A cooking vessel

The present patent application relates to a cooking vessel provided with steel bottom piece for use in cooking tops with induction hobs.

The purpose of the present invention is to improve the cooking vessels used on induction hobs, of the type composed of a vessel made of aluminum alloy (and alloys) provided with a steel bottom piece.

As it is known, cooking vessels made of aluminum and aluminum alloys cannot be used on induction hobs. For such a reason, said vessels are normally provided with a ferromagnetic plate made of ferritic stainless steel (normally AISI 430) on the bottom of the aluminum vessel.

Said ferromagnetic plate is joined to the bottom of the vessel by means of different technologies, such as braze welding or impact bonding; consequently, a bi-metallic plate, e.g. a plate made of two materials with different thermal conductivity, is created on the bottom.

When heated on the induction hob the bottom of the cooking vessel tends to suffer thermal expansion, with consequent increase of the concavity value of the bottom both during use and afterwards.

More precisely, said unpleasant thermal expansion occurs during the heating of the vessel, but also continues after the vessel has cooled down and returned to ambient temperature.

The purpose of DE 200 15 128 is to improve heat distribution in a cooking vessel. The cooking vessel is provided with a bottom with depressions on the lower surface that comes in contact with the cooking hob. In order to additionally increase heat distribution, a material with high heat diffusion properties, such as aluminum or copper, is encapsulated between the lower wall and the bottom of the cooking vessel and disposed on a stainless steel base body. The layers of different materials are joined with braze welding technique. The depressions realized on the external side (lower side) of the bottom are provided also on the internal side (upper side) of the bottom, in order to contain the braze welding paste during welding by means of brazing.

W091/15142 describes a cooking vessel provided with a bottom with grooves, cavities and ribs (not through slots) designed to expand the bottom differently compared to the same bottom without cavities, so that the concavity of the bottom is controlled within predetermined values. Such a configuration of the bottom of the cooking vessel is designed to reduce deformation under the effect of heat transmitted by traditional non-induction cooking tops. In fact, the aforementioned patent relates to cooking vessels that are completely made of aluminum, without the addition of ferromagnetic materials susceptible to the electromagnetic field generated by the inductor situated under the induction cooking hob.

EP 0 268 186 describes a cooking vessel provided with special grooves on the bottom, which are designed to cause the evaporation of liquids that may come out of the cooking vessel and penetrate between the cooking hob and the bottom of the cooking vessel.

It must be considered that in order to be used at best, cooking vessels must have concavity values measured before, during and after heating in compliance with the acceptability range established by universally acknowledged standards (first of all, the DIN standards).

For cooking vessels designed for use on induction hobs the reference standard is established by the "Class Induction" Swiss association. For the two macrofamilies of bodies, low vessels (frying pans, crepe makers, grills, etc.) and high vessels (casseroles, pots, etc.), the standard establishes the heating time (required only for vessels classified as high) and concavity values both during heating and after cooling.

The initial concavity value (CO) measured at ambient temperature must have a value comprised between zero and [6 % ₀ x (d)], being (d) the diameter of the bottom piece in question; said concavity value must not exceed the value of [(CO) + 6 % ₀ (d)] both during heating up to 180°C and for the 4 following minutes, maintaining the temperature stable at such a value, and then return to a value lower than [6 % ₀ x (d)] when the temperature of the cooking vessel has returned to ambient temperature.

The heating speed, which is required only for cooking vessels classified as high vessels, must be included in a predetermined time interval established by the standard according to the diameter of the vessel (the time/diameter table is available in the Class Induction standards).

In order to reduce unwanted bending and guarantee the return of concavity to said values once the cooking vessel has cooled down, steel bottom pieces have been devised for fixing to the vessel, provided with holes and/or slots with different geometry and surface distribution.

Steel bottom pieces of cooking vessels available on the market are shown in Figs. 1 to 9.

Said holes and/or slots provided on the steel bottom pieces have been devised in order to reduce bending, although results are satisfactory.

The purpose of the present invention is to remedy the drawbacks of the prior art, by devising an aluminum cooking vessel for use on induction hobs, which is provided with a bottom piece adapted to avoid deformation during cooking.

These purposes are achieved according to the invention with the characteristics claimed in the attached independent claim 1 .

Advantageous embodiments appear from the dependent claims.

The purpose of the present invention is a cooking vessel provided with a steel bottom piece, which comprises a central portion and a series of radial slots.

For purposes of clarity the description of the cooking vessel according to the invention continues with reference to the attached drawings, which are intended for purposes of illustration only, and not in a limiting sense, wherein:

Figs. 1 to 9 are bottom views of the bottoms of cooking vessels according to the prior art;

Fig. 10 is an axial sectional view of a cooking vessel turned upside down and an exploded view of the bottom piece of the invention;

Fig. 10A is an enlarged, partially interrupted, view of Fig. 10; Fig. 1 1 is the same view as Fig. 1 0, except for the fact that the bottom piece is assembled on the bottom of the cooking vessel ;

Fig. 1 1 A is an enlarged, partially interrupted, view of Fig. 1 1 ;

Fig. 1 2 is a bottom view of the bottom piece fixed to the bottom of the cooking vessel according to the invention ; and

Fig. 1 3 is a top view that shows in detail one of the slots provided in the bottom piece of the cooking vessel according to the present invention.

With reference to Figs. 1 0 and 1 2, an aluminum cooking vessel (1 00) is disclosed, which comprises a bottom (101 ) and side walls (1 02) extending from the bottom (1 01 ) in such manner to define a compartment to contain food to be cooked.

The cooking vessel of the invention also comprises a bottom piece (1 ) composed of a plate or disk made of ferromagnetic material for use on induction hobs. Advantageously, the bottom piece (1 ) is made of ferritic stainless steel.

The bottom piece (1 ) is applied on the lower surface of the bottom (1 01 ), e.g. on the external surface with respect to the cooking compartment, in such manner that the bottom piece (1 ) is in contact with the induction hob. For illustrative purposes, the bottom (1 01 ) has thickness of approx. 4.8mm before being fixed to the bottom piece (1 ). Instead, the bottom piece (1 ) has thickness of approx. 0.3-0.7mm.

Also referring to Fig. 1 2, the bottom piece (1 ) is provided with a central portion (B) and a series of through slots (1 1 ) extending radially from the central portion (B). All slots (1 1 ) have the same dimensions and the same angular spacing.

The bottom piece (1 ) is fixed to the aluminum bottom (1 01 ) of the cooking vessel by means of pressure fixing process.

The pressure fixing process is composed of the following steps:

• surface preparation of the two sides of the bottom piece (1 ) and bottom (1 01 ) to be fixed, in such manner that both sides have average surface roughness (Ra) not lower than 1 μιη ; • centering and partial fixing by means of tacking of the steel bottom piece (1 ) coaxially to the bottom (101 ) of the aluminum cooking vessel;

• heating of the bottom piece (1 ) - bottom (101 ) assembly in such manner to reach an equilibrium temperature of approx. 300°C÷350°C;

· pressing by means of a press in order to guarantee a force proportional to the diameter of the steel disk of the bottom piece (1 ) to be fixed to the aluminum bottom (101 ), said pressure being indicatively 5-20 Ton/0(mm).

During the pressure fixing process, the aluminum of the bottom (101 ) penetrates in the through slots (1 1 ) of the bottom piece (1 ) and is disposed flush with the external surface of the bottom piece (1 ). In this way the external surface of the bottom piece (1 ) is perfectly smooth and flat. Such a process is performed without adding any other material, such as the braze welding paste used in traditional braze welding processes.

It must be considered that said through slots (1 1 ) obtained on the bottom piece (1 ) are not designed to guarantee fixing between the steel of the bottom piece and the aluminum of the bottom, being exclusively designed to reduce bending suffered by cooking vessels used on induction hobs.

The distribution of the slots (1 1 ) must be homogeneous and the percentage ratio between the surface of the slots and the total surface of the steel bottom piece must be comprised between 25% and 30%.

Specifically, it was noted that the higher the number of the slots, with the same percentage between the surface of the slots and the surface of the bottom piece, the higher the concavity control will be.

The bottom piece (1 ) is provided with a central portion (B) that is not crossed by the slots (1 1 ); said central portion (B) is basically circular with radius (R0) comprised between a minimum value R0 = 1 1 mm and a maximum value R0 =13 mm.

The bottom piece (1 ) is basically shaped as a disk with radius (R). Said slots (1 1 ) end before the external edge of the bottom piece (1 ) and are inscribed in a circle with radius (R1 ) comprised between a minimum value (R) - 1 1 .5mm and a maximum value (R) - 7mm; being (R) the radius of the steel bottom piece (1 ).

Referring to Fig. 13, each slot has:

- a total length (L2) equal to (R1 ) - (R0);

- a radius of lower edge (R2) comprised between a minimum value of 0.5mm and a maximum value of 0.6mm;

- a radius of upper edge (R3) that is a function of the radius (R) of the steel bottom piece (1 ) and is given by the following formula:

40.8 42.8

These values for steel disks with radius R comprised between 55mm and 120 mm guarantee, between two adjacent slots, a steel thickness variable from 1 to 1 .2mm that, in addition to guaranteeing material continuity along the radial direction, ensures easy production with any technology (shearing, punching, laser cutting, etc.) starting from a steel solid disk.

The description continues with reference to the table below, which shows the values obtained from tests performed with 10 models of pans available on the market and 2 models of pans according to the invention.

In the table below models 1 to 8 are cooking vessels provided with a steel bottom piece with pattern respectively corresponding to Figs. 1 to 8.

Instead, models 9 and 10 refer to cooking vessels provided with the same steel bottom piece with pattern corresponding to Fig. 9; in this case the two models differ in size.

Model 1 1 is the cooking vessel of the invention, which is provided on the bottom (101 ) with a steel bottom piece (1 ) with pattern corresponding to Fig. 12.

Following is an explanation of the values shown in each column:

Code: identification code of the model of cooking vessel under test; - Bottom thickness mm: total thickness of the bottom (aluminum + steel) in mm;

0 bottom mm: diameter of the steel bottom piece, on which 6 o is calculated, being the limit value established by the Class Induction standard;

C/l : initial concavity, e.g. the maximum value of deflection measured in mm;

6 o bottom: 6 o of the diameter of the steel bottom piece obtained from ø bottom int/1000 ^* 6 calculation;

TO cold: the first column shows the concavity value in mm measured on the cooking vessel before heating, whereas the second column shows the corresponding value expressed in ₀ ;

T1 180°C: the first column shows the concavity value in mm measured when the temperature of 180°C is achieved on the part, whereas the second column shows the corresponding value expressed in ₀ ;

T2 180°C 4': the first column shows the concavity value in mm measured after maintaining the temperature constant on the part at 180°C for 4 minutes, whereas the second column shows the corresponding value expressed in ₀ ; T3 amb: the first column shows the concavity value in mm measured after cooling the part down until ambient temperature is achieved, whereas the second column shows the corresponding value expressed in % ₀ ;

As it can be seen, the only model of cooking vessel with bending value under the threshold value of 6 o during prolonged heating (until 180°C for 4') is the cooking vessel of the invention, and specifically model 1 1 , with 3.46mm.

The only model amongst those available on the market that has maintained said value under 6 o is model 7 (with 4.3mm). However, it can be noted that said model has a steel bottom piece with high thickness of 7.4mm (almost double than model 1 1 ) and, as mentioned above, it is known that said bending is reduced when the thickness of the steel bottom piece is increased.

However, it must be said that such a high thickness makes the cooking vessel (model 7) excessively heavy, also affecting production, storage, transportation costs, and therefore is not a desirable solution.

On the contrary, the model of the invention has a total thickness lower than 5.5mm.

Therefore, model 1 1 achieves all the aforementioned purposes.

Moreover, the time needed to achieve the operating temperature is shown for the model of the invention. To the experts of the field these values demonstrate the excellent operation of the cooking vessel of the invention that, by achieving the desired temperature rapidly, allows for saving on energy with reference to the induction hob.

As an additional confirmation of the advantages of the invention, the time values of model 7 are given, which achieves the operating temperature in double time compared to model 1 1 .