CH717987A1 - Process for treating a surface of a metal support with a view to enameling it. - Google Patents

Abstract

The invention relates to a method for treating a surface of a metal support with a view to enamelling it. This method comprises the following steps: a) forming on said surface an enamel coating called sacrificial enamel (E3); and b) dissolving said sacrificial enamel coating (E4). The invention also relates to a method of enameling a surface of a metal support. This method comprises the treatment of said surface by the treatment method according to the invention then the formation of a decorative enamel coating (E5) on said surface. Finally, the invention relates to an enamelled support capable of being obtained by the enamelling process according to the invention.

Description

The present invention relates to a method of treating a surface of a metal support for its enameling and a method of enameling a surface of a metal support. The support according to the invention is typically a watch or jewelry element such as a dial or a bracelet.

[0002] Traditionally in watchmaking, when it is desired to enamel a metallic element such as a dial, the latter is first degreased, then cleaned, for example by carrying out sandblasting and/or pickling with sulfuric acid.

[0003] Next, to form an enamel coating on the surface of a dial thus prepared, enamel, initially in the form of a powder, is mixed with water to form a paste. This paste is spread on the dial then put in the oven, at a temperature typically between 700°C and 1000°C in order to solidify. The operations of deposition of the paste and of solidification by baking are typically repeated a certain number of times until a multilayer coating having the desired termination (thickness, color, etc.) is obtained.

[0004] A technical difficulty during the implementation of such a process lies in the formation of air bubbles during enameling, these becoming trapped in the enamel. They are unsightly and can become a technical constraint, for example in the case of subsequent polishing of the enamel. Indeed, by polishing the enamel there is a risk of "burst" these bubbles and thus form "craters", it is then necessary to fill them in by preparing them, that is to say on the one hand by filing them to be able to fill them and on the other hand by filling them with enamel (formation of a new enamel paste, filling and firing).

The present invention aims to provide a method for obtaining a metal support at least part of which is covered with enamel, said enamel comprising as few bubbles as possible, in particular as few bubbles as possible visible to the naked eye.

The invention proposes to this end a method for treating a surface of a metal support with a view to its enameling, characterized in that it comprises the following steps: a) forming on said surface a coating of enamel called sacrificial enamel; and b) dissolving said sacrificial enamel coating.

Preferably, step b) comprises at least one cycle for dissolving said sacrificial enamel comprising the following steps: i) subjecting said sacrificial enamel coating to a fluorinated acid; then ii) rinsing it so as to neutralize the acid and to eliminate the metal salts formed.

[0008] Preferably, step i) of the sacrificial enamel dissolution cycle comprises soaking the support covered with sacrificial enamel in a bath of hydrofluoric acid, preferably in a bath of hydrofluoric acid of a concentration comprised between 1 and 70%, preferably 50%. It may in particular be buffered hydrofluoric acid or BHF (Buffered Hydrofluoric Acid). Advantageously, step ii) is carried out with mechanical stirring to promote the detachment of the metal oxide residues formed during step i). It typically includes the use of ultrasound.

In a preferred embodiment of the invention, step ii) breaks down into: a first rinsing step to neutralize said fluorinated acid, preferably for at least ten minutes; a second rinsing step including ultrasound, typically in an ultrasound bath, to remove the metal salt residues, preferably for at least three minutes; and a third rinsing step.

In a preferred embodiment of the invention, step b) comprises at least two successive dissolution cycles, preferably at least three.

The rinsing steps carried out within the scope of the invention are preferably carried out with deionized water.

[0012] The enamel formed in step a) of the process according to the invention is qualified as "sacrificial" because it is intended to be completely dissolved in step b). It is typically a standard enamel, such as those used in watchmaking. Advantageously, said sacrificial enamel is a flux, that is to say an enamel free of metal oxides. As a variant, it can be colored, for example black.

As indicated above, the support of the method according to the invention is made of metal. Insofar as it is at least partly enamelled in step a) then subjected to a treatment aimed at dissolving the enamel formed in step a) during step b), it must be heat resistant and chemically. In fact, it must withstand enamel firing temperatures of at least 700° C. as well as enamel dissolution treatments.

In the context of the present invention, the support is typically made of a metal chosen from noble metals and their alloys. Preferably, said metal is chosen from platinoids, gold and molybdenum as well as their alloys.

The invention also relates to a process for enameling a surface of a metal support comprising the treatment of said surface by the treatment process according to the invention and then the formation of a decorative enamel coating on said surface.

[0016] The expression “decorative enamel” is used here to define an enamel not intended to be dissolved, as opposed to a “sacrificial enamel”.

[0017] Typically, in the context of the present invention, the metal support is a timepiece or piece of jewelery such as a watch face or a bracelet.

Finally, the invention relates to an enameled support obtainable by the process according to the invention.

Other characteristics and advantages of the present invention will appear on reading the following detailed description given with reference to the appended drawings in which: FIG. 1 is a diagram showing the different steps of an enameling method according to a particular embodiment of the invention; FIG. 2 schematically illustrates a dial covered with a layer of sacrificial enamel obtained after the implementation of the first three steps of the method described in FIG. 1; FIGS. 3a to 3c schematically illustrate successive stages of a dissolution cycle making it possible to dissolve the layer of sacrificial enamel illustrated in FIG. 2 and formed during the process illustrated in FIG. 1.

Referring to Figure 1, during a first step E1 of an enamelling process according to a particular embodiment of the invention, a metal support is provided comprising at least one surface to enamel. In the example illustrated in FIGS. 2 to 3c, the support is a timepiece dial 1 and this surface corresponds to the upper face of dial 1, intended to be exposed to the view of the user.

The dial 1 consists of a gold alloy - palladium (Au-Pd).

In a second optional step E2, at least the surface to be enamelled of the dial 1 is cleaned. This step comprises degreasing, for example burning, by treatment with hydrocarbons or by alkaline degreasing followed by several rinses then stripping with sulfuric acid. . During this step E2, sandblasting can also be carried out, typically with tartar, in order to homogenize the surface of the dial.

In a third step E3, an enamel coating 2 called “sacrificial enamel” is formed on the surface to be enamelled. The enamel used is a standard enamel, of the same type as the enamels traditionally used in watchmaking. It initially comes in the form of a powder which when mixed with water becomes a paste. After having spread this paste on the surface of the dial, the latter is put in the oven A passage in the oven, at a temperature typically between 700°C and 1000°C, preferably between 800°C and 900°C, preferably between 800°C °C and 835°C, allows the enamel to solidify.

In the example shown, the sacrificial enamel 2 is a flux.

[0025] The operations of depositing the paste and of solidification by firing can be repeated a certain number of times until a multilayer coating having the desired termination is obtained. Typically, multiple firings are performed. The sacrificial enamel is made in such a way as to cover the entire surface to be enameled and thus avoids its oxidation.

Figure 2 schematically illustrates the dial 10 corresponding to the metal dial 1 covered with a layer of sacrificial enamel 2 obtained at the end of step E3.

In a fourth step E4, the sacrificial enamel 2 of dial 10 is dissolved. This is typically done in several sub-steps, as described below.

A first sub-step of step E4 consists in immersing the dial 10 obtained at the end of step E3 in a 50% hydrofluoric acid bath for approximately 1 to 2 hours to dissolve the enamel. sacrificial 2. This step is preferably carried out on several dials 10 simultaneously, the dials 10 being typically arranged in an openwork basket 3 positioned in a tank 4 containing the acid, as illustrated in FIG. 3a.

A second sub-step of step E4 consists in neutralizing the hydrofluoric acid. In the example illustrated, the basket 3 containing the dials is immersed in a bath of deionized water in cascade. This step, illustrated in FIG. 3b, is continued until neutralization of the hydrofluoric acid.

In a third sub-step of step E4 the basket 3 containing the dials is immersed in an ultrasonic bath (illustrated schematically in Figure 3c). The dials are then rinsed with deionized water. This sub-step makes it possible to detach the metal oxide residues formed during step E3. Alternatively, ultrasound could be replaced by another means of mechanical agitation. However, the use of ultrasound has the notable advantage of being contactless.

[0031] All of the sub-steps of step E4 described above constitute a cycle of dissolution of the sacrificial enamel coating 2. If necessary, the person skilled in the art will successively carry out this cycle of dissolution several times, the the objective being to completely dissolve the layer of sacrificial enamel 2 produced in step E3.

At the end of step E4, a metal dial is obtained, the surface of which to be enamelled has been treated within the meaning of the present invention.

In a fifth step E5, a decorative enamel coating is formed on the surface thus treated. This is a conventional enameling step such as step E3. However, it will generally be preferred to use a colored enamel for this step, at least for the first layers of the coating. We can then make a fondant on top to bring protection and shine to the decorated dial.

[0034] Surprisingly, the so-called decorative enamel of the enamelled universal joint obtained at the end of this step E5 has fewer bubbles, or in any case fewer bubbles visible to the naked eye, than an enamel produced on a simply cleaned dial (via a step such as step E2 for example). Advantageously, the process according to the invention makes it possible to obtain a decorative enamel comprising no bubbles visible to the naked eye. In the context of the present invention, it is considered that a bubble is not visible to the naked eye if its diameter is less than 0.1 mm.

[0035] Although the mechanisms of action are not clearly identified, it is clear that steps E3 and E4 made it possible to physically and/or chemically modify the surface to be enameled of dial 1. These steps constitute a surface treatment with sense of the present invention. They include, among other things, a change in the roughness of the metal surface.

As indicated above, reducing the amount of bubbles in the enamel has an obvious aesthetic advantage and is also particularly advantageous in the case where the decorative enamel is intended to be polished.

The foregoing description sets out to describe a particular embodiment of the invention by way of non-limiting illustration, and the invention is not limited to the implementation of certain particular characteristics which come from be described, such as the fact that the support is a watch dial. It may in fact be any other metal support, one surface of which is intended to be enamelled.

If the support used in the illustrated example is made of an Au-Pd alloy, that if could alternatively be made of another metallic material, preferably of a metal chosen from the noble metals or of an alloy of such metals. These materials have the advantage of resisting acids, of withstanding the firing of the enamel without any change in appearance and of not oxidizing.

Finally, other acids capable of dissolving enamel, fluorinated acids, could be used instead of hydrofluoric acid during step E4.

Claims (12)

1. Method for treating a surface of a metal support for its enameling, characterized in that it comprises the following steps: a) forming on said surface an enamel coating called sacrificial enamel; and b) dissolving said sacrificial enamel coating. 2. Method according to claim 1, characterized in that step b) comprises at least one dissolution cycle of said sacrificial enamel coating comprising the following steps: i) subjecting said sacrificial enamel coating to a fluorinated acid; then ii) rinsing it so as to neutralize the acid and eliminate the metal salts formed. 3. Method according to claim 2, characterized in that step i) of said dissolution cycle comprises soaking the support in a bath of hydrofluoric acid. 4. Method according to claim 2 or 3, characterized in that step ii) rinsing is done with mechanical stirring. 5. Method according to claim 4, characterized in that the mechanical agitation comprises the use of ultrasound. 6. Method according to one of claims 2 to 5, characterized in that step ii) comprises: – a first stage of rinsing with water, preferably for at least ten minutes; – a second rinsing step in an ultrasonic bath, preferably for at least three minutes; and optionally – a third stage of rinsing with water. 7. Method according to one of claims 2 to 6, characterized in that step b) comprises at least three successive dissolution cycles. 8. Method according to one of the preceding claims, characterized in that said support consists of a metal chosen from noble metals and their alloys. 9. Method according to one of the preceding claims, characterized in that said sacrificial enamel is a flux. 10. A method of enameling a surface of a metal support comprising treating said surface by the method according to one of claims 1 to 9 and then forming a decorative enamel coating on said surface. 11. Method according to one of the preceding claims, characterized in that said support is a timepiece or piece of jewelery such as a dial or a bracelet. 12. Enamelled support obtainable by the method according to claim 10 or 11.