The invention relates to a method of deep-drawing a blank to manufacture a container comprising a base with walls joined thereto comprising the steps:

(a) providing a deep-drawing blank comprising a base part that is intended to form a base of the container after deep-drawing, at least two, essentially rectangular wall parts each extending from an adjoining side of the base part and intended to form, after deep-drawing, essentially flat wall parts of the container projecting upwards from the container base, and corner parts which are intended to form curved corner or transition parts after deep-drawing, each extending between two said wall parts, and

(b) deep-drawing the deep-drawing blank to form the containe .

Containers of different sizes are used among other things for packaging foodstuffs such as for example essentially square, PP film coated steel containers with tapering walls for packaging animal foods, often referred to as pet food containers. Such a container is produced by deep-drawing from a deep-drawing blank. After deep-drawing the container generally undergoes some further treatmen . To this end the production process may include the steps of processing the deep-drawn container in order to obtain

an exact flange width on its rim, and beading or turning the flange.

An undesired phenomenon can occur during the deep-drawing of the rectangular containers, namely the occurrence of buckling. This buckling may be differentiated into two types: (a) primary and (b) secondary buckling. Primary buckling, also referred to as wrinkling, is buckling that occurs in the flange of the deep-drawn product. This primary buckling has a disadvantageous effect on the flatness of the closing edge, the so-called sealing edge. If this wrinkling is too large then the container cannot be sealed. Wrinkling can be suppressed by, among other techniques, applying a higher blank-holder force. Secondary buckling, also referred to ^" simply as

"buckling", is buckling in a corner of the container. This secondary buckling, if extensive, results in separation of the film and paint coating on the inside and the outside of the container respectively. This dela ination can cause the base material of the container to corrode. When less extensive, the secondary buckling can lead to damage to the film and/or the paint coating. In this situation too the base material can corrode. One consequence of the corrosion can be that a foodstuff packed in the container is more likely to decay.

The secondary buckling is not easy to prevent by adjusting the blank-holder force during the deep-drawing process because, when deep-drawing a container with

tapering walls, a free forming zone is present in the deep- drawing blank between the punch and the die. If the blank- holder force is too high it can cause fracture of the deep- drawing blank. That might be avoided by varying the blank- holder force, but this is difficult to do in practice due to the high rate of production required for deep-drawing containers.

In order to prevent the problems described, it is desirable to be able to produce a buckle-free container. In addition a buckle-free container is aesthetically more pleasing.

Producing the container in accordance with the method of the invention can reduce or prevent the occurrence of secondary buckling. By reducing the occurrence of secondary buckling less undesired delamination of the coating appears and at the same time an aesthetic effect is obtained as a consequence of an attractive buckle-free container.

In accordance with the invention a method of the type stated in the preamble is now provided, wherein the geometry for the deep-drawing blank is so arranged that a standard value or coefficient K, defined as the quotient of the surface area of the wall parts of the blank and the surface area of the corner parts of the blank, is chosen to be less than or equal to a given critical value.

There are a large number of mutually dependent parameters which affect the secondary buckling, such as for example the shape of the container, the material used, the

tooling used, and the associated process parameters for the deep-drawing, such as for example the blank-holder force. Changing the geometry of the ultimate finished product is the most obvious way of preventing buckling, but for commercial reasons this is often not possible. In the case where the product geometry and the tooling are fixed, in order still to suppress secondary buckling, it has now been found that relatively simple adjustment of the geometry of the deep-drawing blank, for example by cutting away material at suitably selected locations, can lead to a substantial limitation of buckling and so improve the appearance of the container. For these products which are manufactured by deep-drawing, specific standard values are important for determining an area of feasibility. The present invention is based on the finding that the dimensionless parameter or coefficient K, which can be called the blank ratio, has been found to have a predictive value in respect of the occurrence of secondary buckling. This blank ratio is the ratio of the flange surface area of the wall part and the flange surface area for the corner or transition part. It has been found that when K is less than or equal to a critical value, no significant secondary buckling occurs during the deep- drawing of a metal container. The critical K value is dependent, on among other things, the geometry of the

Preferably, the critical value of the standard value or coefficient K is less than or not substantially greater than 1. In this situation the secondary buckling

is reduced compared to the present situation.

More preferably the critical value of the standard value or coefficient K is less than or not substantially greater than 0.9 so that the secondary buckling is reduced still further.

Even more preferably the critical value of the standard value or coefficient K is less than or not substantially greater than 0.85, which has been shown to virtually eliminate secondary buckling at least for one shape of container.

The method in accordance with the invention may be further characterised by including a step wherein the container manufactured by the deep-drawing is processed, for example by taking away material by trimming (edge cutting) , so that a suitable edge is obtained on the container on the side remote from the base of the walls extending away from the base.

The method may include a further step in which said edge is processed into a flange edge, for example by turning or beading.

The method in accordance with the invention can be employed to manuf cture a wide range of containers. For example a container comprising a base part from which extend upstanding peripheral walls comprising two straight wall parts and two rounded corner or transition parts, but also such a container comprising three, four or more such wall parts joined by associated rounded corner or transition parts. The method in accordance with the

invention is particularly suitable for the manufacture of containers comprising a base part which is essentially rectangular, from which extend upstanding peripheral walls comprising four essentially flat wall parts and four curved corner or transition parts joining the wall parts.

More preferably the method in accordance with the invention is employable for manufacturing containers with an essentially square base part. In the case of deep- drawing the deep-drawing blank into the container, the symmetry of the container assists the flow behaviour of the blank material so that an attractive, buckle-free container is produced.

Different sorts of materials or material combinations can be processed in accordance with the method of -the invention. The method is especially suitable for processing a deep-drawing blank into a container, wherein the deep-drawing blank comprises a metal substrate suitable for deep-drawing. The metal substrate can also be provided on at least one face with an organic coating, such as PP or PET.

Preferably the method of the invention is employed for the manufacture of a container by deep-drawing a blank comprising a metal substrate of a packaging steel suitable for deep-drawing. At the same time the method of the invention can be employed for the manufacture of a container by deep- drawing a blank comprising a metal substrate of an aluminium alloy suitable for deep-drawing.

The scope of the invention also extends to buckle-free containers manufactured in accordance with the method of the invention.

By way of example, embodiments of the invention will now be described with reference to the drawings in which:

Fig. IA shows a circular deep-drawing blank and indicates by dash-dotted lines the periphery of an essentially square container to be produced from the blank, 1. Fig. IB shows the adaptation of the blank of Fig.

IA in its flange to modify the relative proportions of the wall parts and the corner parts in accordance with the invention, Fig. 2 shows the wall surface areas of the wall parts and the corner parts of an originally square deep- drawing blank of side length 2X ₀ adapted to the manufacture of an essentially square container in accordance with the invention,

Fig. 3 is a more detailed illustration based on Fig. 2, giving the legend for calculating the cross-hatched surfaces, but because Fig. 2 is symmetrical, Fig. 3 shows only one quadrant, and

Fig. 4 is a chart showing the results of a series of experiments, wherein both the originally round and the originally square deep-drawing blanks of packaging steel were adapted to different K values for the production of essentially square containers with rounded corners.

In Fig. IA a conventional circular deep-drawing

blank is shown with radius r ₀ . The dash-dotted lines indicate the periphery of the container to be produced from the blank, the inner boundary indicating the periphery at the bottom of the container where the wall and corner parts join the base part, and the outer boundary indicating the periphery at the top of the container peripheral walls adjoining the blank flange. B is the side length of the square container, Rι _ower is the radius of the corners at the base of the container, and Ru _pper is the radius of the corners at the top of the container. The flange of the blank extends around the container periphery that is indicated thus.

Fig. IB shows how the circular blank of Fig. IA can be modified for the method of the invention. Parallel to each edge of the indicated periphery of the container to be formed, sectors of the circular blank of height t are cut away from the originally circular shape. The resulting surface area of a wall part the blank flange is shown cross-hatched and indicated by A _{wall f} i^ _ge - The surface area of a corner part is cross-hatched oppositely and indicated

"Y ^■ ^ ^■ corner flange*

In Fig. 2, by way of modification to an originally square deep-drawing blank for a similar square container, at each corner of the blank a triangular portion of height t is cut away. Here too the surface area of a wall part of the flange is shown cross-hatched and indicated by A^n _f i _angc ^{and the} modified surface area of a corner part is cross-hatched oppositely and indicated by

"corner f lange -

Simple formulae may be derived for calculating the surface areas A _{wall fla} nge ^{and A} corner _{f l} ange - ^For example , concerning to the case of the modified square deep-drawing blank :

Awall flange = <**-*> <* ^« -**>

( _, (YoB-a) ) ² --* ² -

^corner flange

in which the symbols used designate the dimensions shown in

Fig. 3 .

The chart of Fig. 4 shows the results of a series of tests in which both round and square steel blanks having different K values were deep-drawn to form essentially square packaging containers with rounded corners, eg. containers suitable for packaging animal food and also termed pet food containers. Along the horizontal axis is the di ensionless standard value K and along the vertical axis is the degree of buckling designated as "buckling area" and expressed in mm ² .

The buckling area was determined by means of a contour graph, in which the deep-drawn container was compared against a .reference container. Tilts C ϋta ii.ei. ^* S iiai ά. uάac Wi ii uiϊTicaS iϋuS Go. 70 Λ

70mm, with corners of 17.5mm radius. The height of the finished product was 25mm. The top of the containers had

dimensions of 79 x 79mm, with corners of 25mm radius. The blank-holder pressure was 2 bar.

It may be inferred from Fig. 4 that for the essentially square container under test there is a transition point at the K value of about 0.85. At values greater than 0.85 increasingly greater secondary buckling occurs; moreover there was a dependence on the shape, that is to say whether an originally round deep-drawing blank is used or a square one. At a K value less than or equal to 0.85 no or little secondary buckling occurred. At a K value less than 0.85 the buckling surface is in a range from 0 to 5mm ² . This spread is a consequence of small variations in, among other things, the deep-drawing process parameters and spread in the test method; in this area there was no secondary buckling visually observed in the corners of the essentially square containers.