EP0766622B1

EP0766622B1 - Method for forming corrugated paper container and container made therefrom

Info

Publication number: EP0766622B1
Application number: EP95923944A
Authority: EP
Inventors: Robin P. Neary
Original assignee: Newark Group Industries Inc; Newark Group Ind Inc
Current assignee: THE NEWARK GROUP, INC.
Priority date: 1994-06-20
Filing date: 1995-06-19
Publication date: 2000-12-06
Anticipated expiration: 2015-06-19
Also published as: JPH10502585A; EP1023992A3; ATE197932T1; DE69519573T2; EP1023992A2; DE69519573D1; EP0766622A1; AU2864295A; WO1995035204A1; CA2193386A1; ES2156208T3

Abstract

A method for forming a corrugated paperboard container and a container made therefrom having a first layer of flat paperboard (12) to which is attached a second layer of contoured paperboard (14), said contoured layer having a plurality of flutes, each said flute including a first flute tip and a second flute tip, the flute rise being substantially vertical at the midpoint between said flute tips and a perforated layer of paperboard (30) having a plurality of openings (31) therein coupled with said fluted layer along said plurality of said flute tips, said forming method comprising, placing the corrugated paperboard into a stamping die, the stamping die having a male die and a female cavity shaped in the shape in which the paperboard is to be formed; pressing the paperboard between the die and the cavity to apply a pressure to the paperboard; applying heat to the paperboard simultaneously with said application of pressure; and removing said die from the cavity. <IMAGE>

Description

BACKGROUND TO THE INVENTION

The present invention relates to a method of forming corrugated paperboard containers, including food containers and food trays and, in particular, employing corrugated paperboard in a novel stamping process, which paperboard has an increased frequency of flutes in its internal layer, such that upon introducing the corrugated paperboard to the stamping process, the paperboard does not break apart and is capable of being moulding to produce a satisfactory unitary structure.
Heretofore, to produce unitary paperboard containers without gluing, manufacturers have attempted to stamp certain types of corrugated paperboard, such as E-flute corrugated paperboard, and have been unable to create a satisfactory unitary construction. Efforts at stamping corrugated paperboard resulted in the paperboard breaking up during the stamping process and have failed to achieve a utilitarian moulded unitary structure.
Accordingly, the use of corrugated paperboard for containers has been limited to an expensive multi-step manufacturing process in which the paperboard must first be printed, then die cut and then pressed trough complex box folding machinery. Accordingly, a corrugated paperboard that would permit pressing and forming (stamping) into a container and a method of pressing and forming corrugated paperboard container would be desirable.
Prior art documents AU-B-54281/86, FR-A-2358505 and EP-A-0341652 teach that the moulding of paperboard with moisture under heat and pressure is known.
US-A-4637811 and US-A-4950524 show prior art relating to a moulding process.
FR-A-2713136 shows prior art teaching of perforations in layers.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the instant invention, a method as defined by the features of claim 1 of forming a unitary container from corrugated paperboard is provided. The method comprises introducing corrugated paperboard having an outer flat layer of paperboard and an internal contoured layer of paperboard having a high frequency, per linear meter of paperboard, of flutes formed from alternating upper and lower curved surfaces into a die press and pressing the paperboard between a die and a cavity to apply pressure to the paperboard. Applying heat to the paperboard simultaneously with the application of pressure.
In a second type of container a three-layered corrugated paperboard tray is provided. The paperboard is formed of a first layer of corrugated paper, a second layer of industrial strength paper, and a third layer of paper having holes formed therein, the corrugated layer being sandwiched between the second layer and third layer. The top paper layer comprises a sheet of paper having a plurality of holes therein resting on the fluted crests of the corrugated layer. The upper surface of the top perforated layer is moisture resistant. The underside of the fluted layer and the bottom layer of the corrugated construction have moisture-resistant barriers to prevent leaking through the entire tray.
Accordingly, it is an object of this invention to provide an improved method for forming a unitary container of corrugated paper.
Another object of the invention is to form a container from corrugated paper by stamping, without the need for a complex box folding machinery or the need for die cutting.
Yet another object of the invention is to form a unitary box from single face or single ply corrugated paper.
A further object of the invention is to provide a container made out of lighter, cheaper and recyclable materials.
Still another object of the invention to provide a method for forming complex shapes by pressing and forming paperboard.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to the following description, taken in connection with the accompanying drawings, in which:
FIG. 1 is a partial perspective view of the paperboard utilized in connection with the invention;
FIG. 2 is a front elevated view of the paperboard constructed in accordance with the invention;
FIG. 3 is a sectional view of stamp used in connection with the method of the invention;
FIG. 4 is a perspective view of a container constructed in accordance with the invention;
FIG. 5 is a perspective view of the paperboard food tray constructed in accordance with another embodiment of the present invention;
FIG. 6 is a partial top plan view of a paperboard utilized in connection with the present invention; and
FIG. 7 is a cross-sectional view taken along line 3-3 of FIG. 5 of the paperboard utilized in connection with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of corrugated paper used to form applicant's box is disclosed in U.S. Patent No. 4,931,346 which is incorporated herein by reference. Reference is made to FIGS. 1 and 2, wherein a paperboard, capable of being pressed and formed (stamped), generally indicated as 10, comprising two layers is depicted. An outer layer 12 is flat paperboard having a thickness in the range of 0.3 mm to 1.5 mm.
An internal layer, generally indicated as 14, of paperboard 10 is contoured, having a thickness in the range of 0.23 mm to 0.5 mm. The internal contoured layer 14 consists of flutes 16, each flute being formed by one of an alternating upper 18 or lower 20 curved surface. Internal layer 14 is glued to an inner surface 22 of the outer layer 12 along the lower curved surfaces 20. Gluing the lower curved surfaces 20 of the internal layer 14 to the inner surface 20 of the outer layer 12 allows for the formation of exposed ridges 24 and grooves 26 (FIG. 2) across the unattached surface of internal layer 14. Internal layer 14 has a high frequency (flutes/meter) of flutes 16 and a corresponding high frequency of ridges 24 on the exposed surface of internal layer 14 allowing the paperboard 10 to be pressed into a unitary structure.
One upper curved surface 18 and an alternating lower curved surface 20 make up one set of flutes 28. Each flute 16 has a height in the range of 0.9 mm to 1.2 mm or 0.035 inches to 0.047 inches, and sets of flutes 28 in the internal layer 14 have frequency within the range of 350-400 sets of flute 26 per linear meter of paperboard 10. In an exemplary embodiment, the number of sets of flutes is 375 per linear meter of paperboard, the flute repeat length to flute height ratio is 2.6 or less, and the corresponding number of ridges on the top surface of the internal layer is 6 to 12 ridges per inch.
In producing the stampable paperboard 10, internal contoured layer 14 has glue applied to the lower curved surfaces 20. Outer layer 12 and internal layer 14 are passed through rollers under a sufficient pressure to compress the two layers together to adhere them. As a result, lower curved surfaces 20 adhere to inner surface 22 of the outer layer 12 and the pressure causes the lower curved surfaces 24 to imbed and become indented into inner surface 20 of outer layer 12 by at least 0.1 mm (0.004 inches). In the preferred embodiment, the amount of indent (or imbedding) is 0.2mm (0.008"). The high frequency of flutes 16 per inch (2,54 cm), and, hence, the corresponding increased number of ridges 24 across the unattached surface of the internal layer 14, causes the flats (between curves 18 and 20) in each of the flute sets 28 to become more vertical and this adds strength during compression so that the paperboard can be pressed and formed. Accordingly, by increasing the number of flutes 16, and hence the number of ridges 24, the ability of the corrugated paperboard to be pressed and formed consequently increases.
The flutes in the stampable paperboard have a higher frequency and higher amplitude when compared with other fluted paper. This maximizes the ratio of flute repeat length to flute height. Because of the more vertical flute rise at the midpoint between flute tips there is a greater rate of change of height over distance so that for a given height of flute rise, the corrugated paperboard will have a smaller repeat ratio when compared to E-flute or F-flute, by way of example, and for a given repeat ratio the described paperboard will have a higher amplitude than E-flute or F-flute corrugated paperboard. In the preferred embodiment the stampable corrugated paperboard has a flute repeat length to flute height ratio of 2.6, whereas in E-flute or F-flute, the flute repeat length to flute height ratio is 2.8.
It is the high frequency of flutes per inch (2,54 cm) in tKe corrugation that specifically allows the paperboard to be molded by the dye pressing method described in detail below. Unlike attempting to mold corrugated paperboard of the prior art, which paperboard does not have such high density fluting in the corrugation and consequently breaks up in the molding process, the corrugated paperboard described above does not break up in the molding/stamping process and, instead, is able to be molded into a unitary structure. This ability to mold the corrugated paperboard into a unitary structure is specifically due to the high density of fluting in the corrugation.
In one preferred embodiment of the present invention, single-face corrugated paperboard comprising a flat outer layer and an internal contoured layer comprising a flute repeat length to flute height ratio of 2.6 is pressed and formed by introducing the single-face corrugated paperboard into a stamp and applying pressure and temperature for an amount of time such that the corrugated paperboard becomes pressed and formed to form a unitary container. The high frequency of flutes and ridges in the internal layer cause the single face paperboard to be stamped by this method to form a satisfactory unitary structure such as a container.
Reference is now made to FIG. 3 which is a stamp used in conjunction with the method for stamping the corrugated paper as shown. In an exemplary embodiment of the method, it is important that the paperboard be flexible and easily deformed without fracturing or cracking. Accordingly, in a first step moisture is added to the paperboard to soften the fibers. Moisture is added until the moisture level of each sheet is 8 to 11%, however, it is best to run as close to the lower end of this range in an exemplary embodiment to prevent excessive moisture forming steam beneath any coating applied to the paperboard in the forming process causing the coating to blister. However, the deeper the stamping the higher moisture content required. An additive such as fluorocarbon is added to the moistening process to achieve about a 1% solution in water. The additive assists in water retention, aids forming and reduces grease wicking in the final process.
The moisture and fluorocarbon applied to the paperboard does not immediately penetrate the sheet so that the sheet remains stiff and difficult to form at first. Additionally, latent moisture may still be laying on top of the sheet, having not been absorbed by the fibers, therefore in an exemplary embodiment, the paperboard sits for forty-eight to seventy-two hours before forming, allowing the board to reach equilibrium and to obtain a more uniform distribution of moisture throughout the cross-section of the sheet.
The paperboard is deep drawn utilizing a stamp, generally indicated as 100. The stamp includes a bolster plate 102 upon which is seated a heated female cavity 104. A plunger is slidably mounted on bolster plate 102 to move into and out of heated female cavity 104.
A top die mounting plate 108 slidably supports a reciprocating platen 110. Platen 110 is attached to a male die 112. Male die 112 moves towards and is received in heated female cavity 104 with the reciprocating motion of platen 110. A draw ring 114 is mounted about male die 112 for positing paperboard prior to stamping.
The method of pressing or stamping the paperboard consist of the basic steps of feeding the paperboard, creasing the paperboard, cutting the paperboard and forming the paperboard. The moistened paperboard is creased in an area where corners will be formed while the paperboard blank is formed in a web. The web is then advanced a predetermined amount centering the creased paperboard between male die 112 and heated female cavity 104. Reciprocating platen 110 pushes male die 112 through the die cavity 104, shearing the registered pre-creased paperboard from the remainder of the web and urging it to fall through the die creating the blank. The pre-creased cut blank falls through and is centered between male die 112 and draw ring 114 and heated female cavity 114 mounted on bolster plate 102.
Draw ring 114 is extended and contacts the paperboard, tensioning the paperboard as male die 112 descends pushing the paperboard into female cavity 104. The draw ring holds the board tightly against the rim of the female cavity to discourage wrinkles from forming on the side panels of the container as it is being drawn. The draw ring 114 further forces excess paper into the corners and insures that neat, even folds are made, following the pre-creased lines. The more tension applied to the paperboard during forming, the neater the folds and fewer wrinkles formed on the side panels.
Once all of the paperboard has been drawn into the heated cavity, the press extends the male die slightly, depressing the female cavity 104 and bolster plate 102 by 1/30 seconds of an inch holding the tray under pressure and heat. In an exemplary embodiment, the pressure force applied is approximately four tons at 48°C (120°F). This allows the moisture in the board to turn into steam and escape through vents provided in the die, setting the paperboard in the shape of the die. Essentially, the folds in the corners are steam ironed into the form of the container, imparting a structural integrity to the container.
The stamp (press) is then opened and plunger 106 is activated entering female cavity 104 ejecting the container from the heated female cavity. The excess paper is then trimmed from the formed container.
FIG. 4 shows a corrugated paperboard container, generally indicated as 200, made by the stamping process of the present invention. The container has a clam shell construction and includes a lower compartment 210 for receiving food and the like and an upper compartment 240 for covering and closing the container. Lower compartment 210 includes a base 212 and four upstanding sidewalls 214, 216, 218 and 220, integrally formed with base 212 along fold line 222 formed during stamping. Wall 214 is coupled to wall 216 along a curve at connecting wall 224 formed during stamping. Similarly, wall 216 is coupled to wall 218 by a curved wall 226, wall 218 is coupled to wall 220 by curved wall 228 and curved wall 220 is formed integrally with front wall 214 by a curved wall 230. Each of curved walls 224, 226, 228, 230 are formed during stamping and easily could be formed as fold lines. Wall 214 is formed with a lip 232 at an acute angle with wall 214 and having slot 234 formed therein.
Upper compartment 240 is similar in construction to lower compartment 210 and includes a top wall 242 and side walls 244, 246, 248 and 250, each of walls 244, 246, 248 and 250 are coupled to top wall 242 along a fold line 252 formed during stamping. Side walls 244, 246, 248 and 250 are also integrally coupled, each adjacent sidewall by a respective curved wall 254, 256, 258 and 260. Side wall 248 of upper compartment 240 and side wall 218 of lower compartment 210 are pivotably coupled together by a hinge 262 formed as a fold line between lower compartment 210 and upper compartment 240 during the stamping process. Upper compartment 240 rotates about hinge 262 in the direction of arrow A to close container 200 upon itself.
Side wall 244 is also formed with a lip 264 having a tab 266 formed thereon. Tab 266 is received within slot 234 to fasten upper component 240 to lower component 210.
In a preferred embodiment, bottom compartment is formed with a receiving lip adapted to receive a lip about upper compartment 240 to further seal in a mating relationship, container 200.
As shown in FIG. 4, upper compartment 240 and lower compartment 210 are each formed from paperboard 10 including outer layer 12 to which is glued one contoured internal layer 14. Contoured interior layer 14 consists of flutes 16 which are formed by alternating upper 18 and lower 20 curved surfaces. In a preferred embodiment the ratio of flute repeat length to flute height is 2.6 or less. Upon gluing the lower curved surfaces 20 of the internal layer 14 to the inner surface of outer layer 20 ridges 24 and grooves 26 are formed on the outer surface of the internal layer 14.
The paperboard employed may be solid bleached sulfate (SBS) or chipboard, or a recycled material. In one embodiment, outer flat layer 12 may be chipboard, and internal layer 14 may be a recycled paperboard medium. In a preferred embodiment, the outer layer 12 has a thickness of 0.007 inches (0,17 mm) and internal contoured layer 14 has a thickness of 0.040 inches (1,02 mm). The two layers together may have a working range from 30 to 60 0,8 - 1,6 mm from the outer surface of outer layer 12 to the ridged top of internal layer 14. As a result, less paperboard is used, the container is light in weight, and although the container maintains its strength and rigidity as if layers were solid paperboard, it, at the same time, possesses the capability of becoming stamped.
As shown, the internal surfaces of walls 212, 214, 216 and 218 as well as 242, 244, 246 and 258 of compartments 210 and 240 are formed with ridges having upper ridges 24 and grooves 26 between ridges 24. There are 6 to 12 ridges per inch, and, in a preferred embodiment, there are 9 ridges per inch. As shown more clearly in FIG. 4, the upper ridges 24 of bottom wall 212 form a raised food-receiving surface on which the food is placed. Any moisture given off by the hot food within the container is received and collected within grooves 26. In this manner, the collected moisture is not absorbed by the food, and container 200 prevents the food from becoming soggy.
In a preferred embodiment, internal layer 14 is coated with a water-based coating prior to stamping. The coating is repulpable, recyclable, and resists the penetration of moisture given off by the hot food. As a result, the molded paperboard container maintains its strength and rigidity.
Reference is made to FIGS. 5 and 6 wherein a portion of a laminated paperboard 10 utilized in constructing a food tray 300, comprising three layers in accordance with another embodiment of the invention is depicted. Two of the layers are similar in construction to the paperboard described above, the difference being the addition of a waterproof barrier. Like numerals are utilized to identify like structures.
A first outer layer 12 is flat paperboard having a thickness in the range .15 mm to 1.0 mm. An internal layer generally indicated as 14 of paperboard 10 is contoured, having a thickness in the range of .15 mm to .25 mm. Internal contoured layer 14 consists of flutes 16, each flute being formed by one of an alternating upper 18 or lower 20 curved surface. Internal layer 14 is glued to an inner surface 22 of the outer layer 12 along the lower curved surfaces 20. Gluing the lower curved surfaces 20 of the internal layer 14 to the inner surface 22 of the outer layer 12 allows for the formation of exposed ridges 24 and grooves 26 across the upper surface of internal layer 14. Internal layer 14 has a high-frequency (flutes/meter) of flutes 16 and the corresponding high frequency of ridges 24 in the upper surface of internal layer 14.
A moisture barrier coating 27 is provided on upper surface 22 of outer barrier layer 12. A similar moisture barrier (not shown) coating 29 is provided on the bottom surface of internal layer 14. The top surface of layer 14 is uncoated and therefore absorbs moisture coming in contact therewith. Barrier coatings 27, 29 create a moisture barrier which retains moisture captured by internal layer 14 within layer 14, and protects the tray from leakage.
One upper curved surface 18 and an alternating lower curved surface 20 make up one set of flutes 28. Each flute 16 has a height in the range of 0.9 mm to 1.2 mm or 0.35 inches to 0.047 inches, and sets of flutes 28 in the internal layer 14 have frequency within the range of 350-400 sets of flute 26 per linear meter of paperboard 10. In an exemplary embodiment, the number of sets of flutes is 375 per linear meter of paperboard, the flute repeat length to flute height ratio is 2.6 or less, and the corresponding number of ridges on the upper surface of internal layer 14 is 6 to 12 ridges per inch. The high flute frequency provides a greater exposed layer surface per meter increasing the area of absorbing surfaces in layer 14.
Perforated top layer 30 is provided above internal layer 14. Layer 14 has a thickness and construction similar to that of outer layer 12. However, perforated openings 34 are provided uniformly on the surface of layer 30. Top layer 30 is glued at an inner surface to curved upper surface 18 of internal layer 14, in a way similar to that of gluing inner surface 22 of outer layer 12 along the lower curved surfaces 20 of internal layer 14. Perforated openings 34 are provided at regular intervals in upper layer 30, which allow liquid to flow through upper layer 30, and come in contact with exposed surfaces on fluted internal layer 14. In an exemplary embodiment, the regularly spaced openings 34 each encompass two complete upper curved surfaces 18 of ridges 24. These holes are spaced at a distance of 1/2 or 12.7 mm in a checkerboard pattern. The top surface of upper layer 30 is coated with a moisture barrier coating 33 protecting the upper surface from liquid being absorbed through the top of layer 30. Thus, all liquid emanating from the product funnels through openings 34 in the upper surface of top layer 30, and is absorbed and trapped by internal layer 14 and the inner surface of top layer 30, and held there without seeping through internal layer 14 by moisture coating barriers 27, 29 and 30.
Paperboard 10, by its construction, is stampable. To form a tray 300 therefrom, internal contoured layer 14 has glue applied to lower curved surfaces 20 and upper curved surfaces 18. Outer layer 12, internal layer 14, and upper layer 30 are passed through rollers under a sufficient pressure to compress the three layers together to adhere them. As a result, lower curved surfaces 20 and upper curved surfaces 18 adhere to inner surfaces 22 and the lower surface of upper layer 30, respectively, and the pressure causes lower curved surfaces 24 and upper curved surfaces 18 to embed and become indented into inner surface 20 of outer layer 12 and the lower surface of top layer 30 by at least 0.1 mm (0.004 inches). In a preferred embodiment, the amount of indent (or embedding) is 0.2 mm (0.008 inches). The high frequency of flutes (16 per inch) and the corresponding increased number of ridges (24 across the upper surface of internal layer 14) causes the flats between curves 18 and 20 in each of the flute sets 28 to become more vertical, and this adds strength during compression so that the paperboard can be pressed and formed. Accordingly, by increasing the number of flutes 16, and hence the number of ridges 24, the ability of the corrugated paperboard to be pressed and formed consequently increases.
In one preferred embodiment of the present invention, triple-layer corrugated paperboard comprising a flat outer layer, and internal contoured layer comprising a flute repeat length to flute height ratio of 2.6, and an upper flat top layer with evenly spaced apertures therein is pressed and formed by introducing the three layer corrugated paperboard into a stamp and applying pressure and temperature for an amount of time such that the three-layer paperboard becomes pressed and formed to form a unitary tray. The high frequency of flutes and ridges in the internal layer causes the triple-layer paperboard to be stamped by this method to form a satisfactory unitary structure such as a food tray.
Tray 300 is then formed as described above in connection with stamp 100 shown in FIG. 3.
FIG. 5 shows a corrugated paperboard tray, generally indicated as 300, made by the stamping process of the present invention. Tray 300 includes a base 312 and four upstanding angled side walls 314, 316, 318 and 320, integrally formed with base 312 along fold line 322 formed during stamping. In a preferred embodiment, these angled sidewalls are disposed at a predetermined angle of 142° to the base. Wall 314 is coupled to wall 316 along a curved wall 324 formed during stamping. Similarly, wall 316 is coupled to wall 318 by a curved wall 326, wall 318 is coupled to wall 320 by curved wall 328, and curved wall 320 is formed integrally with front side wall 314 by curved wall 330. Each of curved walls 324, 326, 328 and 330 are formed during stamping and easily could be formed as fold lines. Walls 314, 316, 318 and 320, as well as curved walls 334, 326, 328 and 330, are formed with a lip 332 at an acute angle with the respective walls.
As shown in FIG. 5, corrugated paperboard container 300 is formed from paperboard 10 including outer layer 12 to which is glued contoured internal layer 14 to which, in turn, is glued top layer 30, including moisture barrier layer 33, and evenly spaced circular apertures 31. Contoured interior layer 14 consists of flutes 16 which are formed by alternating upper 18 and lower 20 curved surfaces. In a preferred embodiment, the ratio of flute repeat length to flute height is 2.6 or less. Upon gluing the lower curved surfaces 20 of the internal layer 14 to the lower surface of outer layer 20, ridges 24 and grooves 26 are formed on the upper surface of internal layer 14. Upon gluing the lower surface of upper layer 30 to upper curved surfaces 18 of internal layer 24, ridges 24 and grooves 26 are partially covered by upper layer 30 in the portions not comprised of regularly spaced perforated openings 34.
The paperboard employed may be solid bleach sulfate (SBS) or chipboard, or a recycled paper or paperboard or container board material. In one embodiment, outer flat layer 12 may be chipboard, internal layer 14 may be a recycled paperboard medium, and upper layer 30 may be chipboard. In a preferred embodiment, the outer layer 12 has a thickness 0.007 inches (0,17 mm), internal contoured layer 14 has a thickness of 0.040 inches (1,02 mm), and upper layer 30 has a thickness of 0.007 inches (0,17 mm). The three layers together may have a working range from 0,85 to 1,70 mm from the outer surface of layer 12 to the upper surface of top layer 30. As a result, less paperboard is used and the container is light in weight.
During use, food is placed in the container. As juices or other fluids leak from the food, they fall through perforated openings 34 in top layer 30. The uncoated lower surface of top layer 30 and the upper surface of internal layer 14 absorb the liquid, removing it from the food. The liquid is retained in internal layer 14 and top layer 30 by moisture barriers 27, 29 and 30 so that tray 10 does not leak over time.
As shown in FIGS. 5 and 7, the internal surfaces of walls 312, 314, 316 are formed with upper layer 30 and circular apertures 31 overlaying ridges having upper ridges 24 and grooves 26 between ridges 24. There are six to twelve ridges per inch, and in the preferred embodiment, there are nine ridges per inch. As shown more clearly in FIG. 5, the perforated upper layer 30 overlaying upper ridges 24 forms a moisture wicking, raised food receiving surface on which food is placed. Any moisture given off by the food within the tray is wicked away from the food through perforated openings 34 in top layer 30 and collected within grooves 26 of internal layer 14. In this manner, the collected moisture is not absorbed by the food, and tray 300 prevents the food from becoming soggy, or from spoiling by remaining in contact with this moisture. There is no longer a need for a liquid absorbing napkin. The tray would operate in a similar manner if top layer 30 had no perforations and was not coated with a moisture proof barrier.
In a preferred embodiment, internal layer 14 is coated with a water-based coating prior to stamping. The coating is repulpable, recyclable and resists the penetration of moisture given off by food. This coating 29 is placed on the bottom of internal layer 14. As a result, the molded paperboard container maintains its strength and rigidity.
Also in a preferred embodiment, a similar coating 33 is placed on the upper surface of upper layer 30. However, this layer is not applied to layer 30 where perforated openings 34 exist. Thus, upper layer 30 becomes impenetrable to moisture, and all moisture is wicked away from the food in tray 300 through perforated openings 34 in upper layer 30.
Although the container formed by the process of this invention is shown to have a particular shape, it should be understood that the stamping process for corrugated paperboard can be used to form a corrugated container of any shape, such as round, square, rectangular or oval, as well as non-container structures such as trays or the like. Advantageously, as a result of the present invention, there is provided a method of stamping corrugated paperboard having an internal contoured layer containing a high frequency of flutes and a food container formed by the method of stamping corrugated paperboard, such food container being able to resist the penetration of moisture, is rigid and strong, is light in weight and collects moisture in troughs or grooves to prevent food from becoming soggy.
By forming paperboard having one layer consisting of 350 to 400 sets of flutes per linear meter of paperboard, a light weight container capable of being formed in a stamping process is provided. By utilizing such high frequency fluted paperboard, and by moistening the paperboard and holding the paper taut prior to pressing and by pressing at a high pressure and high temperature during the cycle, a unitary construction formed of corrugated paperboard is obtainable.

Claims

A method for moulding a corrugated paperboard container (200,300) having a first layer of flat paperboard (12) to which is attached a second layer of contoured paperboard (14), said contoured layer having a plurality of flutes (16), each said flute including a first flute tip and a second flute tip, the flute rise being substantially vertical at the midpoint between said flute tips with the flute repeat length to flute height ratio being no greater than 2.6, said forming method comprising, placing said corrugated paperboard into a stamping die (100), said stamping die having a male die (112) and a female cavity (104) shaped in the shape in which the paperboard is to be formed; pressing said paperboard between said die (112) and said cavity (104) to apply a pressure to said paperboard; providing moisture to said paperboard applying heat to said paperboard simultaneously with said application of pressure; and removing said die (112) from said cavity (104), said paperboard having folds therein and said heat applied to said paperboard creating steam within said cavity (104) to steam iron said folds to form a container and provide structural integrity.
The method of claim 1 wherein the first layer of paperboard (12) has a thickness in the range of 0.3 mm to 1.5 mm.
The method of claim 1 or 2, wherein the contoured layer of paperboard (14) has a thickness in the range of 0.23 mm to 0.5 mm.
The method of any preceding claim further comprising the step of coating at least one layer of said paperboard prior to placing said paperboard into said die.
The method of any preceding claim wherein said pressure force applied is approximately four tons.
The method of any preceding claim wherein said heat is applied at a temperature of about 48°C (120°F).
The method of any preceding claim, further comprising the step of providing a moisture barrier (27) on the upper surface of said layer of paperboard (12) before positioning said fluted layer of paperboard (14) above and in contact with said layer of paperboard (12).
The method of any proceeding claim, further comprising the step of providing a moisture baffler (29) on the lower surface of said fluted layer of paperboard (14) before positioning said fluted layer of paperboard (14) above and in contact with said layer of paperboard (12).
The method of any preceding claim further comprising the step of providing a perforated layer of paper board (30) having a plurality of openings (31) therein coupled to said contoured layer.
The method of Claim 9 further comprising the step of providing a moisture baffler (33) on the upper surface of said perforated layer of paperboard (30) before positioning said perforated layer of paperboard above and in contact with said layer of paperboard (14).
The method of Claim 9 or 10 further comprising the steps of placing said first layer of flat paperboard and second layer of contoured paper board into a stamping die (100), said stamping die having a male die (112) and a female cavity (104) shaped in the shape in which the paper board is to be formed;

pressing said paper board between said die (112) and said cavity (104) to apply a pressure to said paper board;

applying heat to said paper board simultaneously with said application of pressure to affix said perforated layer (30) to said fluted layer (14) to permit fluid communication therebetween; and

removing said die (112) from said cavity (104).
A container formed as a unitary construction by stamping corrugated paper, said container (200) comprising:

a first compartment (210) having a base (212) and sidewalls (214, 216, 218, 220) integrally formed therewith;

a second compartment (240) having a top (242) and sidewalls (244, 246, 248, 250) integrally formed therewith;

hinge means (262) integrally joining said first compartment and said second compartment to allow said upper and lower compartments to move relative to each other about said hinge means;

said first compartment (210), second compartment (240) and hinge means (262) being formed of paperboard, said paperboard having at least a flat layer (12) and a contoured layer (14), said contoured layer consisting of a plurality of flutes (16), each said flute including a first flute tip and a second flute tip, the flute rise being substantially vertical at the midpoint between said flute tips with the flute repeat length to flute height ratio being no greater than 2.6, each flute being formed by alternating upper (18) and lower (20) curved surfaces such that the contoured layer (14) is formed with ridges providing increased strength and rigidity using less paperboard, said contoured surface having 6 to 12 ridges per 2.54 cm (per inch); and

said ridged surfaces are coated with a coating (29) to resist penetration of moisture.
The container of claim 12 wherein the upper edges of said ridges form receiving surfaces on which an object received by said container (200) rests, and grooves formed between said ridges for receiving and collecting moisture given off by said object; and said ridges are coated with a recyclable coating adapted to resist penetration of moisture.
The container as claimed in any of claims 12 or 13 further comprising fastening means (266, 234) for releasably fastening said lower compartment (210) to said upper compartment (240).
The container as claimed in any of claims 12 to 14, wherein said coating is water based coating.
The container of any of claims 12 to 15 further comprising a perforated layer of paperboard (30) coupled with said fluted layer of paper board along said plurality of upper curved surfaces (18) of said fluted layer of paper board.