FR2713113A1 - Process for producing lids and boxes made of metal-polymer-metal complexes with a rolled edge - Google Patents

Abstract

The invention relates to a method for rolling, for crimping, the edge of a cover or a metal-plastic-metal complex over a small diameter without causing a crack in the extended metal layer. or during this folding operation. This process consists of locally reducing the thickness of plastic in the area which must be rolled so that the complex, reduced to a thin layer of plastic between two sheets of metal, behaves roughly like a homogeneous sheet of metal. The plastic pushed back out of the zone intended to be rolled forms a bead either towards the outside of this zone or towards the inside. The process applies to food cans and beverage cans made of a metal-plastic-metal complex as well as to the lids of these cans. The bodies of boxes and lids produced by the method also constitute an object of the invention.

Description

PROCESS FOR MANUFACTURING COVERS AND BOXES
METAL-POLYMER-METAL COMPLEX WITH ROLLING EDGE
TECHNICAL FIELD OF THE INVENTION

The invention belongs to the technical field of manufacturing, by stamping or stamping-drawing from layered metalloplastic composite materials, boxes intended for example for the packaging of beverages or foods, aerosol containers or easy-opening lids or no.

More specifically, the metalloplastic complex used in the present invention is of the metal-polymer-metal type, that is to say in which a polymer sheet is interposed between two metal sheets to which it adheres.

In the text of this patent application, it will be used indifferently for the designation of this complex the terms: metal-plastic-metal, metal-polymer-metal, or more simply, abbreviated, MPM.

DESCRIPTION OF THE PRIOR ART

Many documents describe laminated metalloplastic complexes for the manufacture of food cans, beverage cans or lids. However, the majority of them concern metal-polymer or polymer-metal-polymer complexes, the metal-polymer-metal complexes being more rare.

By way of illustration, mention may be made of the international patent application PCT filed on June 25, 1981 by METAL BOX
LIMITED and published as WO 82/00020 January 7, 1982.

This application gave rise, in particular, to the European patent EP 0 055 719.

This patent discloses a metalloplastic structure composed in its simplest embodiment, of a polyethylene film (called abbreviated PE) bonded to a sheet or sheet of metal. Another embodiment consists of two PE films bonded to opposite surfaces of a metal sheet to form a PE-metal-PE complex. Finally, a third embodiment consists of two sheets or sheets of metal bonded to the opposite surfaces of the PE film.

The PE used, obtained by copolymerization of ethylene and butene-1, is of the linear low density type and it has been found that this particular type whose characteristics are described in the application has the interesting property of adhering directly to the metal without the need to use an adhesive. It suffices to adhere it to the metal by simultaneous application of heat and pressure (heat-sealing).

The metal substrate may be steel, steel having a coating of tin or chromium or chromium / chromium oxide or zinc oxide, aluminum alloy or not, nickel, copper or zinc . It may have undergone a chemical conversion treatment.

In the examples given, sheets of different types of
PE of 100 micrometers thick are thus thermobonded on sheets of different metals: steel, tin-plated steel, chromium-chromium oxide-coated steel, aluminum, 210 micrometers thick. The samples obtained are then shaped hollow articles by pleating, stamping, stamping, drawing walls. The adhesion of the coatings is compared and demonstrates the superiority of linear low density PE.

French patent FR 2,665,887 (PECHINEY EMBALLAGE
FOOD) discloses a stamped, stamped-stretched or flow-wrapped overcap, characterized in that it comprises two aluminum layers bonded together by an adhesive layer of Shore hardness less than 80. The adhesive layer may consist of ethylene copolymer Acrylic acid or polyethylene or polypropylene modified by acid functions. The total thickness of the complex is between 120 and 400 μm with the following distribution in% of the total thickness:
-Outer layer of aluminum: 20 to 50%
adhesive layer: 3 to 30%
inner layer of aluminum: 40 to 60%
PROBLEM
The problem posed to the inventors was that of improving the bodies of stamped boxes, in particular food boxes, drawn-drawn boxes for drinks and their lids, whether easy opening or standard.

The process of shaping the food boxes is the process of stamping-re-pressing, that of the canisters is drawing-drawing, that of the covers is stamping. All these processes allow very high production rates.

In any case, we start with a stamping pass.

We start from a flat circular blank made of steel or aluminum alloy. For the lids, this blank is stamped in one pass. For food boxes, it is stamped either in one pass, or in two or more passes. In the latter case, the first pass gives a cup-shaped blank which is then re-plowed to reduce its diameter and increase its height. Finally, for the beverage cans, the cup-shaped blank obtained by stamping in one or more passes is then stretched using a series of drawing rings, generally three in number, of decreasing inner diameters. . This results in thinning and correlative elongation of the wall. All these techniques are well known to the skilled person.

These boxes and easy-open lids are lined internally with a food polish and externally with one or more decorative layers indicating the nature and brand of the product contained.

Among the elements of the cost price of boxes and lids obtained by stamping or stamping-drawing, the cost of the metal, despite its low weight, intervenes for a preponderant part. The idea has therefore come to researchers to replace part of this metal with a less expensive material: the plastic material. The price of the usual polymers, polyolefins such as polyethylene (PE) or propylene (PP), polyesters (PET), polyamides is generally lower, with an equal thickness, than that of aluminum alloys.

Since the modulus of elasticity and yield strength of most plastics is much lower than that of metals, the substitution of plastic for metal faces several structural problems. In addition to these structural problems, there are process problems, related to the fact that the metal cans are generally manufactured under conditions very different from those used for the implementation of plastics. For example, metal containers are normally manufactured at high speed and at ambient or moderate temperature while the behavior of plastics is such that plastic containers are normally manufactured at lower speed and higher temperature.

Previous research has shown that thin layers of plastic adhering well to a metal sheet can be implemented by simple modifications of conventional metal processing methods. This is because the behavior of the metalloplastic complex during processing is governed by the stronger and thicker metal and because the stresses generated in the thin plastic layer (s) are easily transferred to the sheet. metal because of their good adhesion.

This restriction to relatively thin plastic layers has not been a hindrance in this earlier research because the role of plastic was generally to protect the metal against corrosion and a relatively thin layer of plastic is sufficient for this protection.

To achieve the objective of the present invention which is to reduce the thickness and thus the cost of the metal used, the inventors have found that the plastic layer must be placed between two metal layers and must be thicker than that reached until here in containers made of metalloplastic complex.

The improved toughness of MPM complexes characterized by their elongation at break has successfully stamped MPM structures with much higher relative plastic thicknesses than ever before.

Thus, for the drawn-drawn beverage cans, the inventors have developed a metalloplastic complex consisting of a central layer of thermoplastic polymer of thickness P coated on each of its inner and outer faces with metal foils. respective thicknesses Mi and Me, such that the ratio P / (Mi + Me) is greater than 0.5.

By way of example, the thickness P of the polymer is between 100 and 500 μm and the thickness M 1 or Me of each of the metal sheets is between 25 and 150 μm.

The same metalloplastic complex can be used for stamped food cans and lids. For the boxes, the thickness P of the polymer is between 100 and 500 μm and the thickness M1 or Me of each of the metal sheets is between 25 and 150 μm. For the covers, the thickness P of the polymer is between 80 and 300 μm and the thickness M1 or Me of each of the metal sheets is between 25 and 100 μm.

In order to manufacture a box from a box body, the body is first leveled by shearing the upper part of the walls and then shrinking the upper part. The upper edge must then be rolled over a small radius of curvature to allow crimping the lid after filling the box. However, during this folding operation of the metalloplastic complex according to this small radius, it is found that the metal sheet farthest from the center of curvature, that which is therefore in extension, breaks where the radius is the weakest, another metal sheet remaining intact. This phenomenon, for reasons that would be too long to expose here does not occur with a homogeneous sheet of metal of the same thickness without a polymer layer.

Faced with this problem, the inventors first sought a solution, then quickly hypothesized that this rupture of the expanded metal sheet had no effect on the mechanical strength of the filled box and provided with its cover crimped. What could be feared, in fact, is that a box with a rolled edge in which one of the two metal sheets of the complex is broken, can not withstand the tensile stress created by the internal pressure which tends to tear off the lid. But the stresses created by an internal pressure in the direction of the axis of a cylinder are approximately half of those in the direction perpendicular to the axis.

Thus, if there is enough metal in the complete metalloplastic structure, with its two metal layers, to withstand the stresses in a plane perpendicular to the axis, there is enough metal in the layer left intact to resist axial constraints. This hypothesis has been confirmed by the calculation. In addition, the total thickness in the rim is generally greater than that of the thinnest part of the wall, which still gives a margin of safety.

 It is also possible, to strengthen the box, to choose for the outer sheet a higher thickness or a stronger alloy than for the inner layer. Finally, the outer appearance of the box will not be affected since the broken part of the metal film will be covered by the rolled edge of the lid so that the end user of the box will not even notice it. box will not even notice it.

On the other hand, for lids, whose edges must also be rolled over a small radius for crimping, the problem remains unresolved.

OBJECT OF THE INVENTION
The invention relates to a method for rolling the edge of a MPM complex cover on a small diameter without causing crack in the metal layer extended or during this folding operation.

This process involves locally reducing the thickness of plastic in the area to be rolled so that the complex, reduced to a thin layer of plastic between two sheets of metal, behaves much like a sheet of homogeneous metal.

Although this method, as has been explained above, is not essential for rolling the upper body boxes, it can also be applied. In the remainder of the description and the figures, the lids will be almost alone mentioned, but the method also applies to box bodies.

The bodies of boxes and lids manufactured by the method above are also an object of the invention.

DESCRIPTION OF THE FIGURES
FIG. 1 represents one of the variants of the invention in which the local decrease in the thickness of the plastic is by outwardly pushing.

FIG. 2 represents one of the variants of the invention in which the local decrease in the thickness of the plastic is by displacement towards the center of the cover.

DESCRIPTION OF THE INVENTION
The inventors started from the observation that this phenomenon of cracking of the metal layer in extension observed on the MPM complexes when folded on a small radius does not occur with a homogeneous metal sheet, as was explained above. . They also noticed that the radius of curvature around which one could bend the structure
MPM without breaking any of the layers was even lower than the total thickness of the structure and in particular that the polymer train was weak. They had the idea of reducing the thickness of the central plastic layer only in the outer annular zone where the lid must be rolled.

The method used consists in pressing back the central plastic so that, at the point where this pressure is exerted, the thickness of the polymer is reduced in a proportion of at least 50% such that folding becomes possible without breaking the outer layer of metal.

 Obviously, when the pressure is exerted, the plastic is sufficiently fluid so that it can flow. It must therefore be brought to a temperature above its softening point and, preferably, at its melting point. This is easily achieved by localized heating on an outer ring of the lid. As the plastic discharge operation takes place in the production line of the lids, which operates at a high speed, a rapid heating mode, for example induction heating, will be chosen.

The process comprises two successive and almost simultaneous stages:
localized heating of the edge of the lid at a temperature above that of softening the plastic and, preferably, that of melting;
- Applying pressure on the peripheral area of the cover to push back part of the plastic of this area.

An important question from a practical point of view is the recovery of this expelled plastic. Two possibilities have been experimented.

The first is to expel the plastic to the outside.

In this case, the thus expelled plastic is added to the plastic already present in the outer annular zone. It then contributes to increasing the thickness thereof, thus forming an annular bead with a diameter greater than the diameter of the zone corresponding to the reduction in thickness.

If the area of reduced plastic thickness is very close to the outer diameter of the lid, some of the plastic will actually be expelled outwardly thereby forming an additional plastic ring around the outer diameter of the lid.

Depending on the case, this plastic ring will be used to improve the tightness of the lid on the box or it will be cut so as to separate it from the lid.

The second solution is to push the plastic towards the center of the lid where it will form a slight annular bead that will improve the mechanical properties of the lid and in particular its rigidity.

Figures 1 and 2 illustrate one and the other of these two solutions.

Figure 1 shows the removal of a portion of the plastic to the outside. A circular cover blank in section according to a diameter (1) consists of a plastic core (2) and two metal sheets (3) and (4).

The circular edge of the blank is heated around its entire periphery: zones (5) and (6) so as to soften or even melt the plastic. This zone corresponds to that where the lid will be rolled on a small diameter. A pressure represented by the arrows is then applied over the entire periphery of the blank. The liquid or at least softened plastic then flows outwardly and forms a bead (7) towards the outside of the lid. The area where part of the plastic material has been removed can then be folded over a small radius without cracks being observed.

Figure 2 shows the removal of the plastic towards the center of the lid. A circular cover blank in section according to a diameter (11) consists of a plastic core (12) and two metal sheets (13) and (14). An annular zone of the blank is heated a short distance from the edge (15) and (16) so as to soften or even melt the plastic. A pressure represented by the arrows and acting on the area where the lid is rolled is then applied over the entire periphery of the blank. The liquid plastic or at least softened can not flow outward since the edge itself is not heated and so the plastic has remained solid. It will therefore flow towards the center of the lid and form a bead (17), (18), still located in the heated zone and which absorbs the plastic expelled by the pressure. The edge of the blank where the plastic material has been eliminated completely or almost completely can then be folded over a small radius without cracks being observed.

The method described above can be applied also in these two variants to the upper part of the walls of boxes. The principle remains the same: the upper part of the wall is heated and the pressures, instead of being applied perpendicularly to the plane of the cover, are applied radially.

The easy-opening or standard opening lids and the metal-metal-polymer-type metal-plastic complex boxes according to the invention are characterized in that, at the point where, in order to crimp, the lid or box must be rolled on a small diameter, the thickness of plastic material forming the groin has been reduced by at least 50%.

For both boxes and convergents, it is possible and recommended to use recycled polymers.

EXAMPLES
Example 1

A 140 micron thick polypropylene web was coated on each of its faces with a layer 5 microns thick of an adhesive made of a polypropylene sheet modified with maleic acid. Both sheets of adhesive were applied to the cold sheet by passage between rollers. The composite strip thus obtained was then introduced continuously between two sheets of 80 micrometers thick aluminum alloy 3003, manganese alloy according to the "Aluminum Association" standards and preheated by passing through an oven at a temperature of 200 ° C. in order to melt the adhesive. The MPM complex obtained was then introduced between rollers to thermobond.

From this complex, circular blanks 75.4 mm in diameter were cut. These blanks were then stamped to give lids 65 mm in diameter. Before proceeding with the rolling of the edges, these edges were treated according to the diagram of Figure 1 by heating them by induction at a temperature of 200 C so as to melt the plastic. A pressure of 1500 kPa was applied on the periphery of the lid so as to expel a portion of the central plastic of the heated zone. The edges of the lids were then rolled for crimping. During this operation, no cracks in metal or plastic were highlighted
Example 2
A layer of low density polyethylene 150 micrometers thick was extruded between two sheets of 80 microns of aluminum thickness preheated by passing through a furnace at a temperature of 200 ° C. without the interposition of adhesive. It was then passed between rolls to heat-seal it, and 75.4 mm diameter circular blanks were cut from this complex, and before rolling the edges, these edges were treated according to the diagram in FIG. by induction heating them to a temperature of 200 ° C so as to melt the plastic in an area near the edge, but at a small distance. A pressure of 1500 kPa was applied on the periphery of the lid so as to expel a portion of the plastic core from the heated zone towards the center of the lid. The edges of the lids were then rolled into the emptied area of his plastic groin for crimping. During this operation, no cracks in metal or plastic were highlighted
Example 3
A 150 micron thick polyester layer was extruded between two pre-heated 80 micron thick sheets of aluminum by passing through an oven at a temperature of 280 ° C without the interposition of adhesive.
MPM obtained was then passed between rolls to thermocollate it. From this complex, circular blanks 75.4 mm in diameter were cut. Before rolling the edges, these edges were processed according to the scheme of Figure 2 by inductively heating them to a temperature of 280 ° C so as to melt the plastic in an area near the edge, but at a small distance A pressure of 1500 kPa was applied around the lid to expel some of the groin plastic from the heated area to the center of the lid.
were then rolled into the emptied area of his plastic gum
for crimping. During this operation, no
crack of metal or plastic has been highlighted

Claims (7)

 CLAIMS 1. Easy opening lids or standard metal-polymer-metal type metal-plastic complex covers characterized in that, at the place where, for crimping, they must be rolled over a small diameter, the plastic material forming the The groin has been forced to reduce its thickness by at least 50% at this folding zone with a small radius of curvature. 2. Metal-polymer-metal type metal-plastic complex boxes characterized in that, at the point where, for crimping purposes, said boxes are to be rolled over a small diameter, the plastic material forming the core has been displaced in such a way that to reduce its thickness by at least 50% at this folding zone with a small radius of curvature. 3. A method of rolling the edges of lids or cans of metal-polymer-metal complex-metal complex without cracking the metal of heating the complex, in the area of the lid or box to be rolled, over the point softening, or preferably melting, the plastic and pressing the central plastic so that at the point of pressure the thickness is reduced by at least 50% . 4. Method according to claim 3, characterized in that the plastic is forced out of the lid. 5. Method according to claim 4, characterized in that the plastic discharged towards the outside of the lid forms a bead between the folding zone and the edge of the lid. 6. Method according to claim 4, characterized in that the plastic discharged to the outside of the lid is expelled outside the lid. 7. Method according to claim 3, characterized in that the plastic is expelled towards the center of the lid and forms a bead between the folding zone and the center of the lid.