Abstract:
The present invention generally relates to systems and methods for generating, processing, handling and forming plastic materials into an end product such as, but not limited to, consumables and packaging. The plastic materials are made into preforms or preform sheets, which in turn may be selectively masked to expose desired regions of the same for a saturation process, a heating process, or both. In addition, the various processes may be sufficiently controlled to obtain a desired micro-structure, and in turn obtain desired mechanical, structural, and aesthetic properties in the end product. By way of example, an embodiment of the present invention results in consumable, plastic cups that are lighter weight and more structurally robust less than conventional plastic cups. Further to one or more embodiments of the present invention, various systems for material handling may be utilized to efficiently, timely and cost effectively produce the preforms.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority from U.S. Provisional Patent Application No. 61/837,110, filed on Jun. 19, 2013, and the subject matter of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to methods for producing layered or multi-layered cellular thermoplastic polymer structures, and more specifically to methods for producing lighter weight, selectively foamed plastic end products such as consumables and packaging. 
       BACKGROUND 
       [0003]    For decades, plastics have been made in various forms and used in many different durable and non-durable applications. Plastics may generally be characterized as “thermoplastics” or “thermosetting plastics.” A product made from a thermoplastic becomes pliable or moldable above a specific temperature, and returns to a solid state upon cooling. Most thermoplastics have a high molecular weight with polymeric chains that associate through intermolecular forces, referred to as van der waal forces, thus thermoplastics may be remolded because they can be softened by heating and reformed into new or similar products upon cooling. Thermosetting plastics are formed with permanent or irreversible “cross-linking” bonds that are generated during the curing process. These cross-linked bonds break down or disassociate upon heating and do not reform upon cooling. 
         [0004]    By way of example, non-durable plastics are commonly used in packaging and food service environments. In most cases, plastic packaging or plastic coverings that have been in contact with food are not meant to be recycled or reused, and therefore these products are typically disposed of after a single use. The disposal of non-durable plastics results in large amounts of waste. The recycling of non-durable plastic products requires that the product be cleaned. Cleaning, in turn, uses additional resources and energy. Nevertheless, cleaning processes may degrade the polymeric bonds of thermoplastics and may be insufficient for thermosetting plastics, and in either instance may result in recycling efforts that are less than optimal. 
         [0005]    One type of plastic packaging in common use is referred to as solid or un-foamed plastic packaging, which is a highly dense form of plastic. Solid plastic packaging provides minimal, if any, cushioning for the packaged goods. In addition, solid plastic packaging generally has low thermal insulation properties, and thus does not adequately isolate hot and cold products from users or maintain product temperatures. The high density of solid plastic packaging results in an increased shipping weight, which in turn increases shipping and transportation costs. 
         [0006]    The packaging industry, for example, has been switching to the use of foamed plastic to make packaging or other products. Foamed plastics are produced using a chemical blowing agent that decreases the density over a given volume. The foaming process adds some insulation and cushioning benefits as compared to solid plastics, but creates other problems. 
         [0007]    Foamed plastics are more friable (e.g., brittle, easily crumbled or pulverized, frangible), which makes collecting and recycling more difficult. Styrofoam, a common type of foamed plastic, can be broken into small pellets that are nearly impossible to separate from a waste stream. The surface of foamed plastics is often cellular in nature, and unattractive to consumers. The chemical blowing agents used in producing many foamed plastics include fluorocarbons and/or chlorofluorocarbons, which may be environmentally restricted because of the contamination effects of these materials. The injection of fluorocarbons and/or chlorofluorocarbons into foamed plastics may render them non-recyclable from an environmental perspective. 
         [0008]    Most foamed plastics are produced in the melt state of the polymer. Polymers such as polyethylene terephthalate (PET) and Polylactic acid (PLA) are not commercially feasible for producing foamed products using conventional means. Most foamed plastics are also difficult to thermoform, limiting their industry and commercial applications. Foamed plastics typically have poor barrier properties, so it is difficult to create foamed plastics with layered structures and hence bonded structures may be required, which increases the cost of the end product. 
         [0009]    Microcellular foaming processes that utilize carbon dioxide or other high pressure gases as a foaming agent have been explored to resolve some of the drawbacks mentioned above. The main methods for commercial production of microcellular foams are extrusion foaming, semi-continuous production, and injection molding. Extrusion foaming, much like chemical blowing foaming, requires a high-quality input stream which may make it impossible to utilize recycled foams; the extruded result also may not be drawn deep into cup shapes. Microcellular injection molding cannot create thin walled shapes with a good surface finish. Semi-continuous production is a labor intensive process which consumes an interleaving layer and requires a large amount of plastic. Due to the fact that the plastic loses gas during processing, and that a large roll of plastic takes a long time to process, the properties at the start of a foam roll will differ from those of the end of a roll. 
         [0010]    The problems concerning the utilization of plastics in packaging, in foam and solid form, extend to durable goods as well. A particular area of plastic usage for durable goods is composites, in which multiple phase separated materials are bonded together in order to create a single part with better properties. The center section of these parts is often foam. Most foam materials produced for these applications cannot be recycled after bonding, and are not biodegradable. 
         [0011]    The life cycle of a conventional plastic product, such as a plastic cup, includes obtaining the raw material (e.g., petroleum, corn, plastic scrap, or combination thereof), refining the raw material, generating plastic pellets, processing the pellets into sheets or injection molding them into preforms, heating, shaping using mechanical means and/or air pressure, heating set sheet to increase crystallinity and service temperature, trimming and secondary operations to finalize product, distributing the product for customer consumption, disposing of the product as waste or as a recyclable material. 
       SUMMARY 
       [0012]    In one embodiment, the present invention generally relates to systems and methods for generating, processing, handling and forming plastic materials into an end product such as, but not limited to, consumables and packaging. The plastic materials are made into preforms or preform sheets, which in turn may be selectively masked to expose desired regions of the same for a saturation process, a heating process, or both. In addition, the various processes may be sufficiently controlled to obtain a desired micro-structure, and in turn obtain desired mechanical, structural, and aesthetic properties in the end product. By way of example, an embodiment of the present invention results in consumable, plastic cups that lighter weight and more structurally robust less than conventional plastic cups. Further to one or more embodiments of the present invention, various systems for material handling may be utilized to efficiently, timely and cost effectively produce the preforms. 
         [0013]    In one aspect of the present invention, a method for making plastic end products includes the steps of (1) generating a plurality of plastic preforms, each preform having a inner region defining an area from which an end product will be formed, and each inner region bounded by a periphery region; (2) loading the plurality of preforms into a pressure vessel rack; (3) placing the pressure vessel rack into a pressure vessel; (4) applying a pressure to the pressure vessel rack; (5) while under pressure, saturating the plurality of preforms with a gas for a predetermined sorption time; (6) moving the plurality of preforms from the pressure vessel rack into a heating rack; (7) heating the plurality of preforms; and (8) forming each of the plurality of preforms into a configuration corresponding to a desired plastic end product. 
         [0014]    In another aspect of the present invention, a cross-sectional micro-structure of a plastic apparatus includes an exterior surface; an interior surface; a first intermediate region located between the exterior and interior surface, the first intermediate region configured with a plurality of first foamed cells, wherein the first foamed cells include a plurality of first interstices disposed among a plurality of first plasticized portions; and a second intermediate region located between the exterior and interior surface, the second intermediate region configured with a plurality of second foamed cells, wherein the second foamed cells include a plurality of second interstices disposed among a plurality of second plasticized portions, and wherein the second interstices are larger than the first interstices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings may not be necessarily drawn to scale. For example, the shapes of various elements and angles may not be drawn to scale, and some of these elements may be arbitrarily enlarged or positioned to improve drawing legibility. Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0016]      FIG. 1  is a flow chart of a preform process according to an embodiment of the present invention; 
           [0017]      FIG. 2  is a perspective view a preform having an inner region and a periphery region according to an embodiment of the present invention; 
           [0018]      FIG. 3  is a perspective view of a pressure vessel rack for holding a plurality of preforms according to an embodiment of the present invention; 
           [0019]      FIG. 4  is a close-up, perspective view of the pressure vessel rack of  FIG. 3  according to an embodiment of the present invention; 
           [0020]      FIG. 5  is a perspective view of the pressure vessel rack of  FIG. 3  being placed into a pressure vessel according to an embodiment of the present invention; 
           [0021]      FIG. 6  is a side elevational view the pressure vessel rack of  FIG. 3  being removed from a pressure vessel according to an embodiment of the present invention; 
           [0022]      FIG. 7  is a perspective view of a heating tray assembly for holding preforms according to an embodiment of the present invention; 
           [0023]      FIG. 8  is a flow chart of a large sheet plastic process according to an embodiment of the present invention; 
           [0024]      FIG. 9  is a perspective view of a heating tray assembly for holding a large sheet according to an embodiment of the present invention; 
           [0025]      FIG. 10  is a schematic, perspective view of a conveyance system according to an embodiment of the present invention; 
           [0026]      FIG. 11  is a schematic, perspective view of a conveyance system having a plurality of preforms being processed according to an embodiment of the present invention; 
           [0027]      FIG. 12  is a perspective view of a conveyance system according to another embodiment of the present invention; 
           [0028]      FIG. 13  is a side elevational view of a conveyance system according to yet another embodiment of the present invention; 
           [0029]      FIG. 14  is a perspective view of a conveyance system having an internal conveyance system according to an embodiment of the present invention; 
           [0030]      FIG. 15  is a cross-sectional view of the conveyance system of  FIG. 14 ; 
           [0031]      FIG. 16  is a perspective view of an internal conveyance system according to an embodiment of the present invention; 
           [0032]      FIG. 17  is a close-up view of a portion of the internal conveyance system of  FIG. 16 ; 
           [0033]      FIG. 18  is a perspective view of an extrusion system according to an embodiment of the present invention; 
           [0034]      FIG. 19  is a perspective, cross-sectional view of the extrusion system of  FIG. 18 ; 
           [0035]      FIG. 20  is a perspective view of an injection molding system according to an embodiment of the present invention; 
           [0036]      FIG. 21  is a close-up view showing a micro-structure of a saturated preform or an end product made from the saturated preform according to an embodiment of the present invention; and 
           [0037]      FIG. 22  is perspective view of a plurality of post-forming plugs for making an end product into a desired shape according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known systems and processes for producing plastic products may not necessarily be shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. 
         [0039]    The present invention is generally directed to rapid foaming processes for creating polymer end products that have an integral solid skin. In one embodiment, the processes general include the steps of saturating a thermoplastic disc, sheet or preform; sending the saturated product through a heating cycle, and the forming the product into a final shape. The processes may be utilized with a variety of thermoplastic materials and the final product may include biodegradable fibers and adhesives to create a biodegradable composite product. 
         [0040]    By way of example according to an embodiment of the present invention, plastic material may be obtained as a solid sheet. The sheet is placed into a pressure vessel at saturation pressures in a range of about 200 to about 2,500 pounds-per-square-inch (psi), with a preferred range of about 300 psi to about 1,000 psi, in which a gas used in the pressure vessel may be a gas such as, but not limited to, carbon dioxide. As the sheet absorbs the gas, a gas concentration in the sheet will be highest on an exterior surface of the sheet and the gas concentration will increase over time in the pressure vessel. In one embodiment, the sheet may be left in the pressure vessel to achieve an equilibrium gas concentration throughout its thickness, but preferably the sheet would be extracted from the pressure vessel prior to achieving equilibrium. As the gas concentration increases, crystallization of the sheet will begin to occur. Similar to the gas concentration, the crystallization may be initially higher on the exterior surface, but may reach an equilibrium state if the sheet is left in the pressure vessel long enough. For thin section of relatively fast-absorbing polymers such as, but not limited to, a 0.024 inch thick PLA, a saturation time in a range of about one (1) minute to about one (1) hour may be sufficient. For thicker sections and/or slower absorbing polymers such as, but not limited to, PET, a saturation time in a range of about thirty (30) minutes to about one-hundred (100) hours may be sufficient. 
         [0041]    When the gas trapped in the sheet reaches a certain concentration, the pressure vessel is vented, opened, and the sheet is removed. After removal, the gas trapped in the sheet will begin to disperse into the atmosphere. Because the dispersion process will be fastest on the exterior surface of the sheet, the gas concentration will lower more quickly in the exterior surface as compared to an interior of the sheet. While the initial gas dispersion happens, the gas in the interior will continue to diffuse and make the gas concentration more even throughout the thickness of the sheet. 
         [0042]    The sheet is then heated, which may be accomplished using an infrared heating source according to an embodiment of the invention. As the sheet heats up, the sheet softens and the gas trapped in the sheet increases in fugacity, which is defined as the desire of the gas to escape from a material. The softening (i.e., heating) process may also be referred to as a foaming process. In addition and as the sheet heats up, bubbles begin to nucleate at various sites in the softened sheet to create cells. In areas of higher crystallinity and higher gas concentration, there are more nucleation sites. In areas of lower crystallinity and lower gas concentration, there are fewer nucleation sites. If the exterior surface of the sheet has been allowed to lose a sufficient amount of gas then a skin may be created as the exterior surface. The skin may be solid, approximately solid, or at least have smaller cells as compared to other portions of the sheet that lost a lesser amount of gas during heating. High crystallinity portions of the sheet may require higher temperatures. 
         [0043]    As the temperature of the sheet continues to increase, the sheet continues to soften and the fugacity continues to increase. Within the interior region of the sheet as the gas escapes, the cells may begin to grow, which may be referred to as foaming. Even after the plastic is removed from the heat source, additional foaming may occur as gas continues to escape and the sheet continues to stretch. 
         [0044]    As the cells grow, they stretch local regions (e.g., boundary regions) of the sheet around the growing cells. The growing process may lead to local strain induced crystallization in the sheet, and the strain induces crystallization, which in turn may make the boundary regions more resistant to further growth. The amount of time and ramp up rate of the heating process should strike a balance between cell growth and a wall thickness of the boundary regions. For example, if the wall thickness becomes too thin then the cells may rupture. Strain induced crystallization may be viewed as a self-regulating process to help strike the aforementioned balance. From a micro-structural viewpoint, the heating process may generate additional foaming, increase cell sizes, and/or create oblong shaped cells at desired locations within the sheet. 
         [0045]    Cell growth stops when the sheet reaches an equilibrium state for its temperature or when the sheet has cooled to where it no longer stretches due to internal forces from the gas within the cells. In a combined thermoforming and foaming process, the equilibrium state may be achieved in a mold or tool or even after the sheet has been removed from the mold or tool. Optionally, impacting the sheet with compressed air may also bring about the equilibrium state. 
         [0046]    By way of example and according to an embodiment of the present invention, one process starts with a multi-layered structure, with each layer being either a different type of plastic, or a type of the same plastic with different crystallization characteristics such as, but not limited to, a polyactic acid (PLA) structure, which is non-crystallizable and foamable, an ethylene-vinyl alcohol (EVOH) copolymer structure, which is crystallizable and high barrier, and another PLA structure. The PLA portion may be rapidly foamed while the EVOH portion would provide superior barrier properties. Optionally, the multi-layered structure may include a first plastic not prone to crystallization on a periphery of a part, and a second plastic prone to crystallization at the center of the part. This type of multi-layered structure may yield a part whose internal structure remained rigid at high temperatures, while the non-crystalline periphery may allow a high depth of draw and provide toughness to the part. 
         [0047]    The different layers may expand at different rates and/or by different amounts during heating. By way of example, an internal layer of EVOH or an internal layer of another highly crystalline material may not expand or may expand by only a small amount. Such minor or zero expansion will constrain the exterior surface layers (e.g., the PLA layers), so that a majority of any cell growth should occurs in a thickness direction to prevent waves and wrinkles from being formed in the sheet prior to a post-heat forming process. 
         [0048]    In an embodiment of the present invention, a plastic end product is formed using a preform process. By way of example,  FIG. 1  shows a preform process or preform method  100  for making plastic end products. At step  102 , a plurality of plastic preforms, which may also be referred to as discs, are generated such that each preform includes a inner region defining an area from which an end product will be formed. The inner region is bounded by a periphery region. At step  104 , the preforms are loaded into a pressure vessel rack (hereinafter a PV rack). At step  106 , the PV rack is placed into a pressure vessel. At step  108 , pressure is applied to the PV rack that contains the preforms. Typically, the applied pressure will be greater than one atmosphere, but may vary during the pressure cycle to be equal to or below one atmosphere. At step  110 , the preforms are saturated with a gas such as, but not limited to, carbon dioxide for a predetermined sorption time. The amount of gas and the sorption may be adjusted, as will be explained below, to achieve desired densities, micro-structures, weight, and structural properties for the desired end product. At step  112 , the preforms are moved from the PV rack onto a heating rack. At step  114 , the preforms are heated to a desired temperature and held at that temperature for a desired amount of time—again the temperature and time may be adjusted, as will be explained below, to optimize desired densities, micro-structures, weight, and structural properties for the desired end product. At step  116 , the preforms a mechanically formed into a configuration corresponding to a desired plastic end product. The purpose of the inner and periphery regions, the configuration and operation of the PV rack and the heating rack along with other details of the disc process  100  are explained in more detail below. 
         [0049]      FIGS. 2-7  show the equipment that may be utilized to implement the disc process  100  of  FIG. 1 .  FIG. 2  shows that one or more preforms  200  may be cut from a sheet of plastic. The preforms  200  may be cut from single or multi layered extruded sheets. It may be desirable to partially form (e.g., via mechanical distortion) the preforms  200  into a curved shape to minimize stretching required during final forming and possibly to help control any warping that may occur during the heating process. The preform  200 , which may have any shape, includes the material to make one or more end products. The preform  200  includes an inner region  202  that defines an area from which the end-product will be produced. In the illustrated embodiment the inner region  202  is generally circular to make an end product such as a consumable, plastic drinking cup, but it is appreciated that the inner region  202  may take other shapes depending on the end product desired. The preform  200  also includes a periphery region  204  that bounds the inner region  202 . The periphery region  202  may also take a variety of shapes and functions primarily to structurally support the inner region during saturation, heating and forming operations. 
         [0050]    Supporting small unsupported sections of the preforms  200  during saturation may have process control advantage. As the preforms  200  soften when exposed to the gas, such as carbon dioxide, there may be saturation conditions in which it is desirable to heat the preforms during saturation. 
         [0051]      FIGS. 3 and 4  show the preforms  200  being loaded into the PV rack  206 . The preforms  200  may be loaded into the PV rack  206  with the assistance of automated equipment (not shown) or manually after the preforms have been cut from the plastic sheet. Referring to  FIG. 4 , the PV rack  206  includes receiving slots  208  for receiving the preforms  200 . The receiving slots  208  are separated by walls  210 . A wall thickness  212  of the walls  210  may be selectively sized to allow a desired amount of gas to contact and/or penetrate the inner regions  202  ( FIG. 2 ) of the preforms  200  during the saturation process. The PV rack  206  may have varying designs (e.g., wall thicknesses and shapes) based on the configuration of the desired end product. 
         [0052]      FIGS. 5 and 6  show the PV rack  206  entering ( FIG. 5 ) and exiting ( FIG. 6 ) a pressure vessel  214 . The PV rack  206  is then loaded into the pressure vessel  214  for the saturation process. The pressure vessel  214  and includes a sealable entrance door  216  and a sealable exit door  218 . In addition, and as will be described in more detail below, the pressure vessel  214  may take a variety of shapes and sizes to process multiple preforms  200  and may include internal mechanisms to move the preforms  200  through different sections of the pressure vessel  214 . Multiple pressure vessels  214  may be used at the same time (e.g., in parallel) to produce a desired rate of the desired end product. Within the pressure vessel  214 , the preforms  200  are permitted to be partially or fully saturated depending on a desired gas concentration on the inner regions  202  of the preforms  200 . 
         [0053]      FIG. 7  shows a heating rack  220  having a support tray  222  for receiving and supporting the preforms  200  after they have been removed from the PV rack  206  ( FIG. 6 ). In the illustrated embodiment, the heating rack  220  includes a top tray  224  and a bottom tray  226  that sandwich or trap the support tray  222  during heating and keep the preforms securely in place. The heating rack  220  may be placed into a thermoformer where the preforms  200  are heated and stretched into a desired shape. The preforms  200  expand primarily in thickness, but also somewhat in plane with the plastic sheet. This expansion may cause warping and the top tray  224  and bottom tray  226  operate to constrain the preforms  200  to minimize warping. Warping is undesirable because it can lead to wrinkling during the stretching of the part, and may also affect the amount of heat conduction to the preforms  200 . During heating and stretching, a multi-layered cellular structure is created. 
         [0054]    The heating tray may have edges (not shown) that will assist in lip forming of end products such as, but not limited to cups or other end products. The periphery regions  202  trapped between the top and bottom trays,  224 ,  226  may have less gas trapped in those regions due to the trays acting as a gas and/or heat shield. Accordingly, the periphery regions  202  may be prevented from foaming. 
         [0055]    During heating, the edges periphery regions  202  may be constrained to prevent warping. Warping prior to shaping is undesirable because it may cause non-uniform characteristics in stretched and formed end-products. 
         [0056]    In another embodiment of the present invention and referring to  FIGS. 8 and 9 , a plastic end product may be formed using a large sheet process  300 . At step  302 , a large sheet  312  of a plastic material may be obtained by extruding the sheet or procuring a pre-sized sheet. At step  304 , the sheet  312  is placed between a top tray  314  and a bottom tray  316  to form a heating tray assembly  318 . Both the top and bottom trays  314 ,  316  include apertures  320  for defining a configuration for the end product; whereas the configuration is similar to an exterior profile of the end product to be formed (e.g., round for a cup, square for a packaging box, etc.). The apertures  320  allow the sheet  312  to be saturated in selected areas. At step  306 , the assembly  318  is placed into a pressure vessel. While in the pressure vessel, the assembly  318  is saturated with a gas for a period of time and then removed from the pressure vessel. At step  310 , the sheet  312 , while remaining trapped between the top and bottom trays  314 ,  316 , is heated and formed into a desired plastic end product using thermoforming equipment. The sheet  312  may be transported through the thermoforming machinery utilizing the assembly  318  that was utilized for saturation. 
         [0057]      FIG. 10  schematically shows a conveyance system  400  for a pressure vessel according to another embodiment of the present invention. The pressure vessel may be configured with gate-style valves and pressure locks at the inlet door and exit door. The pressure locks would allow heating tray assemblies (e.g., preforms or sheets) to enter the pressure vessel even though other assemblies are already being processed within the pressure vessel. The conveyance system  400  continually moves the assemblies through the pressure vessel to achieve a desired processing rate without needing to depressurize and re-pressurize the pressure vessel. The conveyance system  400  may take the form of rods attached to the assemblies, a track system that engages the edge of the assemblies, pneumatic actuators, a chain system or a belt system. 
         [0058]    The conveyance system  400  includes an entrance chamber  402  that may be evacuated using a pump  404  that vents to an accumulator and/or carbon dioxide supply reservoir  406 . At the entrance chamber  402 , a first gate valve  408  is opened so the assembly may be inserted. The first gate valve  408  is then closed and the entrance chamber  402  is pressurized using the pump  404 . Next, a second gate valve  410  is then opened and the assembly is moved into a main chamber  412 .  FIG. 11  shows that a plurality of heating tray assemblies  414  may be processed in the main chamber  412 . The assemblies  414  may remain in the main chamber  412  for a controlled amount of time, absorbing gas through diffusion at the surface of the assemblies and thus allowing the gas to permeate areas of the assemblies that will later be heated and formed. 
         [0059]    Referring back to  FIG. 10 , one or more of the assemblies may be moved from the main chamber  412  to an exit chamber  416  by opening a third gate valve  418 . The third gate valve  418  is then closed and the exit chamber  416  is evacuated using the pump  404 . A fourth gate valve  420  may then be opened for the assembly to be removed from the pressure vessel. The assembly  414  may then be held in the exit chamber  416  for a period of time that allows for a desired amount of gas desorption before heating and thermoforming. 
         [0060]    Processing the assemblies directly in the pressure vessel may allow for the injection of carbon dioxide during injection molding or extrusion. In the aforementioned embodiment, the main chamber  412  may be employed to fully saturate parts, the assemblies. Alternatively, the assemblies may be injection molded or extruded with lower gas concentrations and then saturated or desorbed to achieve higher or lower gas concentrations through the surface. 
         [0061]    The temperature in each chamber of the pressure vessel may be controlled independently to generate a desired layered structure for the end product. In one embodiment, the main chamber  412  may be segregated or controlled to be subjected to a low pressure to saturate the assemblies for a period time adequate to achieve a uniform low gas concentration. The low gas concentration would be able to create a large cellular structure to reduce the density of the end product. The assemblies may then be subjected to a higher pressure, as compared to the low pressure, for a shorter period of time to create an integral crystalline skin for the end product. Saturation pressures may be in a range of about 200 psi to about 2,500 psi, with a preferred range of about 300 psi to about 1,000 psi. 
         [0062]    In yet another embodiment, the gate valves may take the form of dynamic seals that allow the entrance and removal of assemblies from pressure vessels without opening and closing the pressure vessel. 
         [0063]      FIG. 12  shows a recirculating conveyance system  500  that includes a drive mechanism  502  configured to engage and supports  504  that receive one or more preforms or preform sheets (not shown). The drive mechanism  502  may take the form of a drive belt or a drive chain. In the illustrated embodiment, the drive mechanism  502  takes the form of a pair of drive belts. A motor  506 , shown schematically, may be mechanically coupled to the drive mechanism  502  to provide a continual advancement of through an oven during heating. The preforms may be selectively added or removed during the continual advancement. 
         [0064]      FIG. 13  shows another embodiment of the recirculating conveyance system  500  having counter-rotating drive mechanisms  502  as indicated by arrows  508 , respectively. Preforms or preform sheets  510  may be fed into the conveyance system  500  and removed therefrom. The supports  504  provide a platform for the preforms  510  during saturation and/or heating while also shielding portions of the preforms during such process. 
         [0065]      FIGS. 14 and 15  show an embodiment of a internal conveyance system  600 , similar to, but not identical to the embodiment illustrated in  FIG. 10 . The conveyance system  600  includes an entrance chamber  602  with entrance gate valves  604  and an exit chamber  606  with exit gate valves  608 . As best shown in  FIG. 15 , a main chamber  610  receives a vertical conveyance system  612 . At least one lower actuator  614 , which may take the form of linear actuators, moves the preforms (now shown) into main chamber  610 . Likewise, at least one upper actuator  616  removes the preforms (not shown) from the main chamber  610 . 
         [0066]      FIGS. 16 and 17  show an embodiment of the vertical conveyance system  612  that includes a continuous belt or chain  618 , pulleys or rollers  620 , and receiver mechanisms  622 . Referring to  FIG. 17 , each receiver mechanism  622  includes an upper plate  624  and a lower plate  626  separated by a space  628  sized to receive a preform or a preform sheet (not shown) and allow adequate saturation thereof. 
         [0067]    The conveyance system  600  in cooperation with the vertical conveyance system  612  allows for efficient and continuous material handling in moving the preforms through at least a saturation cycle or process while adequately supporting the preforms. Now referring briefly back to  FIGS. 14 and 15 , the operation of the conveyance system  600  may include the steps of pressurizing the main chamber  610  while opening the entrance chamber  602  and the exit chamber  606  to atmospheric pressure. The gate valves  604 ,  608  are then closed and sealed after a preform or preform sheet has been placed into the entrance chamber  602 . The entrance and exit chambers  602 ,  606  are then pressurized to the same pressure as the main chamber  610 . The gate valve  604  is opened and the actuator  616  moves the preform into the main chamber  610 . The vertical conveyance system  612  moves the preform either through the main chamber  610  at a rate to achieve a desired amount of saturation. By way of example, a desired amount of time for saturation may be in a range of about 200 psi to about 2,500 psi, with a preferred range of about 300 psi to about 1,000 psi, or even longer depending on the desired end product. The exit chamber  606 , if not already pressurized, may be pressurized to have the same pressure as the main chamber. The gate valve  606  is opened and the preform is translated by the linear actuator  614  to the exit chamber  606  from the main chamber  610 . The exit chamber  606  may then be vented or depressurized and the saturated preform removed from the system  600 . It is appreciated that thee conveyance  600  may be configured in a variety of ways such that the direction of material handling may be accomplished horizontally, vertically, in a non-horizontal or non-vertical direction, or any combination thereof 
         [0068]      FIGS. 18 and 19  show an extrusion system  700  for processing plastic pellets into preforms or preform sheets according to an embodiment of the present invention. The extrusion system  700  includes a pressure vessel  702  coupled to an extrusion chamber  704 . A hopper  706  receives plastic pellets (not shown) and funnels the pellets into the extrusion chamber  704 . The pellets are melted in the extrusion chamber  704  using mechanical pressure, such as an extruder screw, an external heat source, or some combination thereof. While the pellets are melting, a gas port  708  in fluid communication the extrusion chamber  704 , allows the introduction of a gas such as, but not limited to, carbon dioxide into the extrusion chamber  704 . The gas saturates the melting or melted pellets. 
         [0069]    As best illustrated in  FIG. 19 , the pressure vessel  702  includes an extrusion die  710  that mechanically forms the melted pellets into a continuous sheet  712 . Upon exiting the extrusion chamber  704 , the continuous sheet  712  has been uniformly or selectively saturated. In one embodiment, the pressure vessel  702  may be maintained at a pressure level sufficient to prevent foaming while the temperature therein is maintained to allow for sufficient cooling of the continuous sheet  712 . In one embodiment, the saturation pressure level within the pressure vessel  702  is maintained within a range of about 200 psi to about 2,500 psi, with a preferred range of about 300 psi to about 1,000 psi. Selectively controlling the pressure and temperature within the pressure vessel  702  may advantageously minimize or eliminate undesired foaming of the continuous sheet  712  in the pressure vessel  702 . 
         [0070]    In addition, the pressure vessel  702  may include one or more rollers  714  to guide the continuous sheet  712  through the pressure vessel  702 . A cutter  716  operates to cut the continuous sheet  712  into segmented preforms or sheets  718 . A seal or wall  720  located in the pressure vessel  702  separates a pressurizing chamber  722  from an exit chamber  724 . In one embodiment, the wall  720  includes an internal guide  726  that is configured to direct the segmented preforms  718  into the exit chamber  724 . Similarly, an exit guide  728  may operate to direct the segmented preforms  718  out of the exit chamber  724  of the pressure vessel  702 . The exit chamber  724  may be vented to atmospheric pressure or to a gas reservoir (not shown). 
         [0071]      FIG. 20  shows an injection molding system  800  that is structurally similar to the extrusion system  700  of the previously described embodiment. The injection molding system  800  includes a pressure vessel  802 , an injection molding chamber  804 , a hopper  806 , and a gas injection port  808  that function as described above. The injection molding system  800  utilizes an injection molding ram  810  to force the melting or melted pellets into the pressure vessel  802 . The injection molding system  800 , according to an embodiment of the present invention, may produce one or more saturated discs  812  that are systematically expelled or ejected from the pressure vessel  802  at a desired processing rate. Advantageously, the saturated discs  812  may be formed with varying thicknesses or shapes to permit selective, not necessarily uniform, saturation while minimizing or eliminating one or more post-forming operations that may be needed to convert the discs  812  into end-products. 
         [0072]      FIG. 21  shows an asymmetric, cross-sectional micro-structure  900  for a plastic apparatus, such as a plastic end product, which may take the form of a plastic cup. The cross-sectional micro-structure  900  for the plastic apparatus may be produced by one of or a combination of the methods discussed above. For example, the cross-sectional micro-structure  900  may be produced by selectively and controllably foaming and/or heating one or more portions of a preform or a sheet and/or by establishing a desired temperature differential between surfaces of the preform or the sheet. 
         [0073]    The illustrated embodiment of  FIG. 21  shows the micro-structure  900  that includes an exterior surface  902 , an interior surface  904 , a first intermediate region  906  located between the exterior and interior surfaces  902 ,  904 , and a second intermediate region  908  located between the exterior and interior surfaces  902 ,  904 . The first intermediate region  906  takes the form of a multitude of first foamed cells  910  that include first interstices  912  that are surrounded by or disposed among first plasticized portions  914 . The second intermediate region  908  takes the form of second foamed cells  916  that include second interstices  918  that are surrounded by or disposed among first plasticized portions  920 . In the illustrated embodiment, the second interstices  918  are larger than the first interstices  914 , thus making a density of the second intermediate region  908  lower than a density of the first intermediate region  906 . In one embodiment, the exterior surface  902  and/or the interior surface  904  may take the form of a generally solid surface. While  FIG. 21  shows the first intermediate region  906  located adjacent to the exterior surface  902  and the second intermediate region  908  located adjacent to the interior surface  904 , it is appreciated that this ordering may be reversed or otherwise altered. It is also appreciated that one or more additional intermediate regions, such as a third intermediate region, may be included between the exterior surface  902  and the interior surface  904 . 
         [0074]    The micro-structure  900  may advantageously improve bending strength, prevent the spread of colorants during forming, and/or provide a smoother finish. Embodiments of the micro-structure  900  allow a manufacturer to customize the appearance and mechanical properties of the plastic apparatus. 
         [0075]    Controlled heating of the preform or sheet may be used to induce specific properties during the saturation process. As discussed above, heating a single side of the preform or sheet may result in an asymmetric density and cellular distribution. By way of example, such an asymmetric density and cellular distribution may be manipulated to optimize mechanical properties, such as creating a plastic apparatus with an improved bending strength to resist external crushing forces while remaining sufficiently flexible to side loading so the apparatus may be easily removed from a stack. Optionally, one or more colorants or coloring dies may be selectively infused into the micro-structure  900  to create a colored, high density exterior while minimizing undesired density increases of one or both of the intermediate portions  906 ,  908 . Controlling a size of the foamed cells  910 ,  916  may advantageously permit a desired amount of light diffusion or light refraction through or by the plastic apparatus. By way of example, foamed cells  910  having a measured length across a height or width of the foamed cell  910  within a range of about one (1) to about twenty (20) micrometers provides an aesthetically desired amount of light diffusion through the plastic apparatus. It is further appreciated that the size of the foamed cells  910  may operate to give the exterior surface  902  a rougher finish (e.g., large-sized foamed cells  910 ), a matte finish (e.g., medium-sized foamed cells  910 ), or a glossy finish (e.g., smaller-sized foamed cells  910 ). 
         [0076]    In another embodiment, the micro-structure  900  includes an intermediate wall  922  located between the first intermediate region  906  and the second intermediate region  908 . Preferably, the intermediate wall  922  may take the form, generally, of a solid wall having a wall density that is higher than both of the intermediate regions  906 ,  908 . Inclusion of the intermediate wall  922  may advantageously permit a desired amount of resiliency for the plastic apparatus and thus allow it to spring back to a desired shape when loaded. The intermediate wall  922  may also provide a barrier for crack propagation, thus making the plastic apparatus more crack resistant. Alternatively or additionally, the intermediate wall  922  may make the plastic apparatus more resistant to in-plane or out-of-plane expansion or warping during one or more of the manufacturing processes. 
         [0077]      FIG. 22  shows how an end product (not shown), such as plastic cup, may be post-formed from preforms or preform sheets  1002 . In the illustrated embodiment, a plug  1004  is forced into the preform  1002  to create the end product. The plug  1004  may be heated. The amount of force and heat of the plug  1004  may be controlled to achieve a desired thickness and shape of the end product. Trimming and other post-forming operations may also be needed to produce the end product. For example, trimming and/or stamping operations may be needed to produce a lip of a plastic cup. 
         [0078]    In yet another embodiment of the present invention, it may be advantageous to distort or partially distort the preforms or sheets during the heating process. Referring back to  FIG. 2 , the portion of the preform  200  to be distorted would be the inner region  202 . Referring back to  FIG. 9 , the portion of the sheet  312  to be distorted would be the regions exposed by the apertures  320 . The distortion process may be done to increase the uniformity of heating, control warping, and/or to begin stretching the preform or sheet into shape before placing it into a mold. The distortion may be accomplished by increasing air pressure on one side of the preform or sheet, decreasing air pressure on one side of the preform or sheet, establishing a differential pressure across the preform or sheet, directing a mechanical force onto the preform or sheet using temperature controlled plugs, or some combination thereof. Because plastics may behave differently at different strain rates the distortion during the heating process may advantageously reduce overall processing time, advantageously require less forming after heating, or some combination thereof. 
         [0079]    The systems and methods described herein may advantageously shield and constrain a portion of a preform during saturation and/or heating. The systems and methods may be used with a variety of thermoplastics. The saturation process preferably utilizes carbon dioxide gas, but other types of gas may be employed to optimize the mechanical or micro-structural properties of the end product. In one embodiment, the end product may be biodegradable or recyclable. In addition, the end product may have a smooth exterior surface that is aesthetically and structurally better than existing end products made with other methods. The systems and methods described herein may produce an end product that is less expensive than a similar product of solid plastic due to the lower amount of raw plastic material used and a more energy processes. The end product may be lighter in weight, which reduces downstream transportation and inventory costs. Moreover, material left over after processing the preforms or sheets, as described above, may be re-processed and/or re-used to make additional sheets or preforms. 
         [0080]    The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, patent applications and publications referred to in this specification are incorporated herein by reference. Aspects can be modified, if necessary, to employ devices, features, and concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0081]    These and other changes can be made in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all types of plastic end products (e.g., plastic cups, packaging, etc.) and processes for making the same. Embodiments of the processes and/or end products described herein may be utilized individually or in any combination. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.