OVERMOULDING METHOD

Abstract

An overmoulding method for making a part (1) with a decoration (2) made of a metal alloy (8) that is at least partially amorphous. The method includes: providing an bauche (3) with at least one through hole (4) opening onto the overmould (2) to be made, the bauche (3) being made of a first material with low thermal effusivity; positioning the bauche (3) inside an injection mould (6); injecting the liquid metal alloy (8) through the through-hole (4) opening into a cavity of the injection mould (6) and/or of the bauche (3); and moulding the bauche (3) with the overmould (2) to obtain the part (1).

Claims

1. An overmoulding method for making a part (1) with a decoration (2), also known as an overmould, said decoration (2) being made of a metal alloy (8) that is at least partially amorphous, the method comprising the following steps: providing an bauche (3) with a first face (3a) to be overmoulded and a second face (3b) acting as an entry point for injecting the liquid metal alloy (8), said bauche (3) being provided with at least one through hole (4) extending between the first face (2a) and the second face (3b) and opening out onto the overmould (2) to be made, the through hole (4) forming a channel (5) for injecting the metal alloy (8), said bauche (3) being made of a first material with a thermal effusivity less than or equal to 7,000 W K1 m2 s, preferably less than or equal to 3,500 W K1 m2 s, or being at least partially plated with a layer made of said first material; providing an injection mould (6), said injection mould (6) and/or the bauche (3) comprising an impression (7, 10) forming the negative of the overmould (2) to be made; positioning the bauche (3) inside the injection mould (6) with the through hole (4) arranged facing the impression (7) of the injection mould (6) if said injection mould (6) comprises an impression (7); injecting the liquid metal alloy (8) from the second face (3b) of the bauche (3) through the through hole (4) opening onto the impression (7, 10) to obtain the bauche (3) with the overmould (2); and unmoulding the bauche (3) with the overmould (2) to obtain the part (1).

2. The overmoulding method according to claim 1, wherein said injection mould (6) is made of a second material with a thermal effusivity of less than or equal to 7,000 W K.sup.1 m.sup.2 s.sup.1/2, or is at least partially plated with another layer made of said second material.

3. The overmoulding method according to claim 1, wherein the cross-section of the through hole (4) in the first face (3a) is smaller than the cross-section of the overmould (2) projected in the plane of the first face (3a), when the overmould (2) to be made is in positive relief relative to the first face (3a) of the bauche (3).

4. The overmoulding method according to claim 3, wherein the cross-section of the through hole (4) in the first face (3a) is at least 1.2 times smaller than the cross-section of the overmould (2) projected in the plane of the first face (3a).

5. The overmoulding method according to claim 1, wherein the largest dimension of the cross-section of the through hole (4) is comprised between 0.1 and 2 mm.

6. The overmoulding method according to claim 5, wherein the largest dimension of the cross-section of the through hole (4) is comprised between 0.25 and 0.5 mm.

7. The overmoulding method according to claim 1, wherein the first face (2a) is an upper face of the bauche (3) and wherein the second face (2b) is a lower face of the bauche (3) opposite the upper face (2a), the through hole (4) extending between the upper face and the lower face through the thickness of the bauche (3).

8. The overmoulding 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 bauche (3) is comprised between 0.08 and 1.7 mm.

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

10. The overmoulding method according to claim 1, wherein the first material is a ceramic, preferably a zirconia.

11. The overmoulding method according to claim 2, wherein the second material is a polymer.

12. The overmoulding method according to claim 11, wherein the second material is a silicone or a rubber.

13. The overmoulding method according to claim 1, wherein the part (1) is a horology component.

14. The overmoulding method according to claim 1, further comprising a step in which an excess of the at least partially amorphous metal alloy (8) extending from said second face (3b) after the injection of the liquid metal alloy (8) is removed from the second face (3b).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0018] FIGS. 1 to 5 schematically show the steps of the method according to the invention for a relief decoration.

[0019] FIG. 6 shows a variant of the method for a negative decoration.

[0020] FIG. 7 shows a variant of FIG. 3, in which the impression for the overmould is made only in the bauche and the decoration is flush with the surface of the bauche.

[0021] FIG. 8 shows another variant of FIG. 3, in which the impression for the overmould is made in the bauche and in the mould, forming a relief decoration.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The invention relates to an overmoulding method for making a decorated part. The part can, for example, be a horology component. More specifically, it can be an external part chosen among the non-exhaustive list comprising a middle, a back, a bezel, a crown, a button, a wristlet link, a wristlet, a tongue buckle, a clasp, a dial, an applique and a hand. It can also be a movement component chosen among the non-exhaustive list comprising an oscillating mass, a plate bar, a pallet, a wheel and a plate. Typically, the horology component is a dial or a bezel 1 (FIG. 5) with indicators and numerals forming a decoration 2, also referred to hereinafter as an overmould. In the example illustrated, the decoration is in relief, also known as positive decoration. The method is more specifically suited to decoration made from a metal alloy that is at least partially amorphous. At least partially amorphous is taken to mean with more than 50% of amorphous phase. Examples include Pt850 alloy, Pd600 alloy, Vit105 alloy, and Ni53 alloy. It is also particularly well suited for parts made from brittle materials such as ceramics. The part is made of a material with low thermal effusivity. Low thermal effusivity is defined as a value less than or equal to 7,000 W K1 m2 s, or even less than or equal to 3,500 W K1 m2 s. For example, the ceramic could be zirconia, which has a thermal effusivity of 2,400 W K1 m2 s. If the part has a thermal effusivity greater than 7,000 W K1 m2 s, at least part of its surface can be coated with a layer that has a thermal effusivity less than or equal to 7,000 W K1 m2 s, or even less than or equal to 3,500 W K1 m2 s.

[0023] The overmoulding method can be used when overmoulding to produce a positive relief, overmoulding flush with the surface to be decorated or overmoulding to produce a negative relief relative to the surface to be overmoulded. Combinations of the aforementioned overmoulds can also be made.

[0024] The method is illustrated in FIGS. 1 to 6 for one variant and in FIGS. 7 and 8 for other variants. The method will be illustrated below for the variant shown in FIGS. 1 to 6, in which the impression for the overmould is formed in the injection mould, with the difference being that in FIG. 6 the relief is negative rather than positive. For the variants in FIGS. 7 and 8, the concept of the method is the same, with the only change being that the impression for the overmould is formed respectively in the bauche or in the bauche and in the injection mould.

[0025] In addition, the method is illustrated below for an bauche with a multi-face geometry. The method can also be used 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 is to be understood beyond its geometric definition and refers more generally to a portion of the surface of the bauche acting as an entry point for the liquid alloy relative to another portion of the surface of the bauche acting as an exit point for the liquid alloy, with portions that may or may not be connected.

[0026] The method is illustrated in FIGS. 1 to 5 for a positive relief overmould formed of indicators and numerals on a bezel. It should be noted that the values given below are also valid for other parts to be overmoulded, without being limited to a bezel or to a positive relief overmould. Accordingly, in FIG. 6, the relief is negative.

[0027] In a first step (FIG. 1), an bauche 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 overmoulded, and face 3b, which is a face opposite face 3a or optionally a face connected to face 3a, with the opposite face or connected face preferentially being a concealed face once the watchcase has been assembled. In the example illustrated, the visible face 3a is the upper face and face 3b is the opposite face, which is therefore the lower face. This bauche is the part to be overmoulded and it is made of a material with low thermal effusivity. The hole 4 opens out where the overmould is to be applied and forms an injection channel 5 for the passage of the liquid metal or alloy 8 (FIG. 3). Depending on the dimensions of the overmould to be made, the bauche comprises one or more holes through the overmould.

[0028] Preferentially, to keep the material of the bauche from becoming brittle, the through holes have a small cross-section. The cross-section of the hole can be constant or variable along its length. Preferentially, the hole is conical in shape, flaring towards the lower face 3b as shown in FIG. 2, which allows for a finer geometry on the upper face and anchors the overmould in the bauche. The hole can have a circular, oblong or other cross-section. For example, holes with an oblong cross-section can be used to increase the supply section on elongated overmoulds such as indicators. The holes can also not be straight but instead have bends or complex shapes, so that the injection entry can be positioned more flexibly.

[0029] Preferentially, the largest dimension of the hole section is comprised between 0.1 and 2 mm, more preferentially between 0.2 and 1 mm, and even more preferentially between 0.25 and 0.5 mm. If the section varies along the hole, the largest section will be considered and within that section, the largest dimension will be measured.

[0030] For a bezel or a dial, the thickness of the bauche is comprised between 0.3 and 3 mm. For a through hole between the upper face and the lower face of the bauche, this results in a range of hole diameter/length ratios ranging from 0.08 to 1.7 mm for a circular section of the hole with a diameter comprised between 0.25 and 0.5 mm. By way of example, for other parts, this hole diameter/length ratio ranging from 0.08 to 1.7 mm can generally be kept or, in other words, the ratio between the largest dimension of the cross-section of the through hole and the thickness of the bauche is comprised between 0.08 and 1.7 mm for a hole extending between the upper face and the lower face. Moreover, for positive relief overmoulds, the cross-section of the through holes in the upper face is less than or equal to the cross-section of the overmoulds to be filled, projected in the plane of the upper face, to keep the alloy from overflowing the overmould to be filled. Preferentially, the cross-section of the through holes in the upper face is smaller than the cross-section of the decorations projected in the plane of the upper face. More preferentially, the cross-section of the through holes in the upper face is at least 1.2 times smaller than the cross-section of the overmoulds projected in the plane of the upper face.

[0031] In a second step (FIG. 3), the bauche 3 is placed in a mould 6 with an impression 7 in one of its two parts, having a shape corresponding to the shape of the overmould to be made. For positive relief, the impression 7 forms a cavity. As a variant to FIG. 7, the mould can have no impression, and it is the bauche 3 that comprises an impression 10 forming a cavity. As a variant to FIG. 8, the mould 6 comprises an impression 7 and the bauche 3 also comprises an impression 10 forming the cavity with the two cavities designed to communicate. The overmould will therefore fill the cavities of the bauche and of the mould.

[0032] According to the invention, the mould is made of a material preferentially with low thermal effusivity with a value less than or equal to 7,000 W K.sup.1 m.sup.2 s.sup.1/2, preferably less than or equal to 3,500 W K.sup.1 m.sup.2 s.sup.1/2 or it is at least partially plated with a layer of this material with low thermal effusivity. According to the invention, this material can be a ceramic (zirconia, Macor, etc.), a metal (titanium or titanium alloy, etc.) or, preferentially, a polymer such as a silicone or a rubber. In fact, despite the high temperatures of the molten metal during injection, for example 700 C. for a Pt850 alloy or 1,200 C. for a Vit105 alloy, it has been found that a polymer mould can be used with no degradation to the mould. There are several advantages to using polymer moulds: [0033] Their very low thermal effusivity (<500 W K.sup.1 m.sup.2 s.sup.1/2) makes it possible to fill complex geometries, [0034] Their elasticity, particularly in elastomers, allows them to conform precisely to the surface of the horology component, especially the bezel in the example illustrated, and to compensate for its manufacturing tolerances, such as the angles, [0035] Their elasticity also allows the creation of 3D decorations with sunken reliefs that would not be possible with a rigid mould, [0036] The cost of manufacturing a polymer mould is much lower than that of a metal or ceramic mould.

[0037] In a third step, also shown in FIG. 3, the liquid alloy 8 is injected via the through hole 4 acting as an injection channel 5 from the lower face 3b of the bauche 3. The alloy is heated and injected at a temperature equal to or above its solidus temperature when using a conventional injection method. When using a rapid heating method for injecting a preform that is at least partially amorphous (>50%), such as rapid discharge forming, the injection temperature is set so that the viscosity is under 1,000 Pa.s. During injection, the mould can be heated to a temperature below or equal to the glass transition temperature of the amorphous alloy being injected.

[0038] In a fourth step shown in FIG. 4, the mould is removed. The resulting bauche 3, which corresponds to the final or near-final part, has the amorphous metal alloy overmoulds 2 on its upper face 3a. On its lower face 3b, a foot 9 formed with the injection alloy 8 solidified in the injection channel is left. This foot can be kept on the final part. Alternatively, the lower face can be trued to remove this excess material. The resulting part 1 shown in FIG. 5 has an overmould 2 that does not require any truing after injection into the mould. Of course, a finishing step can still be carried out on the overmoulds to obtain specific surface finishes or to machine non-mouldable geometries.

[0039] It should be noted that the method also makes it possible to make an overmould 2 that is flush with the upper face 3a to be decorated; in this case, the cavity in the mould 6 as described above is not present and, if necessary, the bauche 3 comprises an impression 10 (FIG. 7). The method also makes it possible to produce a negative overmould, in which case, instead of the cavity of the injection mould, an impression 7 forms a protrusion that is inserted into the through hole 4 during injection (FIG. 6).