EP1188350A1 - Heating element on an aluminium substrate for household appliance - Google Patents

Abstract

A heating element for a soleplate of an iron, composed of: a substrate constituted by a plate containing aluminum; an insulator covering one surface of the plate; and a heating circuit carried by the insulator, wherein the insulator includes an anodized oxide layer having a thickness between 10 and 200 microns and resistant to abrupt temperature variations of the plate.

Description

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HEATING ELEMENT ON ALUMINUM SUBSTRATE FOR HOUSEHOLD APPLIANCES

The present invention relates to heating elements for irons.

The irons usually comprise an envelope forming a handle generally made of plastic and a hot metallic sole which is applied to the linen to be ironed. The sole of recent irons is composed of a flat metal lower plate heated by an upper molded aluminum body, this body being applied to the internal face of the flat plate and comprising a tubular heating element. For reasons of weight, size, and economy, we sought to remove the molded aluminum body and apply a heating element directly on the inner face of the flat plate of the soleplate of the iron. There is thus known patent EP555159 in which the heating element is a sandwich structure adhering to the sole. However, this structure requires manufacturing times and precautions which lead to insufficiently economical production.

Enamelled steel plates are known on their internal face on which a thin layer circuit or a screen-printed circuit is deposited. However, this process requires the use of glass compositions which are sufficiently insulating to withstand the voltage of the electricity sector. This type of enamel is deposited at high temperature incompatible with an aluminum substrate on the one hand and on the other hand the differential shrinkages after firing of this type of enamel and the aluminum substrate induce tensions at which a flat sole plate of iron cannot resist without deformation.

Patent FR1584094 is known which describes a circuit deposited on an anodized aluminum substrate. But this embodiment where the alumina layer reaches only a few microns is not suitable for a circuit working at the voltage of the electrical sector. Furthermore, use is limited to temperatures lower than those of an iron soleplate. Patent EP120119 describes an aluminum substrate intended to receive a deposited electrical circuit. A thick anodized layer of alumina provides electrical insulation with silicic sealing. But if this substrate can receive an electrical or electronic circuit, there is no indication to suggest that it is adapted to undergo temperatures as high as those undergone by an iron soleplate.

Patent EP058023 also describes a clogging treatment of an anodized aluminum substrate. The alumina layer is sealed with an organometallic product polymerized, for example, by heating. However, the compulsory clogging treatment to obtain good dielectric strength does not constitute redundant security and does not presume resistance to thermal shocks encountered by an iron.

Patent FR2763780 describes a process for manufacturing printed circuits on an aluminum substrate where a thick layer of alumina is formed on the surface to serve as an insulator and can withstand high temperatures without the appearance of defects. Clogging of pores is not mandatory. However, if the circuit can operate when the substrate is brought to high temperature, it is specially adapted to support the implantation of integrated circuits or modules. These components, which the support contributes to cool, have by far the power and instantaneous temperature variations of an iron soleplate.

Document US3805023 describes a heating element having an anodized aluminum substrate. The alumina layer obtained in a weak acid bath is dense in contact with the substrate and constitutes an electrical barrier, while the outside of the layer is porous. In the porosities, a light-sensitive product containing a precious metal has been deposited which, after irradiation through a negative and development, acts as a catalyst for chemical deposition of the conductive track in nickel.

The use for an iron is in no way cited. A priori, the resistive tracks are deposited to replace the catalyst in the pores of the alumina layer. A priori, the external porosities are completely contraindicated for a steam iron soleplate working in a humid environment.

The object of the invention below is a heating element for an iron soleplate iron, this heating element being light, compact, and economical to manufacture.

The object of the invention is achieved by a heating element for an iron soleplate, the substrate of which consisting of a thin plate of pure aluminum or alloyed with silicon or alloyed with magnesium is covered with an insulator on which tracks are deposited. heating, due to the fact that the insulator is an anodic oxidized layer with a thickness between 10 and 200 microns resistant to sudden variations in temperature of the plate.

The use of aluminum allows to benefit from all the advantages of this metal in particular with regard to the temperature distribution on the sole. Aluminum, pure or alloyed with silicon containing practically no magnesium, is also easily enamelled. Advantageously for good heat distribution, the substrate of the heating element covers a large part of the internal surface of the iron soleplate, the dissipated power being able to be high.

Anodizing provides an insulator naturally adherent to the support and whose expansion is compatible with the latter.

Preferably, the oxidized layer is of the type obtained by anodic oxidation carried out in a solution of an acid with low electrical conductivity, with a current density between 1 and 5 A / dm2 at a stabilized temperature.

The use of oxidation in an acid solution with low electrical conductivity similar to that described in patent FR2763780 but applied to a heating element of household appliances, makes it possible to surprisingly obtain good resistance in temperature and in breakdown voltage.

In addition, numerous tests in the workshop have proven the good endurance resistance of this insulation during tests where the iron soleplate is subjected to numerous sudden coolings on a wet felt, tests largely representative of a hot iron soleplate applied to a poodle. In a preferred version, the substrate of the heating element is an enameled iron soleplate prior to anodization on its external face. intended to be in contact with fabrics, this enameling being carried out at a temperature which relaxes the metal.

Anodizing requires a stress relief treatment of the metal. By enameling the soleplate before anodizing, this relaxation treatment is saved. Preferably the enamel is protected from corrosion of the anodizing baths by a peelable protective varnish.

Preferably the pores of the oxidized layer are filled with another metallic oxide obtained by decomposition of a polymerizable organometallic product such as a silane, an organic derivative of titanium or zirconium or germanium or tin.

Although this filling is not essential with all kinds of oxidized layers, it provides additional operating safety.

In another version, the surface of the oxidized layer is impregnated with an organic polymerizable product resistant to temperature such as a polyester or a polyimide.

This other possibility is easier to implement and requires cooking at a lower temperature.

In a preferred version the conductive heating tracks are screen-printed then cooked doughs. In another implementation of a sole heating element, the heating element having an aluminum substrate is brazed on the inner face of the iron sole at a temperature which allows the tracks to be cooked.

This makes it possible to obtain soles whose face in contact with the fabrics is made of stainless steel having a good distribution of the heat on the surface. Brazing is performed when the tracks are freshly deposited which saves cooking of the latter.

In another version, the nickel tracks are deposited on the dense alumina layer by chemical means.

Any known method can be used to deposit an adherent conductive or semi-conductive layer acting as a heating track. We prefer to use known methods which consist in metallizing the entire surface then, using a photosensitive masking resin, making the tracks by attacking the parts of this surface which must not be conductive. The advantage is that this type of process essentially only requires equipment compatible with the manufacture of printed circuits.

The invention will be better understood on reading the examples below.

In a preferred embodiment of the invention, the substrate of the heating element constitutes the soleplate of the iron which is a flat aluminum plate with rounded edges to facilitate sliding on the fabrics. The sole is relaxed by a heat treatment, by annealing at 400 ° C, then degreased, pickled and thoroughly cleaned.

An oxide layer obtained by anodic oxidation in an acid solution with low electrical conductivity is deposited electrolytically on the internal face of the soleplate. When the deposit obtained cannot be used as a bonding layer to a deposit giving good ironing qualities to the external face intended to come into contact with the ironed fabric, this face is masked by usual means in electroplating.

An oxidized layer of sufficient insulation is obtained in solutions of oxalic, citric, lactic or tartaric acid with a current density of 1 to 5 A / dm2 on the part.

In a preferred version, the insulation produced by the oxidized layer is reinforced by an additional redundant insulation for more security.

The additional insulation layer is siliceous in a first embodiment. It can be obtained by thermal decomposition of a silane deposited on the surface of the oxide.

In a second version, the additional insulation is an oxide or a combination of metal oxides such as titanium, zirconium, germanium, tin. It can be obtained by thermal decomposition of known precursors of these metals. In a third version, the additional insulation is a layer of a glass with a low melting point such as those used to make circuits hybrids. This layer can be obtained for example by screen printing of a paste 7802 from the company Metech and baking at 500 ° C. on the oxidized layer.

The electrical circuit includes resistive heating tracks deposited on the oxidized layer. Preferably these tracks are pastes or inks deposited by screen printing and conductive after cooking. One can for example use a polyimide paste loaded with silver reference CM3321 E from the company Protavic which polymerizes in one hour at 150 ° C followed by one hour at 275 ° C.

Advantageously, a polyimide film covers the oxidized face and its heating tracks. This film prevents the whole from being sensitive to humidity. It is obtained, for example, by screen-printed deposition of an IM891 paste from the company Protavic and polymerization on the tracks.

In the simplest version, the outer face of the soleplate comes directly into contact with the fabric. The metal face is only cleaned or polished by mechanical brushing. In a more sophisticated embodiment, the soleplate of the iron is a flat plate with rounded edges so that they are less aggressive, made of aluminum alloyed with silicon where the magnesium level is less than 0.01%. This plate is enamelled on its external face to the iron at a temperature which anneals the aluminum and relaxes it. It is therefore not necessary to carry out a relaxation treatment prior to anodization. The enameling gives good ironing characteristics to the sole. Preferably the enamel is protected at least during anodization by a peelable protective film. The sole has on its internal face insulation and tracks similar to those of the previous example.

When the user goes back with an iron equipped with a soleplate according to the invention, the resistive heating tracks dissipate in the aluminum a significant power which can go up to 2 kW for a soleplate of approximately 2 dm2. Aluminum has the advantage of distributing the heat well over the entire surface. Furthermore, the sole undergoes heating to the set temperature which can be higher than 250 ° C. and can be brought into contact with a damp cloth, even wet. The severe endurance tests show good resistance of the sole according to the invention. In another embodiment, the substrate of the heating element is a flat aluminum plate which is relaxed and which receives an oxidized layer obtained by anodic oxidation in a solution of an acid with low electrical conductivity, as in the previous examples . In the same way, the oxidized layer can receive additional insulation if necessary, then the tracks are deposited and baked if necessary.

The heating element obtained is bonded to the inner face of an iron sole formed by a thin plate of enameled aluminum on its outer face. Advantageously, a silicone adhesive compatible with the temperatures of use of the sole is used. This assembly has the advantage of being compatible with that of existing irons manufacturing comprising a heating body bonded to a sole.

In another embodiment, the substrate of the heating element is a flat aluminum plate which is expanded and which receives an oxidized layer obtained by anodic oxidation in a solution of an acid with low electrical conductivity, as in the previous examples . In the same way, the oxidized layer can receive additional insulation if necessary, before the deposition of the tracks.

The heating element obtained is brazed on the inner face of an iron sole formed by a thin plate of stainless steel. Advantageously, a solder melting at a temperature is used which ensures the cooking of the deposited tracks and / or of the protective deposit, making the operation more economical. This operation is made possible by rigorous control of the atmosphere in which it takes place.

In a different version, the heating element obtained is fixed on the inner face of an iron sole formed by a thin plate of stainless steel, the outer edges of which are rolled to bear on the heating element and maintain it in a known way. Heat transfer between the heating element and the soleplate is provided by a thin layer of heat-resistant conductive adhesive such as silicone adhesive. This gives a sole which has the same advantages as that of the previous example, but with another presentation. In another version, the soleplate of the iron is a flat plate with rounded edges so that they are less aggressive, made of aluminum alloyed with silicon where the magnesium level is less than 0.01%. This plate is enamelled on its external face to the iron at a temperature which anneals the aluminum and relaxes it. It is therefore not necessary to carry out a relaxation treatment prior to anodization. The enameling gives good ironing characteristics to the sole. Preferably the enamel is protected at least during anodization by a peelable protective film. The sole has on its internal face an oxidized insulating layer similar to that of the previous examples. Nickel tracks have been deposited on this insulating layer, preferably not clogged or incompletely clogged. To do this, the surface is sensitized with a solution containing palladium chloride or a commercial precursor distributed for example by the company SHIPLEY. The surface is then soaked in a metallization bath comprising nickel sulphate and reducing agents, and becomes coated with nickel. The surface is then masked with a photosensitive resin, exposed to ultraviolet light through a artwork which reproduces the layout of the desired tracks, and revealed by a sodium bath to remove the exposed parts of the mask. The free surface of nickel is then attacked for example by a solution of iron perchloride which leaves only the tracks. Remnants of masking resin are removed with acetone. These operations, in part compatible with the manufacturing equipment of printed circuits, are carried out according to the rules of the art with the necessary precautions and intermediate rinsing.

The connections are then soldered to the tracks by known means. Compared to the tracks obtained by screen printing, the tracks obtained by chemical deposition of nickel do not require cooking.

The application of this invention is essentially intended for the field of irons, but it is however possible to envisage applying it in a similar manner, with a few obvious adaptations, to the external face of a tank bottom intended for heating water in a kettle or oil in a fryer.

Claims

1. Heating element for an iron soleplate, the substrate of which consists of a thin plate of pure aluminum or alloyed with silicon or alloyed with magnesium is covered with an insulator on which are heated tracks, characterized in that the insulator is an anodic oxidized layer with a thickness between 10 and 200 microns resistant to sudden variations in plate temperature.

2. Heating element according to claim 1 characterized in that the oxidized layer is of the type obtained by anodic oxidation conducted in a solution of an acid with low electrical conductivity, with a current density between 1 and 5 A / dm2 at a stabilized temperature.

3. Heating element according to one of claims 1 to 2 characterized in that the pores of the oxidized layer are filled with another metal oxide obtained by decomposition of a polymerizable organometallic product such as a silane, an organic derivative of titanium or zirconium or germanium or tin.

4. Heating element according to one of claims 1 to 3 characterized in that the surface of the oxidized layer is impregnated with a glass with low melting point.

5. Heating element according to one of the preceding claims, characterized in that it comprises screen-printed heating tracks on the oxidized layer.

6. Heating element according to one of claims 1 to 4 characterized in that it comprises metal heating tracks deposited on the layer oxidized chemically.

7. Heating element according to one of claims 1 to 6 characterized in that it is brazed on the inner face of the iron soleplate at a temperature which allows the cooking of the tracks. Heating element according to one of Claims 1 to 6, characterized in that the substrate of the heating element is an enameled iron soleplate before its anodization on its face intended to be in contact with the fabrics, this enameling being carried out at a temperature that relaxes the metal.