OVERMOULDING METHOD

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No. 24207618.0, filed on Oct. 18, 2024, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The invention relates to an overmoulding method suitable for making an overmould in at least partially amorphous metal alloy, the overmould forming a decoration, also referred to as a pattern, designed to be added to a part or forming a part itself.

TECHNOLOGICAL BACKGROUND

Several documents describe methods for overmoulding a part with amorphous alloy patterns.

For example, documents EP 2 315 673 and CH 715 132 describe methods for inlaying amorphous metal into recesses in the part. These methods are not well suited to making relief overmoulds and require a finishing step to true the surface of the overmoulds.

Document EP 2 370 865 describes a method for decorating a part with a relief decoration made of amorphous metal. The method consists of using a mask through which the metal is injected. This leaves excess amorphous metal on the aesthetic zones, which must therefore undergo a finishing step.

Injection moulding methods pose several problems when it comes to amorphous metal. The mould cannot be heated to a temperature approaching the melting point because the cooling rate must be sufficiently high to prevent crystallisation. Moreover, the use of conventional metal moulds requires injection channels with cross-sections similar to the thickness of the cavity to be filled in order to prevent premature solidification in the channels, which would keep the alloy from reaching the cavity. This negatively affects the flexibility of the geometries of the injected parts, particularly for small parts that have to be filled with a layer of alloy covering the entire face of the part.

SUMMARY OF THE INVENTION

The present invention aims to remedy the aforementioned drawbacks by proposing an overmoulding method which, on one hand, enables injection with no solidification issues in the injection channels and no risk of crystallisation of the amorphous metal and, on the other hand, produces an overmould that does not necessarily require truing after injection.

To this end, it is proposed to start with an ébauche used as an injection tool designed to be overmoulded. The ébauche is made of a material with low thermal effusivity or plated with a layer of a material with low thermal effusivity so as to minimise heat extraction by the latter during the overmoulding process. It is also proposed to inject from a face opposite or connected to another face intended to be overmoulded via through holes provided within the ébauche and opening onto a cavity forming the negative of the overmould to be made. In other words, it is also proposed to inject from a portion of a surface, opposite or connected to another portion of the surface to be overmoulded via through holes provided within the ébauche and opening onto a cavity forming the negative of the overmould to be made. According to the invention, the cavity can be formed in the ébauche and/or in the injection mould holding the ébauche.

The low thermal effusivity of the ébauche guarantees that there is no solidification in the through holes upstream of the cavity and thus that the alloy is injected into the cavity in a liquid state. Preferentially, for optimal results, it is also recommended to use an injection mould made of a material with low thermal effusivity. This enables the alloy to conform perfectly to the shape of the mould cavity and/or of the ébauche to accurately reproduce the geometry and surface finish of the cavity. As a result, the overmoulds are geometrically and aesthetically finished in their as-injected state, with no further truing required.

The use of an ébauche made entirely or partially from a material with low thermal effusivity makes it possible to create through holes with very small cross-sections with no risk of the metal coagulating before reaching the cavity in the mould. These very small through holes keep the ébauche from becoming brittle and enable small overmoulds to be created.

To sum up, the overmoulding method according to the invention makes it possible to achieve greater flexibility in the geometries of the overmoulds, improve the reproducibility of the surface finishes of the mould and/or of the ébauche on the overmould and produce “net-shape overmoulds.” This method therefore makes it possible to produce overmoulds with complex shapes.

More specifically, the invention relates to an overmoulding method for making an overmould forming a decoration to be added to a part or a part itself, said overmould being made of an at least partially amorphous metal alloy, the method comprising the following steps:

• • Providing an ébauche used as an injection tool with a first face to be overmoulded and a second face acting as an entry point for injecting the liquid metal alloy, said ébauche being provided with at least one through hole extending between the first face and the second face and opening out onto the overmould to be made, the through hole forming a channel for injecting the metal alloy, said ébauche being made of a first material with a thermal effusivity less than or equal to 7,000 W K⁻¹ m⁻² s^1/2, preferably less than or equal to 3,500 W K⁻¹ m⁻² s^1/2, or being at least partially plated with a layer made of said first material,
  • Providing an injection mould, said injection mould and/or the ébauche comprising an impression forming the negative of the overmould to be made,
  • Positioning the ébauche inside the injection mould with the through hole arranged facing the impression of the injection mould if said injection mould comprises an impression,
  • Injecting the liquid metal alloy from the second face of the ébauche through the through hole opening onto the impression to obtain the ébauche with the overmould,
  • Unmoulding the ébauche with the overmould,
  • Detaching the overmould from the ébauche to form the decoration to be added or the part itself.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 to 5 schematically show the steps in the method according to the invention.

FIG. 6 shows the overmould forming a decoration added to a bezel.

FIG. 7 shows overmoulds forming a pallet and a wheel, respectively.

FIG. 8 shows a variant of FIG. 3 in which the impression for the overmould is made only in the ébauche.

FIG. 9 shows another variant of FIG. 3, in which the impression for the overmould is made in the ébauche and in the mould.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method for producing an overmould which is a decoration to be added to a part or which is a part itself. The part can also be a horology component, for example. 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. For example, as shown in FIG. 7, the overmould 2 forming the part itself is a pallet 11 or a wheel 12. As a further example, the decoration can be indicators and numerals to be added to a horology component. Typically, an overmould 2, also known as decoration, is added to a dial or a bezel 1 (FIG. 6).

The method is more specifically suited to overmoulds 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.

The method is illustrated in FIGS. 1 to 5 for one variant and in FIGS. 8 and 9 for other variants. The method will be illustrated below for the variant shown in FIGS. 1 to 5, in which the impression for the overmould is formed in the injection mould. For the variants in FIGS. 8 and 9, the concept of the method is the same, with the only change being that the impression for the overmould is formed respectively in the ébauche or in the ébauche and in the injection mould.

In addition, the method is illustrated below for an ébauche 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 ébauche acting as an entry point for the liquid alloy relative to another portion of the surface of the ébauche acting as an exit point for the liquid alloy, with portions that may or may not be connected.

In a first step (FIG. 1), an ébauche 3 used as an injection tool is provided. This ébauche 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 K⁻¹ m⁻² s^1/2, or even less than or equal to 3,500 W^K−1 m⁻² s^1/2. This material can be ceramic (zirconia, Macor, etc.), metal (titanium or titanium alloy, etc.) or a synthetic material such as a polymer, particularly a silicone or a rubber. For example, it could be a ceramic such as zirconia that has a thermal effusivity of 2,400 W K⁻¹m⁻²s^1/2. If the ébauche has a thermal effusivity greater than 7,000 W K⁻¹ m⁻² s^1/2, 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 K⁻¹ m⁻² s^1/2, or even less than or equal to 3,500 W K⁻¹ m⁻² s^1/2.

The ébauche 3 has one or more through holes 4. According to the invention, these through holes act as injection channels and extend between face 3a, which is the face to be overmoulded, and another face 3b, which acts as the injection entry point. This other face can be opposite face 3a or optionally connected to face 3a. In the example illustrated, face 3a is the upper face and face 3b is the opposite face, which is therefore the lower face. 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 ébauche comprises one or more holes through the overmould. Preferentially, to keep the material of the ébauche from becoming brittle, the through holes have a small cross-section. This hole with a small cross-section can be made using a femtosecond laser, for example. The cross-section of the hole can be constant or variable along its length. For example, the hole can have a conical shape flaring towards the upper face 3a as shown in FIG. 2, which enables, for certain variants, easy release of the overmould with the alloy congealed in the injection channel at the end of the process. 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 point can be positioned more flexibly.

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.

For example, for a through hole between the upper face and the lower face of the ébauche, for an ébauche thickness comprised between 0.3 and 3 mm, the range of hole diameter/length ratios will be comprised between 0.08 and 1.7 mm for a circular section with a diameter comprised between 0.25 and 0.5 mm. In general, this hole diameter/length ratio ranging from 0.08 to 1.7 mm can be kept regardless of the thickness of the ébauche. Moreover, 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 overmoulds 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.

In a second step (FIG. 3), the ébauche 3 is placed in a mould 6 with an impression 7 forming a cavity in one of its two parts, having a shape corresponding to the shape of the overmould to be made. As a variant to FIG. 8, the mould can have no impression, and it is the ébauche 3 that comprises an impression 13 forming a cavity. As a variant to FIG. 9, the mould 6 comprises an impression 7 and the ébauche 3 also comprises an impression 13 forming the cavity with the two cavities designed to communicate. The overmould will therefore fill the cavities of the ébauche and of the mould.

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⁻¹ m⁻² s^1/2, preferably less than or equal to 3,500 W K⁻¹ m⁻² s^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:

• • Their very low thermal effusivity (<500 W K⁻¹m⁻²s^1/2) makes it possible to fill complex geometries,
  • Their elasticity, particularly in elastomers, allows them to conform precisely to the surface of the ébauche and to compensate for its manufacturing tolerances, such as the angles,
  • Their elasticity also allows the creation of 3D decorations with sunken reliefs that would not be possible with a rigid mould,
  • The cost of manufacturing a polymer mould is much lower than that of a metal or ceramic mould.

For similar reasons, a polymer material is also a good choice for the ébauche.

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 ébauche 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.

In a fourth step not shown, the mould has been removed.

Then, in a fifth step that can be seen in FIGS. 4 and 5 for two different variants, the overmould 2 has been detached from the ébauche 3. According to the variant in FIG. 4, it is detached by grinding the few attachment points between the overmould and the tool. Next, the overmould is attached to the part to be decorated, as shown in FIG. 6. According to the variant in FIG. 5, the overmoulds 2 with the sprues 10 of amorphous alloy from the injection channels 5 have been detached from the ébauche. The whole part can be ejected as the alloy is removed during solidification. Ejection will be made easier if the through holes are cone-shaped, as shown in FIG. 2. These sprues can form attachment means for adding the decoration to the part. For example, they can form fastening feet to be inserted into the part. In this variant, before ejection, the foot 9 formed on the lower face with the injection alloy 8 solidified in the injection channel is removed. This foot can be removed by mechanically truing the lower face.

According to the invention, the method described above can also be used to manufacture overmoulds which themselves form a part. The overmoulds can therefore be functional parts such as a pallet 11 or a wheel 12 as shown in FIG. 7. Like the decoration to be added, the part can comprise a relief on its lower face formed by the sprues created by the solidification of the amorphous metal in the injection channels.

The method can optionally comprise an additional step, not shown, of finishing the overmoulds to obtain particular surface conditions or to machine non-mouldable geometries.