MOLD FOR PRODUCING WATCH PARTS

This invention relates to a process for producing parts and in particular to a process for producing watch parts and a mold for producing watch parts and a process for manufacturing a mold according to the preamble of the corresponding independent claims.

EP 0 468 467 discloses a process for producing a precision iron or iron alloy part of a sintered body formed by powder molding, said process comprising the steps of forming a homogeneous mixture consisting essentially of an iron or iron alloy powder, and an organic binder, forming the mixture into a molded body having a prescribed form, removing the organic binder from the molded body in an inert atmosphere, reducing said iron or iron alloy powder contained in the molded body to remove oxygen therefrom and form a reduced molded body and sintering the reduced molded body. In the described process the molding is done by injection molding and only iron parts are manufactured.

EP 1 172 453 discloses a method for making a finally shaped forming tool with a structured forming surfaces having a uniform surface structure, said method comprising the following steps: a) making a glass or plastic mold having a molding surface for forming the structured forming surface of the forming tool; b) coating the molding surface of the glass or plastic mold with at least one material so as to form a cladding body in the glass or plastic mold; c) releasing the cladding body with the structured forming surface from the glass or plastic mold; and d) bonding the cladding body to a base body to form the finally shaped forming tool with the structured forming surface. The coating of the mold is done e.g. by thermal spraying. Materials used for the cladding body are e.g. high melting metallic and ceramic materials whereas alloys of nickel/chromium, cobalt-based alloys and carbides are included in these materials.

Several methods are known to produce polymeric films, as for example in US 2008/10176034, where multiple polymeric layers are produced in a film that thereby replicates a mold surface with microstructures. Each layer is made of epoxy powder sprayed onto the mold and melted and cured by an infra red or UV source.

US 5 301 415 discloses a method for forming an object by applying segments of complementary material and deposition material so as to form layers of material. The layers of material form a block containing the object made of deposition material, which is surrounded by complementary material which serves as a support structure during forming. Then, the complementary material is removed. The complementary material is removed by heating the block whereby the complementary material melts and the deposition material does not melt under the heating conditions. The deposition material and complementary material are preferably applied by thermal spray deposition but may be applied by weld deposition, liquid slurry, gravity drop or any manual means such as a hand sifter.

From the prior art several methods and processes for the build-up of a mold consisting of a particular material are known. Furthermore, processes and methods are known for coating the surface of a part. These coating processes are applied at certain conditions of temperature, pressure, and so on. The process conditions depend e.g. on the used coating material.

The post processing and mechanical finishing of parts which are small and have to comprise a precise structure can be difficult. The difficulties increase in case the part, respectively the surface of the part, has to fulfil high requirements regarding hardness, abrasion and wear resistance, and tribologic properties like in the watch technology. Therefore, there is a need for a process to manufacture watch parts made of a variety of materials, and for a corresponding mold and a process for manufacturing the same.

The object of the invention is to provide a process for producing watch parts and a mold for producing such watch parts and a process for manufacturing a mold which allows manufacturing watch parts made of one or more of a variety of materials.

These objects are achieved by a process for producing watch parts and a mold for producing watch parts and a process for manufacturing a mold according to the corresponding independent claims.

The process for producing a watch part comprises in a first step a provision of a mold with a precise mold shape at an inner mold surface, wherein the precise mold shape corresponds to the negative contours of the watch part to be produced and the mold being made of a mold material. In a second step the mold is filled by a spraying method, wherein the used material corresponds to a watch part material. Furthermore the mold is separated from the watch part in a third step.

As a result, small precisely structured parts are manufactured, which contours are defined by the precise mold shape of the inner mold surface. The precision of the process is related to the fact that the precise mold shape is exactly reproduced by the material filled into the mold. Therefore, a moderately strong adhesion between the mold material and the material of the watch part is desired. Accordingly the outside of the watch part is in close contact to the inner mold surface. The precise mold shape is to be preserved during the filling of the mold by the spraying method. Thus, one characteristic of the mold material is on the one hand a thermal resistance against conditions during the spraying of the part material. On the other hand the mold material has to be separable form the watch part without damaging the precise structure of the watch part.

The process is not limited to the production of watch parts, but may also be applied for the production of small parts for other uses. The descriptions of physical elements and the processes given in the present patent application in relation to watch parts are therefore applicable to other types of parts as well. Typically, such parts are of small size and are precisely structured. For example, they exhibit a combination of properties which are achieved by building up the small and precisely structured part from the outside toward the inside. The maximal dimensions of the parts are in the range of 1mm to 5mm. For example, the maximal dimensions of the parts are a thickness of 0.2mm to 1 mm or to 2mm and a diameter of 2mm to 10mm or to 20mm.

In an embodiment, at least one of a spraying method and the part material is modified within a spraying step such that the watch part comprises at least a first portion and a second portion having different material properties. This allows for an abrupt or a gradual change of the material properties of the watch part. In a related embodiment, the first portion and the second portion have the same material properties.

In an embodiment, the first portion is a first spraying layer which is coated at least onto the inner mold surface defining the precise shape of the watch part and the second portion is a second spraying layer which is filled into the mold. Thereby, the entire inner mold surface is coated by the first spraying layer. The filling of the mold takes place until a filling level, and the filling level corresponds to at least the dimension of the watch part. This allows to build up a watch part with defined properties of the surface of the watch part, wherein these properties are determined by the first spraying layer and fulfil requirements imposed by the application of the produced part. The spraying method and/or part material can be further modified in the course of spraying the first and/or second spraying layer as well. This leads to corresponding sub-layers with different material properties.

In an alternative embodiment, the first portion is a first spraying layer which is coated onto a portion of the inner mold surface and the second portion is a second spraying layer which is filled into the mold. Thereby, a portion of the second spraying layer is in contact with a portion of the inner mold surface. This allows to determine the material properties of the surface of the part to be different at different locations on the surface of the part. This is advantageous if for example one portion of the part should be wear resistant and a second portion of the part should exhibit certain friction characteristics, e.g. be slidable, according to the requirements imposed by the application of the produced part.

In an embodiment, an additional reworking step is performed on the watch part before the mold is separated from the watch part. This implies that an adhesion force and/or a form fit between the part and the mold is strong enough to sustain the stress present in the course of the reworking. The reworking step accomplishes for example a grinding or a milling. The grinded part material originates from a filling or coating of the mold above a filling level. This overfilling allows to make sure that a desired minimum dimension of the watch part is obtained. Furthermore, small irregularities at the surface can be eliminated. The reworking especially of small and fragile part is simplified by holding the part in the supporting mold.

The mold has to sustain a high thermal loading in the course of the spraying of the part material into the mold. Under these conditions especially the precise mold shape of the inner mold surface should not change its shape significantly. Therefore, requirements to be satisfied by the mold material are e.g. a good formability (in a second state). Furthermore, e.g. a high mechanical stability and a high heat resistance of the mold material (in a first state) are beneficial to preserve a small and precise structure, which corresponds to the exact negative contours of the part to be produced. All these specifications limit the selection of suitable mold material. In the following a possible variety of materials for the mold is presented.

In an embodiment, the mold material comprises or consists of a polymeric material, in particular a polyoxymethylene (POM) or a polyimide (trade name: Kapton). Therefore, a per se known injection molding (IM) is applicable for the provision of the mold, wherein a master mold comprises a master shape, which corresponds to a precise part shape to be built up in the process for producing a watch part.

In an alternative embodiment, the mold material comprises or consists of a metallic material, in particular copper. Due to the material properties of copper like a high thermal strength and stability in combination with its good thermal conductivity the usage of copper as mold material is advantageous.

In an alternative embodiment, the mold material comprises or consists of a material exhibiting a transition from a solid state at a first pressure (mold material in a first state) to a flowable state at a second pressure (mold material in a second state), with the first pressure being lower than the second pressure. This allows the mold material at the second pressure or a higher pressure to adopt a second state and to flow around a master shape present in a die. Due to a release of the pressure to the first pressure the mold material resolidifies by adopting a first state. The negative contours of the master shape are maintained at the inner mold surface after the master mold is removed.

In an alternative embodiment, the mold material comprises or consists of a salt being flowable at the second pressure and solid at the first pressure, the mold material in particular being an alkali metal halide, in particular potassium bromide or potassium iodide or caesium iodide or a mixture of such inorganic salts. Due to the availability especially of inorganic salts, the production of the mold is cost-effective.

In an embodiment, the spraying step is accomplished by a thermal spraying method, the method in particular being a plasma spraying method or a chemical vapor deposition (CVD) method or a plasma enhanced chemical vapor deposition (PECVD) method. This allows to produce a solid part material with a high-purity and a high-performance. The CVD method is able to produce thin layers by at least one of a chemical reaction and a decomposition of a volatile precursor on the mold. Different CVD methods are distinguishable by the chemical reaction and the process conditions used. Furthermore, more uniform layers can be produced by changing the process conditions towards lower pressure (LPCVD, "low pressure chemical vapor deposition") or lower temperature (PECVD).

In an embodiment, the spraying step is accomplished by Physical vapor deposition (PVD), for example, by one or more of Cathodic Arc Deposition, Electron beam physical vapor deposition, Evaporative deposition, Pulsed laser deposition, and Sputter deposition.

The applied spraying methods can be used to produce the first portion and the second portion of the watch part. Therein, a spraying device with a certain set of adjustable parameters is used. After the first portion is sprayed on the inner mold surface, a defined set of parameters is changed and the second portion is sprayed. The changed set of parameters can for example include the temperature or the character of the sprayed material (gaseous, liquid, solid state). This change of parameters can be executed in a continuous manner or abruptly. Either way, the change of parameters gives rise to a change of material properties.

In an embodiment, the first spraying layer is applied by PECVD as a coating onto the entire inner mold surface comprising the precise mold shape, or onto just a portion of this inner mold surface, at a temperature of 200°C to 800°C. By changing the pressure in a range between 10^-3 mbar and 10 mbar the properties of the coating can be varied. The thickness of the coating can be for example less than 50 micrometers.

In the present text the material the watch part is made of is called part material. The term part material, in the absence of other information, refers to the sprayed material as well as the deposited material, including the first spraying layer and second spraying layer, as well as the material of a so-called 'green part', or a sintered material or the material not yet providing the finally desired properties of the watch part 2.

In an embodiment, at least one portion of the part material comprises or consists of a ceramic-like material, or a pure carbon material, or a metal-oxide, or a metal, or a polymer, or a fluorinated polymer or a hard metal material, in particular a carbide or a nitride, or other materials applicable in a spraying process. This broad variation of part material results in a broad variation of available material properties. In the case of a watch part it is advantageous to have an outer layer (corresponding e.g. to the first spraying layer) which is stable, wear and abrasive resistant. Such properties are for example exhibited by hard materials. Due to these properties, these materials are difficult to process especially if a precise shape has to be worked out. Therefore, it is advantageous to use the first spraying layer in order to provide on the one hand the desired material properties of the surface layer of the watch part. On the other hand the wanted shape can be replicated from the precise mold shape of the inner mold surface.

In an embodiment, the step of separating the mold from the watch part is based on a chemical method and in particular comprises dissolving of the mold material by a solvent, and/or a chemical reaction decomposing the mold material, and/or a melting or burning of the mold material and/or an electrochemical process, in particular a galvanic process. This allows for a very gentle removal of the mold especially the precise mold shape comprises an undercut structure element. Furthermore, this requires that the watch part is resistant against the conditions of a chemical separation.

In an embodiment, the separation of the mold from the watch part is based on a mechanical method and in particular comprises a knocking-off or a breaking of the mold. This allows for a relatively fast removal of the mold especially if the watch part is hard and not fragile.

In an embodiment, the mold comprises an undercut structure. This allows to produce watch parts with a big variety of complex shaped structures.

In an embodiment, the mold including the precise mold shape is completely built up before the spraying step. This allows to apply a homogenous and continuous first spraying layer onto the inner mold surface.

The mold for producing a watch part comprises an inner mold surface with a precise structure. This structure corresponds to a precise mold shape and is coatable by a spraying process. The mold is separable from a coating sprayed onto the inner mold surface. This allows to produce a watch part with a precise structure, as the precise mold shape of the precise mold shape corresponds to the desired watch part structure.

In an embodiment, at least the inner mold surface comprises or consists of a polymeric material, in particular polyoxymethylene (POM) or polyimide, or a metallic material, in particular copper. The inner mold surface can also comprise or consists of a material exhibiting a transition from a solid state at a first pressure (mold material in a first state) to a flowable state at a second pressure (mold material in a second state), with the first pressure being lower than the second pressure and with a melting point being above the conditions present during the spraying process, in particular an inorganic salt. The inorganic salt can be in particular an alkali metal halide, in particular potassium bromide or potassium iodide or caesium iodide or a mixture of such salts. This allows that the mold is stable during the spraying process.

In an embodiment, the inner mold surface is resistant to the conditions present during the spraying process, the process in particular being a plasma spraying method or a chemical vapor deposition method or a plasma enhanced chemical vapor deposition method. This allows to use a large variety of material sprayable by the spraying process.

In an embodiment, the material used for spraying of the inner mold surface comprises or consists of a ceramic-like material, or a metal-oxide, or a pure carbon material, or a pure metal, or a hard metal material, in particular carbide, or nitride, or a fluorinated polymer. This allows to build up the watch part, choosing from and/or combining a variety of material properties.

In an embodiment, the watch part is made up of a single part material with uniform material properties.

In general, the conditions during the spraying of the part material are matched with the properties desired for the application of the watch part. Furthermore, the mold is able to sustain the spraying conditions whereas the mold is removable from the watch part without damaging the watch part.

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, which schematically show:

Figure 1

a master mold, with a mold material being formed in a die and the mold material maintaining the master shape;

Figure 2

a (per se) known spraying device for coating the mold;

Figure 3

a series of stages in the process for producing a precisely structured watch part.

In principle, identical parts are provided with the same reference symbols in the Figures.

Figure 1 schematically shows a master mold 3 comprising at least one die 31 and/or one press plunger 32. At least one of the die 31 and the press plunger 32 comprises a master shape 30. The master shape 30 defines a precise structure of a watch part 2 to be manufactured, in particular at least a part of the shape of the watch part 2.

The master mold 3 is filled with a mold material 12, wherein the mold material 12 can be a metallic material, in particular cooper, or a polymeric material, in particular polyoxymethylene (POM) or polyimide (Kapton), or a material which is solid at a first pressure and becomes flowable at a second pressure, wherein the first pressure is lower than the second pressure. This material, flowable at the second pressure could be in particular an inorganic salt, in particular an alkali metal halide, in particular potassium bromide or potassium iodide or caesium iodide or a mixture of such salts.

The mold material 12 is pressed into the master mold 3 by a pressing force 33 (arrows) and flows around the master shape 30 of the master mold 3. The flowable mold material 12 therefore adopts the negative contours of the master mold 3 and a future watch part 2. In this flowable state the mold material is in a second state 14.

While or after the release of the pressing force 33 the mold material 12 changes its state to a first state 13. The mold material in a first state 13 is a solid state. In the course of and after this re-solidification from the mold material in a second state 14 to the mold material in a first state 13, the mold material 12 preserves the negative contours of the master mold 3.

In a final step the mold 1 is removed from the master mold 3, wherein the master shape 30 corresponding to a precise part shape 20 is replicated as a precise mold shape 10. This step of removing can utilise the application of mechanical pressure, optionally in combination with a thermal treatment effecting a dilatation of the mold 1 and/or the master mold 3. The precise mold shape 10 is embodied by an inner mold surface 11. At least a surface portion of the inner mold surface 11 is defined by the mold material 12 and is resistant against the conditions present during a subsequent building up of the watch part 2.

In an embodiment, the mold material 12 is a material which is solid at a first pressure and becomes flowable at a second pressure, wherein the first pressure is lower than the second pressure. When the pressing force 33 pressing the mold material 12 into the master mold 3 is high enough to reach at least the second pressure, the liquefied mold material 12 flows around the master shape 30. Afterwards the pressing force 33 is lowered, the mold material 12 becomes solid again and maintains the negative contours of the master shape 30.

In an alternative embodiment, the mold material 12 is a metallic or polymeric material. The mold 1 can be manufactured using a ceramic injection molding (CIM), a metallic injection molding (MIM) or plastic injection molding (PIM), wherein the flowable mold material 12 (mold material in a second state 14) is pressed into a master mold 3 and a so-called 'green part' is cooled and de-molded. Traditionally, such a 'green part' resulting from a CIM, MIM or PIM process is fragile and obtains its resistance in an additional sintering step. For the application as a mold 1, with the precise mold shape 10 at the inner mold surface 11, it is sufficient if the 'green part' can sustain the conditions present during the build up of the watch part 2. Therefore in many cases the sintering step can be skipped, which entails that the mold can be easily destroyed after the building up of the watch part 2.

In an alternative embodiment, the mold 1 is undercut. An undercut structure of the inner mold surface 11 can be realised by using at least two press plungers 32, comprising a master shape 30. The press plungers 32 are able to come in close contact with each other and the mold material 12 is pressed aside. The undercut structure of the inner mold surface 11 can also be realised by a mold 1 comprising at least two mold parts. These mold parts can be combined after re-solidification to constitute the uniform mold 1. The undercut structure of the inner mold surface 11 can be realised in various known ways, e.g. by using a dividable master mold 3 and/or additional movable mold parts which can be removed after re-solidification of the mold material 12 without destroying the precise mold shape 10.

Figure 2 schematically shows a spraying device 4, wherein the device comprises a spray gun 41. The material sprayed by the spray gun 41, also called sprayed material 42, corresponds to the material the watch part 2 is made of (part material). The sprayed material 42 is sprayed into the mold 1 with the precise mold shape 10.

The spraying device 4 is in particular used for a thermal spraying process, wherein the thermal spraying process can be in particular a plasma spraying method or a chemical vapor deposition (CVD) method or a plasma enhanced chemical vapor deposition (PECVD) method. The plasma spraying method is performed at low pressure, which includes vacuum plasma spraying (VPS) or low pressure plasma spraying (LPPS). The application of CVD processes is advantageous because the layer coated into a surface is very homogenous and a big variation of materials can be deposited as sprayed material 42.

The watch part 2 may have to exhibit a certain combination of properties like a high mechanical strength (hardness), a high abrasion resistance, a high wear-resistance, a small electrical conductivity and as well a high thermal conductivity. Some of these properties, like the mechanical strength, can create huge problems when manufacturing the part in a conventional manner, especially for small and precise structures. These problems can be overcome by the method presented herein, using a sprayed material 42 which comprises the desired properties. Such sprayed material 42 can be a metal oxide, in particular a ceramic-like material, or a pure metal, or a pure carbon material, or a hard metal material, in particular a carbide or a nitride, or a fluorinated polymer, in particular Teflon (brand name by DuPont Co).

In order to build up a watch part 2, the mold 1 comprising the precisely shaped inner mold surface 11 has to sustain the conditions during the spraying process. The temperature during the spraying reaches, for example, up to 200°C or to 500°C or more. This implies a high thermal resistance of the mold 1 and especially of the inner mold surface 11, where the sprayed material 42 has to replicate the precise mold shape 10.

It can be advantageous if that the sprayed material 42 exhibits a strong adherence to the mold material 12 in order to allow for an exact replication of the precise mold shape 10.

The properties desired for a watch part 2 as listed above are mainly related to the properties of the surface of the watch part 2, therefore it is possible to create the watch part 2 as comprising at least two portions. For example, a first portion corresponds to a surface layer or coating of the watch part 2 and a second portion corresponds to an inner filling of the watch part 2. Hence, the watch part 2 is build up in a reverse order, wherein first the surface layer is sprayed into the precise mold shape 10 and secondly the inner filling is deposited onto the surface layer, e.g. filling up the remainder of the mold.

A variation of the properties of the first coating layer 21 and the second coating layer 22 can be controlled or manipulated by varying the type, consistency, flow rate, deposition rate etc. of the sprayed material 42, and/or the thickness of the deposited layer. One possibility to control the thickness and the quality of the deposited layer is usage of the very same spraying device 4 according to different modi operandi. For instance the spraying device 4 can be operated with, that is, fed with, a gaseous material in order to form a thin layer, but later on a solid material (e.g. as powder) can be used, that is, fed to the same spraying device 4, to form a thick layer for filling the precise mold shape 10. The material supplied in the gaseous and solid state can be the same material, or can be different materials. Also the deposition rate can be regulated and affect the properties and quality of the deposited layer by only using a single spraying device 4.

In an alterative embodiment, the mold 1 can be undercut. In such a case it is advantageous that the spraying device 4 and the mold 1 are movable relative to each other, e.g. in a tumbling motion. The tumbling of at least on of the spraying device 4 and mold 1 ensures that the undercut regions of the mold can be coated with the sprayed material 42 and the precise mold shape 10 can be reproduced.

Figure 3 shows a series of steps in the manufacturing process of a watch part 2. In the first place (figure 3a) a mold is provided, comprising an inner mold surface 11 with a precise mold shape 10. The precise mold shape 10 corresponds to negative contours of the watch part 2.

In a next step (figure 3b) a first spraying layer 21 corresponding to a coating is deposited onto the inner mold surface 11. The mold material 12 has to be resistant against the conditions of the spraying because the precise mold shape 10 has to be preserved. The first spraying step comprises a spraying process, in particular a thermal spraying process, in particular a plasma spraying process or a chemical vapor deposition (CVD) process or a plasma enhanced chemical vapor deposition (PECVD) process. Also plasma spraying methods performed at low pressure are applicable in order to build up the first spraying layer 21. The first spraying layer 21 or coating corresponds to the surface of the future watch part 2 and therefore has to exhibit a high quality and high performance regarding hardness and resistance to wearing and abrasion.

In a further step (figure 3c) a second spraying layer 22 or, in the present case, a filling is deposited onto the first spraying layer 21. There can be differentrequirements on the second spraying layer 22 as compared to the first spraying layer 21. This allows to incorporate complementary material properties in the watch part 2, e.g. with the filling being more elastic, and the coating being harder. Also, depending on requirements, cheaper material can be used for the filling that for the coating. The present example shows two spraying layers, but of course only a single spraying layer or more than two spraying layers with different properties can be applied.

A change of the properties of the spraying layers can be achieved by adapting the coating conditions, such as temperature and/or deposition rate and/or deposition material and/or changing the sprayed material 42 fed to the spraying device 4 from gaseous material to solid material (powder). The composition of subsequent spraying layers can be changed gradually or abruptly. The second spraying layer 22 can be created by filling the mold 1 until at least a filling level is reached.

In a further step (figure 3d) an excess of material, which might be present at the outer surface of the mold or exceeds the filling level, is milled or ground off. The excess of material may comprise part of the second spraying layer 22 but may also comprise part of the first spraying layer 21. The milling or grinding to remove excess material can be performed in a stage when the watch part 2 is still incorporated into the mold 1 and thereby is mechanically held and stabilized by the mold 1.

In a further step (not shown) an additional layer is sprayed onto the milled or grinded surface. The sprayed material can be the material of the first spraying layer 21 or the second spraying layer 22 or a different material.

In a further step (figure 3e) the mold 1 and the watch part 2 are separated from each other. The separation step comprises either a chemical method or a mechanical method for the removal of the mold 1. The chemical method in particular comprises dissolving of the mold material 12 by a solvent, and/or a chemical reaction decomposing the mold material 12, and/or a melting or burning of the mold material 12 and/or an electrochemical process, in particular a galvanic process. The mechanical method in particular comprises a knocking-off or a breaking of the mold 1. The watch part 2 or at least the outer surface of the watch part 2 should be resistant to the conditions present in the course of the separation step.