NO834593L - CARTON CONTAINER, AND PROCEDURE AND APPARATUS FOR MANUFACTURING THIS. - Google Patents

Description

CARTON CONTAINER, AS WELL AS METHOD AND APPARATUS FOR PROCESSING

- POSITION OF THIS

The present invention generally relates to a method and apparatus for forming pressed paperboard products such as paper trays and plates, as well as products produced by such a method.

Shaped fiberboard containers, such as paper plates and troughs, are usually made either by molding fibers from a pulp slurry into the container's desired shape or by pressing a cardboard sheet between pressing dies into the desired shape. Molded pulp articles are relatively strong and rigid after drying but generally have rough surface properties and are not usually coated so that they are easily penetrated by water, oil and other liquids. Pressed paper or cardboard containers, on the other hand, can be decorated and coated with a liquid-proof coating before punching with dies into the desired shape. A large number of paper plates are produced annually by each of these methods at relatively low unit costs. The products are manufactured in many different shapes, rectangular or polygon as well as round and with a multi-chamber configuration.

Pressed paper or cardboard containers tend to

to exhibit less strength and stiffness and in this respect cannot be compared with containers formed by the pulp molding process. Much of the strength and resistance to bending of plate-like containers produced by each of the aforementioned processes resides in the sidewalls and edge areas surrounding the center and bottom portion of the container. In plate-like structures produced by the mass casting process, the side wall and the folded edge of the plate form a uniform, continuous structure that exhibits good resistance to bending as long as they are not destroyed or split. In contrast, containers made by pressing a cardboard sheet will cause this to be deformed and get

.~a change in surface area to form the sheet of it

in desired.three-dimensional form. Fold lines become individual

times placed around the periphery of sheets being formed into deep-pressed products to.allow the paperboard to fold or yield along the lines of weakness or folds to

record reduction in surface area that takes place during pressing. However, the arrangement of fold lines, waves or corrugations in the sheet or blank will result in a shaped product with natural lines of weakness around which the product will bend more easily under less force than if the product was not weakened in this way. Shallow containers such as paper plates can also be formed from cardboard blanks that are not folded or bent, but the pressing operation will cause wrinkles or folds to form in the cardboard material at the edge and side walls of the container in a more or less random manner. These folds will again act as natural lines of weakness in the container around which bends

ing can take place.

In the usual method of pressing cartonbge holders from flat blanks, a sheet or cardboard web is cut to give the blank a circular shape for a plate after which the blank is pressed firmly between upper and lower dies having surfaces conforming to the desired shape of the finished container. The paperboard web is usually coated with a liquid-tight material on an over-

flat and can also have decorations printed under the coating. The upper and lower die surfaces are typically machined so that when they begin compression of the formed paperboard between them, the die surfaces will generally be evenly spaced over the entire surface area of the formed paperboard. The lower die is spring-mounted to limit the maximum force applied to the carton between dies and this force is distributed over the entire area of the carton if the distance between the dies is even. In practice, the machining of the dies will be such that random high and low points are usually formed on the surfaces of the dies, leading to randomly located areas of the carton that are strongly pressed while other areas are unpressed. The dies are generally heated to facilitate the forming and pressing operation. Paper-

or cardboard plates produced in this way have a

good decorative quality and liquid resistance due to the surface coating and they are suitable for large production volumes with relatively low unit costs. However, as indicated above, the plates suffer from a lower than desired level of stiffness and are subject to greater bending during normal household use than is possibly desirable.

Although the problems relating to the stiffness of pressed cardboard containers have long been known, there has been limited success to date in improving the stiffness properties of these products in a commercially practical manner. An example of a method which aims to improve the stiffness of pressed paper plates is shown in US patent no. 3,305,434, in which a method is shown where a cardboard with a very high moisture content, in the range of 15-35% by weight, is pressed between heated pressing dies which are specially designed to allow the escaping water vapor to escape during the pressing operation.

The cardboard blank starting material must thus be relatively soft and easily shaped into the desired shape. Deformation of the mold of the soft and flowable paperboard is prevented by bringing the dies to a stop at which the surfaces of the dies are uniformly separated by a distance approximately equal to or slightly less than the desired thickness of the formed container. The shaped cardboard material dries under the heat and pressure applied via the press forms and the fibers inside the material build up internal bonds whereby the drying helps to maintain the strength and stiffness of the deformed parts of the cardboard material. The apparent limitation of such a process is the complex press forms required to allow the release of water vapor from the pressed board, handling problems for the very moist board and longer production times required due to the time required to allow the removal of water vapor from the board during the press operation, all of which contribute to increasing production costs

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the nings. The cardboard container according to the present

- finning is formed from a fibrous substrate material in such a way that the raised areas of the container are essentially free of the type of fault lines found in cardboard containers pressed in a conventional manner. Examples of products manufactured according to the invention are containers with a bottom wall, an upwardly directed side wall that extends from the bottom wall and an edge that extends from the side wall. The bottom wall of the shaped container has substantially the same thickness and density as the blank, while the edge preferably generally has a density greater than that of the blank and is substantially denser in the areas where creases form in the edge during the initial shaping. The parts of the cardboard that are folded up during forming are essentially of the same thickness as the rest of the container, even if they contain more fiber material and the entire surface of the edge area is essentially smooth. The upward side wall or part of it can also be dense, especially in the areas where folds form. The container can be formed into different geometric shapes used for pressed cardboard products. The rim preferably has a downwardly directed rim portion which is compressed and densified and which has been found specifically to enhance the rigidity of the container structure. The cardboard starting material may be coated in a conventional manner to provide decoration and liquid tightness. Due to the lack of cavities and other lines of weakness, the container according to the invention will have a stiffness that is at least 40% and often 100% greater than conventional containers pressed from the same cardboard output material.

In the method for forming a cardboard sheet blank for the container as described above, the blank material should be selected to have a moisture content before forming in the range of 8-12% by weight, preferably 9.5-10.5% by weight. The blanks are pressed between a pair of complementary dies with a die surface generally in accordance with the shape of the pressed plate, but where adjacent surfaces of the dies in the edge area are closer to each other than at the bottom wall area as the die surfaces approach. During the forming operation, the surfaces of the two dies will engage the cardboard blank and deform the blank into the general shape of the formed product. However, when the die surfaces are still approaching each other, the closer surfaces of the dies at the edge will engage the cardboard in the edge area before the cardboard in the bottom wall part of the blank engages firmly and as a result, very high compression forces will be applied to the edge area and especially to the downwardly extending parts of the edge . Compressive forces may also be applied to the upward sidewall to push out wrinkles and voids formed therein during the initial forming of the container. The moisture in the board helps to weaken the fiber bonds within the board allowing the fibers to come apart and flow under the intense compressive force applied to the edge area, especially at the crease. Movement of the fibers inside the carton under pressure causes wrinkles and other fault lines in the edge to be substantially eliminated so that after the compression molds are removed from the carton, bonds between the fibers will be restored and the edge area of the shaped container will be essentially a whole structure.

Under preferred conditions, the press molds are kept at a temperature in the range of 120-160°C. This temperature range has been found to provide the best conditions for fiber flow and forming under the intense pressures applied by the dies without overheating the blank and causing surface blistering or burning of the board. When the moisture is driven out of the heated cardboard, fiber bonds will form again in the compressed areas. The dies are mounted in a conventional manner so that the movement of the die surfaces towards each other is stopped only by the compression of the cardboard material between them. The force applied to the dies is limited by the spring support for the lower die,

typically with a power of at least 2700 kp and preferably 3600

kp or more for containers in the usual diameter range of 23-25 cm. The largest part of the force between the dies is applied in the edge area of the shaped plate and gear

typical pressures in the pressure area of at least 14 kp/cm and also greater local pressures at the area where the carton initially

folded.

Further purposes and features will be apparent from the following detailed description together with the attached drawings, where: Figure 1 is a perspective view of a plate-like cardboard container according to the invention. Figure 2 is a cross-section of the container according to Figure 1 taken generally along the line 2-2 in Figure 1. Figure 3 is a cross-section of the upper and lower pressing mold used to press the container according to Figure 1 and which shows a flat blank in position between the press forms. Figure 4 is a simplified schematic image showing the clearance between the upper and lower die surfaces in Figure 3 when they are adjacent and press the cardboard blanks between them. Figure 5 is a photomicrograph (140X) of a cross section through the bottom wall portion of a known commercially manufactured cardboard plate. Figure 6 is a photomicrograph (80X) of a cross-section through the center of the edge portion of a known commercially manufactured cardboard plate. Figure 7 is a photomicrograph (80X) of a cross-section of a position adjacent the edge of the frame member for a known commercially available cardboard plate. Figure 8 is a photomicrograph (140X) of a cross-section through the bottom wall part of a cardboard plate produced according to the invention. in in

Figure 9 is a photomicrograph (140X) of a cross-section re-

- nom center of the edge part of a cardboard plate shaped according to the invention.

Figure 10 is a photomicrograph (110X) of a cross-section of a position adjacent to the edge of the frame part of a cardboard plate produced according to the invention.

With reference to the drawings, figure 1 shows a perspective of a cardboard or cardboard container in the form of a plate. This container structure will be described to illustrate the invention, although it can easily be understood that the invention can include many other container geometries. The shape of the plate 10 is typical of commercially produced plates which are distributed to the market: It has a substantially flat, circular bottom wall part 11, an upwardly directed side wall part 12 which serves to hold food and especially sauces on the plate and a curved edge part 13 which extending from the side wall. The plate end parts 11, 12 and 13 are formed as a whole with each other. Differences between

the different parts are best illustrated with reference to the cross-section shown in Figure 2. The flat bottom wall 11 of the plate extends to the position in the plate marked with 15 where the side wall 12 begins to rise upwards. The upward side wall 12 terminates at the position indicated by 16 in Figure 2 where the carton begins to bend up and down around a smaller radius to provide a downward bending edge 13 which terminates at the peripheral edge 17. The edge or flange 13 serves a number of purposes in the paper plate - the product. It provides a more aesthetically pleasing appearance than a plate which only had an upward facing side wall with a terminating edge and it provides a general transverse area that can be grasped by the user when the plate is to be carried. From a structural overall point of view for the plate, the most important function of the edge 13 is to make the plate rigid and more resistant to bending when held by the user. As can be seen from an examination of the cross-section shown in figure 2, the well-

wood shape of the bent edge 13 provide a structure

which is naturally resistant to bending about any radial axis extending from the center of the plate. If the cardboard forming the edge portion 13 is uniform and continuous, the plate will resist bending in the user's fund until the plate is so heavily loaded that the cardboard at the edge 13 is under a tensile load to cause the cardboard to yield and bend. The maximum tensile load in the plate under normal load will lie around a general radial cross-section through the edge area.

Although the theoretical maximum load that one cardboard plate can withstand is related to the breaking strength of the cardboard from which the plate is made, it has been found that plates made by the conventional blank press process have a much lower load capacity than can be expected as a result of folds and wrinkles formed in the edge section. These folds and folds will naturally occur in the incipient edge during forming to accommodate the reduction of the edge area as it draws radially inward during formation of the upwardly bent wall 12. The folds and folds extend radially over the edge and extend through a part of the upward facing side wall 12 which also has a somewhat shrunken surface area. The wrinkles or folds in the edge material create a break in the fiber material at the fold and break many bonds between the fibers and result in a radial fault line in the edge, which will form a natural hinge that is much less resistant to stresses created by loads on the plate than the original carton. As such wrinkles are unavoidable in normal pressing processes, it has not been thought possible until now to significantly increase the stiffness of plates pressed from flat cardboard blanks. Cardboard items, especially those to be deep pressed, are usually provided with a number of radial fold lines to control the number and position of the wrinkles in the formed product. However, such fold lines will not increase the stiffness of the finished product and in fact will usually tend to decrease the stiffness of shallow pressed products, compared to

1 containers that are not folded or folded.

The cardboard plate 10 according to the invention is also formed from a uniform flat blank of cardboard, either folded or unfolded, and thus must undergo folding in the side walls 12

and the edge 13. The resulting fold lines are shown for the sake of illustration at 20 in Figure 1. However, the plate 10 is made in such a way that the cardboard near the edge portions of the folds 20 is tightly compressed and substantially bound together so that the folds 20 at the edge do not form any natural hinge lines or lines of weakness and in fact exhibit a breaking strength which is substantially the same as that of whole carton.

As described in more detail below, the cardboard material in the edge 13 is compacted at the folds and any

cavities or fractures formed in the edge area of the boards 20 during the pressing operation are compressed and new bonds are formed between the tightly compacted fibers in these areas. The entire edge area is preferably densified and gets a something

reduced thickness compared to the bottom part of the plate.

As is also shown in the cross-section in Figure 2, in which the dimensions are exaggerated for illustration purposes, the thickness of the plate 10 at the flat bottom wall 11 and the folded side wall 12 is substantially the same as the nominal thickness of the unpressed blank from which the plate is made. . However, starting at a point marked 16 in the border area between the side wall part 12 and the edge part 13, the cardboard has increased in density and the thickness of the cardboard decreases towards the edge part 17. More particularly, the entire downwardly extending part of the edge, namely the part of the edge from the top 21 to the end edge 17 thus preferably compressed to a thickness which is somewhat smaller than the thickness of the bottom wall. The material at the edge is suitably denser than the cardboard material in the remaining part of the plate and the area at the folds 20 is substantially denser than the bottom wall. In general, the cardboard in the blank has a nominal thickness in the range of 0.25-1.0 mm.

with a basis weight in the range 160-650 g/m^. The density of

the cardboard in the bottom wall and the side wall parts is preferably in the range of 0.66 g/cm 2. •

Containers produced according to the invention have a

much greater rigidity than comparable containers made from corresponding cardboard blank material according to prior art. In order to provide a comparison of the stiffness for different plates produced with a configuration such as the plate 10, a test method has been used in which the resistance force of a plate under a standard bending is measured. The test device used, a "Marks II Plate Rigidity Tester" has an edge-shaped support platform on which the plate rests. A pair of plate guide supports are mounted on the support platform in positions approximately corresponding to the radius of the plate from the top of the edge-shaped platform. The cardboard plate is placed on the support platform with its edges lying against two guide supports so that the platform extends to center of the plate. A straight leveling rod mounted for up and down movement parallel to the support platform is moved down until it contacts the top of the edge on each side of the plate so that it is held lightly between the platform and the horizontal leveling rod. A sensor with a movable force gauge , as a "Hunter Force Gauge" is moved in to a position just in contact with the top of the edge under the leveling rod at the unsupported side of the slab. The sensor is lowered to bend the edge downwards 1.3 cm and the force exerted by the bent plate on the sensor is determined for a typical commercially manufactured cardboard plate with a diameter of 23 cm of a similar shape to the plate 10, the stiffness readings will generally be approx. 60 g or less (using the "Hunter Force Gauge"), while the plate 10 as shown in Figures 1 and 2 and manufactured as described above can be produced with average stiffness measurements of at least 90 g and generally above 100 g.

Figure 3 shows a cross-section of an upper press die 25 and a lower press die 26 which are used to press a flat, circular cardboard blank 27 into the shape of the plate 10.

The construction of the pressure plates 25 and 26 and the equipment on which they are mounted is essentially conventional,

for example as used in presses manufactured by "the Peer-less Manufacturing Company". In order to promote retention and shaping of the blank 27, the press molds are segmented in the manner shown. The lower die 26 has a circular base part 29 and a central circular platform 30 which is mounted

movable relative to the base 29. The platform 30 is edge-operated in a conventional manner and is pressed towards a normal position so that its flat top forming the surface 31 is initially above the forming surface 32 of the base 29. The platform 30 is mounted for sliding movement on the base 29 and where the entire base 29 itself is mounted in a conventional manner on springs (not shown). Because the blank is pressed very hard at its peripheral edge area, moisture in the paperboard which is driven off from it during pressing in the heated pressing nozzles cannot easily escape. To allow the release of this moisture, at least one circular groove 33 is arranged in the surface 32 of the base, which groove is vented to the atmosphere via a channel 34.

In a similar way, the top press mold 25 is segmented into

an outer ring part 35, a base part 36 and a central platform 37 with a flat shaped surface 38. The base part has a curved, symmetrically shaped surface 39 and the outer ring 35

has a curved forming surface. The central platform 37 and the outer ring 35 are slidably mounted on the base 39 and preloaded by springs not shown to the normal position shown in figure 3 in a commercially common way. Press die 25

is mounted reciprocally to and from the lower press mold 26. During the press operation, the workpiece 27 is first placed on top

the flat forming surface 31, generally underlying the bottom wall portion 11 of the plate to be formed and the forming surface 38 makes initial contact with the top of the blank 27 to hold it in place when the forming operation begins. Further downward movement of the die 25 brings the spring-loaded forming surface 40 of the outer ring 35 into contact with the edges of

blank 27 to begin shaping the edges of the blank above

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the underlying surface 32 in the area which will define the bent edge 13 therein. finished plate. However, due to that the ring 40 is. spring-loaded, the cardboard material in the edge material will not be significantly compressed or deformed by the initial forming because the force applied by the forming surface 40 is relatively small and limited by the spring forces applied to the press mold segment 35. Eventually, the press mold 25 will move sufficiently far down so that the platform segments 30 and 37 in the ring segment 35 is fully compressed so that the adjacent parts of

the forming surface 38 and 39 are coplanar and adjacent parts of the surfaces 35 and 40 are coplanar and correspondingly the forming surface 31 will be coplanar with the adjacent part of the forming surface 32. The upper press die 25 continues its downward movement and drives the entire lower press die 26 downwards towards the forces of the springs, not shown, which carry the press mold 26. At the full extent of the downward movement of the upper press mold 25, these will exert a force on each other via the shaped blank 27 which separates them, which force is equal to the force applied by the compressed springs that support the press mold 26.

Thus, the magnitude of the force applied to the workpiece 27 and the distribution over its area can be adjusted by changing the stroke length of the upper die 25.

In a conventional manner, the dies 25 and 26 can be heated with electrical resistance elements not shown and their

temperature is controlled to a selected level by controlling the temperature of the dies by means of thermistors, not shown, mounted as close as possible to the forming surfaces of the dies.

In the press operation for the production of known cardboard plates, the press molds 25 and 26 will be machined so that the forming surfaces 38, 39 and 40 of the press mold 25 will nominally be essentially parallel to the forming surfaces 31 and 32 in the lower press mold 26 at a selected

distance approximately equal to the thickness of the workpiece pressed.

Based on geometric considerations of the press mold surfaces

- it can be seen that the folded-up side wall and any bent-down edge will be exposed to greater compression forces initially if the selected distance at which the press mold surfaces are parallel is less than the workpiece thickness, while the top and the edge, as well as the bottom wall will receive essentially all of the compression forces if the selected parallel distance is greater than or equal to that of the workpiece thickness. In both cases, the force between the dies will be distributed over the entire area of the carton between the dies, including the bottom wall which makes up more than half the area of the pressed plate, except for irregularities in the machining of the die surfaces which cause high or low points.

As indicated above, plates pressed using a uniform surface clearance exhibit relatively low stiffness, mainly as a result of severe breakage of the fibers at the wrinkles in the edge of the plate.

According to the present invention, the forming surfaces 38, 39 and 40 in the upper die 25 are nowhere completely parallel to the forming surfaces 31 and 32 in the lower die 26. The preferred distance between the die surfaces according to the invention is shown in the section in Figure 4 as illustrates a cross-section of two dies close together, substantially in the position they would be in if a cardboard blank were held between them during the pressing operation. Naturally, the relative distance between the die surfaces will depend on the thickness of the cardboard blank. which is formed. However, the topography of the die surfaces can be specified generally by assuming that the circumferential position 41 in the die surfaces at which the sidewall of the plate ends and the rim begins, the die surfaces are separated by a thickness substantially equal to the nominal thickness of the paperboard blank. The die surfaces are preferably shaped so that the distance between the surfaces decreases gradually and continuously from such a reference position

towards the outer end of the edge for the cardboard plate that is formed between the pressing forms. The localization at the press die surfaces corresponding to the outer edge is denoted by 42 in Figure 4 and the localization at the press die surfaces corresponding to the top of the edge in the shaped plate is denoted by 43 in Figure 4. For cartons for plates with conventional thickness, i.e. in the range 0.25-1 .0 mm, it is preferred that the distance between the upper die surface and the lower die surface decreases continuously from the nominal cardboard thickness at position 41 to at least 0.05 mm less than the nominal thickness at position 43 and to at least 0.08 mm less than the nominal thickness at the outer edge position 42. The distance between the upper and lower die at other points that are not along the edge, such as at the center point 44 in the side wall area and the middle part 45 in the rounding between the bottom wall and the side wall at the beginning 46 in the side wall and at the bottom wall 47 is preferably at least so

as large as the nominal thickness of the cardboard sheet. More particularly, the distance between the press die surfaces at the bottom wall is substantially greater than the thickness of the cardboard blank so that the bottom area is exposed to little pressure. As an example for a board blank with a nominal thickness of 0.41 mm, satisfactory die spacings are as follows: Position 42, 0.33 mm, Position 43, 0.36 mm, Position 41, 0.41 mm, Position 44 , 0.48 mm, and at positions 46, 47 and 48 at least 0.51 mm. The relevant press mold clearance can be measured by laying strips of solder radially over the surface of the bottom press mold and pressing the molds together and measuring the thickness of the solder at different positions of the mold surface after pressing.

It will be obvious from the considerations given above of the mold clearance as discussed above that the pressing dies 25 and 26 engage the cardboard blank between them and all or substantially all of the force between the dies will be exerted on the annular area of the pressed blank which generally

lies between the marked positions 41 and 42 in Figure 4. ^The springs on which the lower die 26 is mounted are typical'

controlled so that the full stroke of the upper die

25 will result in a force being applied between the molds in the range of 2700-3600 kp. For a paper plate with the usual diameter of 23 cm after pressing, a force between the forms of, for example, 3200 kp evenly distributed over the area of the plate will lead to a pressure of approx. 7.7 kp/cm over the entire area of the plate. However, with press molds that are shaped according to the invention, as shown in figure 4, where the edge area of the press mold surfaces are separated closer to each other, the force will be concentrated along the edge of the plate. With eri typical width for the edge part,

where the distance between lines 41 and 42 for a 23 cm plate will be approx. 1.25 cm, for example applying a force of 3200 kp between the forms will be concentrated along the edge area and the pressure applied to the carton along the edge will be approx. 37 kp/cm^. As a result of an inevitable small misalignment between the upper and lower form, high and low points in the forms and variations in the cardboard thickness, the pressure applied to the cardboard at certain places along the edge will be less than this maximum value, but will in any case be at least 14 kp/cm which is twice the pressure that would be applied to the edge if the compression forces were distributed evenly over the total area of the pressed plate, which has normally been the case in known pressing operations for the production of cardboard plates.

The compression forces would be even greater at the folds of the carton as these areas are raised above the rest of the carton and contain more fibrous material. These folded areas will make up a small percentage of the edge's area, for example 4-5%, so that the compression forces concentrated in these areas can rise to several hundred kp/cm^ . This immeasurably large pressure serves for a great densification of the fibrous material in the folds at the edge.

The ideal die surface configuration as indicated above is preferably maintained around the entire circumference of the molds so that all mold surfaces are completely symmetrical. Naturally, the practical machining of the forming surfaces will not make it possible to maintain perfect symmetry, nor will it be possible to achieve

at any radial cross-section through a

- practical form to have the preferred die surface distance as indicated above. The most critical tolerances are those within the edge area from position 41 to position 42. It is highly preferred that the shape clearance at the edge is uniform> along a circumferential line around the edge so that all folded areas in the edge are exposed to the same intense compression forces. A satisfactory radial gradient for surface clearance is for the nominal board thickness "N" at position 41, N -0.05 mm at position 43 and N -0.08 mm at position 42. Satisfactory results have been obtained with forms measured to be in accordance with this gradient within + 0.05 mm, while the best results were obtained with forms where the gradient was within 0.03 mm, provided that the distance between the forms at positions 45-47 is at least as great as the nominal cardboard thickness N and preferably 0.08-0.20 mm greater than the nominal thickness N.

By using a die surface configuration as described above, it is possible to impose compressive forces to the edge portion of a magnitude capable of causing plastic deformation of the edge region of the plate when the other conditions of the method are satisfied, particularly with respect to moisture content. in the object being shaped and with regard to the temperature of the press moulds. Under proper process conditions, the fibers in the edge area, especially in the folds, will apparently break the fiber-to-fiber bonds and compress under the very high applied tensile stresses to transform the fiber-to-fiber bonds. Application of these form distances with applied high forces, for example 2700-3600 kp, will lead to a compression of the edge area of 15-20% or more of the workpiece thickness, although the fibrous material tends to return to an uncompressed thickness after the pressure is relieved. Although such high loads can be expected

.to cause wrinkles or localized tearing of the cardboard _..,in the edge area, this will not take place, however. IN

instead, the cardboard material in the edge area will behave as if

it was a plastic, compressible material.

It has been found that the correct moisture levels in the paperboard are a condition for such yielding or plastic behavior inside the paperboard. In addition, the molds are kept at a high but not too high temperature to facilitate the pressing process. The cardboard which is formed into the blanks 27 is conventionally produced by a wet laying process and is typically available in the form of a continuous web on a roll. The cardboard output material preferably has a basis weight in the range 160-650 g/m 2 and a thickness in the range 0.25-1.0 mm. Lower basis weight and less thickness are preferred to facilitate the forming process and from an economic point of view. The cardboard starting material used to make the cardboard sheets is typically made from bleached pulp and is usually double coated with clay on one side. Such cardboard starting material normally has a moisture content in the range of 4.0-8.0% by weight.

The effect of the compression forces on the edge is greatest when correct humidity conditions are maintained within the carton, i.e. at least 8% by weight and less than 12% by weight of water, preferably 9.5-10.5% by weight. The cardboard in this area contains sufficient moisture to deform under pressure but not such a large amount of moisture that water vapor interferes with the forming operation or that the cardboard is too weak to withstand the high applied compression forces. In order to achieve the desired moisture level in the cardboard output material when it comes off a roll, the cardboard is treated by spraying or rolling on a wetting solution, mainly water, although other ingredients such as lubricants can be added. The moisture content can be controlled using a handheld moisture measuring device of the capacity type to ensure that the desired moisture conditions are maintained. It is preferred that the paperboard output is not formed until at least 6 hours after the wetting operation to allow

.that the humidity inside the carton reaches equilibrium.

As a result of the intended end use of cardboard plates

In these cases, the cardboard output material is typically coated on one side

. with a liquid-tight layer.or number of such layers. In addition for aesthetic purposes, the material is often initially pressed before coating. As an example of a typical coating material, a first layer of a polyvinyl acetate emulsion may be applied over the printed board with a second layer of nitrocellulose lacquer applied over the first layer. The starting material is moistened on the uncoated side after all printing and coating steps have been completed. In a typical forming operation, the sheet of cardboard will be fed continuously from a roll via a punching machine, not shown, to form circular blanks 27 which are then introduced into a position between the upper and lower pressing dies 25 and 26. The dies are heated as described above for to facilitate the forming process. It has been found that the best results are obtained if the upper die 25 and the lower die 26, especially their surfaces, are kept at a temperature in the range of 120-160°C + 6°C. These die temperatures have been found to facilitate the plastic deformation of the paperboard in the edge area if the paperboard has the preferred moisture content. At these preferred mold temperatures, the added heat to the workpiece is apparently sufficient to release moisture inside the workpiece below the edge and thereby promote deformation of fibers without overheating the workpiece and thereby causing blisters as a result of steam being released or burning of the workpiece material. It is obvious that the amount of heat applied to the carton will vary with the amount of time that the dies are in press position with the carton. The preferred press mold temperature is based on individual holding times which normally occur at normal production rates of 40-60 presses per minute and higher or lower temperatures for the press molds will generally . necessitate higher and lower production rates, respectively.

The properties of the cardboard containers produced according to the present invention can best be compared with known ones

such produced from similar materials by sub-

search the photomicrographs in figures 5-10.

Figures 5-7 showed different cross-sectional sections through a cardboard plate produced according to the known technique in which the die surfaces are evenly spaced, while Figures 8-10 are cross-sectional sections through a cardboard plate produced according to the present invention. Both paper plates were formed from a low density bleached paper plate stock with a flat weight of 277 g/m and with a thickness of 0.41 mm, which was clay coated on one side and printed on one side with standard printing ink, coated with a first layer of polyvinyl acetate emulsion and further coated with a nitrocellulose varnish. The density of the cardboard was 0.66 g/cm 2 with a "Taber" rigid

hot in the cardboard area, in the machine direction 110-300 units and across the machine direction 50-165 units. The image shown in figure 5 (140X) is through the central part of the known tall-lark structure. It can be seen that there are a number of voids inside the fiber structure which indicate that the cardboard was not sufficiently compacted even though the fiber distribution is relatively even. The thickness of the cross section is approx. 0.41 mm. Figure 6 (80X) is a cross-section through the edge area of the known

plate, generally taken along a circumferential line at the top of the rim. The particular image in Figure 6 is through one of the areas in the edge that has a fold or wrinkle in it. As can be seen from examination of the image according to Figure 6, the cardboard at the wrinkle is clearly cracked, leaving large voids between the fibers and where adjacent fibers are torn apart so that a fault line or a very weak area exists inside the cardboard at the fold. In addition, it is clear that the surface of the cardboard at the wrinkles is discontinuous and that large gaps exist between adjacent parts. The thickness of the cross section at the fold is approx. 0.66 mm and is greater than the original thickness over a certain distance away from the fold.

Figure 7 (80X) is a section along the edge generally along a circumferential line at a position near its outer edge. This j "Ti section shows the termination of one of the wrinkles running re-

nom the edge in a familiar plate, Again it is in wrinkle re-

. advised wide cavities and one rough, discontinuous surface structure. The maximum thickness is approx. 0.51 mm at the crease.

In the image according to figure 8 (140X) is a cross-section through the approximate center of a plate produced according to the invention. A comparison between figure 8 and figure 5 shows that the structure of the cardboard at the center of the pressed plate is essentially similar in both cases, both have a relatively smooth surface and a significant distribution of voids through the fiber matrix inside the cardboard, which is distinctive for the unpressed, low-density board material from which the pressed plates are made. The average thickness is approx. 0.41 mm. Figure 9 (140X) is a photomicrograph taken along a section through the top of the edge part of a plate manufactured according to the invention and where the section lies along a circumferential line through one of the folded or wrinkled areas of the pressed plate. The difference between Figure 9 and Figure 6 is significant. The cardboard in the area through which the section according to figure 9 was taken is very compacted and there is very little empty space between the fibers. The structure in the folded area is in marked contrast to the folded areas in figure 6 where there are open cavities between the cardboard, which causes the strongly weakened state of the edge in this area. The cardboard in the edge shown in figure 9 is compacted and its density is increased so that the cardboard is clearly denser in the center area shown in figure 8. The maximum thickness for this cross-section is present at the two folds shown and is approx. 0.43 mm, which is essentially the same as the bottom wall thickness. Away from the folded area, the thickness of the edge is about the same or somewhat thinner than that of the bottom wall. As the bent area contains significantly more fibrous material than in the rest of the carton, possibly 40-100% more, the density in the folded areas is significantly greater than in the remaining part of the carton.

The surface of the carton in figure 4 is essentially

smooth and continuous, this again in contrast to the discontinuous surface shown in the image according to Figure 6 and the folds inside the carton in Figure 9 are folded back on themselves and the folded surfaces are pressed tightly together. In particular, the bottom surface of the section shown in figure 9 is smooth and continuous instead of being broken up at the wrinkle lines, as shown in figure 6. The coating covering the top surface of the plate is clearly visible in the image according to figure 9 and this coating illustrates well where folding begins at the edge of the plate when it was shaped. However, the very high pressures applied to the edge of the plate have caused virtually all traces of the folds to disappear at the bottom of the carton where the fibers are essentially bound together, leaving only rudimentary traces of the fold at the top of the carton where the coating on the surface prevents entanglement of fibers. The heat and pressure applied during the molding process may be sufficient to cause some melting and surface adhesion between adjacent coated surfaces overlying the fold lines, even though the outer nitrocellulose coating is resistant to heat and pressure.

A cross section through a plate according to the invention taken just inside the outer edge is shown in Figure 10 (110X). Here again it can be seen that the fibers inside the plate are essentially compacted and practically all traces of folds that existed in the edge area during the forming operation have disappeared, except for small areas where tops of the top coating in the folded areas have been laid back on themselves. The bottom of the cardboard surface is again smooth and unbroken, this in stark contrast to the cut in the well-known tall box, as shown in figure 7. As is clear from figure 10, the fibers are densely and closely compressed together and only a few cavities or air pockets are left and the overall structure is densified so that although the edge of the plate becomes progressively thinner towards the edge, as shown in figure 2, the basis weight of the cardboard in this area is essentially uniform as a result of compaction of the fibres. The thickness of the cardboard shown in figure 10 is approx.

0.389 mm, which is 4-5% thinner than the bottom wall. This densification of the plate in the edge area and folding of the folded surface areas against itself to reform the edge into a substantially whole structure leads to a marked increase in stiffness for the plate, as described earlier. Naturally, a successful production of pressed containers according to the present method will require consideration with regard to the pressing process according to good manufacturing technique. In particular, it is necessary to ensure that the upper and lower dies 25 and 26 are properly aligned so that they engage with a blank in between in the desired manner. Such installation technology is a normal part of press maintenance. Observations of pressed plates with the molds can be made to ensure that the molds are properly aligned, which shows up as evenness in the appearance of the folded outer edges of the plate rim. It must be understood that the invention is not limited to the particular construction and arrangement of parts and the particularly described method, but includes all such modified forms which fall within the scope of the following claims.

Claims (22)

1. Procedure for manufacturing a container of a flat blank of a fibrous substrate, characterized by the following steps: (a) forming the blank into one shaped container with a bottom wall, an upward facing side wall extending from the bottom wall and an edge portion extending from the side wall, where folded areas are formed at the edge when this shaped to occupy the reduced area of the edge portion during forming, (b) applying sufficient heat and pressure to the edge portion to fold the folded surfaces of the edge back upon itself and densify the fibrous material in the folded areas of the edge such that the folded areas is compressed to a thickness which is essentially the same as that of the blank from which the container is formed and with a density which is substantially greater than that of the blank. 2. Method according to claim 1, characterized by the further feature that before the blank is shaped, it is moistened to a water content of 8-12% by weight. 3. Method according to claim 2, characterized in that the item is moistened to a water content of 9.5-10.5 weight*. ■ 4. Method according to claim 1, characterized by applying heated surfaces to the edge at a temperature in the range of 120-160°C on the opposite surfaces of the edge. 5. Method according to claim 1, characterized in that a pressure of at least 14 kp/cnr is applied to the edge. 6. Method according to claim 1, charac- In sert in that the edge has a downwardly bent part which is primed to a thickness less than that of the blank. 7. Method for the production of a plate-like, pressed container, characterized by the following steps: (a) providing a flat fibrous substrate with a thickness of 0.25-1.02 mm, a basis weight in the range 160-650 g/m<2 >and with a water content of 8-12% by weight, (b) providing upper and lower dies with mating surfaces defining the desired shape of the finished container therebetween, each mold surface for each mold comprising a bottom wall portion, an upwardly extending side wall portion from the bottom wall portion around its periphery and a bent edge portion extending from the side wall portion, and wherein the mold surfaces are shaped such that the bottom wall portions of the mold surfaces are separated by a distance approximately equal to the thickness of the blank being provided, the distance between the edge portions of the molds being less than the thickness of the provided blank, (c) heating the dies to a temperature in the range of 120-160°C, and (d) pressing the blank between the surfaces of the upper and lower die under a pressure sufficient to cause the fibrous substrate in the edge portion of the pressed blank to be compressed to a thickness less than the original thickness of the blank, while the bottom wall portion of the pressed container remains at substantially the same thickness as the blank. 8. Method according to claim 7, characterized in that the edge parts of the molds are curved and that the surface of the upper and lower mold is shaped so that _ when the surfaces of the press molds are separated in such a way that the points I on the press mold surfaces correspond to the intersection between edges part and the side wall part are separated by a distance essentially equal to the unpressed thickness of the workpiece, then the distance between the upper and lower mold surface at the top of the ring-edge parts is approx. 0.05 mm less than the thickness of the workpiece and the distance between the upper and lower surface of the edge of the workpiece is approx. 0.08 mm less than the thickness of the blank and where the distance between the upper and lower surfaces of the die at the bottom wall and the side wall is equal to or greater than the thickness of the blank. 9. Method according to claim 7, characterized in that the workpiece is pressed with a force of at least 2,700 kp which is applied between the upper and lower pressing forms. 10. Method according to claim 7, characterized in that the subject has a water content in the range 9.5-10.5% by weight. 11. Method according to claim 7, characterized by providing a blank including the steps of providing a fibrous substrate blank with a water content of less than 9% by weight until the cardboard in the blank has a water content in the range of 9.5-10% by weight. 12. Method according to claim 7, characterized in that the upper pressing mold is segmented and has an outer concentric ring part which first comes into contact during the pressing step and presses this to fold over the end edge of the workpiece and retain the workpiece at the edge while the remaining part of the upper die descends and presses the workpiece. 13. Method according to claim 7, characterized by the step of providing a blank including one with a liquid-tight coating on a surface thereof. " 14. Method according to claim 7, characterized in that a carton is provided with a in thickness. of approx. 0.41 mm and a basis weight of approx. 278 g/m2. 15. Apparatus for forming a plate-like container from a circular cardboard blank of selected thickness, characterized in that it comprises upper and lower dies with mating surfaces which are generally shaped as the desired shape of the finished container, having a substantially flat circular bottom wall portion, a concentric upwardly directed side wall portion extending from the bottom wall portion and a concentric, curved, downwardly bent annular edge portion extending from the side wall portion, the surfaces of the upper and lower dies being shaped such that when the surfaces are separated at the position at the boundary between the side wall portion and the rim edge part, at a distance which is essentially equal to the chosen thickness for the cardboard blank, the distance between the upper and lower press mold surfaces decreases substantially continuously outwards along the edge part to such an extent that the mold surfaces are separated approx. 0.05 mm less than at the top of the edge part and are 0.08 mm closer to each other at the outer edge of the edge part and where the distance between the mold surfaces at the side wall and bottom wall parts is equal to or greater than the distance between the mold surfaces at the intersection between the side wall and the edge part. 16. Apparatus according to claim 15, characterized in that the distances between the surfaces of the side wall part and the bottom wall part are at least 0.05 mm greater than the distance between the mold surfaces at the intersection between the side wall and the edge part. 17. Apparatus according to claim 15, characterized in that the selected cardboard blank has a thickness in the range 0.25-1.0 mm. 18. Apparatus according to claim 15, characterized in that the upper die is segmented and has a concentric outer ring part which comes into contact with the workpiece "to first roll over the edge part of the workpiece and hold the workpiece at the edge when the remaining part of the upper die descends down and pushes the subject. 19. Plate-like cardboard container with high rigidity, characterized in that it comprises a bottom wall, a side wall that extends up from the bottom wall along its periphery, a bent edge that extends from the side wall, the bottom wall, the side wall and the edge being formed as a whole of cardboard and the edge has a number of folded areas where the cardboard is folded over and sealed to itself so that the surface of the cardboard which forms the edge is essentially smooth and continuous and the folded areas essentially have the same thickness and are substantially denser than the bottom wall. 20. Container according to claim 19, characterized in that the thickness of the bottom wall is in the range 0.25-1.0 mm and where the edge is approx. 0.025 mm thinner than the bottom wall. 21. Container according to claim 19, characterized by a liquid-tight coating on a surface thereof. 22. Container according to claim 19, characterized in that the bottom wall is essentially flat and has a circular periphery and where the edge is curved and has a downward-facing part which is thinner and denser than the bottom wall.