Patent Publication Number: US-8123885-B2

Title: Method of manufacturing flexible covers

Description:
CROSS-REFERENCE TO RELATED CASES 
     This application claims priority of my copending provisional patent application, Ser. No. 61/153,182, filed Feb. 17, 2009, and is related to my applications, Ser. No. 11/840,019, filed Aug. 16, 2007 on a more versatile cover and method of manufacture, Ser. No. 12/363,528, filed Jan. 30, 2009 on a method of orienting a cover, and Ser. No. 12/369,629 now abandoned, filed Feb. 11, 2009 on a method and dispenser for flexible covers. 
    
    
     BACKGROUND 
     1. Field 
     The field hereof is container covers. More particularly, the present disclosure relates to an improved method of making a flexible container cover with an elastic encircle opening. 
     2. Prior Art 
     There are number of different types of container or plate covering systems of which I am aware. 
     One type is a fixed-size container lid which is sized and manufactured to fit a specific container. For example plastic food storage containers such as those sold under the trademark Tupperware generally have matching plastic lids. This approach requires that a specific matching lid be available to cover the container. The lids cannot be used on a different size or shape container. 
     The second type of product offering and system for covering plates and bowls as well as containers is flexible plastic film and aluminum foil generally provided in a continuous sheet on a roll, such as rolls of plastic wrap sold under the trademark Saran or foil sold under the trademark Reynolds. In order to cover a plate or bowl, the user simply cuts a length of the film or foil and manually molds the cut film or foil over the container. Although this system is flexible enough to cover containers of various sizes and shapes, it is often difficult to cut the film to the correct length and apply it to a container without the film sticking to itself or its edges lifting from the container. Additionally it is difficult to re-cover a container with the same piece of film since plastic films lose their ability to stick to the container after prolonged use, and aluminum foils tear and become wrinkled. Also, foil is not transparent and thus does not allow users to see what is stored in the container. And because it is metallic it cannot be used in some microwave ovens. 
     The third type of bowl, plate, and container cover is a flexible film cover with an elastic band sewn or stitched around the opening. This type of product has been commercially available and for many years. The same or a similar product is also sold as a shower cap. SC Johnson Co. has sold this type of container cover under their trademark Quick Covers. This type of cover is made from a circular sheet of plastic film, also known as a blank, with a strip of elastic sewn around the perimeter. The elastic, being shorter than the circumference of the blank, gathers the edge of the blank to the center and creates a cover similar in appearance to a shower cap. 
     There are three drawbacks to this design. The first is that it is unsanitary and unsightly for use with food items due to the fact that the elastic band is stitched to the plastic so that the thread and the ends of the elastic are not contained within, and often hang from the cover. The elastic is also prone to separating at the stitches after a number of uses. The second drawback is that these covers cannot be mass produced on high speed machinery because the elastic must be sewn around the perimeter of the sheet. The third drawback is that these covers do not fit both elliptical and rectangular containers with similar size openings, nor do they fit elongated rectangular shaped container, and therefore have relatively low versatility. 
     U.S. Pat. Nos. 2,490,451 to Magid (1949) and 3,035,960 to Farkas et al. (1962) disclose covers with an elastic band heat sealed into a hem along the periphery of a circular sheet of plastic film. Although these covers eliminate the unsanitary and undesirable aesthetics of the above stitched type of cover, they do not fit both elliptical and square containers with similar size openings, nor do they fit elongated rectangular containers. Furthermore, the method of manufacturing the covers of both patents requires multiple forming head to form the sheet and the band. Also it is difficult to remove the finished covers from the sealing apparatus, and thus they cannot be made on high speed machinery. Finally, these covers completely enclose the elastic band in a sealed edge which is not functional in a microwave oven because the air trapped within the hermetically sealed hem will expand when heated in a microwave, which can cause the hem to burst, thereby destroying the cover. 
     In my above copending application Ser. No. 11/840,019 I disclose an improved more versatile cover and method of manufacture. This cover is made from a rectangular sheet of plastic film material, or blank, and an elastic band. The sheet has a predetermined amount of material cut from each corner. The sheet is placed onto a table between four posts that extend from the table. The sheet is positioned so that a post lines up with each corner of the sheet. A rubber band is stretched across the four posts into a similar shaped rectangle and rests just above the sheet. The stretched rubber band now has four sides which correspond to the four sides of the sheet. The side edges of the sheet are folded inward and over the stretched band and heat sealed directly to the sheet, sealing the band within four pockets that are formed along the sides of the sheet. Each corner of the stretched band is released from its post, allowing the band to relax and pull the four corners of the sheet toward the center to form the cover. 
     While this cover and its method of manufacture is a substantial improvement over prior covers, there are three areas that could be improved further. The first area in need of improvement is that the forming method requires a cut in each corner to allow room for the elastic band holding post, which leaves an opening in the hem at each corner of the cover exposing a portion of the band. While this eliminates the problem discussed above of a bursting hem when used in a microwave, and does not cause a functional problem because the location of the cut or hole is below the rim of the container being covered, the exposed band has the potential to look like a manufacturing defect to some customers. The second area in need of improvement is the process step of stretching and placing the rubber band around the holding posts. This step requires a high degree of precision to position the stretched rubber band directly above the sheet so that the sides of the sheet can be folded over the band in order to be heat sealed. This step slows the manufacturing process and limits the process capability for high speed automation. There is also a limitation in my above referenced applications in that the cover formed by the folding over an elastic band method requires that the starting blank have substantially straight sides. While a rectangular blank provides maximum flexibility in being able to cover round, rectangular, square, oval, or any other shaped container, there could be a situation where a customer would want a cover made from a round blank. 
     ADVANTAGES 
     Accordingly, some advantages of one or more aspects are to provide an improved method of manufacturing a flexible cover and article thereof that: a) can be round or rectangular in shape and thereby cover both round and rectangular shaped containers, b) eliminates a cut or hole in the hem at each corner which exposes a portion of the elastic band, c) allows air to escape from the hem to prevent the hem from bursting when used in a microwave oven, d) utilizes a forming head which simultaneously forms the sheet and applies the elastic band, e) can be automatically striped from the sealing apparatus, f) can be made on high speed automated machinery, and g) can be integrated into an automated assembly line process. Other advantages of various aspects will be apparent from a consideration of the following description and the accompanying drawings. 
     SUMMARY 
     In accordance with one aspect, an apparatus and method for making a flexible container cover comprises a forming machine with a lower section having: a) a square shaped forming mandrel having a top, four substantially vertical sides, and a heating element extending longitudinally along each side, b) four elongated sealing bar mechanisms each having a top side with a sheet guide and a front side with an elongated strip of rubber attached thereto, and c) a cover ejector plate with plural upwardly extending ejector pins. Each sealing bar mechanism is positioned opposite a respective side of the forming mandrel separated by predetermined distance to create a forming groove, recess or moat around the forming mandrel. Each adjacent sealing bar is separated by a small gap. The cover ejector mechanism is positioned below the forming mandrel so that the ejector pins align with clearance holes cut through the forming mandrel. An upper section having a forming head assembly comprises a) a forming die with sides that define a cavity sized to telescope over the forming mandrel, and b) a band ejector plate with plural downwardly extending ejector pins located along the perimeter. The forming die hangs below the band ejector plate by the use of shafts which extend from the forming die through clearance hole in the ejector plate and are held in place by shaft collars at the top of each shaft. The ejector pins extend downwardly from the ejector plate and are centered within gaps cut into the sides of the forming die. 
     With the forming head assembly in an up position a square sheet of plastic film is positioned on top of the forming mandrel. Each side of the sheet extends past the sides of the mandrel and rest on top of each sealing bar within the sheet guides. An elastic band is stretched and placed around the sides of the forming die. The forming head assembly is actuated downward so that the forming die telescopes over the forming mandrel forcing a marginal portion of the sheet downward into the moat in a V-shaped cross sectional configuration. Next the forming die hits a mechanical stop while the band ejector mechanism continues to move downward causing the ejector pins to push the elastic band off of the forming die and snap securely around the sides of the forming mandrel, thereby holding the sheet in place. Next the forming head assembly is returned to the up position and the sealing bars are actuated inward to push the sides of the sheet against the mandrel to form a hem enclosing the elastic band. As the sealing bars are actuated the heating element is switched on which melts and seals the plastic to form a seam which encloses the elastic band within a continuous hem along the periphery of the sheet. An unsealed pleat is formed within the hem at the location of the gap between adjacent sealing bars. Next the side sealing bars are returned to the outward position and the cover ejector mechanism is raised such that the ejector pins move through the clearance holes in the forming mandrel and push the underside of the plastic sheet upward which automatically strips the formed cover from the forming mandrel. Finally, the ejector plate is retracted back down through the mandrel which strips the cover from the ejector pins. Once released from the ejector pins, the elastic band relaxes to its natural free length, which is less than the length of the perimeter or circumference of the blank. Thus the elastic band draws or gathers the sides and corners inward towards the center, forming the sheet into a hollow cover with an expandable opening. The elastic band is sealed within a continuous hem around the periphery of the sheet having an unsealed pleat which prevents air from being trapped within the hem area when the cover is heated in a microwave. 
     In another aspect a blast of compressed air or mechanical means is used to fill the gaps between adjacent sealing bars to eliminate an unsealed pleat and provide a continuous and hermetically sealed hem containing an elastic band. 
     In another aspect the forming mandrel, forming die and associated parts are configured in an elongated rectangular shape in order to form a cover from an elongated rectangular blank designed to cover round, square or elongated rectangular containers. In yet another aspect the forming mandrel, forming die and related parts are configured in an elliptical shape to form a cover from a round blank. 
     In another aspect a flexible container cover comprises a rectangular sheet of film having an elastic band sealed within a hem around the periphery of the sheet in which pleats are sealed at each corner, but there is an absence of sealed pleats along each side. 
     In another aspect a flexible container cover comprises a sheet of film having an elastic band sealed within a hem around the periphery of the sheet with the hem having an un-pleated, unsealed area to prevent air from being trapped within the hem in order to prevent the hem from bursting when used in a microwave. 
     In yet another aspect a rectangular sheet of film having a straight chamfer cut at each corner is formed into a flexible container cover having curved corners. 
     In another aspect a fully automated process is provided comprising die cutting, elastic band feeding, cover forming and ejecting, and finish product accumulation. In timed sequence a die cutter station is provided which cuts the sheet or blank from a roll of film material. Next the blank is automatically placed onto the forming mandrel of the forming machine. Next an elastic band feeding apparatus stretches and applies an elastic band around the forming die of the forming machine. Next the forming machine forms and ejects a finished cover. Lastly, an accumulator picks up the ejected cover and stacks the cover in an accumulation bin for final packaging. 
     These and other aspects and features of the disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       DRAWING FIGURES 
         FIG. 1A  is a plan view of a sheet blank used to form a flexible container cover. 
         FIG. 1B  is a plan view of an elastic band used to form the cover. 
         FIG. 1C  is a side view of the cover. 
         FIG. 2  is an exploded perspective view of an apparatus for forming covers. 
         FIG. 2A  is a side view of an apparatus for forming covers. 
         FIG. 2B  is a top view of the apparatus taken along line  2 B- 2 B of  FIG. 2A . 
         FIG. 2C  is a bottom view of the apparatus taken along line  2 C- 2 C of  FIG. 2A . 
         FIG. 3A  is a partial side view of a forming die in the down position forcing a sheet for forming the cover into a forming groove. 
         FIG. 3B  is a partial side view of the band ejector in the down position immediately after pushing the elastic band off of the forming die with the band holding the side of the sheet against the side of the forming mandrel. 
         FIG. 4A  is a partial side view of a sealing bar sealing the edge of the sheet to form a hem which encapsulates the elastic band. 
         FIG. 4B  is a top view of sealing bars sealing the edge of the sheet along each side. 
         FIG. 5  is a partial side view of a cover eject mechanism in an up position immediately after stripping the finished cover from the sides of the forming mandrel. 
         FIG. 6  is a partial side view of a finished and loose cover immediately after the cover eject arms are lowered back through the forming mandrel. 
         FIG. 7  is a partial plan view of formed cover in a stretched configuration showing the hem, pleating, and sealing pattern. 
         FIG. 8  is a plan view of apparatus parts used to make a cover from a circular blank. 
         FIG. 9A  is a plan view of an alternative embodiment blank used to make a flexible container cover. 
         FIG. 9B  is a perspective view of the alternative embodiment cover made from the blank shown in  FIG. 9A  covering a rectangular shaped dish. 
         FIG. 10  is a plan view of apparatus parts used to make a hermetically sealed seam. 
         FIG. 11  is a plan view illustrating an automated process for manufacturing flexible container covers. 
     
    
    
     REFERENCE NUMERALS 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 100 
                 Frame 
                 110 
                 cover ejector plate 
               
               
                 110A 
                 assembly line ejector plate 
                 112 
                 cover ejector pin 
               
               
                 114 
                 cover ejector air cylinder 
                 120 
                 square forming mandrel 
               
               
                 120′ 
                 circular forming mandrel 
                 120A 
                 assembly line forming 
               
               
                   
                   
                   
                 mandrel 
               
               
                 120S 
                 mandrel side 
                 120T 
                 mandrel top 
               
               
                 122 
                 cover ejector clearance hole 
                 124 
                 forming moat 
               
               
                 130 
                 sealing bar 
                 130′ 
                 curved sealing bar 
               
               
                 130A 
                 assembly line sealing bar 
                 132 
                 rubber strip 
               
               
                 134 
                 sealing bar air cylinder 
                 136 
                 sealing bar clearance 
               
               
                 136′ 
                 sealing gap 
                 138 
                 blank guide 
               
               
                 140 
                 band ejector plate 
                 140B 
                 band ejector bearing 
               
               
                 142 
                 band ejector air cylinder 
                 144 
                 band ejector pin 
               
               
                 150 
                 forming die 
                 150S 
                 forming die side 
               
               
                 152 
                 clearance slot 
                 153 
                 die cavity 
               
               
                 154 
                 shaft 
                 156 
                 shaft collar 
               
               
                 158 
                 compression spring 
                 160 
                 elastic band 
               
               
                 170 
                 square film sheet 
                 170′ 
                 circular film sheet 
               
               
                 170″ 
                 rectangular film sheet 
                 170CC 
                 chamfered sheet corner 
               
               
                 170HC 
                 curved hemmed corner 
                 170F 
                 folded edge 
               
               
                 170S 
                 sheet side 
                 170U 
                 unsupported portion 
               
               
                 172 
                 sealed pleat 
                 173 
                 unsealed pleat 
               
               
                 174 
                 seam 
                 175 
                 unsealed section 
               
               
                 176 
                 hollow body 
                 177 
                 Opening 
               
               
                 178 
                 hem 
                 180 
                 heating element 
               
               
                 182 
                 heating element gap 
                 190 
                 Cover 
               
               
                 190″ 
                 cover 
                 300 
                 forming head assembly 
               
               
                 300A 
                 assembly line forming head 
                 310 
                 compressed air nozzle 
               
               
                 400 
                 rectangular container 
                 500 
                 turn table 
               
               
                 510 
                 station one 
                 512 
                 station two 
               
               
                 514 
                 station three 
                 516 
                 station four 
               
               
                 520 
                 die cutting apparatus 
                 521 
                 roll of film 
               
               
                 522 
                 band feeding apparatus 
                 524 
                 accumulator apparatus 
               
               
                 526 
                 accumulator bin 
                 600 
                 logo 
               
               
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION 
     First Embodiment—FIGS.  1 - 2 C 
       FIGS. 2 and 2A  are prospective and side views respectively of the apparatus for forming flexible container covers  190  ( FIG. 1C ). The forming apparatus comprises an upper section and a lower section secured to a common frame. The lower section, which is also shown in the top view in  FIG. 2B , comprises a square forming mandrel  120  having a top side  120 T and side walls  120 S. In this first embodiment the side walls are vertical or at a 90° angle from the top side for reasons to be explained. The overall length, width, and shape of mandrel top  120 T is predetermined by the size and shape of the container cover to be produced. An impulse heat sealing unit is constructed by attaching a heating element  180  longitudinally along sidewall  120 S around the perimeter of the mandrel. 
     The heating element is made from a strip of nichrome wire which is commonly used as a heating element in toasters and hair dryers. Other suitable heating element material well known in the art can also be used. The nichrome wire is insulated from mandrel  120  by use of a strip of heat-resistant insulating tape located between the wire and the mandrel (not shown). The top of the wire is also covered by a strip of heat-resistant insulating tape (not shown). A single strip of wire is wrapped around the entire mandrel ending at its starting point with a small gap  182  between the two ends. The free ends of the wire are connected to a low voltage power source (not shown) which can be turned on and off. When a predetermined amount of electrical current is passed through the wire, electrical resistance causes the wire to get very hot, very quickly. There are a number of different sealing technology well known in the art which could be used as an alternative for impulse heat sealing, such as ultrasonic vibration energy (ultrasonic sealing), high-frequency die electric sealing (induction heating), or a the use of a heated sealing clamp or heated bar. 
     Mandrel  120  is supported on frame  100  in a horizontal configuration as shown in  FIG. 2A . Adjacent each side of the mandrel is an elongated sealing bar  130  with a long strip of rubber  132  attached to the front edge which faces mandrel  120 . At the top of sealing bar  130  is a blank guide or stop  138 . Sealing bar  130  is attached to an air cylinder  134  (shown in  FIGS. 2A and 2B  in a retracted position) which is attached to frame  100 . A moat or groove  124  is formed between mandrel sides  120 S and sealing bar rubber strips  132 . As shown in  FIGS. 2 and 2B , moat  124  and sealing bars  130  surround the sides of mandrel  120  except for clearance gaps  136  located at each corner between each adjacent sealing bars for reasons to be explained later. A cover ejector plate  110  is positioned below mandrel  120  and is secured to air cylinder  114  (shown in  FIG. 2A  in a retracted position) which is secured to frame  100 . Ejector plate  110  has four ejector pins  112  which extend vertically and are aligned within four clearance holes  122  which run through mandrel  120  as shown in  FIGS. 2A and 2B . 
     The upper section of the apparatus ( FIGS. 2 ,  2 A and  2 C) comprises forming head assembly  300  which is made up of square forming die  150  which has four sides  150 S which define a die cavity  153  of a size and shape designed to telescope over forming mandrel  120  with a predetermined clearance between the sides of the mandrel and the sides of the die. In this first embodiment the clearance is in the range of 1.27 mm (0.050 inch) to 6.35 mm (0.250 inch) but other clearance distances can be used. Forming die  150  is suspended from a band ejector plate  140  which has eight downwardly extending ejector pins  144  along its periphery. The ejector pins are aligned with eight clearance slots  152  cut into the sides of forming die  150 . Forming die is supported from ejector plate  140  by two linear motion shafts  154  which extend from the forming die through two bearings  140 B in the ejector plate and are held in place by a shaft collar  156  at the top of each shaft. The shafts and bearings allow forming die  150  to move up and down relative to band ejector plate  140 . A compression spring  158  is installed on each shaft between forming die  150  and band ejector plate  140  to provide stability between the two parts. Band ejector plate  140  is attached to air cylinder  142  (shown in  FIG. 2A  in a retracted position) which is attached to frame  100 . Forming head assembly  300  is suspended above and aligned with forming mandrel  120 . 
     Operation 
     First Embodiment—FIGS.  1 - 7   
     In accordance with this first embodiment, a flexible container cover  190  ( FIG. 1C ) is made by placing film sheet or blank  170  ( FIG. 1A ) on forming mandrel top  120 T ( FIGS. 2 ,  2 A and  2 B). Blank  170  can be any flexible film material such as polyethylene, polypropylene, polyester, nylon that is suitable for use in covering bowls, plates, and other objects, or a combination of these materials. Alternatively a biodegradable film can be used, such as polylactic acid (PLA), or polyvinyl alcohol (PVA), which contains starch in order to biodegrade. The size of blank  170  is predetermined to fit a specific size range of container openings. The thickness of blank  170  can be any thickness of film available. Part or all of blank  170  can have optical properties that are clear, transparent, translucent, and opaque. For this first embodiment a square sheet of low-density polyethylene (LDPE) with a chamfer cut at each corner  170 CC and a thickness of 0.0254 mm (0.001 inch) is used. The sides  170 S of the blank extend past the sides  120 S of the mandrel and rest on top of sealing bars  130  within blank guides  138  as shown in  FIGS. 2A and 2B . There is an unsupported portion  170 U of the blank between the sides of the forming mandrel and the tops of the sealing bars. 
     Next an elastic band  160  ( FIG. 1B ) is stretched and placed around forming die sides  150 S as shown in  FIGS. 2 ,  2 A and  2 C. The elastic band is preferably made from any thin elastic material such as latex (natural rubber), or latex-free material, such as polyisoprene, polyurethane, or combination of these and other materials in any color available. The elastic element has an unstretched, relaxed or free length that is less than the length of the perimeter or periphery of the plastic sheet for reasons to be discussed. 
     As shown in  FIG. 3A , forming head assembly  300  is lowered (by activating air cylinder  142   FIG. 2A ) so that forming die  150  telescopes around forming mandrel sides  120 S and stops at a predetermined level. This causes forming die sides  150 S to push the unsupported portion  170 U of the blank into groove or moat  124  that surrounds the mandrel. The plastic sheet extends from the top of the mandrel, downward into the moat area, around the side of the forming mandrel, and back up to the top of the sealing bar in a V-shaped cross-sectional pattern. 
     Next, as shown in  FIG. 3B , with the forming die  150  stationary, band eject plate  140  continues to move downward, compressing springs  158  and causing the band ejector pins  144  to roll or push elastic band  160  off of the forming die. The elastic band then snaps against the mandrel sidewalls  120 S, holding blank  170  firmly in place against the sides of the mandrel. The plastic sheet flows from the top of the mandrel, down the sidewall under and around the elastic band, and then back up to the top of the sealing bar in a generally U-shaped cross-sectional pattern. 
     As shown in  FIGS. 4A and 4B , forming head assembly  300  is returned to its original retracted position by deactivating air cylinder  142   FIG. 2A . Next sealing bars  130  are moved forward by activating air cylinders  134 , pushing the sides of blank  170  inward to form a folded edge  170 F adjacent elastic band  160 . This creates a hem  178  encapsulating the elastic band. Sealing bar clearance  136  ( FIGS. 2 and 2B ) is reduced in size as the bars move forward toward the mandrel, forming sealing gap  136 ′ ( FIG. 4B ). Simultaneous with the extension of sealing bar  130 , electricity is applied to heating element  180 , causing it to become hot to melt and fuse an overlapping portion of the blank to form a seam  174  around the periphery. 
     Turning now to  FIG. 5 , the sealing bars are returned to their original retracted positions. Next cover ejector plate  110  is raised by activating air cylinder  114  so that cover ejector pins  112  extend through clearance holes  122  which are bored through mandrel  120 . The tops of the eject pins contact the underside of sheet  170 , thereby lifting it upward and automatically pushing or stripping it from the mandrel. As discussed above, mandrel side walls  120 S are vertical or at a 90° angle from the top of mandrel  120 T. This allows the cover to be pushed off easily. The mandrel side wall can also have a slight taper with an angle in the range between 70° to 110° from the mandrel top, provided that the taper is gentle enough to allow the cover to be pushed off by the eject pins. 
       FIG. 6  shows the final step of the process in which ejector pins  112  are retracted. This causes cover  190  to be stripped from the eject pins as they return back through clearance holes  122 .  FIG. 6  shows cover  190  after the elastic band has been allowed to relax to its natural free length, which is less than the length of the perimeter or circumference of blank  170 . Thus the elastic band has drawn or gathered the sides and corners inward towards the center, forming the sheet into cover  190  comprising a hollow body  176  with an expandable opening  177 . 
       FIG. 7  illustrates a portion of cover  190  in a fully stretched and open configuration and shows elastic band  160  covered by a continuous hem  178  defined by folded edge  170 F and seam  174  formed by the apparatus and method. Multiple pleats  172  are arranged along each curved hemmed corner  170 HC and are traversed or crossed and sealed by seam  174 . There is an absence of pleats when the cover is stretched along each straight side. The pleats result because the perimeter of the sheet at each corner is larger than the perimeter of moat at each corner, so the sheet folds, creases, or pleats along this area and the pleats are traversed or crossed and sealed by the seam, with one exception. An unsealed pleat  173  is located at a point which corresponds to sealing gap  136 ′ ( FIG. 4B ) between adjacent sealing bars. This unsealed pleat is formed when sealing bars  130  push the sides of the sheet inward ( FIG. 4B ), which causes a small amount of the sheet to fold or crease into gap  136 ′ between adjacent sealing bars, which prevent it from being sealed. This unsealed pleat in finished cover  190  prevents air from being trapped within the hemmed edge, which prevents the hem from exploding when the cover is used in a microwave oven. 
     Other means of leaving an unsealed area in the hem to allow air to escape can be used. For example gap  182  ( FIG. 2A ) formed between the adjacent ends of the wrapped heating element wire can be adjusted to a predetermined size in order to leave a small unsealed section or gap  175  in the seam. Alternatively, a small section of rubber strip  132  can be removed (not shown) which would eliminate sealing pressure at that point and leave an unsealed gap in the seam at a location corresponding to the gap in the rubber strip. The unsealed gap can be formed on a curved or straight section of the cover and need not have a pleat associated with the unsealed gap. Alternatively, one or more holes can be punched into the blank at a location which forms the hem to prevent air from being trapped in the hem.  FIG. 7  shows hem  178  forming a curved corner  170 HC. Corner  170 HC is formed from chamfered corner  170 CC on blank  170  by the pleating effect discussed above. A rectangular cover formed with round corners provides a tighter fit on certain rectangular and oval casserole type dishes, and is an unexpected benefit of the pleating phenomenon. 
     Alternative Embodiments—FIGS.  8  to  10   
       FIG. 8  illustrates an alternate embodiment for making a circular container cover. In order to form a circular cover from the apparatus, a circular blank  170 ′ would be used in combination with a circular forming mandrel  120 ′ and curved sealing bars  130 ′. Similar modifications to the forming die and band eject plate (not shown) would be made. 
       FIG. 9A  illustrates an alternative elongated rectangular blank  170 ″ that can be used to make the elongated rectangular shaped container cover  190 ″ of  FIG. 9B  shown covering a rectangular shaped container  400 . In this example blank  170 ″ has chamfered corners  170 CC and a logo printed across the surface. Once formed chamfered corners  170 CC form hemmed curved corners  170 HC which provide a tight fit on container  400 . Those experienced in the art can make the required size modifications to the apparatus parts to form cover  170 ″ into cover  190 ″. 
       FIG. 10  illustrates a modification which would eliminate unsealed pleat  173  ( FIG. 7 ). In order to prevent a portion of the sheet from creasing or folding into the gap  136 ′ between adjacent sealing bars, a compressed air nozzle  310  can be attached which in timed sequence with the sealing bar activation can shoot a blast of compressed air at the corner of the sheet. This will cause a pleat to fold inward between the closing sealing bar and the forming mandrel, thereby preventing a fold of material from getting caught between adjacent sealing bars. Instead of compressed air, a mechanical finger can also be used in timed sequence to tuck the corner inward and prevent a fold of material from getting caught in gap  136 ′ between adjacent sealing bars. This modification would produce a container cover which has a hermetically sealed hem which some customers may want for sterile applications or to prevent water from migrating into the hem when the cover is washed. 
       FIG. 11  shows an automated assembly line for manufacturing flexible containers covers. The automated assembly line integrates the apparatus parts and process functions disclosed in the first embodiment along with additional processing steps and apparatus well known in the art. The automated assembly line comprises a machine having a rotating circular turn table  500  with four forming mandrels  120 A (similar to forming mandrel  120 ), secured to the turntable and positioned 90° apart. Each forming mandrel is surrounded by sealing bars  130 A (similar to sealing bars  130 ). The mandrels and sealing bars move together on the rotating turntable. There are four fixed positions or stations that correspond to the positions of each of the four mandrels. 
     A first station  510  comprises a die-cutting apparatus  520 . The die cutting apparatus comprises an unwind stand, web guides, and a punching unit all well known in the art. 
     A second station  512  comprises a forming head apparatus  300 A and band feeding apparatus  522 . Forming head  300 A (similar to forming head  300 ) comprises a forming die and band ejector plate. Band feeding apparatus  522  comprises a conveyor of matching sets of hooks or pins which automatically stretch and release rubber bands around the forming die. The rubber bands can initially be put on the hooks manually, or by an automated descrambling apparatus that would descramble individual rubber bands from a jumbled mass and place them on the hooks. Alternatively elastic bands can be formed directly around the forming die from two separate ribbons of elastic as disclosed in U.S. Pat. No. 5,749,989 to Linmann et al., which is incorporated by reference. Alternatively the band feeding can be performed by a worker positioned at this station with a supply of rubber bands; the worker manually stretches and applies the rubber band around the forming die. 
     At a third station  514  no fabrication action is taken. This station therefore provides a dwell or pause to allow the seam to be sealed. 
     A fourth station  516  comprises an eject plate  110 A, (similar to eject plate  110 ), and an accumulating apparatus  524  which is designed to pick up and stack the finished covers after they have been stripped from the forming mandrel. The accumulating apparatus can comprise a reciprocating overhead arm equipped with a vacuum cup for picking up an ejected cover and moving it to an accumulation bin  526 . 
     Starting at first station  510  a roll of film material  521  is loaded on the unwind stand and fed through web guides (not shown) to die cutter  520 . Next the die cutter punches a blank (similar to blank  170 ) and places the blank on top of forming mandrel  120 A. The blank is placed with a mechanical positioner or, alternatively with a blast of compressed air. A series of vacuum holes in the top of the mandrel hold the blank in place when the turntable moves the mandrel to the next station. 
     Next the turntable rotates 90° counter-clockwise (CCW) as shown by the arrows to second station  512  and places the first mandrel with the blank on top under forming head  300 A. Band feeding apparatus  522  (or a worker) next applies a rubber band to the forming die. The forming die is lowered to form the sheet and apply the rubber band as described earlier. The forming head moves up and sealing bars  130 A are actuated along with a heating element (not shown) to form and seal a hem containing the elastic band. 
     Next the turntable indexes another 90° CCW to third station  514  station where sealing bars  130 A continue to be pressed against the sides of the mandrel for a predetermined interval to seal the hem. 
     Finally the turntable moves to the fourth station  516  where sealing bars  130 A are deactivated. Eject plate  110 A is the activated to strip the cover from the mandrel as described for the first embodiment. Accumulator apparatus  524  next moves into position above the loose cover, and with the use of a vacuum picks up the cover and transfers it to accumulation bin  526 . The process is repeated sequentially as each mandrel intermittently indexes to each respective station. If the dwell at each station is 0.75 second and it takes 0.25 second to move a mandrel between stations, the total time to complete one cover is 4 seconds, so the throughput would be one cover per second or 60 finished covers per minute. 
     CONCLUSIONS, RAMIFICATIONS, SCOPE 
     The reader will see that according to the disclosure, I have provided a flexible container cover and fabrication method that can fit containers, plates, and bowls in a variety of shapes and sizes, made from a sheet of film and having a continuous hem covering an elastic band about the periphery of the sheet. The hem can have an unsealed section or unsealed pleat which prevents air from being trapped within the hem so that it does not burst when used in a microwave oven. Alternatively the hem can be hermetically sealed. The process and apparatus can form the flexible container covers from round, elliptical, rectangular, or square blanks. It utilizes a forming head which simultaneously forms the sheet and applies the elastic band and automatically strips the formed cover from the sealing apparatus. The process and apparatus can be integrated into an automated assembly line for the high speed production of flexible container covers. 
     While certain representative embodiments and details have been shown for purposes of illustrating the disclosure, it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed may be made without departing from the scope of the disclosure. For example the starting sheet can have square corners, radius corners, or chamfered corners. Rectangular sheets similar to the sheets shown in my pending application Ser. No. 11/840,019, filed Aug. 16, 2007 and incorporated by reference can be used. These sheets have a section of material removed from each corner. Such sheets or blanks can be formed with the method and apparatus detailed in the current disclosure, producing a cover with an interrupted hem and exposed band at each corner. 
     In lieu of rubber bands, any other elastic band may be used, such as rubber covered by a sleeve, rubber covered by wound thread, elastic plastic, a coiled metal spring (in a sleeve or without a sleeve), etc. 
     The primary graphical alignment indicator means disclosed in my pending application Ser. No. 12/363,528, filed Jan. 30, 2009 and incorporated by reference can be incorporated into the present blanks to allow the user to quickly and easily identify which direction to pull, place, or orient the cover in order to keep the sides parallel with the sides of a rectangular container. 
     The size and shape of the apparatus tooling parts, such as the forming mandrel, forming die, band ejector mechanism, cover ejector mechanism, and sealing bars can all be modified. For example instead of using eight band-ejector pins, any other number of pins can be provided to push the band off of the forming die. In lieu of pins, other pushing or ejecting means can be used. Similarly, instead of using four cover ejector pins, any other number of pins can be provided to push the formed cover off of the forming mandrel. In lieu of pins, other pushing, pulling, or ejecting means can be used, such as compressed air or vacuum. 
     A separate strip of wire or heating element can be secured to each side of the forming mandrel instead of using one long continuous wire. The hem can be sealed by other means of melting and fusing the overlapping plastic. A different color elastic band  160  can be used for different size covers to help differentiate the sizes. For example, a cover sized to fit containers 10.2 cm (4 inches) to 22.9 cm (9 inches) can use a yellow rubber band, while a cover sized to fit containers 22.9 cm (9 inches) to 35.6 cm (14 inches) can use a green rubber band and so forth. Instead of four mandrel positions on the rotating turntable, there could be 8 mandrel positions (45 degrees apart) for the four stations which would reduce the travel time and increase the throughput of the machine. 
     The stations of the automated assembly line process can be configured in a linear layout instead of a circular layout. Instead of a reciprocating arm the accumulator apparatus can be a rotating station with plural picking heads for removing the finished covers from the forming mandrel in timed sequence. 
     In lieu of covering food containers, the cover can be used to cover any other type of container, such as laboratory specimens, mechanical or electronic parts, biological materials, office materials, etc. 
     Thus the scope should be determined by the appended claims and their legal equivalents, and not by the examples given.