WO2006040595A1

WO2006040595A1 - Product packaging

Info

Publication number: WO2006040595A1
Application number: PCT/GB2005/050178
Authority: WO
Inventors: Michael Alan John Feast; Brenton John Zebedee; Ian Henry Balchin; Howard White
Original assignee: Stanelco Rf Technologies Limited
Priority date: 2004-10-16
Filing date: 2005-10-10
Publication date: 2006-04-20
Also published as: GB0423013D0; AR051328A1; TW200616853A

Abstract

A product such as meat or a ready meal (12) is packaged in a tray (14) of aluminium foil coated with a polymeric material, covered with a cover film (34), these being bonded together by dielectric heating between opposed electrodes (18, 24). 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

Product Packaging

This invention relates to a method for packaging a product in a foil tray, suitable for packaging food.

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.

According to the present invention there is provided a method for packaging a product, using a tray comprising aluminium foil coated with a polymeric material, and a cover sheet comprising a polymeric material, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes.

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 have a sufficiently high dielectric loss factor that it is heated by dielectric heating. The use of dielectric welding provides the major advantage that a good seal can be formed despite the presence of contamination on the surfaces. This is not 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.

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.

In the dielectric welding, the electrically conducting material of at least one electrode is preferably separated from the materials to be bonded by one or more layers of electrically insulating material. This layer of insulation acts as a dielectric barrier, and also suppresses heat loss from the material being welded. The electrically insulating material is preferably one that is not dielectrically heated, for example PFA (perfluoro alkoxyalkane) , or polytetrafluoroethylene (PTFE) . As another example, the electrodes may be of aluminium, and be coated with a layer of alumina (by anodising) which is impregnated with PTFE. Alternatively or additionally a sheet of insulating material such as silicone rubber, providing good thermal and electrical insulation, may be interposed between the electrodes and the material to be bonded. The layer of electrically insulating material is preferably no more than 2 mm thick, and may be between 20 and 50 μm.

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.

It will be appreciated that the foil tray may itself act as one of the electrodes. In this case the tray may be supported by a non-conducting frame, as long as means are provided for feeding RF signals to or from the tray.

For example this may be a spring contact arranged to press against the underside of the base of the tray. Such a spring contact may be connected by a lead to the RF generator, or the RF signals may be coupled into the spring contact capacitively.

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 an inductor and 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.

This may for example be such that the impedance presented to the generator remains at a constant value such as 50

Ω.

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:

Figure 1 shows a cross-sectional view of a welding apparatus for packaging a food product; Figure 2 shows a circuit diagram of the matching network of the apparatus of figure 1; and

Figure 3 shows a plan view of a packaging plant.

Referring to figure 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 50 μm thick layer 22 of PTFE, 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 50 μm thick layer 28 of PTFE.

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 which are operated so that the impedance presented to the generator 32 remains at a constant value such as 50 Ω. This ensures that despite changes in the load, excessive voltages are not applied between the electrodes (ie the lower die 18 and the upper die 24) . 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 PTFE 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 acts as a good heat conductor and so 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.

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. Referring now to figure 2, this shows the matching network 30 in more detail. The signal from the generator 32 passes through a monitoring circuit 40 (shown diagrammatically) , an inductor 42, and a variable capacitor 43, and so to the conductor 36 leading to the upper tray 20. A variable capacitor 44 connects the junction between the monitoring circuit and the inductor 42 to earth. The monitoring circuit 40 monitors the radio frequency current and voltage, and adjusts the values of the variable capacitors 43 and 44 using servo-motors 45 and 46 so that the impedance presented to the generator

32 remains at a constant value such as 50 Ω.

It will be appreciated that a welding apparatus may incorporate a different active matching network to that described above. In another modification, 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.

In another alternative, the lower aluminium die 18 is replaced by a die of an insulating material (such as PEEK) defining a recess to accommodate the tray. In this case the tray 14 acts as the lower electrode. The recess may be open at the bottom, as in the aluminium die shown in figure 1, or may be closed at the bottom (the depth of the recess being slightly greater than the depth of the deepest tray 14) . Coupling of the RF signals to the tray 14 may be by a capacitive coupling, or by a spring contact. For example the recess may be provided with a beryllium/copper spring contact in the base of the recess. It will be appreciated that this ensures that trays that are of slightly different depths do not alter the operation of the welding equipment. Coupling of the RF signals to this spring contact may again be by a capacitive coupling, or by direct electrical contact (for example using brushes) . For example the insulating die may rest on and be supported by an aluminium base plate, and the spring contact may be capacitively coupled to this base plate.

If the RF signals are to be capacitively coupled to the tray 14, this may for example utilise a metal dish into which the tray 14 nests, the metal dish matching the shape of the underside of the tray and preferably extending up the sides to within say 10 mm of the rim, and coated with a layer of electrical insulation over its entire surface, for example a 200 μm thick layer of glass-reinforced plastic or of PTFE. This capacitive dish would preferably be resiliently mounted to accommodate any variation in the height of successive trays. Such a capacitive coupling works effectively even if the underside of the tray has a coating of insulating material, such as a contaminant, on it.

The apparatus 10 of Figure 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. The dies 18 and 24 are described as having 50 μm thick coatings 22 and 28 of PTFE on the opposed welding faces, but it will be appreciated that these coatings may be of a different thickness, though typically between 20 μm and 150 μm, and that the coatings may be of a different dielectric material. Furthermore an additional dielectric barrier, such as a layer of silicone rubber 0.5 mm thick, may be provided on one or more of the welding faces; preferably such a layer of silicone rubber is attached with an adhesive on top of the coating 22 on the lower die 18, so that it remains attached to the die 18 throughout a multiplicity of welding operations, but can be removed and replaced when it becomes worn or damaged.

For example, referring now to figure 3, 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 figure 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 (TM)) 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 50 μm thick layer of insulation such as PTFE (as described in relation to figure 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. 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 figure 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 (not shown) 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.

It will be appreciated that, just as in the apparatus of Figure 1, the trays may be supported by an insulating die instead of the metal plate 52, so the trays 14 themselves act as the lower electrode; and as discussed above the RF connections to the trays may include capacitive couplings and spring contacts.

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.

Claims

1. A method for packaging a product, using a tray comprising aluminium foil coated with a polymeric material, and a cover sheet comprising a polymeric material, the method comprising bonding the cover sheet to the tray by dielectric welding between opposed electrodes .

2. A method for packaging a product as claimed in claim

1 wherein the polymeric material coating the aluminium foil is polyethylene.

3. A method as claimed in claim 1 or claim 2 wherein the electrically conducting material of at least one electrode is separated from the materials to be bonded by one or more layers of electrically insulating material.

4. A method as claimed in claim 3 wherein the electrically insulating material comprises perfluoro alkoxyalkane, polytetrafluoroethylene, or alumina impregnated with polytetrafluoroethylene.

5. A method as claimed in claim 4 wherein the electrically insulating material is of thickness between

20 and 50 μm.

6. A method as claimed in any one of the preceding claims wherein the bonding is carried out by dielectric welding using signals from a radio-frequency signal generator that is a solid-state device, and the signals are supplied via a matching network.

7. A method as claimed in claim 6 wherein the matching network is an active matching network, incorporating an inductor and at least one variable capacitor controlled by a servo motor.

8. A method as claimed in any one of the preceding claims wherein the cover sheet comprises amorphous polyethylene terephthalate.

9. A method as claimed in any one of the preceding claims wherein the foil of the tray itself acts as one of the opposed electrodes.

10. A plant for packaging food products in aluminium foil trays with peripheral rims and with a polymer coating, 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, and means to compress a cover sheet comprising polymeric material onto the rim of the or each tray, and to bond the cover sheet to the rim of the tray by dielectric welding between opposed electrodes to which signals from the signal generator are coupled.