Abstract:
The disclosure relates to an apparatus and method for manufacturing a molded article from plastic material feedstock in a continuous manner. The plastic material feedstock is confined between endless belts that define molded surfaces of the article to be molded.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Embodiments of the present invention relate to an apparatus and method for molding plastic articles. More particularly, embodiments of the present invention relate to an apparatus and method for continuous molding of plastic articles from powdered, shredded, or granular plastic. 
         [0003]    2. Related Art 
         [0004]    Although many apparatuses and processes exist by which plastic may be molded or otherwise formed into articles of manufacture, many of these prior art apparatuses and processes require a plastic feedstock that has molding characteristics and uniformity consistent with virgin material. Recycled material may be incompatible with these apparatuses and processes for several reasons. First, recycled plastic material, even if pure, usually has inferior molding properties compared to virgin material due to degradation caused by prior molding and/or environmental exposure. Second, it is generally not practical to identify and segregate post-consumer plastics with a high degree of accuracy. Thus, recycled material usually contains a variable mixture of plastics, each with a different processing temperature range and viscosity profile. Third, non-plastic contaminants such as wood, rubber and metal are often present in recycled plastic and are difficult to separate reliably. These contaminants can lead to poor part quality, production stoppages, and even damage to molding equipment. 
         [0005]    Despite these processing challenges, it is desirable to utilize recycled plastic in the manufacture of plastic articles when possible. Plastic in post-consumer form is a resource already distributed throughout the world and available almost everywhere without additional transportation costs and impacts. Increased use of recycled plastic can potentially reduce both the volume of waste that must be transported to landfills and the amount of oil that must be depleted to produce virgin plastic, thus preserving a finite resource. To capitalize on these benefits, some processes have been developed that are well suited to the molding of post-consumer recycled plastic. For example, U.S. Pat. No. 8,221,668 discloses such a process. However, the existing processes most compatible with recycled plastic materials tend to rely on closed-mold batch processing. These processes require an entire mold to be heated and cooled along with the plastic being molded, and can be scaled only by building additional molds as production rates increase. Heating and cooling a mold can require significant energy input and may limit the achievable process throughput. Building additional molds to increase production rates may be costly. 
         [0006]    Accordingly, it is desirable to provide an apparatus and method capable of overcoming the limitations of the plastic molding processes known in the art. 
       SUMMARY 
       [0007]    The foregoing needs are met to a great extent by the present disclosure, providing a distinct advance in the art of plastic molding. 
         [0008]    Various embodiments of the present invention provide an apparatus and a method for molding a plastic article in a continuous and energy-efficient manner that is scalable and compatible with post-consumer grade recycled plastic feedstock. The present invention is particularly suited to the manufacture of sheet stock and dimensional boards capable of being used in place of natural wood and plywood products in many applications. The feedstock may be granular, powdered, or shredded. The apparatus broadly comprises a frame, a plurality of belts, a plurality of pulleys, a force reacting structure, a drive motor, a plastic material supply hopper, a plastic material applicator, a heater, and a cooler. The frame may support the various components of the apparatus and may maintain them in proper alignment with one another. Each belt may be disposed around a plurality of pulleys and may have at least one planar portion therebetween. A planar portion of each belt may be disposed in proximity to a planar portion of each other belt such that the belts may together define a plurality of molding surfaces capable of cooperatively constraining the plastic material feedstock, moving it en masse, and ultimately forming at least some of the surfaces of an article to be molded. Additional belts may be employed to define additional molding surfaces as desired. The planar portions of the belts disposed together may form a tubular passageway therebetween having a cross section related to the desired cross section of the plastic article to be molded. Plastic material may be drawn from the hopper by the material applicator and deposited on at least one of the belts. Additional hoppers and/or material applicators may be provided to deposit additional types, grades, compositions, or forms of plastic and/or other materials onto the same and/or different belts. One or more of the belts may extend beyond the other belt or belts to facilitate access by the material applicator or applicators thereto. A drive motor may be coupled to one or more of the pulleys to drive one or more of the belts. The belts may be advanced in unison, drawing material deposited on any one of said belts into contact with the molding surfaces presented by all of the belts. It is preferred that the belts cooperate to completely encircle the plastic material during molding, but in some embodiments the belts may only partially encircle the plastic material during molding. In either case, the belts may be further advanced in unison, bringing the material into a portion of the apparatus that is heated by the heater. This may melt or soften the granular, powdered, or shredded feedstock material and form a viscous, flowable mass therefrom. The heat may also cause any blowing agent that may be present in the plastic feedstock or any portion thereof to expand, creating a foam. The force reacting structure may be disposed adjacent to a planar portion of at least one of the belts and may make sliding or rolling contact therewith. The force reacting structure may serve to resist pressure generated during molding and may maintain the planar portion of at least one of the belts in a planar configuration. The belts may be further advanced in unison, bringing the material into a portion of the apparatus that is cooled by the cooler. This may cause the viscous, flowable plastic mass to solidify into a solid mass. The belts may be advanced still further, causing the finished, shaped, and solidified plastic article to begin to emerge from the belts and from the apparatus. The apparatus may be operated on a continuous basis provided that a sufficient supply of plastic material is available in the hopper or hoppers. Therefore, a molded article of indefinite length may be manufactured. The molded article may be cut to a desired length without stopping the process by means of a flying cutter of the type known in the art of extrusion. 
         [0009]    This summary is intended to introduce the present invention in a simplified form. Additional aspects and details of the invention and selected embodiments thereof are disclosed in the detailed description. This summary is not intended to designate particular elements of the claimed subject matter as key or essential. Nor is this summary intended to limit the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a molding apparatus constructed in accordance with an embodiment of the invention. 
           [0011]      FIG. 2  is a schematic diagram of a molding apparatus constructed in accordance with an embodiment of the invention and incorporating two force reacting structures, both comprising shaft-mounted rollers. 
           [0012]      FIG. 3  is cross sectional view showing cut A-A from  FIG. 1 . 
           [0013]      FIG. 4  is a cross sectional view showing cut B-B from  FIG. 2 . 
           [0014]      FIG. 5  is a schematic diagram of a molding apparatus constructed in accordance with an embodiment of the invention and incorporating one force reacting structure comprising recirculating rollers and another force reacting structure comprising shaft-mounted rollers. 
           [0015]      FIG. 6  is a flow diagram illustrating a method for molding a plastic article. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following detailed description makes reference to accompanying drawings that illustrate specific embodiments of the present invention. Separate references to “an embodiment” or “one embodiment” do not necessarily refer to the same embodiment, though they may. The specific embodiments illustrated and/or described in detail in this disclosure are included to enable those skilled in the art to practice the invention. Other embodiments and variations will be apparent those skilled in the art and may be substituted without departing from the scope of the present invention. Therefore, the detailed description that follows should not be construed in a limiting sense. 
         [0000]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a molding apparatus  10  constructed in accordance with an embodiment of the invention is illustrated in  FIG. 1 . The molding apparatus  10  may include a plurality of continuous belts  20 ,  22 ,  24 ,  26  disposed around a plurality of pulleys  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 . Each continuous belt may be disposed around at least two corresponding pulleys. For example, in  FIG. 1 , continuous belt  20  is disposed around pulley  30  and pulley  32 . It is preferred that the continuous belts be made from stainless steel, but other materials having adequate thermal conductivity, mechanical properties, temperature resistance, and release properties may be substituted. The belts may be disposed in proximity to one another and arranged such that a tubular passageway  28  is formed therebetween.  FIG. 3  shows a cross sectional view through the tubular passageway  28  and surrounding belts and pulleys of the embodiment illustrated in  FIG. 1 . It is preferred that the tubular passageway  28  so formed be completely sealed in cross section (as shown in  FIG. 3 ), with each belt disposed in direct contact with adjacent belts. However, in some cases the tubular passageway may be only partially sealed. For example, if only two belts are used, corresponding to belts  20  and  22 , a substantially rectangular tubular passageway may be formed that is sealed on two sides but open on two sides. Alternatively, it is possible that four continuous belts  20 ,  22 ,  24 ,  26  may be used, but gaps may be left at the interfaces therebetween, resulting in a tubular passageway that is not completely sealed. It is preferred that the tubular passageway  28  have four sides as illustrated in  FIG. 3 , though in some embodiments the tubular passageway  28  may have a different number of sides. 
         [0017]    Although the tubular passageway  28  may be of uniform cross section throughout its length, in some embodiments the cross sectional area of the tubular passageway  28  may be locally reduced at a location  48  near the entrance to said tubular passageway  28  as illustrated in  FIG. 2 ,  FIG. 4  and  FIG. 5 . The localized reduction in cross sectional area of the tubular passageway  28  at the location  48  may cause material constrained within or passing through said tubular passageway to be squeezed, compressed, or further constrained, and may prevent pressure generated during the molding process from escaping and/or forcing plastic material through the entrance to the said tubular passageway  28 . The tubular passageway cross section may reach a minimum at a single location, forming a pinch point. Alternatively, the tubular passageway may be maintained at a constant reduced cross section over some portion of the tubular passageway&#39;s length. The entrance to the tubular passageway may receive plastic material in a granular, shredded, or powdered state having a bulk density significantly lower than the solid density of the plastic material itself. The tubular passageway or the entrance thereto may be configured to progressively squeeze the individual particles of plastic together until the region of the tubular passageway having a minimum cross section is reached. The bulk density of the plastic particles in this region may approach the solid density of the plastic material from which the plastic particles are formed. This compacted material may form a plug capable of resisting back-flow of plastic particles and/or gas. In some embodiments, the cross sectional area of the tubular passageway  28  may occur abruptly at the entrance to said tubular passageway. However, it may be advantageous in some cases to reduce the cross sectional area gradually so as to minimize disruption of the plastic particles as they are compressed. 
         [0018]    One continuous belt  20  may be disposed beneath another continuous belt  22 . The lower continuous belt  20  may be longer than the upper continuous belt  22 , allowing the upper surface of each belt to be accessed to facilitate the application of plastic or other materials thereto. 
         [0019]    Plastic material applicators  60 ,  62 ,  64  may be disposed proximate to one or more of the continuous belts  20 ,  22 . It is preferred that the plastic material applicators be disposed above an upper surface of the continuous belts  20 ,  22 . The plastic material applicators may comprise gravity-fed apertures spaced a distance above the upper surface of the continuous belts as illustrated in  FIG. 1 . Granular, powdered, or shredded plastic material may flow through the apertures, coming to rest on the continuous belts and building up thereon. Because of the flow characteristics of granular, powdered, and shredded materials, a stoppage in the flow may be created once the space between an aperture and a continuous belt has been filled. Movement of the continuous belts around their respective pulleys relative to the stationary apertures may cause plastic material to be drawn out from under each aperture, said plastic material being spread across portions of the surfaces of the continuous belts. When plastic material is drawn out from under the apertures, voids may be formed between the apertures and the continuous belts, thus enabling replacement plastic material, urged by gravity, to flow from the hoppers, through the apertures, and onto the continuous belts. In some embodiments of the invention, the plastic material applicators may comprise an active system for metering, conveying, and depositing plastic material onto one or more continuous belts. Metering may be implemented on a volumetric or gravimetric basis and by any means known in the art. In some embodiments, plastic may be applied to at least one continuous belt other than on its upper surface. In some embodiments, as illustrated in  FIG. 1 , a plastic layer  72  may be deposited by one plastic material applicator  62  over another plastic layer  70  previously deposited by a different plastic material applicator  60 . In some embodiments, and as illustrated in  FIG. 1 , a plastic layer  74  may be deposited by a plastic material applicator  64  onto an upper surface of an upper continuous belt  22 . In such an embodiment, the plastic material may be transferred to a lower surface of an upper continuous belt  22  by advancing the belt around pulleys  34 ,  36 . This may cause the plastic material deposited on the upper surface of the upper continuous belt  22  to enter the tubular passageway  28  cooperatively formed by the continuous belts  20 ,  22 ,  24 ,  26  and to contribute to the formation of a molded plastic article  98  therein. 
         [0020]    The plastic material applicators  60 ,  62 ,  64  may deposit different forms, grades, compositions, or types of plastic. In some embodiments, multiple plastic material applicators may deposit identical materials. For example, plastic material applicators  60  and  64  may deposit a first type of plastic material suitable for forming a high density plastic skin, while plastic material applicator  62  may deposit a second type of plastic material suitable for forming a low density core. Although three plastic material applicators  60 ,  62 ,  64  are preferred and are illustrated in  FIG. 1 , a different number of plastic material applicators may be employed. For example, a single plastic material applicator may be used where a plastic article with a single plastic composition is desired. Alternatively, more than three plastic material applicators may be employed if a plastic article composed of more than three distinct layers or material types is desired. Furthermore, material applicators for non-plastic materials and/or manual placement may be employed to incorporate non-plastic materials such as fiber reinforcement into portions of a plastic article molded by the apparatus  10 . For example, natural fibers such as hemp, cotton, flax and the like, synthetic fibers such as fiberglass, carbon, and the like, and recycled fibers of either natural or synthetic origin may be incorporated into a plastic article molded by the apparatus  10 . This may be accomplished by mixing fibers into the plastic supply feeding the plastic material applicators,  60 ,  62 ,  64 , or by placing the fibers as discrete layers on top of skin plastic layers  70 ,  74  where they will end up encapsulated inside the finished plastic part, yet remain near the surface to maximize their structural effectiveness. The fibers may be placed as discrete layers by automated fiber applicators, or by manually placing fibers on the exposed skin plastic layers  70 ,  74 . For example, fibrous material may be unwound from a roll automatically and drawn into the tubular passageway  28  by the motion of the continuous belts. Alternatively or additionally, fibrous materials of non-uniform sizes and shapes, and particularly recycled fibrous materials, may be manually placed on exposed portions of plastic layers such as plastic layers  70  and  74  of  FIG. 1 . Manual placement may enable recycled fibrous material pieces of unpredictable sizes and shapes to be layed as a patchwork to form a fibrous layer of relatively uniform thickness. This may permit otherwise unusable materials to be incorporated into a molded plastic article  98  while maintaining relatively predictable properties. 
         [0021]    The plastic material applicators  60 ,  62 ,  64  may be supplied with plastic material by corresponding plastic material supply hoppers  50 ,  52 ,  54 . The hoppers may be disposed above the plastic material applicators, facilitating gravity flow therebetween. Alternatively, the hoppers may be disposed apart from or below the plastic material applicators, and alternative means of conveyance may be employed to transfer material from the plastic material supply hoppers to the plastic material applicators. In some embodiments, the plastic material applicators may be fed directly by a grinder, shredder, pulverizer, or the like, and an intermediate hopper may not be required. 
         [0022]    A drive motor  14  may be mechanically coupled to at least one of the pulleys  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44  and may cause at least one of the continuous belts  20 ,  22 ,  24 ,  26  to advance at a controlled rate. It is preferable that the motor  14  be coupled to all of the continuous belts forming the tubular passageway  28  so as to ensure that all of the said continuous belts move in unison. Alternatively, multiple motors may be employed to drive the various belts, or some belts may be left undriven. Undriven belts may serve as idler belts and may passively move in unison with the other belts, urged on by drag from the plastic material in contact therewith. 
         [0023]    A heater  80  may be disposed proximate to the tubular passageway  28  and may heat those portions of the continuous belts that are proximate to the heater  80  as well as any plastic material present within the tubular passageway  28  proximate to the heater  80 . The plastic within the tubular passageway  28  may be heated by conduction through at least one of the continuous belts  20 ,  22 ,  24 ,  26  forming the walls thereof. The heater  80  may be disposed proximate to a first portion of the tubular passageway  28  relative to the direction of plastic flow therethrough. The heater  80  may preferably be of the direct flame impingement type, and may comprise a network of tubes through which a gaseous fuel may flow and a plurality of apertures through which said fuel may exit and combust. Flames from the heater  80  may impinge directly on the inner surface of the belt  20 . Although only one heater is shown in  FIG. 1  for clarity, it is preferred that multiple heaters be employed to heat multiple belts at once, thus heating the tubular passageway  28  and the contents thereof from multiple directions. In some embodiments, the heater  80  may be of the hot air, electric, infrared, induction, or other heater type known in the art. 
         [0024]    It may be beneficial for the continuous belts  20 ,  22  or portions thereof proximate to the plastic material applicators  60 ,  62 ,  64  to be heated to an intermediate temperature. The intermediate temperature to which portions of the continuous belts may be heated may be higher than the temperature of those portions of the continuous belts in close proximity to the cooler  84 , but lower than the temperature of those portions of the continuous belts in close proximity to the heater  80 . Heating to the intermediate temperature may promote rapid sintering of the plastic particles deposited on the belts by the plastic material applicators  60 ,  62 ,  64  and may facilitate adhesion of the plastic material to the continuous belts  20 ,  22 . Such adhesion may be particularly beneficial in the case of those embodiments in which plastic material is expected to cling to a continuous belt in an inverted position. For example, plastic material  74  is expected to cling to continuous belt  22  as it travels around pulley  34  just prior to entry of said plastic material  74  into tubular passageway  28  of the embodiment of the invention illustrated in  FIG. 1 . The heat for heating portions of the continuous belts to an intermediate temperature may be derived from the waste heat or exhaust gasses of heater  80 . If the heater  80  is unable to provide sufficient waste heat to preheat the continuous belts adequately, or if better control is desired, one or more auxiliary heater  86  may be employed to preheat portions of the continuous belts  20 ,  22  as required. 
         [0025]    A cooler  84  may be disposed proximate to the tubular passageway  28  and may cool those portions of the continuous belts that are proximate to the cooler  84  as well as any plastic material present within the tubular passageway  28  proximate to the cooler  84 . The plastic within the tubular passageway  28  may be cooled by conduction through at least one of the continuous belts  20 ,  22 ,  24 ,  26  forming the walls thereof. The cooler  84  may be disposed proximate to a second portion of the tubular passageway  28 , following the portion proximate to the heater  80  relative to the direction of plastic flow therethrough. The cooler  84  may preferably be of the water mist type, and may comprise a network of tubes through which water may flow and a plurality of apertures or atomizing nozzles through which said water may emerge as a mist. Mist from the cooler  84  may impinge directly on the inner surface of the belt  20 . Although omitted from  FIG. 1  for clarity, it is preferred that multiple coolers be employed to cool multiple belts at once, thus cooling the tubular passageway  28  and the contents thereof from multiple directions. In some embodiments, the cooler  84  may be of the compressed air jet, water spray, water saturated contact sponge, or other cooler type known in the art. 
         [0026]    An insulative barrier  82  may separate a portion of the continuous belt  20  proximate to the heater  80  from a portion of the continuous belt  20  proximate to the cooler  84 . A frame  12  may be provided to maintain the components of the apparatus  10  in their respective positions. 
         [0027]    A force reacting structure  90 , illustrated in  FIG. 2 , may be disposed adjacent to the tubular passageway  28 . The force reacting structure may serve to resist pressure generated inside the tubular passageway  28  and may resist outward deflection of those planar portions of the continuous belts forming the walls of the tubular passageway. The force reacting structure  90  may comprise a structural support  92  and at least one contact feature  94  for making contact with a planar portion of at least one of the belts. The contact feature  94  may comprise a low-friction surface for making sliding contact with the at least one belt, a plurality of recirculating rollers, a plurality of shaft-mounted rollers, a plurality of balls, or a plurality of other rolling elements for making rolling contact with the at least one belt. Different portions of the apparatus may employ different contact feature types simultaneously. For example, a single apparatus may be constructed with recirculating rollers making contact with one belt, while shaft-mounted rollers make contact with another belt as illustrated in  FIG. 5 . Alternatively or additionally, different contact feature types may be utilized within a single belt. For example, recirculating rollers may be used adjacent to those portions of the belts in which the tubular passage  28  cross section is reduced, while shaft-mounted rollers may be used adjacent to other portions of the same belts. In those embodiments utilizing recirculating rollers as contact features, it may be beneficial for the rollers to be formed from a material having a low thermal conductivity and/or a low heat capacity. This may minimize the energy expended to heat and cool the rollers. The force reacting structure  90  may counteract pressure exerted on the belts from within the tubular passageway  28  during the evolution of gas from a blowing agent present in the plastic material. 
         [0028]    In some embodiments, it may be desirable to apply a release agent to one or more of the continuous belts prior to depositing plastic material thereon. The release agent may serve to enable the plastic article formed by means of the present invention to be removed from the apparatus without damage to either the plastic article or the apparatus. In some cases, release agent may be applied periodically by manual means. However, in some embodiments, a release agent applicator  88  may be employed to continuously and automatically deposit release agent onto one or more of the continuous belts  20 ,  22 ,  24 ,  26  during the operation of the apparatus  10 . The release agent applicator  88  may apply release agent by means of a contact roller, contact sponge, brush, mister, spray nozzle, or the like. It is preferred that the release agent applicator be positioned such that release agent is applied to a continuous belt prior to the auxiliary heater  86  or heater  80  relative to the direction of belt motion through the apparatus  10 . 
         [0029]    Steps of a method  100  for molding a plastic article  98  in accordance with various embodiments of the present invention are illustrated in  FIG. 6 . The steps may be performed in the order shown in  FIG. 6 , or they may be performed in a different order. Some steps may be performed concurrently rather than consecutively and some steps may be omitted. 
         [0030]    Referring to step  101 , a first belt, such as the first continuous belt  20  of the apparatus illustrated in  FIG. 1 , or some portion thereof may be preheated. 
         [0031]    Referring to step  102 , a first plastic material may be deposited onto a first belt, such as the first continuous belt  20  of the apparatus illustrated in  FIG. 1 . 
         [0032]    Referring to step  103 , a first fiber layer may be deposited onto the first plastic material. 
         [0033]    Referring to step  104 , a second plastic material such as the material forming the plastic layer  72  illustrated in  FIG. 1  may be deposited onto the first plastic material such as the material forming the plastic layer  70  in  FIG. 1 . 
         [0034]    Referring to step  105 , a second belt, such as the second continuous belt  22  of the apparatus illustrated in  FIG. 1 , or some portion thereof may be preheated. 
         [0035]    Referring to step  106 , a third plastic material may be deposited onto a second belt, such as the second continuous belt  22  of the apparatus illustrated in  FIG. 1 . 
         [0036]    Referring to step  107 , a second fiber layer may be deposited onto a third plastic material. 
         [0037]    Referring to step  108 , the first belt and second belt may be advanced, transporting plastic material deposited on exposed surfaces of said belts to a position between said belts such that the material is constrained thereby. 
         [0038]    Referring to step  109 , the first belt and second belt may be advanced, transporting plastic material constrained therebetween to a position proximate to a heater. 
         [0039]    Referring to step  110 , the first belt and second belt may be advanced, transporting plastic material constrained therebetween to a position proximate to a cooler. 
         [0040]    Although method steps  102 ,  104 , and  106  refer to a first plastic material, a second plastic material, and a third plastic material, the form and composition of some or all of these materials may be identical. For example, the first plastic material and the third plastic material may be identical and may be suitable for forming the skins of the plastic article  98 . The second plastic material may differ from the first plastic material and the third plastic material, and may be suitable for forming the core of the plastic article  98 .