Patent Publication Number: US-6213301-B1

Title: Plastic container for food products

Description:
CROSS REFERENCE 
     This is a continuation-in-part of U.S. application Ser. No. 08/975,149 filed Nov. 20, 1997. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to plastic containers, and more particularly, to injection-molded containers for containing food products. 
     BACKGROUND OF THE INVENTION 
     One area where the use of plastic containers has become widespread is in the food packaging industry. Accordingly, it is common for these plastic food containers to serve as the end display package in which the product is presented for sale to the customer. Typical of these containers are those used for dairy products such as cottage cheese, sour cream, or the like where an integral body of the container is provided having a sidewall that tapers down to an integral transverse bottom wall with the top opening being closed by a plug fit lid. Normally, the lid has a depending peripheral skirt which locks onto the upper rim of the tapered wall of the container body. One difficulty encountered in the design of closure lids for the containers described above is ensuring that when they are nested in a stack such as for shipping, the stacked lids maintain a constant gap between adjacent skirts of respective stacked lids. Maintaining uniform gap spacing is important to allow them to be efficiently utilized with high-speed automated packaging equipment which position the containers for automatic filling and automatically caps the filled containers with lids taken from the stack by mechanical devices of the equipment. Various rib structures have been employed with lids to provide the requisite spacing, see e.g. U.S. Pat. Nos. 4,826,039 and 5,377,861. 
     Many of these food product plastic containers have their parts formed by a thermoforming process. In thermoforming, a thin plastic sheet is formed into the desired shape by heating and forcing the sheet against a mold to produce a container part having a uniform, very thin, cross-sectional thickness which can result in a part having very flexible walls. In the particular application of interest herein, currently there is a flavored yogurt thermoformed container that has a reverse tapered sidewall with a larger diameter bottom and being open at both the smaller diameter top and at the bottom thereof. A separate bottom closure member is also formed by thermoforming and is spinwelded to the sidewalls to close off the bottom for receiving yogurt therein. The bottom closure includes a base panel and depending skirt wall which is spinwelded to the interior surface of the body wall to permanently attach the pieces together. Thus, unlike the previously described top closure lids which are designed to be opened, the bottom closure for this particular yogurt container does not have a locking skirt which locks onto a rim of the sidewall and which can be opened to gain access to the food therein. Instead, after being filled with yogurt, the top is closed by a flexible foil seal as by an adhesive. To gain access to the yogurt, the seal is peeled open from over the opening at the top of the container sidewall. 
     The thermoformed and spinwelded yogurt containers described above suffer from numerous shortcomings. For spinwelding the bottom closure to the container body wall, both pieces are provided with integral gripping lugs which project relatively far radially inward relative to the body and skirt walls so that they can be grasped by the spinwelding equipment for rotating the two parts relative to each other to create frictional heating for welding the parts together. The bottom closure has the lugs formed on its skirt, and the body sidewall has lugs around the top thereof. The spinwelding technique requires specialized handling and filling equipment that results in a relatively slow production of containers for filling. 
     Since the parts of the above-described yogurt container are thermoformed parts having a constant wall thickness, the radially inward projecting lugs form corresponding indentations on their exterior wall surfaces. Because of the aforementioned display function of the exterior surface of the yogurt container, maximizing the amount of surface area available for printing information, such as product characteristics, e.g. ingredients, nutritional content, or other required information about the product, is an important consideration, especially where the containers are relatively small, such as for example with the preferred 6 oz. (170 g) yogurt containers herein. The lugs at the top of the wall restrict the height of the printing that can be received on the container sidewall. In addition, there are unsightly indentations on the sidewall due to the lugs that are readily visible to the purchaser, and because of the radial extent to which they project into the container interior, they can unduly interfere with removing the food product therefrom, e.g. spooning yogurt out from the container. Accordingly, there is a need for a plastic container for food products such as a yogurt container which is more aesthetically pleasing, and better maximizes the print receiving surface area thereon. 
     Another significant characteristic the containers should possess is the ability to stack with uniform spacing between the container parts and so that while stacked, adjacent parts do not become jammed and wedged together. In addition, the space taken up by a given number of stacked container parts should be minimized. The above described lugs of the prior yogurt container, in addition to their grasping function for spinwelding provide a stacking surface with their bottom flat surfaces. The indentations of the lugs extend relatively far down the sidewalls spaced from the top in both the container body and the bottom closure. This precludes stacking of these container parts in a compact fashion. 
     It is also important that the container be adapted to be used with conventional automated container feeding equipment that is currently employed in container filling assembly and printing operations. Both the upper container wall and the bottom closure lack an annular rim for feeding with mechanical mechanisms or devices such as mechanical fingers, feed screws or shuttles of the automated feeding equipment commonly used with the packaging of dairy products. 
     As previously mentioned, thermoformed plastic containers generally have very thin cross-sectional thickness so that their walls can be very flexible. This is especially true with the bottom closure of the above described yogurt container where the skirt sidewall is relatively thin in thickness and the base panel is quite wide in diameter which can cause the closure member to be very pliable. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an improved, injection-molded plastic container is formed with an upper main body portion ultrasonically welded to a lower body portion. Preferably, each of the body portions has a small annular rim that is abutted and welded together to form the container. The annular rims allow use of conventional, automatic, container feeding apparatus having mechanical devices for insertion between nested rims in a stack of container portions. Thus, an improved, automated process of forming the container and filling the same can be obtained with this new injection-molded container. 
     The upper body portion and lower base portion of the container herein are ultrasonically connected together with there being an energy director area between the upper and lower container portions for facilitating the ultrasonic welding process. In the preferred form, the energy director area is provided between the rims of the container body and base. As the container herein avoids spinwelding the parts together such as in the previously described yogurt container, there is no need to provide lugs for grasping the container parts for relative rotation thus eliminating the large indentations that can be seen on the outer surface of the corresponding body portion of the prior yogurt container. Accordingly, the present container has an exterior surface portion of the frustoconical wall of the container body that is smooth over substantially its entire extent extending substantially between the top and bottom of the frustoconical wall so as to provide a large uninterrupted smooth area on the body wall to receive printing thereon. Thus, the container herein maximizes its surface area for receiving print in contrast to the prior spinwelded container having indentations formed by the gripping lugs over which printing cannot be applied. 
     The base wall of the lower container portion may be provided with a substantially centrally located raised portion that is disposed in the container interior when the upper and lower container portions are ultrasonically attached. The raised portion has inclined surfaces relative to the base wall for distributing loads about the center of the base panel to provide added container strength for meeting the drop test requirements for the container. 
     Because each of the container portions is injection molded, it may have thick cross-sectional areas where needed to add strength to the overall container to meet drop tests. The thermoformed containers cannot add thickened cross-sectional areas because the container is formed from a sheet of uniform cross-sectional thickness. 
     Stacking shoulders disposed adjacent the tops of the upper and lower container portions are preferably provided so as to minimize the distance between the tops of the container portions and their respective stacking shoulders for providing stacking of upper container portions on each other and lower container portions on each other in a compact, vertical arrangement. Accordingly, an increased number of container portions can be stacked in a given space versus the prior spinwelded containers which have stacking surfaces on the bottom of the spinwelding lugs that are further down from the top of the corresponding container portion sidewalls. In contrast to the prior container, the present stacking shoulders are closely adjacent the tops of the container portions to minimize the nest interval between stacked parts for more compact stacking. The present container body and base can have a smaller nest interval over that of corresponding parts of the prior container without causing a problem in separating parts from the stack with the container feeding equipment due to the provision of the rims that are lacking in the prior spinwelded container and which can be readily engaged by the equipment&#39;s mechanical devices for separation. In addition, with respect to the open-ended container body, there is less of a concern with developing a vacuum type seal between closely stacked parts having tapered walls that would make it more difficult to properly separate the stacked container bodies in a stack with small nest intervals. 
     In a preferred form, the stacking shoulders of the lower container portions have inner edges which outwardly taper in a downward direction for camming with the top of the frustoconical sidewall for self-centering the lower container portions in a stacked, vertical arrangement. 
     The lip projecting radially inward from the top of the container sidewall includes an inner annular portion terminating in a free edge and an outer face forming a wedge shape, with the outer face of the inner portion extending upwardly at a small acute angle to the outer face of an outer annular portion. Enhanced sealing is provided between the lip and the thin seal member and eliminating the requirement of gripping on an outwardly projecting roll rim over which the seal member must be crimped to provide sealing attachment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of an upstanding main body portion of a plastic container for containing food products in accordance with the present invention showing a frustoconical wall of the container body having a larger diameter at its bottom and a radially outward projecting annular rim thereat; 
     FIG. 2 is a plan view of the container body of FIG. 1; 
     FIG. 3 is an enlarged sectional view of the rim of the container main body showing the rough surface at the bottom thereof for ultrasonically welding with a corresponding rim of a lower base portion of the container; 
     FIG. 4 is an enlarged sectional view of an inner stacking ring of the container main body and showing a portion of a seal member sealingly attached over the top opening of the container body; 
     FIG. 5 is an elevational view partially in section showing two container body portions stacked one on top of the other in a compact, vertical arrangement; 
     FIG. 6 is a bottom plan view of the container lower base portion having inner stacking ribs formed therearound; 
     FIG. 7 is a cross-sectional view taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is an enlarged sectional view of a raised dome provided on the container base portion; 
     FIG. 9 is an enlarged sectional view of the frustoconical sidewall of the container base portion and showing an annular rim projecting radially outward from the bottom of the wall and including an energy director area of the rim for ultrasonically attaching the rims of the container body and base portions together; 
     FIG. 10 is an enlarged sectional view of one of the stacking ribs of the container base portion; 
     FIG. 11 is an enlarged sectional view taken along line  11 — 11  of FIG. 6; 
     FIG. 12 is a sectional view showing two container base portions stacked one on top of the other in a compact, vertical arrangement; 
     FIG. 13 is a fragmentary, cross-sectional view showing two container bodies stacked together; 
     FIG. 14 is a fragmentary, sectional view showing an alternate form of two container base portions stacked one on top of the other in a compact, vertical arrangement; and 
     FIG. 15 is a fragmentary, enlarged, sectional view showing an alternate form of the radially inward projecting lip of the container main body and showing a portion of a seal member sealing attached over the top opening of the container body. 
     FIG. 16 is a sectional view showing two container bases portions stacked one on top of the other in a canted arrangement. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The container described herein is a two-part container having an upper main body portion  10  (FIG. 1) and a lower base portion  12  (FIG. 6) which is to be attached at the bottom of the main body portion  10  so as to form an open top container for being filled with food products. The preferred application is as a container for yogurt and the exemplary dimensions set forth herein for the container portions  10  and  12  are for a container that is filled with 6 oz. (170 g) of flavored yogurt; however, it will be understood that size of the container portions and the dimensions can be varied from that described herein and still fall within the scope of the present invention. After being filled, the open top is then sealed by a thin seal member  14  which can be adhered to the top of the main body portion  10  for sealing the food product in the interior  16  of the container. 
     One important advantage conferred by the upper and lower portions  10  and  12  of the present container is that they are stackable in a very compact and stable manner so as to conserve space during transport of large volumes of these container portions and to enable them to cooperate with presently available automated container feeding equipment that is currently being used with plastic food containers for processing thereof. Both the container body  10  and base  12  are provided with lower annular flanges or rims  18  and  20 , respectfully, which project radially outward from the bottom of the respective container portions. As can be seen by reference to FIGS. 5 and 12, the respective rims  18  and  20  provide distinct structure on the container body portions  10  and  12  for readily allowing mechanical devices (not shown) of the feeding equipment to fit between adjacent rims in the stack for separating the container body portions in the stack for further processing. In addition, the rims  18  and  20  can include an energy director area therebetween to facilitate ultrasonic bonding of the rims  18  and  20  together to attach the container base  12  to the container body  10 , as will be more fully described hereinafter. 
     A significant advantage in using an ultrasonic bonding between the upper and lower container portions is that of speed of operation and the elimination of specialized spinning equipment to spin one piece relative to the other piece. 
     Another significant advantage of providing the ultrasonic bonded rims  18  and  20  over the spinwelding process used to attach the prior yogurt containers is that the exterior surface  22  of the container body  10  can be smooth over substantially its entire extent providing a large uninterrupted, smooth area for receiving printing thereon. This is in direct contrast to the prior spinwelded container which, as previously described, has spinwelding is lugs formed around the top of the corresponding container body for being grasped by the spinwelding equipment. The lugs create large indentations in the container surface which limit the height of printing on the container exterior surface and thus do not maximize the surface area for printing as with the present ultrasonically welded container. 
     The details of the illustrated and preferred embodiment of the container portions  10  and  12  will next be more specifically described. Referring to FIG. 1, the container body  10  has a sidewall  24  which has a generally frustoconical shape so that it tapers from a smaller diameter top  26  to a larger diameter bottom  28  at which the annular rim  18  is formed and projects radially outward therefrom. As previously mentioned, the bottom  28  is closed off by the container base  12  which is ultrasonically welded thereon leaving a mouth  30  at the sidewall top  26  open for being filled with the food product into the container interior  16 . 
     As best seen in FIG. 4, the container body sidewall  24  includes a short upper thick wall section  32  and a lower thinner wall section  34  therebelow. By way of example and not limitation, the preferred thickness for the thick wall section  32  can be approximately 0.020 inch (0.508 mm) and the preferred thickness of the thinner wall section  34  can be approximately 0.016 inch (0.406 mm). The thickness of the rim  18  at the bottom of the wall section  34  is approximately 0.023 inch (0.584 mm). To achieve the different wall thicknesses for the sections  32  and  34  and rim  18 , the container herein is preferably injection molded. 
     Injection molding the present container portions  10  and  12  is advantageous over the prior thermoformed yogurt container portions in that thermoforming generally involves the stretching of a uniform thickness sheet of plastic that causes thin areas at some locations and a loss of strength at a location where more strength may be desired. In an injection-molded container, increased wall thickness may be added where desired to provide the desired strength. Further, thermoforming does not allow the control of a radius or the forming of sharp corners that can be achieved with injection molding; and these can lead to the telescoping together of containers during shipping or in certain drop tests. For example, as best seen in FIG. 3, the flange  18 , to be sonically welded, is made thicker in cross-section than the cross-section of container sidewall  24 . Also, a corner  19  joining the sidewall  24  to the flange  18  is made substantially thicker than the container wall or the flange to add mass to the corner in order to make it stronger. Thus, more plastic can be added selectively to the weldable flange  18  and to the corner  19  without adding to the mass of the rest of the container, as would be necessary in a thermoformed container. The thermoformed walls for these containers typically range between 0.010 inch (0.254 mm) and 0.016 inch (0.406 mm) thick. Also, because the injection-molded walls need not have a uniform thickness throughout, this allows different portions of the wall to be designed with various thicknesses and shapes such as the above-described container dewall  24  having thick and thinner sections  32  and  34 , thereof. 
     Returning to FIG. 4, it can be seen that the thick wall section  32  stands substantially vertical or at a very slight incline relative to the lower thinner wall section  34  which tapers at more of an angle, e.g. 3.5° from vertical, down to the larger diameter bottom  28  of the sidewall  24 . The preferred distance from the top  26  of the thick wall section  32  of the sidewall  24  down to the bottom  28  of the flange rim  18  is approximately 3.473 inches (8.821 cm). In addition, the thick wall section  32  steps down from the thinner wall section  34  at exterior shoulder  36  so as to have a smaller diameter relative to the progressively increasing top to bottom diameter of the wall section  34  with the distance from the shoulder  36  to the container body top  26  being short relative to the distance from the shoulder  36  down to the rim  18 . The step of the shoulder  36  is approximately 0.015 inch (0.381 mm) so that the exterior surface  32   b  of wall section  32  is substantially axially aligned with interior surface  34   a  of wall section  34  at the top thereof. A radially inward projecting lip  38  is provided at the upper end of the wall section  32  and terminates radially distal from the wall section  32  around the container mouth opening  30 . 
     In injection molding of the container upper body portion  10 , forces occur during the stripping of the body portion from the mold as the core is pulled out that tend to form a crease in the sidewall  32  above the shoulder  36 . To add strength to the sidewall  32 , to resist such creasing, a series of ribs  37  are formed on the outside surface of the container extending upwardly, as viewed in FIG. 4, from the shoulder  36 . The ribs  37  add more mass and strength. The preferred ribs have a 4:1 slope with the ribs being four times as high as wide in the radial direction. 
     For the preferred yogurt container by way of example and not limitation, the exterior diameter of the vertical thick wall section  32  is approximately 2.061 inches (5.235 cm); and the exterior diameter of the tapered thin wall section  34  at its smallest diameter top portion adjacent transition region  33  is approximately 2.094 inches (5.319 cm) with the interior diameter being approximately 2.064 inches (5.243 cm), and at its largest diameter at the bottom  28 , it has an interior diameter of approximately 2.475 inches (6.287 cm). The outer diameter of the radial rim  18  is approximately 2.64 inches (6.706 cm). The annular lip  38  can extend radially from the top  26  of thick wall section  32  for approximately 0.165 inch (4.191 mm) so that the mouth  30  has a diameter of approximately 1.725 inch (4.382 cm). The distance from the shoulder  36  to the top  26  of the thick wall section  32  is approximately 0.310 inch (7.874 mm), and the distance down from the shoulder  36  to the top of the rim  18  is approximately 3.140 inches (7.976 cm) so that the total height of the container body  10  in the preferred 6 oz. (170 g) yogurt container embodiment is approximately 3.473 inches (8.821 cm), as previously mentioned. As is apparent, because of the location of the shoulder  36  at the transition region  33  high up along the sidewall  24  so that there is a relatively short upper thick wall section  32 , there remains a much longer distance for the smooth wall section  34  so as to provide the food container herein with a large, uninterrupted surface area on its exterior sidewall surface  22  for receiving printing thereon. It has been found that in the preferred 6 oz. (170 g) yogurt container, the wall section  34  provides for approximately 2.87 inches (7.29 cm) of printing height which is about 10% greater than that afforded with the prior thermoformed and spinwelded container. The median printable circumference around the wall section  34  is 7.17 inches (18.21 cm), and less a {fraction (3/16)} inch (4.76 mm) vertical gap, the printable circumference is 6.99 inches (17.48 cm). 
     The foil seal member  14  is adhered over the container top  26  so as to seal the mouth  30  and extends on the upper surface  38   a  of the lip  38  around the substantially right angle corner  40  formed between the lip  38  and wall section  32  and down towards the shoulder  36  on the exterior surface of the wall section  32  stopping at end  14   a  thereof. As the foil seal member  14  stops short of the exterior shoulder  36 , a gripping space  41  is provided around the bottom of the wall section  32  between the seal member bottom end  14   a  and the exterior shoulder  36 . Thus, to open the container by removal of the foil seal member  14 , a person can insert their finger into the gripping space  41  for engaging the seal member end  14   a  with their fingers and peeling it from the exterior of the upper wall section  32  and lip  38  to remove the seal member  14  from across the container mouth  30  for accessing the food product e.g. yogurt, in the container interior  16 . 
     For stacking of container bodies  10 , the container sidewall  24  has an interior stacking shoulder or ring  42  formed integrally thereon and projecting radially into the container interior  16  substantially radially aligned with the exterior shoulder  36  so that the stacking ring  42  is provided at the transition region  33  of the sidewall  24  relatively close to the top  26  of the container body  10  between the container wall sections  32  and  34 . As best seen in FIGS. 4 and 13, the bottom surface  42   a  of the stacking ring  42  is spaced below the top surface of the shoulder  36  preferably by approximately 0.020 inch (0.508 mm). The container bodies  10  are stacked with the top lip  38  of the lower container body  10   b  abutting the flat bottom surface  42   a  of the stacking ring  42  of the upper container body  10   a  as shown in FIG.  13 . The distance of the nest interval D 1  (FIG. 5) between rims  18  of adjacent container bodies  10   a  and  10   b  will be substantially equal to the short distance between the top of the lip  38  and the ring bottom surface  42   a  which in the preferred form with the dimensions of the container body  10  as set forth earlier where the container is used as a yogurt container with the container interior filled with 6 oz. (170 g) of yogurt, gives a nest interval D 1  of 0.330 inch (8.382 mm) that is substantially less than the interval provided with the prior spinwelded yogurt container utilizing the lugs as the stacking structure. With the small nest interval D 1  of 0.330 inch (8.382 mm), there will be a small air gap of approximately 0.004 inch (0.102 mm) between adjacent wall sections  34  of container bodies  10   a  and  10   b  in the stack. Thus, the present container bodies  10  can be stacked in a much more compact, vertical arrangement providing for substantial savings in transportation costs in that a much greater number of container bodies  10  can be stacked in a prescribed space. This allows, for instance, a greater number of stacked container bodies  10  to be put into a carton box for shipping. 
     As mentioned, the container body  10  and base  12  are preferably both integral injection-molded pieces. Injection molding allows these plastic parts to be formed with more intricate shapes and walls and allows more control over the shape and cross-sectional thicknesses of the plastic so as to enhance the performance and durability of these plastic parts such as when subject to drop testing. In this regard, the transition region  33  between the thick wall section  32  and the thin wall section  34  has a fairly intricate shape with sharp angled corners on the exterior flat shoulder  36  and the inner stacking ring  42 . For example, as can be seen in FIG. 4, the stacking ring surface  42   a  intersects with an inclined surface  44  at sharply angled corner  46  radially distal from the slightly radiused right angle juncture of the surface  42   a  with the inner surface  34   a  of the wall section  34 . 
     In the preferred 6 oz. (170 g) yogurt container, the corner  46  is radially spaced from the inner wall surface  34   a  a short distance of 0.034 inch (0.8636 mm) so as not to unduly interfere with scooping of food from the container interior  16  while still providing for secure and stable stacking, as more fully discussed herein. The angled inclined surface  44  extends upwardly and radially outward from the corner  46  to the interior surface  32   a  of the wall section  32 . As this transitional region  33  is thicker from between the corners of the shoulder  36  and ring  42  relative to wall sections  32  and  34 , it also adds strength to the container wall  24 . The intricate shape of the wall including the transition area  33  between the wall sections and the sharp corners and flat surfaces thereof are readily produced by injection molding whereas thermoforming a container having these types of sharply angled surface features would be much more difficult, if not impossible. 
     Additionally, the container body  10  is molded with an inner circumferential bead  48  which is used to strip the container body from the mold. The bead  48  is raised from the wall section inner surface  32   a  and is integral therewith, and likewise the bead  50  is raised from the wall section inner surface  34   a  and is integral therewith. Preferably the bead  48  is spaced down from the bottom of the lip  38  to its radial inwardmost point by a distance 0.100 inch (2.54 mm) with the radial spacing of this inwardmost point from the wall section interior surface  32   a  being approximately 0.005 inch (0.127 mm). 
     The stacking ring  42  extends continuously around the inner circumference at the transition area  33  of the sidewall  24  so as to present an unbroken flat surface  42   a  to be engaged with the flat top surface of the top lip  38  for stacking in a secure and stable manner. The continuous stacking ring surface  42   a  is molded flat to be abutted with a corresponding flat surface on the top of the lip  38  of an adjacent stacked container. Such an arrangement in the stack presents less of a risk of slippage of the engaged surfaces off from each other, especially when considered in conjunction with the thicker, stronger reinforced wall  24  of the container body  10 . This is particularly important when the container bodies  10  are loaded in the stack as wedging and jamming of the stacked container portions can prevent separation by the mechanical devices of automatic feeding equipment with which the container portions  10  and  12  are to be utilized. Thus, the stacking structure including the ring surface  42  and the lip  38  of the present container body  10  is effective to maintain a constant and small uniform gap or nest interval D 1  between stacked container bodies  10  while keeping them stacked in a secure and stable manner. 
     Similar to the container body  10 , the container base  12  is an injection-molded part so that it can be formed with a more intricate shape including having integral walls with different thicknesses. Referring to FIGS. 7 and 9, the container base includes an upper panel wall  52  and a frustoconical sidewall  54  which depends from the periphery of the panel wall  52  at the top  56  of the wall down to its bottom  58  at which annular rim  20  is formed. Similar to rim  18 , the rim  20  projects radially outward from the wall bottom  58 . The panel wall  52  is preferably slightly thicker in cross-sectional thickness than both the frustoconical sidewall  54  and the annular rim  20 , with annular rim  20 , in turn, being slightly thicker than the frustoconical sidewall  54 . 
     For example, in the preferred form as a yogurt container, the container base  12  has a base panel  52  with a thickness of 0.018 inch (0.457 mm), a frustoconical sidewall  54  with a thickness of 0.016 inch (0.406 mm) and an annular rim  20  with a thickness of 0.016 inch (0.406 mm). The sidewall  54  tapers at about 7.5° from the vertical and has a vertical height from top  56  to the bottom of the rim  20  of approximately 0.500 inch (1.27 cm). The diameter across the panel wall  52  is preferably approximately 2.334 inches (5.928 cm) with the diameter across the outer edges of the rim  20  being larger, preferably on the order of 2.640 inch (6.706 cm). As discussed earlier, the base panel wall  52  has a relatively large diameter in comparison to the height of the sidewall  54  so that the panel  52  is more easily deflected when subject to loads due to the large span across the top  56  of the sidewall  54 . By injection molding the panel wall  52  so that it is thicker than the sidewall  54 , the panel wall  52  is provided with enhanced rigidity so as to be better able to withstand drop tests. To assist in withstanding drop tests and preventing any cracking of the walls  52  and  54  of the base member  12 , a raised dome  60  is integrally formed at the center of the panel wall  52 , as best seen in FIG.  8 . The dome  60  has an arcuate inclined surface  62  relative to the flat panel wall  52  so that any loads on the center of the panel  52  will be substantially distributed thereabout so as to avoid stressing the panel center with direct loads thereon. The dome  60  adds strength and improved load bearing capacity such as when filled containers are subjected to drop tests. Instead of flexing the center of the panel wall  52 , the loading created by the food product in the dropped containers will be distributed over the panel. 
     The dome  60  is preferably provided with a wall having different thicknesses with annular inclined portion  64  being thicker than the panel wall  52  and the top enlarged portion  66  being thicker than the annular portion  64 . So, for example, with the preferred panel thickness of 0.018 inch (0.457 mm), the annular wall portion  64  will have a thickness of approximately 0.022 inch (0.559 mm) and the enlarged top portion  66  will have a thickness of approximately 0.032 inch (0.813 mm) at its thickest point at the center of the base  12 . By providing the central top portion  66  as the thickest portion of the dome  60 , the dome  60  itself is stiffened against flexing at its highest central point over the base panel wall  52  so as to improve its load distributing characteristics. Also, similar to body wall  24  having bead  48 , base sidewall  54  includes an integral raised bead  67  (FIG. 9) formed on the wall surface  54   a  to aid in pulling the container base  12  from the mold. At its radially inwardmost point, the bead  67  is preferably spaced approximately 0.093 inch (2.362 mm) down from the bottom surface  52   a  of panel wall  52  and radially inward approximately 0.005 inch (0.127 mm) from wall surface  54   a.    
     For stacking purposes, the container base wall  52  has integral circumferentially spaced stacking ribs  68  that are adjacent the top  56  of the sidewall  54  so as to minimize the  35  nest interval D 2  when the container bases  12   a  and  12   b  are stacked, as shown in FIG.  12 . The ribs  68  are integrally formed at the junction of the panel wall horizontal surface  52   a  and frustoconical wall inclined surface  54   a  and project down from the horizontal surface  52   a  of the panel wall  52  a short distance so that when the upper horizontal surface  52   b  of the panel wall  52  of the lower base  12   b  engages with the generally flat bottom surface  68   a  of the ribs  68  of the upper base  12   a , the nest interval D 2  between the container bases  12  is minimized. More specifically, the nest interval D 2  between the stacked bases  12  will be approximately equal to the distance from the bottom surface  68   a  of the stacking ribs  68  to the horizontal upper surface  52   b  of the panel wall  52 . With the preferred panel wall thickness of 0.018 inch (0.457 mm) between lower and upper wall surfaces  52   a  and  52   b , the preferred distance between the stacking rib bottom surface  68   a  and the panel wall lower horizontal surface  52   a  is approximately 0.182 inch (4.623 mm) so as to provide a 0.200 inch (5.08 mm) nest interval D 2  between adjacent stacked container bases  12 . The stacking interval D 2  is significantly less than the stacking interval of the corresponding container bases in the prior yogurt container so that the container bases  12  herein can be stacked in a more compact, vertical arrangement providing advantages similar to the container body  10  in terms of a number of container bases  12  that can be shipped in a given amount of space such as in a box carton. Because of the small nest interval D 2 , the stacked bases  12   a  and  12   b  have a small air gap of approximately 0.0058 (0.1473 mm) inch between adjacent base sidewalls  54 . 
     An additional advantageous characteristic of the stacking ribs  68  over the prior lugs is that the ribs  68  project inwardly relative to the base walls  52  and  54  without the corresponding indentation formed in the walls exterior surfaces as with the thermoformed lugs. Accordingly, the ribs  68  provide a support and stiffening function as they gusset the walls  52  and  54  relative to each other so as to enhance the strength at the top juncture  56  between the base panel  52  and the frustoconical sidewall  54 . In this manner, the stacking ribs  68  serve to stiffen the container base walls  52  and  54  so as to improve their performance when subjected to drop tests, as previously described. In the preferred yogurt container form, the ribs  68  project approximately 0.075 inch (1.905 mm) radially inward along the horizontal wall surface  52   a  from the frustoconical wall surface  54   a , and as previously mentioned, project downwardly along the inclined wall surface  54   a  approximately 0.182 inch (4.623 mm) from the panel lower horizontal surface  52   a . The ribs  68  can have sides  68   b  and  68   c  provided with a slight draft, such as 1° from the vertical, to converge towards the rib bottom surface  68   a  with a thickness between sides  68   b  and  68   c  being approximately 0.020 inch (0.508 mm). 
     In an alternate preferred yogurt container form as shown in FIG. 14, the ribs  68  can have inner edges  68   d  which taper outwardly in a downward direction to facilate self-aligning of the stacked container bases  12 . In particular, the spacing of the interconnection of the edge  68   d  with the horizontal surface  52   a  from the wall surface  54   a  is considerably greater than the spacing of the interconnection of the edge  68   d  with the rib bottom surface  68   a  from the wall surface  54   a  and in the most preferred form is generally double the spacing of the interconnection of the edge  68   d  with the rib bottom surface  68   a  from the wall surface  54   a . Specifically, the spacing of the interconnection of the edge  68   d  with the horizontal surface  52   a  from the wall surface  54   a  is generally equal to the distance the ribs  68  project downwardly along the inclined wall surface  54   a , namely approximately 0.182 inch (4.623 mm) whereas the spacing of the interconnection of the edge  68   d  with the rib bottom surface  68   a  from the wall surface  54   a  is approximately 0.075 inch (1.905 mm). In the most preferred form, the edge  68   d  is linearly straight between its interconnection with the surfaces  52   a  and  68   a  (with the interconnections being radiused in the preferred form) and extends at an angle of approximately 550 from rib bottom surface  68   a , with the surfaces  52   a  and  68   a  being generally parallel. 
     When two or more container bases  12  are properly stacked, as illustrated in FIGS. 12 and 14, the container bases  12  are centered with respect to one another, and the requisite spacing between the rims  20  of the container bases  12  is maintained around the entire peripheries of the rims  20 . However, due to vibrations and handling associated with automated packaging equipment, two container bases  12   a  and  12   b  may try to become slightly off-center with respect to one another. In particular, the upper container base  12   a  could cant relative to the lower container base  12   b . As an example referring to FIG. 12, the upper container base  12   a  could be positioned such that on the right side the rims  20  generally abut and the interconnection between the panel wall  52  and the sidewall  54  of the lower container base  12   b  abuts with the inner edge of the rib  68  of the upper container base  12   a , while on the left side, the rims  20  are spaced generally equal to or slightly less than the height of the sidewall  54  of the lower container base  12   b  and the interconnection between the panel wall  52  and the sidewall  54  of the lower container base  12   b  abuts with the inclined surface  54   a  adjacent to the rim  20  of the upper container base  12   a . This canting of the container bases  12   a  and  12   b  may prevent mechanical fingers of automated feeding equipment from moving between the rims  20  of adjacent container bases  12  but also may result in wedging of the container bases  12  together so that realignment may be difficult, requiring individual, manual separation and replacement. 
     The ribs  68  of FIG. 14 are advantageous in preventing the tendency of the container bases  12  from moving off center. Particularly, the edges  68   d  act as camming surfaces to move the container bases  12  to be self-centered and to have equal peripheral spacing between the rims  20 . In particular, with the edges  68   d  being angled from a vertical orientation, there are force components urging the upper container base  12   a  back to a centered position relative to the lower container base  12   b , which would not exist for the ribs  68  of FIGS. 6,  7 ,  11 , and  12  having generally vertical inner edges. In the event that it is noticed that some of the container bases  12  are canted within a stack of container bases  12  as shown in FIG. 16, the operator can run a hand along the edge of the stack which should raise and lower the container bases  12  relative to one another and the container bases  12  should self center. 
     To ultrasonically attach the container base  12  to the body  10 , the respective annular rims  18  and  20  are welded to form welded plastic areas between the rims for connecting the base  12  to the body  10 . More specifically, the rim  18  of the container body  10  has a lower surface  18   a  that is provided with a roughened irregular texture to facilitate ultrasonic bonding to the container base flange  20 . In this regard, the container base rim  20  is provided with an upstanding annular energy director  70  projecting from its upper surface  20   a , preferably approximately 0.015 inch (0.381 mm) high, and having a rounded top triangular cross-sectional shape, as best seen in FIG.  9 . Accordingly, when a single container body  10  and a container base  12  are removed from their respective stacks such as by the mechanical devices of the processing equipment in engagement with the rims  18  and  20  and brought together in the ultrasonic fixture, the annular rims  18  and  20 , and more particularly the textured bottom surface  18   a  and the energy director  70  will be brought into high frequency vibration with one another with the energy concentrated at the rounded top of the energy director  70 . The molded texture surface  18   a  increases the abutting surface area between it and the raised energy director  70  so as to increase the frictional heat generated between the vibrating surfaces improving the melt of the energy director areas and bond quality between the rims  18  and  20 . The preferred energy director adds more mass of plastic to the flange  20  and has a 0.010 inch (0.254 mm) radius on its top with sloping walls  71  (FIG. 9) defining a 60° angle therebetween and a height of 0.015 inch (0.381 mm). Once the container base  12  is ultrasonically welded to the container body  10 , the automatic feeding equipment can take the welded container by the attached rims  18  and  20  to a filling station for being filled with food products and then on to a sealing station where the foil seal member  14  is adhered over the open mouth  30  of the filled container. 
     As previously mentioned, the container herein allows the mechanical devices of automated feeding equipment to be readily implemented with the radially projecting welded rims  18  and  20 . Also, as previously discussed, the ultrasonic attachment of the container portions  10  and  12  eliminates the spinwelding process used to attach the prior yogurt container portions so that the present container, and of particular importance, the main body portion  10  thereof, no longer need include relatively large indented lugs that are provided towards the top of the container. In this manner, substantially the entire extent of the container body exterior surface  22  can be utilized to receive print thereon maximizing the surface area of the container body  10  used for this purpose. The container body  10  and base  12  herein are substantial improvements in terms of their strength for withstanding the drop tests to which they are subjected and for their processing in that they are readily adapted for use with currently employed automated container feeding equipment. Moreover, both the container body  10  and base  12  are provided with stacking structure so that their stack intervals between adjacent stacked pieces is reduced so that more parts can be stacked in a given space, and so that the stacking can occur in a more secure and stable manner. Even with the small nest intervals D 1  and D 2  of the respective container body  10  and base  12  herein, the rims  18  and  20  allow the mechanical devices of the processing equipment to readily separate the parts in a trouble free fashion from the stack. 
     In an alternate preferred yogurt container form as illustrated in FIG. 15, the radially inward projecting lip  38  is shaped to facilitate sealing with the seal member  14 . In particular, the lip  38  includes a first, outer annular portion  74  having parallel, planar, upper and lower faces  74   a  and  74   b  which are generally horizontal and generally perpendicular to wall section  32  and surfaces  32   a  and  32   b . The first portion  74  integrally terminates in a second, intermediate annular portion  76  having parallel, planar, upper and lower faces  76   a  and  76   b . The upper and lower faces  76   a  and  76   b  extend at an obtuse angle of approximately 120° to upper and lower faces  74   a  and  74   b , respectively. The second portion  76  integrally terminates in a third, inner annular portion  78  having parallel, planar, upper and lower faces  78   a  and  78   b . The upper and lower faces  78   a  and  78   b  extend at a small acute angle of approximately  70  to the upper and lower faces  74   a  and  74   b . The third portion  78  terminates in a free edge  78   c  which extends from an apex of the upper face  78   a  towards wall section  32  at an obtuse angle approximately equal to the obtuse angle between upper and lower faces  76   a  and  76   b  to upper and lower faces  74   a  and  74   b  and specifically approximately 120° to the upper and lower faces  74   a  and  74   b , with the face  78   a  and the edge  78   c  forming a wedge shape. In particular, the lip  38  does not include a section extending from the third portion defining a generally vertical surface concentric to the second portion  76  and specifically an inwardly sloping frustoconical surface. The free edge  78   c  is parallel to and radially spaced inward of faces  76   a  and  76   b  in the preferred form. 
     The thicknesses of the first portion  74  between faces  74   a  and  74   b , of the second portion  76  between faces  76   a  and  76   b  and of the third portion  78  between faces  78   a  and  78   b  are equal, are slightly greater than the thickness of wall section  32  between surfaces  32   a  and  32   b  and, in the most preferred form, are approximately 0.025 inches (0.635 mm). The length of the upper face  78   a  between edge  78   c  and upper face  76   a  is approximately 0.060 inches (1.524 mm). The length of the upper face  76   a  between upper faces  74   a  and  78   a  is less than the thickness of the portions  74 ,  76 , and  78  and in the preferred form is approximately 0.010 inch (0.254 mm). The vertical spacing perpendicular to the upper face  74   a  between the upper face  74   a  and the interconnection between the upper face  78   a  and the free edge  78   c  is generally equal to the thickness of the portions  74 ,  76 , and  78  and is approximately 0.025 inches (0.635 mm). The interconnections between surface  32   b  and face  74   a , faces  74   a  and  76   a , surface  32   a  and face  74   b , faces  74   b  and  76   b , faces  76   b  and  78   b , and face  78   b  and edge  78   c  can be radiused. 
     The face  74   a  of the lip  38  is molded flat to be abutted with the unbroken flat surface  42   a  of an adjacent stacked container body  10 . Thus, although not entirely flat as the lip  38  of FIGS. 4 and 13, the lip  38  of FIG. 15 allows the bodies  10  to be stacked in a secure and stable manner and also obtains the other advantages of the present invention. In the preferred form where the container body  10  is injection molded, gate area imperfections  82  can be created on the face  74   a  after the removal of the sprue in the molding process, with the gate area imperfections  82  located intermediate the face  76   a  and the radially outward portion of the face  74   a.    
     The thin seal member  14  is adhered to the radially outward portion of face  74   a  and the face  78   a , and in the preferred form, the portion of the thin seal member  14  radially beyond the face  74   a  is not sealed but simply pressed tightly against the exterior surface  32   b . The stepping down of the thick wall section  32  from the thinner wall section  34  provides a recess for receipt of the peripheral portions of the seal member  14  to prevent unintentional catching of the peripheral edge bottom end  14   a  of the seal member  14  which may result in accidental partial removal or unsealing of the seal member  14  from the lip  38 . It should be specifically noted that the exterior surface  32   b  is of a smooth, cylindrical shape in the preferred form and that an outwardly projecting roll rim or shoulder is not required around which the seal member  14  is crimped to grip the seal member  14  on the top of the body portion  10 . In particular, the sealed container of the preferred form of the present invention is advantageous as providing a cleaner appearance as the seal member  14  also has a cylindrical appearance on the exterior surface  32   b  and has less of a tendency for the peripheral edge bottom end  14   a  to flare outwardly as can occur when the seal member  14  is crimped over a convoluted surface. Additionally, the sealing operation and specifically the mechanism of the processing equipment is simplified when the seal member  14  is pressed flat against the exterior surface  32   b  which is cylindrical compared to if the seal member  14  were crimped against an exterior surface of a convoluted shape. 
     The lip  38  of FIG. 15 is arranged so that the seal member  14  will tend to extend between the face  78   a  and the radially outward portion of face  74   a  without abutment with the face  76   a  and the radially inward portion of face  74   a . In particular, the seal member  14  is sealed on the lip  38  in a manner to extend or bridge over the gate area imperfections  82  on face  74   a . If the seal member  14  were forced down on the gate area imperfections  82  during the sealing of the seal member  14  on the lip  38 , the gate area imperfections  82  can undesirable puncture the seal member  14 . The height differential provided by the portion  76  intermediate the portions  74  and  78  creates a void between the seal member  14  and the lip  38  so that the seal member  14  spans over the gate area imperfections  82 . This span effect is also enhanced by the upward angling of the portion  78  relative to the first portion  74  and the horizontal, with the upward angling of the portion  78  also enhancing the area of sealing between the seal member  14  and the face  78   a . 
     The preferred shape of lip  38  of FIG. 15 according to the teachings of the present invention is believed to insure that sealing occurs between the seal member  14  and the lip  38  around the entire periphery of the lip  38 . This in turn allows the portion of the seal member  14  not to be sealed with the exterior surface  32   b  so that the consumer can readily separate the peripheral bottom end  14   a  of the seal member  14  from the wall section  32  for ease of grasping and pulling to completely remove the seal member  14  from the main body portion  10  when it is desired to consume the yogurt or other food product contained in the interior  16  of the container. Adhering the seal member  14  to its bottom end  14   a  upon the body portion  10  may make separating the peripheral bottom end  14   a  from the container difficult when it is desired to remove the seal member  14  from the container. Also, the enhanced sealing provided by the shaping of the lip  38  according to the teachings of the present invention contributes to the ability to eliminate the requirement of gripping of the seal member  14  on an outwardly projecting roll rim or shoulder over which the seal member  14  must be crimped to provide sealing attachment. 
     While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.