GB2260960A - Vacuum packaging - Google Patents

Abstract

A process for forming a sealed package comprising a rigid tray-like container 10 for a product such as food requiring isolation from atmospheric conditions for a substantial time, includes the steps of applying to each container 10 a planar shoot of lid material 22A, positioning each container in a forming chamber 32 with the lid clamped along a peripheral line thereof, establishing a differential pressure across the lid material to stretch the material away from the container into a smoothly-curved arc as seen in cross-section, advancing the container into a sealing chamber 42 and there evacuating both the interior and exterior of the container, and then (after sealing of the lid to the container mouth) increasing the pressure above said container to force the lid material down from its arc-shaped configuration into the interior of said container and into close intimate pressure contact with the upper surface of the product. <IMAGE>

Description

METHOD AND APPARATUS FOR PRODUCING A SEALED PACKAGE CAPABLE OF LONG-TIME SHELF LIFE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to processes and methods for making sealed packages of food products and the like which must be maintained isolated from atmosphere for long periods of time such as a year or more. More particularly, this invention relates to such processes wherein the packages are formed by repetitive machine operations functioning at high speed.

2. Background of the Invention Packaging machines of many different types have been used over the years for carrying out techniques for making sealed packages, typically evacuated packages where food products are involved. Commonly, such packages have been made from continuous sheets of plastic packaging material, usually a laminate of several layers of different kinds of plastic interleaved with layers of adhesive. For many years, such packages were made by thermoforming techniques, wherein the plastic sheets were heated to a quite high temperature sufficient to effect softening of the material so that it can readily be formed by pressure or other application of moderate force into a desired shape.

Such thermoforming was employed not only to make the conventional flexible or semi-rigid tray-like containers to receive the product, but also to form the top or lid, especially when the top was "back formed" into a shape wherein the central face of the top was recessed down into the container to engage the upper surface of the product.

One example of the many prior art references disclosing such techniques is U.S. Patent 4,201,630, issued to R. A.

Mahaffy et al on May 6, 1980.

More recently, it has been found very desirable to be able to sterilize and/or cook the products contained in sealed packages after the package has been completed.

Such sterilization/cooking treatment often employs very high temperatures. One beneficial effect of such hightemperature treatment is that bacteria are killed in food products, thus preventing the onset of food spoilage. An adverse effect however is that packaging material suitable for the previously-used thermoforming techniques as described above cannot be used in the newer processes without damage to the packaging material due to the high-temperature heat treatment after the package has been completed or during heating prior to consumption in the home. Accordingly, new package arrangements utilizing different kinds of packaging materials have had to have been developed for accommodating the new high-temperature post-formation package-treatment processes.

These new package arrangements generally comprise a fully rigid tray-like container (as distinguished from so-called semi-rigid containers) formed of some type of plastic such as polypropylene or crystalizable polyester and having a planar marginal sealing surface around the container opening. A top or lid is sealed to such marginal sealing surface to complete the package. For the newer processes, this lid typically will incorporate at least a layer of metal foil to serve as a barrier. Commonly, the lid will be made from a laminated sheet of packaging material comprising successive layers, for example, of: polyester/adhesive/aluminum foil (barrier)/adhesive/film sealant. Various alternatives to such laminates are well known in the art including some not containing foil.

Generally, it is not possible to fill the container trays to the very top, due to handling and filling-control considerations, and also due to the variation in volume taken up by different products having the same net weight or calorie content. Thus in carrying out such packaging procedures, there always will be some "head space" above at least most of the upper surface of the product when a planar lid is assembled to the container to form the completed package.

When a package having a relatively large head space is evacuated, the package tends to collapse under atmospheric pressure, thereby creating damaging stresses in various parts of the package assembly. To avoid that difficulty, it is known to fill the head space with a volume of inert gas to eliminate or reduce the negative pressure to a practical level. This procedure is difficult to use, especially where the packaged products are not uniform in size and/or shape. Also, the product in such a gas-filled package is not well constrained against movement within the package. Moreover, expansion of the gas during post-fill thermal processing often causes damage to the package.

It has been known for some time to avoid or minimize such gas-fill problems by forming the lid in such a way that it extends down into the head space sufficiently deeply to engage the upper surface of the product. Such a lid configuration effectively immobilizes the product, and additionally provides for good heat transfer through the lid during post-fill sterilization or cooking. One proposal for so forming a lid is disclosed in U. S. Patent 4,424,659 (J. A. Perigo et al). Still other proposals have been made of techniques for achieving the desired result.

The prior art proposals however have not been capable of achieving the desired goal satisfactorily. A principal problem with prior art techniques has been unacceptable package failure rate due to excessive stresses on the seals between the lid and the marginal areas of the container tray. Excessive seal stresses can occur during the evacuation and sealing process, or when the completed package is heated during sterilization or cooking, or during shipment. Such seal stresses can break the seal so as to destroy the package, or can result in what has been referred to as seal creep, wherein the component parts are relatively shifted along the sealed region, thereby unacceptably reducing the capacity of the seal to prevent transmission of atmospheric fluids/gases into the package interior.

SUMMARY OF THE INVENTION In a preferred embodiment of the invention, to be described hereinafter in detail, there is provided a packaging apparatus arranged to carry out a new packaging procedure for eliminating excessive seal stress with its resultant injury to package integrity. In carrying out this new technique in its preferred embodiment, a series of product filled rigid tray-like containers first are advanced into a sealing region of the packaging machine. A sheet of packaging material is laid down over such filled containers, with the packaging material being of such composition as to be capable of serving as a lid for the new type of packages to be post-fill processed at quite high temperatures.Typically, such lid material will include a metal foil as one of its layers, to provide for a very effective barrier against fluid transmission through the lid for a very long time such as a year or more. This feature, in conjunction with processing at high-temperatures sufficient to kill bacteria, results in very long shelf life, approaching that of conventional canned food products.

The product-filled containers together with their overlaid lid material are advanced successively into a forming chamber of the packaging machine. In that chamber, the lid material is clamped firmly to a planar marginal sealing surface around the opening of each container tray. While so clamped in the forming chamber, a differential pressure is developed across the lid material so as to press the material up and away from the container tray. By such pressure-forming, the lid material is gently shaped into an arc-like, bowed configuration. This bowing out of the material is effected with only very small stresses at any region of the material within the peripheral line where the lid is clamped to the container. This gentle stretching leaves the material free from any regions of stress concentration.

To achieve that result, the lid material is stretched within the region interiorly of the clamped peripheral line where sealing engagement is made with the marginal sealing surface of the container. The stretching action is carried out as uniformly as possible throughout the interior region of the lid, such that the stretching at any one point is effectively the same as at any other point in that region. Only moderate-angle bending of the material is permitted at the region of joinder with the peripheral seal-line. The uniformly dispersed stretching action assures that the stresses resulting from stretching are distributed quite evenly throughout the lid material within the periphery of the lid.

Upon completion of this stretching phase, the container/lid assembly is transferred to a sealing chamber, with the lid material still distended up away from its container tray in an arc-shaped configuration (as seen in cross-section). The sealing chamber then is closed, and vacuum is developed in the interior of the chamber, above and below the container/lid assembly, as well as in the interior of that assembly. The peripheral region of the lid material then is heat-sealed to the marginal sealing surface of the container tray.

Thereafter, the pressure in the sealing chamber above the container/lid assembly is raised, as by venting the chamber interior to atmosphere, so as to develop a differential pressure across the lid material to force that material down into the container. The pressure applied is sufficient to establish forcible contact between the lid material and substantially the full upper surface of the product, with a significant amount of pressure being applied to the product. The force of this pressure engagement may rearrange the product, and may, for example, smooth out the product upper surface, if the product is of such a nature to be so rearranged.The previous upward forming of the lid assures that there will be sufficient lid material to reach down into the head space of the container and to fully contact the upper surface of the product without placing any significant stresses on the seal.

After the lid material has been pressed fully down against the product, the sealing chamber is opened, and the completed (sealed) package is removed. Due to the forming procedures employed, the completed package will be highly resistant to damage from subsequent processing such as sterilization/cooking at high temperatures, and other post treatment to which such packages commonly are subjected.

Still other objects, aspects and advantages of the invention will in part be pointed out in, and in part apparent from, the following detailed description of the invention considered together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A and 1B are vertical longitudinal sections showing the arrangement of a packaging machine for carrying out the processes of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring first to the right-hand edge of Figure 1B, filled tray-like containers 10 are presented to the input region 12 of an automatic packaging machine generally indicated at 14. The containers are moved with an intermittent indexing motion through the machine in a leftward direction from the input region, and being presented at various operating stations as will be described. (For some applications, the machine could be arranged as a continuous-motion type, well known in this art.) The containers 10 are conventionally shaped, typically rectangular or oval, with planar marginal sealing surfaces 16 in the form of flanges around the upwardly-facing openings, and filled with product 17 which may be a mounded product.

Preferably, the containers 10 are so-called preforms, meaning that they were previously formed at another facility. The containers are supported for movement by a series of carriers or dies 18 of a type known in the art.

Alternatively, the packaging system could be provided with apparatus for making such containers from a continuous web of material, in known fashion.

The containers 10 are made of rigid plastic material, e.g. polypropylene, crystalizable polyester or other suitable material, having a thickness of .020 to .050 inches. Such material will not be physically distorted by ordinary stresses encountered in packaging processes, and thus the containers will retain their original shape throughout the machine procedures as will be described hereinafter, and also throughout subsequent post-seal processing and shipment.

The containers 10 first move into a web lay-down station 20 where a continuous sheet of packaging material 22 is applied over the openings of the containers to serve (after forming as described) as a lid for the respective container. This material typically will be a laminate, e.g., having a succession of layers such as (starting from the top): (1) 48 gauge polyester, (2) adhesive, (3) metal foil (e.g. aluminum with a thickness of .00035 to .001), (4) adhesive, (5) sealant layer, and sometimes (6) a heatseal coating. The first polyester layer (1) permits printing of product information on the package, and provides abrasion protection which may particularly be needed for the subsequent layer of metal foil. Although the metal foil itself may serve as a satisfactory barrier to fluid diffusion, it may be advisable to include a further layer beneath the foil to enhance the barrier properties of the laminate.

The adhesive layers (2) and (4) may be any of many known materials used for such purposes. The film sealant heat-seal layer (5), to provide a final permanent heat-seal to the marginal sealing surface of the container, may for example be poly-propylene.

Each container 10 together with its lid material 22 then is indexed into a forming station 30. That station comprises a forming chamber 32 with vertically separable upper and lower elements 34, 36 of conventional construction. After the container is in correct position, indexing movement stops. The chamber elements 34, 36 are forced together, using seal press motion to clamp the lid material to the marginal sealing surface 16 of the respective container.

With the forming chamber elements 34, 36 in clamp position, vacuum is developed above the lid material 22 to draw that material up into a dish-shaped or arc-shaped configuration, as generally illustrated in the drawing at 22A. This forming process preferably is carried out without heating, e.g., at normal room temperature, such as 70OF. To assist in this forming of the lid, air pressure may be developed within and/or below the container 10. In some cases, no vacuum may be used and only air pressure from below may be utilized for forming. The degree of forming may be regulated by controlling the pressure differential between the regions above and below the lid material.

The upward forming of the lid material 22 advantageously is accomplished by stretching all portions of the material which are within the peripheral area defined by the clamping line around the marginal sealing surface 16 of the container. This stretching action is developed uniformly throughout that interior region of the lid material.

The interior region of the lid remains free from any concentration of stretching stresses in any localized part of that region, as previously had been the result in earlier processes used for thermoforming plastic material to make vacuum packages where the stretching particularly was concentrated in the marginal side regions destined to become side walls of the back-formed lid.

The forming of the lid material 22 causes it to be gently bulged upwardly into the somewhat dish-shaped configuration 22A, as seen in cross-section in Figure 1B.

To assure minimal stress concentrations which might ultimately cause rupture of the seal (yet to be formed, as described below), the lid material 22 is formed upwards in a limited fashion, avoiding any sharp-angle bends of the material. In particular, the angle between the lid material 22 and the associated planar sealing surface 16 of the container should not exceed 45 , and preferably should be no greater than 25".

After completion of stretching, the formed volume in the lid should in any event be greater than or at least equal to the unfilled head space in the container 10 with the product in place. For example, with a head space of 50cc, the available formed volume of the stretched lid material might appropriately be 6Occ.

After such stretched-lid formation, the forming chamber 32 is opened to permit removal of the composite container/lid assembly. This assembly then is advanced to an evacuate-and-seal station 40, with the lid material still in its upwardly extended dish-shaped configuration.

The evacuate-and-seal station includes a sealing chamber 42 of conventional construction having two vertically separable elements 44, 46. After the container/lid assembly is in place in that chamber, the chamber elements are closed to seal the chamber interior from outside atmosphere.

With the evacuate-and-seal chamber 42 closed, vacuum is applied throughout the chamber, including the interior of the container/lid assembly. The vacuum level may be regulated if required by the characteristics of the product. Some inert gas may also be introduced into the package interior if appropriate for the particular packaging process involved.

After evacuation, a heated seal bar 48 is driven down to engage the lid material 22, just outboard of the peripheral line of the bowed lid 22A. This effects a heatseal to the marginal sealing surface 16 of the container 10. The seal can be either a fusion or a peelable type.

Thereafter, the chamber 42 is vented to atmosphere. If desired, the chamber region above the package can be vented first to achieve a differential action. In any event, pressure developed in the chamber forces the formed lid 22A down into the container 10, with the lid being pressed against the upper surface of the product firmly and fully across the entire area of the product surface.

Because the lid 22A had initially been formed in a gentle arc-shaped configuration with the stretching stresses being distributed essentially uniformly over all of the lid material within the interior region within the peripheral seal line, the intimate engagement of the lid with the product is effected without producing significant tensile stresses at the peripheral regions where the heatseal is formed. Thus the chances of seal disruption due to such stresses or due to subsequent package processing are greatly reduced. Moreover, it becomes possible to utilize thinner packaging material, thus reducing costs.

With the package thereby completed, the chamber 42 is opened and the package is transferred to the cooling station 50 where it is exposed to either chilled plate cooling or to atmospheric conditions for one cycle, permitting the seals to cool off. Thereafter, the package is transferred to the trim station 60 where excess packaging material is removed so that the package can be readied for shipment.

Although a preferred embodiment of the invention has been disclosed herein in detail, it is to be understood that this is for the purpose of illustrating the invention, and should not be construed as necessarily limiting the scope of the invention since it is apparent that many changes can be made by those skilled in the art while still practicing the invention claimed herein. For example, although in the preferred embodiment the lid is formed from a continuous web of packaging material 22, it may be advantageous in certain applications to provide the lids as preforms. In that case, the central portion 22A of the preform will be deformed upwardly, as shown in Figure 1B, so that it can subsequently be reversed in the evacuateand-seal station 40 so as to extend down into the container 10 to engage the packaged product. Other variations will be evident to those skilled in this art.

Claims (11)

What is Claimed is:

1. A process for packaging a product subject to post-packaging heat treatment, said process forming a sealed package of the type comprising a rigid container having an opening at its top leading to the container interior and carrying a product such as food requiring isolation from atmospheric conditions for a substantial time, said container including a flat marginal sealing surface around said opening, said package further comprising a lid having a generally planar central region and sealed along a line around its outer periphery to said marginal sealing surface, said lid including a metallic foil sufficiently thin to assure that it is flexible enough to be readily shaped into a curved configuration while still providing a strong isolation barrier to substantially prevent diffusion of fluid material through the lid; said container and said lid forming a composite container/lid assembly; said process comprising the steps of: advancing into a packaging machine a series of said rigid containers filled with product and each having a lid including a metallic foil; applying to each container in sequence a planar sheet of said lid material including metallic foil for serving as said lid for said container to form said composite container/lid assembly; positioning each container together with its lid material in a forming chamber in said packaging machine and with the lid being clamped along a peripheral line thereof to the marginal sealing surface of the container to effect sealing engagement therebetween;; establishing a differential pressure across said lid material for each container to form said material in a direction away from the plane of said planar sheet of material and so as to be moved away from the associated container, said forming being effected by stretching said lid material within the interior region of said material defined by said outer peripheral line where the lid is to be sealed to said marginal sealing surface, said material of said interior region being stretch-formed into a smoothly-curved arc; at some stage in the process, applying heat to the region of said outer peripheral line of said lid material and to said marginal sealing surface of said container/lid assembly to effect a seal therebetween; advancing said container/lid assembly into a sealing chamber at a sealing station of said packaging machine;; closing said sealing chamber to seal its interior from outside atmosphere with the container/lid assembly clamped together at said marginal sealing surface; evacuating the spaces above and below said container/lid assembly while simultaneously evacuating the interior of said container; increasing the pressure above said container/lid assembly to force said lid material down from its arc-shaped configuration into the interior of said container and into close intimate pressure contact with the surface of said product adjacent said opening; and opening said sealing chamber and removing the sealed package for further processing.

2. The process of Claim 1, wherein said lid material when being formed into a smoothly-curved arc is stretched at all points in the region located within said peripheral line.

3. The process of Claim 2, wherein the stretching at all of said points is essentially equal, so as to be free of significant regions of stress concentration.

4. The process of Claim 1, wherein said stretching of the lid material is carried out at room temperature.

5. The process of Claim 1, wherein the upward forming of said lid material is limited by applying a pressure differential across said lid material such as to assure that the angle between the lid material and the marginal sealing surface does not exceed 45 .

6. The process of Claim 5, wherein the stretching is limited to assure that said angle does not exceed 25o.

7. The process of Claim 6, wherein the lid material is stretched sufficiently to provide a formed volume at least equal to or slightly greater than the unfilled head space in the container with the product in place.

8. The process of Claim 1, wherein the upwardlystretched arc shape of said lid material is maintained as the container/lid assembly is advanced into said sealing station chamber.

9. The process of Claim 1, wherein the product placed in the container is pressure-shapable; the contour of the surface of said product being rearranged by the pressure contact of said lid to provide a smoothed surface in full-area contact with said lid.

10. The process of Claim 5, including the step of maintaining the material of said lid away from engagement with any part of said forming chamber while said lid is being stretched away from said container into said smoothly-curved arc.

11. The process of claim 1, substantially as herein described with reference to the accompanying drawings.