FIELD OF THE INVENTION 
       [0001]    The current invention is directed to collapsible plastic storage containers, in particular to containers made from load bearing rotomolded plastic parts having a twinned wall structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    Collapsible shipping and storage containers are well known in the art. Collapsibility is desirable in order to minimize the space requirements of the container when it is empty and not in use. Collapsibility improves storage and shipping options and mitigates shipping costs. 
         [0003]    Collapsible containers typically include removable or foldable side or end walls, a base and optionally a roof. Generally, shipping containers are constructed from wood, metal or plastic parts. Metal containers are durable with high load bearing capacities, but are heavy. Plastic containers are lighter, but are not as durable, are typically much smaller in size and have reduced load bearing capacity. As a result, the storage capacity of plastic containers can be limited. 
         [0004]    For example, a collapsible shipping container made of plastic is described in U.S. Pat. No. 4,630,746. Each part of the container has a “meshed” structure made by injection molding. The container has two opposing side walls that are inwardly foldable along a vertically hinged axis. In contrast, U.S. Pat. No. 3,870,185 teaches a collapsible plastic container having side walls that are inwardly foldable along a horizontally hinged axis. The plastic containers taught are not suitable for storage and transport of heavy loads. 
         [0005]    A metal container having inwardly folding horizontally hinged side walls is disclosed in U.S. Pat. No. 5,190,179. The container also has end walls that pivot to within the container where they are stored when the container is collapsed. The metal container contains fork lift tines to allow for its movement when collapsed or erected. Other metal containers having similar features are disclosed in U.S. Pat. Nos. 4,577,772, 3,570,698, 4,848,618, and 4,214,669. The weight of these metal containers and their component parts often requires a forklift or other specialized equipment in order to collapse and erect the container, as well as for movement of the containers from one location to another. 
         [0006]    In light of the above, there remains a need for a collapsible container having good load bearing properties and high storage capacity but without the added weight or corrosion problems of metal component parts. 
       SUMMARY OF THE INVENTION 
       [0007]    The current invention overcomes the problems associated with the prior art by providing a collapsible container that is relatively lightweight and has a high load bearing capacity. 
         [0008]    The current invention provides a collapsible container comprising: a roof; a base; two opposing end walls, each end wall comprising an upper and a lower end wall panel pivotally attached to the roof and the base respectively, the upper end wall panel being horizontally pivotally attached to the lower end wall panel to permit inward folding of each end wall; two opposing removable side walls fitting between the roof and the base; the end walls and the side walls being a twinned wall load bearing structure defined by inner and outer wall members made of plastic; whereby the container is collapsible by removing the side walls and inwardly folding the end walls to draw the roof toward the base. 
         [0009]    In an embodiment of the invention, the side walls and the end walls are rotomolded plastic components. 
         [0010]    In an embodiment of the invention, at least one of the side walls and end walls has a plurality of concave indentations forming an internal arch between the inner and outer wall members of the twinned wall structure. 
         [0011]    In an embodiment of the invention adjacent edges of the end walls and the side walls engage each other through mating tongue and groove formations. 
         [0012]    In an embodiment of the invention the side walls are partitioned into at least two removable side wall panels. 
         [0013]    In an embodiment of the invention the side wall panels of the container engage the roof and base along their upper and lower edges respectively by a complimentary tongue and groove means. The tongue and groove means allows each side wall panel to slidably engage the roof and the base. 
         [0014]    In an embodiment of the invention the tongue and groove means between the side wall panels and the roof has a section dimensioned to loosely engage a side wall panel aligned with the section, so that each side wall panel can be removed from the container when aligned with the section. 
         [0015]    In an embodiment of the invention the base has a recessed area that holds each of the side wall panels when the container is collapsed. 
         [0016]    In an embodiment of the invention the container has a locking means that prevents removal of a side wall panel that is aligned with a section dimensioned to loosely engage a side wall panel. 
         [0017]    In an embodiment of the invention the base has depending tine slots. 
         [0018]    In an embodiment of the invention the roof has protrusions that are complimentary to the tine slots so that a plurality of containers can be stacked one on top of the other without slippage. 
         [0019]    The inventive containers are collapsible to minimize space requirements during transport, and are easily moved without requiring specialized equipment. 
         [0020]    The inventive containers are weatherproof and stackable when collapsed or erected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1   a  shows a perspective view of an embodiment of the current invention. 
           [0022]      FIG. 1   b  shows an exploded perspective view of an embodiment of the current invention. 
           [0023]      FIG. 2  shows a cross sectional perspective view of a lower end wall panel in an embodiment of the current invention. 
           [0024]      FIG. 3  shows a perspective view of a lower end wall panel in an embodiment of the current invention. 
           [0025]      FIGS. 4   a  and  4   b  show perspective views of the roof in an embodiment of the current invention. 
           [0026]      FIGS. 5   a  and  5   b  show perspective views of the base in an embodiment of the current invention. 
           [0027]      FIG. 6  shows a cross sectional elevation view of the roof, end wall and base in an embodiment of the current invention. 
           [0028]      FIG. 7  shows an exploded perspective view of an end wall in an embodiment of the current invention. 
           [0029]      FIG. 8  shows a perspective view of an embodiment of the current invention. 
           [0030]      FIG. 9  shows a cross sectional plan view of the end walls and side walls of an embodiment of the current invention. 
           [0031]      FIG. 10   a  shows a partial perspective view of an embodiment of the current invention. 
           [0032]      FIG. 10   b  shows a partial perspective view of an embodiment of the current invention. 
           [0033]      FIG. 11  shows a perspective view of an embodiment of the current invention. 
           [0034]      FIGS. 12   a  and  12   b  show perspective views of a locking means in an embodiment of the current invention. 
           [0035]      FIG. 13  shows a perspective view of an embodiment of the current invention. 
           [0036]      FIG. 14  shows a perspective view of an embodiment of the current invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The current invention describes collapsible plastic containers having improved storage and load bearing capacity. 
         [0038]    In the current invention, the use of the terms “end wall” and “side wall” is arbitrary and is used only to distinguish one set of opposing container walls from the other. It will be recognized by a person skilled in the art that the side walls can be designated as the end walls and vice versa and that the side walls can be the same length as the end walls, or they may be longer or shorter than the end walls. 
         [0039]    The terms “mate” or “mating edges” includes complimentary tongue and groove means, interlocking offset edges, abutting offset edges and the like, but does not include abutting parallel edges. 
         [0040]    The term “removable” is meant to encompass container components that are in their entirety removable from the container without an attachment point to any other component of the container. In contrast, the term “integral” is meant to encompass container components that have at least one point of attachment to at least one other component of the container regardless of their orientation or configuration. 
         [0041]      FIG. 1   a  shows a preferred embodiment of the current invention. The container  1  has a roof  5 , a base  10 , two opposing integral end walls  15 , and two opposing removable side walls,  20 . The opposing end walls are each comprised of an upper end wall panel  25  and a lower end wall panel  30  pivotally attached to the roof  5  and the base  10  respectively. The upper end wall panels  25  are pivotally attached to lower end wall panels  30  along a horizontal axis  35  by hinges  40 . The axis  35  extends horizontally across the vertical midpoint or centre of the end walls  15 . The side walls  20  which are fully removable fit between and engage the roof  5  and the base  10 . 
         [0042]    In a preferred embodiment the ends walls and the side walls are rotomolded plastic parts having a twinned wall structure defined by an inner wall member  45  and an outer wall member  50 . The roof and base are also preferably made of rotomolded plastic and optionally have a twinned wall structure. The twinned wall structure has a void space between inner and outer wall members  45  and  50  respectively as shown in  FIG. 2 . 
         [0043]    Rotomolding techniques are well known in the art and are particularly well suited to the production of large or hollow plastic parts having complex shapes. 
         [0044]    With reference to  FIG. 3 , the load bearing capacity of the side walls and the end walls can be increased by incorporating a plurality of concave indentations  55 , each indentation forming an internal arch,  60  between inner  45  and outer wall  50  members. As shown in  FIGS. 1 and 3 , the concave indentations  55  are substantially horizontal. It will be recognized by a person skilled in the art that the concave indentations can face inward as when in the outer wall member, or outward as when in the inner wall member. A combination of inward and outward facing concave depressions is also contemplated by the current invention. 
         [0045]    Without wishing to be bound by theory, the arches defined by the concave indentations provide an internal “roman arch” which resists deformation of the walls by compression forces. As the walls are compressed, the internal arch resists inward or outward flexing of the walls in a direction approximately perpendicular to the direction of the compression forces. 
         [0046]    In a preferred embodiment of the current invention, the side walls  20  are partitioned into at least two removable side wall panels having mating or abutting adjacent edges. 
         [0047]    Preferably, the side wall panels have upper and lower edges that engage the roof and base respectively by a complimentary tongue and groove means. The tongue and groove means allows each side wall panel to slidably engage the roof and the base. 
         [0048]    In a preferred embodiment of the invention, the end walls  15  and the side walls  20  engage each other along mating tongue and groove formations. The formations may have any suitable shape, provided that a groove within the tongue and groove formation of an end wall or a side wall is in alignment with a tongue within the tongue and groove formation of an adjacent side wall or end wall respectively. 
         [0049]    With reference to  FIGS. 4   a  and  4   b , the roof  5  has two downwardly extending end skirting walls  65 . The upper end wall panels  25  are pivotally attached to the end skirting walls by one or more hinges  41 . Similarly, the roof  5  has two downwardly extending side skirting walls  70  which engage the side walls  20 . There is an upper groove  75  disposed in each downwardly extending side skirting wall. The upper grooves  75  form part of a tongue and groove means between the side wall panels and the roof. 
         [0050]    With reference to  FIGS. 5   a  and  5   b , the base  10  has spaced apart top and bottom surfaces  80  and  85  joined by a pair of base end walls  90  and a pair of base side walls  95 . The base also has two upwardly extending end retaining walls  100 . The top surface  80  that is between the end retaining walls  100  provides a recessed area for storing side wall panels. The lower end wall panels  30  are pivotally attached to the end retaining walls  100  of the base by hinges  42 . The base has a pair of lower grooves  105 . Each lower groove is disposed in the top surface  80  of base  10  adjacent to each base side wall  95 . The lower grooves  105  form part of a tongue and groove means between the side wall panels and the base. 
         [0051]    In a preferred embodiment the base has a plurality of “kiss offs”. The term “kiss off” refers to a point in a twinned wall component at which the inner and outer wall members  45  and  50  pinch together to make contact with one another. Use of “kiss offs” in other parts of the collapsible container  1  is also contemplated by the current invention. It is well known in the art that “kiss offs” increase the structural rigidity and strength of a twinned wall component. 
         [0052]    In a preferred embodiment, each side wall panel has a pair of offset perimeter edges that slidably engage upper grooves  75  and lower grooves  105  in the side skirting walls  70  of roof  5  and the upper surface  80  of base  10  respectively. 
         [0053]    A reversed tongue and groove means, in which grooves present in the upper and lower edges of the side wall panel slidingly mate with a tongued track on the roof and base respectively, is also contemplated by the current invention. 
         [0054]    With reference to  FIG. 6 , the upper end wall panels  25  are pivotally attached to the end skirting walls  65  by one or more than one offset hinge  41 . The lower end wall panels  30  are pivotally attached to the retaining walls  100  of the base by one or more than one offset hinge  42 . The upper end wall panels are horizontally pivotably attached to the lower end wall panels by one or more than one offset hinge  40 . 
         [0055]    The upper end wall panels  25  and the lower end wall panels  30  have abutting or mating adjacent edges. In a preferred embodiment, an outwardly offset pair of lower edges on the upper end wall panels  25  mate with an inwardly offset pair of upper edges on the lower end wall panels  30  when the container is erect, as shown further in  FIGS. 7 and 8 . This configuration of mating offset edges prevents water from entering the container along the horizontal axis  35  and provides a stop against the outward folding of the end walls beyond a substantially vertical position. 
         [0056]    With reference to  FIGS. 8 and 9 , the end walls  15  and the retaining walls  100  have on one lateral side, perpendicular corner extensions  114  and  115  respectively which mate with an edge of an adjacent side wall  20  to define a pair of vertical corner sections. The corner extensions  114  and  115  have a first lateral groove  130  disposed therein which mates with an edge of an adjacent side wall  20 . Non-mating, abutting adjacent edges can also be used as can a reversed tongue and groove interaction, in which the groove,  130  is present in an adjacent edge of the side wall. 
         [0057]    To facilitate inward folding of the end walls along axis  35 , the corner extensions  114  on upper and lower end wall panels  25  and  30 , have beveled upper and lower ends respectively. The corner extension  115  on the end retaining walls have a squared upper end. In one embodiment of the invention, the corner extensions  114  and  115  on the lower end wall panels and the end retaining walls respectively have adjacent beveled and squared end surfaces which evenly abut one another as shown in  FIG. 10   a . This arrangement leaves a small space between corner extensions  114  and  115 . Preferably, the corner extensions  114  and  115  on the lower end wall panel and the end retaining wall respectively have adjacent beveled and squared end surfaces which abut one another unevenly as shown in  FIG. 10   b . The upper end of corner extension  115 , inward of grove  130 , has an upwardly extending portion  116  which overlaps with the space between corner extensions  114  and  115 , outward of groove  130 , providing additional weatherproofing for erected container  1 . The corner extension  114  on the lower end wall panel  30  has a cutout  117  inward of groove  130 , which mates with the upwardly extending portion on corner extension  115  when the container is collapsed. 
         [0058]    With reference to  FIG. 9 , the side walls  20  have on one lateral side a perpendicular corner extension  118  which mates with an edge of an adjacent end wall  15  to define a second pair of vertical corner sections. The corner extensions  118  have a second lateral groove  135  disposed therein which mates with an edge of an adjacent end wall  15 . Non-mating, abutting adjacent edges can also be used as can a reversed tongue and groove interaction, in which the groove,  135  is present in an adjacent edge of the end wall. 
         [0059]    As shown in  FIGS. 1 and 9 , each of the side walls  20  can be partitioned into first  21 , second  22  and third  23  sequentially adjacent side wall panels. First and second side wall panels,  21  and  22  are generally flat, while the third side wall panel  23  has an L-shaped cross section the short side of which defines corner extension  118 . The first  21  and second  22  side wall panels have a pair of offset perimeter edges that slidingly engage the upper groove  75  and the lower groove  105 . An edge of the first wall panel  21  also engages the first lateral groove  130  in corner extensions  114  and  115  of an adjacent end wall and retaining wall respectively. The first and second side wall panels are equivalent but flipped relative to one another so that adjacent offset edges of the first side wall panel and the second side wall panel mate with one another. The perpendicular corner extension  118  of the third side wall panel  23  has disposed therein a second lateral groove  135  which mates with the edge of an adjacent end wall. The side wall panel  23  also has a pair of offset perimeter edges that mate with an adjacent edge of the second door panel  22 , with the lower groove  105  of the base  10  and with the upper groove  75  of the roof  5 . 
         [0060]    As shown in  FIG. 11 , the side wall panels  23  provide structural support for an erected container, when the side wall panels  21  and  22  have been removed to provide access to the container. 
         [0061]    In a preferred embodiment of the current invention, the tongue and groove means between the side wall panels and the roof will have at least one section dimensioned to loosely engage a side wall panel aligned with the section, so that a side wall panel can be removed from the container when aligned with the section. 
         [0062]    With reference to  FIGS. 4   a  and  4   b , the section  120  in each upper groove  75 , is dimensioned to allow upward displacement of side wall panels  21  and  22  when aligned with the section. Upward displacement of a side wall panel allows for disengagement of the tongue and groove means holding the side wall panel in place between the roof  5  and the base  10 . As a result, side wall panels  21  and  22 , which are laterally slidable in grooves  75  and  105 , can be removed from the container by alignment with the section  120  followed by disengagement of the tongue and groove means by upward displacement. The location of section  120  is indicated by a cutout portion  131  in the side skirting walls  70 . The side wall panels  23  have an L-shaped cross section and are not laterally slidable for alignment with the section  120 . To permit removal of side wall panel  23 , a section  121  aligned with each side wall panel  23  is present in each upper groove  75 . The section  121  permits upward displacement of the side wall panels  23  thereby disengaging the tongue and groove means so that side wall panels  23  can be removed from container  1 . 
         [0063]    The container has at least one locking means that prevents upward displacement of a side wall panel that is in alignment with a section dimensioned to loosely engage a side wall panel (i.e. sections  120  and  121 ) so that the side wall panel cannot be removed from the container. 
         [0064]    In an embodiment, the locking means comprises one or more than one dead bolt, which locks adjacent side wall panels to one another. 
         [0065]    In another embodiment, the locking means is a cam disk  125  in communication with a side wall panel and a section dimensioned to loosely engage a side wall panel. The cam disk is rotatable between a locked position and an unlocked position as shown in  FIGS. 12   a  and  12   b  respectively. When in a locked position, the cam disk  125  reduces the vertical dimension of the section dimensioned to loosely engage a side wall panel, to block upward displacement of a side wall panel. When in an unlocked position, the cam disk  125  does not alter the dimensions of the section dimensioned to loosely engage a side wall panel, thereby allowing upward displacement of a side wall panel. 
         [0066]    In one embodiment of the invention, the cam disk can be rotatably attached to a side wall panel adjacent to or within the upper tongue and groove means. 
         [0067]    A combination of dead bolts and cam disk locks can also be used in the present invention, as can other locking means that are well known in the art. 
         [0068]    In an embodiment of the current invention, the end skirting walls of the roof have downwardly extending flanges  132  which overhang the end walls when the container is erect and which seal the ends of a collapsed container by a mating or abutting engagement with the retaining walls of the base. In a preferred embodiment, the retaining walls  100  have a lip  133  that engages the flanges  132  on the roof when the container is collapsed. The flanges  132  help to waterproof the container  1 . 
         [0069]    In order to facilitate movement of container  1 , the base has base cut outs  135  which accommodate a pair of tines or prongs approaching the container along an axis substantially perpendicular to the side walls or substantially perpendicular to the end walls. The cut outs  135  form depending tine slots and can be integrally molded in base  10  with spacing to accommodate the tines of a forklift, a hand push pallet mover or hand jack or other suitable device having tines which engage complimentary slots. 
         [0070]    The roof can have protrusions  140  which are complimentary to the cut outs  135  in the base, so that a plurality of collapsed or erect containers may be stacked one on top of the other without slippage. Optionally, a gully  145  can be provided in the protrusions  140  of roof  5  to allow liquid to escape a confined area  150  defined in the roof by the protrusions. 
         [0071]    In an embodiment of the invention, at least one of the ends walls has a plurality of rain proof cutouts  155  comprising upwardly deepening vertical troughs which extend from an upper end of the end wall  15  to below the bottom edge of the downwardly extending flanges  132 . The troughs provide airflow into erect container  1  while the shape of cutouts  155  prevents water from entering the container  1 . 
         [0072]    In an embodiment, the top surface  80  of the base  10  immediately inward of lower groove  105  is vertically higher than the top surface  80  of the base  10  immediately outward of lower groove  105 , so that liquid entering the lower groove  105  will not flow into the container  1 . Optionally, the lower grooves  105  may contain drainage holes. 
         [0073]    The side wall panels  21 - 23  and the end wall panels can have one or more than one handle. 
         [0074]    The hinges  41 ,  42  and  43  can be made of rotomolded plastic. The hinges  40 ,  41  and  42  have male and female components that can be optionally integrally molded with the upper and lower end wall panels, the roof or the base. 
         [0075]    The incorporation of an insulating material between the inner and outer wall members  45  and  50  respectively, of each twinned wall component is also contemplated by the current invention. 
         [0076]    The base  10  can be reinforced by any method known in the art to strengthen plastic pallets, including for example the incorporation of metal, plastic or composite reinforcing bars, ribbing, columns, posts or studs. 
         [0077]    The dimensions of the container are not of particular importance, however, by way of example only, the container can be 8 feet high by 8 feet wide by 6 feet deep. In a another non-limiting embodiment the minimize size of the container will be 6 feet high by 4 feet wide by 6 feet deep. 
         [0078]    The load bearing capacity of container  1  will depend on the dimensions of the container, but by way of example only, a container with the dimensions 8 feet high by 8 feet wide by 6 feet deep, will have a load bearing capacity of up to about 3000 lbs. 
         [0079]    The weight of an unloaded erect or collapsed container of the current invention will depend on the dimensions of the container, but by way of example only, the weight of a container 8 feet high by 8 feet wide by 6 feet deep, will be less than 2500 lbs, preferably from about 500 to 1500 lbs. 
         [0080]    The container  1  can be moved using standard hand pushed pallet movers or hand jacks. A forklift, crane or other especially heavy equipment is not a requirement for moving the container  1 , although they may also be used. 
         [0081]    To collapse the container the side wall panels are removed, followed by inward folding of each end wall along horizontal axes  35 , to draw the roof  5  closer to the base  10 . A partially collapsed container is shown in  FIG. 13 . The fully collapsed state of container  1  is shown in  FIG. 14 . Optionally, the side wall panels can be placed in the recessed area between the upwardly extending end retaining walls  100  on the top surface  80  of the base  10  before the container is collapsed. This allows the side wall panels to be stored within the collapsed container. 
         [0082]    To erect the container, the above steps are reversed: the roof  5  is lifted away from the base  10 , causing the end walls  15  to fold outward until they are substantially vertical. Next, the side wall panels are fed through a section in each upper groove  75 , which is dimensioned to loosely engage a side wall panel, in order to engage the tongue and groove means. Optionally, only side wall panels of the type having an L-shaped cross section (i.e. side wall panels  23 ) are added to the container in order to leave openings for access to the interior of an erected container. Optionally, side wall panels having a generally flat shape (i.e. side wall panels  21  and  22 ) are also added to partially close or fully close the container. 
         [0083]    The roof  5  can be lifted manually, with a forklift, a jack or other suitable means. For example, a forklift can be used to erect the end walls, followed by the addition of at least one side wall panel to maintain the container in an erected position. The method used by a person skilled in the art to erect the container will depend on the dimensions and weight of the container. 
         [0084]    It will recognized by persons skilled in the art, that the above description represents only specific embodiments, and that various modifications can be made without diverging from the scope of the invention described.
Disclosed is a collapsible, portable container having a roof, a base, two foldable end walls and two removable side walls. The end walls and side walls of the container are load bearing elements having a twinned wall structure made of rotomolded plastic. The container is collapsed by removing the side walls and inwardly folding the end walls to draw the roof closer to the base. The load bearing capacity of the end walls and the side walls can be increased by arched indentations formed within the twinned wall structure.