Method for manufacturing a cardboard tray and cardboard tray

Abstract

The invention relates to a cardboard tray folded out of a single sheet, which tray comprises: - a bottom; - an upstanding wall arranged along the periphery of the bottom, the upstanding wall having an upper edge defining the access opening of the cardboard tray; - plastic foil lined against the inside of the cardboard tray and extending at least on the bottom and the upstanding wall; wherein the upstanding wall is composed out of at least first upstanding wall part sections bordering the bottom and a second upstanding wall part sections bordering the upper edge, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is larger than the angle of the cord extending between the peripheral edge bordering the respective first upstanding wall part section and the upper edge with the normal vector of the bottom.

Description

[0001]Method for manufacturing a cardboard tray The invention relates to a method for manufacturing a cardboard tray laminated with a plastic foil, which method comprises the steps of: - providing a sheet of cardboard and folding the sheet into a tray comprising a bottom and an upstanding wall arranged along the periphery of the bottom, the upstanding wall having an upper edge defining the access opening of the cardboard tray, - providing a plastic foil over the access opening of the cardboard tray; - heating the plastic foil; and - laminating the heated plastic foil against the inside of the cardboard tray, such that the plastic foil extends at least on the bottom and the upstanding wall, wherein different zones of the plastic foil are heated to different temperatures to control the local elasticity in the plastic foil. Such a method is for example known from EP 2441697. The upstanding wall is composed out of flat wall part sections, which extend from the bottom to the upper edge. As a result the angle between the bottom and the upstanding wall is close to 90°. When arranging the plastic foil into the folded cardboard tray, the plastic foil will stretch unevenly. In particular at the edges and corners of the cardboard tray the degree of stretch of the plastic foil is substantial compared to the other parts where the plastic foil is lined against the inside of the cardboard tray. In order to ensure that the plastic foil does not rupture at the corners and edges of the cardboard tray, a relative thick plastic foil is used. However, legislation requires that the amount of plastic remains under 5% in weight compared to the cardboard, in order to allow the tray to be discarded as cardboard waste. So, using a thicker plastic foil requires a heavier cardboard in order to comply to the legislation. This however increases the costs of the cardboard tray and is undesired. US 2015217884 discloses a similar tray as disclosed in EP 2441697. According to US 2015217884 the edge of the plastic foil is kept at a relative low temperature, below the softening temperature of the foil, compared to the center of the plastic foil. As a result the edge of the plastic foil will not adhere to the horizontal flange of the tray, which is beneficial when sealing a closing foil onto the horizontal flange. The tray of US 2015217884 also has the disadvantage that a relative thick plastic foil is needed in order to avoid rupture at the corners and edges of the cardboard tray. It is therefore an object of the invention to reduce or even remove the above mentioned disadvantages. This object is achieved with a method according to the invention, which is characterized in that zones heated to a relative low temperature compared to the temperature of other zones of the plastic foil, correspond with corners in the cardboard tray. By heating the plastic foil in zones differently, the plastic foil will stretch differently locally. This allows for more control on how the plastic foil is stretched when laminated into the cardboard tray. As at corners of the cardboard tray, the plastic foil typically tends to stretch the most, heating said parts of the plastic foil to a lower temperature, ensures that said portion will get less stretched and that other parts of the plastic foil will get stretched more, than when the plastic foil is heated evenly. As a result a more even thickness of the plastic foil is obtained. This allows for a thinner plastic foil to be used compared to when no specific differently heated zones for the corners are applied, such as in EP 2441697 and US 2015217884. In an embodiment of the method according to the invention the difference in temperature between a zone heated to a relative low temperature corresponding with a corner in the cardboard tray and a zone of the plastic foil corresponding to a wall adjacent to a corner is at least 10°C. In a preferred embodiment of the method according to the invention the heated plastic foil is pre-stretched by pulling the plastic foil into a heated dome before laminating the heated plastic foil against the inside of the cardboard tray. With the method, the plastic foil is stretched evenly by pulling the plastic foil into a heated dome. The plastic foil is then further stretched, when the foil is laminated into the cardboard tray. As the foil is already evenly pre-stretched, the extremes in stretching of the plastic foil will be reduced. In addition the plastic foil, while being pre- stretched can already be heated in different zones to different temperatures. In a preferred embodiment of the method according to the invention the upstanding wall is composed out of at least first upstanding wall part sections bordering the bottom and a second upstanding wall part sections bordering the upper edge, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is larger than the angle of the cord extending between the peripheral edge bordering the respective first upstanding wall part section and the upper edge with the normal vector of the bottom. With this embodiment, the first upstanding wall part sections will have a larger angle with the bottom, compared to a prior art cardboard tray. Using at least a first and second wall part section for the upstanding wall, allows for the edges and corners to approximate a curved edge or corner, in which the angles are less sharp. When using more than two wall part sections, the shape of the tray will even more approximate a curved edge further contributing to achieving an even thickness of the plastic foil throughout the tray. As a result, the plastic foil, when lined into the cardboard tray will stretch more evenly, such that the thickness of the plastic foil at the edges between the bottom and first wall part sections as well as between the first and second wall part sections, is less reduced in relation to the thickness of the plastic foil at the remaining portions of the cardboard tray. This allows for reduction of the overall thickness of the plastic foil and for a reduction of the weight of the cardboard, while still complying to the above mentioned legislation. The cardboard tray according to the invention can thus be manufactured lighter and at lower costs, than a cardboard tray according to the prior art. The more the tray approximates a semi sphere, by increasing the number of wall part sections, the thinner the plastic foil can be. In a preferred embodiment of the method according to the invention the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is within the range of 15° – 60°, preferably within the range of 15° – 45°. When the plastic foil is heated and lined against the inside of the cardboard tray, the foil will stretch the most at the edges with the bottom. Having the angle between the bottom and the first upstanding wall part section in the range of 15° – 60°, preferably within the range of 15° – 45° ensures that the variation of stretch in the plastic foil is substantially reduced. Another embodiment of the method according to the invention further comprises a peripheral, horizontal flange arranged to the upper edge of the upstanding wall. A horizontal flange provides for additional strength to the cardboard tray and allows for sealing of a lid onto the flange, after the cardboard tray has been filled with for example food stuff. In a further preferred embodiment of the method according to the invention the bottom of the cardboard tray has a polygonal shape, preferably an octagonal shape. Having a polygonal shape, such as an octagonal shape also reduces the sharpness of the angles between the edges of the bottom and accordingly of adjacent first upstanding wall part sections. This further reduces the amount in which the plastic foil is stretched when being laminated to the inside of the cardboard tray. In yet a further preferred embodiment of the method according to the invention along each side of the polygonal shape a first upstanding wall part section is arranged along a first fold line. Preferably, along an edge of the first upstanding wall part section, opposite of the first fold line, a second upstanding wall part section is arranged along a second fold line. The resulting cardboard tray approximates a semi sphere shape. When the plastic foil is heated and stretched by pressing with a pressure difference, the plastic foil will try to take the shape of a semi sphere. So, if the folded cardboard also approximates a semi sphere, the differences in local stretch of the plastic foil will be minimized. In still a further preferred embodiment of the method according to the invention, the first upstanding wall part sections are, along the periphery of the bottom, alternately triangular shaped with the first fold line being the base of the triangular shape and the apex of the triangular shape bordering a corner between two adjacent second wall part sections. As a result the number of first upstanding wall part sections is double from the number of second upstanding wall part sections. The triangular shaped first upstanding wall parts form a transitional shape between the horizontal bottom and the more vertical second upstanding wall part sections, such that the preferred sphere shape is even more approximated. In still a further preferred embodiment of the method according to the invention the plastic foil is at least heated to a temperature above the softening temperature of the plastic foil, such that the heated plastic foil is laminated fully against the inside of the cardboard tray. By having the plastic foil at a temperature at least above the softening temperature, it will be ensured that the plastic foil will stick to the cardboard and thus can be laminated thereon. The invention further relates to a cardboard tray manufactured with the method according to the invention, which tray comprises: - a bottom; - an upstanding wall arranged along the periphery of the bottom, the upstanding wall having an upper edge defining the access opening of the cardboard tray; - plastic foil lined against the inside of the cardboard tray and extending at least on the bottom and the upstanding wall; wherein the upstanding wall is composed out of at least first upstanding wall part sections bordering the bottom and a second upstanding wall part sections bordering the upper edge, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is smaller than the angle between the normal vector of the cord, extending between the peripheral edge bordering the respective first upstanding wall part section and the upper edge, and the normal vector of the bottom, wherein along each side of the polygonal shape a first upstanding wall part section is arranged along a first fold line wherein the first upstanding wall part sections are, along the periphery of the bottom, alternately triangular shaped with the first fold line being the base of the triangular shape and the apex of the triangular shape bordering a corner between two adjacent second wall part sections. The shape of this cardboard tray approximates the shape of a sphere, which already minimizes the stretch differences in the plastic foil liner. By using this cardboard tray with the method according to the invention, the stretch at the corners is even further reduced, which allows for an even thinner plastic foil liner to be used. These and other features of the invention will be elucidated in conjunction with accompanying drawings. Figure 1 shows a perspective view of a first embodiment of a cardboard tray for use with the method according to the invention. Figure 2 shows a cross-section of the cardboard tray according to figure 1. Figure 3 shows a perspective view of an embodiment of a cardboard tray according to the invention. Figures 4A – 4B show schematically the steps of an embodiment of the method according to the invention. Figure 5 shows a schematic top view of a step of the method according to the invention. Figure 6 shows a top view of a part of the cardboard tray of figure 3. Figures 7A and 7B show top views of embodiments of the horizontal flange of a cardboard tray according to the invention. Figure 1 shows a first embodiment of a cardboard tray 1 for use with the method according to the invention. The tray 1 has an octagonal bottom 2 with along each straight edge an upstanding wall part 3, 4 composed out of a first upstanding wall part section 3 and a second upstanding wall part section 4. The upstanding wall parts 3, 4 form the upstanding wall having an upper edge 5 defining the access opening of the tray 1. The access opening is bordered by a horizontal flange 6 arranged arranged to the upper edge. Figure 2 shows a cross-section of the cardboard tray 1 according to figure 1. The angle α is formed between the normal vector of the first upstanding wall part section 3 and the normal vector of the bottom 2. The angle β is formed between the normal vector of the cord 7, extending between the peripheral edge 8 of the bottom 2, bordering the respective first upstanding wall part section 3 and the upper edge 5, and the normal vector of the bottom 2. The angle α is smaller than the angle β, such that the upstanding wall part 3, 4 approximates a curved wall part. This ensures, that when a plastic foil is laminated on the inside of the cardboard tray 1 against the bottom 2 and the upstanding wall part sections 3, 4, the local stretch in the plastic foil is reduced compared to when the tray would have had a single straight upstanding wall part between the peripheral edge of the bottom 2 and the upper edge 5. Figure 3 shows a perspective view of a second embodiment of a cardboard tray 20 for use in the method according to the invention. The cardboard tray 20 has an octagonal bottom 21 with first upstanding wall part sections 22, 23 arranged on each straight edge of the octagonal periphery of the bottom 21. The first upstanding wall part sections 23 form corners, while the first upstanding wall part sections 22 are provided with second upstanding wall part sections 24, of which the upper edges 25 form a rectangular access opening. The access opening is bordered by a horizontal flange composed out of flange sections 26. For each of the first upstanding wall part sections 22, 23 applies that the angle between the normal vector of each first upstanding wall part section 22, 23 with the normal vector of the bottom 21 is smaller than the angle between the normal vector of the cord, extending between the peripheral edge of the bottom 21 bordering the respective first upstanding wall part section 22, 23 and the upper edge 25, and the normal vector of the bottom 21. This corresponds to the angles as explained in figure 2 for the cardboard tray 1 shown in figure 1. Figures 4A – 4B show schematically the steps of an embodiment of the method according to the invention. A cardboard tray 1, 20 as shown in figure 1 or 3 is arranged in a lower mold 30 with the flanges 6, 26 resting thereon. A plastic sheet 31 is provided over the tray 1, 20 and an upper mold 32 with a dome shaped cavity 33 is arranged there above. The upper mold 30 is heated, such that the plastic foil 31 is heated. Insulating elements are positioned in the upper mold 30 to provide zones of lower temperature in the plastic foil 31 which will correspond at least to the corners of cardboard tray 1, 20. In figure 4B, a pressure difference is arranged over the plastic foil 31, such that the plastic foil 31 is urged into the dome shaped cavity 33 of the upper mold 32. This causes the plastic foil 31 to be pre-stretched. In figure 4C the pressure difference over the plastic foil 31 is reversed, such that the heated, pre- stretched plastic foil 31 is urged into the cardboard tray 1, 20 and is lined against the inside thereof. By pre-stretching the plastic foil 31 the plastic foil 31 is stretched evenly and when the foil 31 is lined into the cardboard tray 1, 20, the additional stretching is limited. This contributes, in addition to the shape of the cardboard tray 1, 20, and in addition to the zones of different temperature to a more even thickness of the lined plastic foil 31 allowing for a reduction in thickness of the plastic foil 31 compared to the prior art. Figure 5 shows a schematic top view of a step of heating the plastic foil of the method according to the invention. A heated plate 40 can be used instead of the upper mold 32 with the dome shaped cavity 33 in the method shown in figures 4A-4C. The heated plate 40 had three concentric zones 41, 42, 43, of which the temperature can be controlled separately. This allows for the plastic foil to be heated in three different zones. These concentric zones could also be integrated into the upper mold 32 of figures 4A – 4C. In figure 5 a top view of a cardboard tray 1 according to figure 1 is shown. The first heating zone 41 corresponds with the bottom 2, the second heating zone corresponds with the first upstanding wall part sections 3, while the third heating zone 43 corresponds with the second upstanding wall part sections 4 and the flange 6. By having the second heating zone 42 at a lower temperature, than the first and third heating zones 41, 43, the plastic foil being lined against the first upstanding wall part sections 3 is less flexible, such that the stretchability is reduced for this zone. This also contributes to a more even stretching of the plastic foil and allows for using a thinner plastic foil compared to the prior art. Figure 6 shows a top view of a part of the cardboard tray 20 of figure 3. In figure 6 the heating zones 70 – 73 for different parts of the tray are shown. These heating zones 70 – 73 are positioned outside of the edges and corners of the tray 20 and may be heated to a higher temperature, than the remaining parts of the plastic foil. This ensures that the plastic foil can be stretched more at the flat surfaces of the bottom and upstanding wall portions due to the higher temperature at the zones 70 – 73, while at the corners and edges between these flat surfaces, the plastic foil is stretched less. Figures 7A and 7B show top views of embodiments of the horizontal flange of a cardboard tray. Figure 7A shows a cardboard tray 50 with a bottom 51, first upstanding wall sections 52 and second upstanding wall sections 53, similar to the cardboard tray of figure 1 and 2. Horizontal flange parts 54, 55 are arranged to the upper edge of the upstanding wall 52, 53. The horizontal flange parts 54, 55 abut at the edges 56, which have a complimentary wave shape, such that the flange parts 54, 55 hook into each other. Figure 7B shows a further embodiment of a cardboard tray 60 with a bottom 61, first upstanding wall part sections 62, second upstanding wall part sections 63 and horizontal flange parts 64, 65. The abutting flange part sections 64, 65 are connected to each other by a jigsaw puzzle piece connector 66.

Claims

Claims 1. Method for manufacturing a cardboard tray laminated with a plastic foil, which method comprises the steps of: - providing a sheet of cardboard and folding the sheet into a tray comprising a bottom and an upstanding wall arranged along the periphery of the bottom, the upstanding wall having an upper edge defining the access opening of the cardboard tray, - providing a plastic foil over the access opening of the cardboard tray; - heating the plastic foil; and - laminating the heated plastic foil against the inside of the cardboard tray, such that the plastic foil extends at least on the bottom and the upstanding wall, wherein different zones of the plastic foil are heated to different temperatures to control the local elasticity in the plastic foil, characterized in that zones heated to a relative low temperature compared to the temperature of other zones of the plastic foil, correspond with corners in the cardboard tray.

2. Method according to claim 1, wherein the difference in temperature between a zone heated to a relative low temperature corresponding with a corner in the cardboard tray and a zone of the plastic foil corresponding to a wall adjacent to a corner is at least 10°C.

2. Method according to claim 1, wherein the heated plastic foil is pre-stretched by pulling the plastic foil into a heated dome before laminating the heated plastic foil against the inside of the cardboard tray.

3. Method according to claim 1 or 2, wherein the upstanding wall is composed out of at least first upstanding wall part sections bordering the bottom and a second upstanding wall part sections bordering the upper edge, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is larger than the angle of the cord extending between the peripheral edge bordering the respective first upstanding wall part section and the upper edge with the normal vector of the bottom.

4. Method according to claim 3, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is within the range of 15° – 60°, preferably within the range of 15° – 45°.

5. Method according to any of the preceding claims, wherein the folded tray further comprises a peripheral, horizontal flange arranged to the upper edge of the upstanding wall.

6. Method according to any of the preceding claims, wherein the bottom of the folded tray has a polygonal shape, preferably an octagonal shape.

7. Method according to claim 6, wherein along each side of the polygonal shape a first upstanding wall part section is arranged along a first fold line.

8. Method according to claim 7, wherein along an edge of the first upstanding wall part section, opposite of the first fold line, a second upstanding wall part section is arranged along a second fold line.

9. Method according to claim 7 or 8, wherein the first upstanding wall part sections are, along the periphery of the bottom, alternately triangular shaped with the first fold line being the base of the triangular shape and the apex of the triangular shape bordering a corner between twoadjacent second wall part sections.

10. Method according to any of the preceding claims, wherein the plastic foil is at least heated to a temperature above the softening temperature of the plastic foil, such that the heated plastic foil is laminated fully against the inside of the cardboard tray.

11. Cardboard tray manufactured with the method according to claim 1, which tray comprises: - a bottom; - an upstanding wall arranged along the periphery of the bottom, the upstanding wall having an upper edge defining the access opening of the cardboard tray; - plastic foil lined against the inside of the cardboard tray and extending at least on the bottom and the upstanding wall; wherein the upstanding wall is composed out of at least first upstanding wall part sections bordering the bottom and a second upstanding wall part sections bordering the upper edge, wherein the angle between the normal vector of each first upstanding wall part section with the normal vector of the bottom is smaller than the angle between the normal vector of the cord, extending between the peripheral edge bordering the respective first upstanding wall part section and the upper edge, and the normal vector of the bottom, wherein along each side of the polygonal shape a first upstanding wall part section is arranged along a first fold line characterized in that the first upstanding wall part sections are, along the periphery of the bottom, alternately triangular shaped with the first fold line being the base of the triangular shape and the apex of the triangular shape bordering a corner between two adjacent second wall part sections.

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