The invention refers particularly to a process for the production of cooking apparatus that comprises a stainless steel container to which is fixed a covering element with high coefficient of thermal conductivity in general. The electric heating resistance is arrange to contact the covering element in such a way as to allow uniform thermal conductivity between the same resistance and the base of the stainless steel container.

Furthermore, the finished product is completed with other auxiliary devices, for instance thermostats, switches or safety devices in general, as well as with coverings, covers and the like to make the apparatus suitable for sale, both from the point of view of complying with the safety norms, and from the esthetical point of view.

STATE OF THE ART An example of a process for the production of similar cooking apparatus, as well as of the apparatus produced according to such process, is described in the European Patent application N° EP-A- 0307555. The process described in this document involves the friction welding of a covering element in aluminum to the external base of the stainless steel container and the simultaneous forming in the originally flat covering element of a groove sufficiently deep to

house the heating resistance inside the same groove. The welding and the simultaneous forming of the groove reduce the number of steps necessary to give a similar manufactured article and theoretically would reduce the production costs of the same.

It is necessary to consider that the process made known in the cited European Patent application requires very high operating pressures and temperatures precisely to obtain the forming of the groove during the welding together of the container and the aluminum covering. This inevitably involves considerable energy loss, above all because of the need to form a sufficiently deep groove starting from a substantially flat plate. Moreover, the severe operating conditions of the plant for the production of manufactured articles according to the process described in the cited European Patent application accelerates the wear and tear in the same plant, increasing the frequency of breakdowns, malfunctions, as well as maintenance interventions, i. e. those events that require stopping the plant with consequent slowdown of production. It is enough to remember that operating pressures of not less than 600 tons are necessary to obtain the forming of a similar groove in the welding step, carrying the materials subjected to welding and forming to temperatures of around 600°C.

Another drawback associated with a similar process is due in particular to the need to perform the operation of welding and forming at the above mentioned particularly elevated temperatures. In fact, the quality of the product obtained at elevated temperature varies as a function of the composition of the alloy of aluminum used to form the covering and the respective groove. Therefore, if the operating temperature is not controlled

adequately in relation to the variety of the material used, a high number of rejections is possible. A partial remedy for this drawback requires the employment of graphite or of release liquid products during the process of welding and forming, but it is well known that that involves the generation of highly toxic compounds which are harmful for the health of the operators. It is therefore necessary to impose all the necessary mesures to guarantee the safety and quality in the work environments in which this technique is used.

Furthermore, it should be remembered that the forming of a sufficiently deep groove starting from a flat plate requires a considerable quantity of material. It is in fact necessary to start from a flat plate having a thickness greater or equal to 4 mm and a surface area substantially equal or slightly less than the surface area of the base wall of the stainless steel container to be able to extend the covering material over all the surface of the container and get simultaneously a covering having at least a thickness of 2 [mm] and a groove sufficiently deep (around 8-10 mm) to house the electric heating resistance.

Therefore, all the advantages deriving from doing the operations of welding and simultaneously forming the groove would be jeopardized by the drawbacks and by the costs that a similar process involves.

It is further appropriate to underline that the manufactured article obtained with the known process from the cited European Patent application is not very efficient in use. Indeed, only one portion of the radiating surface of the resistance is in contact with the aluminum covering, i. e. only the portion of the surface in contact with the inside surface of the groove in which it is inserted; the

remaining surface portion of the resistance is only in contact with the air. A complete transfer of the energy released by the resistance to the aluminum covering is practically impossible since part of the same energy is lost to the surrounding environment through the portion of radiant surface in contact with the air.

DESCRIPTION OF THE INVENTION This being stated, one object of the present invention is to propose a process that allows an electric cooking apparatus to be produced at particularly limited costs.

Another object of the present invention is to propose a process for the production of an electric cooking apparatus which does not impose severe strains on the production plant.

A further object of the present invention is to propose an electric cooking apparatus that is considerably more efficient from the heat exchange point of view than those of known type.

These objects are achieved by the present invention, which relates to a process for the production of an electric cooking apparatus, in which the apparatus comprises at least one stainless steel container having at least one base wall, at least one covering element in a material having a high coefficient of thermal conductivity fixed externally to the container on its base wall, and at least one electric heating resistance placed in contact with the covering element, characterized by comprising the steps of: -shaping the covering element in the form of a shell having at least one base wall and at least one side wall; -fixing the stainless steel container and the shell together by joining their respective base walls; -inserting the resistance inside the shell; and

-folding at least a portion of the side wall of the shell toward the inside to fix the resistance in contact with the inside base wall of the shell.

The process according to the present invention allows the production of the container and the covering element separately without requiring the forming of a groove during the fixing of the covering element to the stainless steel container. As well as avoiding considerable strains on the production plant, the process according to the present invention allows the production costs to be reduced considerably in terms of both the quantity of material needed to produce the covering element and of fixing the heating resistance in contact with the same covering element.

In accordance to a preferential embodiment of the process according to the invention, the covering element is produced in aluminum or alloys thereof and is shaped in the form of a shell, i. e. in the form of a container having a base wall and a side watt, by means of a cold pressing operation.

After the covering element is shaped, the stainless steel container and the covering element in the form of a shell are fixed together by means of friction welding of their respective external base walls, requiring only particularly limited pressures and temperatures to effect the welding.

Projecting elements that facilitate the centering of the resistance inside the base wall of the shell are preferably formed during the welding step, as well as projecting elements are formed for the support and/or the fixing of a thermostat onto the base of the shell.

In the same way, it is possible to form simultaneously at least one pair of projecting support elements located in proximity to the

position of the terminal portions of the resistance, in such a way as to allow the electrical terminal contacts for connecting the resistance to be suitably spaced from the base wall of the shell.

Subsequent to the welding operation, a portion of the side wall of the shell is preferably removed in correspondence of the position from which the terminals for electrically connecting the resistance will stick out. After the insertion of the resistance into the shell, the remaining portion of the side wall of the same shell is folded onto the resistance by means of a cold folding operation to completely envelope the external surface of the resistance for all its length except in proximity of the terminals for the electrical connection of the resistance.

The advantages obtainable using the process according to the present invention are considerable in terms of energy-saving and of limited strain on the plants, compared to the known type of processes. Operating pressures of around 400 tons and temperatures around 400°C are sufficient to obtain the manufactured article according to the process of the present invention. It also considerably reduces the frequency with which it is necessary to apply release products or graphite which could give rise to problems of health and safety in the work environment.

Furthermore, since the forming of a groove to house the resistance element is no longer necessary, the covering material requirement of aluminum or alloys thereof is reduced by around 30% compare to that required in processes already known in the art.

The invention also relates to an electric cooking apparatus, of the type comprising at least one stainless steel container with at least one base wall, at least one covering element made of a material

having a high coefficient of thermal conductivity fixed externally to the container on its base wall, and at least one electric heating resistance placed in contact with the covering element, characterized in that the covering element consists of a shell having at least one base wall and at least one side wall; and that at least one at least one portion of the side wall of the covering element is folded to fix the resistance in contact with the inside base wall of the shell.

Preferably, the folded portion of the side wall of the shell envelopes completely the electric resistance except in proximity to the electrical connecting terminals, in such a way that all the energy released by the resistance is absorbed by the covering element. This gives highly efficient thermal exchange between the heating resistance and the covering element.

BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages of the present invention will be more evident from the following description, made for illustrative but not limiting purpose, with particular reference to the enclosed schematic drawings in which: -Figure 1 is a perspective view from the bottom of a cooking apparatus according to a possible embodiment of the present invention; -Figure 2 is a section view of a detail of the cooking apparatus shown in Figure 1; -Figure 3 is a in cross section view of a covering element in the raw form before being shaped into a shell; -Figure 4 is a cross section view of a covering element, already shaped as a shell, before being fixed to a container to form a

cooking apparatus according to the present invention; -Figure 5 is a cross section view illustrating a step of the process during which a covering element shaped as a shell is fixed to a container to form a cooking apparatus according to the present invention; -Figure 6 is a plan view, in reduced scale, of a shell after its fixing to the container and the forming of projecting elements on its inside base wall ; -Figures 6A-6D are enlarged views in cross section of some possible embodiments of the projecting elements formed on the inside base wall of the shell ; -Figure 7 is a perspective view that illustrates the position of an electric heating resistance in a covering element shaped as a shell and fixed to a container; -Figure 8 is an enlarged view of a detail of a cooking apparatus obtained by means of the process according to the present invention; and -Figure 9 is an enlarged view of a detail of a cooking apparatus obtained according to another possible embodiment of the invention.

BEST MODES FOR CARRYING OUT THE INVENTION Figure 1 shows a cooking apparatus according to the present invention, which comprises a cooking container 1-produced for instance in stainless steel-and a covering element 2 produced in a material with a high coefficient of thermal conductivity-for instance aluminum or alloys thereof. As it is well known in the art, the use of stainless steel containers for the production of cooking apparatuses is certainly preferable, above all for hygienic reasons,

but steel has the drawback of not allowing uniform thermal conductivity to all the base of the container. For this reason, it is now common practice to fix a covering element, generally produced in aluminum alloy, firmly on the base of the stainless steel container for the purpose of enabling more uniform thermal conductivity from the heat source to the base of the container. Also in the case of the cooking apparatus according to the present invention, the covering element 2 in aluminum alloy is joined externally to the stainless steel container 1 on its base wall. A source of heat is provided by an electric heating resistance 3 placed in contact with the covering element 2.

The covering element 2 is preferably produced in the form of a shell 20 (Figure 4) with at least one base watt 22 and one side wall 23. As best illustrated in the section view of Figure 2, the side wall 23, or at least one portion of it, is folded to tightly fix the resistance 3 in contact with the base wall 22 of the shell 20.

According to an advantageous embodiment of the invention, the folded portion of the side wall 23 of the shell 20 envelopes completely the electric resistance 3 except in proximity of the electrical connection terminals 30. Therefore, the side wall 23 is folded to cover preferably all the external surface of the resistance 3 that is not in contact with the base wall 22 of the shell 20, i. e. up to bring the side wall 23 substantially in contact with the base wall 22 as illustrated in the detailed view of Figure 2. This allows the transfer of substantially all the heat released by the resistance 3 to the covering element 2 so making the cooking apparatus according to the present invention more efficient than the known type of apparatus.

The container 1 and the covering element 2 are preferably joined together by means of friction welding between their respective external base walls. Preferably, but not necessarily, the base walls of the shell 20 and of the container 1 are substantially equal in size and surface area.

The process for the production of a cooking apparatus according to the present invention comprises a forming step for a shell 20 (Figure 4) by means of cold pressing a flat plate 15 (Figure 3) in aluminum alloy, in such a way as to draw a shell 20 having a base wall 22 and a side wall 23.

Figure 5 shows the welding step, during which the container 1 and the shell 20 are set reciprocally with their external base walls in contact and pressed between the respective dies 101 and 120 of a friction welding plant. In particular, the die 120 is suitably shaped to allow the forming of a plurality of projecting elements on the base wall 22 of the shell 20 during the same welding step.

In particular, as is shown in more detail in Figure 6 and in the Figures 6A-6D, there are projecting elements 25 inside the base wall 22 that facilitate the centering of the electric resistance 3 during its insertion into the shell 20, as well as a plurality of projecting elements 26 that allow the later fixing of a thermostat (not shown) to the base wall 22.

The number and the position of the projecting elements 25 and 26 as shown in Figure 6 naturally vary according to the type and the dimensions of the resistance 3 and of the thermostat to be fixed to the shell 22.

With regard to the shape of the projecting elements 26 for fixing a thermostat, Figures 6A-6C illustrate some alternative embodiments that could be used according to the present invention. For instance,

the projecting element 26 shown in Figure 6A comprises a spacer portion 126a and a pin 226a located on its top. A thermostat could be positioned on the suitably located projecting elements 26 with its respective fixing holes inserted on the pins 226a. The ends of the pins 226a are then riveted to complete the fixing of the thermostat to the base wall 22 of the shell 20. In Figure 6B, the supporting element 26 projecting from the base wall 22 comprises a simple pin 226b devoid of any spacer portion and it is used in the same way as already illustrated for the pin 226a of the embodiment shown in Figure 6A. The projecting element 26 of Figure 6C comprises instead a spacer portion 126c in which a blind hole 226c is drilled to receive a self-tapping screw (not shown) by means of which the fixing in a detachable way of a thermostat could be effected.

The projecting element 25 shown in Figure 6D comprises a simple protuberance that, together with others similarly suitably located, facilitates the centering of the resistance 3. The forms and the dimensions of the projecting elements 25 and 26 can naturally vary depending on the different requirements of fixing the thermostat and centering the resistance 3. Furthermore, different types of projecting elements could be created simultaneously on the base wall 22 to adapt the same cooking apparatus to receive various types of resistance and of thermostats.

In the following steps of the process according to the invention, before inserting the resistance 3 in the shell 20 thus formed, a portion of the side watt 23 is removed to allow subsequent access to the electrical connection terminals 30 of the resistance 3. After inserting the resistance 3 into the shell 20, the manufactured article substantially assumes the appearance of the embodiment

illustrated in Figure 7, with the electrical connection terminal 30 of the resistance 3 located in correspondence to the portion of side wall previously removed. The correct position of the resistance 3, which can also have dimensions slightly smaller than the contour of the shell 20, is facilitated by the presence of the projecting centering elements 25.

The wall 23 of the shell 20 is then folded toward the inside in such a way as to cover the resistance 3 entirely for all its length except in proximity of its ends comprising the electrical connection terminals 30. The folding of the wall 23 is preferably effected cold.

After having performed this operation, the extremity portions of the resistance 3 are preferably subjected to a light folding for spacing the electrical connection terminals 30 from the base wall 22 of the shell 20, until the configuration shown in Figure 8 is reached.

According to an alternative embodiment of the present invention shown in Figure 9, support elements in relief 27 could be provided which allow the extremity portions of the resistance 3 to maintain their distance from the base watt 22.

The projecting support elements 27 could be easily obtained during the friction welding step (Figure 5) between the shell 20 and the container 1, using the same technique used to obtain the projecting elements 25 and 26. The presence of the support elements 27 would also allow the extremity portions of the resistance 3 to fold up automatically during the step in which the side watt 23 of the shell 20 is folded, avoiding the need to provide a further step of working.

Various modifications could be provided without going outside the scope of the present invention. In particular, the various elements that comprise the cooking apparatus can have different forms of

embodiments from those shown schematically in the Figures. For instance, the container and the shell could have a substantially rectangular shape in plan, or the container could only consist of a substantially flat plate.

Furthermore, the shell 20 can also be formed in such a way that its side wall 23 is already devoid of the lacking portion corresponding to the position of the electrical connection terminals 30 of the resistance 3.

Another possible form of carrying out the process with respect to the illustrated process could provide for the cutting and the folding of a portion of the side watt 23, instead of its removal, corresponding to the extremity portions of the resistance 3. In other words, the portion of side wall could be cut only along vertical lines leaving it therefore connected to the base wall 22. The subsequent folding or forming of this portion can allow similar spacers, for instance, to those shown with the reference number 27 in Figure 9.