Where the following text refers to assembly parts, this is also understood to include components of such assembly parts, such as two edges of the same piece of sheet.

Various techniques are known for joining together assembly parts of a can, for example for attaching a lid to a free edge of a can body. The oldest technique consists in soldering the lid to this edge. It is also known to join lid and the edge of the can body by means of flanging or with beaded edges, in which case an airtight closure is obtained by firstly applying a plastic adhesive in the area of the join. In patent WO 97/12808, it is proposed to coat the material of the can body and of the lid in advance with a thermoplastic material which is locally melted after the edge of the can body and the edge of the lid have been beaded over together.

Beading over the edge of the can body and of the lid together after the can has been filled is regarded as an operation which is mechanically difficult to carry out. For this reason, there is a desire for a process in which any preliminary mechanical working of the lid and of the edge of the can body can remain limited to operations carried out before the can is filled.

In this connection, a method has been found which is also suitable for joining together assembly parts other than a can body to a lid. In particular, the novel process appears also to be suitable for producing a cylindrical can body from a planar metal sheet which has been bent into a cylinder, without the need for a welding operation, a soldering operation or a folding operation.

The invention therefore consists in the fact that, in the known process, the assembly parts are firstly given the shape in which they are to be assembled, and that they are then positioned opposite one another with an overlap and with the layers of plastic facing towards one another, and are held pressed together, without further deformation, along a joining seam which is to be formed, heat being applied locally in the joining seam up to the temperature at which the layers of plastic are united, after

which the joining seam is cooled rapidly and, if necessary, the pressure is then removed.

It should be noted that the assembly parts can be held pressed together by means of external means, but also that they can be pressed together as a result of their shape.

In the latter case, of course, it is not necessary to remove the pressure from the joining seam after it has been cooled.

It has been found that this process results in a join which is sufficiently strong for applications for thin-walled metal cans for packaging purposes. In order to ensure that this strength is sufficient, the overlap between the assembly parts must be at least 1 mm wide, the layers of plastic must be at least 5 pm thick, the assembly parts should be pressed together with a pressure of between 1 and 6 MPa, the layers of plastic should be composed of at least one layer and the composition of the layers of plastic should be selected from the group consisting of optionally pigmented thermoplastics, comprising polyesters, polyolefins, polyamides and their copolymers. This group, includes, for example, PET (polyethyleneterephthalate) and polypropylene. In this context, pigments are also understood to include organic colorants. Preferably, in this case, the overlap is between 2 and 4 mm wide and the layers of plastic are between 25 and 50 p. m thick.

It has been found that a can produced in this way has stronger joining seams when PET is used than when polypropylene is used in the layers of plastic. However, if the can produced is subjected to an intensive sterilization treatment after it has been filled, it has been found that the strength of a join containing PET decreases considerably. Therefore, if such a sterilization treatment is employed, it is preferable to use polypropylene as the material for the layers of plastic. It should be noted that when comparing the effect with and without sterilization, the conditions observed when simulating sterilization were always a temperature of 120°C for 60 min.

As has already been noted, the invention also relates to a thin-walled metal can for packaging purposes.

In particular, the invention relates to such a can comprising assembly parts made from thin metal sheet which is provided, on at least one side, with a layer of thermoplastic, the assembly parts being assembled with an overlap and with the layers of plastic facing towards one another, and the join between the assembly parts comprising at least one joining seam which is formed by local fusion of the overlapping layers of plastic.

The invention also relates to a thin-walled metal can for packaging purposes if it is produced using the process described above.

In particular, the invention also relates to a thin-walled metal can, the body of which is in the form of a cylinder, and this body comprising a planar metal sheet which

has been bent into a cylinder and is provided, on both sides, with a layer of thermoplastic, the joining seam comprising an overlap of two edges of the said metal sheet over a width of between 2 and 4 mm.

Alternatively, the invention is also applied to a thin-walled metal can, the body and base of which comprise a single piece and form one assembly part, and a lid forms another assembly part. According to the invention, the body and the lid, at the location where they adjoin one another, are both provided with cylindrical faces which fit along one another and are joined together over a width of at least 1 mm and which join comprises an adhesion layer which is formed by fusion of a layer of thermoplastic on both cylindrical faces.

It should be noted that European Patent 0,575,267 has disclosed a can and a lid which are attached to one another along two faces which fit againt one another.

However, this document relates to a plastic can and a plastic lid which are adhesively bonded to one another, in a removable manner, along essentially horizontal face edges.

A can of this nature and a lid attachment of this nature are not suitable for enclosing food or drinks which may have to be sterilized. Also, a can of this nature is not suitable for holding contents which are pressurized, for example CO2-containing drinks.

The above-described metal can according to the invention may be formed in various ways. The join between the body and the lid may be reinforced, allowing it to be formed with a greater dimensional accuracy, if at least one of the cylindrical faces which fit along one another forms part of a beaded-over edge of the assembly part in question. When the join has been made, this stronger design also means that it is less susceptible to complete or partial detachment of the join brought about by external forces. Consequently, the can is less vulnerable if handled roughly during transport.

These advantages which have been described are enhanced still further if the beaded- over edge is flattened so that it is parallel to the cylindrical face which forms part of it.

Then when the layers of thermoplastic on the two cylindrical faces are fused together, those parts of the beaded-over edge which have been flattened against one another can also fuse together, resulting in a greater strength.

The increased strength provided by the beaded-over edge can be enhanced still further by beading this edge over twice.

It should be noted that British Patent 1,536,671 has disclosed a metal can with a lid which can be attached thereto, with a beaded-over edge also being provided on the body and on the lid. However, these beaded-over edges together serve to form a mechanical force-fitting closure between body and lid. Furthermore, a small plastic rim is provided in order for the lid to be closed off in an airtight manner. This plastic is not applied to the sheet metal material in advance in order to join together, in a structurally

fixed manner, two cylindrical faces which fit along one another by fusing the layer of plastic.

In a further embodiment of the can according to the invention, one of the assembly parts is provided, beyond the cylindrical face in the axial direction, with a delimiting edge against which the other assembly part bears. This can considerably facilitate positioning the lid on the edge of the body when closing off the can. Lid and body can only be correctly located with respect to one another in one position, specifically if one of the assembly parts is bearing against the delimiting edge in the other assembly part. This measure also facilitates attaching the two assembly parts to one another.

A preferred embodiment of the metal can designed in this way consists in the fact that the lid is provided with an edge which has been beaded over twice and forms two cylindrical faces which are parallel to and separate from one another and bear against the two sides of the free end of the body of the can.

The invention will now be explained with reference to a number of figures, in which: Figs 1 a and b show the dimensions of a tensile test bar.

Fig. 2 illustrates a can body produced according to the invention.

Fig. 3 shows a cross-sectional view of a detail of a thin-walled metal can according to the invention.

Figs 4,5,6 and 7 show a similar detail for different embodiments of a metal can.

A tensile test is used in order to be able to demonstrate the strength of a join produced according to the process. The tensile test bar used in this test is shown in plan view in Fig. la and in longitudinal section in Fig. lb. The tensile test bar is obtained by joining two plastic-coated metal strips 1 and 2 together using the novel process. The strips 1 and 2 have a length L of 200 mm and a width W of 20 mm. The metal strips themselves, 3 and 4, are made from steel with an electrolytically applied layer of chromium (ECCS). The steel base consists of continuously annealed steel strip with a hardness (R30T) of T57. Two strip thicknesses are used in the tests, namely 0.12 mm and 0.27 mm. The strips are coated on both sides with a layer of thermoplastic (denoted in Fig. lb by reference numerals 5; 6; 7; and 8). Tests are described using four different types of plastic, denoted by test codes PET 1; PET 3; PP 1 and PP 2. The PET plastics are of the polyethylene terephthalate type, and the PP plastics are of the polypropylene type. The various test codes indicate the following: PET 1 is a homopolymer based on predominantly terephthalic acid and ethylene glycol;

-PET 3 is a copolymer comprising predominantly terephthalic acid and another di-acid or a di-alcohol; -PP 1 is a random copolymer comprising polypropylene and 0-5 % polyethylene, with a random distribution; -PP 2 is a polypropylene homopolymer.

Table 1 shows: the test code; the layer thickness in microns; the metal coating of the steel strip to which the plastic is applied, and the thickness of the steel strip in millimetres.

In all the tests, the strips 1 and 2 were attached to one another with an overlap O of 3 mm. When a PET coating was used, the bonding temperature employed was between 255 and 270°C, whilst with the PP coatings it was 220°C. During bonding, the strips were pressed together, at the location of the overlap, with a pressure of 240 N for 5 sec. Both the heating and the subsequent cooling of the test bar were carried out as quickly as possible, the cooling being effected by submerging the test bar in water.

Table 1 Test code Characterization of the Plastic Metal Steel plasticcoating coating coating thickness thickness (mm) l (llm) PET 1 homopolymer 30 ECCS 0.27 PET 3 copolymer 30 ECCS 0.27 random copolymer 40 ECCS 0.12 PP 2 homopolymer 40 ECCS 0.12 Tensile tests were carried out using a tensile strength testing bench of a commercially available type in which the tensile test bar was extended at a constant rate. Tests were carried out both on test bars which were subjected to the tensile test immediately after production and with test bars which were first subjected to the conditions of a sterilization treatment for a can filled with a foodstuff before they were subjected to the tensile test. In this case, as is standard for a sterilization treatment of this nature, the test bar was held under pressure in water of 120°C for 60 minutes.

Three types of test bars were made. The first was made from steel strip with a thickness of 0.27 mm, a metal ECCS coating and with a plastic coating comprising PET 1 applied to one side and a plastic coating comprising PET 3 applied to the other side, in accordance with Table 1 (test code PET 1/3). At the location of the overlap

between the strips 1 and 2, the PET 1 and PET 3 coatings faced towards one another.

Two other types of test bars were based on ECCS material with a thickness of 0.12 mm; in one case, both sides were coated with PP 1 and in the other case both sides were coated with PP 2.

Table 2 summarizes the results of the tensile tests carried out with these three bars, giving both the test results after the test bars had and had not been exposed to sterilization conditions.

Table 2 Test code Sterilized Shear force per mm width of the test bar N/mm ll PET1/3 no 90. 50 yes 57. 87 PP 1 no 35. 81 yes 36.61 PP 2 no 40. 48 yes 41. 16 It can be seen from Table 2 that in all the cases described the measured shear force per mm width of the test bar at rupture of the test bar was still sufficiently high for it to be possible to accept that this joining method results in secure can joins.

Fig. 2 indicates how a can body can be produced, using the novel process, from a planar sheet 9 which has been bent into a cylinder and is provided with a thermoplastic coating layer on both sides. The edges 10 and 11 of the cylindrical sheet overlap one another by, for example, 3 mm, and then the sheet edges, in a similar manner to the tensile test described above, are pressed together in the direction of the arrows and are then heated and rapidly cooled again.

Figures 3 to 6 show four different embodiments of a detail of a join according to the novel process between a can body 12 and a lid 16 which is attached thereto. The figures show a cross section on one side of the lid 16 at the join location. In this case, the can body is of the seamless type.

It may be obtained by deep-drawing or wall ironing of sheet material 13 which has first been coated with layers of plastic 14 and 15. The material of the lid 16 also comprises metal sheet 17 which has been coated with layers of plastic 18 and 19. The layers of plastic may for their part be of the same type as those described in the tensile

test. It has been found that the metal sheet (13; 17) may again be of the ECCS type.

However, it may also, for example, comprise tin-plated or galvanized steel or steel which is not provided with any metal coating layer whatsoever. The metal sheet may also consist of a metal other than steel, such as aluminium.

Lid 16 and the top end of body 12 are shaped in such a manner that they have cylindrical faces 20 and 21 which fit against one another. By firstly heating the joining seam formed in this way and then cooling it again, those parts of the layers of plastic 15 and 19 which bear against one another can be made to adhere to one another. This figure also indicates how the free end of body 12 is beaded over at location 22, after which the beaded edge is flattened.

Fig. 4 shows a design which corresponds to Fig. 3 but in which the beaded-over edge has been formed into an edge 23 which is beaded over twice.

Fig. 5 shows a beaded-over edge 24 which is not beaded over outwards, but rather inwards. This figure also shows that the adjoining part of the body beneath the beaded edge is widened outwards, thus forming a delimiting edge 25, against which lid 16 bears at location 26. As a result, when attaching the lid 16, the position of the lid with respect to the beaded edge is clearly defined, thus facilitating accurate positioning of the lid with respect to the body 12.

Fig. 6 shows a variant of the structure shown in Fig. 5. In this case, the edge of the lid 16 is beaded over twice in such a manner that at the location where the lid adjoins the free end of body 12 two separate, parallel cylindrical faces 29 and 30 are formed, which both bear against the free end of body 12. As a result, it is possible, after heating, to obtain a double bond between body and lid. Also, in this case, the bend 27 in the beaded edge forms a boundary, at location 31, for the lid when it is pushed over the edge of the body.

Fig. 7 shows another variant of the design shown in Fig. 5. Positioning the lid 16 upside-down means that both cut edges of body and lid 16 cannot be seen from the outside. This method also makes the volume of the can slightly greater.