Abstract:
A method of thermoforming shells so that their cut lips face inward toward the shells permits the production of a number of novel thermoformed products having superior nesting and de-nesting qualities and having the ability to fit together to produce a variety of closed shell forms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to thermoforming and thermoformed containers and in particular to a thermoformed container having an inwardly extending cut lip.  
           [0002]    Thermoforming is a process of heating a thermoplastic sheet to working temperature and then forming it to a finished shape by means of heat or pressure. In a typical thermoforming system set up for continuous forming, the sheet is heated in an oven then moved to a forming press. There the softened plastic material is forced against the mold surface by vacuum or air pressure until it sets. During the forming process, a sheet clamp may hold the sheet flat at the edges of the mold. A single mold may be used or matched male and female molds may be used to assist the forming process.  
           [0003]    When the material has set, it may be moved to a trim press where a steel rule or matched punch and die cut the part from the sheet. The use of separate dies can cause some misregistration between the molded part and the cutting die, and accordingly a cut in-place configuration may be used where the sheet is formed and trimmed in the same station.  
           [0004]    Thermoforming can produce extremely thin-walled parts with low tooling costs. Nevertheless, there are a number of limitations to the thermoforming process. First, the cut lip of the flange may give the products an unfinished look. To the extent that the cut lip reveals the thinness of the material, packaging using thermoforming techniques may look inexpensive.  
           [0005]    Even with cut in-place systems, it is difficult to control the dimension and alignment of the cut lip, limiting the application of thermoforming in products where a tight tolerance must be held on this outer dimension.  
           [0006]    The light weight and low cost of thermoformed products make it critical that the products be nested for economical shipping. Nevertheless, the implicit symmetry to a thermoformed sheet causes nested products to stack tightly, making de-nesting a problem.  
           [0007]    The cut lip of the thermoformed product interferes with attempts to create a closed container with tight sealing.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a novel thermoforming process that permits the construction of thermoformed containers where the cut lip extends inward rather than outward as is typical. As a result, a wide variety of new thermoformed products and products having a radically different outward appearance may be produced. By combining the inwardly facing cut lip with an outwardly extending rim, closed stronger containers with substantially smooth outlines may be easily produced.  
           [0009]    Specifically, the present invention provides a method of producing thermoformed shapes with inwardly extending flanges involving the positioning of a heated sheet of thermoplastic material over a cavity having an upper lip and having an undercut portion beneath the lip. The heated sheet of thermoplastic is then drawn down over the upper lip and into the cavity and into the undercut portion. Next, a cutting die is passed into the upper lip to separate the thermoplastic material at the upper lip and the cavity is separated at the undercut portion to extract the formed thermoplastic material.  
           [0010]    Thus, it is one object of the invention to provide a method of manufacturing thermoformed containers that does not require the product to have an outwardly extending lip.  
           [0011]    The process produces a novel container having a base surrounded by integrally formed side walls extending upward to a cut lip, the walls and base formed of a single thermoplastic sheet and defining a volume, and the cut lip extending inward about the volume.  
           [0012]    It is one object of the invention to provide a new thermoformed product without the normal outwardly extending cut lip. By facing the cut lip inward, the outer dimensions of the shape are accurately controlled by the cavity size, permitting designs with much greater precision in outside dimension. The cut lip is also made less visible, providing a more substantial, three-dimensional visual appearance to the manufactured product.  
           [0013]    The upwardly extending walls may include a ridge below the cut lip, the ridge protruding away from the volume. The ridge may be sized to support the container on a lower surface of the ridge against a cut lip of a second identical container with the bases of the two containers spaced apart when the containers are nested together.  
           [0014]    Thus it is another object of the invention to provide a thermoformed shape that may be nested in a naturally spaced apart configuration amenable to de-nesting. The ridge and inwardly facing cut lip allows an arbitrary separation between the thermoformed parts when nested, a spacing otherwise difficult to achieve in thermoforming.  
           [0015]    The ridge may have at least one wall obtusely angled with respect to an adjoining portion of the side wall so that a gap is formed between the ridges of the two containers when they are nested.  
           [0016]    It is yet another object of the invention to provide an ample gap between containers for automatically or manually de-nesting the containers.  
           [0017]    The combination of inwardly extending cut lip and outwardly extending ridge permits a closed thermoformed container to be produced in which a second container, having a ridge sized to receive and fit over the ridge of the first container, may be thereby attached to the first container.  
           [0018]    Thus it is another object of the invention to provide a closed container produced of thermoformed parts having positive attachment between the components and good sealing. The interfitting of the two ridges provides an attachment between two thermoformed shells that is substantially seamless to the eye.  
           [0019]    Conversely, one of the shells may have a vertically extending flange to permit easy separation of the two shells when one shell is used as a lid or the like. One of the shells may incorporate a terrace beneath the first ridge in the side wall, displacing the ridge inwardly by substantially the thickness of the thermoplastic sheet. The terrace may be positioned near the cut lip of the engaging second shell when the two shells are placed together, thereby causing that cut lip to lie substantially flush with the side wall and to resist separation of the shells by a catching of the cut lip.  
           [0020]    Thus another object of the invention is to provide a tamper resistant seal between two thermoformed shells fastened into a closed container.  
           [0021]    The foregoing and many other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration an preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a perspective view of a thermoformed container produced according to the present invention, having an inwardly extending cut lip;  
         [0023]    [0023]FIG. 2 is a cross-sectional view of the container of FIG. 1 taken along line  2 - 2  of FIG. 1 showing the inwardly extending cut lip and an outwardly extending ridge positioned beneath the cut lip;  
         [0024]    [0024]FIG. 3 is a fragmentary cross-sectional view of a thermoforming machine for producing the container of FIGS. 1 and 2, showing the drawing of a sheet of thermoplastic material into a cavity having an undercut portion;  
         [0025]    [0025]FIG. 4 is a second view of the machine of FIG. 3 showing a die cutting of the cut lip of the container of FIGS. 1 and 2 as produced in the thermoforming machine, with a retraction of portions of the cavity to release the part therefrom;  
         [0026]    [0026]FIG. 5 is a fragmentary cross-section of the container of FIG. 2 when nested with other similar containers showing the action of the ridge and cut lip in holding the containers with their bases in separation;  
         [0027]    [0027]FIG. 6 is a fragmentary view of FIG. 5 showing the effect of the shape of the ridge in providing a gap between containers suitable for a de-nesting pawl or the like;  
         [0028]    [0028]FIG. 7 is a cross-sectional fragmentary view of the upper edge of a container produced by the present invention attached to a second container also according to the present invention to connect together at interengaging ridges into a single unit;  
         [0029]    [0029]FIG. 8 is a figure similar to that of FIG. 7 showing the use of a hemicircular rather than rectangular interengaging ridges;  
         [0030]    [0030]FIG. 9 is a figure similar to that of FIGS. 7 and 8 showing the use of interengaging ridges with two different shapes to provide dual lines of sealing between the first and second containers;  
         [0031]    [0031]FIG. 10 is a figure similar to FIGS. 7 through 9 where the second container has an vertically rather than inwardly extending flange to assist in separating the containers;  
         [0032]    [0032]FIG. 11 is a figure similar to FIGS. 7 through 10 having a recessed terrace for receiving the inwardly extending cut lip of the second container so as to provide a seamless outer profile that is tamper resistant;  
         [0033]    [0033]FIG. 12 is a figure similar to FIGS. 7 through 11 showing placement of a flexible release tab between the two containers that may be pulled upward to release the containers from one another;  
         [0034]    [0034]FIG. 13 is a figure similar to FIGS. 7 through 12 showing the use of a rotatable cam element for separating the interengaging ridges of the two containers;  
         [0035]    [0035]FIG. 14 is a figure similar to FIGS. 7 through 12 showing the position of a cut-out near the ridge of the second container and an embossment near the ridge of the first container, extending through the cut-out when the containers are assembled together, permitting opposed finger pressure to release the containers from one another;  
         [0036]    [0036]FIG. 15 is a figure similar to FIGS. 7 through 13 showing a three-part container having an insert for supporting a product against an upper clear container base;  
         [0037]    [0037]FIG. 16 is a front elevational view of an example container using the present techniques;  
         [0038]    [0038]FIG. 17 is a cross-section through the container of FIG. 16 along line  17 - 17  showing the interfitting of two halves and an internal plastic liner;  
         [0039]    [0039]FIG. 18 is a front perspective view of a cubical container such as may be produced by the present technique;  
         [0040]    big.  19  is a cross-sectional view through the container of FIG. 18 alone lines  19 - 19  showing the interfitting of the container halves;  
         [0041]    [0041]FIG. 20 is a front perspective view of a three-part container employing the techniques of the present invention;  
         [0042]    [0042]FIG. 21 is a cross-sectional view of the container of FIG. 20 along lines  21 - 21  showing the three elements of the container. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]    Referring now to FIGS. 1 and 2, a thermoformed container  10  includes a generally planar base  12  attached at its periphery to upstanding side walls  14  which terminate at an upper cut lip  16 . The base  12  and walls  14  enclose a volume  18  with the cut lip  16  generally extending inward about the volume  18 . In a conventional thermoformed product, the cut lip  16  would extend outward away from volume  18  so as to be clear from the walls  14  for a die cutting operation. Positioned beneath the cut lip  16  is a ridge  20  extending about the circumference of the upper edge of the container  10  and protruding outward away from the volume  18 .  
         [0044]    Referring now to FIG. 3, the container  10  may be formed on a thermoforming machine  22  having a cavity  24  generally conforming in shape to the outer surface of the container  10 . The cavity  24  is positioned to provide a recess under a forming table  26 , the latter which provides a planar upper surface across which a sheet of thermoplastic material  28  may slide.  
         [0045]    The heated sheet  28  may be drawn down into the cavity  24  as shown by dotted outlines  28 ′ by means of a relative vacuum provided beneath the sheet  28 ′ in the cavity  24  through one or more orifices or nested by another portion of the machine (not shown).  
         [0046]    In the drawing operation, the sheet  28 ′ is formed over an upper lip  30  defining the edge of the cavity  24 . The cavity  24  is formed from three mold components: the lip  30  (being part of the table  26 ) which also provides a first-half of an undercut portion  32 , a second mold portion  34 , which abuts the underside of the lip  30  and provides a second half of the undercut portion and a surface shaping the side walls  14 , and a third mold portion  36  which defines the shape of the base  12  and may move independently of the second mold portion  34  as will be described. The undercut portion  32  forms the ridge  20  in the finished container.  
         [0047]    The interface between the lip  30  and the second mold portion  34  is aligned with the extreme outermost extent of the ridge  20  to permit the container  10  (to be formed in the cavity  24 ) to be removed from the cavity  24  by a separation of lip  30  and the second mold portion  34 . During the forming process, a die  40  is positioned above the cavity  24  with a vertical cutting edge  42  aligned with the lip  30 .  
         [0048]    Referring now to FIG. 4, after the sheet  28  has been drawn down to conform with the inner surfaces of the cavity  24 , including being drawn into the undercut portion  32 , it is allowed to cool below its glass transition temperatured thus ensuring that it will retain its shape once pressure has been released. At this time, the mold portion  34  and  36  move downward together as indicated by arrow  43  while die  40  cuts through sheet  28  at lip  30  releasing the container  10  from sheet  28 .  
         [0049]    Once the sheet  28  has been cut through by the shearing action between lip  30  and cutting edge  42 , die  40  stops moving whereas mold portions  34  and  36  continue moving downward until the cut lip  16  is clear from the structure of the lip  30 . At this time, mold portion  34  continues downward motion, but mold portion  36  is stopped by ejector peg  46  pushing upward on mold portion  36  with respect to mold portion  34  ejecting the container  10  from the mold cavity  24 . The container  10  may then be moved laterally into a collection bin (not shown).  
         [0050]    Die  40  then moves upward and sheet  28  again moves out over the cavity  24  which is reassembled by the upward motion of mold portions  34  and  36 . The splitting of the portions  34  and  30  at the outermost extent of the ridge  20  allows removal of the container  10  from the cavity  24  without interference with the mold components.  
         [0051]    Referring again to FIG. 2, the dimension  50  from the center of the container  10  to its outermost extent, representative of an outside dimension of the container lo, will be accurately controlled by the dimensions of the cavity  24 . The accuracy of dimension  52 , representing the distance between the center of the container  10  and the cut lip  16 , need not control of the closeness of fit between the container and the cover. Accordingly, designs that require a precise outside dimension and accurately located outside rim will be possible with the present invention. Furthermore, because the cut lip  16  is displaced from the outside dimension, the upper edge of the container  10  presents a smooth profile suitable for many containers as will be described. The cut lip  16  may not be smooth.  
         [0052]    Referring now to FIG. 5, one aspect of the present invention is that it permits the design of containers  10  that may be readily de-nested after they have been nested together. Generally, thermoformed containers are nested for efficient shipping. Because thermoformed containers are manufactured of a single, essentially constant thickness sheet it is normally very hard to keep the nested containers from fitting into one another so tightly as to prevent their ready de-nesting. In particular, the bases  12  and side walls  14  may abut so closely as to prevent air flow between these surfaces, thus creating a vacuum when the containers are to be de-nested. Further, the cut lip  16  of the container (normally facing outward) may abut making it difficult to selectively grasp a single container for de-nesting.  
         [0053]    As shown in FIG. 5, the present container  10  may be nested into a second identical container  10 ′ which may in turn be nested into yet a third identical container  10 ″. In this case, rather than the bases  12 ,  12 ′ and  12 ″ abutting, the depth of nesting of the containers is controlled by the interference between the ridges  20  and the cut lips  16  of the next lower container. By proper dimensioning of the ridges  20  and the cut lip  16 , the amount of spacing between the containers  10 ,  10 ′ and  10 ″ may be precisely controlled to prevent two dense of nesting and to space bases  12  apart.  
         [0054]    Further, referring to FIG. 20, the ridges  20  may be shaped so that a generous gap  56  exists between adjacent containers (for example,  10 ′ and  10 ″) to permit a de-nesting pawl  58  to easily fit between containers  10  and  10 ′ in the gap  56 , for example, to provide compatibility with an automatic de-nesting device. Because dimensions  50  and  52  (shown in FIG. 2) and the shape of the ridges  20  are essentially independent, great flexibility in the amount of nesting may be obtained. Critical to this feature of the invention is that the ridge  20  have at least one wall obtusely angled with respect to an adjoining portion of the side wall  14  so as to be able to rest on the inwardly extending cut lip  16 .  
         [0055]    Referring now to FIG. 7, a first container  10 ( a ) may provide a rectangular ridge  20  as viewed in cross-section, having a top and bottom radially extending wall generally parallel to the base  12  flanking a vertically extending wall generally perpendicular to the base  12 . The cut lip  16  in this case is at the inner edge of the upper radially extending wall.  
         [0056]    A second container  10 ( b ) may be fabricated having a ridge  60  formed as a vertically extending wall  61  extending downward from a base  12 ( b ), the base  12 ( b ) and wall  61  abutting the upper radial wall and vertically extending wall of the ridge  20 , respectively.  
         [0057]    The lower extent of the vertical wall  61  may include an inwardly extending radial wall  62  terminating in a cut lip  16 ( b ) and abutting the lower radial wall of the ridge  20 .  
         [0058]    A slight deformation of the thermoplastic material of the containers  10 ( a ) and  10 ( b ) permits this second container  10 ( b ) to serve as a lid to container  10 ( a ), ridge  60  snapping in place over ridge  20 . The ability to precisely control the outer dimension of the ridge  20  and ridge  60  permits the interfitting of these containers  10 ( a ) and  10 ( b ) as shown. Note that the cut lips  16 ( a ) and  16 ( b ) need not be precisely located for sealing to occur between containers  10 ( a ) and  10 ( b ).  
         [0059]    A cohesive or adhesive may be placed between ridge  20  and ridge  60 ; however, mechanical force will normally hold the containers  10 ( a ) arid  10 ( b ) together.  
         [0060]    Referring to FIG. 8, containers  10 ( a ) may instead employ ridge  20  and ridge  60  that are hemicircular in cross-section to provide a smoother contour of the ridges to assist in the snapping of the containers  10 ( a ) and  10 ( b ) together. Further, the radius of ridge  20  may be slightly greater than that inner radius of ridge  60  so as to eliminate play in a vertical direction. Similarly, the radius of the container  10 ( b ) analogous to dimension  50  in FIG. 2 may be somewhat lest than that radius on container  10 ( a ) to provide a tight seal, the difference in radius being accommodated by a slight unrolling of the ridge  60 .  
         [0061]    Referring to FIG. 9, conversely the ridge  20  of container  10 ( a ) may be given a slightly smaller cross-sectional radius than the ridge  60  of container  10 ( b ), and ridge  60  may be given an angular cross-section so as to promote contact between ridge  20  and ridge  60  at two rings of contact  63  at which a cohesive or adhesive may be placed to provide a double-sealing of the containers.  
         [0062]    Referring to FIG. 10, the cut lip  16 ( b ) of container  10 ( b ) may extend vertically so as to provide a gap  64  between container  10 ( a ) and  10 ( b ), providing a purchase for a user&#39;s finger such as may assist in the separation of containers  10 ( a ) and  10 ( b ) once they are connected.  
         [0063]    On the other hand, separation of the containers  10 ( a ) and  10 ( b ) may be intentionally made more difficult, for example, for the production of a tamper-proof container, by forming in the side wall  14 ( a ) of container  10 ( a ) a depressed terrace  66  displaced toward the center of container  10 ( a ) by a distance substantially equal to the thickness of the sheet from which container  10 ( b ) is formed. The terrace  66  is positioned near the cut lip  16 ( b ) when the containers  10 ( a ) and  10 ( b ) are assembled, so that the terrace  66  causes the cut lip  16 ( b ) to lie substantially flush with the side wall  14   a  to resist a catching of cut lip  16 ( b ) to separate of the two containers.  
         [0064]    Referring now to FIG. 12, a flexible tab  69  such as a cloth or plastic strip, may be attached to one of the containers  10 ( a ) and  10 ( b ) to extend outward between the ridge  20  and ridge  60  to be grasped by a user of the containers  10 ( a ) and  10 ( b ) and pulled to assist in separating these containers.  
         [0065]    Separation of the containers may also be promoted by means of a cam disk  68  having an operator  70  protruding through an aperture  73  in the base  12  ( b ) of container  10  ( b ). The operator is accessible to a user and when rotated also rotates the cam disk  68  which is positioned on the inside of the closed containers  10 ( a ) and  10 ( b ), with a portion extending between ridge  20  and ridge  60 . As the cam disk  68  is rotated, its thickness between ridge  20  and  60  increases, separating the two containers  10 ( a ) and  10 ( b ) without a pulling on the containers.  
         [0066]    Referring now to FIG. 14, the upper radial wall of ridge  20  may include an upwardly extending embossment  72  providing a button which may protrude through an aperture  74  cut in the base  12 ( b ) of container  10 ( b ). The embossment  72  so formed may be pressed downward as indicated by arrow  75  by finger pressure while upward force indicated by arrow  76  may be exerted against the cut lip  16 ( b ) of container  10 ( b ), also by finger pressure. The result is that both containers may be pushed apart with a simple one-handed action, without the need for a firm grip, for example, on the lower container  10 ( a ) as would be required for pulling container  10 ( a ).  
         [0067]    Referring now to FIG. 15, a three-part container may be constructed having a lower and upper container  10 ( a ) and  10 ( b ), respectively, locked together by means of ridges  60  and  20 . A third container  10 ( c ) is held between them and provides a ridge  78  just fitting into the inside of ridge  20 . The container  10 ( c ) has a side wall  14 ( c ) which continues upward from ridge  78  across the cut lip  16 ( a )of container  10 ( a ) and along the side wall  14 ( b ) of container  10 ( b ) to have its base  12 ( c ) run parallel to base  12 ( b ) of container  10 ( b ). The base  12 ( c ) may include one or more pockets  80  to contain product for display through a transparent container  10 ( b ). The pockets  80  may conform to the product to arrange it for attractive display through the container  10 ( b ).  
         [0068]    Referring now to FIGS. 16 and 17, the containers  10 ( a ) and  10 ( b ) need not function as base and lid, but may form two halves of a general container such as a dispensing is container  82  shown in FIG. 16. Such a container may have a front and rear face comprised of containers  10 ( a ) and  10 ( b ). A separately formed nozzle  84 , which may be injection molded, can be attached to a hole formed in the containers  10 ( a ) and  10 ( b ), respectively.  
         [0069]    Here, the containers  10 ( a ) and  10 ( b ) provide both a product storage volume  86  and a handle  88  and permit great flexibility in surface design and package dimension. Referring to the cross-section of FIG. 17, the halves  10 ( a ) and  10 ( b ) are assembled using the tamper-resistant joint generally described with respect to FIG. 11, where the cut lips  16 ( b ) of container  10 ( b ) rest in terraces created in container  10 ( a ).  
         [0070]    Product  89  contained within the container  82  may be held loosely in the container  82  or may be contained in a flexible plastic bag  90  as may be preferred for very fine powdered or liquid product  89 .  
         [0071]    Referring now to FIGS. 18 and 19, containers  10 ( a ) and  10 ( b ) may together form a rectangular parallelepiped container  92  without external flanges. In this case, the division between containers  10 ( a ) and  10 ( b ) divides the parallelepiped along the diagonals of two opposed faces and follows edges of the other two pairs of faces.  
         [0072]    Referring to the cross-sectional view of FIG. 19, container  10 ( a ) has a base  12 ( a ) that is simply the corner between two of the faces of the container  92  and the side walls  14 ( a ) proceed upward at approximately 90° angles from each other. The ridges  20  and  60  fit together with the seam of FIG. 11 to provide corners between two of the faces.  
         [0073]    In the embodiment shown, the container  92  may be adapted for use as a dispenser with a depression  94  formed in two of the faces of container  10 ( b ) as may be covered by an “L” cap  96  fitting around the corner of the two faces and attached by an adhesive sticker at one side to provide a hinge  98 . An aperture may be cut in the depression  94  underneath the “L” hinge  98  to provide access to products stored inside the volumes contained by halves  10 ( a ) and  10 ( b ).  
         [0074]    Referring now to FIGS. 20 and 21, a second three-part container may be formed where the third component provides not a product-supporting insert, but a cap holding the other  10  two components together. Specifically, shells  10 ( a ) and  10 ( b ) may be fit together as has been previously described. A ridge  102  providing a depression extending inward toward the volume contained by containers  10 ( a ) and  10 ( b ) may be formed in containers  10 ( a ) and  10 ( b ) so as to produce a channel running around the container  100  crossing both containers  10 ( a ) and  10 ( b ). A third container  10   c  in this case has inwardly extending cut lips  16 ( c ) fitting into the channel formed by ridges  102  to cover an end of the combined containers  10 ( a ) and  10 ( b ), thereby serving to hold them together, the ridge  102  and cut lip  16 ( c ) serving to hold the container  15 ( c ) in place.  
         [0075]    Referring now to FIGS. 22 and 23, the ability to produce a cut lip  16  facing inward toward the volume of the thermoformed container permits a dead hinge construction (i.e., a hinge without a flexure of the thermoplastic material) to be used. Here, a ridge  20  of container  10 ( a ) is formed into a relatively large cylindrical form having hemicircular cross-section and conforming to an expanded ridge  60  on container  10 ( b ). The cylindrical form may be bisected by a circular disk  104 , integrally molded with ridge  20  and shown in dotted lines in FIG. 22. Ridge  20  including disk  104  and ridge  60  extend about the axis of their cylinders for greater than 180° to provide a hinging action where they retain their connection as the remaining portions of containers  10 ( a ) and  10 ( b ) are separated in rotation about the cylinder axis. The remaining opposite side of containers  10 ( a ) and  10 ( b ) form the hinge  104  of container  103  and may include the release mechanism described with respect to FIG. 14.  
         [0076]    Referring now to FIGS. 24 and 25, a container  106  may be formed from a box shaped container  10  having a rectangular ridge  20  as was described with respect to FIG. 7 where the inwardly extending cut lip  16 ( a ) extends only along three sides of the upper edge of the container  10 . A paperboard or card stock backer plate  108  may be inserted beneath the radially inward extending cut lip  16 , thereby being retained as a cover to container  10 . The cardboard backer plate  108  may have downwardly extending ears  110  attached at opposite edges that will slide beneath cut lip  16 . The ears may be folded upward as indicated by arrows  112  against the backer plate  108  so that the backer plate  108  may slide over the container  10  and beneath the radial upper edge leading to the cut lip  16  and then fold downward into the recess formed by ridge  20 , thus preventing the backer plate  108  from sliding out again without significant effort.  
         [0077]    The above description has been that of a preferred embodiment of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.