Overmolding process

Abstract

The invention relates to an overmolding process for producing an article (1) with a decoration (2), also called overmolding, said decoration (2) being made of a metallic alloy (8) that is at least partially amorphous. The process comprises the steps of: - Providing a blank (3) having at least one through hole (4) opening into the overmolding (2) to be produced, said blank (3) being made of a first material having low thermal effusivity or being at least partially coated with a layer made of said first material, - Providing an injection mold (6), - Positioning the blank (3) within the injection mold (6), - Injecting the metallic alloy (8) in liquid form through the through hole (4) opening into a cavity of the injection mold (6) and / or the blank (3), - Demolding the blank (3) with the overmolding (2) to obtain article (1).

Description

Technical field of the invention

[0001] The invention relates to an overmolding process adapted to produce an overmolding in at least partially amorphous metallic alloy, said overmolding forming a decoration, also called a motif, on an article made of a material with low thermal effusivity. Technological background

[0002] Several documents describe processes for overmolding an article with patterns in amorphous alloy.

[0003] Examples include documents EP 2 315 673 and CH 715 132, which describe processes for embedding amorphous metal in recesses made in the article. These processes are poorly suited for creating raised overmoldings and require a finishing step to rectify the surface of the overmoldings.

[0004] Document EP 2 370 865 describes a method for decorating an article with a relief design made of amorphous metal. The method involves using a mask through which the metal is injected. This results in excess amorphous metal in the areas to be decorated, and these areas must therefore undergo a finishing step.

[0005] In injection molding processes, several problems arise with amorphous metals. It is not possible to heat the mold to a temperature close to the melting point because the cooling rate must be sufficiently high to prevent crystallization. Furthermore, the use of conventional metal molds necessitates injection channel cross-sections similar to the thickness of the cavity to be filled, to prevent premature solidification within the channels, which would prevent the alloy from reaching the cavity. This negatively impacts the flexibility of the injected part geometries, particularly for small parts where a sheet of alloy must be used to fill an entire face of the part. Summary of the invention

[0006] The present invention aims to overcome the aforementioned shortcomings by proposing an overmolding process which, on the one hand, allows injection without solidification problems into the injection channels and without risk of crystallization of the amorphous metal and, on the other hand, provides an overmolding which does not necessarily require rectification after injection.

[0007] To this end, it is proposed to start with a blank of the article made from a material with low thermal effusivity in order to minimize heat extraction during overmolding. It is also proposed to inject from the opposite or adjacent face to the face to be overmolded via through holes made in the blank and opening into a cavity forming the negative of the overmolding to be performed. In other words, it is also proposed to inject from a portion of a surface opposite or adjacent to another portion of the surface to be overmolded via through holes made in the blank and opening into a cavity forming the negative of the overmolding to be performed. According to the invention, the cavity can be made in the blank and / or in the injection mold that holds the blank.

[0008] The low thermal effusivity of the blank ensures that the through-holes upstream of the cavity do not solidify, thus allowing the alloy to flow into the cavity in a liquid state. Preferably, for optimal results, it is also recommended to use an injection mold made of a material with low thermal effusivity. This allows the alloy to perfectly conform to the shape of the mold cavity and / or the blank, faithfully reproducing the geometry and surface finish of the cavity. Consequently, the overmoldings are finished, both geometrically and aesthetically, in their as-injection state, eliminating the need for any further machining.

[0009] This process is particularly well-suited for overmolding blanks made from materials with low thermal effusivity, such as certain ceramics (zirconia, glass, enamels, etc.), certain metals or metal alloys (grade 2 titanium, grade 5 titanium, Inconel, etc.), polymers (rubber, PEEK, etc.), natural materials (wood, minerals, etc.), composites, and materials with any thermal effusivity but coated with a low thermal effusivity layer. Using a blank made entirely or partially from a low thermal effusivity material allows for the creation of very small through holes without the risk of hardening before reaching the cavity. These very small through holes prevent weakening the ceramic or any other material used and allow for the creation of small overmoldings.

[0010] In summary, the overmolding process according to the invention allows for greater flexibility in the geometries of injected parts, improves the reproduction of the mold's surface finish on the injected part, and enables the production of two-material parts with "net-shape overmolding." This process thus makes it possible to create overmoldings with complex shapes.

[0011] More specifically, the invention relates to an overmolding process for producing an article with a decoration, also called overmolding, said decoration being in a metallic alloy that is at least partially amorphous, the process comprising the steps of: Provision of a blank with a first face intended to be overmolded and a second face serving as an entry point for the injection of the liquid metal alloy, said blank being provided with at least one through hole extending between the first face and the second face and opening at the level of the overmolding to be carried out, the through hole forming a channel for the injection of the metal alloy, said blank being made of a first material having a thermal effusivity less than or equal to 7000 WK-1 m-2 s1 / 2, preferably less than or equal to 3500 WK-1 m-2 s1 / 2 or being at least partially coated with a layer made of said first material, Provision of an injection mold, said injection mold and / or the blank comprising an impression forming the negative of the overmolding to be carried out,Positioning of the blank within the injection mold with the through-hole aligned with the mold cavity if the injection mold has a cavity; Injection of the liquid metal alloy from the second face of the blank through the through-hole opening into the cavity to obtain the blank with the overmolding; Demolding of the blank with the overmolding to obtain the finished part. Brief description of the figures

[0012] THE figures 1 to 5 schematically represent the steps of the process according to the invention for a relief decoration. figure 6 represents a variation of the process for negative decoration. figure 7 represents a variant of the figure 3 where the mold cavity is formed only in the blank and where the decoration is flush with the surface of the blank. figure 8 represents another variant of the figure 3where the imprint for overmolding is made in the blank and in the mold forming a relief decoration. Detailed description of the invention

[0013] The invention relates to an overmolding process for producing a decorated article. The article may, for example, be a watch component. More specifically, it may be a case component chosen from the non-exhaustive list including a case middle, a case back, a bezel, a crown, a pusher, a bracelet link, a bracelet, a pin buckle, a clasp, a dial, an applique, and a hand. It may also be a movement component chosen from the non-exhaustive list including an oscillating weight, a bridge, an anchor, a wheel, and a mainplate. Typically, the watch component is a dial or a bezel 1 ( fig.5) with indices and numbers forming a design 2, also referred to below as overmolding. In the illustrated example, the design is in relief, also called a positive design. The process is particularly well-suited for a design made in a metallic alloy that is at least partially amorphous. At least partially amorphous means having more than 50% amorphous phase. Examples include Pt850, Pd600, Vit105, and Ni53 alloys. It is also particularly well-suited for an article made of a fragile material such as ceramic. The article is made of a material with low thermal effusivity. Low thermal effusivity is defined as a value less than or equal to 7000 WK-1 m-2 s1 / 2, or even less than or equal to 3500 WK-1 m-2 s1 / 2. As an example, the ceramic can be zirconia which has a thermal effusivity of 2400 WK-1 m-2 s1 / 2.If the article has a thermal effusivity greater than 7000 W K-1 m-2 s1 / 2, it is possible to apply at least in part to its surface a layer which has a thermal effusivity less than or equal to 7000 W K-1 m-2 s1 / 2, or even less than or equal to 3500 W K-1 m-2 s1 / 2.

[0014] The overmolding process allows for positive relief overmolding, overmolding flush with the surface to be decorated, or negative relief overmolding relative to the surface to be overmolded. It is also possible to create combinations of the aforementioned overmolding techniques.

[0015] The process is illustrated in figures 1 to 6 for a variant and to figures 7 and 8 for other variants. The process will be illustrated below for the variant of figures 1 to 6 where the cavity for overmolding is formed in the injection mold, with the difference that at the figure 6 The relief is in negative, not positive. For the variants of figures 7 and 8The concept of the process is the same, with the only change being that the cavity for overmolding is respectively made in the blank or in the blank and in the injection mold.

[0016] Furthermore, the process is illustrated below for a blank with a multi-faceted geometry. It is also possible to implement the process for a geometry with a single surface, such as a surface of revolution in the case of a torus. In this case, the term "face" used below will be understood beyond its geometric definition and will more generally designate a portion of the blank surface serving as the entry point for the liquid alloy relative to another portion of the blank surface serving as the exit point for the liquid alloy, with these portions being either connected or disconnected.

[0017] The process is illustrated in figures 1 to 5for a positive relief overmolding consisting of indices and numbers on a bezel. It should be noted that the values ​​given below are also valid for other overmolded items, not limited to a bezel or a positive relief overmolding. Thus, at the figure 6 The relief is in negative.

[0018] In a first step ( fig.1), a blank 3 with one or more through holes 4 is provided. These through holes act as injection channels and extend between face 3a, which is the visible face intended to be overmolded, and face 3b, which is a face opposite face 3a or possibly a face adjacent to face 3a, with the opposite or adjacent face preferably being a hidden face after assembly of the watch case. In the illustrated example, the visible face 3a is the upper face and face 3b is the opposite face, which is therefore the lower face. This blank is the part to be overmolded and is made of the low thermal effusivity material. The hole 4 opens at the location where the overmolding is to be and defines an injection channel 5 for the passage of the liquid metal or alloy 8 ( fig.3 Depending on the dimensions of the overmolding to be carried out, the blank includes one or more through holes for overmolding.

[0019] Preferably, to avoid weakening the blank material, the through holes have a small cross-section. The hole cross-section can be constant or vary along its length. Preferably, the hole has a conical shape flaring towards the lower face 3b as shown in the figure 2 This allows for a finer geometry on the top face and anchors the overmolding to the blank. The hole can have a circular, oblong, or other cross-section. For example, holes with an oblong cross-section are suitable for increasing the flow area on elongated overmoldings such as indices. Holes may also be non-straight but rather have bends or complex shapes to allow for more flexible positioning of the inlet for injection.

[0020] Preferably, the largest dimension of the hole's cross-section is between 0.1 and 2 mm, more preferably between 0.2 and 1 mm, and even more preferably between 0.25 and 0.5 mm. If the cross-section varies along the hole, the largest cross-section will be considered, and within that cross-section, the largest dimension will be measured.

[0021] For a bezel or dial, the blank thickness is between 0.3 and 3 mm. For a through hole between the top and bottom faces of the blank, this results in a diameter-to-length ratio of 0.08 to 1.7 mm for a circular cross-section diameter of 0.25 to 0.5 mm. Generally, as an example, for other items, this diameter-to-length ratio of 0.08 to 1.7 mm can be maintained; in other words, the ratio between the largest dimension of the cross-section of the through hole and the blank thickness is between 0.08 and 1.7 mm for a hole extending between the top and bottom faces.Furthermore, for a positive relief overmolding, the cross-sectional area of ​​the through holes on the top face is less than or equal to the cross-sectional area of ​​the overmolding to be filled, projected onto the plane of the top face to prevent the alloy from overflowing beyond the overmolding. Preferably, the cross-sectional area of ​​the through holes on the top face is less than the cross-sectional area of ​​the decoration projected onto the plane of the top face. Even more preferably, the cross-sectional area of ​​the through holes on the top face is at least 1.2 times smaller than the cross-sectional area of ​​the overmolding projected onto the plane of the top face.

[0022] In a second step ( fig.3 ), the blank 3 is placed in a mold 6, one part of which has an impression 7 with a shape corresponding to the shape of the overmolding to be produced. For a positive relief, the impression 7 forms a cavity. As an alternative for the figure 7The mold may be without an impression, and it is the blank 3 that has an impression 10 forming a cavity. As an alternative for the figure 8 The mold 6 has an impression 7, and the blank 3 also has an impression 10, forming the cavity with the two cavities intended to communicate. The overmolding will therefore fill the cavities of the blank and the mold.

[0023] According to the invention, the mold is made of a material preferably having low thermal effusivity with a value less than or equal to 7000 WK⁻¹ m⁻² s⁻¹ / ², preferably less than or equal to 3500 WK⁻¹ m⁻² s⁻¹ / ², or it is at least partially coated with a layer of this low thermal effusivity material. According to the invention, this material can be a ceramic (zirconia, Macor, etc.), a metal (titanium or titanium alloy, etc.), or preferably a polymer such as silicone or rubber. Indeed, despite the high temperatures of the molten metal during injection, for example 700°C for a Pt850 alloy or 1200°C for a Vit105 alloy, it has been found that it is possible to use a polymer mold without degrading it. There are several advantages to using a polymer mold: Their very low thermal effusivity (< 500 WK -1 < m -2 < s 1 / 2 < ) allows them to fill complex geometries. Their elasticity, especially for elastomers, allows them to precisely conform to the surface of the watch component, and in particular the bezel for the example illustrated, and to compensate for the manufacturing tolerances of the latter, for example the angles. Their elasticity also allows the creation of 3D decorations with undercuts that would not be demoldable with a rigid mold. The manufacturing cost of a polymer mold is much lower than that of a metal or ceramic mold.

[0024] In a third stage also represented at the figure 3The liquid alloy 8 is injected through the through hole 4, acting as an injection channel 5, from the lower face 3b of the blank 3. The alloy is heated and injected at a temperature equal to or above its solidus temperature in the case of a conventional injection molding process. In the case of a rapid-heat injection molding process for a preform that is at least partially amorphous (> 50%), such as rapid discharge forming, the injection temperature is set so that the viscosity is below 1000 Pa·s. During injection, the mold can be heated to a temperature lower than or equal to the glass transition temperature of the injected amorphous alloy.

[0025] In a fourth visible stage at the figure 4The mold is removed. The resulting blank 3, corresponding to the final or near-final article, has overmoldings 2 in amorphous metal alloy on its upper face 3a. On its lower face 3b, a foot 9 formed with the injection alloy 8 solidified in the injection channel remains. This foot can be retained on the final article. Alternatively, the lower face can be ground to remove this excess material. The article 1 thus obtained is visible at the figure 5 This presents an overmolding process that does not require rectification after injection into the mold. However, it is still possible to perform a finishing step on the overmoldings to obtain specific surface finishes or to machine non-moldable geometries.

[0026] It should be noted that the process also allows for the creation of an overmolding 2 flush with the upper face 3a to be decorated; in which case, the cavity of the mold 6 mentioned above is non-existent and possibly the blank 3 has an impression 10 ( fig.7 ). The process also allows for negative overmolding, in this case, instead of the injection mold cavity, an impression 7 forms a protrusion that fits into the through hole 4 during injection ( fig.6 ).

Claims

1. Overmolding process for producing an article (1) with a decoration (2), also called overmolding, said decoration (2) being in a metallic alloy (8) that is at least partially amorphous, the process comprising the steps of: - Providing a blank (3) with a first face (3a) intended to be overmolded and a second face (3b) serving as an entry point for the injection of the metallic alloy (8) in a liquid state, said blank (3) being provided with at least one through hole (4) extending between the first face (2a) and the second face (3b) and opening at the level of the overmolding (2) to be performed, the through hole (4) forming a channel (5) for the injection of the metallic alloy (8), said blank (3) being made of a first material having a thermal effusivity less than or equal to 7000 W K-1 m-2 s1 / 2, preferably less than or equal to 3500 W K-1 m-2 s1 / 2 or being at least partially coated with a layer made of said first material,- Provision of an injection mold (6), said injection mold (6) and / or the blank (3) comprising a cavity (7, 10) forming the negative of the overmolding (2) to be produced, - Positioning of the blank (3) within the injection mold (6) with the through hole (4) positioned opposite the cavity (7) of the injection mold (6) if said injection mold (6) has a cavity (7), - Injection in liquid state of the metal alloy (8) from the second face (3b) of the blank (3) through the through hole (4) opening onto the cavity (7, 10) to obtain the blank (3) with the overmolding (2), - Demolding of the blank (3) with the overmolding (2) to obtain the article (1).

2. An overmolding method according to the preceding claim, wherein said injection mold (6) is made of a second material having a thermal effusivity less than or equal to 7000 WK -1 m -2 s 1 / 2 , preferably less than or equal to 3500 WK-1 m -2 s 1 / 2 or being at least partly coated with another layer made of said second material.

3. Overmolding method according to any one of the preceding claims, wherein the cross-section of the through hole (4) at the level of the first face (3a) is less than the cross-section of the overmolding (2) projected into the plane of the first face (3a), when the overmolding (2) to be made is in positive relief with respect to the first face (3a) of the blank (3).

4. Overmolding method according to the preceding claim, wherein the cross-section of the through hole (4) at the level of the first face (3a) is smaller by a factor of at least 1.2 compared to the cross-section of the overmolding (2) projected into the plane of the first face (3a).

5. Overmolding method according to any one of the preceding claims, wherein the largest dimension of the cross-section of the through hole (4) is between 0.1 and 2 mm, preferably between 0.2 and 1 mm.

6. Overmolding method according to the preceding claim, wherein the largest dimension of the cross-section of the through hole (4) is between 0.25 and 0.5 mm.

7. Overmolding method according to any one of the preceding claims, wherein the first face (2a) is an upper face of the blank (3) and wherein the second face (2b) is an lower face of the blank (3) opposite the upper face (2a), the through hole (4) extending between the upper face and the lower face through the thickness of the blank (3).

8. Overmolding method according to claim 7, wherein the ratio between the largest dimension of the cross-section of the through hole (4) and the thickness of the blank (3) is between 0.08 and 1.

7.

9. Overmolding method according to claim 7 or 8, wherein the through hole (4) forms a cone flaring from the upper face to the lower face.

10. Overmolding process according to any one of the preceding claims, wherein the first material is a ceramic, preferably a zirconia.

11. Overmolding process according to any one of claims 2 to 10, wherein the second material is a polymer.

12. Overmolding process according to the preceding claim, wherein the second material is a silicone or a rubber.

13. Overmolding method according to any one of the preceding claims, wherein article (1) is a watch component.

14. Overmolding process according to any one of the preceding claims, comprising a step of removing at the level of the second face (3b) an excess of the metal alloy (8) at least partially amorphous protruding from said second face (3b) following the injection in liquid state of the metal alloy (8).

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