EP0202969A1 - Flat heating element with electrical resistance, and article equipped with such an element - Google Patents

Abstract

The flat electrical resistance heating element is intended principally for a household article having a heating surface. The heating element comprises a heating resistance cut out from a metal sheet (1) which is coated with a substrate (2) of electrically insulating material resistant to the heating temperature and adherent to the metal sheet (1) and to the material of which the support (5) of the heating surface of the article is made. Use principally in base-heated kitchen utensils and irons.

Description

The present invention relates to a flat electric resistance heating element, in particular for a household article comprising a heating surface.

The invention also relates to heating articles comprising such a heating element.

The invention preferably applies, but is not limited to, cookware with a heated bottom, cooking plates, plate warmers, radiators and radiant panels and irons.

Culinary articles with a heated bottom generally comprise a tubular and shielded electrical resistance fixed by crimping to the bottom of the article. This method of attachment is complex and expensive. In addition, it does not make it possible to obtain an excellent thermal contact between the resistance and the bottom of the article nor to obtain a homogeneous distribution of the temperature.

In the case of electric irons, the heating sole is generally made of aluminum cast on an armored electrical resistance. The production of such soles is also complicated and expensive.

In addition, as in the case of articles with a heated bottom, the electrical resistance is in contact with the soleplate in a generally U-shaped line, so that it does not allow good homogeneity of the temperature distribution to be obtained.

In the current state of the art, this difficulty is remedied by making relatively thick soles. However, such a solution considerably affects the cost as well as the weight of the article, so that it is necessary to have recourse to relatively expensive light alloys.

The object of the present invention is to remedy the drawbacks of known embodiments by creating a flat heating element which is easy to integrate into the heating article, which makes it possible to ensure a uniform distribution of the heating over the heating surface of the article and which is inexpensive to manufacture.

According to the invention, this electric resistance heating element, in particular for a household article comprising a heating surface, comprises a heating resistance cut from a metal sheet and is characterized in that this sheet is coated in a substrate of electrically insulating material, resistant at the heating temperature and capable of adhering to the metal foil and to the material constituting the support of the heating surface of the arcle.

The fact that the resistor is cut from a metal sheet, makes it possible to obtain a high ohmic value per unit of area and above all to distribute the heating temperature uniformly over the entire surface of the resistor. Furthermore, such resistors cut from a metal sheet are inexpensive to produce.

The fact that this resistive sheet is coated in an insulating substrate adhering to this sheet and to the material constituting the support of the heating surface of the article, makes it possible to solve in a simple and effective manner, the problems of the insulation of the resistive sheet and the attachment of the heating element to the heating article.

According to an advantageous version of the invention, the metal sheet is chosen from the following resistive metals: stainless steel, ferronickel alloy and nickel-chromium alloy.

The sheets produced in these alloys can have thicknesses between 0.08 and 0.015 mm, while having sufficient mechanical properties to withstand without damage the various manipulations necessary for the manufacture of the heating element.

According to a preferred version of the invention, in the case of a heating element intended for heating to a temperature between 400 and 500 ° C., the metal sheet is comprised between two layers of felt of ceramic material impregnated with a polymerizable resin under the action of heat, these two layers of felt being bonded together and adhering to the metal sheet, the resin being preferably chosen from polyimide, phenolic and silicone resins.

According to another version of the invention, the metal sheet is between two layers based on alumina projected in the form of plasma.

According to another version of the invention, the metal sheet is between two layers of enamel.

In the case of the use of layers of ceramic felt impregnated with resin, the bond between the resistive sheet and the support is obtained during the polymerization of the resin. The two layers of ceramic felt provide excellent electrical insulation of the resistive sheet and give the heating element excellent heat resistance. After polymerization of the resin which impregnates the layers of felt, these become hard and resistant to mechanical shock. At the same time, the resin allows the heating element to adhere to a metal support.

The application of a layer of alumina projected in the form of plasma, or a layer of enamel which is baked, also makes it possible to obtain an excellent bond between the resistive sheet and the support as well as excellent electrical insulation. .

The invention also applies to heating articles comprising a support to which a heating element according to the invention is fixed.

This support can be chosen from the following materials: aluminum, stainless steel, mild steel, ceramic, vitrocrystalline material and glass.

The heating element can be fixed to the support by means of a resin layer which may correspond to that which permeates the substrate, when the latter is for example based on ceramic fiber felt.

The heating element can also be fixed to the support by means of a plate applied with pressure against this heating element and fixed to said support.

According to another version of the invention, the heating element is integrated into the support when it is made, for example by polymerization of the resin contained in the substrate.

Other features and advantages of the invention will appear in the description below.

In the appended drawings, given by way of nonlimiting examples:

• . Figure 1 is a sectional view of the various layers of a flat heating element according to the invention, before assembly of these layers;
• . Figure 2 is a partial plan view of the resistance obtained from a cut sheet;
• . Figure 3 is a plan view similar to Figure 2 relating to another embodiment of the resistor;
• . Figure 4 is a sectional view of the completed heating element, the different layers being assembled;
• . Figure 5 is a sectional view showing the heating element and a support, before assembly;
• . Figure 6 is a view similar to Figure 5 showing the completed hot plate;
• . Figure 7 is a view similar to Figure 1, concerning a first alternative embodiment;
• . Figure 8 is a sectional view showing the element according to this finished variant;
• . Figure 9 is a sectional view showing the element according to this variant and a support, before assembly;
• . Figure 10 is a sectional view showing the completed heating plate;
• . Figure 11 is a sectional view showing the different layers of a second alternative embodiment and a support, before assembly;
• . Figure 12 is a sectional view showing the completed heating plate;
• . Figure 13 is a sectional view showing the different layers of a third embodiment, a cover plate of the heating element and a support, before assembly;
• . Figure 14 is a sectional view showing the completed heating plate;
• . Figures 15 to 18 are sectional views showing the different stages of the manufacture of a fourth alternative embodiment, the heating element being integrated into the support;
• . Figures 19 to 21 are sectional views showing the different stages of the manufacture of a fifth alternative embodiment;
• . Figure 22 is a sectional view showing the different layers of a sixth embodiment, before assembly;
• . Figure 23 is a sectional view of the completed heating plate;
• . Figure 24 is a sectional view showing the different layers of a seventh alternative embodiment, before assembly;
• . Figure 25 is a sectional view of the completed heating plate;
• . Figures 26 to 29 are sectional views showing the different stages of manufacture cation of an eighth embodiment
• . Figure 30 is a sectional view showing a ninth alternative embodiment;
• . Figure 31 is a sectional view showing a tenth embodiment.

In the embodiment of FIGS. 1 and 4, the heating element comprises a resistor 1 cut from a metal sheet, comprised between two layers 2 based on felt of ceramic fibers loaded with polyimide resin. This felt consists of ceramic fibers and mica flakes at a rate of 70 to 80% of the product, the resins acting as a binder when they are polymerized while the electrical insulation is mainly due to the above materials.

The metal sheet 1 is for example a sheet of stainless steel, of ferro-nickel alloy or of nickel-chromium alloy having a thickness of between 0.08 and 0.015 mm. This sheet 1 is cut so as to form therein (see FIG. 2) a ribbon of constant width which may include at certain points connecting straps intended to be cut selectively in order to vary the length of the resistance and thus obtain characteristics. different from the resistive circuit or allow the said characteristics to be adjusted on demand.

The metal sheet can also be cut as shown in Figure 3, so as to form a continuous resistive strip the sinuous contour.

The metal sheet 1 is preferably treated chemically or mechanically to improve the adhesion of the layers 2.

In this example, this adhesion is obtained by heating the assembly constituted by the sheet 1 sandwiched between the two layers 2, so as to polymerize the polyimide resin contained in these layers. This polymerization has the effect of binding the two layers 2 together through the openings 3 made in the sheet 1 (see FIG. 4), and of binding these two layers 2 to this sheet 1.

The two layers 2 can for example be made of the material described in French patent No. 80 23 943 of November 5, 1980.

To avoid adhesion of the layer 2 with the heating surface used to polymerize the resin, the outer face of this layer is covered by a metal sheet 4, for example aluminum which at the same time has a thermal or self-diffusing effect. radiant and mechanically protects layer 2.

The heating element obtained, as shown in FIG. 4, can be fixed on a metal support 5 made of aluminum, stainless steel, mild steel, ceramic or vitrocrystalline material or glass.

This fixing can be obtained by placing a layer of polyimide resin 6 between the support 5 and the heating element and by heating the assembly so as to polymerize this resin.

The hot plate obtained, as shown in FIG. 6, can constitute a cooking plate, the heating bottom of a culinary article, the soleplate of an iron, etc.

In the case of FIGS. 1, 4, 5 and 6, the thickness of the layers has been greatly exaggerated for the clarity of the figures. In reality, the thickness of the heating element will generally not exceed one millimeter.

The assembly thus produced can withstand temperatures reaching and even exceeding 350 ° C., ensuring a perfectly uniform distribution of the temperature over the entire surface of the heating plate.

In the case of the embodiment according to FIG. 7, the resistive sheet 1 is placed between two layers, each comprising a layer 7 of polymerized samicanite (laminate of sheets of paper impregnated with phenolic resin loaded with mica flakes). Each layer 7 of samicanite is coated with a layer of phenolic resin 8, not polymerized. The resistive sheet 1 is placed between the two layers 8 of unpolymerized phenolic resin.

After polymerization of the two layers 8 of phenolic resin, this resin completely coats the resistive sheet 1 and binds together the two layers 7 of samicanite through the openings cut in the resistive sheet 1, as indicated in FIG. 8.

To fix the heating element to the support 5, a layer 9 of ceramic felt loaded with polyimide resin is inserted between them as indicated in FIG. 9. The polymerization of the polyimide resin contained in the layer 9, obtained by heating, binds the heating element to the support 5.

As in the case of the embodiment shown in Figure 6, the hot plate shown in Figure 10 withstands temperatures above 400 ° C while having a perfectly uniform distribution of temperature, which thus avoids any risk of localized hot spot.

In the embodiment of Figure 11, the cut resistive sheet 1 is as in the embodiment according to Figure 1 placed between two layers 2 of ceramic felt impregnated with polyimide resin. Of on either side of the two layers 2, two composite layers are applied, each comprising a layer 7 of samicanite coated with a layer 8 of phenolic resin, the latter being adjacent to the layer of felt 2. Another layer 10 of phenolic resin is applied between the layer 7 of samicanite adjacent to the support 5 and the latter.

To fix the different layers to the support 5, the assembly is heated to polymerize the layers 8, 10 of phenolic resin and the polyimide resin contained in the felt layers 2. The various layers of the heating element are thus joined together (see figure 12), at the same time as it is fixed to the support 5, while ensuring excellent thermal insulation between the cut resistive sheet 1 and the support 5.

As in the previous embodiments, the thickness of the heating element does not generally exceed 1 mm, so that the thickness of insulating material between the support 5 and the resistive sheet 1 practically does not affect the thermal transmission properties .

Furthermore, in the case of the embodiment according to FIG. 12, like that of FIG. 10, the layer 7 of samicanite disposed opposite the support 5 mechanically protects the resistive sheet 1 and avoids any risk of sticking of the assembly with the walls of the mold used to mold and heat treat this assembly.

In the case of the embodiment according to FIG. 13, the heating element is composed, as in the embodiment of FIG. 1, by two layers 2 of ceramic felt impregnated with polyimide resin applied on either side of the resistive sheet 1 This heating element instead of being glued to the support 5, is pressed against the latter by means of a counter plate 11, for example made of aluminum, the curved edge 11 a of which is fixed by means of a sealed weld 12 to support 5 (see figure 14).

Once the counter plate 11 is welded to the support, the assembly is heated to polymerize the polyimide resin contained in the layers 2, so as to bond these simultaneously to the resistive sheet 1, to the support 5 and to the counter plate 11.

In this embodiment, the support 5 can be the aluminum bottom of a cooking utensil. The counter plate 11 fixed in a sealed manner to this bottom makes it possible to preserve the resistive sheet 1 from any contact with humidity.

In the embodiment illustrated in Figures 15 to 18, the resistive sheet 1 is fixed and electrically insulated from the support 5 by enamel.

In a first step (see FIG. 15), a layer 13 of a conventional enamel slip for aluminum is applied to the support 5, for example of aluminum, and this layer is then dried.

In a second step (see FIG. 16), the cut resistive sheet 1 is placed on the layer 13 of enamel slip.

In a third step (see FIG. 17), a second layer 14 of enamel slip is applied to the resistive sheet 1 which is then dried.

In a final step (see Figure 18), the two layers of enamel 13 and 14 are baked simultaneously. During this baking, the two layers of enamel 13 and 14 bond together through the openings in the sheet. resistive 1 by completely coating it and hanging on the support 5.

As in the case of the previous embodiments, the support 5 can be the bottom of a cooking utensil. The enamel layers 13, 14 effectively protect the resistive sheet 1 against impact, withstand heating at a temperature above 400 ° C and provide excellent thermal insulation.

In the embodiment of Figures 19 to. 21, the connection between the resistive sheet 1 and the support 5 is also obtained by means of enamel.

In a first step (see FIG. 19), the support 5 (for example the stainless steel sheet of a bottom of a cooking utensil) is covered with a layer 15 of enamel slip. Likewise, the surface of an aluminum counterplate 16 is covered by a layer 17 of enamel slip. After drying of the two layers 15 and 17 of enamel slip, in a second step - (see FIG. 20), the support 5 is applied against the counterplate 16 so that the resistive sheet 1 is covered on both sides by the slip layers 15 and 17. A third layer 18 of enamel slip is also applied to the face of the counterplate 16 opposite the support 5.

In a third step (see FIG. 21), the three layers of enamel 15, 17, 18 are fired simultaneously. A heating base is thus obtained comprising a resistive sheet 1 sandwiched between the two metal plates 5, 16 one of which is covered by an enamel layer 18. The two enamel layers 15, 17 provide an excellent connection between the two plates 5 and 16, perfectly insulate the resistive sheet 1 and provide an excellent seal for the latter. In addition, these enamel layers allow the heating base to withstand temperatures exceeding 400 ° C.

In the embodiment of Figures 22, 23, the support 5 is the bottom of stainless steel sheet of a cooking utensil. Against this support 5, is fixed by a brazing layer 19, a first aluminum plate 20. Against the latter, is fixed by means of a second brazing layer 21, a second aluminum plate 22. This second plate 22 is stamped so as to form between it and the first plate 20, a recess 23 adapted to receive the cut resistor 1 a shown in FIG. 3. This resistor 1a is placed between two layers 24, 25 of insulating and refractory cement.

Before hardening of the cement of the layers 24, 25, the stampings formed on the plate 22 are crushed towards the first plate 20 (see right part of FIG. 23), so as to ensure the cohesion of the cement.

In this version, the refractory cement can be replaced by a paste based on powder of refractory and insulating material such as alumina or magnesia. To obtain a connection between the powders, they can be heated so as to make them sinter.

In the embodiment of Figures 24 and 25, the resistive sheet 1 is sandwiched between two layers 26 of ceramic fiber felt. These two layers of felt 26 are impregnated on their faces adjacent to the resistive sheet with a silicone resin 27.

One of these layers of felt 26 is impregnated on its face adjacent to the support 5 with a second layer of silicone resin 28.

By heating the assembly, the layers 27, 28 of silicone resin polymerize by simultaneously ensuring the connection between the two layers of ceramic fiber 26 through the openings of the resistive sheet 1 and the connection of these two layers 26 of ceramic fiber with the support 5.

In the embodiment of FIGS. 26 to 29, the resistive sheet 1 is linked to the support 5 by means of alumina sprayed in the form of plasma.

In a first step (see FIG. 26), a first layer of alumina 29 is projected in the form of plasma.

In a second step (see FIG. 27), the resistive sheet 1 is deposited on the alumina layer 29.

In a third step (see FIG. 28), the resistive sheet 1 is covered with a second layer of alumina 30 which adheres the first layer 29 through the openings in the resistive sheet 1.

In a last step (see FIG. 29), the second layer of alumina 30 is covered with a layer of thermosetting resin 31 which protects it against mechanical shock.

In the embodiment of Figure 30, there is shown a low temperature heating container made by molding polyester resin loaded with glass fibers.

First, a part 32 of the thickness of the bottom of the container is molded, then the resistive sheet 1 is placed on it. In a second step, the rest 33 of the container is molded on the first part 32.

In the embodiment of FIG. 31, an iron soleplate 34 has been shown on which a heating element 35 according to the invention is applied, for example of the kind of that shown in FIGS. 4 or 8.

In this example, the sole 34 is produced by a simple sheet of chromed steel. On the heating element 35, a counter plate 36 is applied which is pressed on the heating element 35, by a crimping 37 carried out along the edge of the soleplate 34. An excellent thermal contact is thus obtained between the heating element 35 and sole 34.

Since the heating element 35 extends over practically the entire inner surface of the sole 34, the whole of the latter is heated in a uniform manner.

Furthermore, given the relative thinness of the sheet forming the sole 34, it is possible to obtain a very rapid rise in temperature. Indeed, due to the extra-flat section of the heating element 35 compared to the diameter of the usual wire, a thermal contact of much better quality is obtained and self-rigidity of the whole of the resistance which has a mechanical resistance. correct.

In addition, such an embodiment of the sole 34 is very simple and economical. It also has the advantage over conventional thick soles of being much lighter than the latter.

Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to them without departing from the scope of the invention.

Thus, the invention can be applied to heating supports made of a material other than metal, such as ceramic, glass or vitro-crystalline materials used for the top of cookers.

The heating element can also be applied to perforated flat supports in the form of a frame, for heating with an air flow. In this case, the support can be made of insulating material or composite and reinforced material (insulating on a steel element for example).

The heating element can also be applied to solid and perforated supports of three-dimensional shape, for example for heating a hollow container through the bottom and the side wall or heating with an air flow, as in the hair dryer case.

In the above description, there have been described embodiments of heating elements intended for heating to temperatures between 400 and 500 ° C. Of course, the invention also applies to heating elements intended for heating to temperatures below the above-mentioned values and for example between 100 and 200 ° C. In this case, the substrate in which the cut resistor is coated can be produced by a layer of paint or varnish supporting a temperature between 100 and 200 ° C. This substrate can also be made of a thermoplastic or thermosetting material itself constituting the wall of the heating article.

Claims (16)

1. flat heating element with electrical resistance, in particular for a household article comprising a heating surface, this heating element comprising a heating resistance cut from a metal sheet (1, 1a), characterized in that this metal sheet is coated in a substrate ( 2, 7, 8, 13, 14, 15, 17, 24, 25, 26, 29, 30, 32, 33) of electrically insulating material, resistant to the intended heating temperature and adhering to the metal foil (1, the ) and which can adhere to the material constituting the support (5) of the heating surface of the article. 2. Heating element according to claim 1 characterized in that the metal sheet (1, 1a) is chosen from the following resistive metals: stainless steel, ferro-nickel alloy and nickel-chromium alloy. 3. Heating element according to one of claims 1 or 2, characterized in that the metal sheet (1) is between two layers (2, 7) of felt of ceramic material impregnated with a polymerizable resin under the action heat, these two layers of felt being bonded together and adhering to the metal sheet. 4. Heating element according to claim 3 characterized in that the resin is chosen from polyimide, phenolic and silicone resins. 5. Heating element according to claim 1, characterized in that the metal sheet (1) is between two layers (29, 30) based on alumina sprayed in the form of plasma. 6. Heating element according to claim 1, characterized in that the metal sheet (1) is between two layers (13, 14; 15, 17) of enamel. 7. Heating element according to claim 1, characterized in that the resistive metal sheet (1) is between two layers (7) of laminate of sheets of paper impregnated with phenolic resin and charged with flakes of mica, these two layers being bonded together and adhering to the metal foil. 8. Heating element according to one of claims 1 to 7, characterized in that one of its faces is covered by a layer (4, 7, 11) of a material resistant to the heating temperature and adhering to the substrate. 9. Heating element according to claim 8, characterized in that said layer is a metal sheet or plate (4, 11). 10. Heating element according to claim 8, characterized in that said layer is a sheet or plate of insulating material (7). 11. Heating article comprising a heating element according to one of claims 1 to 10, characterized in that it comprises a support (5) to which the heating element is fixed. 12. A heating article according to claim 11, characterized in that the support (5) is chosen from the following materials: aluminum, stainless steel, mild steel, ceramic, vitrocrystalline material and glass. 13. Heating article according to one of claims 11 or 12, characterized in that the heating element is fixed to the support (5) by means of a resin layer (6). 14. Heating article according to one of claims 11 or 12, characterized in that the heating element (35) is fixed to the support (34) by means of a plate (36) applied with pressure against this heating element and fixed to said support. 15. Heating article according to one of claims 11 or 12, characterized in that the heating element is integrated into the support (5) at the time of its preparation. 16. A heating article according to claim 15, characterized in that the heating element is integrated into the support (5) by polymerization of the resin contained in the substrate (2).

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