MADE OF DIFFERENT MATERIAL

The invention relates to a container, in particular for the heating of food in microwave ovens or the like, comprising a thin-walled container body of material essentially opaque to microwave energy, e.g. aluminium foil or thin aluminium strip, at least one side (wall, cover or base) of the body having a wall section of material transparent to microwave energy, e.g. synthetic material, which is inserted into the container body, the wall section being securely joined at its edge to the container body by means of flanging or folding.

Such a container is already known (US 4 689 458) . In the case of this known container, the container body, consisting of metal foil, has an opening in its base which is closed by a wall section inserted therein of synthetic material transparent to microwave energy. The attachment of this wall section of synthetic material to the metal foil body is achieved by providing this body with a fold at the edge of its base opening which is open towards the opening and into which the edge of the wall section is inserted. If necessary, the edge of the wall section can also be glued or sealed to the fold of the container body to prevent the edge of the wall section from undesirably sliding out of the open fold. However, this gluing or sealing is an additional time-consuming step that increases the cost of manufacture of the container, which is very undesirable in the case of a mass-produced article.

A permanently flanged or fold-ed-joint between the wall section and the container body in which the edge of the inserted wall section is also flanged or folded has .not succeeded up to now due to the problem of the synthetic material of the wall section having such a resiliency that, for the minimum wall thickness necessary for the required stability of the wall section, the synthetic material edge of this wall section opens up the joint. With an appropriate wall thickness of the container body, this can actually be prevented to a large extent, but in the case of a mass-produced article such as the container in question, a wall thickness which is greater than that required for adequate stiffness of the body is uneconomical.

In the case of a container of the generic type initially mentioned, it is therefore the object of the invention to design the flanged or folded joint between the container body, usually of metallic foil, and the wall section inserted therein, which mostly consists of synthetic material, in such a manner that it alone, without any additional gluing or sealing, forms a well sealing and durable, secure joint without an overdimensioning of the wall thickness of the body being necessary and without having to doubt the required rigidity of the insertable or inserted wall section. According to the invention, this is achieved in a simple manner in that the inserted wall section is thinner at its joining edge, which is flanged to or folded with the container body, than is necessary in its remaining surface area within the joining edge, and less rigid than the wall thickness of the container body in the region of its flanged or folded connection with the inserted wall section, and in fact suitably that much thinner to the extent that the thin joining edge of the inserted wall section does not open up the flanged or folded joint on account of the resiliency of its material. To this end, the joining edge of the wall section can have a thickness which corresponds at most to half of the thickness of the container body in the joint area.

~ Z - Additionally, or instead of the thin structuring of the bent edge of the inserted wall section, it is, however, also possible to provide this edge with a reduced moment of resistance in a different manner, such as for example by edge cut-outs, which weaken the restoring force of the synthetic material edge when it bends back from its flanged or folded position to the extent that the synthetic material edge can no longer open the flanged or folded joint.

Within its edge attachment, the inserted wall section can be provided with a preferably grid-shaped coating, print or embedding of a material which influences the passage of microwaves and/or reflects thermal radiation.

The invention also relates to a process for the manufacture of a wall insert of synthetic material, which can be permanently flanged or folded at its edge together with a container body to manufacture a container according to the afore-mentioned inventive structure. This process is characterized in that initially, a synthetic material blank is manufactured with an edge bent at right angles, the external dimensions of which correspond to the desired external dimensions of the wall insert, and that this edge bent at right angles is subsequently seized at its outer end and stretched in a drawing process in its direction of stretching while reducing its wall thickness. To implement this process, a deep-drawing device can be used which has a gripping device with two clamping jaws to grip the synthetic material blank inside its bent edge, wherein a clamping device can be provided on the side of the one clamping jaw facing away from the other clamping jaw for seizing the outer end of the bent edge of the blank, the clamping device itself consisting of two clamping jaws which seize this edge on both its sides and which can be moved towards and away from each other as well as mutually in the direction of stretching of the edge of the blank, relative to the gripping direction.

Further suitable embodiment device are characterized in the dependent claims.

In the drawing, the manufacture according to the invention of a flanged joint between the body of a container and the wall section inserted in its base as well as an embodiment of a deep-drawing device for the manufacture of this wall section are schematically shown, these being described in detail in the following, wherein:

Fig. 1 shows the container body with an inserted wall section in cross-section before the manufacture of the flanged joint.

Fig. 2 shows a section through the finished flanged joint of the container body and the inserted wall section.

Figs. 3, 4, 5 and 6 show cross-sections in partial view through an exemplified embodiment of a device for the manufacture of such a wall section in different operating positions as well as the shape of the work piece obtained in each of the operating positions of this device.

Fig. 7 shows the last process step of the device shown in Fig. 3 - 6 in a larger transverse section with the wall section shown in its final form in the device.

Fig. 1 shows the body 1 of a container consisting of metal in section which has an opening in its base that is closed by a wall section 2 of synthetic material inserted therein. To securely attach the wall section 2 to the container body 1, both the container body and the wall section 2 each

have an edge 3, 4 bent at rig ^~ htS-angles which, in the i-ni•ti•a,l position, i.e. before joining by flanging, lie flat against each other (Fig. 2). While the edge 3 of the body surrounding the opening of the container body has the same wall thickness as the body in its other areas, the wall thickness of the edge 4 of the inserted wall section 2 is significantly thinner than its surface area 6 inside this edge which covers the base opening in the body 1.

In the case of a container in which food is to be cooked, the container body can consist for example of an aluminium strip of ca. 120 _/ um which has been formed into the shape of the container body by deep-drawing. The wall section inserted into this container body can consist of any synthetic material which is suitable for food. If it consists of a polyester, e.g. polyethyleneterephtalate (PET), the edge 4 of the wall section can have a wall thickness of about δO.um. If it consists of a polypropylene, the edge 4 can have a wall thickness of about 180.urn, since polypropylene after folding or flanging of the edge of the wall section with the container body has a somewhat lower shape stability and lower resiliency than polyester, which in the cold state is especially difficult to deform. In both cases, the wall section inside the edge 4 can have a thickness of about 250,um. These different thicknesses of the adjacent edges 3 and 4, shown in Figs. 1 and 2, facilitate that these edges, which consist of different materials, can be easily formed with the flanged shape shown in Fig. 1 and that the restoring force in bending of the synthetic material edge 4 is low enough that the metal edge 3 can resist this force, so as to prevent the opening of the flanging. The thickening 5, shown at the outer end of the edge 4 of the wall section 2 in Figs. 1 and 2, results from the manufacturing technique and is explained further below.

The deep-drawing device, shown as an example in Figs. 3 to

7, for the manufacture of such a wall section 2 of synthetic material, which can be inserted into a container body 1, has a fixed cylindrical or almost cylindrical housing 10 provided with an opening 11 at its upper side. The upper clamping jaw 12 of a gripping device 12, 13 can be moved up and down within this opening, the gripping device serving to clamp a synthetic material blank 17 used for the manufacture of the insertable wall section. The lower clamping jaw 13 is securely fastened in the housing 10. The upper clamping jaw 12 of the gripping device can be moved towards the lower clamping jaw, and the upper clamping jaw has at its edge a protrusion 14 which laterally overlaps the outer periphery of the lower clamping jaw in the position in which it is moved towards the lower clamping jaw (Fig. 4). The protrusion 14 is provided with a cutting edge 15 (Fig. 7) which, together with the opposing edge 16 at the edge of the opening 11 of the housing 10, serves as a cutting device to cut the blank 17 into its required dimensions, the insertable wall section 2 being then manufactured out of this blank.

A clamping device 18, 20 is provided in this device beneath the lower clamping jaw 13 and consists of a reciprocating internal clamping body 18 and an external clamping ring 20, which can be formed of several segments and moved in a holder 21 towards and away from the peripheral area 19 of the internal clamping body. The outer clamping ring 20 can be moved up and down together with its holder 21 and the clamping body 18.

The device shown in Figs. 3 - 7 operates as follows: The upper clamping jaw 12 of the gripping device 12, 13 is initially pulled out of the edge of the opening 22, of the housing 10, formed as a cutting ring, until a starting workpiece 23, for example in the form of a synthetic material strip pre-heated to 130° to 140°C, is placed with its sides on the upper edge 22 of the housing 10 under the clamping jaw. The upper clamping jaw 12 is then moved from

its upper end position above the cutting position shown in Fig. 3 into its lower end position shown in Fig. 4, in which case its cutting edge 15 passes the opposing cutting edge 16 of the housing 10 while cutting the blank 17 shown in Fig. 3 out of the workpiece. The clamping jaw 12 carries along the blank 17 during its further downward movement in order to clamp it between itself and the lower clamping jaw 13 (Fig. 4). In this case, the protrusion 14 moves laterally past the peripheral surface of the lower clamping jaw 13 so that the blank 17 receives an edge 24 which is bent at right angles.

Subsequently, the outer clamping ring 20 moves out of its initial position inside its holder 21 shown in Figs. 3 and 4 into the clamping position shown in Fig. 5, in which it clamps the outer end of the bent edge 24 of the blank 17 between itself and the internal clamping body 18. In this manner, an appropriate indentation 27, shown in the bottom of Fig. 5, is produced in the bent edge 24 of the synthetic material blank 17. In this clamping position, the internal clamping body 18 and the external clamping ring 20 move downwards out of their upper end position, shown in Figs. 3, 4 and 5, in which case they move away from the lower clamping device 13, and the bent edge 24 is stretched by stretch forming and subjected to a significant thinning of its wall thickness in the process. In this case, a thickening of the material can remain at the extreme end of the stretched edge 26 (see number 5 in Fig. 1), the thickening being caused by the securing of this end between the clamping jaws 18 and 20.

The insertable wall section manufactured with this device has therefore obtained its final shape, which is shown in the bottom of Fig. 6.