DE69427408T2 - DEEP-DRAWN AND STRETCHED CONTAINER OF METAL PLASTIC AND THEIR PRODUCTION METHOD - Google Patents

Description

Background of the invention

The present invention relates generally to the manufacture by drawing and ironing of cans for packaging beverages or foodstuffs and containers for aerosols. More specifically, the present invention relates to new and improved metal-plastic laminate structures of the type involving metal-polymer-metal, that is, in which a layer of polymer is placed between two metal sheets to which it is adhered.

Hereinafter, the terms metal-plastic-metal, metal-polymer-metal, or more simply the abbreviation MPM are used without particular distinction to identify the new and improved laminate materials of the present invention.

There are numerous documents that describe metal-plastic layer structures or constructions. The majority of these concern metal-polymer or poly-metal-polymer structures, although metal-polymer-metal structures are less common.

Of these documents describing MPM components, none teach a preferred range of materials and thicknesses that are particularly suitable for producing drawn and ironed cans for beverages or foodstuffs.

By way of illustration, WO 82/00020 published on January 7, 1982 describes a metal-plastic construction which, in its simplest embodiment, is a polyethylene (PE) film attached to a metal foil or to a metal plate. Another embodiment includes two films of PE attached to opposite sides of a metal plate to form a PE-metal-PE complex. Finally, a third embodiment consists of two metal plates or films attached to opposite surfaces of a PE film. The polyethylene used, obtained by low pressure copolymerization of ethylene and of butene-1, is of a low density linear structure type (LLDPE) with a density of between 0.91 and 0.94. The LLDPE described in this application has the interesting property of adhering directly to the metal without the need for an adhesive. It is enough to stick it to the metal by applying heat and pressure at the same time (welding).

The metallic substrates described in this application include: steel, steel with a thin coating of chromium, chromium/oxide or zinc, untreated or nickel, copper or zinc treated aluminium. It may have undergone a chemical conversion treatment.

Although there is no explicit teaching regarding the desired absolute or relative examples of simple metal-plastic structures, the published application describes films of various polyethylene types of 100 µm (micron) thickness welded to the plates of various metals, such as steel, tin-plated steel, chromium-chromium oxide coated steel or aluminum of 210 µm (micron) thickness. The resulting samples are then formed into hollow articles by folding, punching, drawing and wall ironing. The adhesion of the coatings is compared, and demonstrates the superiority of linear low density polyethylene (LLDPE).

French patent FR 2 665 887 (Pechiney Emballage Alimentire) describes a capsule designed to fit over a cork, which is manufactured by drawing, drawing and ironing or projection stretching, and which is characterized in that it comprises two layers of aluminum held together by an adhesive layer having a Shore hardness of less than 80. The adhesive layer can be composed of an ethylene-acrylic acid, of polyethylene or of polypropylene modified with acid functionality. The complex has a total thickness of between 120 and 400 µm (microns), with the following percentage distribution of the total thickness:

Outer aluminum layer 20 to 50%

Adhesive layer 3 to 30%

Inner aluminum layer 40 to 60%

European patent application EP-A-0 046 444 assigned to Schwerzerische Aluminium AG describes an MPM composite (laminate) film in which the plastic layer can be as thick as the two metal layers combined. A stated condition for achieving deep drawability is that the plastic core layer and the metal layers are selected such that when the composite is stretched, the weight carried by the plastic core is greater than that carried by each of the metal sheet strips. This This condition is achieved by means of an oriented or stretched plastic layer. It is also stated that the soft or semi-rigid thin aluminum sheet strip is particularly suitable. Drawing and ironing are not taught as a suitable process, but rather deep drawing and deep drawing by elongation, which are the conventional processes for such semi-rigid containers.

A similar approach for MPM constructions is described in European patent application EP-A-0 034 781, filed in the name of BASF Aktiengesellschaft. The inventors of this application roll the plastic foil before combining it with the metal foil to give the plastic ductile properties that are more like certain selected metals.

A metal-plastic-metal structural laminate that can be formed into various useful articles is described in European application EP-A-0134958, filed in the name of Dow Chemical Company. This invention is characterized in part by a very wide range of individual layer thicknesses, total thicknesses and thickness ratios. It is further characterized by an ability of the laminate to withstand at least some degree of stretch deformability as measured in a standard laboratory test, the ability to be bent to a certain sharp radius without cracking, and a certain degree of thermal stability.

The patent contains no reference to any drawn or drawn and ironed forms, nor to the ability of these laminate structures to be formed by both a drawing process and a drawing and ironing process. The laboratory test described is a type of biaxial stretch test in which the material is evenly stretched while the edge is retained, so as to make the material thinner. While such a stretch forming process is conventionally used to make flat parts such as vehicle dashboards, it is not used to make food or beverage cans. A conventional drawing process, such as that used to make food cans, or the initial steps to make beverage cans, allows the material to flow from the edge and results in little or no reduction in thickness.

US Patent 3,298,559 issued to Continental Can Company describes laminated metal-plastic containers, such as cake pans, which are cold drawn in conventional press molds. Of the metal-plastic containers described, some are of the MPM type. Although wide ranges of thicknesses of the metal and plastic layers are claimed, there is no teaching on the importance of the ratio of these thicknesses. These examples, covering the MPM structures have a plastic to total metal thickness ratio between 5 and 9. The metal layers in the MPM examples are described as soft or untempered. There is no indication that these containers can be formed by a drawing and ironing process or that drawn containers suitable for food cans can be produced.

Task given

The task set to the inventors was to improve a can for gaseous drinks. The can generally has a volume of about 33 centilitres and a cylindrical shape, is closed by an easily openable end, and is coated on the inside with a varnish approved for foodstuffs and on the outside with one or more decorative layers indicating the type and brand of the product contained. These coatings are generally applied after the can has been formed.

The most commonly used forming process is the deep drawing and ironing process. This process allows very fast production cycles, which has enabled the enormous worldwide distribution of this type of can.

This process, well known to those skilled in the art, comprises, as its name indicates, a first series of one or more deep-drawing operations and a second series of one or more ironing operations. One begins with a flat round disc of steel or aluminum alloy of a thickness of about 300 µm (microns). This disc is first deep-drawn to form a flat initial cup using a device represented by Fig. 1a. The initially flat disc (1) is shown there during deformation. It is pressed between a fixed die plate (2) and a pressure plate (3). The descent of the punch (4) driven by a piston allows the formation of the cup, which undergoes practically no reduction in thickness. The cup is then formed by a second drawing operation (Fig. 1b), the reference numerals corresponding to the same elements as in Fig. 1a.

The walls of this cup are then drawn using a series of generally three ironing rings with decreasing internal diameters. Fig. 2 shows one of the three ironing rings and illustrates how it works. The cup (5) is placed with a slight clearance over the punch (6), which causes it to penetrate the inside of the ring (7), whose internal diameter is smaller than the external diameter of the cup. This results in a thinning and a corresponding lengthening of the wall. The size of the extension or The stretching is set by the difference between the external diameter of the cup and the internal diameter of the ring. The deformation capacity of the cup does not allow the final height of the can to be achieved in a single ironing operation; as stated above, it is usual to arrange three rings in a row, which are passed one after the other by the cup with the same punch stroke.

Among the cost factors of the beverage can produced by the process described above, the metal costs represent a decisive part, despite its low weight. Therefore, the idea arose among researchers to replace part of the metal with a less expensive material: plastic.

The elastic modulus and elastic limit of most plastic materials being much lower than those of metals, the substitution of plastic for metal presents several structural problems. In addition to these structural problems, there are process problems linked to the fact that metal cans are generally manufactured under conditions that are significantly different from those used to mold plastic. For example, metal containers are typically manufactured at high speeds and at ambient or moderate temperatures, while the behavior of plastic is such that plastic containers are typically manufactured at lower speeds and at higher temperatures.

Previous researchers have shown, in particular as described in WO82/00020, that thin plastic layers that adhere well to a metal foil are able to be formed by simple modifications of the conventional metal forming processes. This can be explained by the fact that the behavior of the metal-plastic assembly during forming is determined by the stronger and thicker metal and by the fact that the stresses generated in the thin plastic layer or layers are easily transferred to the metal foil due to their good adhesion.

The limitation to relatively thin plastic layers was not a problem in previous research, since the role of the plastic was to protect the metal against corrosion and that a relatively thin layer of plastic was sufficient for this protection.

Although WO82/00020 states that, if desired, the laminates can be made with metal or foil bonded to opposite sides of the polyethylene film, there is no indication that thicker plastic layers are desirable or possible in such MPM structures. If a conceived MPM structure were to use the 100 µm (micron) plastic film and If two 210 µm (micron) metal foils described in this patent were used, the ratio of the plastic core thickness to the total metal thicknesses would be less than 0.24. This low ratio, as will be shown, is less than that needed to achieve the desired cost savings.

In addition to using layers of plastic thin enough to be dominated by the metal during molding, researchers have used two approaches. The first, shown in French patent FR 1414475 and U.S. patent 4,390,489, is to begin molding from a heated material, as would be used in a conventional plastics process such as thermoforming.

The second approach is to work with metal-plastic structures in which the respective materials are chosen in such a way that the plastic core dominates the molding and the deformation of the aluminum follows the deformation of the plastic. In EP-A-0 046 444 described above, this condition is specified in terms of the load carried by the plastic core being greater than that carried by any of the metal strips. This is achieved by using soft or semi-hard aluminum strips and using an oriented or stretched plastic layer.

In EP-A-0 034 781, the inventors cold roll the plastic film to make it even more dominant compared to a given metal foil. This allows them to use a slightly stronger metal foil. Although the inventors do not report their results in terms of the percent of the load carried by the metal, this percentage can easily be calculated for each of the layered structures shown in the example, assuming that the highest metal load is equal to that of the metal foil when tested alone. This calculation shows that the metal would carry only 16.4% of the tensile load in the structure using the cold rolled foil and 20.8% of the control using a thicker layer of the same plastic film without cold rolling. Again, no mention is made of drawn and ironed containers, nor of the ability of the layered structure to successfully undergo the wall ironing process.

According to US Patent 3,298,559, the inventors do not cold roll or otherwise orient the plastic, but in each of the MPM examples given they specify that the aluminum foils are soft (with 0.07 to 0.15% C) or uncured. In addition, the ratio of plastic thickness to total metal thickness in each MPM example is at least 5:1. Although the inventors do not provide the percentage of weight supported by the metals nor the mechanical data from which it can be calculated, This combination of soft aluminum and high plastic to aluminum thickness ratio means that the plastic layer will dictate the molding process.

As described below, the use of soft metal alloys, or the use of a thin plastic layer compared to the total thickness of the metal would not meet the objective of the present inventors to provide a major reduction in material costs for a container which at some point must withstand internal pressure or other significant mechanical stresses.

The present inventors also found that an unoriented plastic core layer is preferred over an oriented plastic core in terms of the ability to withstand the deeper draws and especially the subsequent stretch forming steps required for deep drawn and ironed beverage and food cans.

In order to achieve the aim of the present invention, which is to reduce the thickness and consequently the cost of the material used, the inventors found that the plastic layer to be placed between two metal layers should be thicker than that used so far in containers made of metal-plastic structures. It is known from other types of mechanical structures to use a cheaper material, or a material of lower density, as a central layer between two outer layers made of a stronger and stiffer material. Such "sandwich" structures are known to achieve a flexural rigidity approaching that of a single layer of the more massive material of the same thickness as the total thickness of the sandwich.

Although the less resistant central material contributes to the bending stiffness of the structure, it contributes little to the tensile strength of the sandwich. This limits the possible reduction in the total thickness of the two outer metal layers. The tensile strength of a structure with relatively thin metal walls, such as a container, is called membrane strength.

The inventors found that the pressure at which the base of a rigid container, such as a can for gaseous beverages, can begin to change from a concave shape as seen from the outside to a convex shape depends on a complex function of the bending stiffness and the membrane strength. This pressure is generally referred to as the bottom buckling pressure. The formula for this function of the two types of strengths depends on the exact shape of the concave dome and on the shape of the part of the base that connects the dome to the bottom of the container wall.

The soil buckling pressure (P) can be expressed for a single metal layer as a function of thickness by the formula:

P = ken

where: k = material dependent proportionality constant, e = thickness of the material, n = exponent varying between 1 and 2, dependent on the geometry of the base. If the exponent is close to 1, it indicates that the buckling pressure is more sensitive to the membrane strength, if the exponent is close to 2, it indicates that the bottom buckling pressure is more sensitive to the bending stiffness. For most beverage can bases, the exponent is between 1.2 and 1.9. The closer the exponent is to 2, the lower the plastic thickness required for a given thickness of the outer metal layers.

Fig. 3 shows the plastic thickness ep required for a total thickness of the two metal layers en to achieve the same bottom buckling pressure as with an all-metal structure with a thickness of 330 um (microns). As can be seen from the various curves, much less plastic thickness is required for an exponent n = 1.7, for which the bending stiffness is the most important factor, than in the case of an exponent n = 1.2, for which the membrane strength is the most important factor.

It can also be seen from Fig. 3 that for a given base design, and hence for a given value of n, there is a range of plastic thicknesses ep and corresponding total thicknesses of the two metal layers em that will provide the required floor buckling resistance. For a design with an exponent of, for example, 1.5, all acceptable combinations of the abscissa and ordinate correspond to any point on the curve labeled 1.5.

In general, the points on the left of each curve correspond to the most economical structures, since they incorporate less of the expensive metal and more of the cheap plastic.

It should also be noted that these points have a thickness ratio of the thickness of the plastic to the total thickness of the metal that is greater than that realized in the prior art.

It was expected that manufacturing containers using conventional metal forming processes such as deep drawing and ironing would be relatively more difficult starting from an MPM structure with structures containing less metal and more plastic. One reason for this assumption was that as much during deep drawing as during ironing, the MPM structure is subjected to tensile loads, and one would think that according to existing technology, the elongation at break of the MPM structure would be the same as that of an all-metal structure. At this elongation, the plastic material, due to its low modulus, would bear a smaller portion of the tensile loads caused by deep drawing and ironing. To test the generally accepted assumption of equal elongation at break, the inventors performed uniaxial tensile tests on various structures with varying plastic thicknesses and a constant thickness of 100 µm (microns) for each layer of the outer aluminum alloy.

They were surprised to observe that the elongation at break, as shown in Fig. 4, increased with the thickness of the plastic, reaching a maximum near 300 µm (microns) of plastic, which is a ratio P/(Mi + Me) of 1.5.

Although the explanation for this surprising increase in elongation at break is not entirely clear, examination of the samples after breakage shows that it is related to the ability of the plastic to distribute the concentration of stresses arising from the onset of necking of one of the outer metal layers. The plastic distributes the stresses over a large surface of the opposite outer layer, thus preventing the necking of the first outer layer from propagating to breakage. If the plastic layer is relatively thin, this concentration of stresses is transferred to a relatively small surface of the opposite outer layer, resulting in simultaneous necking of the two layers. If the plastic layer is thicker than the optimum value, it appears that the plastic is less able to transfer the stress concentration to the opposite outer layer, and necking proceeds in a sequential manner in the two metal layers.

The improvement in the toughness of the MPM structure, characterized by its elongation at break, has allowed successful deep drawing and ironing of MPM structures with relative plastic thicknesses significantly greater than ever achieved. As stated above, the economic balance for a given bottom buckling pressure is significantly more favorable with such higher plastic thicknesses.

Using data from the same tensile specimens used to produce Fig. 4, in which each metal foil was 100 microns thick and made of aluminum alloy 3003 with a tensile strength of 239 MPa, the inventors calculated the range of total load carried by the metal foil. The percentages varied from 99% with a 55 µm (micron) thick core to 82% with a 420 µm (micron) thick core. Core.

Summary of the invention

In accordance with these and other objects, the present invention provides new and improved metal-polymer laminates and constructions useful for forming can bodies and cans. The nature and thicknesses of the layers of the laminates are particularly adapted to provide the required mechanical characteristics of the intended metal cans, particularly for packaging gaseous beverages or foods, as well as their means for forming by deep drawing and ironing.

The present invention also aims to provide a method for producing metal cans by deep drawing or iron-drawing of metal-plastic constructions of the MPM type, which are intended for packaging food products or beverages.

Short description of the drawings

Fig. 1a and 1b schematically show two deep-drawing processes of a round disk according to the state of the art;

Fig. 2 illustrates ironing and deep drawing of the wall of a deep-drawn bowl to transform it into a can;

Fig. 3 shows isobars for the soil buckling pressure P, the total thickness of the metal em is the abscissa and the thickness of the intermediate plastic layer ep is the ordinate;

Fig. 4 shows the variation in elongation at break of an MPM structure in which each of the outer metal foils has a thickness of 100 µm (microns) as a function of the thickness of the central polymer layer;

Fig. 5a and 5b show two preferred shapes of the base of the stamp according to the invention ; and

Fig. 6 shows mold components for the second deep drawing process according to the invention.

Description of the preferred embodiments

Can bodies composed of a base and a wall, the generatrices being perpendicular to the base of a metal-plastics structure of the metal-plastics-metal type, intended in particular to contain beverages, and the objects of the invention are characterized in that the metal-plastics structure comprises a central layer of thermoplastic polymer of thickness P coated on its inner and outer surfaces with metal foils of respective thicknesses Mi and Me, such that the ratio P/(Mi + Me) is greater than 0.5, and in that the bodies are manufactured by deep drawing and ironing, and in that their wall is thinned with respect to the bottom of the can body.

According to the same explanation method of the above-mentioned problem, the ratio P/(Mi + Me) is preferably between 0.7 and 2.5, and most preferably between 1 and 2.

In an advantageous method of realizing the metal-plastic structure, the central polymer layer has a thickness of between 100 and 500 µm (microns) before ironing and each of the metal foils has a thickness of between 25 and 150 µm (microns) before ironing. These thicknesses are obviously less in the thinned walls of the body. In another advantageous method, this plastic layer is essentially not oriented other than by random orientation normally created during casting or blowing of a plastic film.

The polymer that forms the central layer is chosen from one of the following thermoplastics: polypropylene, low and high pressure polyethylene, polyester and polyamide. It is interesting to note that the polymer is not in contact with the food product or drink in the containers and it is therefore possible and advisable to use recycled polymers. Tests have been carried out with recycled polyester and polypropylene and have given completely satisfactory results.

The metal is either tinned or untinned steel coated with chromium, zinc, nickel or chromium-chromium oxide or an alloy of aluminium, with aluminium alloys being preferred. It is further preferred that the tear strength of the metal foil, when tested alone and in tension, is greater than 185 MPa.

The specific materials and thicknesses of the layers are preferably selected so that most of the load is carried by the combined metal layers when the Initial foil is pulled with unidirectional tension. Preferably, the percentage of load carried by the combined metal layers should be greater than or equal to 70%.

The metal foils can have different thicknesses or be made of different metals. One can use a metal foil corresponding to the outside of the can that is thicker than that corresponding to the inside of the can, for reasons explained below, or one can choose an alloy that has better corrosion resistance for the foil corresponding to the inside of the can and an alloy that has better mechanical strength for the foil corresponding to the outside of the can.

A layer of suitable adhesive with a thickness of between 1 and 20 µm (microns) can be inserted between the central layer of polymer and the foil or foils of metal, the thickness of the adhesive being included in the total thickness of the polymer P.

The adhesive inserted between the polymer and the metal can be either a thermosetting polymer, for example a polyurethane or an epoxy, or a thermoelastic polymer, such as polyolefins modified in a conventional manner by an ethylene acid (hydroxysuccinic acid, crotonic acid, etc.), ethylene-acrylic resins (EAA), polyesters, or copolymers of moxiomers corresponding to the polymers indicated above.

The adhesion of the metallic foils to the central polymer layer is obviously an important characteristic of the metal-plastic structures and of the can bodies made from these structures. This adhesion is measured by the peel strength, the force required to detach a strip of metallic foil of a given width from its polymer carrier, and which is therefore expressed in force per unit length. The structures intended for the manufacture of deep-drawn and iron-drawn can bodies should have a peel strength of more than 0.4 Newton per millimetre.

The metal-plastic structures can in turn be coated on one or two sides with a lacquer or a polymer film without deviating from the system of the present invention.

Another object of the present invention relates to the finished cans starting from the bodies or shapes whose characteristics are indicated above. To make a can starting from a can body, one first proceeds by trimming the body in height by shearing off the upper parts of the wall and then by necking down this upper part. The upper edge should then be rolled to a small bending radius, to allow the end to be beaded after the can has been filled. During this process of bending the metal-plastic assembly according to this small radius, it is observed that the metal foil further away from the centre of the bend, which is in stretching, breaks at the point where the radius is smallest, leaving the other foil intact. This phenomenon does not occur in a homogeneous metal of the same thickness without a polymer layer, for reasons that would take too long to explain here. Faced with this problem, the inventors first looked for a solution, but then quickly put forward the theory that this tearing of the metal foil in stretching has no influence on the mechanical strength of the can that has been filled and beaded. What can be feared is in fact that a can with a rolled flange in which one of the two metal foils is torn would not be able to withstand the tensile stresses generated by the internal pressure, which tend to detach the end. However, the internal stresses in the axial direction of a pressurized cylinder are approximately half those in the direction perpendicular to the axis. Consequently, if there is enough metal in the entire metal-plastic construction with its two layers of metal to resist the stresses in a plane perpendicular to the axis, there is enough metal in the remaining intact layer to resist the axial stresses. This theory has been confirmed by calculations. In addition, the total thickness in the rim is generally greater than that of the thinnest part of the wall, giving an additional excess of safety. It is also possible to reinforce the can by selecting a greater thickness or a stronger alloy for the outer foil than for the inner layer. Finally, the external appearance of the can will not be affected since the broken part of the metal foil will be covered by the folded edge of the end in such a way that the end user of the can will not even notice it. A final can made of metal-plastic metal-polymer-metal, having a rolled upper edge in which the metal foil is torn at the greater distance from the center of curvature, which is consequently in extension, at the place where the radius is smallest, constitutes a second object of the invention.

In addition to the flanging technique, it is also possible to attach the cover to the metal-plastic can using any other known technique, such as heat sealing or gluing.

The invention also relates to a method for producing deep-drawn and iron-drawn cans intended for packaging beverages, characterized in that it comprises the following steps:

a) producing a strip of a metal-plastic structure comprising successively a metal layer a thermoplastic polymer layer a second metal layer a polymeric adhesive layer capable of being inserted between each metal layer and the polymer layer.

b) Cutting round slices from the strip.

c) deep drawing the discs to give cups in two successive passes, each of the two passes preferably being carried out with a cylindrical punch having a circular base in which the generatrix is connected to the base by a radius of curvature of between 5 and 10 mm, and the second pass preferably being carried out with an entry angle of the rigid crosshead to the horizontal of between 10 degrees and 70 degrees.

d) Ironing of the wall of the bowls, achieved by means of a series of ironing rings, preferably 4, of which the first, called calibration, reduces the wall thickness by only between 2 and 25%.

The articles of metal-plastic construction of step a) of the invention are manufactured by various known processes. The most commonly used are coextrusion, heat sealing and induction bonding. The last two processes are preferably carried out on a continuous production line fed with plastic films and metal strips.

Direct coextrusion consists of extruding the central polymer layer and, on either side of this central layer, two thin layers of adhesive between two metal foils that are continuously unrolled and form the outer layers. The composite product thus obtained then passes between the rollers to achieve adhesion between the various layers. This technique is undoubtedly only used in the case of thermoplastic adhesives.

Hot-melt bonding consists in starting from a polymer composite strip comprising a central polymer layer covered on each of its sides by the adhesive layer, here also thermoplastic, and inserting this strip between two metal foils. Hot-melt bonding is ensured by passing the composite product thus obtained between two rollers heated to a temperature sufficient to melt the adhesive layer or at least to soften it sufficiently in such a way as to ensure adhesion between the polymer core and the metal foils.

Finally, induction bonding consists of coating the inner sides of the two metal foils with a thermosetting adhesive by a known method, and applying these foils on both sides to the central strip of polymer by means of the rollers.

The molding of the cans comprises a first step c which generally consists of one or more successive drawing operations using a device as shown in Fig. 1. In order to adapt the working conditions to the particular case of metal-plastic structures of the MPM type, the researchers were led to favor a particular shape of the stamping base and the entry shape of the stamping plate, which cooperate to ensure drawing of the structure without molding cracks, folds or detachments.

The punch, which has the general shape of a rotary cylinder, presents, according to one of the preferred methods of the invention, an axial section whose generatrix is connected to the base of the punch by a circular arc with a radius of between 5 and 10 mm. This connection can be made directly on the base of the punch or by an intermediate piece, as can be seen in the section of a second circular arc whose center is on the axis of rotation of the punch. Figs. 5a and 5b illustrate the two variations indicated above.

Fig. 5a represents the simplest form of the embodiment. The punch 9 is considered in section through the axis; it takes the form of a cylinder of rotation around the axis 10. The generatrix 11 is connected to the base by a circular arc of radius R1 which falls between 5 and 10 mm (for example 8 mm for a punch with a diameter of 85 mm). This arc generates a section of a torus by rotation.

Fig. 5b represents a slightly more complex shape; the generatrix is connected to the base by a first circular arc 14 of radius R1 between 5 and 10 mm, which is connected tangentially to a second circular arc of large radius R2 (15) centered on the axis of the punch. The circular arc (15) creates a spherical dome and the circular arc (14) a section of a torus. For example, R1 can be of the order of 6 mm and R2 of the order of 250 mm.

The other preferred method of the invention works in conjunction with the special shape of the stamp described above, whereby this also special shape of the entry of the stamp plate is used for the second deep-drawing pass. Conventional stamp plates for deep-drawing have the shape shown schematically in Fig. 1a, which represents the first deep-drawing pass, which, as described above, starts from a flat circular disk. Fig. 6 shows the following deep-drawing passes according to the invention, the starting material of which is no longer a disk, but a deep-drawn form. The cup (24) is in the deep-drawing process, its initial diameter corresponds to the upper part (25) and is being reduced to its final diameter (26) determined by the space between the punch (27) and the punch plate (28). Accordingly, the height of the walls increases without ironing taking place in the traditional sense of the word, a significant reduction in thickness. An internal pressure plate (29) is arranged inside the initial cup.

The inventors found that the angle formed by the generatrix of the entry cone with the horizontal plane perpendicular to the axis of the punch is crucial for deep drawing metal-plastic structures of the MPM type. This angle should be between 10 degrees and 70 degrees, and preferably around 60 degrees.

The forming of the cans includes a final step (d) consisting of a deep-drawing operation which allows the wall to be lengthened with a corresponding thinning of the wall. This operation is shown schematically in Fig. 2.

The Applicant discovered that it was possible to iron draw the walls of a cup drawn from an MPM disk without causing defects such as cracks or delamination, preferably by processing it in four iron draw passes in succession, instead of the three usually carried out, and particularly according to the teaching of European Patent Application EP 0402006. More preferably, the first of these iron draw passes is a simple calibration pass, with a thickness reduction ratio of between 2 and 25%.

The following examples illustrate the various steps of the invention in specific cases.

example 1

A strip of polypropylene with a thickness of 300 µm (microns) was coated on each of its sides with a layer of 10 µm (microns) of an adhesive consisting of a layer of polypropylene modified with maleic acid. The two films of adhesive were cold applied to the film by passing between two rollers. The composite strip thus obtained was then continuously inserted between two sheets of 100 µm (microns) thick aluminum alloy 3003, a manganese alloy according to the standards of the "Aluminium Association", each being unwound from a roll and passed through a furnace at a temperature of 200ºC for preheating in such a way as to remove the adhesive. melt. The MPM structure obtained is then passed between rollers exerting a pressure of about 4000 kPa and then wound on a reel. Starting from this structure, round discs with a diameter of 140 mm are cut out. The discs were then deep-drawn in two successive passes using a punch similar to that in Fig. 5a with a radius R1 = 8 mm. For the second deep-drawing pass, a punch plate was used for which the angle alpha was 60º. The first pass gave cups with an external diameter of 86 mm and a height of 35 mm. The second pass gave cups with an external diameter of 67 mm and a height of 56 mm. These cylindrical cups, supported by a punch, are finally deep drawn by a pass through a tool which irons out a series of four rings of successively smaller diameter to give beverage can cups with a diameter of 66 mm and a height of 130 mm. The respective reductions in wall thickness, expressed as a percentage of the initial wall thickness, are as follows: 20%, 40%, 52% and 57%. Careful examination of these cups showed no signs of any cracking of the metal or plastic. No delamination between the metal and the plastic was observed.

Example 2

An MPM composite strip was prepared by coextrusion of a core between two sheets of the same aluminum alloy 3003 as in Example 1, but with a thickness of 80 µm (microns), comprising polypropylene with a thickness of 250 µm (microns) and, on either side of this core, an adhesive layer comprising maleic acid-modified polypropylene with a thickness of 10 µm (microns). Adhesion was achieved by passing between two rollers heated to 200ºC while applying a pressure of 4000 kPa. Some beverage cans were prepared under the same conditions as in Example 1. Examination of these cups showed no cracks in the metal or plastic. No delamination was observed between the metal and the plastic.

Example 3

An MPM composite strip was prepared by coextrusion under the same conditions and the same components as in Example 2, but using polypropylene for the core recycled from cans made with the same MPM construction. Although the recovery of the polypropylene from the used cans did not allow the separation of the adhesive from the polymer, the can obtained was of excellent quality and also showed neither cracks nor delamination. Under the same conditions as In Example 1, beverage can bowls were manufactured. Examination of these bowls showed no cracks in the metal or plastic. No delamination between the metal and the plastic was observed.

Example 4

An MPM assembly was prepared under the same conditions as those of Example 2, comprising in sequence: a 3003 alloy film of 80 µm (micron) thickness, an amorphous polyethylene terephthalate adhesive layer of 10 µm (micron) thickness, a polyethylene terephthalate layer of 200 µm (micron) thickness, another amorphous polyethylene terephthalate layer of 10 µm (micron) thickness, and finally another 3003 alloy film of 80 µm (micron) thickness. The beverage can bowls were prepared under the same conditions as in Example 1. Examination of these bowls showed no cracks in the metal or plastic. No separation between the metal and the plastic was observed.

Example 5

Example 5 concerns the same manufacture of cups as in Example 4, with the significant difference that the polyethylene terephthalate used came from the recycling of used plastic bottles. These bottles were ground after washing and drying and fed into the feed hopper of an extruder. No quality problem was observed either in the assembly or on the cups obtained by deep drawing and ironing.

Claims (11)

1. A drawn and ironed can body comprising a base and an upright side wall, the base and the side wall having a Mi-P-Me type layered structure, where Mi and Me are inner and outer metal foil layers respectively and P is a central polymer layer, and the ratio of the thicknesses P/(Mi and Me) is greater than 0.5 and the thickness of the side wall is less than the thickness of the base. 2. Can body according to claim 1, characterized in that the central polymer layer is substantially unoriented. 3. Can body according to claim 1 or 2, characterized in that the thickness of the central polymer layer P is between 100 and 500 µm and the thickness of each of the metal layers Mi and Me is between 25 and 150 µm. 4. Can body according to one of claims 1 to 3, characterized in that the peel resistance of the metal layers against the plastic layers in the layered structure is greater than 0.4 N/mm. 5. A can body according to any one of claims 1 to 4, wherein Mi and Me are metal foils independently selected from foils of steel, tinned steel, chromium, zinc or nickel coated steel, chromium-chromium oxide coated aluminum and aluminum alloys. 6. Can body according to one of claims 1 to 5, wherein the polymer layer P comprises a thermoplastic polymer selected from the group consisting of polypropylene, low density polyethylene, high density polyethylene, polyesters and nylon. 7. A can body according to any one of claims 1 to 6, further comprising an adhesive layer having a thickness of between 1 and 20 µm interposed between the central polymer layer P and each of the film layers Mi and Me, the thickness of the adhesive being included in the total thickness of the polymer layer P. 8. Can body according to one of claims 1 to 7, wherein the side wall portions have a upper rolled region in which the metal foil Mi is stretched and torn at the location where the radius of curvature is smallest. 9. Can body according to one of claims 1 to 8, characterized in that the metal foil Me, which forms an outer surface of the can body, is thicker than foil layer Mi, which forms an interior of the can body. 10. A process for producing can bodies with reduced metal content, the process comprising: providing a sheet of Mi-P-Me laminate, wherein Mi and Me are inner and outer metal foil layers, respectively, and P is a central polymer layer, and wherein the ratio of thicknesses P/(Mi and Me) is greater than 0.5; cutting slices (1) from the sheet; drawing the discs in at least one drawing operation to form a bowl (5, 24); ironing the bowl in at least one ironing operation to form a can body comprising a base portion and an upright side wall portion (25), the upright side wall portion having a smaller overall thickness than the base portion. 11. The method of claim 10, wherein in the laminate sheet the polymer layer P has a thickness between 100 and 500 µm and each of the metal foils Mi and Me independently has a thickness between 25 and 150 µm.