Method for producing a tray for food products

Field of application

0001The present invention generally refers to the field of plastic materials for the food packaging industry.

0002In detail, the invention regards a sheet of plastic material, a tray for the holding of a food product obtained from such sheet of plastic material, a packaging comprising a such sheet in particular for packaging a food product in a protective or modified atmosphere and a method for producing them.

Prior Art

0003It is known that the food packaging sector is particularly careful to the consumer needs and, therefore, several new packages are constantly released into the market.

0004Packages comprising a tray or bowl of foamed polystyrene are widely available for solid food products such as meat, fish, cheese, fruits and vegetables as well as oven, food shops and confectionary products.

0005In particular, trays essentially made of a single layer of foamed polystyrene are used. Such trays serve as simple containers for holding the packaged food product, or used are trays having opened cells structures which allow, among others, also the absorption of the liquids released by the food, especially meat and fish.

0006In this type of packages, closure is generally obtained by winding an extensible plastic film around a tray holding the food product.

0007In such case, the shelf-life of the food package thus packaged, lasts only a few days, generally from two to four days.

0008Furthermore, known are packages sealed in a modified or protective atmosphere; such packages extend the periods of preservation of the food product held therein.

0009Such type of package essentially comprises a tray made of a foamed polystyrene layer generally coated in the inner part by a first plastic film having barrier properties against gases or vapours, which is in turn sealed along a peripheral strip to a second plastic film having barrier properties, in such a manner to entrap a preset gas or gas mixture within the package between the two plastic films.

0010A distinctive characteristic of the trays with barrier properties consists in having slightly inclined (almost vertical) side walls, in order to increase their capacity and that is their internal volume.

0011Such solution is necessary given that the content of the package, that is the food product arranged in the tray, cannot project beyond the peripheral upper edge of the tray itself sealed on which is the closure plastic film, as it usually occurs on the contrary in the packages comprising closed trays by wrapping using an extensible film.

0012In any case, such trays, in all the cases considered beforehand are quite standardised comprising a foamed portion with a very low density, generally comprised in the range between 30 and 50 grams/litre and in any case not exceeding 60 grams/litre, and thicknesses of the walls comprised between 3.5 and 6 millimetres which are capable of ensuring a god rigidity of the tray using small amounts of plastic material.

0013As known, such trays are obtained by means of thermoforming a sheet made of a foamed polystyrene layer coupled with a plastic film having barrier properties, in which the polystyrene layer (foamed portion) has a density comprised between 40 and 80 grams/litre.

0014In cases where trays of foamed plastic material are not used, the known technique also provides for packages of the type in modified or protective atmosphere, comprising trays essentially made of polypropylene, polyethylene terephthalate, high impact polystyrene or other non-foamed resins which allow making transparent, opaque and coloured products.

0015Though advantageous and meeting the objective, trays of the type considered above are not free of drawbacks which in the specific case should be linked to the characteristics which determine its advantages.

0016In particular, the packages comprising trays of foamed polystyrene guarantee a certain degree of thermal insulation, useful when transporting the package, for example, from the point of purchase to home.

0017Furthermore, such trays are made with a limited amount of plastic material which is less, given the same capacity of the tray (content), than the one used for trays made of non-foamed plastic material, which - alongside a lower unit cost - also entails a lower environmental impact and a lower exploitation of resources in general.

0018On the contrary, right due to the fact that they are made of foamed plastic material, such trays are particularly bulky thus leading to a negative impact on storage and transport.

0019Furthermore, in case of trays intended for packaging in a protective or modified atmosphere, the overall dimensions required, for storing, stacking and transporting the trays, is even greater due to the lower inclination of the walls as described previously.

0020Furthermore, given that the packages in a protective or modified atmosphere are generally packaged, that is completed, in the factories instead of being completed at the large-scale sales points where the package can be purchased, often desired of the trays used is an easy unstacking on automatic systems which, on the contrary, the trays made of foamed plastic material provided by the known art are not capable of ensuring.

0021As a matter of fact, the walls of such type of tray are quite thick and slightly inclined (almost vertical) walls and, therefore, it is arranged in a disorderly manner in vertical rows or stacks, making the automatic unstacking in the packaging process quite complicated.

0022On the other hand, with regard to the packages in modified or protective atmosphere comprising trays made of non-foamed plastic material, it should be noted that the latter are less bulky with respect to the former and they therefore have a smaller overall dimension, with the ensuing advantage in terms of storage and transport.

0023Furthermore, such trays are also generally quite orderly when arranged in vertical stacks and thus they do not entail particular unstacking difficulties in food package industrial production lines.

0024Nevertheless, trays of this type are subject to the drawback implying that in order to obtain the rigidity merely sufficient to allow its use, and in any case undoubtedly lower than the one of the trays made of foamed plastic material, an amount of plastic ranging between 30% and 100% higher is used, with the consequent increase of costs, greater environmental impact and generally a higher exploitation of resources.

0025Furthermore, trays of this type do not guarantee not even the slightest thermal insulation of the product held therein, an insulation which - as observed beforehand - is often required by the consumer.

0026Therefore, it would be desirable to have a tray having the positive characteristics of the known trays made of foamed plastic material and the positive characteristics of the known trays made of non-foamed plastic material, which is simultaneously capable of overcoming the abovementioned drawbacks or disadvantages.

Summary of the invention

0027A first objective of the present invention is that of providing a tray for a food product, in particular for a package in a modified or protective atmosphere, provided with a good structural rigidity and dimensional stability, additionally capable of allowing an accurate forming even when it comes to small details.

0028A second objective of the present invention is that of providing a tray of the abovementioned type, capable of ensuring both an orderly and tidy stacking as well as having a small volumetric overall dimension, and an easy unstacking in particular on the automatic systems when making food packages in a modified or protective atmosphere.

0029A third objective of the present invention is that of providing a tray of the abovementioned type which has a limited weight and therefore a limited production cost and a limited environmental impact and which is capable of guaranteeing a given thermal insulation.

0030A further objective of the present invention is that of providing a package in a modified or protective atmosphere for a food product comprising a tray of the type considered above.

0031These and further objectives according to the invention are meet by a sheet of plastic material comprising a foamed extruded polystyrene layer coupled with a flexible plastic film having barrier properties against gases, characterised in that said foamed extruded polystyrene layer has a density comprised between 150 and 350 grams/litre, preferably between 180 and 220 grams/litre, even more preferably about 200 grams/litre, and a thickness equivalent to or smaller than 2.2 mm.

0032In detail, according to the present invention provided is a sheet of plastic material of the abovementioned type, a tray for the holding of a food product obtained by means of thermoforming - from such sheet of plastic material - a package in a protective or modified atmosphere for a food product comprising a such tray and a related manifacture method Further characteristics and advantages of this invention shall be clearer from the description of some embodiments of a sheet of plastic material, a tray for a food product obtained from such sheet and a package in a protective or modified atmosphere for a food product comprising a such tray, provided hereinafter with reference to the attached drawings provided for illustrative and non-limiting purposes.

Brief description of the drawings

0033In such drawings: <ul id="ul0001" list-style="dash"><li><figref idref="f0001">Figure 1</figref> shows, represented in a schematic manner, three vertical rows made of a same number of trays stacked respectively corresponding to: a) trays of the known type, made of foamed plastic material, having barrier properties against gases; b) trays according to the present invention, made of foamed plastic matrial, having barrier properties against gases; c) trays of the known type, made of non-foamed plastic material, having barrier properties against gases.</li><li><figref idref="f0001">Figure 2</figref> shows an enlarged cross-sectional view, respectively, of: a) a part of the tray of <figref idref="f0001">figure 1-a</figref>); and b) a part of the tray of <figref idref="f0001">figure 1-b</figref>).</li></ul>

Detailed description

0034Referring to the abovementioned <figref idref="f0001">figures, 1</figref> indicates a tray for a food product according to the present invention.

0035In particular, the tray 1 is illustrated in <figref idref="f0001">figure 1-b</figref>) stacked in a vertical row and compared with stacks of trays of the known type; while shown in <figref idref="f0001">figure 2</figref> is a portion of some of the abovementioned stacks in which the trays are illustrated according to a cross-sectional view.

0036The tray 1 is of the bowl type and it essentially comprises a bottom 2 and side walls indicated with 3 having a raised peripheral edge 4.

0037The tray 1 is essentially made of a foamed extruded polystyrene layer and of a plastic film of the known type having barrier properties against gases or vapours, such film being coupled with said foamed extruded polystyrene layer at the internal surface of the tray, not shown in the figures.

0038A such plastic film is for example made of a multilayer plastic film in which at least one of the layers is made of EVOH (ethyl vinyl alcohol) or PVDC (polyvinylidene chloride), or other polymers or polymer mixtures with low permeability to gases or vapours with a thickness comprised between 2 and 12 microns, in such a manner to be impermeable to gases.

0039Therefore, the tray 1 is particularly indicated to packaging a food product in a protective or modified atmosphere, as better observed hereinafter in the description.

0040According to the invention, the tray 1 is obtained by means of thermoforming a sheet comprising a foamed extruded polystyrene layer, on whose side or surface thereof the abovementioned plastic film impermeable to gas is coupled, whose foamed extruded polystyrene layer has a density comprised between 150 and 350 grams/litre, preferably comprised between 180 and 220 grams/litre, or even better about 200 grams/litre and a thickness equal or smaller than 2.2 millimetres.

0041Thermoforming a sheet of plastic material, just like coupling a flexible plastic film on a plastic layer of foamed plastic material, are performed according to conventional techniques well known to the known art and, thus, not described here in detail unless the details are useful to a man skilled in the art to understand the invention.

0042With regard to the abovementioned sheet of plastic material, the sheet is obtained according to the invention as follows.

0043Firstly, prepared is a polystyrene-based mixture, which is supplied into an extruder which can be of various types, that is, one single-screw, one double-screw, tandem single-screw or tandem primary double-screw extruders.

0044The polystyrene-based mixture is plasticized and homogenised inside the extruder by means of heating, then, when the mixture is well plasticized, a foaming agent of the known type such as for example butane is introduced into the extruder, then the mixture is cooled and extruded.

0045In particular, according to the invention, a foamed polystyrene sheet is extruded with a density comprised between 150 and 350 grams/litre, preferably between 180 and 220 grams/litre, even more preferably about 200 grams/litre, and a thickness equivalent or smaller than 2.2 mm.

0046The extrusion occurs through a die preferably having a circular crown-shaped exit opening, thence obtaining the abovementioned sheet in form of a tubular piece which is fitted or slid eXternally with respect to a cylindrical spindle which has the,purpose to gauge , that is, give the final dimension to the extruded tubular piece.

0047According to a further characteristic of the invention, the ratio between the diameter of the adjustment spindle and the diameter of the circular crown-shaped die is comprised between 1.5 and 2.2 and preferably is about 2.

0048It should be observed that the value of the abovementioned ratio is substantially lower (about half) than the one generally used for the production of foamed polystyrene sheets according to the known art, and it allows to obtain a foamed extruded polystyrene sheet having a structure with perfectly integral cells, slightly oriented along a transverse direction.

0049The spindle is cooled in such a manner to consolidate the foamed polystyrene tubular sheet, which is then cut according to one or more generatrixes to form a flat foamed extruded polystyrene sheet coupled on which is the abovementioned plastic film having barrier properties.

0050Thus obtained is the abovementioned sheet of plastic material comprising the foamed extruded polystyrene layer coupled with the plastic film having barrier properties, which is then wound in coils which are subsequently forwarded to the thermoforming to obtain the trays which are finally used to make the sealed packages in a protective or modified atmosphere according to the present invention.

0051In particular, the coupled sheet is thermoformed in a conventional manner, thus obtaining trays generally having side walls with a thickness equivalent to or smaller than 2.5 mm and a foamed portion density (polystyrene layer) comprised between 80 and 200 grams/litre, preferably between 90 and 110 grams/litre, mainly depending on the density of the foamed portion of the abovementioned sheet of plastic material, and depending on the thermoforming operation parameters.

0052The trays are then separated by means of die cutting or other known cutting systems.

0053Obtained in this manner are single trays 1, having a given thermal insulation capacity, ready for storage and then forwarded to the site where the food packages are made in a modified or protective atmosphere.

0054In such sites, the abovementioned package is completed according to the known operations. Such operations essentially comprise arranging a food product in the tray 1, followed by thermowelding on the peripheral edge 4 of the tray 1 a further plastic film having barrier properties (closure plastic film).

0055In detail, such further plastic film having barrier properties is thermowelded, along a peripheral strip, to the first plastic film having barrier properties which was coupled to the foamed extruded polystyrene layer, at the peripheral edge 4 of the tray, in such a manner that a preset modified or protective atmosphere is entrapped between the two plastic films having barrier properties.

0056The modified or protective atmosphere is made of a preset gas preferably selected from the group comprising nitrogen, argon, oxygen, helium, carbon dioxide and their mixtures.

0057The modified or protective atmosphere according to the invention is entrapped in the package according to the invention in a known manner, for example by extracting at least partially, before thermowelding the two plastic films, the air comprised between them followed by introducing, still in the space comprised between the abovementioned films, the preset gas or gas mixture.

0058In this case, the method or process is referred to as vacuum/gas, in which the modified atmosphere is provided through vacuum by means of conventional type of apparatus.

0059Alternatively, the air is replaced solely through ejection by means of insufflating the preset gas or gas mixture, according to the so-called "gas washing" method.

0060In the latter case, the preset gas or gas mixture is insufflated between the first and the second plastic film arranged one over the other.

0061The insuflated gas ejects the air present between the two plastic films, in turn occupying the space comprised between them and remaining subsequently entrapped in the sealed package when the plastic films are welded to each other.

0062The invention shall now be illustrated in further detail through a non-limiting example, described in which is the embodiment of a sheet of plastic material and the embodiment of a tray obtained from such sheet according to the invention.

Example

0063- A mixture consists of a crystal polystyrene resin in granules of the Styron^™ 686E type sold by Dow Plastics, charged with nucleating additives (talc) at a percentage equivalent 0.6% with respect to the total weight of the resin, was supplied into a tandem extruder comprising a first 60 mm diameter cylinder and a second 90 mm diameter cylinder, having a capacity of 100 kg/h.

0064The mixture was plasticized in a portion of the extruder kept at 190-210°C, then butane was added to the plasticized polymer -as a foaming gas - at a quantity equivalent to 2% of the total of the resin.

0065The polymer charged with the foaming gas was then cooled in the secondary cylinder up to a temperature of about 125° C and extruded through a die having a circular crown-shaped exit opening with an average diameter of 110 mm, obtaining a tubular shaped foamed extruded polystyrene sheet.

0066The tubular shaped foamed extruded polystyrene sheet was fitted and cooled on a cylindrical adjustment spindle kept at about 10° C having a diameter of 215 mm, thus equivalent to 1.95 times greater than the average diameter of the circular crown-shaped die and subsequently, after being cut longitudinally, it was coupled, through hot rolling, with a plastic film having barrier properties against gases containing EVOH and having a thickness of 60 microns.

0067Thus obtained, according to the present invention, is a sheet of plastic material comprising a foamed extruded polystyrene layer coupled with a flexible plastic film having barrier properties against gases, in which the foamed portion of the sheet of plastic material, that is, the foamed extruded polystyrene layer has a density equivalent to 200 grams/litre and a thickness equivalent to 1.3 mm.

0068The sheet of plastic material thus obtained was introduced into a thermoforming machine provided with a preheating furnace kept at about 160° C and, thence, shaped in a mould, kept at a temperature of 20° C, for 3.5 seconds obtaining trays according to the invention having a density of the foamed portion (polystyrene layer) equivalent to 110 grams/litre, an average thickness of the walls of about 2 mm and a depth of 50 mm.

0069Furthermore, the abovementioned trays were obtained in regular shapes and with writings on the bottom, representing among others the trademarks, perfectly imprinted and of good quality.

0070The trays thus obtained, arranged in rows, had a good overall dimension equivalent to about a half of the one of foamed polystyrene trays of the same dimensions made according to the known art.

0071Furthermore, the mechanical resistance of the abovementioned trays was ascertained and such trays resulted widely sufficient to bear both the packaging operations and the subsequent handling to which such trays are generally subjected.

0072Unstacking tests were also performed on automatic unstacking machines which showed a fairly flawless operation.

0073A first advantage achieved by the present invention consists in having trays essentially made of foamed plastic material adapted for packaging a food product in a modified or protective atmosphere, which when stacked in vertical rows have an overall dimension corresponding to about a half of the trays provided for by the known art, and almost equivalent to the one of trays made of non-foamed plastic material, as illustrated in <figref idref="f0001">figure 1</figref>.

0074A second advantage lies in the accuracy and order of the stacking observed in the trays of the invention (<figref idref="f0001">figure 1 and 2</figref>), which can be compared to the one of the trays made of non-foamed plastic material (<figref idref="f0001">figure 1</figref>).

0075Such stacking order and accuracy make the trays according to the invention also particularly suitable for unstacking on automatic systems, contrary to what occurs with traditional trays made of foamed plastic material having barrier properties.

0076A further advantage lies in that the weight of the trays according to the invention, which in the case of the trays of <figref idref="f0001">figure 1</figref>, measuring 180 x 250 x 30 mm, is about 13.7 grams each, that is it can be compared, if not equivalent, to the one of trays made of foamed plastic material, while it is lighter than the about 18.7 grams of weight of the trays of the same dimensions made of non-foamed plastic material, provided for by the known art.

0077As considered beforehand, the weight of a tray should be directly linked to the amount of plastic material used to make it, and thus the cost of the tray itself in terms of production and transport.

0078As far as rigidity is concerned, the one observed on the trays according to the invention is only slightly lower than the one of the traditional trays made of foamed plastic material, but nonetheless absolutely sufficient for the type of packaging intended for such trays, given that it was proved higher with respect to the rigidity of the known type trays of non-foamed plastic material.

0079Also the thermal insulation that the trays according to the present invention are capable of guarantee is slightly lower than the one of traditional trays made of foamed plastic material but nevertheless sufficient to provide a good thermal protection to the content.

0080Furthermore, a tray made of foamed extruded polystyrene according to the present invention allows a well defined forming even when it comes to small details which at times supplement a food product package, such as for example writings and trademarks which are often a guarantee for the consumer.

0081Still, it should be added that when making a tray according to the present invention, it is advantageously possible to use the same thermoforming moulds used for making the trays made of foamed plastic material, of the known type.

0082Therefore, the present invention achieves all the advantages described above without requiring new equipment which may have a negative impact in terms of costs.

0083It is understood that the invention described above in its different embodiments comprising a sheet of plastic material, a tray for a food product obtained from such sheet, a package in a modified or protective environment for a food product , comprising a such tray and the relative manufacture method, shall be subjected, with the aim of meeting specific and required needs, by a man skilled in the art, to various modifications all of which fall within the scope of protection of the invention, as defined by the following claims.