Abstract:
The present invention is directed to a device for the production of a cellular wood plastic composite material comprised of an orifice that conducts the composite material from the adapter of the extruder to the transition die plate in such a manner that a uniform flow of material reaches the transition die plate; a transition die plate that further directs the flow of material to a flow restriction die plate in a manner ensuring that equal amounts of material are delivered to all areas of the flow restriction die plate; a flow restriction die plate that provides sufficient resistance to material flow to increase the melt pressure of the portion of the material that is upstream in relation to the flow restriction die plate and controls the temperature increase caused by this restriction by dividing the flow into numerous suitably sized and shaped streams; a compression die plate that fuses the separate streams issuing from the flow restriction die plate into a single stream of material and maintains the melt pressure at a level which will prevent premature development of cells in the material; a shaping die plate that is designed to shape the material in such a way that the fully expanded material will approximate the shape of the desired profile and to control the rate of cell development and expansion so that large numbers of uniform cells are produced.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     The application claims priority to U.S. Provisional Application entitled “Extruded Cellulose-Polymer Composition and System for Making Same,” Ser. No. 60/844,827, filed Sep. 15, 2006, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is directed to an extruded wood-polymer composite material suitable for use in place of natural wood and to a die, machine and process for manufacturing the composite material.  
       DESCRIPTION OF THE PRIOR ART  
       [0003]     U.S. Pat. No. 5,516,472 to Layer is incorporated by reference in its entirety. Layer teaches the use of a stranding die in the extrusion of a synthetic wood comprised predominantly of a thermoplastic material and cellulosic filler. The stranding die produces a plurality of strands which are then compressed and fused together in a molding die. The compressed mass then passes through a shaping die plate where the final profile shape is established. This arrangement does not allow for expansion of the profile after it exits the shaping die plate and, as a result, does not give the reduction in density desired in a cellular composite.  
         [0004]     Extrusion of cellular, i.e., foamed, wood plastic composites depends on the formation of gas bubbles or cells within the composite matrix. If the gas can be introduced under pressure, it will be more evenly dispersed throughout the matrix resulting in a more uniform cell structure with an increased number of cells. This type of structure is accepted as being more desirable. Development of pressure in the extrusion process is a result of restriction of the flow of the extrudate. The first problem encountered was development of suitable pressure while producing a profile with a large enough cross section to be useful. Obviously, a large profile necessarily involves a lesser degree of restriction of the extrudate flow than a small profile.  
         [0005]     It is possible to increase the amount of restriction when extruding a large profile by dividing the flow into multiple channels. One method of dividing the flow is presented in U.S. Pat. No. 3,573,152 to Wiley et al. Wiley et al. teach the use of multiple orifices in a die for the purpose of increasing pressure in a molten plastic mass containing a foaming agent when the opening of the die is too large to produce the pressure required by the foaming process. These orifices are spaced so that the desired degree of expansion will occur when the molten plastic streams expand due to the presence of gas bubbles produced by the blowing agent and occupy the space between the orifices. A portion of the die downstream from the orifices may contain the expanded streams so that they conform to some desired shape but do not retard the expansion. Very low density foams are made by this process.  
         [0006]     Division of the flow into multiple channels causes a secondary problem. The divided flows must be fused back together again to form a cohesive structure. During extrusion of the unfilled polymer foams described by Wiley et al., the expanding streams readily adhere to one another. Addition of wood and/or other fillers to the polymer introduces a difficulty. The resulting composite streams will not adhere sufficiently without additional compression because the amount of polymer on the surface has been reduced by the added filler. The polymer streams produced by the process revealed by Wiley et al. are too widely spaced to produce a cohesive mass necessary for the production of a cellular composite profile with a unified structure. Compression of the strands or streams after the orifice is required.  
         [0007]     The stranding plate revealed by Layer will produce a flow restriction by dividing the extrudate flow, but the orifice size must be adapted to produce optimum conditions for cell formation. The molding die presented in Layer must also be adapted to produce the amount of compression necessary to create a cohesive mass without increasing the temperature of the extrudate.  
         [0008]     The purpose of this invention is to provide a means of creating sufficient restriction of extrudate flow to cause the increase in pressure necessary for effective cell formation when extruding a large profile while at the same time providing a means of producing a profile with a cohesive structure.  
       SUMMARY OF THE INVENTION  
       [0009]     Referring now to the figures for an exemplary version of the system of the present invention, the present invention is directed to a die system  14  for extruding a cellular, foamed extrudate from an extruder  12 , the extruder  12  including an exit opening  13 , and forming a composite molded extrudate product having a substantially uniform cell structure from a mixture of organic fibrous material and thermoplastic material. The die system  14  includes the following plates:  
         [0010]     a. an adapter die plate  20 , removably connected to the extruder  12  for receiving the extrudate from the exit opening  13  of the extruder  12 , the adapter die plate  20  including a front opening  22 , a rear opening  23  and a flow channel  26  connecting the front opening  22  to the rear opening  23 ;  
         [0011]     b. at least one transition die plate  30  adjacent the adapter die plate  20 , the transition die plate  30  including a front opening  31 , a rear opening  33  and a flow channel  32  connecting the front opening  31  to the rear opening  33 ;  
         [0012]     c. a flow restriction die plate  60  adjacent the transition die plate  30 , the flow restriction die plate  60  having a front opening  65 , a rear opening  67  and a flow channel  63  connecting the front opening  65  to the rear opening  67 , wherein the flow channel  63  includes a plurality of stranding channels  68  to divide the flow of extrudate, wherein the stranding channels  68  have a diameter and length to provide sufficient resistance pressure to the extrudate flow such that the resistance pressure of the extrudate entering the adapter die plate  20  is increased, wherein the increase in resistance pressure alters the extrudate entering the adapter die plate  20  such that the extrudate entering the adapter die plate  20  is characterized by increased uniform cell structure and lower density, wherein flow channel  32  of the transition die plate  30  is shaped such that the flow of extrudate to the flow restriction die plate  60  ensures that equal amounts of extrudate are delivered to the stranding channels  68 ;  
         [0013]     d. a compression die plate  70  adjacent the flow restriction die plate  60  and comprising a front opening  72 , a rear opening  74  and a flow channel  76 , wherein the compression die plate front opening  72  is adjacent the flow restriction die plate rear opening  67 , wherein the compression die plate flow channel  76  is shaped to reform the extrudate into a single stream of extrudate and wherein further the compression die plate flow channel  76  is shaped to maintain the melt pressure of the extrudate at a level which will prevent premature development of cells in the extrudate material;  
         [0014]     e. a shaping die plate  80  adjacent the compression die plate  70  and comprising a front opening  82 , a rear opening  84  and a flow channel  86 , wherein shaping die plate front opening  82  is adjacent the compression die plate rear opening  74  and wherein the shaping die plate flow channel  86  is shaped to approximate the shape of the desired profile of a final extruded product  16  and to control the rate of cell development and expansion so that large numbers of uniform cells are produced, wherein ratio of the volume of flow channels  32 ,  46  and/or  56  of the transition die plates  30 ,  40 , and/or  50  to the volume of the shaping die plate  80  ranges from 1.05:1 to 3.45:1.  
         [0015]     The present invention is also directed to a process for forming a cellular, foamed extrudate from an extruder  12 , the extruder  12  including an exit opening  13 , and forming a composite molded extrudate product  16  having a substantially uniform cell structure from a mixture of organic fibrous material and thermoplastic material, the process comprising the following steps:  
         [0016]     a. mixing the cellulosic material and thermoplastic material in a hopper  10 ;  
         [0017]     b. forwarding the mixed material to an extruder  12  to form an extrudate;  
         [0018]     c. passing the extrudate through an adapter die plate  20  removably connected to the extruder  12  for receiving the extrudate from the exit opening  13  of the extruder  12 , the adapter die plate  20  including a front opening  22 , a rear opening  23  and a flow channel  26  connecting the front opening  22  to the rear opening  23 ;  
         [0019]     d. passing the extrudate through at least one transition die plate  30  adjacent the adapter die plate  20 , the at least one transition die plate  30  including a front opening  31 , a rear opening  33  and a flow channel  32  connecting the front opening  31  to the rear opening  33  wherein the transition die plate flow channel  32  has a shape designed to transform the extruded material discharged from the flow channel  26  of the adapter die plate  20  to a shape more generally approaching that of a finished extruded product;  
         [0020]     e. passing the extruded material through a flow restriction die plate  60  adjacent the transition die plate  30 , the flow restriction die plate  60  having a front opening  65 , a rear opening  67  and a flow channel  63  connecting the front opening  65  to the rear opening  67 , wherein the flow channel  63  includes a plurality of contiguous stranding channels  68  to divide the flow of extrudate, wherein the stranding channels  68  have a diameter and length designed to increase the resistance pressure to the extrudate, wherein the increase in resistance pressure alters the extrudate entering the adapter die plate  20  such that the extrudate entering the adapter die plate  20  is characterized by increased uniform cell structure and lower density, wherein flow channel  32  of the transition die plate  30  is shaped such that the flow of extrudate to the flow restriction die plate  60  ensures that equal amounts of extrudate are delivered to the stranding channels  68 ;  
         [0021]     f. passing the extruded material through a compression die plate  70  adjacent the flow restriction die plate  60 , the compression die plate  70  comprising a front opening  72 , a rear opening  74  and a flow channel  76 , wherein the compression die plate front opening  72  is adjacent the flow restriction die plate rear opening  67 , wherein the front face  72  of the flow channel  76  of the compression die plate  70  has a profile equal to the profile of the area of all of the channels  68  within the flow channel  63  in the flow restriction die plate  60  plus the area of the metal that defines the areas between the multiple channels  68  together which make up the flow channel  63  of the flow restriction die plate  60 , wherein the compression die plate flow channel  63  is shaped to reform the extrudate into a single stream of extrudate and wherein further the compression die plate flow channel  76  is shaped to maintain the melt pressure at a level which will prevent premature development of cells in the material;  
         [0022]     g. passing the extruded material to a shaping die plate  80 , the shaping die plate  80  adjacent the compression die plate  70  and comprising a front opening  82 , a rear opening  84  and a flow channel  86 , wherein shaping die plate front opening  82  is adjacent the compression die plate rear opening  74  and wherein the shaping die plate flow channel  86  is shaped to approximate the shape of the desired profile of a final extruded product  16  and to control the rate of cell development and expansion so that large numbers of uniform cells are produced; and  
         [0023]     h. cooling the extruded product material in the cooling tank  18 .  
         [0024]     The present invention is also directed to a composite molded product having a substantially uniform cell structure from a mixture of cellulosic material and thermoplastic material having the following characteristics:  
         [0025]     a. a density of from 0.50 gm/cc to 0.90 gm/cc,  
         [0026]     b. a flexural modulus of elasticity ranging from 100 ksi to 250 ksi,  
         [0027]     c. a coefficient of linear thermal expansion (CLTE) ranging from 24.5×10 −6  in/in-° F. to 32.0×10 −6  in/in-° F., and  
         [0028]     d. a filler to resin ratio ranging from 0.75:1 to 1:1.  
         [0029]     One of the main advantages to this process is that the final molded product has virtually no expansion after it leaves the molding die. This is due to the low temperature processing in the extruder and die system as well as the unique design of the plates.  
         [0030]     The unique die system of the present invention allows the combined starting materials to bond into a shaped, homogeneous product wherein the final extruded product has a desired reduction in cell density by creating a sufficient restriction of extrudate flow to cause the increase in pressure necessary for effective cell formation when extruding a large extrudate profile and provide a means of producing a profile with a cohesive structure.  
         [0031]     The objects and advantages of the invention will appear more fully from the following detailed description of the preferred embodiment of the invention made in conjunction with the accompanying drawings and attachments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1  is a flow chart illustrating the process of the present invention.  
         [0033]      FIG. 2  is an exploded perspective view of the die system of the present invention illustrating each of the die plates.  
         [0034]      FIG. 3  is a cross-sectional view of the die system of the present invention illustrating each of the die plates.  
         [0035]      FIG. 4A  is a cross-sectional view of the adapter die plate  20  of  FIG. 3  taken along lines  4 A- 4 A of  FIG. 4D .  
         [0036]      FIG. 4B  is a front elevated view of the adapter die plate  20  of  FIG. 4A .  
         [0037]      FIG. 4C  is a cross-sectional view of the adapter die plate  20  of  FIG. 4A  taken along lines  4 C- 4 C of  FIG. 4D .  
         [0038]      FIG. 4D  is a perspective view of the adapter die plate of  FIG. 4A .  
         [0039]      FIG. 5A  is a cross-sectional view of the transition die plate  30  of the present invention taken along lines  5 A- 5 A of  FIG. 5D .  
         [0040]      FIG. 5B  is a front elevated view of the transition die plate  30  of  FIG. 5A .  
         [0041]      FIG. 5C  is a cross-sectional view of the transition die plate  30  of  FIG. 5A  taken along lines  5 C- 5 C of  FIG. 5D .  
         [0042]      FIG. 5D  is a perspective view of the transition die plate  30  of  FIG. 5A .  
         [0043]      FIG. 6A  is a cross-sectional view of the transition die plate  40  of the present invention taken along lines  6 A- 6 A of  FIG. 6D .  
         [0044]      FIG. 6B  is a front elevated view of the transition die plate  40  of  FIG. 6A .  
         [0045]      FIG. 6C  is a cross-sectional view of the transition die plate  40  of  FIG. 6A  taken along lines  6 C- 6 C of  FIG. 6D .  
         [0046]      FIG. 6D  is a perspective view of the transition die plate  40  of  FIG. 6A .  
         [0047]      FIG. 7A  is a cross-sectional view of the transition die plate  50  of the present invention taken along lines  7 A- 7 A of  FIG. 7D .  
         [0048]      FIG. 7B  is a front elevated view of the transition die plate  50  of  FIG. 7A .  
         [0049]      FIG. 7C  is a cross-sectional view of the transition die plate  50  of  FIG. 7A  taken along lines  7 C- 7 C of  FIG. 7D .  
         [0050]      FIG. 7D  is a perspective view of the transition die plate  50  of  FIG. 7A .  
         [0051]      FIG. 8A  is a cross-sectional view of the flow restriction die plate  60  of the present invention taken along lines  8 A- 8 A of  FIG. 8D .  
         [0052]      FIG. 8B  is a front elevated view of the flow restriction die plate  60  of  FIG. 8A .  
         [0053]      FIG. 8C  is a cross-sectional view of the flow restriction die plate  60  of  FIG. 8A  taken along lines  8 C- 8 C of  FIG. 8D .  
         [0054]      FIG. 8D  is a perspective view of the flow restriction die plate  60  of  FIG. 8A .  
         [0055]      FIG. 9A  is a cross-sectional view of the compression die plate  70  of the present invention taken along lines  9 A- 9 A of  FIG. 9D .  
         [0056]      FIG. 9B  is a front elevated view of the compression die plate  70  of  FIG. 9A .  
         [0057]      FIG. 9C  is a cross-sectional view of the compression die plate  70  of  FIG. 9A  taken along lines  9 C- 9 C of  FIG. 9D .  
         [0058]      FIG. 9D  is a perspective view of the compression die plate  70  of  FIG. 9A .  
         [0059]      FIG. 10A  is a cross-sectional view of the shaping die plate  80  of the present invention taken along lines  10 A- 10 A of  FIG. 10D .  
         [0060]      FIG. 10B  is a front elevated view of the shaping die plate  80  of  FIG. 10A .  
         [0061]      FIG. 10C  is a cross-sectional view of the shaping die plate  80  of  FIG. 10A  taken along lines  10 C- 10 C of  FIG. 10D .  
         [0062]      FIG. 10D  is a perspective view of the shaping die plate  80  of  FIG. 10A . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0063]     Reference is made to Layer for an explanation of the general practice of composite wood extrusion techniques. With the exception of the actual cellular composite extrusion die of the present invention and unless otherwise noted, Layer is incorporated herein to describe the standard extrusion practices.  
         [0064]     The present invention is directed to a wood-polymer composite product, as well as the process and machine for making the product. The invention is specifically directed to the combination of a low-temperature extruder and the die system.  
         [0000]     Cellulosic Materials:  
         [0065]     The cellulosic fibrous-polymer composite material of the present invention is characterized by having a higher cellulosic fiber content than normally recognized in the prior art. While the prior art normally requires a material content including approximately 50% fiber to 50% thermoplastic material, the material of the present invention preferably has a higher fiber content. The material can have up to a near 1:0 fiber/thermoplastic content by employing the continuous low temperature extrusion process of the present invention and the proper mix of starting materials. The basic process requires mixing of basic types of raw materials including cellulosic fibers and thermoplastic materials. Cross-linking agents and process lubricants may also be included in the basic mixture.  
         [0066]     One advantage of the present invention is that it can incorporate virtually any kind of waste cellulosic material from sawdust to pond sludge and newspapers. As described earlier, any cellulosic material may be used as a raw material including old newspapers, alfalfa, wheat pulp, wood chips, wood particles, wood flour, wood flakes, wood fibers, ground wood, wood veneers, wood laminates, kenaf, paper, cardboard, straw, and other cellulosic fibrous materials. The cellulosic fibrous material may also comprise refined cellulose such as cotton or viscous and plant fibers such as kenaf, bamboo or palm fiber, straw or any other cellulosic fibrous material. Prior to being combined with the other starting materials, the cellulosic materials should be dried to a moisture content between approximately 1% and 9%. A preferred moisture content is no more than 2%. Drying technologies are known to the art. A suitable example is a desiccant dryer manufactured by Premier Pneumatics, Inc. (Allentown, Pa.).  
         [0000]     Thermoplastic Materials:  
         [0067]     The thermoplastic materials serve primarily as a process fluidizer. Most types of thermoplastic materials may be used, examples of which include multi-layer films, virgin thermoplastics such as polyethylene, polypropylene, poly-vinyl chloride (PVC), low density polyethylene (LDPE), copoly-ethylene-vinyl acetate and waste plastic sawdust from other industries as well as other recyclable polymer materials. Although thermoplastic materials are a preferable component in the make-up of the starting materials, it is not required. As long as the starting material includes a sufficient amount of cross-linking agents and lubricants to “plasticize” the mixture in the extruder, the starting materials do not necessarily require the use of thermoplastic materials.  
         [0068]     The ratio of cellulosic fibers to the thermoplastic material is, therefore, between approximately 4:1 and 1:0. Preferably the ratio between the cellulosic fibers to the thermoplastic material is approximately 1:1.  
         [0000]     Cross-Linking Agents:  
         [0069]     The cross-linking agent serves to strengthen the bond between the several strands of the cellulosic fibers into a final homogenous product. The cross-linking agents bond across the pendent hydroxy groups on the cellulose molecular chain. Cross-linking agents must have the characteristics of forming a strong bond at relatively low temperatures. Examples of cross-linking agents include polyurethanes such as isocyanate, phenolic resins, unsaturated polyesters and epoxy resins and combinations of the same. The phenolic resins may be any single stage or two stage resin preferably with a low hexane content. Although the starting material may comprise a cross-linking agent to strengthen the bonds between the cellulosic fiber strands, the cross-linking agent is not required to form the final product contemplated by the inventive process as long as thermoplastic and cellulosic materials are included in the starting material.  
         [0000]     Lubricants:  
         [0070]     Lubricants, which are common commercial lubricants known to the art of plastic processing, behave as a process aid. Examples of typical lubricants include zinc stearate, which is an internal lubricant and paraffin-type wax, which is an exterior lubricant.  
         [0000]     Other Materials:  
         [0071]     Other materials, which can be added, are known to the art of extrusion and include accelerators, inhibitors, enhancers, compatibilizers and blowing agents. Accelerators, inhibitors, enhancers and compatibilizers are agents which control the speed at which the cross-linking agents work. Accelerators are added to increase the speed of the cross-linking reaction. Examples of accelerators include amine catalysts such as Dabco® BDO (Air Products, Allentown, Pa.) and DEH40® (Dow Chemical). Inhibitors are added to retard the speed of the cross-linking reaction. Examples of known inhibitors include organic acids such as citric acid. Enhancers are used to increase the reactivity between components. Examples of enhancers include cobalt derivatives. Compatibilizers are used to form a more effective bond between cellulosic materials and thermoplastics. Examples of compatibilizers include ethylene-maleic anhydride copolymers. Blowing agents are added to decrease density. An example of a blowing agent is CELOGEN® TSH (Uniroyal Chemical).  
         [0072]     There are many formulation recipes which can be prepared for the starting mixture.  
         [0073]     The following table includes four examples (expressed in pounds of material):  
                                                                                         RECIPE   I   II   III   IV                                        Wood Flour   25.00   25.00   25.00   25.00           Polyethylene   15.00   12.50   15.00   7.50           Zinc Stearate   .75   1.50   1.00   1.25           Wax   .50   .50   .50   .75           Phenolic Resin   1.50   .00   .00   8.50           Isocyanate   .50   1.00   .00   .00           Epoxy Resin   .00   .00   2.50   .00           Catalyst   .00   .00   .075   .00                      
 
         [0074]     The preferred formulation is as follows:  
                                                                 MATERIAL   AMOUNT (PARTS)                                        Wood Flour (40 Mesh)   100.0           Polyethylene (HDPE)   40.0           Zinc Stearate   3.0           External Wax   2.0           Phenolic Resin   6.0           Blowing Agent   1.5                      
 
         [0075]     The wood flour is dried to 2% moisture content or less. The polyethylene (HDPE) and polyurethane are mixed in a ribbon blender until absorbed, approximately five minutes. The remaining ingredients are added to the mixture, and blended for approximately three minutes or until evenly mixed under conditions known to the art.  
         [0076]     Referring now to the figures, wherein the same reference numbers relate to the same or similar features throughout the figures,  FIG. 1  illustrates a flow diagram of the process of the present invention.  
         [0000]     Hopper:  
         [0077]     In the first step, the cellulosic fiber and thermoplastic raw materials are first shredded according to methods known to the art, physically mixed with cross-linking agents and process lubricants in a blender  8 , and subsequently placed into a feed hopper  10 . The cellulose materials are comminuted by conventional particle reduction equipment known to the art. These may include grinders, ball mills, choppers or other equipment capable of reducing the fiber to a flour of a distinct particle size or range of sizes. A 40-mesh flour appears to be the best form but good results have been obtained with both coarser and finer materials.  
         [0078]     The mixing of the materials prior to loading the extruder  12  may be accomplished by any simple mixing device. No heat is required during mixing and only an even distribution of the various components is required. A drum tumbler may be used for small quantities or a larger batch-type mixer such as a ribbon blender known to the art may be used.  
         [0079]     A typical feed hopper used in this process may be a gravity feed, starve feed or force feed (also known as a “crammer”) hopper, depending on the flow characteristics of the particular compound.  
         [0000]     Extruder  12 :  
         [0080]     This mixture of raw materials is then delivered to a heated extruder  12 . The extruder  12  utilizes low temperature mixing and extruding. This is unique in that most plastic mixing processes require mixing at a plasticization temperature, which is quite high. The present mixing temperature is substantially lower, preferably around 180° F. (82° C.). The material passing through the extruder creates a mass of homogenous material at a certain temperature, approximately 1850-200° F. (850-93° C.) depending upon the particular compound. The present invention can be processed with any capacity extruder. A counter-rotating and intermeshing twin screw, high pressure, extruder manufactured by Cincinnati Milacron (CM-55-HP) may be used in the preferred embodiment. Preferably, the process is accomplished by twin screw extruders, which are heated to process temperatures sufficient to blend the product together into a homogenous mixture at low temperature.  
         [0000]     Temperature:  
         [0081]     In the low temperature, high pressure extruder  12 , the materials are blended, heated and then forced into a die system. The extruder  12  of the present invention requires only that the product be brought to a blending or homogenizing temperature, which is less than plasticization temperatures. The temperature of the extruder  12  is controlled by the extrusion speed, external extruder heaters, shearing action and heaters in the die system and monitored by thermocouples and other monitoring circuits. The purpose of the thermocouples is to monitor the heat at each station. The bulk temperature is significantly lower, e.g., about 150°-200° F. (660-93° C.) than the “true melt” of the thermoplastic fluidizers.  
         [0000]     Flow Rate:  
         [0082]     The flow rate of the extruder  12  may be between about 100 and 2500 pounds per hour. In the preferred embodiment the flow rate is approximately 600 pounds per hour with a temperature at approximately 180° F. (82° C.). The product leaving the extruder  12  is essentially unbounded round stock. Various sized extruder orifices are available with a range from 25 millimeters (mm) to 72 mm. In the preferred embodiment a 38 mm orifice is used.  
         [0000]     Die System  14 :  
         [0083]     The materials are blended, heated and then extruded into a die system  14 . The die system  14  is made up of a series of die plates, which will be explained below with reference to  FIGS. 2-10 . The unique die system  14  allows the starting materials to bond and form a shaped-homogeneous product. Each of the plates can be made of materials known to the art to accomplish the necessary purpose. Typical materials include cast iron and stainless steel.  
         [0084]     The volume of extrudate allowed into the die system  14  is controlled by the adapter die plate  20 , which is illustrated in detail in  FIGS. 4A-4D , and further by the shapes of the transition die plates  30 ,  40  and  50 , illustrated in  FIGS. 5A-5D  through  FIGS. 7A-7D , respectively, the flow restriction die plate  60 , as illustrated in  FIGS. 8A-8D , the compression die plate  70 , as illustrated in  FIGS. 9A-9D , and the shaping die plate  80 , as illustrated in  FIGS. 10A-10D . The flow restriction die plate  60  is the fundamental part of this invention. The accompanying dies are designed to provide the material flow required to make the flow restriction die plate  60  effective.  
         [0000]     Adapter Die Plate  20 :  
         [0085]     Extruded material enters the cellular composite extrusion die system  14  through the adapter die plate  20 . The adapter die plate  20  serves as a conduit through which material passes from the extruder  12  to the transition die plates  30 ,  40 , and  50 .  
         [0086]     The adapter die plate  20  connects the die system  14  to the exit opening  13  of the extruder  12 . As illustrated in  FIGS. 4A-4D , the adapter die plate  20  includes a front face  24 , a rear face  25 , and a flow channel  26 . The flow channel  26  narrows in diameter from the front face  24  to the rear face  25 . The flow channel  26  passing through the adapter die plate  20  is designed to direct the flow of material equally to all areas of the transition die plates  30 ,  40 , and  50 . Typically, flow channel  26  is available in sizes ranging from 50 mm to 300 mm.  
         [0087]      FIG. 4B  shows a front elevated view of the adapter die plate  24 . The front face  24  of the adapter die plate  20  comprises a cylindrical front opening  22 , which can accommodate a twin screw extruder  12 , at the end of the flow channel  26  nearest the extruder  12  and an oval-shaped rear opening  23 . Bolt holes  29  are contained near the edge of the front face  24  of the adapter die plate  20  to secure the adapter die plate  20  to the extruder  12 .  
         [0088]      FIG. 4D  shows the adapter die plate  20  in perspective view. The rear face  25  of the adapter die plate  20  comprises an oval orifice  27  which is located at that end of the flow channel  26  which is nearest to the transition die plate  30 . The rear face  25  of the adapter die plate  20  further comprises an extended portion  28  which is designed to nest within a shallow flow channel  32  which comprises part of the transition die plate  30 . A second set of bolt holes  34  is contained within the transition die plate  30  to mesh with the bolt holes  29  in the extended portion  28  of the rear face  25  of the adapter die plate  20  to secure the transition die plate  30  to the adapter die plate  20 .  
         [0000]     Transition Die Plates:  
         [0089]     As illustrated in  FIGS. 5-7 , the transition die plates  30 ,  40 ,  50  are designed to transition and direct the flow of extruded material to all areas of the flow restriction die plate  60  at a uniform rate. Although three transition die plates  30 ,  40 ,  50  are illustrated in the figures, it is within the scope of the present invention to use one or two transition die plates in the die system  14 . If one or two of the transition die plates are used, the width of the plates will generally be thicker. Splitting the transition die plate into three separate die plates  30 ,  40 ,  50  allows for easier machining of the complex shapes required in this transition die plate.  
         [0090]     It is also important that a venturi effect not be created in the flow of material in any section of the adapter die plate  20  or transition die plates  30 ,  40 ,  50  as this would cause a localized decrease in pressure which would result in premature cell formation.  
         [0000]     Transition Die Plate  30 :  
         [0091]     The extruded material is reshaped and slightly expanded in the transition die plate  30 , illustrated in  FIGS. 5A-5D . Generally, the transition die plate  30  is a circularly-shaped metal plate, approximately one and one-half inches thick having a front face  31 , a rear face  33  and bolt holes  34 . The bolt holes  34  extend from the front face  31 , through the die plate to the rear face  33 . They are used to assemble all of the various dies into the die system  14  and may be located in the same position on each die plate. As illustrated in  FIG. 2 , a bolt  15  is adapted to pass though the bolt holes in each respective die plate to secure the die plates together.  
         [0092]     The transition die plate  30  also includes an oblong flow channel  32 , previously described with reference to the adapter die plate  20 . The opening  35  of the channel  32  is essentially the same shape as the rear opening  23  of the adapter die plate  20 , which allows the continuous flow of the extrudate from the adapter die plate  20  through the transition die plate  30  when the die plates  20  and  30  are seated next to each other. The flow channel  32  transforms the extrudate discharged from the flow channel  26  of the adapter die plate  20  to a shape more generally approaching that of the finished product  16 , illustrated in  FIG. 2 .  
         [0093]     Similarly, the function of the transition die plates  40  and  50  serves to transform the extruded material to the finished shape and equalize the flow rate at the outer edges of the extruded material with the flow rate at the center of the extruded material.  
         [0000]     Transition Die Plate  40 :  
         [0094]     Referring now to  FIGS. 6A-6D , the transition die plate  40  is similar in appearance to transition die plate  30 , having a front face  42  with an opening  44  having generally the same dimensions and shape as the rear opening  36  of transition die plate  30  for continuous flow of the extruded material. The oblong flow channel  46  expands slightly in size such that the size of the rear opening  48  is larger in size than the front opening  44 . Similar to the transition die plate  30 , the transition die plate  40  is equipped with bolt holes  41  to join the transition die plate  40  to the rest of the die system  14 .  
         [0000]     Transition Die Plate  50 :  
         [0095]     Referring to  FIGS. 7A-7D , transition die plate  50  is similar in appearance to transition die plates  30  and  40  in that transition die plate  50  has a front face  52  with an opening  54  having generally the same dimensions and shape as the rear opening  48  of transition die plate  40  for continuous flow of the extruded material. The oblong flow channel  56  expands slightly in size such that the size of the rear opening  58  at the rear face  53  is larger in size than the front opening  54 . Similar to the transition die plates  30  and  40 , the transition die plate  50  is equipped with bolt holes  51  to join the transition die plate  50  to the rest of the die system  14 .  
         [0000]     Flow Restriction Die Plate  60 :  
         [0096]     Referring now to  FIGS. 8A-8D , the flow restriction die plate  60  consists of a flat plate  62  having a front face  64 , a rear face  66  and a flow channel  63  which comprises multiple parallel disposed openings or channels  68  that may be in the form of cylinders, slots, or other shapes. The flow of extruded material is divided into separate streams passing through the flow channel  63 . This increases the resistance to flow of the material as the separate streams have a greater amount of surface area for any given volume than one large stream. The resistance to the flow of material through the multiple channels  68  causes an increase in pressure within the material upstream.  
         [0097]      FIGS. 8A-8D  show the multiple apertures  68  contained within an oblong-shaped area similar to the shape of the rear opening  58  of transition die plate  50 . All of the apertures  68  may be substantially round, are contiguous through the material and are substantially parallel to each other and maintain a constant shape from the front face  64  to the rear face  67 . One preferred embodiment of the flow restriction die plate  60  contains apertures  68  which are approximately one-eighth of an inch in diameter. The aperture area of individual strands may be constant throughout a part or may vary indicating the desired density or volume requirements at certain part locations.  
         [0098]     The number, size and length of the openings  68  in the flow restriction die plate  60  are tailored to each individual foam profile and can be designed to produce an optimal pressure increase for a specific range of volume flow rates. This increase in pressure is an essential part of the production of a cellular composite material. Flow of the material against this resistance also produces heat. The flow restriction die plate  60  is most efficient when flow is equally divided between the multiple openings  68  so that the heat produced is uniformly distributed. The adapter die plate  20  and transition die plates  30 ,  40 ,  50  are designed to provide a uniform flow of material to the flow restriction die plate  60 . Similar to the other plates, the flow restriction plate  60  includes bolt holes  69  for assembly. The flow restriction die plate  60  creates pressure within the extruder  12  which not only disperses the gas or vapor that forms the cells, making the cell structure uniform, but also enhances the incorporation of the wood flour into the thermoplastic matrix. This enhanced incorporation imparts valuable properties to the extrudate.  
         [0099]     The pressure drop in a cylindrical flow channel  68  can be described by the following relationship:            
         [0000]     Where:  
         [0100]     L is the length of the cylinder,  
         [0101]     M is the consistency of the material flowing through the cylinder (consistency is related to viscosity by the Power Law of viscous flow—M is the Power Law constant),  
         [0102]     R is the radius of the cylinder,  
         [0103]     Q is the volumetric flow rate (cubic centimeters per second for example),  
         [0104]     π=3.14159 . . . , and  
         [0105]     n is the Power Law exponent.  
         [0106]     The term “pressure drop” refers to the difference in pressure between the entrance and exit of a flow channel. In simpler terms this means that:  
         [0107]     1. The pressure drop in a cylindrical flow channel increases as the length of the flow channel and the consistency, i.e., viscosity, of the fluid increases and decreases as the size, i.e., radius, of the flow channel increases.  
         [0108]     2. The pressure drop in a cylindrical flow channel increases as the flow rate increases. However, this relationship is affected by the Power Law exponent describing the fluid. The Power Law exponent is a measure of what happens to the fluid as it flows faster or slower. Water, for instance, has a Power Law exponent of 1. The viscosity of water is not affected by how fast it is moving. Polymers, i.e., plastics, which are used in the present invention, are shear thinning fluids. This means that the faster they flow, the less viscous they are. Stated another way, the faster they flow, the thinner they are. The exponent in this case is less than 1. The Power Law exponent of the cellular composite material of the present invention is typically about 0.24.  
         [0109]     3. The pressure drop in a cylindrical flow channel decreases with the cube of the radius of the flow channel. However, this relationship is also affected by the Power Law exponent.  
         [0110]     Thus, the pressure an extrudate is subjected to as it enters the flow restriction die plate  60  is related to: 1) the amount of material, i.e., the volumetric flow rate, coming out of the extruder  12  divided by the number of flow channels  68  in the flow restriction die plate  60 . This is the flow rate through an individual channel  68  where the equation above describes the pressure drop; 2) the length of the individual flow channels  68 ; 3) the radius of the individual flow channels  68 ; and 4) the characteristics of the fluid itself.  
         [0111]     Therefore, as the length of the flow channels  68  is increased, the pressure is also increased. Further, as the radius of the flow channels  68  is increased, the pressure is increased. As more flow channels  68  are added to the flow restriction die plate  60 , the flow  10  rate through each flow channel  68  is decreased and the pressure is decreased. Conversely, as the number of flow channels  68  is decreased, the flow rate through each flow channel  68  is increased, which increases the flow pressure.  
         [0112]     Pressure is important because every gas has a certain pressure at a given temperature at which temperature it is soluble, i.e., dissolved, in the fluid into which it is mixed. For the present invention, it is preferred to have gases that will create the cellular structure of the composite of the present invention to be dissolved in the composite extrudate for a couple of reasons. First, the cells are dispersed more completely. Second, when the cells are dissolved, the cells that are formed when the gasses come out of solution will be more numerous and therefore smaller.  
         [0113]     A similar relationship holds for rectangular flow channels  68 . In this case the height of the channel  68  is analogous to the radius of a cylindrical flow channel. As the height of the channel  68  increases, the pressure decreases. Length, flow rate, consistency, and Power Law exponent have the same effects as they did in the case of a cylindrical flow channel.  
         [0114]     The heat generated by flow of the fluid through the flow channels is another important consideration. Heat is important because cellular composites are composed of a large number of cells or bubbles. The cells grow until their walls become so thin that they break. The cells may break open to the exterior or they may break open and combine with adjacent cells, a process known as coalescence. The strength of the cell walls in cellular composites is directly related to the temperature of the composite. Higher temperatures weaken the cell walls. The temperature of a fluid flowing through a cylindrical channel  68  is directly related to the length of the flow channel  68 , the consistency of the fluid, and the velocity of the fluid, i.e., the higher the flow rate, the higher the velocity. The temperature is inversely related to the radius of the flow channel  68 . This means that more flow channels  68 , i.e., less flow in each, cause less temperature increase as do larger flow channels  68 . Longer channels  68  cause more temperature increase. Thus, the same factors that cause an increase in temperature, which is deleterious to the system, cause an increase in pressure, which is beneficial. Thus, a balance must be achieved between the flow rate, based on the number of channels  68 , and the length and radius of the channels  68  such that the pressure is high enough to dissolve the gasses used to produce the cells and to keep the temperature low enough to keep the cells intact.  
         [0115]     The volume of the transition die plates  30 ,  40 , and/or  50  is related to the amount of extrudate material available to flow through any given channel  68  of the flow restriction die plate  60 . The amount of material available to flow through any given channel  68  depends on the number of channels  68 , the pressure forcing the extrudate material through the transition die plates  30 ,  40 , and/or  50 , and the restrictions placed on the material as it tries to find a flow path. If the volume of the transition die plates  30 ,  40 , and/or  50  is too small, the extrudate will be forced to flow at a high velocity and will tend to rush through the center of the flow restriction die plate  60 . The material flowing through the channels  68  in the center will be traveling faster than the material flowing through the outer portions of the flow restriction die plate  60  causing the material in the center to overheat. If the volume of the transition die plates  30 ,  40 , and/or  50  is too large, the extrudate will become stagnant in some areas cutting down the effective area of the flow restriction die plate  60  and causing the material in areas that are not stagnant to flow faster than necessary, again causing overheating.  
         [0116]     In both cases, the finished profile will contain large voids and gas pockets due to breaking of the cells. Thus if the volume of the transition die plates  30 ,  40 , and/or  50  is too small for the number of flow channels  68  and the pressure of the composite flow, the flow will be directed towards the center of the flow restriction die plate  60  more than towards the sides. This will increase the heat generated in the center of the composite flow. If the volume of the transition die plates  30 ,  40 , and/or  50  is too large, material will tend to rest in the outer potions of the transition die plates  30 ,  40 , and/or  50  and flow more rapidly than necessary in the center. By balancing the volume of the transition die plates  30 ,  40 , and/or  50  with the amount of material passing through it in a given time interval, the flow through all channels  68  can be equalized. This will make the temperature increase in each flow channel  68  more equal so that each individual channel  68  is less likely to overheat.  
         [0000]     Compression Die Plate  70 :  
         [0117]     As illustrated in  FIGS. 9A-9D , the compression die plate  70  includes a front face  72 , a rear face  74 , and a flow channel  76  having a front opening  78 , and a rear opening  79 . The compression die plate  70  is designed to mold the extruded material passing out of the individual flow channels  68  of the flow restriction die plate  60  back into one mass of extruded material and to create a linear pressure drop between the rear opening  67  of the flow restriction die plate  60  and the shaping die plate  80 .  
         [0118]     At some point between the rear opening  67  of the flow restriction die plate  60  and the die system  14  exit, the pressure under which the extruded material is contained will drop to a level where cells will begin to form in the material. Stated previously, extrusion of cellular, i.e., foamed, wood plastic composites depends on the formation of gas bubbles or cells within the composite matrix. If the gas can be introduced under pressure, it will be more evenly dispersed throughout the matrix resulting in a more uniform cell structure with an increased number of cells. This type of structure is desirable. Development of pressure in the extrusion process is a result of restriction of the flow of the extrudate. The compression die plate  80  acts to fuse the separate streams issuing from the apertures  68  of the flow restriction die plate  60  into a single stream of material and maintain the melt pressure at a level which will prevent premature development of cells in the material.  
         [0119]     The strands are compressed and shaped in the compression die plate  70 . The heated outer surface of each of the strands acts to anneal the strands together. In addition, as the individual strands are compressed against each other, the localized high temperatures on the outer surface of each strand cause the bonding of the thermoset materials to pendent hydroxy units on the cellulose molecular chain. If cross-linking agents are included in the starting material, the cross-linking agents act to form an exothermic reaction on the outer surface of each strand thereby facilitating the bonding of the thermoset materials to pendant hydroxy units on the cellulose molecular chain. Similar to the other plates, the compression die plate  70  includes bolt holes  71  for assembly.  
         [0120]     The front opening  72  of the flow channel  76  of the compression die plate  70  is a large profile equal to the area of all of the channels  68  within the flow channel  63  in the flow restriction die plate  60  plus the area of the metal that defines the areas between the multiple channels  68  together which make up the flow channel  63  of the flow restriction die plate  60 . The profile tapers, i.e., becomes smaller, rapidly to a size equal to that of the front face  82  of the shaping die plate  80 .  
         [0121]     As the extrudate travels through this tapered flow channel  76 , the same amount of material must travel through the rear opening  79  of the compression die plate  70  that becomes progressively smaller. To do this, the extrudate material must move faster. This is extensional flow and causes an increase in pressure in the compression die plate  70 . Since the speed of the material is increasing, more heat will be generated. The flow channel  76  of compression die plate  70  should taper at a rate between 15% and 30% per unit length. If the amount of taper is less than 15%, the pressure in the compression die plate  70  will be low, the material passing through the individual flow channels  68  of the flow restriction die plate  60  will not fuse together and voids will be present in the final product  16 . If the taper is greater than 30% the acceleration will be too great causing a build up of heat that will cause the cells to break down and again create voids in the material.  
         [0000]     Shaping Die Plate  80 :  
         [0122]     As illustrated in  FIGS. 10A-10D , the shaping die plate  80  includes a front face  82 , a rear face  84 , and a flow channel  86  having a front opening  85  and a rear opening  87 . The initiation of cell formation should occur at the front face  82  of the shaping die plate  80 . Because of the high viscosity and low melt strength of the composite, it is advantageous for expansion to be initiated at a very slow rate. This helps to disperse the gas which forms the cells and prevent the sudden formation of large gas pockets within the extruded profile. This is accomplished by initiating cell formation at the channel  86  near the front face  82  or entrance of the shaping die plate  80  and controlling the rate of expansion within the shaping die plate  80  so that numerous cells have been initiated when the material exits the rear face  84  of the shaping die plate  80 . This is also the external exit of the cellular composite die system  14 .  
         [0123]     The composite material will continue to expand for some time after exiting the die system  14 . The amount of expansion is not the same in all directions but is related to the distance from the center of mass of the profile to the point of expansion. The exit of the shaping die plate  80  at the rear face  84  is designed to shape the material in such a way that the fully expanded material will approximate the shape of the desired profile. Similar to the other die plates, the shaping die plate includes bolt holes  81  for assembly.  
         [0124]     In the present invention, the volume of flow channels  32 ,  46  and/or  56  of the transition die plates  30 ,  40 , and/or  50  is related to the volume of the shaping die plate  80 . A ratio of 2:1 (transition die volume:shaping die volume) is optimal. The ratio can range from 1.05:1 to 3.45:1. At a ratio less than 1.05:1 the volume is too small and center flow heating will occur. At a ratio greater than 3.45:1 the volume of the transition die is too large and stagnation will occur.  
         [0125]     One of the main advantages to this process is that the molded product has virtually no expansion after it leaves the molding die. This is due to the low temperature processing in the extruder and die system.  
         [0126]     Shaping die plates  80  of any shape are contemplated within this invention, including decorative household moldings such as crown moldings, chair rails, baseboards, door moldings, etc., picture frames, furniture trim and other products mentioned in this application. In the shaping die plate  80 , the final shape is maintained. If cross-linking agents are included in the starting material, the cross-linking agents continue to react in the shaping die plate  80 , thereby bonding the individual strands together.  
         [0127]     Cooling Tank  18 :  
         [0128]     After the molded product  16  leaves the shaping die plate  80 , it is fed to a vented cooling tank  18 , which is a conveyor system (known to the art) for conveying the material through a cooling process which may be under negative pressure especially if the product has hollow cores. A representative conveyor-type cooling tank is produced by Cincinnati Milacron. The cooling tank  18  may include a vacuum water bath in the preferred embodiment. The length of the molded product  16  is determined by the length of the cooling tank. Therefore, another advantage of the molded product  16  is that it has potentially unlimited length in that it can continually be extruded from the system.  
         [0129]     The molded product  16  is cooled in the vented cooling tank  18  and transported over rollers (not shown) by a pulling mechanism (not shown) known to the art. The cooled molded product  16  is then cut to the desired lengths using conventional means.  
         [0130]     The molded product  16  can then be covered with a vinyl material, plastic laminate, paint or other suitable coverings known to the art. An inline crosshead extrusion die, known to the art, may be installed down-stream of the puller to apply a capstock of known compounds as an exterior finish.  
         [0000]     Cellulose Plastic Composite  
         [0131]     Because of the designed flow characteristics, the die system  14  creates a flow restriction with minimal frictional heating of the extruded composite material. The flow restriction die plate  60  creates pressure within the extruder  12  which not only disperses the gas or vapor that forms the cells, making the cell structure uniform, but also enhances the incorporation of the wood flour into the thermoplastic matrix. This enhanced incorporation imparts valuable properties to the composite.  
         [0132]     The cellulose plastic composite extrudate produced with the cellular composite extrusion die  14  has the following unique properties. This composite material may be based on thermoplastic resins that are not typically classified as engineering resins. Polyethylene is an example. The composite material produced from polyethylene with this invention may have a density of from 0.50 gm/cc to 0.90 gm/cc, preferably 0.65 gm/cc to 0.75 gm/cc, compared to the resin itself which has a density of 0.95 gm/cc. The flexural modulus of elasticity of this composite material ranges from 100 kilo-pounds per square inch (ksi) to 250 ksi compared to the resin itself which has a flexural modulus of 150 ksi. The cellular material produced is lower in density than a traditional composite material, such as STRANDEX material (Strandex Corporation, Madison, Wis.). The traditional material has a density of 1.12 to 1.18 grams per cubic centimeter. The lower density makes the cellular material easier to fasten, cut, and shape.  
         [0133]     The polyethylene based composite produced using this invention has a coefficient of linear thermal expansion (CLTE) of 24.5×10 −6  in/in-° F. to 32.0×10 −6  in/in-° F. compared to the resin itself which has a CLTE of 70×10 −6  in/in-° F. This composite material contains low cost filler at ratios of from 0.75:1 to 1:1 (filler to resin ratios).  
         [0134]     The incorporation of wood fiber into the polyethylene composite allows the composite to be cut, planed, machined, and fastened in the same way as wood while being resistant to termites and fungal decay and having a low density. The polyethylene based composite produced with this invention has improved impact resistance compared to other cellulose plastic composites. Swelling induced by the uptake of moisture is lower in this composite (0.93% following 24 hr. immersion) than in wood (2.6% for ponderosa pine) or higher density composites (1.15% for STRANDEX composite). This combination of properties in a polyethylene based cellulose plastic composite is unique.  
       EXAMPLE  
       [0135]     Reference is now made to the following example.  
         [0136]     A formulation containing 100 parts per hundred resin (phr) high density polyethylene and 67 phr wood flour has a consistency (Power Law constant) equal to 28.15 Pa-sec and a Power Law exponent equal to 0.36. These values may be determined by any standard method of determining the rheology of a polymer extrudate that behaves as a Power Law fluid.  
         [0137]     Once these values are known they may be used to calculate the pressure produced when the extrudate flows through channels of various sizes and shapes. A general equation describing the pressure produced when a Power Law fluid flows through a channel of some simple cross section was developed by Kozicki (Kozicki, W., et al., “Non-Newtonian flow in ducts of arbitrary cross section”, Chemical Engineering Science, 1966, vol. 21, pp. 665-679:  
         Δ   ⁢           ⁢   P     =         LM     R   h       ⁡     [       2   ⁢     Q   ⁡     (     a   +   bn     )             R   h     ⁢   An       ]       n         
 
 Where: 
 
         [0138]     ΔP=the drop in pressure between the entrance of the channel and the exit of the channel.  
         [0139]     L=the length of the channel  
         [0140]     M=the Power Law constant of the extrudate  
         [0141]     R h =the hydraulic radius (area/perimeter) of the channel cross section  
         [0142]     Q=the volumetric flow rate  
         [0143]     a and b are shape factors dependent on the geometry of the channel section  
         [0144]     n=the Power Law exponent of the extrudate  
         [0145]     If a cellular composite profile with the nominal dimensions of a 1×4 (0.75″×3.5″) is desired and the desired rate of production and density of the profile are 6 ft./minute and 0.9 g/cubic centimeter respectively then the system for production of this profile according to this invention may be designed in the following way.  
         [0146]     The density of an extrudate made from the given formulation will be about 1.12 g/cubic centimeter. The desired production rate is given as 189 cubic inches of profile with a density of 0.9 g/cc per minute (0.75″×3.5″×6 ft/min×12″/1′). This is equivalent to 3097 cubic centimeters of profile with a density of 0.9 g/cm 3  per minute. The amount of extrudate at a density of 1.12 g/cm 3  required to produce this amount of profile is 2489 cubic centimeters per minute or 41.5 cm 3 /second. This is the desired volume flow rate.  
         [0147]     The desired change in density from 1.12 g/cm 3  to 0.9 g/cm 3  will result in a three dimensional expansion. 1 cm 3  of material with a density of 0.90 g/cm 3  will result from the expansion of 0.8036 cm 3  of material with a density of 1.12 g/cm 3 . 0.8036 cm 3  is the volume of a 0.930 cm cube. Therefore the cross section of the shaping die should be 0.75″×0.93=0.70″ by 3.5″×0.93=3.26″. This is equivalent to 1.77 cm by 8.27 cm.  
         [0148]     Assuming that it has been determined experimentally that the gas used will begin to escape and nucleate (initiate bubble formation) at a pressure of 1000 kPa and that nucleation should occur at the entrance of the shaping die, then the length of the shaping die can be calculated to be 10 cm using the general equation above with shape factors a=0.3358 and b=0.8428. (a and b may be calculated using the methods presented by Kozicki.  
         [0149]     If the pressure required to dissolve the gas in the extrudate is 12000 kPa then a pressure drop of approximately 10000 kPa must occur in the flow restriction die. This pressure drop may be obtained through the use of 15 rectangular flow channels measuring 2.54 cm by 0.5 cm (a=0.3441 and b=0.8531) with a length of 7.6 cm. The flow channels would be arranged with their long dimensions parallel, separated by 0.2 cm walls. The dimensions of the entire field of flow channels would then be 2.54 cm in height by 10.3 cm in width.  
         [0150]     The entrance to the molding die would conform to the dimensions of the flow channel field. The area of this cross section would be 26.16 cm2. The exit of the molding die conforms to the dimensions of the shaping die with a cross sectional area of 1.77 cm×8.27 cm=14.64 cm 2 . Following the preferred reduction of 15% to 30% of the molding die volume per unit length the length of the molding die will be between 2.9 cm and 1.5 cm.  
         [0151]     The preferred ratio of the open volume of the transition die to the open volume of the shaping die is 2.1:1. The entrance of the transition die conforms to the dimensions of the exit of the adaptor die while the exit conforms to the dimensions of the flow channel field of the flow restriction die. The simplest shape connects these two cross sections with straight lines. This shape may be constructed and the resulting volume analyzed through the use of a computer aided drafting (CAD) program. Such programs are well known to die designers and machinists. The length of the transition die may be determined through a trial and error process.  
         [0152]     It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.