which positions the outer member and the mandrel relative to each other. 
     According to another aspect of the invention, there is provided an extrusion die for extruding biodegradable material, the extrusion die comprising: a cylindrical mandrel; a cylindrical outer ring positioned around the mandrel; an annular extrusion orifice between the mandrel and the outer ring; a member in communication with at least one defining member of the annular extrusion orifice which produces angular relative movement between the outer ring and the mandrel; a flow control device which controls flow of biodegradable material through the extrusion die, wherein the flow control device comprises a mechanism which translates the outer ring to adjust the width of the annular extrusion orifice; and a positioning device which positions the outer ring and the mandrel relative to each other. 
     According to a further aspect of the invention, there is provided an extrusion die for extruding biodegradable material, the extrusion die comprising: a mandrel; an outer member positioned near the mandrel; an extrusion orifice between the mandrel and the outer member; a mounting plate having a flow bore which conducts biodegradable material toward the extrusion orifice, wherein the mandrel is fixedly mounted to the mounting plate and the outer member is movably mounted to the mounting plate; a shearing member which moves the outer member relative to the mandrel in a direction having a component transverse to an extrusion direction of biodegradable material through the extrusion orifice; a flow control device which controls flow of biodegradable material through the extrusion die, wherein the flow control device comprises a flow control channel upstream of the extrusion orifice, wherein the flow control channel throttles flow of the biodegradable material through the die, wherein the mandrel is attached to the mounting plate with at least one spacer between, wherein the mounting plate and the mandrel define the flow control channel; and a positioning device which positions the outer member and the mandrel relative to each other, wherein the positioning device comprises a shifting device for moving the outer member and the mandrel relative to each other and a fixing device which fixes the relative positions of the outer member and the mandrel. 
     According to another aspect of the invention, there is provided an improved process for the extrusion of biodegradable material wherein the extrusion comprises flowing the biodegradable material in a flow direction through an orifice, the improvement comprising: moving or shearing the biodegradable material, in a direction having a component transverse to the flow direction, during extrusion; controlling the flow rate of biodegradable material through the extrusion die during extrusion, wherein the controlling comprises adjusting the head pressure of the biodegradable material in the extrusion die and adjusting at least one cross-sectional area of a biodegradable material flow path within the extrusion die; and modifying the orifice geometry. 
     According to another aspect of the invention, there is provided a process for manufacturing biodegradable shaped products of increased strength, the process comprising: extruding a biodegradable material, wherein the extruding comprises moving the biodegradable material in a first direction through an orifice to produce an extrudate; modifying the orifice geometry; shearing the biodegradable material, in a second direction having a component transverse to the first direction, during the extruding; controlling the flow rate of biodegradable material through the extrusion die during the extruding, wherein the controlling comprises adjusting the cross-sectional area of an extrusion orifice and wherein the controlling further comprises adjusting the cross-sectional area of a biodegradable material flow path at a location upstream of the extrusion orifice; compressing the extrudate; and molding the compressed extrudate of biodegradable material into a structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is better understood by reading the following description of non-limitative embodiments, with reference to the attached drawings wherein like parts in each of the several figures are identified by the same reference character, and which are briefly described as follows. 
     FIG. 1 is a cross-sectional view of an embodiment of the invention fully assembled. 
     FIG. 2 is a cross-sectional view of an embodiment of the die fully assembled with centering and flow control devices. 
     FIG. 3 is an exploded perspective view of the several parts which comprise the die shown in FIG.  2 . 
     FIG. 4 is a cross-sectional exploded view of a mandrel, mounting plate and spacers. 
     FIG. 5 is a cross-sectional exploded view of a gap adjusting ring, a bearing housing and an end cap. 
     FIG. 6 is an exploded cross-sectional view of a seal ring, an outer ring and a die wheel. 
     FIG. 7A is a cross-sectional side view of an embodiment of the invention having a motor and belt for rotating an outer ring about a mandrel. 
     FIG. 7B is an end view of the embodiment of the invention as shown in FIG.  7 A. 
     FIG. 8 is a side view of a system for producing molded objects from biodegradable material, the system comprising an extruder, a rotating extrusion die, a cylindrical extrudate, rollers, and molding devices. 
     FIG. 9 is a flow chart of a process embodiment of the invention. 
     FIG. 10A is a perspective view of a cylindrical extrudate of biodegradable material having helical extrusion lines. 
     FIG. 10B is a perspective view of a sheet of biodegradable material produced from the extrudate shown in FIG.  10 A. 
     FIG. 11 is an end view of an embodiment of the invention for rotating the die wheel of the rotating die, the device having a rack gear. 
     FIG. 12A is a perspective view of a cylindrical extrudate having sinusoidal extrusion lines. 
     FIG. 12B is a top view of a sheet of biodegradable material produced from the extrudate shown in FIG.  12 A. 
     FIG. 13 is an end view of a device for rotating the die wheel of an embodiment of the invention wherein the system comprises a worm gear. 
     FIG. 14A is a perspective view of an extrudate of biodegradable material wherein the extrudate is cylindrical in shape and has zigzag extrusion lines. 
     FIG. 14B is a top view of a sheet of biodegradable material produced from the extrudate shown in FIG.  14 A. 
    
    
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of the inventions scope, as the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a cross-section view of an embodiment of the invention is shown. The die  1  is made up of several discrete annular members which share the same longitudinal central axis  3 . A mounting plate  20  is located in the center of the die  1  and is the member to which most of the remaining parts are attached. At one end of the mounting plate  20 , an extruder adapter  10  is attached for connecting the die  1  to an extruder (not shown). A backplate  11  is attached between the extruder adapter  10  and the mounting plate  20 . At an end opposite to the extruder adapter  10 , several spacers  100  are positioned in counter sunk holes in the mounting plate  20  at various locations equidistant from the longitudinal central axis  3 . A mandrel  30  has counter sunk holes which correspond to those in the mounting plate  20 . The mandrel  30  is fixed to the mounting plate  20  with the spacers  100  between, the spacers being inserted into the respective counter sunk holes. On the same side of the mounting plate  20  as the mandrel  30 , a seal ring  40  is inserted into an annular spin channel  22  of the mounting plate  20 . At the periphery of the mounting plate  20 , the mounting plate  20  has a bearing portion  71  which extends around the seal ring  40 . An end cap  80  is attached to the distal end of the bearing portion  71  of the mounting plate  20  to lock the seal ring  40  in the spin channel  22 . An outer ring  50  is attached to the seal ring  40  around the outside of the mandrel  30  to form an extrusion orifice  5  between the outer ring  50  and the mandrel  30 . Finally, a die wheel  90  is attached to the outer ring  50 . As described more fully below, a motor and drive system drive the die wheel  90  to rotate the outer ring  50  about the mandrel  30 . 
     Biodegradable material is pushed through the die  1  under pressure by an extruder (not shown) which is attached to the extruder adapter  10 . The biodegradable material passes through flow bore  23  which conducts the material through the extruder adapter  10  and the mounting plate  20  to a central location at the backside of the mandrel  30 . The biodegradable material is then forced radially outward through a disc-shaped cavity called a flow control channel  4  which is defined by the mounting plate  20  and the mandrel  30 . From the flow control channel  4 , the biodegradable material is pushed through the extrusion orifice  5  defined by the mandrel  30  and the outer ring  50 . According to one embodiment of the invention, the biodegradable material is forced through the extrusion orifice  5 , the die wheel  90 , outer ring  50  and seal ring  40  are rotated relative to the stationary mounting plate  20  and mandrel  30 . 
     Referring to FIGS. 2 and 3, cross-sectional and exploded views, respectively, of an embodiment of the invention with orifice shifting and flow control devices are shown. The die  1  is made up of several discrete annular members which share the same longitudinal central axis  3 . A mounting plate  20  is located in the center of the die  1  and is the member to which most of the remaining parts are attached. At one end of the mounting plate  20 , an extruder adapter is attached for connecting the die  1  to an extruder (not shown). A gap adjusting ring  60  is placed concentrically around the cylindrical exterior of the mounting plate  20 . A bearing housing  70  lies adjacent the gap adjusting ring  60  and the mounting plate  20 . A seal ring  40  is placed within the bearing housing  70  and is inserted into an annular spin channel of the mounting plate  20 . At an end opposite to the extruder adapter  10 , several spacers  100  are positioned in counter sunk holes in the mounting plate  20  at various locations equidistant from the longitudinal central axis  3 . A mandrel  30  has counter sunk holes which correspond to those in the mounting plate  20 . The mandrel is fixed to the mounting plate  20  with the spacers  100  between. An outer ring  50  is attached to the seal ring  40  around the outside of the mandrel  30  to form an extrusion orifice  5  between the outer ring  50  and the mandrel  30 . Finally, a die wheel  90  is attached to the outer ring  50  for rotating the outer ring  50  about the mandrel  30 . 
     Referring to FIG. 4, a cross section of the mounting plate  20 , spacers  100  and the mandrel  30  are shown disassembled. The mounting plate  20  is basically a solid cylinder with a cylindrical flow bore  23  cut in the middle along the longitudinal central axis  3 . One end of the mounting plate  20  comprises a mounting shoulder  21  for engagement with the extruder adapter  10  (shown in FIGS.  2  and  3 ). Opposite the mounting shoulder  21 , the mounting plate  20  has a annular spin channel  22  for receiving the seal ring  40  (shown in FIGS.  2  and  3 ). Between the cylindrical flow bore  23  at the center and the spin channel  22 , the mounting plate  20  has a disc-shaped flow surface  25 . The mounting plate  20  also has several mounting plate counter sunk holes  24  for receiving spacers  100  such that the counter sunk holes  24  are drilled in the flow surface  25 . In FIG. 4, only two counter sunk holes  24  are shown because the view is a cross section along a plane which intersects the longitudinal central axis  3 . All of the mounting plate counter sunk holes  24  are equidistant from each other and from the longitudinal central axis  3 . 
     According to one embodiment of the invention, the mandrel  30  is a bowl shaped structure having a base  31  and sides  32 . As shown in FIG. 4, the mandrel  30  is oriented sideways so that the central axis of the mandrel is collinear with the longitudinal central axis  3  of the die. Unlike the mounting plate  20 , which has a flow bore  23  through the center, the mandrel  30  has a solid base  31 . The outside surface of the base  31  is a base flow surface  33 . The mandrel  30  has several countersunk holes  34  which are cut in the base flow surface  33 . In FIG. 4, only two mandrel countersunk holes  34  are shown because the view is a cross-section along a plane which intersects the longitudinal central axis  3 . All of the mandrel countersunk holes  34  are equidistant from each other and from the central axis  3 . The inside of the mandrel  30  is hollowed out to reduce its overall weight. 
     Spacers  100  are used to mount the mandrel  30  to the mounting plate  20 . Each of the spacers  100  comprise male ends  102  for insertion into mounting plate and mandrel countersunk holes  24  and  34 . Of course, the outside diameter of the male ends  102  is slightly smaller than the inside diameters of mounting plate and mandrel countersunk holes  24  and  34 . Between the male ends  102 , each of the spacers  100  comprise a rib  101  which has an outside diameter larger than the inside diameters of the mounting plate and mandrel countersunk holes  24  and  34 . The rib  101  of each spacer  100  has a uniform thickness in the longitudinal direction to serve as the spacer mechanism between the assembled mounting plate and mandrel. 
     The mandrel  30  is attached to the mounting plate  20  with mandrel bolts  36 . The mandrel bolts  36  extend through the base  31  of the mandrel  30 , through the spacers  100  and into treaded portions in the bottom of the mounting plate counter sunk holes  24 . While the heads of the mandrel bolts  36  could be made to rest firmly against the inside of the base  31  of the mandrel, in the embodiment shown, the mandrel bolts extend through risers  35  so that the heads of the mandrel bolts  36  are more accessible from the open end of the mandrel  30 . In this embodiment, one end of each of the risers  35  rests securely against the inside of the mandrel base  31  while the other end of each riser is engaged by the head of a mandrel bolt  36 . 
     Referring to FIG. 5, a cross-sectional view of the gap adjusting ring  60 , the bearing housing  70 , and the end cap  80  are shown disassembled. The gap adjusting ring  60  is a ring shaped member having a longitudinal central axis  3  and an inner diameter slightly greater than the outside diameter of the mounting plate  20  (shown in FIGS.  2  and  3 ). The gap adjusting ring  60  also has several lock screws  61  which extend through an inner portion  62  of the gap adjusting ring  60  for engagement with the mounting plate  20  once the gap adjusting ring  60  is placed around the outside of the mounting plate  20 . Also, the gap adjusting ring  60  has an outer portion  63  for engagement with the bearing housing  70 . At the outer edge of the outer portion  63 , the gap adjusting ring  60  has shifting lugs  64  which are attached via lug bolts  65 . In the embodiment shown, four shifting lugs  64  are attached to the outer portion  63  of the gap adjusting ring  60 . The shifting lugs  64  are spaced around the gap adjusting ring  60  so that one is at the top, bottom, and sides, respectively. The shifting lugs  64  extend from the outer portion  63  in a longitudinal direction for positioning engagement with the bearing housing  70 . The shifting bolts  66  poke through the shifting lugs  64  in the part of the shifting lugs  64  which extend from the outer portion  63  in the longitudinal direction. The shifting bolts  66  poke through in a direction from outside the die toward the longitudinal central axis  3 . Finally, the gap adjusting ring  60  has threaded holes  67  at various locations around the outer portion  63  for receiving screws  74 . 
     The bearing housing  70  is an annular ring which has a longitudinal central axis  3 . The bearing housing  70  has a bearing portion  71  and a support portion  72 . The support portion  72  is annular with is greatest cross-section in a direction transverse to the longitudinal central axis  3 . The bearing housing  70  is attachable to the gap adjusting ring  60  by the support portion  72  which engages the outer portion  63  of the gap adjusting ring  60 . In the embodiment shown, this engagement between the bearing housing  70  and the gap adjusting ring  60  is accomplished by screws  74  between these two members. The support portion  72  has several slip holes  75  which protrude through the support portion  72  in a longitudinal direction. In one embodiment, twelve slip holes  75  are positioned equidistant from each other around the support portion  72  and are positioned equidistant from the longitudinal central axis  3 . The inside diameter of each slip hole  75  is larger than the outside diameter of screws  74  so that there is substantial “play” between the screws  74  and the slip holes  75 . While the slip holes  75  are larger than the screws  74 , the slip holes  75  are small enough so that the heads of the screws  74  securely engage the support portion  72  of the bearing housing  70 . 
     The other major part of the bearing housing  70  is the bearing portion  71  which is an annular section having its greatest thickness in the longitudinal direction. The interior surface of the bearing portion  71  is a bearing surface  76  for engaging lateral support bearings  42  (shown in FIG.  6 ). The bearing surface  76  supports the lateral support bearings  42  in a plane normal to the longitudinal central axis  3 . Protruding from the bearing surface  76  near the support portion  72 , the bearing housing  70  has a bearing housing lateral support flange  73  which supports a lateral support bearing  42  of the seal ring  40  (shown in FIG.  6 ). 
     When the bearing housing  70  is attached to the gap adjusting ring  60 , the relative positions of the two devices may be adjusted. In particular, during assembly, the shifting bolts  66  of the gap adjusting ring  60  are relaxed to provide enough space for the support portion  72  of the bearing housing  70 . The bearing housing  70  is then placed directly adjacent the gap adjusting ring  60  with the support portion  72  within the extended portions of shifting lugs  64 . The screws  74  are then inserted through the slip holes  75  and loosely screwed into threaded holes  67  in the gap adjusting ring  60 . The shifting bolts  66  are then adjusted to collapse on the support portion  72  of the bearing housing  70 . The shifting bolts  66  may be adjusted to push the bearing housing  70  off center relative to the gap adjusting ring  60 . Because the slip holes  75  are larger than the screws  74 , the shifting bolts  66  freely push the bearing housing  70  in one direction or the other. By varying the pressure of the shifting bolts  66  against the outer surface of the bearing housing  70 , the bearing housing  70 , seal ring  40  and outer ring  50  may be perturbed from their original positions to more desirable positions. Once the desired relative position of the bearing housing  70  to the gap adjusting ring  60  is obtained, the screws  74  are tightened to firmly attach the two members. 
     The end cap  80  is preferably a ring which has a longitudinal central axis  3 . The interior portion of the end cap  80  is a stabilizer  81  and the exterior is a fastener flange  82 . Fastener holes  83  are drilled in the fastener flange  82  for inserting fasteners which secure the end cap  80  to the bearing portion  71  of the bearing housing  70 . The outside diameter of the stabilizer  81  of the end cap  80  is slightly smaller than the inside diameter of the bearing portion  71  of the bearing housing  70 . This allows the stabilizer  81  to be inserted into the bearing portion  71 . At the distal end of the stabilizer  81 , there is an end cap lateral support flange  84  which supports a lateral support bearing  42  (shown in FIG.  6 ). Therefore, when the end cap  80  is securely fastened to the bearing housing  70 , the bearing housing lateral support flange  73  and the end cap lateral support flange  84  brace the lateral support bearings  42  (shown in FIG. 6) against movement in the longitudinal directions. 
     Referring to FIG. 6, a cross-sectional view of the seal ring  40 , the outer ring  50  and the die wheel  90  are shown disassembled. The seal ring  40  is a cylindrical member having a longitudinal central axis  3 . The seal ring  40  has an interior diameter which decreases from one end to the other. At the end of the seal ring  40  which has the smallest inside diameter, the seal ring  40  has a notch  47  for engaging the outer ring  50  as discussed below. On the outside of the seal ring  40 , there are four superior piston rings  41  for engaging the mounting plate  20  and the end cap  80  (both shown in FIGS.  2  and  3 ). The seal ring  40  also comprises two lateral support bearings  42 . The lateral support bearings  42  are separated by a bearing spacer flange  43  which is positioned between the two lateral support bearings  42 . The seal ring  40  further comprises two retaining rings  44  which are positioned on the outsides of the lateral support bearings  42 . Thus, the seal ring  40  is assembled by slipping one of the lateral support bearings  42  over each end of the seal ring  40  until they are each adjacent opposite sides of the bearing spacer flange  43 . Next, retaining rings  44  are slipped over each end of the seal ring  40  until they snap into grooves  45  at the outsides of the lateral support bearings  42 . Thus, the lateral support bearings  42  are secured between the bearing spacer flange  43  and the retaining rings  44 . Finally, the superior piston rings  41  are placed in piston slots  46 . 
     The outer ring  50  is a cylindrical member having a longitudinal central axis  3 . The outer ring  50  has a ring portion  51  and a fastener flange  52 . Longitudinal holes are cut through the fastener flange  52  for inserting fasteners which secure the outer ring  50  to an end of the seal ring  40 . The outside diameter of the ring portion  51  is slightly smaller than the inside diameter of the notch  47  of the seal ring  40 . This allows the outer ring  50  to be assembled to the seal ring  40  by inserting the ring portion  51  into the notch  47 . The inside diameter of the ring portion  51  tapers from the end which attaches to the seal ring  40  to the other. At the end of the ring portion  51  having the smallest inside diameter, the outer ring  50  comprises a lip  53  which defines one side of the extrusion orifice  5  (shown in FIG.  2 ). 
     The die wheel  90  is a cylindrical member with a wheel flange  92  and a drive section  93 . Holes are drilled through the wheel flange  92  for inserting wheel fasteners  91  which secure the die wheel  90  and the outer ring  50  to the seal ring  40 . The drive section  93  is a device which engages a drive mechanism for rotating the die wheel  90 . In the embodiment shown in the figure, the drive section is a pulley for engaging a drive belt. 
     Assembly of the complete die  1  is described with reference to FIGS. 2 and 3. First, the extruder adapter  10  is secured to the mounting plate  20  with a back plate  11  between. Next, with further reference to FIG. 4, several spacers  100  are placed in the mandrel  30  by inserting a male end  102  of each spacer  100  into a mandrel counter sunk hole  34 , until all the mandrel counter sunk holes  34  have a spacer  100 . The mandrel  30  is then placed adjacent the mounting plate  20  with the protruding male ends  102  of the spacers  100  being inserted into the mounting plate counter sunk holes  24 . The mandrel  30  is then attached to the mounting plate  20  with spacers  100  between the mandrel bolts  36 . In particular, the risers  35  are slipped over the shanks of the mandrel bolts  36  and the mandrel bolts  36  are inserted through the mandrel base  31 , the mandrel counter sunk holes  34 , the spacers  100 , and the mounting plate counter sunk holes  24 . The bottoms of the mounting plate counter sunk holes  24  are threaded so that the mandrel bolts  36  may be screwed into the mounting plate  20 . The mandrel bolts  36  are then screwed into the threaded bottoms of each mounting plate counter sunk hole  24  to fasten the mandrel  30  to the mounting plate  20 . With further reference to FIG. 5, the gap adjusting ring  60  is slipped over the exterior of the mounting plate  20 . The lock screws  61  are then tightened against the exterior of the mounting plate  20 . The bearing housing  70  is then positioned with the support portion  72  against the outer portion  63  of the gap adjusting ring  60 . The shifting bolts  66  are adjusted to center the bearing housing  70  about the longitudinal central axis  3  and the screws inserted through slip holes  75  and tightened into the threaded holes  67  of the gap adjusting ring  60 . Next, with further reference to FIG. 6, the seal ring  40  having superior piston rings  41 , lateral support bearings  42  and retaining rings  44  attached thereto, is rotatably attached to the bearing housing  70 . In particular, the seal ring  40  is inserted into the bearing housing  70  and then into the spin channel  22  of the mounting plate  20 . The seal ring  40  is pushed all the way into the spin channel  22  of the mounting plate  20  until the first of the lateral support bearings  42  rests firmly against the bearing housing lateral support flange  73 . In this position, two of the four superior piston rings  41  form a seal between the seal ring  40  and the spin channel  22  of the mounting plate  20 . The seal ring  40  is held in this position by inserting the stabilizer  81  of the end cap  80  into the bearing portion  71  of the bearing housing  70 . The end cap  80  is pushed all the way into the bearing housing  70  until the end cap lateral support flange  84  contacts the second of the lateral support bearings  42  of the seal ring  40 . Once in place, the end cap  80  is fixed to the bearing housing  70  by inserting fasteners through the fasteners holes  83  of the fastener flange  82  and into the bearing portion  71  of the bearing housing  70 . The interior surface of the stabilizer  81  of the end cap  80  engages the remaining two superior pistons rings  41  of the seal ring  40  so that the seal ring  40  is completely stabilized and allowed to spin freely about the longitudinal central axis  3 . With the end cap  80  securely fastened to the bearing housing  70 , the seal ring  40  is securely fastened in the lateral direction between the lateral support flanges  73  and  84 . With the seal ring  40  securely in place, the outer ring  50  and die wheel  90  are then attached to the end which protrudes from the mounting plate  20 . In particular, the ring portion  51  of the outer ring  50  is inserted into the notch  47  of the seal ring  40  and the wheel flange  91  of the die wheel  90  is positioned adjacent the fastener flange  52  of the outer ring  50 . Wheel fasteners  91  are then inserted through the wheel flange  92  and the fastener flange  52  and locked into the seal ring  40 . 
     Once assembled, both the extruder adapter  10  and the mounting plate  20  further comprise a flow bore  23  which extends from the extruder (not shown) to the flow surface  25 , as shown in FIGS. 2 and 4. Thus, the die  1  operates such that biodegradable extrudate material is pushed by the extruder through the flow bore  23  until it reaches the base flow surface  33  of the mandrel  30 . The biodegradable extrudate then flows radially outward around the spacers  100  between the flow surface  25  of the mounting plate  20  and the base flow surface  33  of the mandrel  30 . This disc-like space between the mounting plate  20  and the mandrel  30  is the flow control channel  4 . From the flow control channel  4 , the biodegradable extrudate then enters a cylindrical space between the seal ring  40  and the mandrel  20  and is pushed through this space toward the extrusion orifice  5  between the mandrel  30  and the outer ring  50 . As the biodegradable extrudate moves toward the extrusion orifice  5 , the die wheel  90  is rotated to rotate the outer ring  50  and seal ring  40  around the stationary mandrel  30 . Thus, the biodegradable extrudate is twisted by the rotating outer ring  50 . As the extrudate exits the extrusion orifice  5 , a tubular product of twisted biodegradable material is produced. As described fully below, because the seal ring  40  is rotatably mounted within the bearing housing  70 , the seal ring  40  may be made to rotate about the mandrel  30  as the extrudate is pushed through the orifice  5 . 
     Flow of the biodegradable material through the die  1  is controlled in two ways: (1) adjusting the width of the flow control channel  4 , and (2) controlling the size of the extrusion orifice  5 . Regarding the flow control channel  4 , as noted above, biodegradable material is passed from the extruder through a flow bore  23  in the mounting plate  20  until it reaches the base flow surface  33  of the mandrel  30 . From the central location, the biodegradable material is pushed radially outward between the base flow surface  33  of the mandrel  30  and the flow surface  25  of the mounting plate  20 . Of course, as the biodegradable material flows between the surfaces through the flow control channel  4 , it passes around each of the spacers  100  which separate the mandrel  30  and the mounting plate  20 . The width of the flow control channel  4  is adjusted by using spacers which have larger or smaller ribs  101  (See FIG.  4 ). In particular, if it is desirable to decrease flow of the biodegradable material through the flow control channel  4 , spacers  100  having ribs  101  which are relatively thin in the longitudinal direction are inserted between the mounting plate  20  and the mandrel  30 . Alternatively, if it is desirable to increase a flow rate of biodegradable material through the flow control channel  4 , spacers  100  having ribs  101  with relatively larger thicknesses in the longitudinal direction are inserted between the mounting plate  20  and the mandrel  30 . Therefore, in a preferred embodiment, the die  1  has several sets of spacers  100  which may be placed between the mounting plate  20  and the mandrel  30  to control the width of the flow control channel  4 . 
     Additionally, flow of the biodegradable material through the extrusion orifice  5  is controlled by altering the width of the extrusion orifice  5 . The thickness of the extrusion orifice  5  between the mandrel lip  37  and the outer ring lip  53  is adjusted by sliding the gap adjusting ring  60 , the bearing housing  70 , the seal ring  40 , and the outer ring  50  along the longitudinal central axis  3  out away from the stationary mandrel  30 . Since the interior diameter of the ring portion  51  of the outer ring  50  is tapered from the end which attaches to the seal ring  40 , the outer ring  50  has its smallest interior diameter at the outer ring lip  53 . To produce a biodegradable extrudate with a very thin wall thickness, the gap adjusting ring  60  is pushed all the way onto the mounting plate  20  until the outer ring lip  53  is directly opposite the mandrel lip  37 . To produce a thicker biodegradable extrudate, the gap adjusting ring  60  is moved slightly away from the mounting plate  20  along the longitudinal central axis  3  in the direction of direction arrow  6  (shown in FIG.  2 ), so that the outer ring lip  53  is positioned beyond the mandrel lip  37 . Thus, a wider section of the ring portion  51  is adjacent the lip  37  of the mandrel  30  so that the extrusion orifice  5  is thicker. Once the desired orifice size is obtained, lock screws  61  are screwed into the gap adjusting ring  60  to re-engage the mounting plate  20 . This locks the gap adjusting ring  60 , the bearing housing  70 , the seal ring  40 , and the outer ring  50  in place to ensure the thickness of the extrusion orifice  5  remains constant during operation. A thicker extrusion orifice  5  increases flow through the die. 
     Referring to FIGS. 7A and 7B, side and end views of portions of an embodiment of the invention for rotating the outer ring of the die are shown, respectively. The mandrel  30  is attached to the mounting plate  20  so that the mandrel  30  is locked in place. The seal ring  40  and outer ring  50  are rotatably mounted around the mandrel  30 . A die wheel  90  is also attached to the outer ring  50 . All of these members have longitudinal central axes which are collinear with longitudinal central axis  3 . The device also has a motor  110  which has a drive axis  113  which is parallel to longitudinal central axis  3 . Attached to a drive shaft of motor  110 , there is a drive wheel  111 . The motor  110  and drive wheel  111  are positioned so that drive wheel  111  lies in the same plane as the die wheel  90 , the plane being perpendicular to the longitudinal central axis  3 . Opposite the drive wheel  111 , the system further has a snubber wheel  115  which is also positioned in the perpendicular plane of the drive wheel  111  and the die wheel  90 . The snubber wheel  115  has a snubber axis  116  which is also parallel to the longitudinal central axis  3 . Thus, the drive wheel  111  and the snubber wheel  115  are positioned at opposite ends of the system with the die wheel  90  between. A drive belt  112  engages the drive wheel  111 , the die wheel  90  and the snubber wheel  115 . The snubber wheel  115  has no drive mechanism for turning the drive belt  112 . Rather, the snubber wheel  115  is an idle wheel which only turns with the drive belt  112  when the drive belt  112  is driven by the motor  110 . The snubber wheel  115  serves only to evenly distribute forces exerted by the drive belt  112  on the die wheel  90 . Because the drive wheel  111  and snubber wheel  115  are positioned on opposite sides of the die wheel  90 , forces exerted by the drive belt  112  on the die wheel  90  are approximately equal in all transverse directions. If the snubber wheel  115  were not placed in this position and the drive belt  112  engaged only the drive wheel  111  and the die wheel  90 , a net force would be exerted by the drive belt  112  on the die wheel  90  in the direction of the motor  110 . This force would pull the die wheel  90  and thus the outer ring  50  out of center from its position about the stationary mandrel  30 . Of course, this would have the detrimental effect of producing an extrudate tube of biodegradable material which would have a wall thickness greater on one side than on the other. Therefore, the snubber wheel  115  is positioned in the system to prevent the die wheel  90  from being pulled from its central location around the mandrel  30 . 
     In a preferred embodiment, the drive belt  112  is a rubber belt. Alternatively, chains or mating gears may be used to mechanically connect the motor  110  to the die wheel  90 . A typical one-third horse power electric motor is sufficient to produce the necessary torque to drive the drive belt  112 . Further, the gear ratios between the drive wheel  111  and the die wheel  90  are such that the die wheel  90  may preferably rotate at approximately 15 rotations per minute. Depending on the particular gear system employed, alternative embodiments require more powerful motors. 
     Referring to FIGS. 8 and 9, system and method embodiments of the invention are described for producing a biodegradable final product, respectively. The system  130  has a hopper  131  into which biodegradable material is initially placed (step  140 ). The hopper  131  supplies (step  141 ) biodegradable material to an extruder  132  which pressurizes (step  142 ) and cooks (step  143 ) the biodegradable material. The extruder  132  pushes (step  144 ) the biodegradable material through an extrusion die  1 . The extrusion die  1  is an embodiment of the rotating extrusion die of the present invention and is driven by a motor  110  with a drive belt  112 . As the biodegradable material is pushed (step  144 ) through the extrusion die  1 , an outer ring of the die  1  is rotated (step  145 ) around an inner mandrel. The biodegradable material is pushed (step  146 ) from the extrusion die  1  through an extrusion orifice to form a cylindrical extrudate  15 . The cylindrical extrudate  15  is then pulled (step  147 ) from the extrusion orifice by a pair of press rollers  133 . Next, the press rollers  133  flatten (step  148 ) the cylindrical extrudate  15  into a sheet  17  of biodegradable material. The sheet  17  of biodegradable material is then molded (step  149 ) between corresponding molds  134  to form the biodegradable material into final products. The shaped final products are then deposited in bin  135 . 
     According to alternative embodiments of the invention, it is desirable to stretch the cylindrical extrudate  15  as it exits the extrusion orifice  5 . This is accomplished by rotating the press rollers  133  slightly faster than a speed necessary to keep pace with the exit rate of the cylindrical extrudate  15  from the extrusion orifice  5 . As the press rollers  133  rotate faster, the cylindrical extrudate  15  is pulled by the press rollers  133  from the extrusion orifice  5  so that the cylindrical extrudate  15  is stretched in the longitudinal direction before it is flattened into a flat 2-ply sheet. 
     Referring to FIG. 10A, an example of a biodegradable extrudate from the extrusion die of the present invention is shown. The extrudate  15  exits from the extrusion orifice  5  (see FIG. 2 for die components) as a cylindrical structure. Typically, while not meant to be limited thereby, it is believed the polymer chains of the biodegradable material are aligned in the direction of extrusion to produce an extrudate which has its greatest structural integrity in the extrusion direction. If the extrudate  15  exits the extrusion orifice  5  as the outer ring  50  is rotated around the mandrel  30 , the extrudate  15  orients along extrusion lines  16 . 
     Preferably, the cylindrical extrudate  15  is collapsed to form a sheet of biodegradable material having two extrudate layers. As shown in FIG. 10B, a perspective view of a sheet of extrudate material produced from the tubular extrudate of FIG. 10A is shown. The sheet  17  is produced simply by rolling the extrudate  15  through two rollers to compress the tubular extrudate  15  into the sheet  17 . The sheet  17  consequently comprises extrusion lines  16  which form a cross-hatch pattern. The sheet  17  is comprised of two layers, one of which previously formed one side of the tubular extrudate  15  while the second layer of the sheet  17  previously formed the other side of the extrudate  15 . Therefore, because the extrusion lines  16  were helically wound around the extrudate  15 , when the sheet  17  is formed, the extrusion lines  16  of the two layers run in opposite directions. The extrusion line angle  18  of the extrusion lines  16  may be adjusted by controlling the flow rate of the extrudate  15  from the extrusion orifice  5  of the die  1  (see FIG. 2 for die components), and controlling the speed of angular rotation of the outer ring  50  about the mandrel  30 . If it is desirable to increase the extrusion line angle  18 , the die is adjusted to increase the angular speed of the outer ring  50  relative to the mandrel  30 , and/or to decrease the flow rate of the extrusion material from the extrusion die. As noted above, the flow rate of the biodegradable material through the die is controlled by adjusting the size of the extrusion orifice  5  and/or the flow control channel  4 . 
     According to one embodiment of the invention, the outer ring  50  of the die  1  is made to rotate in both clockwise and counter-clockwise directions about the mandrel  30  to produce a biodegradable extrudate wherein the extrusion lines have a wave pattern. To produce this extrudate, the outer ring  50  is first rotated in one direction and then rotated in the opposite direction. Depending on the rates of direction change, the pattern produced is sinusoidal, zigzag, or boxed. The periods and amplitudes of these wave patterns are adjusted by altering the rate of rotation of the outer ring  50  and the flow rate of the biodegradable material through the extrusion die  1 . 
     Many different drive systems are available for alternating the direction of rotation of the outer ring  50 . For example, the motor  110  of the embodiment shown in FIGS. 7A and 7B is made to alternate directions of rotation. As the motor  110  changes directions of rotation, the drive wheel  111 , drive belt  112  and die wheel  90  consequently change directions. 
     Alternatively, as shown in FIG. 11, the die wheel  90  is a spur gear with radial teeth parallel to the longitudinal central axis  3 . The teeth of the die wheel  90  are engaged by teeth of a rack gear  117 . Opposite the rack gear  117 , an idler gear  124  is engaged with the die wheel  90  to prevent the rack gear  117  from pushing the outer ring  50  out of alignment with the mandrel  30  (See FIG.  2 ). The rack gear  117  is mounted on a slide support  118  and moves linearly along a slide direction  120  which is transverse to the longitudinal central axis  3 . The slide support  118  is connected to a drive wheel  111  via a linkage  114 . In particular, one end of the linkage  114  is connected to an end of the slide support  118  and the other end of the linkage  114  is connected to the drive wheel  111  at its periphery. The slide support  118  is braced by brackets  125  so that slide support  118  is only allowed to move along slide direction  120 . As the drive wheel  111  rotates clockwise around rotation direction  119 , the linkage  114  pushes and pulls the slide support  118  back and forth along slide direction  120 . The back and forth movement of the slide support  118  rotates the die wheel  90  and the outer ring  50  alternatively in clockwise and counter-clockwise directions. 
     Since the linkage  114  is connected to the drive wheel  111  at its periphery, as noted above, the alternative clockwise and counter-clockwise rotation of the outer ring  50  is a sinusoidal oscillatory type motion. Thus, this embodiment of the invention produces a biodegradable extrudate  15  with extrusion lines  16  which have a sine wave pattern as shown in FIG.  12 A. As described above, the extrudate  15  is rolled into a sheet  17  having two layers as shown in FIG.  12 B. The period of the sine waves are identified by reference character  19  and the amplitude is identified by reference character  14 . The period  19  and amplitude  14  of extrusion lines  16  may be adjusted by controlling the flow rate of the extrudate  15  from the extrusion orifice  5  of the die  1  (see FIG. 2 for die components), and controlling the speed of angular rotation of the outer ring  50  about the mandrel  30 . If it is desirable to increase the period of the sine waves, the die is adjusted to decrease the angular speed of the outer ring  50  relative to the stationary mandrel  30 , and/or to increase the flow rate of the extrusion material from the extrusion orifice  5 . As noted above, the flow rate of the biodegradable material through the die is controlled by adjusting the size of the extrusion orifice  5  and/or the flow control channel  4 . Further, if it is desirable to increase the amplitude  14  of the sine waves, the angular range of motion of the outer ring  50  is increased so that the outer ring  50  rotates further around the stationary mandrel  30  before it stops and changes direction. While many parameters may be altered to produce this result, a simple modification is to use a drive wheel  111  which has a relatively larger diameter. 
     A similar embodiment of the invention which rotates the outer ring in clockwise and counter-clockwise directions is shown in FIG.  13 . As before, the die wheel  90  is a spur gear with radial teeth parallel to the longitudinal central axis  3 . The teeth of the die wheel  90  are engaged by teeth of a worm gear  122  which is positioned with its axis of rotation transverse to the longitudinal central axis  3 . Opposite the worm gear  122 , an idler gear  124  is engaged with the die wheel  90  to prevent the worm gear  122  from pushing the outer ring  50  out of alignment with the mandrel  30  (see FIG.  2 ). The worm gear  122  is driven by a motor  110  with a transmission  121  between. A drive shaft  123  of the motor  110  is connected to a power side of the transmission  121  and the worm gear  122  is connected to a drive side of the transmission  121 . While the motor  110  rotates the drive shaft  123  in only one direction, the transmission  121  rotates the worm gear  122  in both clockwise and counter-clockwise directions. Further, in one embodiment, the transmission  121  rotates the worm gear  122  at different speeds even though the motor  110  operates at only one speed. A similar embodiment comprises a motor and transmission which drive a pinion gear which engages the die wheel  90 . Since the worm gear  122  is rotated at a constant speed in each direction, this embodiment of the invention produces a biodegradable extrudate which has a zigzag pattern of extrusion lines  16 . 
     Since the motor  110  runs at constant angular velocity and the transmission is used to change the direction of rotation of the worm gear  122 , the alternative clockwise and counter-clockwise rotation of the outer ring  50  is an oscillatory type motion. Thus, this embodiment of the invention produces a biodegradable extrudate  15  with extrusion lines  16  which have a linear oscillatory wave pattern or zigzag wave pattern as shown in FIG.  14 A. As described above, the extrudate  15  is rolled into a sheet  17  having two layers as shown in FIG.  14 B. The period of the zigzag waves are identified by reference character  19  and the amplitude is identified by reference character  14 . The period  19  and amplitude  14  of extrusion lines  16  is adjusted by controlling the flow rate of the extrudate  15  from the extrusion orifice  5  of the die  1  (see FIG. 2 for die components), and controlling the speed of angular rotation of the outer ring  50  about the mandrel  30 . If it is desirable to increase the period of the zigzag waves, the die is adjusted to decrease the angular speed of the outer ring  50  relative to the stationary mandrel  30 , and/or to increase the flow rate of the extrusion material from the extrusion orifice  5 . As noted above, the flow rate of the biodegradable material through the die is controlled by adjusting the size of the extrusion orifice  5  and/or the flow control channel  4 . Further, if it is desirable to increase the amplitude  14  of the zigzag waves, the angular range of motion of the outer ring  50  is increased so that the outer ring  50  rotates further around the stationary mandrel  30  before it stops and changes direction. While many parameters may be altered to produce this result, a simple modification is to control the transmission  121  to allow the worm gear  122  to run longer in each direction before reversing the direction. 
     While the particular embodiments for extrusion dies as herein shown and disclosed in detail are fully capable of obtaining the objects and advantages herein before stated, it is to be understood that they are merely illustrative of the preferred embodiments of the invention and that no limitations are intended by the details of construction or design herein shown other than as described in the appended claims. 
     LIST OF CHARACTER DESIGNATIONS 
       1 . Die 
       3 . Longitudinal Central Axis 
       4 . Flow Control Channel 
       5 . Extrusion Orifice 
       6 . Direction Arrow 
       10 . Extruder Adapter 
       11 . Back Plate 
       14 . Extrusion Wave Amplitude 
       15 . Extrudate 
       16 . Extrusion Lines 
       17 . Sheet 
       18 . Extrusion Line Angle 
       19 . Extrusion Wave Period 
       20 . Mounting Plate 
       21 . Mounting Shoulder 
       22 . Spin Channel 
       23 . Flow Bore 
       24 . Countersunk Holes 
       25 . Flow Surface 
       30 . Mandrel 
       31 . Mandrel Base 
       32 . Mandrel Sides 
       33 . Base Flow Surface 
       34 . Countersunk Holes 
       35 . Risers 
       36 . Mandrel Bolts 
       37 . Mandrel Lip 
       40 . Seal Ring 
       41 . Superior Piston Rings 
       42 . Lateral Support Bearings 
       43 . Bearing Spacer Flange 
       44 . Retaining Rings 
       45 . Grooves 
       46 . Piston Slots 
       47 . Notch 
       50 . Outer Ring 
       51 . Ring Portion 
       52 . Fastener Flange 
       53 . Outer Ring Lip 
       55 . Outer Die Structure 
       60 . Gap Adjusting Ring 
       61 . Lock Screws 
       62 . Inner Portion 
       63 . Outer Portion 
       64 . Centering Lugs 
       65 . Lug Bolts 
       66 . Centering Bolts 
       67 . Threaded Holes 
       70 . Bearing Housing 
       71 . Bearing Portion 
       72 . Support Portion 
       73 . Lateral Support Flange 
       74 . Screws 
       75 . Slip Holes 
       76 . Bearing Surface 
       80 . End Cap 
       81 . Stabilizer 
       82 . Fastener Flange 
       83 . Fastener Holes 
       84 . Lateral Support Flange 
       90 . Die Wheel 
       91 . Wheel Fastener 
       92 . Wheel Flange 
       93 . Drive Section 
       100 . Spacer 
       101 . Rib 
       102 . Male Ends 
       110 . Motor 
       111 . Drive Wheel 
       112 . Drive Belt 
       113 . Drive Axis 
       114 . Linkage 
       115 . Snubber Wheel 
       116 . Snubber Axis 
       117 . Rack Gear 
       118 . Slide Support 
       119 . Rotation Direction 
       120 . Slide Direction 
       121 . Transmission 
       122 . Worm Gear 
       123 . Drive Shaft 
       124 . Idler Gear 
       125 . Brackets 
       130 . Biodegradable Product Producing System 
       131 . Hopper 
       132 . Extruder 
       133 . Press Rollers 
       134 . Molds 
       135 . Bin