This invention relates to a pressure plate for use in forming a carton bottom, to a method of forming a carton bottom, and to a carton.

Various formations of packaging container bottom have been proposed which enable the container to stand upright stably. Examples are disclosed in DE-U1-29705940; W0-A1- 01/87720; WO-A1-2008/089959; and US-B1-647102 . For example, the latter discloses an apparatus for the formation of packaging from unitary blanks, comprising a first tool having a convex end surface, the convex end surface having edges that define a first centre segment and a first pair of triangular segments, and a second tool having a concave end surface, the concave end surface having edges that define a second centre segment and a second pair of triangular segments. Each end surface is of opened U-shape with rounded edges. The blanks may be composed of a relatively rigid sheet of paper coated or otherwise covered with a thermoplastics.

Packaging cartons containing flowable substances, particularly liquid packaging cartons, are conventionally made from a packaging laminate comprised of paperboard and innermost and outermost layers of moisture barrier plastics, possibly with the interposition of an oxygen barrier layer. Such cartons have a tendency, after filling and top-sealing, for the middle part of the external surface of the bottom of the carton to protrude slightly downwards beyond the corners of the external surface, and are thus unstable. Therefore, the filled and sealed carton is rendered less desirable by an end customer, particularly since it is known that bulging of a carton can indicate deterioration of the contents. In the case of cartons made by a method including folding of blanks, proposals have been made for so forming the carton bottom, the shape of which is determined by the shapes of respective active surfaces of a mandrel cap and a pressure plate which co-operate together, that the middle of the external surface of the carton bottom is recessed relative to the corners of that surface. One example is disclosed in US-A-4 , 838 , 847 where the recess in the external surface is substantially pyramidal and comprises a small, flat, square, uppermost face and four slanting trapezoidal faces extending downward from the uppermost face. Another example is disclosed in US-A-5, 482, 204 in which that external surface is formed to an approximate lenticular geometry that is concave within the bottom corners of the carton, the lenticular geometry being a substantially symmetrical stepped pyramid of diminishing area step-planes. In both of those prior documents, corners additional to those of the bottom of the carton itself are formed and this risks damaging the bottom seals and the barrier layer (s) of the laminate packaging material, especially where the additional corners so formed are close to the corners of the carton bottom. Here, it has to be borne in mind that, in the formation of the latter corners, there are already risks of damaging the barrier layer (s) .

According to one aspect of the present invention, there is provided a pressure member for applying pressure to outside surfaces of carton bottom panels, said member having a pressing surface of which a middle is in the form of an outwardly curved dome.

According to another aspect of the present invention, there is provided a method of shaping outside surfaces of carton bottom panels, comprising applying pressure to said outside surfaces so as to produce a carton bottom having an external surface of which a middle is in the form of an inwardly curved dome.

According to a further aspect of the present invention, there is provided a carton of which a bottom has an external surface of which a middle is in the form of an inwardly curved dome.

Owing to the present invention, it is possible to shape the middle of the bottom without forming additional corners. Moreover, the peripheral edge of the dome can be spaced significantly from the carton bottom corners and thereby not promote fracture of the barrier layers in the regions of those corners. Furthermore, all other things being equal, a dome shape is stronger than- a pyramidal shape. Nevertheless, it is still possible for the carton to have a high stability when on a flat, horizontal surface.

Advantageously, for a carton which is of substantially square horizontal section, the dome, in plan view, is substantially circular, whereas, if the carton is oblong rectangular in horizontal section, the dome will itself be of a rounded, oblong form in plan view. Preferably, the curved dome in the external surface of the bottom of the carton closely approaches the edges of the external surface.

The carton may be other than rectangular in horizontal section, for example triangular or octangular.

Each dome is advantageously continuously curved in three dimensions perpendicular to each other and preferably has a width (or minimum width) to height ratio of between about 40:1 and about 10:1, very preferably approximately 20:1. In use of the pressure member, which will usually be in the form of a pressure plate, in forming the bottom of the carton it will usually co-operate with a mandrel member, generally in the form of a mandrel cap, with the mandrel cap having an external surface shaped complementarily to the shape of the dome of the pressure plate. Various forms of sealing of the carton bottom may be employed, such as ultrasonic, induction, high-frequency and hot air.

In order that the invention may be clearly and completely disclosed, reference will now be made, by way of example, to the accompanying drawing, in which: -

Figure 1 shows a diagrammatic axial section, taken on the line I-I of Figure 2 , through a pressure plate and a mandrel cap of a packaging machine for flowable substance, in a condition in which they have formed a bottom of a carton, with the carton being shown fragmentarily ; and

Figure 2 shows a diagrammatic top plan view of the items shown in Figure 1.

Referring to the drawing, there is seen a metal, e.g. steel, mandrel cap 2, a plastics-coated paperboard carton 4, and a pressure plate 6 which consists of a metal base plate 8 and a relatively resilient facing plate 10 of a relatively hard thermoplastics, such as polyurethane . Alternatively, although not so shown, the pressure plate 6 could be made of a metal softer than steel, for example aluminium and the mandrel cap 2 also made of a softer metal .

The pressure plate 6 has a pressing surface 12 of which the middle 14 is in the form of an outwardly curved part-spherical dome having a width to height ratio of approximately 20:1. As can be seen from Figure 2, the carton 4 is of square horizontal cross-section, with a free end edge 16 of a major, outer, substantially rectangular, bottom panel 18 overlapping a minor, outer, substantially rectangular, bottom panel 20. The pressure plate 6 has a pressing surface 22 of which the middle is in the form of an outwardly curved dome 24. The dome 24 is very shallow, having a width to height ratio of approximately 20:1, is circular in outline and, as can be seen from Figure 2, closely approaches the four edges of the external surface of the bottom of the carton 4. The external surface of the mandrel cap 2 is formed with an inwardly curved dome complementary to that of the plate 6, and with recesses 25 for entry of air to discourage build-up of vacuum between the cap 2 and the carton 4 when the carton is stripped from the mandrel, which such vacuum would tend to oppose. Screws 26 attach the mandrel cap 2 to the body (not shown) of the mandrel, whilst the pressure plate 6 is formed with screw holes 28 whereby it is attached to the drive (not shown) for the pressure plate. Bar-form or cylindrical staking arrangements (not shown) may also be provided at the mandrel cap 2 and the pressure plate 6 to form stake points in the carton bottom.

In use, the carton bottom is heat-and-pressure sealed, but the co-operation between the mandrel cap 2 and the pressure plate 6 forms a corresponding dome in the middle of the carton bottom. In the external surface of the carton bottom there do remain four corner regions 30 which, nevertheless, tend to be pulled inwards by the action of the dome 24, so that the carton will be supported upon a horizontal surface substantially at the four corners and the edges of its bottom, so that the carton should stand stably upon a horizontal surface. A particular advantage of use of a relatively resilient material for the plate 10 is that it is more readily able to adapt itself to the panel edges in the external surface of the carton bottom and thus to promote pressure equilibrium over the inwardly domed middle of the carton bottom.

Another advantage of the apparatus described with reference to the drawings is that it is easy to retro-fit, since an existing mandrel cap can usually be readily replaced by the cap 2 and an existing pressure plate can usually be readily replaced by the plate 6.