EP3448599B1 - Device for shell-moulding a metal alloy - Google Patents

Description

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

The pressure shell molding of a metal alloy makes it possible to obtain a finished part directly during molding and is used in very large series for the manufacture of numerous parts used in consumer products such as supports or housings, in particular smart phone, tablet computer, camera, but also parts subject to high stresses, especially in the automotive industry, such as fuel injection bars, or hydraulic distributors without these examples being limiting. Typically, the parts liable for this process are of complex shape, combining zones of very variable thickness and comprising zones of small thickness. These parts must be produced while respecting tight appearance and precision constraints, while maintaining production rates compatible with mass production. According to this process, the material constituting the future part is brought to a suitable temperature, then is injected under pressure into the cavity of a mold resistant to the molding temperature and comprising two or more metal shells. The mold is preheated to a temperature lower than the temperature of the injected material, so that said material cools in contact with the walls of the mold. The part is cooled in the mold to a demolding temperature, the temperature at which the mold is opened and the solidified part is ejected from the mold. Before making a new part, the mold being open, the surfaces constituting the cavity of said mold are sprayed with a release agent, generally an aqueous product, ensuring the absence of attachment or bonding of the future molded part to the walls of the mold. The mold is then closed and the cycle begins again. As an example of implementation, the metal is injected at a temperature between 550 ° C and 650 ° C depending on the grade of material and the type of molding: in liquid phase or in thixomoulding, while the mold is preheated at a temperature of 300 ° C. The figure 1 , relating to the prior art represents, figure 1A , an example of a thermal cycle corresponding to the process described above, showing the evolution of the temperature (102) at the surface of the cavity of a mold as a function of time (101), evolution obtained by installing a temperature probe on one of the surfaces delimiting the mold cavity, or even by means of an infrared thermography of said surface, said mold being made of a tool steel of type DIN 1.2343 (AISI H11, EN X38CrMoV5-1) and being intended for molding of a fine piece of magnesium alloy, the projected surface of the imprint being 200 x 300 mm ² . According to this prior art, the mold is preheated by means of an oil circulation in conduits made for this purpose in the mold. During the casting step (110), the metal is injected into the mold. Said mold is preheated to a nominal preheating temperature (105), frequently of the order of 1/3 to 1/2 of the casting temperature expressed in ° C, so that said metal solidifies on contact with the walls of the mold . During a demolding step (120), the mold is opened, then the part is extracted from the mold during an ejection step (130). During these steps, the temperature of the cavity is kept close to the preheating temperature. During a spraying step (140), a release agent is sprayed onto the surfaces of the molding cavity. The mold is then closed and the temperature regulation means thereof are used during a heating step (150) to bring the latter to the nominal temperature (105) for preheating, which heating step is continues until the cycle begins again. The spraying step (140) considerably reduces the temperature of the surfaces of the molding cavity, so that conventional means of heating the mold, in particular by circulation of oil, do not allow the nominal temperature (105) to be reached. adapted preheating, while respecting the targeted production rates.

Indeed, in the case of heating by oil circulation, the thermal energy transmitted by the oil to the mold is a function of the temperature difference between the mold and oil, so that the closer the mold temperature gets to the oil temperature, the less efficient this transfer is. As the oil circulates at a temperature equal to or slightly higher than the nominal preheating temperature, the time to reach this temperature again is conditioned by the heat exchanges between the oil and the mold, which take place over periods which are not compatible with target rates.

So, figure 1B , the temperature reached on the surfaces of the molding cavity after the preheating step, decreases from 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 at the ^10th cycle is no longer than 195 ° C and 185 ° C during the 14 ^th cycle. By way of example, the duration of the cycle is of the order of a minute, the duration of the ejection step (130) is of the order of 8 seconds and the duration of the step (140) d spraying and closing the mold is of the order of 10 seconds. These durations being variable according to the molded material, the volume and the complexity of the part as well as the means implemented. The rates corresponding to these times do not allow the mold to rise in temperature by heat exchange with the oil in circulation. Indeed, the rise to the target preheating temperature, in the time considered, implies a heat transfer power of several tens of KW, which cannot be achieved by exchange with the oil in circulation, more particularly when the difference temperature between the heating oil and the mold is reduced. It is also not possible to achieve the dissipation of such a heating power on the molding surfaces by conductive exchange with heating resistors.

Thus, according to these same measurements, the maximum rate of heating of the molding surfaces during step (150) is reduced as the temperature difference between the oil and the mold is reduced, down to speeds of l 'order of a few degrees per minute on the last tens of degrees of preheating.

The temperature of the molding surfaces of the cavity being cooler, the metal cools more quickly on contact with them and loses more quickly in fluidity which results in defects in the quality of the part produced, in particular appearance defects or material shortages, especially in thin areas.

The document US 2016/101460 discloses a molding process comprising a step of spraying with a mold release agent the molding surfaces of a cavity delimited by the two parts of a mold. During the spraying step, in order to avoid thermal shocks on the molding surface and the risks 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 describes a mold with independent heating and cooling, the heating being carried out by induction by means of inductors extending in hoses formed in the mold. This document does not describe spraying operations on the molding surfaces, or controlling the cooling of these surfaces during their spraying.

1. a. deux matrices comprenant chacune un bloc portant une surface moulante, de sorte que lesdites surfaces moulantes délimitent une cavité moulante ;
2. b. dans au moins une des matrices, un inducteur cheminant dans un boyau pratiqué dans le bloc portant la surface moulante ;
3. c. un générateur pour alimenter par un courant à haute fréquence ledit inducteur de sorte à chauffer les parois du boyau (340) ;
4. d. l'inducteur étant placé à une distance d de la surface moulante de sorte que la conduction de chaleur de la paroi du boyau comprenant l'inducteur à la surface moulante, à travers l'épaisseur dudit bloc, conduise à une distribution uniforme de la température sur la surface moulante ;

le procédé comprenant les étapes de :

• i. remplissage a cavité moulante par injection du métal dans ladite cavité, ladite cavité étant préchauffée à une température nominale de préchauffage T1 par la circulation d'un courant électrique à haute fréquence dans l'inducteur ;
• ii. solidification du métal dans la cavité moulante ;
• iii. ouverture du moule et éjection de la pièce ;
• v. aspersion des surfaces moulantes de la cavité moulante, le moule étant ouvert, par un agent démoulant ;
• vi. fermeture du moule et chauffage de la cavité à la température T1 ;

lequel procédé comprend après l'étape iii) d'ouverture du moule et avant l'étape v) d'aspersion des surfaces moulantes, une étape consistant à :
iv. chauffer par induction les surfaces moulantes de la cavité alors que la pièce n'est plus en contact avec lesdites surfaces, et poursuivre ce chauffage durant étape v) d'aspersion.The invention aims to remedy the shortcomings of the prior art and to this end relates to a process for the shell molding of a metal in a cavity, using a mold comprising:

1. at. two dies each comprising a block carrying a molding surface, so that said molding surfaces define a molding cavity;
2. b. in at least one of the dies, an inductor traveling in a hose made in the block carrying the molding surface;
3. vs. a generator for supplying a high frequency current to said inductor so as to heat the walls of the hose (340);
4. d. the inductor being placed at a distance d from the molding surface so that the heat conduction from the wall of the hose comprising the inductor to the molding surface, through the thickness of said block, leads to a uniform distribution of the temperature on the molding surface;

the process comprising the steps of:

• i. filling with 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 electric current in the inductor;
• ii. solidification of the metal in the molding cavity;
• iii. opening of the mold and ejection of the part;
• v. spraying the molding surfaces of the molding cavity, the mold being opened, by a release agent;
• vi. closing the mold and heating the cavity to temperature T1;

which process comprises, after step iii) of opening the mold and before step v) of spraying the molding surfaces, a step consisting in:
iv. induction heating the molding surfaces of the cavity while the part is no longer in contact with said surfaces, and continue this heating during step v) of spraying.

Thus, the combination of the induction heating means and the early start of this heating before and during spraying make it possible to at least partially compensate for the temperature loss linked to the spraying of the surfaces of the cavity. Unlike the prior art means of heating which require heating the mold as a whole, induction heating concentrates its effects on the molding surfaces and thus makes it possible to uniformly heat these surfaces in a very short time, while the mold is opened, providing in said surfaces a heating power of several tens of KW, without effect of the temperature of said surfaces on the heating efficiency. Thus, the time required to restore the suitable preheating temperature on the surfaces of the cavity is reduced and the initial molding conditions are kept from cycle to cycle, without interruption or drop in rate.

The invention is advantageously implemented according to the embodiments and the variants set out below, which are to be considered individually or according to any technically operative combination.

According to one embodiment of the process which is the subject of the invention, this comprises, between step i) and step ii), forced cooling of the molding cavity. This embodiment thus makes it possible to fill the cavity at a preheating temperature. high, ensuring the fluidity of the material and the uniform filling thereof, while controlling the cooling cycle of the material and limiting the influence of the cooling time on the cycle time.

According to one embodiment, forced cooling is carried out by the circulation of a heat transfer 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, out of reach in the long term by heating systems by oil circulation or by electrical resistance, in the targeted cycle times, are particularly suitable for the use of magnesium alloys, alloys of aluminum or zinc alloys, without these examples being limiting, the high preheating temperatures also having a beneficial effect on the mechanical and metallurgical characteristics of the parts, with in particular the obtaining of finer grains or the 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 makes it possible to achieve such a heating speed with reduced energy consumption and this independently of the surface of the mold.

Advantageously, the temperature of the molding surfaces reached during step iv) and before step v) is greater than T1. This controlled overheating of the molding surfaces when the part is no longer in contact with said surfaces, makes it possible to limit the minimum temperature reached during spraying. Thus the heating during step v) is faster.

Advantageously, the molding cavity being brought to a temperature of between 200 ° C and 400 ° C, the metal alloy used by the process which is the subject of the invention is a magnesium alloy of type AM20, AM50, AM60 or AZ91D. Thus, the process which is the subject of the invention allows the molding of such materials, which are reputed to be difficult to mold under cycle time conditions compatible with mass production.

Advantageously, the metal alloy is an aluminum and silicon alloy comprising less than 2% of silicon, for example an alloy of the Al-Mg-Si-Mn type. This type of aluminum alloy is anodisable, has a higher solidification start temperature than conventional Al-Si foundry alloys, which results in better mechanical characteristics and increased temperature stability, to the detriment of its ease of molding. The process which is the subject of the invention allows the use of such a material in a reproducible manner under mass production conditions.

The process which is the subject of the invention is also suitable for the shell molding of zinc alloys of the Zamac type, injection molded under pressure in a hot chamber for the production of parts in large series.

The process which is the subject of the invention is suitable for molding metal alloys, injected in the liquid phase during step i). It is also suitable for the thixomolding of these alloys, injected in the semi-solid phase during step i).

Advantageously, the block carrying the molding surface is made of a steel of HTCS 130 type. The high thermal conductivity and thermal diffusivity of this steel allow more reactive temperature regulation of the molding surfaces.

According to an alternative embodiment of the method of the invention, the block carrying the molding surface is made of a non-ferromagnetic material, in which the hose comprising the inductor is lined with a layer of a material of high magnetic permeability. This embodiment is more suitable for pressure shell molding of materials with a high melting temperature, or capable of reacting chemically with ferrous metals at the casting temperature.

• la figure 1, relative à l'art antérieur montre ,selon des diagrammes temps-température, l'évolution de la température des surfaces de la cavité moulante d'un moule de moulage en coquille sous pression préchauffé par une circulation d'huile, figure 1A au cours d'un cycle de moulage, et figue 1B au cours d'une pluralité de cycles de moulage successifs ;
• la figure 2 est une vue schématique en coupe des matrices délimitant la cavité moulante d'un outillage adapté au moulage par injection d'un matériau métallique ;
• la figure 3 représente, selon une vue en coupe, un exemple de réalisation de l'une des matrice d'un outillage selon l'invention adapté au moulage par injection d'un matériau métallique ;
• la figure 4 représente selon une vue de détail un exemple de réalisation de l'installation des inducteurs dans une matrice, telle que représentée figure 3, constituée d'un matériau non-ferromagnétique ;
• et la figure 5 illustre un cycle thermique des surfaces moulantes d'un moule de moulage en coquille sous pression par la mise en oeuvre de l'outillage et du procédé objets de l'invention en comparaison du cycle thermique représenté figure 1A.

The invention is set out below according to its preferred, non-limiting embodiments, and with reference to Figures 1 to 5 , in which :

• the figure 1 , relating to the prior art shows, according to time-temperature diagrams, the evolution of the temperature of the surfaces of the molding cavity of a pressure molding shell mold preheated by an oil circulation, figure 1A during a molding cycle, and fig 1B during a plurality of successive molding cycles;
• the figure 2 is a schematic sectional view of the dies delimiting the molding cavity of a tool suitable for injection molding of a material metallic ;
• the figure 3 shows, in a sectional view, an embodiment of one of the matrix of a tool according to the invention suitable for injection molding of a metallic material;
• the figure 4 shows a detailed view of an embodiment of the installation of inductors in a matrix, as shown figure 3 , made of a non-ferromagnetic material;
• and the figure 5 illustrates a thermal cycle of the molding surfaces of a mold for molding in a pressure shell by the implementation of the tooling and of the method which are the subject of the invention in comparison with the thermal cycle shown figure 1A .

Figure 2 , according to a schematic diagram of the production of the tool which is the subject of the invention, it comprises two dies (210, 220) and means (not shown) for bringing said dies apart and moving them apart, so as to close and open the mold. When the mold is closed, a molding cavity is formed, a cavity delimited by the molding surfaces (211, 221) of said dies.

Only the elements of the tooling essential to the implementation of the invention are described here, the other characteristics of the tooling being known to those skilled in the art in the field of pressure shell molding. Thus, the dies of the tooling object of the invention include in particular conduits for supplying the molded material into the molding cavity of the tooling as well as means for ejecting the molded part after it has solidified.

Figure 3 , according to an exemplary embodiment of the tool object of the invention, one of the dies (210), and preferably the two dies, include induction heating means comprising a plurality of hoses (340) in which pass inductors performing an induction circuit. Said inductors (341) consist, for example, of a copper tube or braid, isolated from the walls of the matrix by a ceramic tube (342), for example a silica sheath, transparent screw with respect to the magnetic field generated by said inductors. Copper braid inductors are preferred for following winding paths with small radii of curvature. The path of the inductors is determined in particular by thermal simulation in order to obtain a uniform distribution. temperature on the molding surface, while ensuring a heating time of said molding surface as small as possible.

Advantageously, the matrix (210) is made in two parts (311, 312). Thus, the hoses (340) for the passage of the inductors are produced by grooving said parts before their assembly.

One or more cooling conduits (350) are formed in the matrix (210), by drilling or by grooving and assembly, as for the hoses receiving the inductors. This conduit (350) allows the circulation, by appropriate means, of a heat transfer fluid in said matrix in order to ensure its cooling. Said heat transfer fluid circulates in said conduits 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 in the gas phase, if said fluid is air or another heat transfer gas. Advantageously, the cooling circuit includes a refrigeration unit (not shown) for cooling the heat transfer fluid to a temperature below ambient temperature. The circulation of the heat transfer fluid makes it possible to cool the matrix (210) and more particularly the molding surface (211). According to alternative embodiments, the cooling duct (350) is placed on the same plane as the inductors and is at an equivalent distance from the molding surface, or the cooling duct (350) is placed at a greater distance from the molding surface as the inductors, the latter then being between the cooling duct and the molding surface, this embodiment favoring the heating speed over the cooling speed, or else, the cooling duct is positioned between the molding surface and inductors, this embodiment favoring the cooling rate. The circulation of the heat transfer fluid and the induction heating can be used jointly for the purpose of regulating the temperature or the cooling rate. A temperature sensor (360), for example a thermocouple, is advantageously placed near the molding surface (211) in order to measure its temperature and to, if necessary, control the heating and cooling conditions. The use of oil as coolant for cooling makes it possible to cool the mold in the conditions for implementing a pressure shell molding of a light alloy of aluminum, magnesium, or zinc, cooling in the gas phase is advantageous for higher processing temperatures as encountered for copper, titanium or nickel alloys.

The block (311) of material comprising the molding surface (211) is sufficiently thick, so that the hoses (340) in which the inductors (341) are placed are spaced a distance d from said molding surface, so that that -this is heated, at least in part, by conduction of the heat produced by the rise in temperature on the walls of said hoses (340), this rise in temperature resulting from the circulation of a high frequency electric current in the 'inductor (341). Thus, the temperature distribution, resulting from the implementation of induction heating, is uniform over said molding surface. The distance d is for example determined by digital simulation of the heating as a function of the properties of the materials present. Although the network of hoses (340) receiving the inductors (341) is here represented as extending in a plane, said hoses are, depending on the intended application, advantageously distributed in the thickness of the block (311) around the tight surface.

The block (311) carrying the molding surface (211) is made of a metallic material in order to have a thermal conductivity and a thermal diffusivity sufficient for the implementation of the heating and cooling phases of the process which is the subject of the invention. Advantageously, said material is ferromagnetic, for example a martensitic or ferrito-martensitic steel, the Curie temperature of which is equal to or higher than the preheating temperature targeted for the molding process. By way of example, for the pressure shell molding of a light alloy, the block (311) carrying the molding surface is made of steel of type DIN 1.2344 (AISI H13, EN X40CrMoV5-1) or DIN 1.12343 (AISI H11, EN X38CrMoV5-1). Advantageously, said block consists of a tool steel as described in the document EP 2,236,639 and commercially distributed under the name HTCS 130® by the company ROVALMA SA, 08228 Terrassa, Spain. This steel has high thermal conductivity and thermal diffusivity, which reduces cycle times.

The inductors (341) are connected to a high frequency current generator, typically a frequency between 10 kHz and 200 kHz, by means (not shown) capable of tuning the resulting resonant circuit, in particular, but not exclusively, a box capacitors and an impedance matching coil, as described in the document WO 2013/021055 . The high frequency current generator and the tuning means of the resonant circuit are selected so as to provide an induction heating power of the molding surface (211) of the order of 100 kW. According to variant embodiments, depending in particular on the size of the mold, the two dies constituting the mold are connected to the same high frequency generator or to two different generators.

Figure 4 , according to another embodiment, the material constituting the block (311) carrying the molding surface of the matrix is not ferromagnetic. In this case, according to an exemplary embodiment, the hoses comprising the inductors (441) are lined with a layer (443) of steel of high magnetic permeability and advantageously retaining its ferromagnetic properties up to high temperature, for example 700 ° C. . Thus, the magnetic field produced by the inductor (441) is concentrated in the liner (443) which rises rapidly in temperature and transmits this temperature by conduction to the matrix. The heat being transmitted by conduction to the molding surface, the judicious arrangement of the inductors makes it possible, as previously, to ensure a uniform temperature on this molding surface. According to exemplary embodiments of this variant, the block (311) carrying the molding surface consists of copper, an austenitic stainless steel or a nickel-based alloy resistant to high temperature of the INCONEL 718® type, without that these examples are not limiting.

When the block (311) is made of ferromagnetic steel, the heating action of the inductors is divided between direct heating by induction 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 heating modes depends on the distance d. When the block (311) is made of a non-ferromagnetic material, a similar effect is obtained by depositing, on the molding surfaces, a ferromagnetic coating, for example a coating with nickel base.

Figure 5 , comparing the thermal cycles (501, 502) undergone by the molding surfaces, between the thermal cycle (501) resulting from an oil-circulation heating mold and the thermal cycle (502) resulting from the implementation of the tool object of the invention shows that the time (520) required to obtain the preheating temperature (105) from the start of the phase (140) of spraying the molding surfaces is reduced. This effect is linked to the ability to provide a greater heating power on the molding surfaces by the induction heating means, in comparison with the means of the prior art, and thus to obtain a faster heating rate, on the order of 5 ° Cs ^-1 on said molding surfaces, with a projected surface imprint of 200 x 300 mm ² and a heating power of the order of 100 kW. In addition, the use of induction heating makes it possible to trigger the heating of the molding surfaces during the step (130) of ejection of the part at a time (510) after the ejection of the part, but before the start. of the spraying step (140). This early initiation of induction heating is carried out when the molding surfaces are approximately at the nominal preheating temperature (105) of the molding cavity. Said heating has the effect of bringing said surfaces to a temperature (505) higher than said preheating temperature (105), so as to limit the drop in temperature following the spraying operation (140). The heating power supplied by the inductors on the molding surfaces is sufficient to obtain this heating without slowing down the ejection step (130) and without delaying the spraying step (140). Thus, the combination of the anticipated start of the heating, of the overheating of the molding surface at a temperature (505) higher than the nominal preheating temperature (105), makes it possible, on the one hand, to obtain the temperature (105) of preheating aimed at the molding surfaces, in the cycle time targeted, and thus ensuring the consistency of the quality of the parts produced over the successive cycles and thus reducing the scrap rates. In addition, this same combination of means and method of implementation makes it possible to carry out the molding cycle in a reduced time (530) compared with the prior art, the heating power provided being greater and independent of the temperature. heated surfaces, thus bringing a gain in productivity by at the same time as improving the reliability of the process. Since the molding surfaces are at a temperature close to the nominal preheating temperature (105) when the anticipated heating of said surfaces is triggered, the implementation of this measure by means of oil circulation heating would have no effect, the proximity of the temperatures of the oil in circulation 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 means for cooling the molding surface of the tooling object of the invention makes it possible to regulate the temperature of the mold and of the charge of molded material during step (110). of casting. Thus, the tooling object of the invention makes it possible to inject the metal alloy into a warmer mold, to ensure better filling thereof, while ensuring sufficiently rapid cooling of the material, in particular to avoid the appearance of porosity or an uneven grain size. Unlike the prior art, where the thermal kinematics of the casting phase (110) is dictated by passive thermal exchanges between the mold and the material, the implementation of the tooling object of the invention makes it possible to regulate, at least in part, this kinematics. Thus, the method implemented by means of the tooling object of the invention makes it possible to improve the intrinsic quality of the parts molded by this method.

The ability to preheat the molding surfaces to a higher temperature and to maintain and regulate this temperature during the casting step (110), allows the use of alloys with a higher solidification start temperature, while by ensuring the filling of the molding cavity, in particular of aluminum alloys comprising less than 2% of silicon, hypoeutectic compared to the AlSi system, while maintaining production rates comparable to those obtained for eutectic or quasi-eutectic alloys. Thus, the process and the tools which are the subject of the invention facilitate the use of alloys with higher mechanical characteristics, in particular the Al-Si-Mg, Al-Mg-Si and Al-Mg-Si-Mn alloys, and the implementation by molding in large series of aluminum alloys suitable for anodized finishing.

The effects of the process which is the subject of the invention using a tool comprising an induction heating 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 the tools which are the subject of the invention are presented as applied to one of the dies, these are applicable to all of the dies delimiting the molding cavity of the tool. According to exemplary embodiments, the inductors ensuring the heating of the molding surfaces of said matrices are connected to a single high frequency current generator or to generators dedicated to each matrix.

The above description and the exemplary embodiments show that the invention achieves the aim, and makes it possible, with regard to the prior art, to increase the production rates, to improve the repeatability of the quality of the parts molded, to improve the metallurgical quality and the quality of production of said parts and to open up the possibility of using more difficult flowability materials under the same conditions of productivity and repeatability.

Claims (10)

1. Method for shell-moulding a metal in a cavity, implementing a mould comprising:

   a. two dies (210, 220) each comprising a block (311) carrying a moulding surface (211, 221), such that said moulding surfaces delimit a moulding cavity;

   b. in at least one of the dies, an inductor (341, 441) running through a pipe (340) arranged in the block (311) carrying the moulding surface;

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

   d. the inductor (341, 41) being positioned at a distance d from the moulding surface such that the conduction of heat from the wall of the pipe (340) comprising the inductor to the moulding surface, through the thickness of said block (311), produces a uniform distribution of the temperature over the moulding surface;

   the method comprising the steps of:

   i. filling (110) the moulding cavity by injecting metal into said cavity, said cavity being preheated to a nominal preheating temperature T1 (105) by the circulation of a high-frequency electric current in the inductor (341);

   ii. solidifying the metal in the moulding cavity;

   iii. opening (120) the mould and ejecting (130) the part;

   v. spraying (140) the moulding surfaces of the moulding cavity, the mould being open, with a demoulding agent;

   vi. closing the mould and heating (150) the cavity to temperature T1 (105);

   characterised in that it comprises after step iii) of opening the mould and before step v) of spraying the moulding surfaces, a step of:
   iv. heating via induction the moulding surfaces of the cavity while the part is no longer in contact with said surfaces, and continuing this heating during step v) of spraying.
2. Method according to claim 1, comprising between step i) and step ii) a forced cooling of the moulding cavity.
3. Method according to claim 2 wherein the forced cooling is carried out by the circulation of a heat transfer fluid in a conduit (350) arranged in the mould.
4. Method according to claim 1, wherein the temperature T1 (105) is comprised between 200° C and 400° C, preferably between 250° C and 300° C.
5. Method according to claim 4, wherein the temperature (505) of the moulding surfaces reached during step iv) and before step v) is greater than T1 (105).
6. Method according to claim 4, wherein the metal is an alloy among:

   - a magnesium alloy of the AM20, AM50, AM60 or AZ91D type, or

   - an aluminium alloy comprising less than 2% of silicon, in particular of the Al-Mg-Si-Mn type, or

   - an aluminium, magnesium and copper zinc alloy of the Zamac type.
7. Method according to claim 6, wherein the metal is injected in liquid phase during step i).
8. Method according to claim 6, wherein the metal is injected in semi-solid phase during step i).
9. Method according to claim 1, wherein the block (311) carrying the moulding surface is formed from steel of the HTCS 130 type.
10. Method according to claim 1, wherein the block carrying the moulding surface is formed from a non-ferromagnetic material, wherein the pipe comprising the inductor is lined with a layer (443) of a material of high magnetic permeability.

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