Patent Application
20130175279
1. Field of the Invention
This invention relates generally to the field of packaging, and more specifically to the field of retortable plastic containers. More specifically, the invention relates to an improved retortable container that is more dimensionally stable during the sterilization process than were conventional predecessor containers.
2. Description of the Related Technology
Certain products require sterilization during the packaging process in order to inhibit the growth of bacteria. Products requiring sterilization include foods such as milk, yogurt and various sauces and prepared foods, as well as certain pharmaceutical products. Thermal processing, sterilization, canning and retorting are all terms referring to the process of taking a food product, already sealed in its container, and heating it to a specific temperature for a specific time. The objective is to kill spoilage organisms and pathogenic bacteria, thus preserving the food and allowing it to be stored unrefrigerated for extended lengths of time.
There are multiple designs for retorting food containers, including batch systems and continuous systems. In a batch system, containers are placed in crates or baskets, which are then loaded into a vessel into which the heating medium is introduced. This method is the oldest and most traditional and also the most versatile in the range of products and container sizes it can handle. In a continuous retort system, a conveyor is used to continuously transport the containers to be sterilized through a heating chamber that contains the heating medium. There are advantages to each method depending on individual processing operations and, just as important, the type of food being processed.
Traditionally, products that require heat sterilization have been packaged in glass containers, which are relatively stable at elevated temperatures and pressures. However, in recent years plastic retortable containers have come into use. Plastic containers tend to be less expensive than glass containers and safer in many respects because they will not shatter when dropped. Unfortunately, plastic containers may lack the column strength that is necessary to avoid deformation of the sidewall of the container when a number of containers or palettes of containers are stacked during transportation or in packaging or retail facilities. While it is possible to increase the strength of a plastic container by increasing the thickness of the sidewall, doing so also increases manufacturing costs by increasing the amount of plastic material that is required. Lightweighting is an important consideration in the design of plastic containers, including plastic cans, because plastic material tends to be relatively expensive.
Many plastic containers also lack the requisite circumferential or hoop strength that is required to avoid excessive deformation when the contents of the container becomes pressurized, such as during a heat sterilization process.
The most common commercial procedure for heat sterilizing canned foods is a retort process in which filled but unsterilized sealed cans are placed in a retort chamber that is injected with steam and held at a predetermined elevated temperature (typically between about 210° F. to about 260° F.) for a predetermined period of time. Conventional plastic containers have been considered unsuitable for packaging applications in which heat sterilization is required, because the heat and pressurization that is inherent to such processes has the tendency to cause irreversible damage and deformation to the sidewall of the plastic can.
The temperatures of the retort process are elevated enough to temporarily increase the internal pressurization of the container. Plastic retortable containers accordingly have been designed to permit limited and reversible controlled flexure of one or more surfaces in order to accommodate the internal volumetric changes that are inherent to the retort sterilization process. U.S. Pat. No. 5,217,737 to Gygax et al. discloses a retortable plastic container that has a flexible bottom portion to accommodate internal volumetric changes. Other retortable containers that have been in commercial use have a champagne style bottom portion that is designed to permit a certain amount of flexure. However, when using a continuous retort process the flexure of retortable plastic containers must be limited so that it will not interfere with the process of conveying the container through the continuous retort system. Typically, such conveyors require at least two dimensionally stable points of contact on the container.
Is also important that the bottom portions of such containers retain their dimensional stability after heat sterilization so that they will not rock when placed on a horizontal support surface such as a table or retail shelf. Containers that are manufactured using an extrusion blow molding process typically have a mold parting line or seam. In order to prevent the mold parting seam from affecting the stability of the container when it is resting on a horizontal surface, recesses or tunnels are commonly integrated into the container bottom design in the area about the parting seam. In addition, differential cooling that occurs during molding process can result in warping of the container bottom that can result in rocking when the container is placed on a horizontal surface. Such rocking can be exacerbated by additional plastic deformation that occurs during the heat sterilization process. In the past, some designs have compensated for this warping or distortion by molding the container bottom to have a slight undulation. However, the presence of the undulation sometimes acted as an initiation locus or migration point for failure of the container during the heat sterilization process.
A need exists for an improved retortable container that exhibits improved dimensional stability and strength during the heat sterilization process without significantly adding to material costs.
Accordingly, it is an object of the invention to provide a retortable container that exhibits improved dimensional strength and stability during the heat sterilization process without significantly adding to material costs.
In order to achieve the above and other objects of the invention, a retortable plastic container according to a first aspect of the invention includes a main body and a bottom. The bottom defines a raised inner portion and at least one substantially flat bottom support surface. The substantially flat bottom support surface is curved and positioned near a radially outermost edge of the bottom when viewed in bottom plan. A groove is defined in the substantially flat bottom support surface.
These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.
Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to
Referring again to
The retortable plastic container 10 is preferably fabricated using an extrusion blow molding process from a plastic material that is compatible with the heat sterilization process, most preferably polypropylene. Most preferably, a multilayer material containing polypropylene and additional layers, such as a barrier layer with low oxygen permeability, may be used to form the sidewall 17 of the plastic container 10.
Referring now to
Groove 24 is preferably shaped so as to be substantially concentric with respect to the curvature of the substantially flat bottom support surface 22, as is best shown in
As
The groove 24 preferably has a maximum width WG, which is best shown in
The groove 24 preferably has a curved inner surface 32 that has an average radius of curvature RG, as is best shown in
The groove 24 also preferably has a maximum depth DG, also best shown in
As
As
Preferably, a ratio WG/WT of the maximum width of the groove 24 to the average thickness of the side wall 17 is substantially within a range of about 0.1 to about 1.0, more preferably substantially within range of about 0.2 to about 0.85 and most preferably substantially the range about 0.3 to about 0.65.
Referring again the
The first sidewall portion further has a height H1. Preferably, a ratio H1/DMAX of the height H1 of the first sidewall portion to the maximum outer diameter DMAX of the container 10 is substantially within a range of about 0.015 to about 0.040, more preferably substantially within a range of about 0.020 to about 0.035 and most preferably substantially within a range of about 0.025 to about 0.030.
The second sidewall portion 42 is unitary with the first sidewall portion 40 and a connected thereto by a concave fillet having an average radius of curvature RB. Preferably, a ratio RB/DMAX of the average radius of curvature RB to the maximum outer diameter DMAX of the container 10 is substantially within a range of about 0.0045 to about 0.065, more preferably substantially within a range of about 0.007 to about 0.04 and most preferably substantially within a range of about 0.0092 to about 0.02.
The presence of the groove 24 and the division of the support surface 22 into inner and outer portions in conjunction with the truncated cone shape of the first sidewall portion 40 has the effect of stiffening the bottom 14 during the heat sterilization process, and also has the tendency to preserve the flatness of the support surface 22 both during the heat sterilization process and during subsequent use of the container 10. The stiffening effect is particularly pronounced when using polypropylene.
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
