Product packaging

Abstract

A product such as meat or a ready meal (12) is packaged in a tray (14) covered with a cover film (34), these being bonded together by dielectric heating between opposed electrodes (18, 24). The dielectric heating is carried out through at least one barrier layer (22, 28) comprising a foam material of a polymeric material that is not susceptible to RF. The film (34) may be of laminated form, for example comprising a polymer such as APET laminated to a bonding layer of polyethylene. This welding process avoids heat wastage, it can provide narrower seals, and enables a good seal to be obtained despite contamination of the surface of the tray.

Description

This invention relates to a method for packaging a product in a tray, suitable for packaging food, and to an apparatus for performing this method.

The packaging of foodstuffs in an aluminium foil tray is known, and some such foil trays are coated with a polymer coating. Such a polymer-coated foil tray may be covered and sealed by a sheet of a plastic material that can be heat sealed to the rim of the tray. The polymer used to coat the foil is typically polyethylene. To enable heat sealing, the cover sheet would typically be of laminated form, incorporating a heat-sealable layer of for example polyethylene. It may also incorporate layers of other polymers, such as amorphous polyethylene terephthalate (APET), polyvinylchloride (PVC), polyamide (PA), or polyvinylidene chloride (PVdC). Although heat sealing is widely used, it has disadvantages, particularly the problems encountered where there is contamination on the rim, and the inevitable heat losses from the hot welding elements.

The packaging of foodstuffs in a plastic tray is also widely known, and a suitable material for such a tray is the polyester, polyethylene terephthalate (PET), which may be either crystalline (CPET) or amorphous (APET). Other tray materials include polypropylene, or polyvinyl chloride (PVC). The selection of material for the tray depends upon the nature of the contents, and so on whether the tray has to be heat resistant. For example polypropylene trays can be used for foodstuffs which are to be microwaved, whereas for foodstuffs to be heated in a conventional oven CPET trays are preferable. Such a tray may be covered and sealed by a transparent sheet of a plastic material that can be heat sealed to the rim of the tray. And as with an aluminium tray, to enable heat sealing, the cover sheet would typically incorporate a heat-sealable layer of for example polyethylene, and may also incorporate layers of other polymers, such as APET, PVC, PA, or PVdC. Similarly, to enable heat sealing, the tray is also typically of laminated form, with a heat-sealable layer on its upper surface; such a heat-sealable layer may be referred to as a surface bonding layer. The cost of the cover sheet for such packages would be less, and recycling easier, if the surface bonding layer could be omitted. These benefits would also arise from using a tray with no surface bonding layer.

According to the present invention there is provided a method for packaging a product, using a tray and a cover sheet, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes, wherein at least one of the electrodes is covered by a barrier layer comprising a foam material of a polymeric material.

The invention also provides an apparatus for performing such a method.

Preferably this foam layer is self-adhesive, so that it can be fixed to the electrode. Polymeric materials that are not significantly susceptible to RF include PTFE, polypropylene and polyethylene, but the preferred material for the foam is acrylic. The suitability of this foam material is contrary to previous expectations that barrier materials should be non-porous; experience had been that small air gaps sometimes led to sparks and arcing, with consequential damage to the material being sealed. The foam structure does not appear to give this problem. Preferably the polymeric material of the foam structure has a low dielectric loss factor (which is the product of the dielectric constant and the loss tangent), so it does not get hot; for example values of loss factor less than about 0.05 are considered to be low, and such materials are not considered to be weldable, while materials with a loss factor less than 0.01 would be considered to be inactive to RF.

Preferably the foam layer is itself covered with a a non-porous polymer layer of a polymer that has a low susceptibility to RF, and a suitable material for this purpose is a polyimide tape. This too may be self-adhesive.

This dielectric bonding process provides the major advantage that a good seal can be formed despite the presence of contamination on the surfaces. This is not usually possible by heat sealing. For example, contamination by fats, oils (such as tuna fish oil), or aqueous solutions, do not prevent the formation of a good quality seal. It is not necessary for there to be any surface bonding layer on a polymeric tray, or any adhesive.

When dealing with a foil tray, it will be appreciated that the polymeric material on the foil, and/or the polymeric material of (or on the sealing surface of) the cover sheet, must of course have a sufficiently high dielectric loss factor that it is heated by dielectric heating. The use of dielectric welding to provide the seal provides the major advantage that a good seal can be formed despite the presence of contamination on the surfaces.

Although the two items are referred to as a tray and a cover sheet, the cover sheet may comprise a layer of foil as well as a layer of polymeric material. However, preferably the cover sheet is much more flexible than the foil tray. The cover sheet may also comprise a layer of paper or card, bonded to a suitable polymeric material.

The foam layer is preferably no more than 2 mm thick, and may be about 1 mm thick.

The radio frequency supply may in principle be at a frequency between 1 MHz and 200 MHz, usually between 10 MHz and 100 MHz, but stringent limits are imposed on any emitted radio waves. In practice therefore the choice of frequency may be more limited. For example the supply frequency may be 27.12 MHz, or 40.68 MHz.

Preferably the radio-frequency signal generator is a solid-state device, and the signals are supplied via a matching network. The matching network preferably is an active matching network, incorporating at least one variable capacitor controlled by a servo motor; it monitors the radio frequency current and voltage, and automatically adjusts the value of the or each variable capacitor in accordance with variations in the load.

The invention also provides a plant to seal film to trays by this method; this may be achieved by modifying a conventional plant.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a cross-sectional view of a welding apparatus for packaging a food product; and

FIG. 2 shows a plan view of a packaging plant.

Referring to FIG. 1, a welding apparatus 10 is shown, partly diagrammatically, for packaging a food product 12 (such as a ready meal) in a stiff, generally rectangular tray 14 of aluminium foil with a surface bonding layer of polyethylene, that has rounded corners and a peripheral lip 16. The apparatus 10 includes a lower aluminium die 18 which defines a generally rectangular aperture 20 into which the tray 14 locates, and the upper surface of the die 18 is coated with a 1 mm thick barrier layer 22 (described below), so that when the tray 14 is located in the aperture 20 its lip 16 is supported by the upper surface of the layer 22 on the die 18. An upper aluminium die 24 has a 5 mm deep recess 25 of the same shape as the aperture 20, surrounded by a ridge 26 with a flat lower surface which is also coated with a barrier layer 28.

The barrier layers 22 and 28 both consist of a high-strength VHB (trade mark) double coated acrylic foam tape, of thickness 1 mm, stuck to the respective die 18, 24, and covered by a strip of polyimide film such as Kapton (trade mark) of thickness 50 μm (0.002 inches). The acrylic material has a dielectric loss factor of 0.028 (at 1 kHz), which is low, while that for the polyimide is 0.0018 (at 1 kHz), so the polyimide is inactive to RF.

The lower die 18 is earthed, while the upper die 24 is connected via a conductor 36 through an active matching network 30 to a solid-state RF generator 32. The matching network 30 incorporates variable capacitors controlled by servomotors. This ensures that despite changes in the load, the circuit remains tuned to the operating frequency. The earth to which the lower die 18 is connected is the earth of the matching network 30.

In use of the apparatus 10, a tray 14 containing a food product 12 is located into the aperture 20. A film 34 of amorphous polyethylene terephthalate (APET) with a coating of polyethylene on its lower surface is placed on top of the tray 14, and the upper die 24 is lowered so that the film 34 and the lip 16 are sandwiched between the barrier layers 22 and 28 on the dies 18 and 24. The generator 32 is then activated (for example for 1.5 seconds), such that the polyethylene of the film 34 is welded to the polyethylene coating the lip 16 of the tray 14. The upper die 24 is then lifted up, and the sealed tray 14 bonded to the film 34 is removed.

This sealing process is surprisingly effective, despite the presence of the aluminium foil which is a good heat conductor and so acts as a heat sink. The effect of the radio frequency is to generate heat within the coatings of polyethylene on the foil tray 14 and on the cover film 34, and so to bring about welding at the interface; unlike heat sealing there is no requirement for heat to diffuse into the polymer, and indeed the temperature will be at its highest in the vicinity of the interface. The foam barrier layers 22 and 28 may also become warm from the effect of the RF, which helps to reduce heat loss from the materials that are being welded.

It should be appreciated that the sealing film 34 may be a multi-layer laminate, with APET providing strength, and one or more other polymers such as ethylene/vinyl alcohol copolymer (EVOH) laminated to it to provide or enhance particular properties such as oxygen impermeability. For example the film 34 might comprise an upper layer 15 μm thick of APET; a 3 μm thick oxygen barrier layer of EVOH; a lower layer 15 μm thick of APET; and a 50 μm thick layer of polyethylene; these layers may be bonded together by thin layers of adhesive. The sealing process is remarkably effective, and can provide a good seal despite the presence of contamination on the lip 16, such as traces of blood, fat or oil from the product 12.

Although the foil tray 14 is described as having a surface bonding layer of polyethylene, this bonding layer may comprise different organic materials, for example being a mixture of polymers, such as a modified ester of succinic acid of vegetable origin.

It will be appreciated that a welding apparatus may differ from the described above, for example the upper die 24 may be connected to earth, and the lower die 18 be connected to the RF generator 32 via the conductor 36 and the network 30.

The apparatus 10 of FIG. 1 is shown as having a single aperture 20 so that a single tray 14 can be sealed in one operation. In a modification a lower die might provide a plurality of apertures so that a plurality of trays 14 can be supported, the upper die providing a corresponding plurality of projecting flat ridges so that the lips 16 of the trays 14 are sandwiched between the dies. In each case the barrier layers 22 and 28 are provided on each die, and remain attached to the die throughout a multiplicity of welding operations; they can be removed and replaced when they become worn or damaged.

For example, referring now to FIG. 2, there is shown a plan view of a conventional foil tray sealing plant modified so that the sealing can be performed in accordance with the present invention. The plant 50 comprises a continuous belt made up of rectangular aluminium plates 52 each defining four apertures 54 to locate trays 14 (as shown in FIG. 1), the plates 52 being linked together by chains 56 on each side of the belt. Trays are placed in each aperture 54, and are filled with the desired food product; the plates 52 carrying the filled trays move (in the direction of the arrow A) step-wise, passing under a covering module 58 in which the trays are covered by a cover film, and the cover film is sealed to the lip of each tray. The covering module 58 seals the cover film to the four trays in one plate 52, this sealing operation taking a few seconds, and then the next plate 52 is moved into position under the covering module 58.

Conventionally such a tray sealing plant would utilise heated plates and high pressure to bring about the sealing. In contrast the present invention uses dielectric heating to form the seal. Each plate 52 is supported by an insulating frame 60 of acetal (e.g. Delrin™) strips, those at the end being 25 mm thick, so it is electrically isolated. Each plate 52 is machined so as to define a 7 mm wide flat-topped rim 62 around each aperture 54 which projects 3 mm above the remainder of the plate 54, and this rim 62 is covered with a 2 mm thick acrylic foam layer covered with polyimide film (as described in relation to FIG. 1). The covering module 58 is also electrically isolated from the remainder of the plant 50 by insulating acetal blocks. The covering module 58 is connected to the earth of the matching network 30. As in the apparatus of FIG. 1, the covering module 58 includes an upper plate which defines ridges that correspond to the rims 62, and these ridges are also covered with an insulating barrier layer of 2 mm thick acrylic foam covered with polyimide film. A copper spring strip (not shown) contacts the underside of the plate 52 carrying the trays undergoing sealing, the copper spring strip being connected via a conductor 36 and a matching network 30 to a signal generator 32 (as described in relation to FIG. 1).

Thus as each plate 52 moves into position under the covering module 58, the covering module 58 presses the cover film on to each tray, and the lips of the trays and the cover film are sandwiched and compressed between the rims 62 and the corresponding ridges in the upper plate which forms part of the covering module 58. The signal generator 32 is activated for 2 seconds so that an RF signal is applied between the earthed upper plate forming part of the covering module 58, and the plate 52 which is live. This seals the film to the lip of each tray. The total dwell time may be 5 seconds, so that the welded seal is held compressed for another 3 seconds after application of the RF energy. The pressure is then released and the next plate 52 in the belt moved into position.

When packaging food products this plant 50 avoids the inevitable energy wastage associated with the use of conventional heated elements. In addition it enables a good seal to be formed despite the presence of water, fat or other contaminants on the lip of the tray. It does not require high pressures compressing the film onto the lip, and for example a pneumatic air supply at 80 psi (530 kPa) is ample compared to almost twice that air pressure as used conventionally; this may supply air to a 6 inch bore cylinder, providing a total load of about 1000 kg. The total sealing area for each plate 52 may for example be about 100 cm². It has been found to provide better results in burst tests than conventional heat sealing, and the proportion of sealed trays where there is a leak is significantly less than with conventional heat sealing: the proportion of leakers can be less than 1%, whereas with conventional heat sealing it may be as many as 5%, and as much as 20% where the food product includes a sauce (because traces of sauce on the rim prevent heat sealing). Because the heat is generated within the polymeric material, the trays and their contents are not significantly heated, so there is a much shorter curing time (as the welded material cools down), and so the welding process is markedly quicker than with heat sealing. Furthermore the quality of the seal is not significantly affected even if the trays are subjected to a rapid freezing procedure immediately following the welding step. If the strength of the seal is to be adjusted, for example to enable the film to be peeled off subsequently, this may be achieved by changing the power supplied by the signal generator 32.

Because of the improved seal produced by the present invention, the width of the seal can be reduced from the conventional value of about 6 mm down to 3 mm; indeed the width could be further reduced to less than 1 mm, for example 0.5 mm or less. This enables smaller rims to be used, saving material, transport costs and shelf space. Indeed the tooling profile (of the opposed electrodes) may be modified so as to create a seal that is designed to fail at a particular location around the rim, for example to avoid the need to puncture the film before heating the contents in a microwave oven.

Although the invention has been described above in relation to sealing a film to an aluminium foil tray, it will be understood that it is equally applicable when sealing a film to a polymeric tray, for example of polyethylene terephthalate.

Claims

1. A method for packaging a product, using a tray and a cover sheet, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes, wherein at least one of the electrodes is covered by a barrier layer comprising a foam material of a polymeric material.

2. A method as claimed in claim 1 wherein the foam layer is self-adhesive.

3. A method as claimed in claim 1 wherein the foam layer is of acrylic foam.

4. A method as claimed in any claim 1 wherein the foam layer is itself covered with a non-porous polymer layer.

5. A method as claimed in claim 1 wherein the tray comprises aluminium foil.

6. An apparatus for packaging a product, using a tray with a rim, and a cover sheet, the apparatus comprising a die defining a recess to locate the tray, the die acting as an electrode, and an opposed electrode whereby the rim and the cover sheet can be located between the die and the opposed electrode to be subjected to dielectric welding, the apparatus also comprising means to supply RF signals between the die and the opposed electrode, wherein at least one of the electrodes is covered by a barrier layer comprising a foam material of a polymeric material.

7. An apparatus as claimed in claim 6 wherein the foam material is an acrylic foam.

8. An apparatus as claimed in claim 6 wherein the foam layer is itself covered with a non-porous polymer layer.

9. A method of dielectric welding using opposed electrodes, wherein at least one of the electrodes is covered by a barrier layer comprising a polymeric foam material.

10. A plant for packaging food products in trays, wherein each tray is supported in an aperture in a plate, wherein each plate is electrically insulated from the remainder of the plant, the plant also incorporating a radio-frequency signal generator, means to connect the plant to earth and means to connect the plate to the signal generator, so the trays are welded to a cover film by dielectric welding, wherein the dielectric welding is carried out through a barrier layer comprising a foam material of a polymeric material.