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Technical field of application

The invention relates to the packaging sector of food products for human consumption. More specifically, the invention relates to a production process for a food tray which is particularly (but not exclusively) suitable for use in controlled atmosphere packaging.

Existing technology

In order to extend the expiry date of a packaged food product, it is usual to place the foodstuff in a tray with a hermetic seal. Without a hermetic seal to the tray, contamination of the foodstuff is possible due to the presence of air, which can accelerate the degradation process.

For foodstuff packaging, a shaped tray is generally used, comprising a bottom, raised side walls and a perimeter edge flange, adapted to define a compartment for containing food.

Said perimeter edge flange is normally a flange with a horizontal edge turned towards the outside of the containment compartment, and necessary for attaching a lid or a sealing film.

To ensure a reliable seal against leaks of liquids or gas to or from the inside, a flat, step-free flange needs to be used, so that the sealing film can be sealed along the entire length of the circumferential flange.

Most trays used for food packaging are typically produced by thermoforming a sheet of plastic. In this case, the thermoforming process requires a sheet of plastic to be heated and pushed into a mould, and then cooled so that it maintains a tray shape over time. After being filled with food, the tray is hermetically sealed using a sealing film.

Recently, to tackle the problem of pollution and reduce the use of plastic, there in an increase in demand for food trays made of paper, flat cardboard or more generally cellulose fibre.

The most traditional process for the production of cardboard food trays involves the following phases:

- providing a sheet of cardboard;

- applying a thermo-adhesive substance to areas of said sheet;

- die-cutting and creating creases on said sheet;

- folding said die-cut sheet along said creases and overlapping flaps of previously adhesivised cardboard to form a shaped tray, while simultaneously applying a source of heat to reactivate said thermoadhesive substance;

- possibly lining the inside of said shaped tray with a film of plastic material, necessary in the case where the tray is used for food packaging in a controlled atmosphere where the compartment for containing the solid and/or liquid foodstuffs is filled with nitrogen.

Disadvantageously, the side walls and perimeter edge flange of the tray have overlapping parts, which are stuck together to keep the sheet of cardboard folded in the shape of the tray. These overlapping parts create steps and areas with different thicknesses that, especially along the perimeter edge flange to which the sealing film will later be attached, cannot guarantee successful sealing of the compartment and therefore hermetic sealing of the tray.

Even in the case where the tray is lined with plastic material, the aforementioned problem persists, because there is an increased risk that there will be irregularities and empty spaces between the plastic film lining said perimeter edge flange and the sealing film heat-sealed thereon to close the compartment, thereby losing the tray’s air-tight seal and allowing outside air to enter the packaging and contaminate the foodstuff.

These trays also have other disadvantages.

To save space, it is preferable to store the trays, once they have been mass produced, nested one on top of the other to form stacks. However, during storage and transport, unwanted interference often occurs between the trays in the stacks, making it difficult during subsequent packaging stages on an automatic packaging line, to pick up a single tray without jams.

Presentation of the invention

The present invention aims to overcome these drawbacks, by providing a production process for a food tray that is effective, quick and economical.

Another aim is to provide a food tray that is air-tight and water-tight, where attachment of the sealing film is reliable and safe, and to provide a tray that is economical, sturdy, can be stacked, and is not bulky.

These aims are achieved with a production process for a cardboard food tray comprising the following steps:

- providing a sheet of cardboard;

- applying a thermo-adhesive substance to areas of said sheet;

- die-cutting and creating creases on said sheet;

- folding said die-cut sheet along said creases and overlapping flaps of previously adhesivised cardboard and applying a source of heat to reactivate said thermo-adhesive substance, to form a shaped tray, comprising a bottom, side walls and a perimeter edge flange, adapted to define a compartment for containing food;

- positioning said tray in succession with other trays already shaped to create a stack of stored trays, characterised in that:

- said cardboard is a corrugated cardboard;

- said production process comprises the phase of mechanically pressing at least said perimeter edge flange so as to crush said corrugated cardboard and flatten said flange, making its surface flat even at the flaps of overlapping cardboard.

According to alternative aspects of the invention, said mechanical pressing of said flange takes place at the same time as said folding of said die-cut sheet or subsequent to the shaping of the tray.

Advantageously, said mechanical pressing of said perimeter edge flange comprises the phase of exerting a differentiated pressure along said perimeter edge flange, maximum at the flaps of overlapping cardboard and minimum or not at all on the remaining portions.

In a preferred variant, said mechanical pressing of said perimeter edge flange comprises the use of a die-type mould and a punch adapted to compress at least said perimeter edge flange, where the residual distance between punch and die, when said mould is in the closed position, is selected based on the thickness of said corrugated cardboard.

In particular, said residual distance between punch and die is less than or equal to the thickness of said corrugated cardboard.

According to a preferred variant of the invention, said production process comprises the phase of creating on said tray, by means of said mechanical pressing, a side-skirt, in other words an edge folded downwards, projecting from said perimeter edge flange, and orthogonal thereto.

In a particularly preferred variant of the invention, said production process comprises the phase of creating on said tray discontinuous protruding structures, adapted to act as spacing means between adjacent stacked trays.

According to a possible variant, said phase of die-cutting said cardboard sheet comprises the phase of making cuts, delimiting areas belonging to said perimeter edge flange, where said areas have an open outline, and said phase of forming said tray comprises the phase of folding the portions of cardboard defined by said cuts to create support teeth.

In particular, said phase of making cuts delimiting areas with an open outline involves four areas of the die-cut cardboard sheet, located at the four corners of said perimeter edge flange. According to an alternative variant, said phase of folding said diecut cardboard sheet comprises the phase of deforming defined portions of said bottom and/or said side walls, according to at least two sets of positions, to create bosses in non-corresponding positions between said two sets.

Advantageously, said phase of folding said die-cut cardboard sheet comprises the phase of deforming at least one defined portion on at least two opposing side walls of said tray near said bottom or near said perimeter edge flange.

According to a further aspect of the invention, said production process comprises the phase of applying a film of plastic material to the inner surface of the containment compartment of said tray and on said perimeter edge flange.

In a preferred variant of the invention, said corrugated cardboard comprises at least one sheet of baking paper turned towards said containment compartment of said food tray.

A further object of the invention is a food tray obtained using a production process as claimed.

The invention offers several advantages.

The flat shape of the perimeter edge flange of the tray is a fundamental characteristic for making the tray, when covered by sealing film, at least air-tight and water-tight, and gas-tight in the presence of nitrogen and an internal film of plastic material.

The compressibility of the corrugated cardboard together with the step of pressing the perimeter edge flange makes it possible to obtain this flatness, thereby eliminating the steps created by the overlapping of flaps of cardboard when forming the tray.

Pressing of the perimeter edge flange is not uniform along the flange, but is advantageously differentiated: it is clear that where the flaps are overlapping, and therefore where there is a double layer of corrugated cardboard, the pressure exerted will be greater than on those portions of the flange consisting of only a single layer of corrugated cardboard.

Using a die-type mould and punch to achieve said pressing makes it possible, with the correct sizing, to be able to diversify the pressure exerted on the die-cut sheet in the flange area and on the body of the tray, which must not be subjected to the same pressure, otherwise it could be weakened or extraction of the already formed tray could be compromised.

Since the pressure on the overlapping flaps of the flange needs to be at a maximum, while around the more central portions of the flange consisting of only a single layer of corrugated cardboard the pressure to be exerted needs to be zero, or minimal, the residual distance between punch and die will be at a maximum equivalent to the thickness of the corrugated cardboard.

The presence of the side-skirt also gives dimensional and structural stability to the tray, and particularly to the perimeter edge flange.

Once pressed, and thus weakened due to the crushing of the corrugated cardboard, said perimeter edge flange could tend disadvantageously to bend during subsequent steps to apply the thermo-shrinking sealing film: said side-skirt lends stiffness to the flange and helps prevent its deformation.

Furthermore, the side-skirt, whose flaps on the die extend beyond those of the flange, make it possible to absorb any small errors in positioning the die under the mould, thereby guaranteeing a flange with a constant width, and flatness, thus making it possible to obtain a welding surface for the tray sealing film that is always sufficient for the purpose, and a constant tray size.

The sheet of baking paper forming at least one face of the corrugated cardboard, the face that when in use constitutes the inner surface of the tray, offers the possibility to use the tray as an instrument for cooking and/or heating, both in traditional ovens and in microwave ovens, and also allows consumption of the food directly from the tray.

The pressing phase may occur either in line, during the tray production process, while it is being formed, or subsequently, with the tray already formed, prior to it being filled.

A further advantage of the solution according to the invention is that the tray obtained is created to be easily stackable in a nested manner, without the risk of interference in the stack of trays during storage and transport.

Each single tray can be easily taken from the stack, thereby allowing the use of a reliable automated process with a tray filling and packaging machine, thus reducing both the number of possible jamming situations and process execution times. The protruding discontinuous structures create spacers and support points for adjacent trays stacked one on top of the other.

The unaligned and staggered arrangement of the protruding discontinuous structures means that, during tray production phases and during subsequent packaging phases, when the trays are stored in stacks, if they are placed one on top of the other, they are not compacted together, but rather a distance is maintained between them that is useful for the grippers of machines for picking up each individual tray.

Even more advantageously, the protruding discontinuous structures, obtained by deformation of defined portions of the tray, and therefore without cuts and openings on the corrugated cardboard, make the tray integral in all its parts, and not subject to possible leaks of liquids and/or pieces of food. The integrity of the tray guarantees additional safety by removing the risk of access to the contents and tampering with the foodstuff.

Brief description of the drawings

The advantages of the invention will become more evident from the following description of a preferred embodiment thereof, given by way of a non-limiting example, and with the help of the accompanying drawings, where:

Figures 1 and 2 show an axonometric view from above and below, respectively, of a food tray obtained using the production process according to a variant of the invention;

Figures 3 and 4 show a transversal cross-section of a detail of Figure 1 , with proportions altered for clarity of representation, during two phases of the production process;

Figures 5 and 6 a side view and an axonometric view from below, of the food trays shown in Figure 1 stacked on top of each other;

Figure 7 shows an overhead plan view of an open die-cut sheet of the food tray shown in Figure 1 , during a phase of the production process according to a variant of the invention;

Figure 8 shows an overhead plan view of a food tray obtained using the production process according to a variant of the invention;

Figure 9 a cross-section along a vertical plane of the food trays according to the variant shown in Figure 8 stacked on top of each other;

Figures 10 and 11 show an overhead schematic axonometric view and a detailed vertical cross-section view, respectively, of a food tray obtained using the production process according to a further variant of the invention;

Figure 12 shows an overhead plan view of an open die-cut sheet of the food tray shown in Figure 10, during a phase of the production process.

Detailed description of a preferred method of actuation of the invention

With reference to Figures 1 -7, there is shown a cardboard food tray 1 obtained by means of a production process according to a possible variant of the invention.

Said tray 1 has a box-like shape with a substantially rectangular design, and comprises a bottom 5, four raised side walls 6 projecting from said bottom 5 that define a compartment 8 for containing food, and a perimeter edge flange 7 extending horizontally on the outside of said containment compartment 8. It is clear that said tray 1 can also have a different shape, with a polygonal or circular design.

Said perimeter edge flange 7 is used, during the actual process of packaging food products inside the tray 1 , for applying a sealing film (not illustrated) adapted to act as a lid for said tray 1 , for example by heat-sealing or gluing.

The production process involves the use of a sheet 2 of corrugated cardboard.

Excellent results have been obtained with a thin-wave type of corrugated cardboard, with thicknesses of thin paper sheets with an overall specific weight lower than the weight of traditional trays made of flat cardboard, useful for giving sturdiness and strength to the final tray.

One of the sheets of paper that constitute the corrugated cardboard, in technical jargon known as the cover, in particular the sheet that in a shaped tray 1 will be turned towards the inside of the food containment compartment 8, is a sheet of baking paper, advantageously adapted to remain in contact with the food when cooking the foodstuff directly inside the tray 1 .

In any case, various types of paper can be used to produce the sheets of corrugated cardboard.

The production process involves applying to defined areas of said sheet 2 of corrugated cardboard a thermo-adhesive substance, glue or heat-reactive paint, which in subsequent phases will allow the overlapping and stable joining of flaps of previously adhesivised cardboard and retention of the shape of the tray 1 over time. The application of a source of heat during the folding phase will enable the thermo-adhesive substance to be activated.

A fundamental phase of the production process involves the diecutting of the sheet 2 of cardboard according to a defined outline (Fig. 7) and the creation of creases 3 that define the folding lines for the components that will form the tray 1 (bottom 5, side walls 6, perimeter edge flange 7).

In the variant illustrated, the die-cut sheet 4, which will be used to create a tray with a rectangular design, comprises areas corresponding to the bottom 5, to the four side walls 6, to the portions of perimeter edge flange 7, and to four connecting corner areas 13.

With particular reference to the simplified tray illustrated in Figures 10 and 11 , the die-cut sheet 4 of cardboard (Figure 12) also comprises end flaps 16 that, when the tray 1 is assembled, will create a side-skirt 17, in other words a downward folded edge, projecting from said perimeter edge flange 7 and orthogonal thereto.

According to a possible variant of the production process that is the object of the invention, said die-cutting phase also comprises the phase of making cuts 10, at the portions of the four connecting corner areas 13 which will go to make up said perimeter edge flange 7, having an open C-shape outline. Said cuts 10 will create discontinuous structures protruding from said perimeter edge flange 7, useful for keeping the trays 1 raised and separated from each other when nested to form a stack 9.

The actual phase of forming the tray 1 first of all involves the phase of folding said die-cut sheet 4 along said creases 3 overlapping it in part.

At said connecting corner portions 13, said die-cut sheet 4 comprises flaps 14 of corrugated cardboard from the flange 7, adapted to overlap reinforcing structure 15 for joining the perimeter edge flange 7, and flaps 14’ of corrugated cardboard adapted to overlap one another to join the side walls 6.

The thermo-adhesive substance previously applied to the sheet 2 of cardboard allows the flaps 14’, and the flaps 14 overlapping the reinforcement structures 15, to remain joined to one another thereby giving the tray 1 its box-like shape.

The above description of the overlapping of the flaps 14, 14' and the reinforcement structures 15 is also valid for the simplified version of the tray 1 illustrated in Figures 10 and 11 .

Said phase of forming said tray 1 also comprises the phase of folding the portions of cardboard defined by said cuts 10. Said portions of folded cardboard thus appear as teeth 11 , projecting from the perimeter edge flange 7, and acting as a support for the tray 1 when stacked.

As is evident from the die-cutting shown in Figure 4 and from

Figures 5 and 6 illustrating a plurality of stacked trays 1 , there are four such cuts 10 and they are located in end areas of the reinforcement structures 15 of the connecting corner portions 13 of the die-cut sheet 4, which will go to make up the four corners of said perimeter edge flange 7.

It is clear that in trays with a polygonal design there may be more than four support teeth 11 , distributed along the flange 7, but still at the corners, while in trays with a circular design the teeth 11 may be distributed on the flange 7 equidistant from one another (for example there may be four teeth 11 located at the four cardinal points).

The sealing film may be applied on the flange 7 externally with respect to said teeth 11 or, should a water-tight or air-tight seal be required, the teeth 11 may be positioned externally with respect to the weld perimeter of the sealing film.

The main phase of the tray 1 production process characterising the invention is shown schematically in Figures 3 and 4, and consists of the mechanical pressing at least of said perimeter edge flange 7, so as to crush said corrugated cardboard and flatten said perimeter edge flange 7 and make its surface flat even at the flaps 14 of cardboard overlapping the reinforcement structures 15.

Should a side-skirt 17 also be present, said mechanical pressing will also include the side-skirt.

The thickness of the corrugated cardboard wave offers a space to allow differential yielding between the single-layer portion of perimeter edge flange 7 and the double-layer portion created by the overlapping flaps 14, in order to equalize the final thickness of both layers and thus obtain a flat perimeter edge flange 7 without steps.

Said mechanical pressing of said perimeter edge flange 7 therefore comprises exertion of a differentiated pressure along said perimeter edge flange 7, maximum at the flaps of overlapping cardboard and minimum or not at all on the remaining portions, in other words those single-layer portions of corrugated cardboard.

Said mechanical pressing of said perimeter edge flange 7 comprises the use of a die-type mould and a punch adapted to compress said die-cut sheet 4 at least on said perimeter edge flange 7 and possibly on said side-skirt 17.

The differentiated pressure on the flange can be obtained through the correct choice of residual distance between the mould punch and die, when said mould is in the closed position.

In detail, said residual distance between die and punch is selected based on the thickness of said corrugated cardboard 4 and is equivalent at a maximum to the thickness of said corrugated cardboard. Said mechanical pressing of said perimeter edge flange 7 may occur at the same time as said phase of folding said die-cut sheet 4 and forming the tray 1 or subsequent to formation of the tray 1 , during a pre-packaging phase.

The production process according to the invention may also comprise an additional phase of internal lining of the tray 1 with a film of plastic material (not shown). This application renders the tray 1 impermeable and air-tight, and is chosen preferably in the case of packaging in a protective atmosphere, where a gas mixture is introduced into the tray 1 before the sealing film is applied, in order to increase and prolong the conservation of food.

Said phase of applying said film in plastic material involves both the inner surface of the containment compartment 8 of said tray 1 , and said perimeter edge flange 7. In this case, the sealing film covering the tray 1 will be heat-sealed onto the film in plastic material along said perimeter edge flange 7.

The production process that is the object of the invention comprises working phases that can be carried out in sequence on a production line machine, or in a discontinuous manner with separate phases and different machines.

It is clear that a particularly advantageous possibility offered by the process according to the invention is the possibility of simultaneously conducting the phase of forming the tray 1 and the phase of pressing at least the perimeter edge flange 7 (and possibly the side-skirt 17 when present), so as to be able to provide a food packaging company with a tray that is ready to be filled and sealed, without giving the company the burden of pressing the perimeter edge flange prior to sealing with the covering sealing film.

The simultaneous phases of forming the tray 1 and pressing at least said perimeter edge flange 7 are carried out by a single die-type mould and punch, appropriately sized and calibrated to exert greater pressures in specific areas of the flange without damaging the other zones of the die-cut sheet 4.

With particular reference to Figures 8 and 9, there is shown a type of food tray 1 produced using a possible variant of the production process.

In this case, the process differs from the one described above only in terms of the phases of creating the discontinuous protruding structures adapted to act as spacing means for the stacked trays 1 .

Said discontinuous protruding structures do not involve making an opening in the corrugated cardboard but only a localized deformation. This deformation, without breakage, is also possible due to the compression capacity of the corrugated cardboard.

In detail, the production process envisages that, during the phase of folding and forming the tray 1 , there is included a phase of locally deforming defined portions of said bottom 5 and/or said side walls 6, by means of pressure, to form bosses 12.

Said bosses 12 can be achieved by means of relief or depression moulding, depending on whether the bosses 12 protrude on the inside or outside of the tray, guaranteeing a continuity of material and ease of production.

To achieve the aim of anti-compacting when stacking the trays 1 it is sufficient to position at least one boss 12 on at least to opposing side walls 6 of said tray 1 .

Said bosses 12 can be arranged alternatively along said side walls 6 near said bottom 5 or near said perimeter edge flange 7, close to the creases 3.

In the variant shown, there are four of said bosses 12 located on the two longer opposing side walls 6, and they are placed near the bottom 5 of the tray 1 .

The bosses 12 appear as structures projecting towards the inside of the tray 1 , but nothing prevents the creation of bosses 12 protruding towards the outside of the tray 1 .

In an alternative variant (not shown), a boss 12 could be located on each side wall 6.

It is clear that to avoid compacting of the trays 1 on top of each other, the bosses 12 between adjacent trays 1 need to have different, staggered positions, i.e. non-corresponding positions. The production process envisages using localised pressure to deform bosses 12 on said bottom 5 and/or said side walls 6 according to at least two sets of positions, to be carried out on two separate die-cut sheets 4, to create two trays 1 with protruding discontinuous structures 10 that are respectively staggered and non-corresponding with one another.

Lastly, the corrugated cardboard chosen and the production process that is the object of the invention also make it possible to create an eco-friendly tray 1 , with recyclable paper, and in some variants even entirely biodegradable, with consequent benefits for the environment.