EP3336221A1 - Method for producing a grid for an electric device - Google Patents

Abstract

The invention relates to a method for producing a lattice intended for mounting on an electrical appliance, from a metal sheet stamped in lattice form, which is etched in a bath with an acidic solution for rounding of punched edges, and then coated in a black chrome bath with a black chrome layer.

Description

The present invention relates to a method for producing a grid intended for mounting on an electrical appliance.

The presently in question grid is preferably used as Abschirmgitter in the door of a microwave oven. On the one hand, it serves as protection against the microwaves, thus keeping the environment shielded despite the electromagnetic field in the cooking chamber of the microwave oven. On the other hand, due to the lattice-shaped design nevertheless a view into the cooking chamber possible, which may be desired by the user.

The present invention is based on the technical problem of specifying an advantageous method for producing a grid for an electrical appliance.

• in einem Bad mit einer sauren Lösung zur Verrundung von Stanzkanten angeätzt wird, und anschließend
• in einem Schwarzchrombad mit einer Schwarzchromschicht überzogen wird.

According to the invention this object is achieved by a punched in lattice metal sheet

• etched in a bath with an acidic solution for rounding of punched edges, and then
• coated in a black chrome bath with a black chrome layer.

Preferred embodiments can be found in the dependent claims and in the entire disclosure, wherein in the representation of the features is not always distinguished in detail between device and process or use aspects; In any case, implicitly, the disclosure must be read with regard to all categories of claims.

On the one hand, this combination of features makes it possible to produce a cost-effective product, but on the other hand it is also functional, robust and visually appealing is. The use of a stamped metal sheet may, for example, provide advantages over a wire grid in terms of stability and also freedom of design (hole size), especially since stamping is a cost-effective process in volume. However, the inventor has found that the punched edges embedded in the metal sheet by stamping can cause local elevations of the electric field, ie field peaks, for example also in the case of a subsequent galvanic coating. There may be problems not only in terms of the shielding function per se.

Initial attempts of the inventor went to round the punching edges by applying a correspondingly thicker black chrome layer or a thicker layer composite (taking into account one or more layers between the black chrome and the metal sheet, see below) at least in fact something, but what, for example. The process duration and also the layer stability may be disadvantageous. According to the invention, the metal sheet is therefore etched to round the punched edges in the acid solution, cf. the following disclosure regarding further process details. The inventor has found that although the coating generally has a leveling effect, esp. But the black chrome can detect any unevenness or defects much more clearly, for example. Compared to glossy chrome. In general, the black chromium layer is preferably the last layer applied to the metal sheet (ie it is not coated in turn).

In a respective "punching edge", an edge along which a respective punching slug has been cut is abutted against a surface of the metal sheet. "Punch" means a punched-out piece of rubbish. The "surfaces" of the metal sheet are opposite to each other with respect to a thickness direction in which the sheet thickness is taken. In the directions perpendicular to the thickness direction, the metal sheet has its surface extension. The acid solution acts directly on the metal of the stamped metal sheet, so this is still uncoated at this time. In general, the etching in the acidic solution preferably takes place without current, which can also offer advantages in terms of the expense.

After etching, the metal sheet is preferably rinsed, preferably with water.

As far as in the present case of a "coating" of the metal sheet with a layer is mentioned, this generally does not imply a cover closed all over, but is, for example, also a masking and only partially coating conceivable. Preferably, the entire metal sheet is coated. Irrespective of this, the layer with which the metal sheet is "coated" must not border directly on the metal sheet, but the metal sheet may also have previously been coated with a further layer. The black chromium layer is preferably applied to a previously deposited on the metal sheet layer, for example. On a nickel layer, see below. Insofar as reference is made to the "metal sheet", this is to be read without any indication to the latticed punched part (after punching). The metal sheet may, for example, have a thickness of at least 0.5 mm, further and particularly preferably at least 1 mm or 2 mm, with possible upper limits (independent of this), for example at not more than 10 mm, 8 mm, 6 mm or 4 mm (increasingly preferred in the order of entry).

As already mentioned, the grid in a preferred embodiment is a screening grid, preferably for a microwave oven. The rounded edges may also be advantageous in terms of this shielding function. The openings punched out for lattice formation can have, for example, in each case a diameter of at most 10 mm, 8 mm, 6 mm, 4 mm, or 2 mm, possible lower limits (independently thereof), for example at least 0.2 mm, 0, 5 mm or 1 mm can lie. "Diameter" in this case generally does not have to imply a circular shape, but is to be read as the mean value of the smallest and largest extent, which corresponds to the circle diameter in the case of the preferred circular geometry.

In a preferred embodiment, the Abschirmgitter is provided for mounting in the door of the microwave oven. The screening grid can be enclosed in the assembled door between two panes. The discs are preferably each transparent (transparent), as material can be provided plastic or glass. In general, this can be a particular advantage of black chrome plating in that it allows a largely glare-free view of the oven. There are, for example, hardly any disturbing reflections of light, the grid appears visually in the background and is not perceived as disturbing by a user. Insofar as, for example, a black-painted lattice is conceivable as an alternative to the present subject matter, in detail the lacquer may be more susceptible to damage than the black chrome.

In a preferred embodiment, the black chromium layer is electrodeposited. The rounding of the edges beforehand may in this case be advantageous in particular because the electric field applied during the deposition becomes more uniform. In this way, it is possible to prevent field peaks which could lead to locally uneven deposition (even if the metal sheet has already been coated with a layer during black chrome plating).

In a preferred embodiment, the metal sheet is made of a possibly low-alloy steel, preferably made of a non-alloy steel. On the one hand, this can offer the material costs in terms of advantages; on the other hand, such a steel can also be easier to punch and etch, because, for example, the punching tool becomes less worn. It can also reduce the etching time and thus the throughput can be increased. However, since the steel sheet is then coated with the black chrome, it is well protected in the application and the use of a lower quality steel does not give a disadvantage.

In the case of the "possibly low-alloyed" steel, the alloying elements may in total account, for example, for a content of at most 5% by mass, with at most 3% by mass further and at most 1% by mass being particularly preferred. The steel can also be completely free of alloying additives. In general, conceivable alloying additives may, for example, be aluminum, niobium, vanadium, titanium and / or chromium. In general, an at least chromium-free steel is preferred. Also, independently of this, a limited carbon content of Being of interest, for example of not more than 0.8%, 0.4% or 0.1% (increasingly preferred in the order in which they are mentioned); Within the scope of the technically feasible, a carbon-free steel may also be preferred (for technical reasons, lower limits may, for example, be at least 0.001% or 0.01%).

In a preferred embodiment, the acidic solution comprises sulfuric acid.

In a preferred embodiment, the acidic solution comprises hydrogen peroxide, for example in an amount of at least 50 ml / l (milliliter / liter), 75 ml / l, 100 ml / l, 125 ml / l or 150 ml / l with ( independent of these) upper limits of, for example, at most 300 ml / l, 275 ml / l, 250 ml / l, 225 ml / l or 200 ml / l (in each case in the order of naming increasingly preferred).

In a preferred embodiment, the acidic solution comprises ammonium hydrogen fluoride. Its volume percentage may, for example, be at least 3%, 4% or 5%, with possible upper limits (independent of the lower limits), for example, not exceeding 30%, 25% and 20% (respectively in the order of naming increasingly preferred).

Preferably, a solution of hydrogen peroxide and ammonium hydrogen fluoride (see the preceding paragraphs with respect to the preferred proportions) is added so much sulfuric acid that the pH then at least 2.5, preferably at least 3, and (independently) at most 4.5, preferably at most 4, is located. Preferably, the acidic solution of hydrogen peroxide, ammonium hydrogen fluoride and sulfuric acid to said or appropriately adjusted parts and ultrapure water.

In a preferred embodiment, subsequent to the black chrome plating in alkaline solution is cleaned with ultrasound treatment. This makes it possible to reduce the chromium VI load, which may be of special interest, especially with regard to the microwave oven and therefore the immediate proximity to the food preparation. But also with others at least in general conceivable Areas of application, for example in the case of a cover grille for a loudspeaker, may be due to the possible contact with the human skin, the chromium-VI freedom meaning.

The inventor has observed in comparative experiments that the black chrome layer may have a cloudy appearance in the case of previously unrounded punching edges after the ultrasonic cleaning, the ultrasonic treatment seems to indicate any irregularities in particular. Conversely, a particularly advantageous interaction of the etching before coating with the ultrasonic cleaning after coating to the effect that a particularly uniform in appearance even black chromium layer results.

Independently of this, after black chromium plating, this purification step is preferably carried out in an alkaline solution in a pH range between 9 and 13, with pH values below 12 or below 11 being preferred, for example at pH 10. The solution may optionally also contain surfactants , These improve the cleaning properties, but do not make the ultrasonic treatment unnecessary. Preferably, the ultrasonic treatment is carried out in a temperature range between 50 and 60 ° C. The ultrasound treatment itself preferably lasts at least 30 s. The ultrasound power can range between 0.5 and 2 W / I.

Another aspect of the invention relates to possibilities of adjusting the gloss or mattness of the black-chromed surface. Here are to be created by fotomatten to high-gloss layers of play. To set certain degrees of matting here is provided to produce the initial metallization of the surface by applying a matte nickel layer on a smooth surface of the metal sheet by electrodeposition without organic matting additives and further applying a Sulfamatnickelschicht.

The basic idea of this aspect of the invention is to apply a matte nickel layer on a smooth sheet metal surface and adjust the mattness over the thickness of the nickel layers. This is aimed at galvanic nickel layers, where no organic matting additives are used. Rather, in a preferred embodiment, it may be a known Wattsche nickel layer, which is technically simple and easy to control.

On the matte nickel layer then another sulfamate nickel layer should be applied. This has the advantage and the function of slightly rounding off the more or less grain-like structure of the matt nickel layer and thus making it less rough and susceptible to soiling. Instead of the sulfamate nickel layer, for example, a semi-gloss nickel layer can also be applied to the matt nickel layer, ie a Wattsche nickel layer from a bath with a brightener additive, preferably saccharin, and, for example, also with a wetting agent.

Overall, the interaction of the depending on the strength of the matte nickel layer and the rounding explained by the Sulfamatnickelschicht an optically attractive and especially well reproducible silk gloss. This is in no way inferior in its optical quality to the known nickel layers with organic matting additives. In addition, in this method, the degree of gloss or degree of matting can be easily and easily controlled by various galvanic parameters, in particular by treatment time and / or current intensity. The need to filter the solutions for filtering organic matting additives is also eliminated. However, the use of conventional nickel layers with a degree of mattness which can be adjusted by means of organic matting additives is likewise possible within the scope of the present invention.

The matt nickel plating is preferably applied as a known and technically well-controlled Wattsche nickel layer, ie as galvanic Nickel layer without organic matting additives. This results in a microscopically bulbous layered structure in which the tuber sizes and tuber intervals can be set via the current intensity and / or coating time, which ultimately determine dullness. A maximum dullness arises when the tubers are practically close to each other.

The electrodeposition of a sulfamate nickel layer is also conventional and known. The corresponding solutions contain nickel sulfamate, that is, the salt of amidosulfuric acid. The sulfamate nickel coating rounds and reinforces the aforementioned nodular or otherwise matt nickel layer, but does not really level it. In particular, the sulfamate nickel layer also enhances the grain size without changing fundamentals at the granularity referred to above as "nodular". It thus receives the matte character, possibly only slightly increases the gloss, but above all provides increased material strength for reasons of stability and resilience and for better wiping sensitivity or better dirt-repellent properties. The roughness reduced by the rounding provides less grip to soiling.

A particular advantage of this embodiment is that the degree of gloss or degree of matting can be adjusted by galvanic parameters in a very simple manner and also has the desired effect after black chromium plating. Different optical properties can be generated with one and the same basic process, that is, the same solution compositions, identical baths, etc. In particular, can be adjusted from batch to batch simply on the current, or even cheaper over the treatment time, the dullness. The thicker the matt nickel layer is, the higher the degree of matting results. This also applies after the application of the following black chrome layer.

In a preferred embodiment, a cathodic activation of the previously applied nickel layer is additionally provided before the black chrome plating (preferably, therefore, the sulfamate nickel layer). The cathodic activation can improve the quality Improve the black chrome plating and lead to less cracking, which also improves the cleaning options with ultrasound. The cathodic activation can be carried out in alkaline or acidic solution, with a pH between 11 and 13 or 1 and 2. Preferably, a combination, because the alkaline cathodic activation gives good results, a subsequent acid cathodic activation occasional difficulties with the carryover of residues of the alkaline solution into the acidic black plating bath helps avoid.

In a preferred embodiment, the metal sheet stamped in lattice form has a surface area of at least 20 cm ² , wherein further lower limits are at least 50 cm ² , 100 cm ² , 200 cm ² , 300 cm ² , 400 cm ² , 500 cm ² , 600 cm ² , 700 cm ² , 800 cm ² , 900 cm ² and 1000 cm ² respectively (in increasing order of preference). Upper limits may be (independently of), for example, at most 5000 cm ² , 4000 cm ² , 3000 cm ² and 2000 cm ² . The area is thereby taken from that area which is bordered by the outer periphery of the metal sheet, so it is not only based on the remaining of the punching surface of the metal sheet, but also flow the punched holes.

The invention also relates to a shielding grid provided for mounting in the door of a microwave oven, which is made in a presently disclosed manner from a stamped sheet metal whose punching edges are rounded by etching and which is coated with a black chrome layer.

Furthermore, the invention also relates to a microwave oven whose door has a corresponding Abschirmgitter.

In the following, the invention will be explained in more detail with reference to an exemplary embodiment, wherein the individual features in the context of the independent claims in other respects may be hindering the invention and continue to distinguish not in detail between the different categories of claims.

First, the lattice shape is punched from a sheet steel with a thickness of about 1 mm. The steel is chrome-free and has at most a low carbon content. To round the punched edges, the stamped sheet metal is then introduced into a bath with an acidic solution and etched. The acidic solution is obtained from 100 ml / l to 200 ml / l hydrogen peroxide with 5% to 20% ammonium hydrogen fluoride, the pH is adjusted to 3 to 4 with sulfuric acid and then filled with ultrapure water. The rounding of the punched edges is advantageous, for example with regard to the subsequent galvanic coating, because non-rounded edges cause electric field spraying and thus may result in uneven deposition.

After etching, the metal sheet is rinsed with water, then it is fed to the coating process. In this case, first a Wattsche nickel layer and then a sulfamate nickel layer are deposited, which forms the basis for a satin finish. Subsequently, in an aqueous solution of 60 g / l sodium hydrogen sulfate at a pH of about 1.8 is cathodically activated (at 3 A / dm ² for 30 s). The solution contains relatively small amounts of surfactants and fluorides to support the cleaning and activation function.

Subsequently, a black chrome layer is electrodeposited with a known bath. For example, the company Schlötter provides appropriate formulations and the necessary chemicals. In, for example, 450 g / l chromic acid, about 1 g / l potassium nitrate and (preferably to saturation) potassium hexafluorosilicate and barium carbonate may be added. The latter in an amount sufficient to precipitate the sulphate impurities present in the chromic acid (as sparingly soluble barium sulphate). The deposition time is typically in the range of a few minutes.

Chromium VI oxides and polychromates are initially present on the black chromium plating layer. These impurities can be removed very successfully be through a first simple water bath and then an ultrasound assisted cleaning in an alkaline solution at pH 10 and about 50 - 60 ° C for preferably at least 1 - 2 min. According to experience, treatments beyond 5 minutes do not bring any significant improvements. After the ultrasonic treatment in the alkaline solution again a water bath is run through.

The grid is then installed as a screen in the door of a microwave oven, where it allows on the one hand due to the lattice shape a view into the oven. The black chrome plating can help to avoid disturbing light reflections. On the other hand, the grid shields the cooking chamber with respect to the microwaves to the outside.

Claims (15)

Method for producing a grid intended for mounting on an electrical appliance,
of a grid-shaped sheet metal, the - Etched in a bath with an acid solution for rounding of punched edges, and then - coated in a Schwarzchrombad with a black chrome layer. The method of claim 1, wherein the grid is a shielding grid for a microwave oven, preferably for a microwave oven. A method according to claim 2, wherein the screening grid is intended to be mounted in the door of a microwave oven. A method according to any preceding claim, wherein the black chromium layer is electrodeposited in the black chromium bath. Method according to one of the preceding claims, in which the metal sheet is made of a possibly low-alloyed steel. A method according to any one of the preceding claims, wherein the acidic solution comprises H ₂ SO ₄ . A method according to any one of the preceding claims, wherein the acidic solution comprises H ₂ O ₂ . A method according to any one of the preceding claims, wherein the acidic solution comprises fluoride ions. Method according to one of the preceding claims, in which the black chromium layer applied in the black chromium bath is cleaned in alkaline solution with ultrasound treatment, in particular at a pH of at least 9 and at most 13, in particular at a temperature of at least 50 ° C and at most 60 ° C and in particular at an ultrasonic power of at least 0.5 W / I and at most 2 W / I. Method according to one of the preceding claims, in which the metal sheet is coated after etching and before being introduced into the black chromium bath with a nickel layer, in particular a galvanic nickel layer. The method of claim 10, wherein the metal sheet after etching and before being introduced into the black chromium bath is coated with at least two nickel layers, preferably first a wadding nickel layer and then a sulfamate nickel layer. The method of claim 10 or 11, wherein the nickel layer is cathodically activated prior to introduction into the black chromium bath. Method according to one of the preceding claims, in which the metal sheet has a surface area of at least 20 cm ² taken within its outer circumference as a continuous surface. Shielding grid for mounting in the door of a microwave oven, produced in a method according to claim 3, optionally also in conjunction with a further one of claims 4 to 13. A microwave oven with a door having a screen grid according to claim 14.