The invention relates to a method and apparatus for pressure die casting, commonly known by the English term of "die casting" of a metal alloy. The invention is more particularly, but not exclusively, dedicated to the field of molding in liquid phase or in thixocasting a light alloy based on magnesium or aluminum. Thixomolding is die casting the metal into a semi-solid state, that is to say at a casting temperature at which the liquid and solid phases coexist.

The pressure die casting of a metal alloy allows to obtain a finished part directly to the molding and is used in very large series for the manufacture of many parts used in consumer products such as carriers or casings, including smart phone, tablet computer, camera, but also parts subjected to high stresses, particularly in the automotive industry, such as fuel injection ramps or hydraulic valves without these examples are limiting. Typically, providers parts from this process are of complex shape, combining very variable thickness and areas having low thickness areas. These parts must be carried out respecting the constraints of appearance and precision tight while maintaining production rates compatible with mass production. According to this method, the material constituting the future part is heated to a suitable temperature and is then injected under pressure into the cavity of a molding temperature resistant mold and comprising two metal shells, or more. The mold is preheated to a temperature lower than the temperature of the injected material so that said material cools in contact with the mold walls. The part is cooled in the mold to a mold release temperature, at which temperature the mold is opened and the workpiece, solidified, is ejected from the mold. Before making a new part, the mold being open, the surfaces forming the cavity of said mold are sprayed with a mold release product, generally an aqueous product, ensuring the absence of coupling or bonding of the future molded part on the mold walls. The mold is then closed and the cycle repeats. As an example implementation, the metal is injected at a temperature between 550 ° C and 650 ° C depending on the material grade and molding type: liquid phase or thixomolding, while the mold is preheated at a temperature of 300 ° C. Figure 1, relating to the prior art is, Figure 1A, an exemplary thermal cycle corresponding to the method described above, showing the evolution of the temperature (102) to the surface of the cavity of a mold according time (101), changes achieved by installing a temperature sensor on one of the surfaces defining the mold cavity, or by means of2. According to this prior art, the mold is preheated by means of oil circulation in conduits practiced for this purpose in the mold. During step (1 10) of casting, the metal is injected into the mold. Said mold is preheated to a temperature (105) nominal preheating, frequently of the order of 1/3 to 1/2 of the casting temperature expressed in ° C, so that said metal solidifies in contact with the mold walls . In a step (120) of mold, the mold is opened and the part is removed from the mold in a step (130) for ejecting. During these steps, the cavity temperature is maintained near the preheating temperature. In a step (140) spraying a release agent is sprayed onto the surfaces of the molding cavity. The mold is then closed and the temperature regulating means thereof run out during a heating step (150) to bring it to the temperature (105) nominal preheating heating step which continues until restarting the cycle. The spraying step (140) significantly reduces the temperature of the surfaces of the molding cavity, so that the conventional means of heating the mold, in particular by circulation of oil, do not achieve the temperature (105) nominal suitable for preheating, while respecting the targeted production rates. heating step which continues until restarting the cycle. The spraying step (140) significantly reduces the temperature of the surfaces of the molding cavity, so that the conventional means of heating the mold, in particular by circulation of oil, do not achieve the temperature (105) nominal suitable for preheating, while respecting the targeted production rates. heating step which continues until restarting the cycle. The spraying step (140) significantly reduces the temperature of the surfaces of the molding cavity, so that the conventional means of heating the mold, in particular by circulation of oil, do not achieve the temperature (105) nominal suitable for preheating, while respecting the targeted production rates.

Indeed, in the case of heating by circulation of oil, the thermal energy transmitted by the mold oil depends on the temperature difference between the mold and the oil, so that the higher the mold temperature approaches the temperature of the oil and less transfer is effective. The oil flowing at a temperature equal or slightly higher than the nominal temperature preheating, the time to reach that temperature is again conditioned by heat exchange between the oil and the mold, which are realized on the unsupported lengths with rates referred.

Thus, Figure 1 B, the temperature reached on the surfaces of the molding cavity after the preheating step, reduces cycle to cycle. For example, for an oil temperature in flow 250 ° C, and a nominal preheating temperature under 230 ° C, the temperature (106) effective preheating during the 10 th cycle is no longer than 195 ° C and 185 ° C during 14 thcycle. For example, the cycle is of the order of one minute, the duration of the ejection stage (130) is of the order of 8 seconds and the duration of step (140) spray and closing the mold is of the order of 10 seconds. These times are variable depending on the molded material, the volume and complexity of the piece and the means used. The speeds corresponding to these times do not allow the ascent of the mold temperature by heat exchange with circulating oil. Indeed, the rise in the preheating target temperature, time considered, involves a thermal power transfer of tens of KW, which can not be reached by exchange with circulating oil, especially when the difference temperature between heating oil and the mold is reduced. It is not possible to reach the dissipation of such a heating power of the molding surfaces by conductive exchange with hot-resistant.

Thus, according to these measures, the maximum rate of heating of the molding surfaces during the step (150) reduces as the temperature difference between the oil and the mold is reduced down to the speed of order of a few degrees per minute over the last ten degrees warm.

The temperature of the molding surfaces of the cavity being colder, the metal cools more quickly in contact with the latter and loses faster fluidity resulting from quality defects of the part produced, including

blemishes or absences of material, particularly in areas of low thickness.

Document US 2016/101460 discloses a molding process comprising a spray step for a release agent of the molding surfaces of a cavity delimited by the two parts of a mold. During the spraying step to avoid thermal shocks on the molding surface and the risk of cracking, due to the high cooling rate imposed by the spraying of the release agent, this document recommends a pre- cooling of said surfaces by means of the circulation of a fluid in the mold.

The document US2016 / 101551 discloses a self-heating and cooling mold, the heating being carried out by induction by means of inductors extending in casings formed in the mold. This document does not describe spraying operations molding surfaces or cooling control of these surfaces during their spraying.

The invention aims to remedy the shortcomings of the prior art and relates for that purpose a method of die casting of a metal in a cavity, using a mold comprising:

at. two matrices each comprising a block with a molding surface, so that said molding surfaces define a molding cavity;

b. in at least one of the matrices, an inductor traveling in a casing formed in the block on the molding surface; c. a generator for supplying a high frequency current to said inductor so as to heat the walls of the casing (340); d. the inductor being located at a distance d from the molding surface so that heat conduction from the wall of the hose comprising the inductor to the molding surface through the thickness of said block, lead to a uniform temperature distribution on the molding surface;

the method comprising the steps of:

i. filling a molding cavity by injecting the metal into said cavity, said cavity being preheated to a temperature

nominal preheating T1 by the circulation of a high frequency electrical current in the inductor;

ii. solidification of the metal in the molding cavity; iii. mold opening and part ejection;

v. spraying the molding surfaces of the molding cavity, the mold being opened by a release agent;

vi. closing the mold and heating of the cavity to the temperature T1;

which process comprises after the step iii) opening the mold and prior to step v) of spray of the molding surfaces, a step of:

iv. inductively heating the molding surfaces of the cavity while the workpiece is no longer in contact with said surfaces, and continue during this heating step v) of spray.

Thus, the combination of induction heating means and the projected initiation of this heating before and during the sprinkling it possible to compensate at least partly the temperature loss due to spraying of the surfaces of the cavity. Unlike the means of the prior art that require heating to heat the mold in its mass, induction heating concentrates its effects on the molding surfaces and thus can heat evenly these surfaces in a very short time, while the mold is opened, by providing said surfaces in a heating power of several tens of KW, no effect of the temperature of said surfaces on the heating efficiency. So,

The invention is preferably implemented according to the embodiments and variations described below, which are considered individually or in any technically operative combination.

According to one embodiment of the method of the invention, the latter comprises between step i) and step ii) forced cooling of the molding cavity. This embodiment thus makes it possible to fill the cavity at an elevated preheat temperature, ensuring the fluidity of the material and the uniform filling thereof, while controlling the material of the cooling cycle and by limiting the influence of time cooling the cycle time.

According to one embodiment, the forced cooling is performed by circulation of a coolant fluid in a conduit formed in the mold.

Advantageously, the temperature T1 is between 200 ° C and 400 ° C, preferably between 250 ° C and 300 ° C. These preheating temperatures, unattainable in length by heating systems by oil circulation or electrical resistance in the target cycle time, are particularly suitable for the implementation of magnesium alloys, alloys aluminum or zinc alloys, without these examples be limiting, high preheating temperatures also have a beneficial effect on the mechanical and metallurgical characteristics of the parts, including obtaining finer grains or absence of porosity .

Advantageously, the heating rate during step vi) is greater than 2 ° C. s 1 and preferably of the order of 5 ° C. s "1 . The concentration of the heating action on the walls of the molding cavity achieves such a heating rate with a reduced energy consumption and this independently of the mold surface.

Advantageously, the temperature of the molding surfaces reached during step iv) and prior to step v) is greater than T1. This controlled overheating of the molding surfaces while the workpiece is no longer in contact with said surfaces, limits the minimum temperature reached during spraying. As warming in step v) is faster.

Advantageously, the molding cavity is heated to a temperature between 200 ° C and 400 ° C, the metal alloy implemented by the method of the invention is a type of AM20 magnesium alloy, AM50, AM60 and AZ91 D . Thus the method of the invention allows the molding of such materials deemed difficult to mold in cycle time conditions compatible with a mass production.

Advantageously, the metal alloy is an alloy of aluminum and silicon comprising less than 2% of silicon, for example an alloy of Al-Mg-Si-Mn.

This type of aluminum alloy is anodizable, has a higher solidification start temperature as alloys of Al-Si foundry conventional, which results in better mechanical properties and increased temperature stability, to the detriment of its ease of molding. The method of the invention allows the implementation of such a material reproducible manner in mass production conditions.

The inventive subject method is also suitable for die casting of the Zamac type zinc alloys, injection molded under pressure in a hot room for the production of parts in large series.

The method of the invention is suitable for molding of metal alloys, injected in the liquid phase in step i). It is also suitable to Thixomolding these alloys injected in semi-solid phase in step i).

Advantageously, the block with the molding surface is composed of a type of steel HTCS 130. The high thermal conductivity and thermal diffusivity of the steel enable a more responsive temperature regulation of molding surfaces.

According to an alternative embodiment of the subject tools of the invention, the block carrying the molding surface is formed of a non-ferromagnetic material, wherein the casing comprising the inductor is jacketed with a layer of a magnetic material high. This embodiment is more suitable for mold casting under pressure of high melting temperature materials, or capable of reacting chemically with the ferrous metal at the casting temperature.

The invention is explained below according to its preferred embodiments, in no way limiting, and with reference to Figures 1 to 5, wherein:

- Figure 1 relating to the prior art shows, according to time-temperature diagrams, changes in the temperature of the molding cavity surface of a pressure die casting mold preheated by circulating oil, 1A during a molding cycle, and fig 1B in a plurality of successive molding cycles;

- Figure 2 is a schematic sectional view of the matrices defining the molding cavity of a tool suitable for injection molding of a material

metallic ;

- Figure 3 shows, in a sectional view, an exemplary embodiment of one of the matrix of a tool according to the invention suitable for injection molding a metallic material;

- Figure 4 shows in a detail view an embodiment of the inductors installation in a matrix, as shown in FIG 3, consisting of a non-ferromagnetic material;

- and Figure 5 illustrates a heat cycle of the molding surfaces of a pressure die casting mold by the implementation of the tooling and the method objects of the invention compared to the thermal cycle shown in Figure 1A.

Figure 2, in a block diagram of an embodiment of the tooling object of the invention, it comprises two dies (210, 220) and means (not shown) for said dies toward and away one of the other, so as to close and open the mold. When the mold is closed, a molding cavity is formed, cavity defined by the molding surfaces (21, 1 221) of said matrices.

Only elements of the essential tools for the implementation of the invention described here are the other features of the tool being known to the expert in the field of pressure die casting. Thus, the tooling matrices object of the invention include the intake ducts of the molded material in the molding cavity of the tool as well as means for ejecting the molded part after solidification.

Figure 3, according to an embodiment of the tooling object of the invention, one of the dies (210), and preferably both matrices include induction heating means comprising a plurality of casings (340) in which cheminent inductors realizing an induction circuit. Said inductors (341) are, for example, made of a tube or of a copper braid, isolated from the die wall by a tube (342) ceramic, e.g., a sheath of silica, transparent screw -to-vis the magnetic field generated by said inductor. copper braid inducers are preferred for monitoring winding paths having small radii of curvature. The path of the inductors is determined in particular by thermal simulation in order to

Advantageously, the matrix (210) is made in two parts (31 1, 312). Thus, the casings (340) for the passage of the inductors are formed by grooving said parts before their assembly.

A conduit or plurality of conduits (350) for cooling are arranged in the matrix (210), by drilling or by scribing and assembly, as for casings receiving inductors. This duct (350) allows the movement, by suitable means, a heat transfer fluid in said matrix to provide cooling. Said coolant circulates in said ducts at a temperature very much lower than the temperature T1 in order to ensure rapid cooling. According to alternative embodiments, the heat transfer fluid circulates in the liquid phase, for example if said fluid is an oil, or gas phase, if said fluid is air or other coolant gas. Advantageously, the cooling system includes a refrigeration unit (not represented) for cooling the coolant to a temperature below room temperature. The circulation of the coolant used to cool the die (210) and more particularly the molding surface (211). According to alternative embodiments, the conduit (350) for cooling is arranged on the same plane as the inductors and is located at a distance equivalent to the molding surface, or the duct (350) for cooling is placed at a higher distance the molding surface as the inductors, the latter being then between the cooling pipe and the molding surface, this embodiment favoring the heating rate compared to the rate of cooling, or, the cooling duct is positioned between the molding surface and the inductors, this embodiment favoring the cooling rate. The circulation of the coolant and induction heating are used together for the purpose of regulating the temperature or the cooling rate. A temperature sensor (360), for example a thermocouple, is preferably placed near the molding surface (21 1) in order to measure its temperature and, where appropriate, enslaving the heating conditions and cooling. The use of oil as a cooling heat transfer fluid ensures the cooling of the mold under the conditions of implementation of a die casting under pressure of a light alloy of aluminum, magnesium, or zinc,

The block (31 1) of material comprising the molding surface (21 1) is sufficiently thick, so that the casings (340) in which are placed the inductors (341) are distant by a distance d of said molding surface, to that the latter is heated, at least in part, by conduction of the heat produced by elevating the temperature of the walls of said casings (340), this temperature rise of the use of a high frequency electric current in the inductor (341). Thus, the temperature distribution resulting from the implementation of the induction heating is uniform over said molding surface. The distance d is for instance determined by numerical simulation of the heating based on the properties of the materials present.

The block (31 1) on the molding surface (21 1) is made of a metallic material to have a thermal conductivity and a sufficient thermal diffusivity for the implementation of the phases of heating and cooling of the method of the invention. Advantageously said material is ferromagnetic, for example steel or a martensitic-ferritic martensitic whose Curie temperature is equal to or higher than the preheating target temperature molding process. For example, for die casting under pressure of a light alloy block (31 1) on the molding surface is composed of a type of steel DIN 1.2344 (AISI H13, IN X40CrMoV5-1) or DIN 1.12343 (AISI H1 1 IN X38CrMoV5-1). Advantageously, said block is made of a steel of Tooling as described in EP 2236639 and distributed commercially under the name HTCS 130® by the company ROVALMA SA, 08228 Terrassa, Spain. This steel has a thermal conductivity and high thermal diffusivity, which reduces the cycle time.

The inductors (341) are connected to a high frequency current generator, typically a frequency between 10 kHz and 200 kHz, by means (not shown) able to tune the resonant circuit resulting, particularly, but not exclusively, to a cabinet capacity and a matching coil impedance, as described in WO 2013/021055. The high-frequency current generator and the tuning means of the resonant circuit are selected so as to provide a heating power by induction of the molding surface (21 1) of the order of 100 kW. According to alternative embodiments, depending in particular on the size of the mold, the two dies forming the mold are connected to the same high-frequency generator or to two different generators.

Figure 4, in another embodiment, the block constituting material (31 1) on the molding surface of the matrix is ​​not ferromagnetic. In this case, according to an exemplary embodiment, the casings comprising inductors (441) are lined with a layer (443) of high magnetic permeability of steel and advantageously retaining its ferromagnetic properties to high temperature, eg 700 ° C . Thus, the magnetic field produced by the inductor (441) is concentrated in the liner (443) rapidly rises in temperature and transmits this temperature by conduction in the matrix. The heat is transmitted by conduction to the molding surface, the judicious arrangement inductors allows, as before, to ensure a uniform temperature on the molding surface.® , without these examples are limiting.

When the block (31 1) consists of a ferromagnetic steel, inducers of heating action is split between a direct induction heating of the molding surfaces and the conduction of heat from the walls of the conduits (340) comprising the inductors. The distribution of energy between these two modes of heating depends on the distance. When the block (311) consists of non-ferromagnetic material, a similar effect is obtained by depositing, on the molding surfaces, a ferromagnetic coating, for example a coating

nickel.

5, the comparison of thermal cycles (501, 502) suffered by the molding surfaces between the heat cycle (501) resulting from a heating mold by circulating oil and thermal cycling (502) resulting from the implementation work of the tooling object of the invention, shows that the time (520) required for the temperature (105) of preheating from the beginning of phase (140) spraying the molding surfaces is reduced. This effect is related to the ability to provide the molding surfaces greater heating power by induction heating means, in comparison of the means of the prior art, and thus obtain a heating faster speed, the order of 5 ° Cs "1 of said molding surfaces, of2and a heating power of approximately 100 kW. In addition, the use of induction heating can trigger heating molding surfaces during step (130) for ejecting the piece at a time (510) subsequent to the ejection of the part, but prior to the beginning step (140) of spray. This early onset of the induction heating is performed when the molding surfaces are approximately at the temperature (105) face of preheating the molding cavity. Said heating causes wearing said surfaces at a temperature (505) higher than said temperature (105) for preheating, in order to limit the consequent drop in temperature in step (140) of spray. The heating power delivered by the inductors on the molding surfaces is sufficient to achieve this without slowing heating step (130) for ejecting and without delaying step (140) of spray. Thus, the combination of the earlier start of the heating, the overheating of the molding surface at a temperature (505) higher than the temperature (105) nominal preheating allows, firstly, to ensure obtaining the temperature (105) preheating referred on the molding surfaces, in the cycle time in question, and thus ensure the constancy of the quality of the parts produced during successive cycles, thus reducing the scrap rate. Moreover, this same combination of resources and implementation method,

same time as improving the reliability of the method. The molding surfaces being at a temperature close to the temperature (105) nominal preheating when the anticipated heating said surfaces is triggered, the implementation of this measurement by means of a heating circulating oil would have no effect, the near the temperature of the circulating oil and that of the mold does not allow the realization of a heat exchange between the oil and the material constituting the mold.

The combination of induction heating means and cooling means of the molding surface of the tooling object of the invention is used to regulate the mold temperature and the molded filler material during the step (1 10 ) casting. Thus, the tooling object of the invention is used to inject the metal alloy in a hot mold, to ensure a better filling thereof, while ensuring a sufficiently quick cooling of the material, particularly to avoid the occurrence of porosities or inhomogeneous grain size. Unlike the prior art, where the thermal kinematic phase (1 10) of melt is dictated by the passive heat transfer between the mold and the material, the implementation of the tooling object of the invention allows regulate, at least in part, this kinematics. Thus, the method implemented by means of the tooling object of the invention allows to improve the intrinsic quality of the castings by this process.

The ability to pre-heat the molding surface at a higher temperature and to maintain and regulate the temperature during step (1 10) of casting, allows the implementation of alloys whose solidification start temperature is higher, while ensuring the filling of the molding cavity, in particular aluminum alloys comprising less than 2% silicon, relative to hypoeutectic AlSi system, maintaining production rates comparable to those obtained for eutectic or near-eutectic alloys. Thus, the process and tooling object of the invention facilitate the implementation of alloys with higher mechanical characteristics, in particular the Al-Si-Mg alloys, Al-Mg-Si and Al-Mg-Si-Mn, and carried out by molding in large series of alloys

The effects of the method of the invention implementing a tool

comprising an induction heater and described above are not limited to the molding surfaces of the tool but also apply to the material supply channels formed in the matrix. Although the method and tools objects of the invention are described as applied to one of the dies, they are applicable to all of the matrices defining the molding cavity of the tool. According to exemplary embodiments, the inductors ensuring the heating of the molding surfaces of said dies are connected to a single high-frequency current generator or generators dedicated to each matrix.

The above description and the embodiments show that the invention achieves the intended purpose, and allows, compared to the prior art, increase production rates, improve the repeatability of the quality of parts molded, improve the metallurgical quality and the quality of execution of said pieces and open up the possibility of implementing flowability harder materials on the same terms of productivity and repeatability.

CLAIMS

Method for die casting a metal in a cavity, using a mold comprising:

at. two dies (210, 220) each comprising a block (31 1) having a molding surface (211, 221), so that said molding surfaces define a molding cavity;

b. in at least one of the matrices, an inductor (341, 441) running in a casing (340) formed in the block (31 1) on the molding surface;

c. a generator for supplying a high frequency current to said inductor (341, 441) so as to heat the walls of the casing (340);

d. the inductor (341, 41) being placed at a distance d from the molding surface so that heat conduction from the wall of the casing (340) comprising the inductor to the molding surface through the thickness of said block ( 31 1), leads to a uniform distribution of temperature on the molding surface;

the method comprising the steps of:

i. filler (1 10) the molding cavity by injecting the metal into said cavity, said cavity being preheated to a preheating temperature rating of T1 (105) by the circulation of a high frequency electrical current in the inductor (341); ii. solidification of the metal in the molding cavity;

iii. opening (120) of the mold and ejecting (130) of the workpiece;

v. sprinkling (140) of the molding surfaces of the molding cavity, the mold being opened by a release agent;

vi. closing the mold and heating (150) of the cavity at the temperature T1 (105);

characterized in that it comprises, after step iii) opening the mold and prior to step v) of spray of the molding surfaces, a step

consists in :

iv. inductively heating the molding surfaces of the cavity while the workpiece is no longer in contact with said surfaces, and continue during this heating step v) of spray.

The method of claim 1, further comprising between step i) and step ii) forced cooling of the molding cavity.

The method of claim 2 wherein the forced cooling is performed by circulation of a coolant fluid in a conduit (350) formed in the mold.

The method of claim 1, wherein the temperature T1 (105) is between 200 ° C and 400 ° C, preferably between 250 ° C and 300 ° C.

The method of claim 4, wherein the temperature (505) of the molding surfaces reached during step iv) and prior to step v) is greater than T1 (105).

A method according to claim 4, wherein the metal is an alloy of:

a magnesium alloy AM20 type AM50, AM60 and AZ91 D, or

an aluminum alloy comprising less than 2% of sillicium, in particular of Al-Mg-Si-Mn, or

an aluminum zinc alloy, magnesium and copper Zamac type.

A method according to claim 6, wherein the metal is injected in the liquid phase during step i).

A method according to claim 6, wherein the metal is injected in a semi-solid phase during the step i).

The method of claim 1, wherein the block (31 1) on the molding surface is composed of a type of steel HTCS 130.

The method of claim 1, wherein the block on the molding surface is formed of a non-ferromagnetic material, wherein the casing comprising the inductor is jacketed with a layer (443) of a high magnetic permeability material.