CH722221A2 - Overmolding process - Google Patents

Abstract

The item is, for example, a watch component.

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, the overmolding forming a decoration, also called a motif, intended to be applied to an article or forming an article as such.

Technological background

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

[0003] EP 2 315 673 and CH 715 132, which describe processes for embedding amorphous metal in recesses made in the article, can be cited. These processes are not well suited for producing raised overmoldings and require a finishing step to rectify the surface of the overmoldings.

[0004] From document EP 2 370 865, a method for decorating an article with a relief design made of an amorphous metal is known. The method consists of using a mask through which the metal is injected. There is therefore an excess of amorphous metal in the aesthetic areas, and these areas must then undergo a finishing step.

[0005] In injection molding processes, several problems arise with amorphous metal. 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 geometries of the injected parts, 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 in 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 serving as an injection mold intended to be overmolded. The blank is made of a material with low thermal effusivity or coated with a layer of a material with low thermal effusivity in order to minimize heat extraction by the latter during overmolding. It is also proposed to inject from a face opposite or adjacent to another face intended to be overmolded via through holes made within 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 intended to be overmolded, via through holes made within the blank and opening into a cavity forming the negative of the overmolding to be performed. According to the invention, the cavity can be provided in the blank and/or in the injection mold receiving 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 proposed 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 to faithfully reproduce the geometry and surface finish of the cavity. Consequently, the overmoldings are finished from a geometric and aesthetic point of view in the as-injection state, and therefore no further machining is necessary.

[0009] Using a blank made entirely or partially from a material with low thermal effusivity allows for very small through holes without the risk of the metal solidifying before reaching the mold cavity. These very small through holes prevent weakening the blank and allow for the creation of small overmoldings.

[0010] In summary, the overmolding process according to the invention allows for greater flexibility in the geometry of the overmoldings, improves the reproduction of the surface finish of the mold and/or the blank on the overmolding, and enables the creation of "net-shape overmoldings." This process thus makes it possible to produce overmoldings with complex shapes.

[0011] More specifically, the invention relates to an overmolding process for producing an overmolding forming a decoration to be applied to an article or to an article as such, said overmolding being in a metallic alloy that is at least partially amorphous, the process comprising the steps of:

–

Provision of a blank serving as an injection tool with a first face intended to be overmolded and a second face serving as an entry point for the injection of the metallic alloy in the liquid state, 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 metallic alloy, said blank being made of a first material having a thermal effusivity less than or equal to 7000 W K<-1>m<-2>s<1/2>, preferably less than or equal to 3500 W K<-1>m<-2>s<1/2> or being at least partly coated with a layer made of said first material,

–

Provision of an injection mold, said injection mold and/or the blank including an impression forming the negative of the overmolding to be carried out,

–

Positioning of the blank within the injection mold with the through hole positioned opposite 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 mold cavity to obtain the blank with the overmolding,

–

Demolding the blank with the overmolding,

–

Separation of the overmolding from the blank to form the decoration to be added or the article as such.

Brief description of the figures

Figures 1 to 5 schematically represent the steps of the process according to the invention.

Figure 6 represents the overmolding forming a decoration applied to a bezel.

Figure 7 shows overmoldings forming respectively an anchor and a wheel.

Figure 8 represents a variant of Figure 3 where the cavity for overmolding is made only in the blank.

Figure 9 represents another variant of Figure 3 where the cavity for overmolding is made in the blank and in the mold.

Detailed description of the invention

[0013] The invention relates to a method for creating an overmolding that is either a decorative element to be applied to an article or an article in itself. 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. For example, as shown in Figure 7, the overmolding 2 forming the article in itself is an anchor 11 or a wheel 12. The decoration may, again by way of example, be indices and numerals to be applied to a watch component. Typically, the overmolding 2, also called decoration, is applied to a dial or bezel 1 (fig.6).

[0014] The process is more specifically adapted for overmolding 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.

[0015] The process is illustrated in Figures 1 to 5 for one variant and in Figures 8 and 9 for other variants. The process will be illustrated below for the variant shown in Figures 1 to 5, where the cavity for overmolding is formed in the injection mold. For the variants shown in Figures 8 and 9, the concept of the process is the same, with the only difference being that the cavity for overmolding is formed, respectively, in the blank or in both the blank and 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 portions that may or may not be connected.

[0017] In a first step (Fig. 1), a blank 3 serving as an injection tool is made available. This blank is made of a material having low thermal effusivity. Low thermal effusivity is defined as a value less than or equal to 7000 W K⁻¹ m⁻² s⁻¹/², or even less than or equal to 3500 W K⁻¹ m⁻² s⁻¹/². This material can be a ceramic (zirconia, Macor, etc.), a metal (titanium or titanium alloy, etc.), or a synthetic material such as a polymer, and in particular a silicone or a rubber. For example, it could be a ceramic such as zirconia, which has a thermal effusivity of 2400 W K⁻¹ m⁻² s⁻¹/². If the rough has a thermal effusivity greater than 7000 W K<-1>m<-2>s<1/2>, it can be at least partially coated with a surface layer which has a thermal effusivity less than or equal to 7000 W K<-1>m<-2>s<1/2>, or even less than or equal to 3500 W K<-1>m<-2>s<1/2>.

[0018] The blank 3 is provided with 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 intended to be overmolded, and another face 3b, which serves as the entry point for injection. This other face may be opposite face 3a or possibly adjacent to face 3a. In the illustrated example, face 3a is the upper face and face 3b is the opposite face, which is therefore the lower face. The hole 4 opens at the location where the overmolding is to be performed 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 performed, the blank has one or more through holes per overmolding. Preferably, to avoid weakening the blank material, through holes have a small cross-section. This small cross-section hole can be produced, for example, by a femtosecond laser. The hole cross-section 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 Figure 2. In some variations, this allows for easy release of the overmolding with the alloy solidified in the injection channel after the process. The hole can have a circular, oblong, or other cross-section. For example, holes with an oblong cross-section are suitable for increasing the feed area on elongated overmoldings such as indices. The holes can also be non-straight but rather have bends or complex shapes to allow for more flexible positioning of the injection entry point.

[0019] 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.

[0020] For example, for a through hole between the top and bottom faces of the blank, for a blank thickness between 0.3 and 3 mm, the diameter-to-length ratio range of the hole will be between 0.08 and 1.7 mm for a circular cross-section diameter between 0.25 and 0.5 mm. Generally, this diameter-to-length ratio of 0.08 to 1.7 mm can be maintained regardless of the blank thickness. Furthermore, the cross-section of the through holes at the top face is less than or equal to the cross-section of the overmoldings to be filled, projected onto the plane of the top face to prevent the alloy from overflowing beyond the overmolding. Preferably, the cross-section of the through holes at the top face is less than the cross-section of the overmoldings projected onto the plane of the top face. More preferably, the cross-section of the through holes at the top face is smaller by a factor of at least 1.2 compared to the cross-section of the overmoldings projected into the plane of the top face.

[0021] In a second step (Fig. 3), the blank 3 is placed in a mold 6, one of whose two parts has an impression 7 forming a cavity with a shape corresponding to the shape of the overmolding to be produced. Alternatively, in Figure 8, the mold may be without an impression, and the blank 3 has an impression 13 forming a cavity. Alternatively, in Figure 9, the mold 6 has an impression 7, and the blank 3 also has an impression 13 forming the cavity, with the two cavities designed to communicate. The overmolding will therefore fill the cavities of the blank and the mold.

[0022] 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 W K<-1>m<-2>s<1/2>, preferably less than or equal to 3500 W K<-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 W K-1 m-2 s1/2) allows them to fill complex geometries.

–

Their elasticity, particularly for elastomers, allows them to precisely conform to the surface of the blank, and to compensate for manufacturing tolerances, such as angles,

–

Their elasticity also allows for the creation of 3D decorations with undercuts that would not be possible to demold with a rigid mold.

–

The manufacturing cost of a polymer mold is much lower than that of a metal or ceramic mold.

[0023] For similar reasons, the choice of polymer material will also be appropriate for the blank.

[0024] In a third step, also shown in Figure 3, the 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 higher than its solidus temperature in the case of using a conventional injection process. In the case of using a rapid-heat injection 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 less than 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 step not shown, the mold is removed.

[0026] Next, in a fifth step, visible in Figures 4 and 5 for two different variants, the overmolding 2 is separated from the blank 3. According to the variant in Figure 4, it is separated by grinding the few attachment points between the overmolding and the tool. Then, the overmolding is attached to the article to be decorated, as shown in Figure 6. According to the variant in Figure 5, the overmoldings 2, along with the sprues 10 of the amorphous alloy from the injection channels 5, are separated from the blank. The assembly can be detached by shrinking the alloy during solidification. Detachment will be facilitated if the through holes have a conical geometry as shown in Figure 2. These sprues can form attachment means for attaching the decoration to the article. For example, they can form attachment feet to be inserted into the article. In this variant, before the removal of the foot, the foot 9 formed on the underside with the injection alloy 8 solidified in the injection channel is removed. This foot can be removed by mechanically grinding the underside.

[0027] According to the invention, it is also possible to use the process described above to manufacture overmoldings which, as such, constitute an article. Thus, the overmoldings can be functional parts such as an anchor 11 or a wheel 12 as shown in Figure 7. Just as with the applied decoration, the part can have on its underside a relief resulting from the cores formed by the solidification of the amorphous metal in the injection channels.

[0028] The process may optionally include an additional, unshown finishing step on the overmoldings in order to obtain particular surface conditions or to machine non-moldable geometries.

Claims (14)

1. Overmolding process for producing an overmolding (2) forming a decoration to be applied to an article (1) or to an article as such, said overmolding (2) being in a metallic alloy (8) that is at least partially amorphous, the process comprising the steps of: – Providing a blank (3) acting as an injection tool 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 produced, 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, 13) 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 form of the metal alloy (8) from the second face (3b) of the blank (3) through the through hole (4) opening onto the cavity (7, 13) to obtain the blank (3) with the overmolding (2), – Demolding of the blank (3) with the overmolding (2), – Separation of the overmolding (2) from the blank (3) to form the decoration to be applied or the article itself. 2. 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 W K<-1>m<-2>s<1/2>, preferably less than or equal to 3500 W K<-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 overmolding (2) and at least one sprue (10) formed by the solidification of the liquid alloy (8) in the injection channel (5) are separated from the blank (3), the assembly with the overmolding (2) and the sprue (10) forming the decoration to be added or the article as such. 4. Overmolding method according to the preceding claim, wherein the assembly is separated by detachment. 5. 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). 6. 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). 7. 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. 8. 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. 9. 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 inferior face of the blank (3) opposite said upper face (2a), the through hole (4) extending between the upper face and the inferior face through the thickness of the blank (3). 10. Overmolding method according to claim 9, 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. 11. Overmolding method according to claim 9 or 10, wherein the through hole (4) forms a cone flaring from the lower face to the upper face. 12. Overmolding process according to any one of claims 2 to 11, wherein the first material and/or the second material is a polymer. 13. Overmolding process according to the preceding claim, wherein the first material and/or the second material is a silicone or a rubber. 14. Overmolding process according to any one of the preceding claims, wherein the article is a watch component.