Hermetically sealed container and test for seal leakage thereof

Abstract

 An hermetically sealed container such as used in packaging of foods, powders, or material where ingress of moisture or contaminants is to be prevented, comprises a dished sidewall (61) having a peripheral flange (64), a sidewall (74) with a peripheral part engaging said flange and a seal formed between said peripheral flange and peripheral part which comprises two regions (76,79) of sealing material extending one alongside the other provided on said part and/or said flange and an evacuated cavity (70) provided by a channel formed in the peripheral flange or part which separates the regions of sealing material. Methods of testing such seals where the channel is pressurised or evacuated are also disclosed.

Description

[0001] This invention relates to hermetically sealed containers and to methods of testing for seal leakage thereof.

[0002] Laminated plastics film is in general use for packaging. For example a plastics/thermoplastics laminate is conveniently made up in the form of hermetically sealed containers. The seals in such containers are formed usually by bonding two or more layers of the laminate to each other under heat and pressure. Also a plastics/metal foil/thermoplastics laminate film, which possesses reduced gas permeation properties, is widely adopted for use in the lids of hermetically sealed, moulded plastics tray containers. The seals of such tray containers are heat sealed to the flange formed in the tray. Instead of being formed as a laminate the thermoplastic layer can be applied all over or locally as a liquid coating for heat sealing.

[0003] A high standard of integrity against leakage is demanded in many such types of container. Examples include vacuum packs where food is kept fresh in a partial vacuum or controlled gas atmosphere or sealed containers which prevent ingress of moisture to inhibit damage in hygroscopic powders. Alternatively, the seals of lidded containers are frequently relied upon to preserve medically sterilised goods from contamination prior to use, or to guarantee that bacterial, microbial or fungal etc. infection of pasteurised food or drink is prevented.

[0004] As the integrity of such packs have increasingly come to be relied upon, damaged goods tend increasingly to be returned to the point of sale. Also the damage or disease that might be caused by infected goods has been increasingly recognised and a measure of quality assurance after packaging is demanded. Generally speaking the integrity of the raw laminate or film tray stock is good and the quality of the seal is the factor which normally gives rise to container leakage and infection. The trend therefore is for both the responsibility for seal leakage and the burden of proof of seal integrity to fall upon the manufacturer who carries out the packaging process. There is a demand for seal designs and seal manufacturing machinery that continuously monitor and test seal integrity in production, and a demand to monitor the condition of the seal integrity through the distribution chain and by the ultimate consumer.

[0005] An object of the present invention is to provide an hermetically sealed container with an improved form of seal. A further object is to provide a method of testing hermetically sealed containers for seal leakage thereof.

[0006] The present invention consists in an hermetically sealed container having a first dished sidewall formed with a peripheral flange, a second sidewall having a peripheral part which engages the peripheral flange of said first sidewall to form a closed storage chamber between said sidewalls, and a seal formed between said engaging peripheral flange and peripheral part, said seal comprising two regions of sealing material extending one alongside the other provided on one at least of said engaging peripheral flange and peripheral part which bond together said flange and said part, and evacuated cavity means provided by channel means formed in one at least of said peripheral flange and peripheral part and separating said regions of sealing material.

[0007] Suitably, said channel means may comprise an endless channel. Advantageously said channel means comprise a channel and an enlarged channel part with which opposite ends of said channel communicate, said enlarged channel part enabling testing of said seal.

[0008] In one form, the container is characterised in that adjacent said enlarged channel part is a depression formed in a compliant part of one of said peripheral flange and peripheral part within the sealing region located outwardly of said channel.

[0009] Suitably, said depression is formed with an aperture for connection to the interior of the depression of vacuum drawing means.

[0010] The invention also consists in an hermetically sealed container comprising a tray of plastics material formed with a peripheral flange and a lid for said tray formed of plastics film overlying said tray and having a peripheral part formed with an endless channel therein and secured by a seal to said peripheral flange, said seal comprising two regions extending along said channel on opposite sides thereof and provided on at least one of said peripheral flange and peripheral part and said channel being pressurised to enable testing of said seal.

[0011] The invention further consists in an hermetically sealed container comprising a top and a tray and having a peripheral flange in which is formed a channel, a channel enlargement with which opposite ends of the channel communicate and adjacent said channel enlargement a depression formed with an aperture for connection thereto of vacuum forming means to enable evacuation of said channel and channel enlargement, said top having a peripheral part secured to said peripheral flange by an hermetic seal comprising two regions of sealing material extending along respective opposite sides of said channel and provided on at least one of said peripheral flange and said peripheral part and said channel enlargement enabling testing of said seal.

[0012] The invention additionally consists in the method of testing an hermetically sealed container for seal leakage thereof, said container having an enclosing sidewall part at least of which comprises a compliant diaphragm, characterised by placing an inverted test cup having detecting means therein against said compliant diaphragm to form a closed cavity therebetween, varying the pressure in said cavity and determining from displacement of or change of force on said diaphragm embraced by said cup sensed by said detecting means whether there is leakage of said seal. Suitably, the pressure variation in said cavity is effected by drawing a vacuum therein.

[0013] Suitably, the sealed container so tested may be a vacuum pack.

[0014] The invention also consists in the method of testing an hermetically sealed container for seal leakage thereof, said container comprising a first dished sidewall formed with a peripheral flange and a second sidewall having a peripheral part which engages said peripheral flange, one of said sidewalls including a compliant diaphragm and a seal formed between said peripheral part and said peripheral flange, characterised by placing an inverted test cup having detecting means therein against said compliant diaphragm to form a closed cavity therebetween, varying the pressure in said cavity and determining from any displacement of or change of force on said diaphragm embraced by said cup sensed by said detecting means whether there is leakage of said seal.

[0015] The latter method may be employed where in said hermetically sealed container said first sidewall comprises a tray formed from plastics material which includes said peripheral flange there being formed in said flange a channel and a channel enlargement with which opposite ends of said channel communicate, and said second sidewall comprises a film of compliant plastic material the peripheral part of which overlies and sealingly engages with said peripheral flange along opposite sides of said channel and forms a chamber with said channel enlargement. With such a container the method is characterised by placing against said chamber in contact wi,th said second sidewall an inverted test cup having detecting means therein, to form a closed cavity with said second sidewall, varying the pressure in said cavity and determining from displacement of or change of force on said second sidewall embraced by said cup whether there is leakage of said seal.

[0016] The invention also includes the method of testing an hermetically sealed container comprising a first sidewall provided by a tray formed from plastics material which includes a peripheral flange formed with a channel and a second sidewall having a peripheral part which overlies and sealingly engages with said flange along opposite sides of said channel, said channel being defined in part by a flexible diaphragm, characterised by placing against said diaphragm an inverted test cup having detecting means therein to form a closed cavity with said diaphragm, varying the pressure in said cavity and determining from displacement of or change of force on said diaphragm whether there is leakage of said seal.

[0017] The invention still further consists in the method of testing an hermetically sealed container for seal leakage thereof, said container comprising a first dished sidewall formed with a peripheral flange and a second sidewall having a peripheral part which engages and forms a seal with said peripheral flange, there being formed in said peripheral flange or peripheral part a channel containing air or gas at least at atmospheric pressure which divides said seal into separated regions extending along respective opposite sides of said channel, characterised by causing an increase in pressure in said channel and determining whether there is leakage of said seal arising from said channel pressure increase.

[0018] In one form of this method, said channel pressure increase is caused by imparting a displacement to said channel and determining whether there is leakage of said seal by checking after a predetermined interval whether said displacement has increased.

[0019] In another form the channel pressure increase is caused by applying a force to said channel and determining whether there is leakage of said seal by monitoring the force of reaction to said applied force exerted by said channel for a reduction thereof over a fixed period.

[0020] The invention also consists in the method of testing an hermetically sealed container for seal leakage thereof, said container comprising a tray formed with a peripheral flange and a top comprising a compliant film having a peripheral part sealed to said flange. characterised by mounting said container in a test chamber containing detecting means located in the mounted position of said container for detection of movement of or change of force on said compliant film varying the pressure in the test chamber and monitoring said film for movement thereof or change of force thereon by said detecting means. Suitably, varying the pressure is effected by drawing a vacuum in the test chamber and the detecting means sense movement of the film caused by the vacuum drawn.

[0021] The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

FIGURE 1 is a side elevation of one form of hermetically sealed container according to this invention;

FIGURE 2 is a fragmentary view to a scale larger than that of Figure 1 showing details of the seal of the container of Figure 1;

FIGURE 3 is a side view of the container of Figures 1 and 2 located for testing of the seal thereof;

FIGURE 4 is a side view of another form of hermetically sealed container according to this invention;

FIGURE 5 is a fragmentary side view of the container of Figure 4 inverted and located for testing of the seal thereof;

FIGURE 6 is a perspective view of a further form of hermetically sealed container according to this invention;

FIGURE 7 is a fragmentary plan view of the container of Figure 6;

FIGURES 8, 9, 10 and 11 are fragmentary sectional elevational views showing details of the structure of the container of Figure 6 and the means and manner of testing the seal thereof;

FIGURE 12, is a sectional side elevation illustrating the manner of testing the seal of a further form of hermetically sealed container; and

FIGURE 13 is a sectional side elevation illustrating the manner of testing and the seal of yet a further form of hermetically sealed container.

[0022] Referring first to Figures 1 and 2, a flexible pack container 10 is made from a plastics laminate film 12. Typically the container is employed to wrap powders in a vacuum pack, or foodstuffs, such as meats or other perishables which are packaged in a sterilised or at least a pasteurised condition, the pack then being sealed.

[0023] The container 10 has dished sidewalls 14 and 16 which together provide a storage chamber 17 and are formed with respective peripheral parts or flanges 18 and 20 which face one another. The plastics film 12 is made with a thermoplastic film coating 22 which is meldable by application of heat. The coating 22 is applied on the inside face of the film 12 to at least one and, preferably, as shown, both of the sidewalls 14 and 16. Whilst the coating can be applied to the entire surface of the film 12 which forms the inner surfaces of the sidewalls of the container, it may, as shown be applied on to the region of the flanges 18 and 20 between which the peripheral bond 24 of the container is formed by heat and pressure applied hermetically to secure the flanges together.

[0024] The peripheral bond 24 consists of an inner and outer peripheral seal 26 and 28 separated by a channel 30 formed by facing channel parts 32 and 34 provided respectively in the flanges 18 and 20. The channel 30 is preferably continuous round the periphery of the container 10 in order that the entire perimeter seal can be tested at the same time. Other forms of seal can be formed in similar fashion to that described, such as a line channel separated seal in which the channel is sealed at its ends, or a double ring seal formed by two parallel channels separating three continuous perimeter seals. Also the channel 30 need only be formed in one of the flanges 18 and 20.

[0025] In manufacture of the container 10, two sheets 14 and 26 are made from thermoplastic coated film stock 12 and the dished shape and channels are imparted by conventional plastics forming involving heat and pressure or vacuum in a forming tool.

[0026] In subsequent forming steps the two sheets 14, 16, now in the sidewall form illustrated, are filled with ingredients at a filling station at which pasteurising or other conditioning may be performed, and sealed by heat and pressure applied by a heated anvil (not shown) at a sealing station. The heat and pressure at the sealing station causes softening of the thermoplastic coating 22 which flows to form the innerand outer peripheral seals 26 and 28 between the flanges 18 and 20.

[0027] The pack 10 may contain air or a preservative gas at or near atmospheric pressure whilst the channel 30 formed by the channel parts 32 and 34 may, to facilitate testing of the seal as hereinafter described, be either pressurised or evacuated. The inner and outer perimeter heat seals may accordingly be formed or sealed one after the other at successive sealing stations when the pack can be evacuated and the ring cavity filled with gas or air under positive pressure. A further embodiment is described by reference to Figure 6 in which the channel 30 is evacuated.

[0028] When the seals of a plastic laminate container are formed, there exists a possibility that the seal will not be properly formed throughout its length or periphery causing the seal to leak. This may occur if for example a fold occurs in one of the laminate sheets or if the thermoplastic coating 22 is not continuous throughout the flange area of the seal. Alternatively, some of the container ingredients or dust may be deposited on the seal line so as to inhibit the formation of a good seal.

[0029] It is therefore advantageous after forming the seal to test the integrity of the seal. Since the channel 30 is small in volume compared with the volume of the container and since it is a simple process to apply a relatively high pressure to the channel by applying an external displacement to the walls of the channel part 32, 34, the seal integrity of the peripheral seals adjoining the channel 30 may be tested more quickly than the seal integrity of a container with a simple flange seal.

[0030] A method of testing the integrity of the peripheral seals 26, 28 adjacent the channel 30 is illustrated in Figure 3. The pack 10, formed with the channel 30 containing air or gas at least at atmospheric pressure between peripheral seals 24, is placed between a stationary anvil 36 and movable anvil 38 in a test station. The anvil 38 is forced towards anvil 36 trapping the channel 30 round the whole length of the channel, until a predefined separation X between the platen faces 40 and 42 respectively of the anvils 36 and 38 is established. The force F separating the faces is measured with a load cell 44 at time intervals t. If there is any progressive leak from the seals 22 or either of them the successive values of the pressure in the load cell progressively reduce, thereby indicating a leak from the channel 30. Alternatively, the separation could be monitored under a constant force, a reduction in the separation indicating seal leakage.

[0031] Precautions are preferably taken that the pressure in the channel 30 at the predefined separation is not such as to rupture or cause to creep the materials of the channel or seals and that the stiffness of the cavity walls provides only a small part of the reactive load between the anvil faces 40 and 42. The period t over which the seal integrity test is performed depends upon the statistics of seal leaks. Since for the most part leakage holes are above a minimum size and the probability of small leakage holes is very low the time t can usually be limited to a few seconds. The time is however substantially less than the period needed to test a simple flange seal of a vacuum pack using a pressure test.

[0032] An alternative embodiment of a container with a leak testable seal according to the invention is illustrated in Figures 4 and 5. A tray pack container 50 is constructed from a flange tray 51 made by processes including injection, co-extrusion, vacuum forming or foam moulding or is drawn from a metal foil or metal foil-plastics laminate. A sealing film 52 is then coated on the tray flange 54 with sufficient thickness to planarize the upper surface of the flange and provide a film suitable for forming a heat seal.

[0033] The flange 54 may be formed with a peripheral channel during the moulding or drawing operations, but this design concept is described in connection with Figure 6, 7 and 8.

[0034] In this embodiment, however, a lid part 56 of the container 50 seals the container with inner and outer peripheral seals 57 and 59 on opposite sides of a channel 58 formed in the lid, the channel being at or above atmospheric pressure. The lid is also formed of a metal foil, metal-plastics or plastics laminate. Alternatively a thermoplastic film may be used for the lid and may also be coated at least in the region of the peripheral seals with a meldable bond layer.

[0035] In manufacture the lid 56 is cut or stamped out round the periphery at the same time stamping, or otherwise impressing, the peripheral groove 58. The tray and lid are then progressively delivered stations for filling, pressurising of the channel 58, sealing and initial testing of the seal.

[0036] A tray pack container of the general type described is often used to pack precooked frozen foods, which must be maintained pasteurised or sterile; or industrial or medical products which are to be kept particularly clean and dust free or medically sterile. Any leak in the peripheral seals may admit agents causing contamination, ingress being accelerated by temperature cycling during the container shelf life. It is therefore desired to form a leak testable seal in this type of tray container to identify and remove the containers that leak.

[0037] The parallel peripheral seals 57 and 59 between the lid 56 and the tray 51 in the region of the flange 54, which are separated by the channel 58 are formed in the sealing station simultaneously or successively, and then testing the seals for seal integrity takes place in a subsequent leak testing station.

[0038] The seal integrity test method which is illustrated in Figure 5 employs the same principles as that illustrated in Figure 3. The flange 54 is compressed between the faces 40 and 42 of the anvils 36 and 38 and the pressure in the load cell 44 is monitored, so that any change in pressure during a test period indicates a fall in the gas pressure in the channel 58, which initially was at least at atmospheric pressure, and therefore a leak through one of the adjoining seals 57 and 59. Since the pressure induced in the channel is high and its volume small, the test period to indicate seal integrity is jelatively short compared with the pressure test of a simple flange seal.

[0039] As in the case of Figure 3, the separation between the anvils could be monitored over a period under a constant applied force, reduction of the separation indicating leakage of the seal.

[0040] An alternative embodiment of tray pack container 60 is described with reference to Figures 6 to 8. Although of general application the container 60 has particular application for a package for ambient shelf stable or cook-chill foods. The ambient shelf stable packages are retorted to effect sterilisation and to inhibit possible bacterial or fungal growth when stored at ambient temperatures; the cook-chill foods are precooked or pasteurised and sealed and intended to be kept chilled (i.e. not deep frozen). If such food packs are not securely sealed, but gradually leak during the product shelf life, the food may become contaminated. Thermal cycling of the product during its shelf life can also cause infections to be induced by way of any leakage hole. A leak testable seal provides a method of ensuring that the container is correctly produced and remains in demonstrably good condition until use.

[0041] The channel 66 of container 60 of this embodiment is distinguished from that of Figures 1 to 5 in that it is evacuated to a low pressure and sealed.

[0042] One advantage of an evacuated city 68 of channel 66 is that the test pressure does not have to be applied by an anvil at a test station. The pressure difference across the perimeter seals on opposite sides of the channel is inherent. Thus if the package is sealed before the cooking process, and the latter occupies a period of say 50-90 minutes, and the vacuum in the channel 66 is then tested after cooking, the test pressure has effectively been applied throughout cooking and therefore for a period over 10³ seconds longer and also at a higher pressure differential at high temperature than is conveniently performed by a seal leakage test applied at a test station in the filling line. The test for leaks applied to the present embodiment is therefore significantly more sensitive than that of the prior embodiment.

[0043] A second advantage of an evacuated channel cavity 68 is that when the container 60 is opened by opening the seal thereof disposed outwardly of the channel 66, the inward flow of air into the channel is made audible. The purchaser of the food pack therefore receives an audible suction sound similar to that obtained when a can (tin) is opened. This provides a familiar indication and reassurance of seal quality. The failure of either seal is also made readily visible in that the diaphragm in place of being drawn concave by the channel vacuum, advances to a position level with the lid. It is then readily observable by the consumer. A spring located under or in a diaphragm defining part of the channel can also provide an eyen more visible or tactile indication of seal integrity to the consumer.

[0044] The tray pack container 60 of Figures 6 to 8 is based on a flanged tray 61, which is moulded or vacuum formed. The flange 64 of the tray is formed with peripheral channel 66, cavity 68 of which, after sealing on opposite sides thereof, is evacuated.

[0045] The flange 64 is extended in area 65 in one or more corners of the tray and in this or each of these areas contains a channel enlargement in the form of a circular cavity 70 which connects with opposite ends of the peripheral channel 66. Adjacent to cavity 70 is a second cavity 72 having a hole 73 which penetrates the material of the flange 64. The top surface of the flange is planar, including an area 78 which separates the cavities 70 and 72, as indicated in detail in Figures 9 and 10.

[0046] After filling, suitably with a food product, the tray pack container 60 is sealed with a film or lid 74. The film lid 74 is a laminate coated with a thermoplastics meldable layer and sealing is carried out by application of heating to the film and pressure to seal the film lid against the flange over the entire lid area of the flange contacted by the film 74 except the area between the cavities 70 and 72. Accordingly an inner peripheral seal 76 has been completely formed, but an outer peripheral seal 79 which is separated by the channel 66 from the seal 76, is incomplete in the area 78 separating the cavities.

[0047] To evacuate the channel cavity 68 formed between the tray 61 and the lid 74 a hollow suction needle 80 is inserted through the hole 63 in the base of cavity 62 until a rubber seal ring 82 seals the hole. The film lid 74 is lifted by the needle in the area of cavity 72 and the area 78 between the cavities 70 and 72. Suction applied via the needle 80 then exhausts the cavity 68 of the channel 66 until the film 74, as indicated in Figure 10 is depressed by external atmospheric pressure acting on the film. After evacuation of the cavity, the film 74 is heated and pressure is applied in the area 65, sealing the area 78 which separates the two cavities 70 and 72, and the suction needle 80 is then withdrawn.

[0048] The vacuum formed in the channel cavity 68 may now be tested, by application of a test vacuum cup which is placed over cavity 70 against the upper face of the film lid 74. The vacuum present in the channel cavity 68, when the film is lifted in this area is indicated by the test vacuum applied to the other side.

[0049] Figure 11 shows a test cup 90 placed in contact over the cavity 70 against the upper face of the film lid 74. An external source of vacuum is applied to the cup to gradually remove air from the test cup, the residual pressure in the cup being monitored. When the test pressure is lower than that in the channel cavity 68, the film 74 over the cavity, instead of being drawn inward is pressed outward, the movement being indicated by a contact strain gauge 94. The test pressure at the time of movement indicates the internal pressure in the cavity 68 and any progressive change in this pressure is employed to indicate the seal integrity.

[0050] After cooking the tray pack ingredients, a second measurement of the vacuum level in the channel cavity by the same test process indicates the condition of the seal after processing of the ingredients. If the pressure in the channel cavity has risen, this will be due to a leak in one or both of the peripheral seals 76 and 79 adjoining the cavity 68. Experience will prove the seal quality is adequate to maintain a vacuum throughout the shelf life of the product. Each individual pack can carry data relating to its seal integrity which can be recorded suitably in a computer.

[0051] When the seal is broken and the consumer lifts the lid the ingress of air into the ring cavity vacuum space may be made audible by an insert 69 of moulded profile in channel 66 which emits a characteristic noise or tone. An additional cavity may be formed in the ring cavity to augment the flow volume of air through the insert.

[0052] For some packages the head space above the contents is evacuated, suitably, to accelerate heating during cooking or sterilisation. In such a case, there is no need for the cavity 72 since the channel cavity 68 is evacuated at the time the head space is evacuated. It will also be appreciated that instead of using the test cup to form an evacuable cavity with the membrane over the cavity 70, the cup could engage any flexible membrane or diaphragm used to define part of the channel 66.

[0053] Referring now to Figure 12, a container 100 is filled with goods 102 that are easily contaminated, such as medically sterile goods; or a hygroscopic powder, where ingress of moisture needs to be inhibited; or milk or meat products on which a fungal, bacterial or microbial culture could grow if admission of such a contaminant were gained. To prevent such infection the container is sealed with a heat sealed compliant film diaphragm 103.

[0054] To test the seal between the container and diaphragm, the container is supported in a test chamber 104. The chamber may be part of a manually loaded test cell or part of an automatic carousel test station. Adjacent the film diaphragm is placed a spring contact 106 and a detector 108 that detects displacement of the spring contact. The detector may for example be optically or magnetically energised.

[0055] A source of vacuum 110 is applied to the test chamber 104 controlled by a solenoid 112 and it will be noted that the container seal as well as the diaphragm is exposed to the vacuum in the chamber 104. On drawing a vacuum by energising the solenoid a reduction takes place in the pressure in the test chamber, the rate of change of pressure being predetermined. At the same time the detector 108 is monitored.

[0056] In the event that the detector on application of vacuum senses movement of the diaphragm when the pressure in the chamber 104 falls below that in the container 100, this indicates a good seal: if however, the detector senses no diaphragm movement, this signals a leak in the container or its seal so that gas flow in and out of the container occurs at the same time as gas flow out of the chamber 104 so that no pressure differential across the diaphragm is established and no movement of the diaphragm takes place. If it is required to detect a slow leak in the container, this is achieved by application of a gradually applied vacuum, where again, if there is no leak, the detector senses movement of the diaphragm 103.

[0057] Figure 13 shows an alternative embodiment of the invention applied to detect leakage of a vacuum pack container 200. In the example the vacuum pack consists of sidewalls 202 and 203 of flexible film which are heat sealed at respective flanges 204 and 205 thereof after filling. Typically powders are packaged in this manner, the package being sealed so that the vacuum is maintained from the time of manufacture until sale, i.e. for a period of several weeks.

[0058] To test the seal integrity of the vacuum pack, the state of vacuum in the pack is checked after a suitable period e.g. 15 minutes (10³ seconds approximately). Preferably the residual vacuum in packs from the same packing line are compared one after another. The test detects the majority of leaking packs which have a relatively large aperture although the method does not detect packs with slow leaks. Slow leakage rates are in practice rare.

[0059] To test the state of vacuum of the vacuum pack container 200, it is placed in sealing engagement against the rim 210 of a test cup or chamber 212, which provides with the pack a cavity 211, the compliant diaphragm 214 formed by the wall 203 being disposed taut across the rim and the chamnber being supported on the pack contents. A spring contact 216 attached to the side of the cavity 211 is disposed adjacent to the diaphragm as the chamber 212 is located on the pack. Movement of the contact 216 is detected by a detector 218. Alternatively, the detector is in the form of a strain gauge 220 which detects an outward force exerted on the contact 216 by the film diaphragm 214 due to pressure difference across the said diaphragm. A source of vacuum 222 is connected to the test chamber controlled by a solenoid 224.

[0060] On energising the solenoid 224 a vacuum is drawn in the test chamber 212 and is selectively increased until the detector 218 indicates diaphragm displacement. This occurs when the test vacuum is just in excess of the vacuum in the pack. Alternatively, on application of the vacuum the strain gauge 220 indicates the force exerted on the contact 216 by the film diaphragm. This indicates the pressure differential across the diaphragm 214 so that, in the case where a leak is present, as air is drawn into the container pack the vacuum therein diminishes and the pressure differential between the pack and the chamber is therefore less than in the case where no leak is present.

[0061] From a series of test packs, a running mean pressure in a test series, which causes diaphragm displacement, is readily detected. This enables a running threshold to be set below which, if an individual pack is found to indicate diaphragm displacement, a leakage is indicated. At the same time a mean running value of the pressure at which diaphragm displacement is indicated provides detailed statistics of the process uniformity.

Claims

1. An hermetically sealed container having a first dished sidewall formed with a peripheral flange, a second sidewall having a peripheral part which engages the peripheral flange of said first sidewall to form a clpsed storage chamber between said sidewalls, and a seal formed between said engaging peripheral flange and peripheral part, said seal comprising two regions of sealing material extending one alongside the other provided on one at least of said engaging peripheral flange and peripheral part which bond together said flange and said part, and evacuated cavity means provided by channel means formed in one at least of said peripheral flange and peripheral part and separating said regions of sealing material.

2. A container as claimed in Claim 1, characterised in that said channel means comprise an endless channel.

3. A container as claimed in Claims 1, characterised in that said channel means comprise a channel and an enlarged channel part with which opposite ends of said channel communicate, said enlarged channel part enabling testing of said seal.

4. A container as claimed in Claim 3, characterised in that adjacent said enlarged channel part is a depression formed in a compliant part of one of said peripheral flange and peripheral part within the sealing region located outwardly of said channel.

5. A container as claimed in Claim 4, characterised in that said depression is formed with an aperture for connection to the interior of the depression of vacuum drawing means.

6. A container as claimed in any one of the preceding claims, characterised in that said storage chamber contains preservative gas.

7. A container as claimed in any preceding claim, characterised in that said second sidewall is dished and said peripheral part thereof comprises a flange and said channel means comprise channel parts formed respectively in facing parts of the flanges of said first and said second sidewalls.

8. A container as claimed in any preceding claim, characterised in that one at least of said sidewalls comprises laminated plastics film.

9. A container as claimed in any preceding claim, characterised in that one at least of said sidewalls comprises a laminate having layers of plastic foil, metal foil and thermoplastic foil.

10. A container as claimed in any preceding claim, characterised in that said regions of sealing material are provided in a layer of thermoplastic material laminated to said one at least of the peripheral flange and peripheral part.

11. A container as claimed in Claim 10, characterised in that said regions are provided in respective layers of thermoplas tic material laminated to the peripheral flange and the peripheral part.

12. A container as claimed in any preceding claim, characterised in that said channel means are provided by opposed channel parts respectively formed in the peripheral flange and the peripheral part.

13. A container as claimed in any one of Claims 1 to 12, characterised in that said channel means contain spring means which closely engage and thereby impart a profile to a diaphragm defining part of said channel means, said profile being removed if predetermined pressure increase takes place in said channel means.

14. An hermetically sealed container comprising a tray of plastics material formed with a peripheral flange and a lid for said tray formed of plastics film overlying said tray and having a peripheral part formed with an endless channel therein and secured by a seal to said peripheral flange, said seal comprising two regions extending along said channel on opposite sides thereof and provided on at least one of said peripheral flange and peripheral part and said channel being at least at atmospheric pressure to enable testing of said seal.

15. A container as claimed in Claim 14, characterised in that means disposed in said channel are adapted upon breaking of the container seal to cause an audible signal.

16. A container as claimed in Claim 15, characterised in that said channel includes a channel enlargement and said audible signal causing means are located where said channel meets said channel enlargement.

17, An hermetically sealed container comprising a top and a tray having a peripheral flange in which is formed a channel, a channel enlargement with which opposite ends of the channel communicate and adjacent said channel enlargement a depression formed with an aperture for connection thereto of vacuum forming means to enable evacuation of said channel and channel enlargement, said top having a peripheral part secured to said peripheral flange by an hermetic seal comprising two regions of sealing material extending along respective opposite sides of said channel and provided on at least one of said peripheral flange and said peripheral part and said channel enlargement enabling testing of said seal.

18. The method of testing an hermetically sealed container for seal leakage thereof, said container having an enclosing sidewall part at least of which comprises a compliant diaphragm, characterised by placing an inverted test cup having detecting means therein against said compliant diaphragm to form a closed cavity therebetween, varying the pressure in said cavity and determining from displacement of or change of force on said diaphragm embraced by said cup sensed by said detecting means whether there is leakage of said seal.

19. The method of testing as claimed in Claim 18, characterised by effecting pressure variation in said cavity by drawing a vacuum therein.

20. The method of testing as claimed in Claim 18 or 19, characterised in that said sealed container is a vacuum pack.

21. The method of testing as claimed in Claim 18 or 19, characterised in that a detecting means arc adapted to detect movement of said diaphragm.

22. The method of testing an hermetically sealed container for seal leakage thereof, said container comprising a first dished sidewall formed with a peripheral flange and a second sidewall having a peripheral part which engages said peripheral flange, one of said sidewalls including a compliant diaphragm and a seal formed between said peripheral part and said peripheral flange, characterised by placing an inverted test cup having detecting means therein against said compliant diaphragm to form a closed cavity therebetween, varying the pressure in said cavity and determining from any displacement of or change of force on said diaphragm embraced by said cup sensed by said detecting means whether there is leakage of said seal.

23. The method of testing as claimed in Claim 22, in which in said hermetically sealed container said first sidewall comprises a tray formed from plastics material which includes said peripheral flange there being formed in said flange a channel and a channel enlargement with which opposite ends of said channel communicate, and said second sidewall comprises a film of compliant material the peripheral part of which overlies and sealingly engages with said peripheral flange along opposite sides of said channel and forms a chamber with said channel enlargement, characterised by placing against said chamber in contact with said second sidewall an inverted test cup having detecting means therein, to form a closed cavity with said second sidewall, varying the pressure in said cavity and determining from displacement of or change of force on said second sidewall embraced by said cup whether there is leakage of said seal.

24. The method of testing as claimed in Claim 23, characterised by varying the pressure in said cavity by drawing a vacuum therein and monitoring the pressure in said cup until the pressure therein is lowered below the pressure in said channel thereby moving said second sidewall part embraced by said cup such movement being sensed by said detecting means.

25. The method of testing as claimed in Claim 24, in which said container encloses contents requiring cooking, characterised by performing before and after cooking of the container contents within the container said vacuum drawing and sidewall displacement detecting operations.

26. The method of testing an hermetically sealed container comprising a first sidewall provided by a tray formed from plastics material which includes a peripheral flange formed with a channel and a second sidewall having a peripheral part which overlies and sealingly engages with said flange along opposite sides of said channel, said channel being defined in part by a flexible diaphragm, characterised by placing against said diaphragm an inverted test cup having detecting means therein to form a closed cavity with said diaphragm, varying the pressure in said cavity and determining from displacement of or change of force on said diaphragm whether there is leakage of said seal.

27. The method of testing an hermetically sealed container for seal leakage thereof, said container comprising a first dished sidewall formed with a peripheral flange and a second sidewall having a peripheral part which engages and forms a seal with said peripheral flange, there being formed in said peripheral flange or peripheral part a channel containing air or gas at least at atmospheric pressure which divides said seal into separated regions extending along respective opposite sides of said channel, characterised by causing an increase in pressure in said channel and determining whether there is leakage of said seal arising from said channel pressure increase.

28. The method claimed in Claim 27, characterised by causing said pressure increase by imparting a displacement to said channel and determining whether there is leakage of said seal by checking after a predetermined interval whether said displacement has increased.

29. The method claimed in Claim 27, characterised by causing said pressure increase by applying a force to said channel and determining whether there is leakage of said seal by monitoring the force of reaction to said applied force exerted by said channel for a reduction thereof over a fixed period.

30. The method of testing as claimed in Claim 28 or Claim 29, characterised by imparting said displacement to the channel along the whole or substantially the whole length thereof.

31. The method of testing as claimed in Claim 29, characterised by monitoring said force of reaction by means of a load cell.

32. The method of testing an hermetically sealed container for seal leakage thereof, said container comprising a tray formed with a peripheral flange and a top comprising a compliant film having a peripheral part sealed to said flange, characterised by mounting said container in a test chamber containing detecting means located in the mounted position of said container for detection of movement of or change of force on said compliant film varying the pressure in the test chamber and monitoring said film for movement thereof or change of force thereon by said detecting means.

33. The method claimed in Claim 31, characterised by drawing a vacuum in said test chamber and monitoring said film for movement thereof by said detecting means.

34. The method of testing as claimed in Claim 33, characterised by employing strain gauge means for detecting movement of said compliant film.

35. The method of testing as claimed in Claim 33, characterised by employing optically or magnetically actuated detecting means for detecting movement of said compliant film.

36. The method of testing as claimed in any one of Claims 18 to 35, characterised by recording on each container tested data enabling the integrity of the container seal during testing to be ascertained.

Drawing