PACKAGING TRAY WITH SEALING FLANGE

Abstract

 A packaging tray (10) comprising a receiving body (16) surrounded by a peripheral flange (18), the flange (18) having an upper surface (20) for receiving a sealing film, wherein the upper surface (20) includes a single continuous rib (24) raised above the upper surface (20) and extending to surround the receiving body (16).

Description

[0001] This invention relates to the field of containers and, in particular, to a packaging container for foodstuffs of the type in which a plastic tray is closed with a sealing film.

[0002] Containers used in food packaging are widely known. In many applications, in particular the packaging of meat, chicken and fish, a plastic tray is sealed with a thin lidding film. The tray material is generally polyethylene terephthalate (PET), which is transparent in its natural form, may be coloured and readily undergoes a thermoforming process. Trays that allow the packaged contents to be clearly seen and inspected can therefore be fabricated in bulk, rapidly and economically. Moreover, PET is widely recycled, making it less damaging to the environment than many alternatives.

[0003] Where PET is less than ideal is in the sealing of a film to the top of the tray. The seal in many food packaging applications is critical but it has proven difficult to meet the requirements of production line efficiency in sealing a thin film to PET. In many cases, the seal is required not only to protect the food, but also to contain a modified atmosphere within the packaging. Such Modified Atmosphere Packaging (MAP) may contain higher levels of inert gases such as nitrogen and / or carbon dioxide within the container in order to reduce food wastage by enhancing the shelf life and / or the appearance of products packed within.

[0004] For many years, a laminate of polyethylene (PE) was provided on the top surface of a PET tray. PE is a material that is easily melted in order to create a bond with an overlaid film. This is of course advantageous in a high-throughput packing plant in which there is a requirement to rapidly and reliably fix a sealing film to a filled tray. The use of PE is disadvantageous in that a PET tray with PE laminate is more difficult to recycle. The presence of PE in the recycling process causes PET to turn cloudy or milky, which renders it less desirable for packaging applications. With more emphasis being placed on the recyclability of plastic materials, there has recently been a drive to find alternative methods of sealing a PET container with a film lid.

[0005] Within the food packaging industry, the drive for innovation is subject to the constraints of the food packing process. Substantial investment is required in setting up the machinery that automates the filling and sealing processes. It will therefore be a barrier to its uptake if any new design of tray requires significant modification to the machinery in which a plant owner has invested.

[0006] EP 2 459 459 describes a PET tray with a base and continuous sidewall that has a sealing flange formed peripherally on the upper side of the sidewall. A layer of adhesive is applied to the sealing flange in order to attach the sealing film by the application of pressure and heat. By applying the adhesive to only the flange region, the proportion of contaminant added to the PET material is significantly reduced, aiding recycling.

[0007] Other prior art methods reduce contamination further and directly bond a PET film with the PET container. In theory, bonding identical materials should result in a particularly good bond. The problem with this approach is that it is necessary to melt the material in the region of the join in order to enable bonding on re-solidification. PET is a material with a relatively high melting point and so forming an adequate bond requires a longer time at elevated temperatures, or heating to a higher temperature. A packing plant running at production speed will therefore fail to seal correctly a far higher proportion of PET - PET lidded containers that similar containers with a PET - PE seal. The increased waste is a hindrance to packing with a PET sealing film. Alternative adaptations, such as reducing the speed of the packing process, result in a significant reduction in throughput and so are also not desirable.

[0008] Both these prior art approaches inevitably result a seal that is prone to weakness. There is a risk that either the PET - PET bond is insufficient as a result of the stringent process requirements or, in the drive to reduce the contamination of PE, the bonding surface area is reduced. In both cases, it is vital therefore that the bonding area is kept free of any matter that may interfere with the seal. In practice, a modern packing plant runs at a high assembly speed and it is unavoidable that during the filling of a PET tray with food to be packed, some part of the foodstuff will drip, drop, leak or splash onto the sealing flange. This spillage interferes with the bonding between film and tray, reducing the effectiveness and reliability of the seal created.

[0009] This problem is addressed in US 2021/0002018, which describes a packaging tray in which the sealing area of the flange is provided with a surface profiled with a series of projections and depressions. If there is any spillage onto the sealing surface during the packing process, the profiled surface encourages contaminating matter to flow into the depressions. The projections are raised upwards of the depressions and so present a sealing surface that is relatively free of contamination. This design of tray reduces the likelihood of any contaminant interfering with the seal between flange and film. Movement of contaminant into the depressions is further assisted by the sealing process itself. After the sealing film is placed over the top of the tray, it is conventional to form the seal by applying heat and pressure in the form of a heated sealing or bonding bar that is pressed onto the film in the region of the contact surface. The pressing action encourages further movement of any contaminant away from the projections and into the depressions, where it is retained below the sealing surface.

[0010] The surface structure disclosed in US 2021/0002018 is one in which the depressions form a series of troughs that run along the flange surface parallel with the perimeter of the tray. It is also known to align the depressions in a herringbone fashion extending from a central line of the flange.

[0011] This design however is not ideal: the sealing surface area is reduced in that the film is now only sealed to the projecting parts of the flange surface. Moreover, matter is retained in the sealing area, which can cause a discoloration of this area in the packed product.

[0012] There is accordingly a perceived need for an alternative design of packaging tray that is suitable for processing with existing packing plant machinery to an equivalent level of productivity and efficiency, and in which the sealing surface is better protected from interference in its ability to form a bond with a sealing film.

[0013] According to a first aspect, the present invention provides a packaging tray comprising a receiving body surrounded by a peripheral flange, the flange having an upper surface for receiving a sealing film, wherein the upper surface includes a single continuous rib raised above the upper surface and extending to surround the receiving body. In comparison with the prior art, the tray of this invention provides for advantageous sealing with a lidding film. In creating the seal between the tray and film, heat and pressure is applied to the flange by the sealing bar or iron. This causes the tray material at the flange surface and the film in this region to melt and, when the bar is removed, the bond is formed. This process is performed very quickly, in line with the requirement to maintain a high throughput within the plant. Inevitably, with operational speed so high, there will be the occasional tray in which the lid fails to seal properly. It is, of course, desirable to keep the failure rate to a minimum. The single rib of the tray of the present invention, provides a continuous narrow surface that protrudes above the flange. As this is clamped, the heat and pressure from the sealing bar is concentrated in this very narrow region of the flange. The result is a quick and effective seal, which is therefore less likely to fail. In addition, as the bar pushes down on the rib, any contaminating foodstuff that may have splashed or leaked onto the flange during the filling procedure, will be pushed off the rib and encouraged to move towards the flange edges. In other words, the single rib promotes the removal of contaminants from the flange and therefore from the sealing region. This is in contrast to prior art designs in which the contaminant is trapped between protrusions and so remains on the flange.

[0014] In a second aspect, the present invention provides a packaging tray comprising a receiving body surrounded by a peripheral flange, the flange having an upper surface for receiving a sealing film, wherein the upper surface includes:

a continuous rib raised above the upper surface and extending to surround the receiving body; and

first and second arrays of side ribs, each side rib extending from the continuous rib towards a respective edge of the peripheral flange.

[0015] The side ribs further assist with the removal of contaminants. Any contaminant that is either pushed from the continuous rib, or that splashes directly on to the side ribs, will be drawn either by gravity or by the clamping pressure into channels between the side ribs. As the side ribs are pressed downwards by the sealing clamp, the channels narrow and so contaminants are pushed along these channels in an outwards direction, towards and over the flange edges. Encouraging the removal of contaminants from the flange is in stark contrast to the prior art approach described in US 2021/0002018, which retains contaminant on the flange, albeit within depressions and out of contact with a sealing part of the surface. The seal effected on the prior art design is therefore over a smaller surface area (depressions storing contaminant cannot seal), making it less effective in comparison with the seal of the present invention for the same size flange. Moreover, any contaminant that is retained in the vicinity of the sealing surface during the sealing process will absorb heat that would otherwise be applied to the sealing surface, with a consequent reduction in the effectiveness of the seal. Finally, retention of the contaminant on the flange will lead to discoloration and therefore a less aesthetically pleasing packed product.

[0016] Movement of contaminants off the sealing flange is further encouraged by tapering the side ribs. It is therefore preferred that each side rib tapers from a first height at the continuous rib to a second height at the edge, the first height being greater than the second height. It is further preferred that each side rib tapers from a first width towards the continuous rib to a second width at the edge, the first width being greater than the second width. This configuration of side ribs means that the channels between the side ribs are narrower and higher in the region closer to the continuous rib. As the sealing bar is pushed downwards, the side ribs are initially squeezed outwards at a region adjacent the continuous rib, thereby narrowing the channels in this region. As clamping continues, the side ribs are progressively squeezed along their lengths, with a corresponding progression in the narrowing of the channels. As a result, contaminants within the channels are moved away from the continuous rib and pushed towards the flange edges.

[0017] The first array of side ribs may be offset from the second array of side ribs, an offset direction being along the continuous rib. In comparison with prior art flanges that have no surface patterning, both the continuous and side ribs of this present design provide additional reinforcement to the flange. Thermoformed trays have a tendency to warp if subjected to high temperatures and pressures, such as experienced while sealing a lid. Reinforcing the flange with ribs accordingly makes the tray more robust and reduces the likelihood of warping as it undergoes the sealing process. The reinforcement provided by the side ribs is improved by having them offset longitudinally, which avoids hinge points developing across the continuous rib.

[0018] The continuous rib preferably has a continuous area that protrudes above the side ribs, the continuous area having a lateral width of between 3 and 15%, and more preferably between 7 and 9%, of the width of the flange.

[0019] Within the first and second arrays, each side rib may be evenly spaced from its neighbours. Moreover, the side ribs are preferably separated by a distance of around 20 to 40% of the width of the flange. Ideally, the density of side ribs around the flange is between 87 and 89 per 100 cm, regardless of the size of the tray.

[0020] Ideally, the continuous rib is positioned substantially equidistant from the respective edges of the peripheral flange. This arrangement reduces the chances of contaminant being left on the flange and so in a position to interfere with the seal. If the rib were off-centre then contaminant that fell to the larger side would have to be drawn a longer distance to the flange edge in order to be removed from the flange. Contaminant on this side would therefore be less effectively cleared.

[0021] Preferably, the receiving body and peripheral flange are unitary in construction, the tray being fabricated from a plastic material using one of: a thermoforming process, injection moulding or blow moulding. The plastic material is preferable polyethylene terephthalate (PET).

[0022] The invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a packaging tray in accordance with an embodiment of the present invention;

Figure 2 is a representation of the sealing flange of the packaging tray shown in Figure 1;

Figure 3 is an enlarged view of the packaging tray shown in Figure 1, showing the sealing flange in more detail; and

Figure 4 is a cross-sectional view of the packaging tray, showing sealing flange detail.

[0023] With reference to Figure 1, a packaging tray 10 comprises a base 12 and upstanding sidewalls 14 that together form a receiving body 16 in which contents can be placed for packing. On top of the sidewalls 14, a peripheral flange 18 surrounds the receiving body 16, extending substantially horizontally and parallel to the base 12. The flange 18 has an upper surface 20 on which a sealing film may be placed and bonded to the surface 20 by the application of heat and pressure.

[0024] The tray 10 is generally unitary in construction, fabricated in batches by a thermoforming process from a single sheet of plastic material, typically PET. The sheet is generally of a thickness in the range 300 to 800 µm and the PET material can be transparent, or coloured. Such technology is well known in the packaging field.

[0025] Turning now to Figures 2, 3 and 4, the flange 18 is shown in more detail. Figures 2 and 3 are perspective views in line drawing and rendered representations respectively. As illustrated, the flange 18 extends longitudinally around a perimeter defined by the tops of the sidewalls 14 and laterally outwardly. At a laterally central position, a raised rib 24 runs along the upper surface 20 of the flange 18, forming a continuous structure that follows the perimeter of the tray 10. Side ribs 26, 28 extend laterally from both sides of the central rib 24 towards the edges of the flange. A first series 26 of side ribs extend from the central rib 24 to an outer edge 30 of the flange 18. A second series 28 of side ribs extend from the central rib 24 to an inner edge 32 of the flange 18. Intermediate the side ribs 26, 28 is a corresponding arrangement of gaps 34, i.e. regions that are not raised above the surface 20 of the flange. As best seen in Figure 2, the first series of side ribs 26 is offset a small amount from the second series 28 in a direction parallel to the central rib 24. Each side rib 26, 28 tapers both in height and width as it extends away from the central rib 24. That is, at the central rib 24, each side rib 26, 28 is at its maximum height above the flange surface, which is slightly less than the height of the central rib 24. The side ribs 26, 28 merge with the central rib 24 and are at their widest configuration in the vicinity of the lateral edges of the central rib 24.

[0026] Such a patterning of the sealing surface may be applied in the course of production of PET trays. During a typical production process, each tray is clamped tightly at its sealing flange as it is held in a thermoforming station. The heated tray is then shaped using a vacuum and plug combination to press it against a suitably shaped mould. At this stage of the process, a pattern etched into the clamp that holds the sealing flange will be imparted to the flange upper surface 20.

[0027] This particular design of flange upper surface 20 provides three significant advantages over the prior art. The first two relate to the sealing process itself and the third to the removal of contaminants from the sealing surface.

[0028] It is noted that the central rib 24 provides a continuous area 36, best seen in Figure 4, that protrudes above all other parts of the sealing flange 18, including the side ribs 26, 28. During a standard sealing process, the sealing film is placed on top of the tray, overlying the sealing flange 18. A hot sealing clamp then approaches the flange, moving on to its upper surface 20. The clamp remains in position for a short period, pressing the sealing film on to the flange before its movement is reversed and the heat and pressure are removed. This part of the process is particularly fast, with the seal typically being effected in a time scale of 0.5 - 2 s. In sealing a packaging tray in accordance with this invention by this method, the part of the flange that first meets the sealing clamp is the central raised rib 24. The heat and pressure from the sealing clamp is therefore concentrated in a very narrow region of the flange. The result is a quick and effective seal between the continuous area 36 of the central rib and the sealing film. As the central rib 24 encircles the packaging tray, this quick seal completely seals the contents within the tray 10.

[0029] The second advantage is that the rib patterning 24, 26, 28 provides additional reinforcement to the flange, which makes tray more robust to the sealing process. As a result of the thermoforming process by which a flat sheet of PET is moulded into a three-dimensional shape, residual strains are present in the tray. During sealing, the flange is subjected to the stresses of clamping and heating. At higher temperatures in particular, the residual strains make the tray liable to warp. The reinforcement provided by the ribs permits the tray to be subjected to higher temperatures without warping. Sealing at higher temperatures, in turn, reduces the likelihood of seal failure.

[0030] The third advantage of the sealing flange in accordance with this invention relates to the removal of contaminants, which indirectly also aids the sealing process as it is described above. In the first instance, any contaminant that falls or splashes onto the sealing surface during the packing process, will be encouraged by gravity to move off any raised portion and into the gaps 34 between the side ribs. When the tray 10 moves on to the sealing process, this is therefore where the greater part of any contaminant will be located. As the sealing clamp moves downwards onto the upper surface 20, the central rib 24 is first forced downwards, as described above, and the clamp next presses against the side ribs 26, 28. As stated previously, these ribs 26, 28 are configured to be higher and wider at the end that contacts the central rib 24. This part is therefore forced downwards first, the side ribs 26, 28 consequently splay outwards at the central-rib end, narrowing the gaps 34 therebetween. Pressure is increased in this region of the gaps 34 between the side ribs, relative to the pressure at the opposite ends (edge ends) of the ribs 26, 28. Any contaminant located between the side ribs is therefore drawn laterally outward along the upper surface 20, away from the central rib 24 towards the flange edges 30, 32. The sealing action therefore enhances removal of contaminant from the upper surface 20. Moreover, if any contaminant remains on the side ribs 26, 28, not having been drawn by gravity into the intermediate gaps 34, the increasing pressure from the sealing clamp will encourage its movement off the raised parts of the ribs 24, 26, 28 and into the lower regions 34 between.

[0031] A sealing flange in accordance with this invention is designed to draw any contaminant that is present on the flange off the sealing surface and over the edge of the tray. That is, in comparison with the prior art, contaminant is far more likely to either fall back into the tray or be removed from the tray. This allows formation of a more effective seal as well as improving the appearance of the packed product in the sealing region. The sealing film may be any of a range of materials, although it is preferred that the majority of it will be formed of PET for compatibility with a PET tray and recycling considerations. The film may be multi-layered, with some layers providing an antifog function, a printable surface, or the like.

[0032] Clearly, the size and number of side ribs 26, 28 will depend on the size of the packaging tray. Tray sizes are usually selected in accordance with the packing plant in which they are to be filled. For example, a plant may be specifically configured to fill and seal a tray of 238.5 mm by 166.5 mm, which will have a flange of 7.5 mm lateral width. Surface design can be varied however, provided that the tray can still be processed by the machinery of the packing plant. In one exemplary flange design, the central rib 24 is raised 0.3 mm above the flange upper surface 20, with its raised continuous section 36 extending substantially parallel to the surface across a width of 0.57 mm. Each side rib 26, 28 has a length of 3.2 mm and a width of that tapers from 1.4 mm at the base of the central rib 24 to 0.9 mm at the flange edges 30, 32. The height of the side ribs 26, 28 also tapers: from 0.3 mm, in accordance with the central rib height, to 0 mm at the flange edges 30, 32. Separation between side ribs 26, 28 (pitch) is 2.27 mm. It is envisaged that this pitch value will not change significantly between tray sizes, the preferred density of side ribs being 88 (± 1) ribs per 100 cm. This arrangement is found to perform best with regard to contaminant removal and flange reinforcement.

Claims

1. A packaging tray (10) comprising a receiving body (16) surrounded by a peripheral flange (18), the flange (18) having an upper surface (20) for receiving a sealing film, wherein the upper surface (20) includes a single continuous rib (24) raised above the upper surface (20) and extending to surround the receiving body (16).

2. A packaging tray (10) comprising a receiving body (16) surrounded by a peripheral flange (18), the flange (18) having an upper surface (20) for receiving a sealing film, wherein the upper surface (20) includes:

a continuous rib (24) raised above the upper surface (20) and extending to surround the receiving body (16); and

first (26) and second (28) arrays of side ribs, each side rib extending from the continuous rib (24) towards a respective edge (30, 32) of the peripheral flange (18).

3. The packaging tray (10) of claim 2 wherein each side rib (26, 28) tapers from a first height at the continuous rib (24) to a second height at the edge (30, 32), the first height being greater than the second height.

4. The packaging tray (10) of claim 2 or 3 wherein each side rib (26, 28) tapers from a first width towards the continuous rib (24) to a second width at the edge (30, 32), the first width being greater than the second width.

5. The packaging tray (10) of any one of claims 2 to 4 wherein the first array of side ribs (26) is offset from the second array of side ribs (28), an offset direction being along the continuous rib (24).

6. The packaging tray (10) of any one of claims 2 to 5 wherein the continuous rib (24) has a continuous area (36) that protrudes above the side ribs (26, 28), the continuous area (36) having a lateral width of between 3 and 15% of the width of the flange (18).

7. The packaging tray (10) of claim 6 wherein the continuous area (36) has a lateral width of between 7 and 9% of the width of the flange (18).

8. The packaging tray (10) of any one of claims 2 to 7 wherein within the first (26) and second (28) arrays, each side rib is evenly spaced from its neighbours.

9. The packaging tray (10) of claim 8 wherein the side ribs are separated by a distance of around 20 to 40% of the width of the flange (18).

10. The packaging tray (10) of claim 8 wherein the density of side ribs (26, 28) in each array is within the range 87 to 89 per 100 cm.

11. The packaging tray (10) of any preceding claim wherein the continuous rib (24) is positioned substantially equidistant from the respective edges (30, 32) of the peripheral flange.

12. The packaging tray (10) of any preceding claim wherein the receiving body (16) and peripheral flange (18) are unitary in construction.

13. The packaging tray (10) of claim 12 wherein the tray is fabricated from a plastic material.

14. The packaging tray (10) of claim 13 wherein the tray (10) is fabricated by a thermoforming process, injection moulding or blow moulding.

15. The packaging tray (10) of claim 13 or 14 wherein the plastic material is polyethylene terephthalate (PET).

16. The packaging tray (10) of claim 1 wherein the upper surface (20) further comprises first (26) and second (28) arrays of side ribs, each side rib extending from the continuous rib (24) towards a respective edge (30, 32) of the peripheral flange (18).

Drawing