EP0344042B1 - Device for transporting a molten metal composition by high pressure and cooling the piston for a vertical pressure die casting machine - Google Patents

Description

The invention relates to a device for transferring a molten metal composition under high pressure, this system having a piston provided with a cooling system.

Known pressure molding apparatuses mostly have a molten metal injection system arranged so as to vertically inject molten metal into molds.

• un module supérieur au niveau duquel peuvent être installés le moule et le dispositif de démoulage;
• un module inférieur dont les principaux éléments sont le cylindre d'injection, le piston d'injection et le dispositif d'alimentation.

They generally consist of two modules:

• an upper module at the level of which the mold and the mold release device can be installed;
• a lower module, the main elements of which are the injection cylinder, the injection piston and the supply device.

In the present text, the term “supply device” means a device into which the melt composition is introduced.

The main difficulty of the high pressure molding method is to prevent the molten metal from cooling too much during its transfer into the mold. Indeed the cooled metal sees its viscosity increase, which seriously harms the maintenance of the pressure during the pressing operation and as a corollary to the quality of the molded parts.

Numerous pressure molding devices are known from the state of the art.

The most elaborate are provided with feeding devices aimed at reducing the transfer time by acting on the feeding mode of the molten metallic composition.

Patent FR 2 446 145 relates to a machine for casting under vertical pressure in which the fusion takes place directly within the machine by means of an electromagnetic coil. The metal is introduced while it is solid by a mechanical arm. The melting chamber is arranged coaxially under an injection cylinder to reduce the time between the melting of the metal and its injection into the mold.

Patent EP 164,301 describes a vertical pressure molding apparatus comprising two modules. The lower module consisting of the piston and the injection cylinder is mounted in rotation on an axis so that the molten composition can be poured directly into the injection cylinder and, by a simple angular movement, brought quickly under the upper module in order to undergo the pressing operation inside the mold. The supply device therefore comprises the injection cylinder itself.

These devices make it possible to reduce the time and the distance for transferring the molten metal but inevitably generate a risk of thermal shock at the place where the molten metal falls. The duration of life of the parts on which the molten metal is poured (that is to say mainly the piston and the walls of the injection chamber), is considerably reduced.

To avoid this excessive heating of the piston and of the walls of the injection chamber, various devices for cooling the piston have been developed.

During the pressing operation, the piston pushes the metal back into the mold at high speed so as to guarantee proper filling of the latter.

As we have just explained, the introduction of molten material into the injection chamber has the effect of causing significant heating, on the one hand of the front wall of the injection piston serving as a base for the injection chamber. injection, on the other hand of the same walls of the chamber.

One of the other problems inherent in pressure molding will therefore be to control the temperatures of the injection chamber and of the piston at the time of loading and during the pressing operation.

The different expansions of the injection piston and of the walls of the injection chamber, due to structural characteristics, cause the space existing between these two parts to increase, the space into which the liquid metal more or less penetrates during pouring or during injection. The effect of this flow is to considerably increase the friction between these two parts and to slow down the speed of the piston in the cylindrical bore of the injection chamber and necessarily exert a harmful influence on the quality of the cast parts and on the lifetime of the apparatus, in particular when the penetration of the metal causes seizure tending to jam the two moving parts.

It is known in the state of the art various methods consisting in cooling the injection piston to limit the differences in expansion of the piston and of the injection chamber.

French Patent No. 2,416,754 describes a device for cooling the injection piston using a cooling water circuit inside the piston connected to a water distribution network and coupled to a water flow control.

European patent EP 116 906 describes an injection piston provided with a more elaborate internal cooling system consisting of a pipe delimited by the internal walls of the piston and by a mouthpiece provided with a tube for feeding the cooling liquid.

These devices allow uniform cooling of the entire front wall of the piston. Their main drawback is that they induce excessive cooling of this front wall, which has the consequence of causing partial solidification of the metal which comes into contact with it. This semi-solidified metal is transformed into crusts which disturb the pressing operation and cause a variation in the filling time of the mold, which affects the quality of the molded parts.

The technical problem which the invention proposes to solve is complex.

The transfer time of the molten metal must be fast to ensure good filling of the mold and good quality of the molded parts. However, the metal to be molded must not have an excessively high temperature in order to avoid the problems of expansion and metallurgy explained above which adversely affect the life of the press and the metallurgical quality of the molded parts.

It is therefore necessary to cool the metal, but it is also necessary not to cool it too much, while ensuring good transfer into the mold, which implies that no seizure should occur between the piston and the injection chamber.

The subject of the present invention is a device for the transfer under pressure of a molten metal composition provided with a piston with a privileged peripheral cooling zone making it possible to obtain, for the very short time of the transfer of the molten metal composition, self-construction a seal while avoiding the problems relating to thermal expansion of the piston and seizing on the walls of the injection chamber.

The object of the invention is to produce a tight seal between the piston and the wall of the injection chamber, so as to avoid any flow of molten material while ruling out the possibility of excessive cooling of the front part of the piston, in order to overcome the drawbacks mentioned in the introduction.

This molding apparatus can be used for molding materials having a melting point above 400 ° C, i.e. metals and alloys. In the present description, the designation "metallic composition" should be understood as including metals and alloys, in particular composite alloys with a metallic matrix. The working pressures generated by the piston vary from 200 to 800 bars.

The device for transferring molten metal composition according to the invention consists of an upper module, produced according to a method known from the prior art and in which the mold can be placed, and of a lower module comprising a device and a piston.

The piston has a privileged cooling zone located at the periphery of its front wall, this sliding piston with a predetermined clearance inside the supply device containing the metallic composition to be discharged.

More precisely, the front wall of this piston is provided with a peripheral chamfer forming with the front wall an acute angle less than 45 ° and preferably between 25 ° and 35 ° and delimiting with the wall of the injection chamber a frustoconical cavity . The piston is also provided with an internal cooling circuit comprising a peripheral groove hollowed out in the front wall of the piston, this groove making it possible to transform the peripheral chamfer mentioned above into a privileged cooling zone.

Another characteristic of the invention resides in the fact that the front wall of the piston has a thickness which is at least twice as great in its central part as in its peripheral part. The central part is thus much less cooled than the peripheral part.

According to a preferred variant of the invention, the front wall of the piston is coated with a heat shield. This shield consists of a layer of a material having, on the one hand a sufficiently low thermal conductivity coefficient, that is to say less than that of the piston head, and on the other hand a hardness greater than that of the piston head. The materials preferably used are carbides and nitrides such as chromium carbide or tungsten carbide, of the type sold by Union Carbide under the brand UCAR.

The piston ring is also entirely or partially coated with thermal protection to improve the sliding between the piston and the wall of the chamber and also having an anti-wear role.

According to another preferred aspect of the invention, the cooling zone is in connection with a cooling circuit the flow of which is adjustable:
- At first, corresponding to the period preceding the introduction of the molten molding material, the flow rate of the coolant is low so as not to excessively cool the piston, while avoiding the risks of vaporization;
- Secondly, during which the molten molding material is introduced, the flow is high to ensure good cooling of the preferred peripheral zone of the piston.

This increase in flow rate accelerates the formation of the tight structure tight seal in the frustoconical cavity, while preventing the coolant from passing into the vapor phase.

The device for supplying molten metal can be any system described in the prior art, such as the device described in patent EP 164,301. In this patent, the molten metal is introduced directly into the injection cylinder. However, according to a preferred variant which makes it possible to accelerate the formation of the sealed peripheral seal according to the invention, this supply system resides in a bottomless container which can slide horizontally and sealingly on a heating floor from a loading position. to a molding position and vice versa. When the container is in the molding position, it constitutes the upper part of the injection chamber.

• La figure 1 représente, en coupe frontale, le module inférieur de l'appareil de moulage selon l'invention équipé du système d'alimentation préférentiel ;
• La figure 2 représente une vue en coupe du piston de la figure 1 ;
• La figure 3 représente une vue en bout de la tête du piston de la figure 1 ;
• La figure 4 représente une vue latérale de la tête d'un piston construit selon une variante préférentielle.

The invention is hereinafter more precisely described with reference to the accompanying drawings by way of non-limiting exemplary embodiment:

• Figure 1 shows, in front section, the lower module of the molding apparatus according to the invention equipped with the preferential feeding system;
• 2 shows a sectional view of the piston of Figure 1;
• Figure 3 shows an end view of the head of the piston of Figure 1;
• FIG. 4 represents a side view of the head of a piston constructed according to a preferred variant.

The pressure molding apparatus described below is made of steel alloyed with 5% chromium, commonly used in the treated state in the production of molds.

According to Figure 1, the upper module produced by a known method is not shown. Only the mold 7 is shown here in thin dashed lines.

The press body 9 consists, on the one hand of a frame 1 surmounted by an asbestos seal 34 and equipped with a heating hearth 2 and in which is arranged along a vertical axis an injection cylinder 3 and d on the other hand, a bottomless container 5 which can slide on the floor 2 in a horizontal movement so as to be placed coaxially with the injection cylinder 3.

The bottomless container is shown in the molding position, while the loading position is shown in thin phantom.

The piston 4 slides vertically in the injection cylinder and is here shown in the low position.

The bottomless container 5 delimits a cylindrical space which constitutes the upper part of the injection chamber 6, the part bottom of this chamber being delimited by the injection cylinder 3. The internal diameter of the bottomless container 5 is the same as that of the injection cylinder 3.

The walls of the bottomless container 5 are thermally insulated and treated with anti-adhesion. This bottomless container 5 is further provided with an internal preheating system 8.

The bottomless container slides by a horizontal movement at variable speed on the heating floor 2 also thermally insulated and treated non-stick. The sole 2 is provided with a heating means 33.

According to this preferred structure of the supply system, the sole 2 is only heated on one part, corresponding approximately to the area in contact with the container when the latter is in the loading position. This position is shown in thin dashed lines.

The thermal insulation and the anti-adhesion treatment of the walls of the bottomless container 5 and of the surface of the heating sole 2 are produced by adding a layer of rectified tungsten carbide, followed by smoothing with graphite. The carbides and nitrides preferably used are chromium carbide, tungsten carbide, bromine nitride and tungsten nitride.

The tightness of the container 5 without sliding bottom on the heating floor 2 is obtained by metal-metal contact of the isolated areas on the floor 2 and on the walls of the container 5.

The injection cylinder 3 has a position coaxial with the container when the latter is in the molding position. The inner surface of this cylinder is also coated with thermal protection and treated with anti-adhesion in the same way as the heating sole 3 and the bottomless container walls 5.

The interior of the press body, that is to say the injection chamber 6 delimited on the one hand by the injection cylinder 3 on the other hand by the internal walls of the container 5 is therefore entirely graphite.

The chamber graphitation operation is regularly carried out using a graphite fill. The sole surface is also permanently graphitized using a graphite tank.

According to FIG. 2, the injection piston 4 according to the invention consists of a rod 10, a body 11 and a head 12.

The piston rod 10 comprises at its end a body 13 provided with a shoulder 14. The body 11 is screwed into the body 13 of the piston until a stop position defined by shoulder 14. The seal between the body of the piston 11 and the body of the rod 13 is produced by means of an O-ring 15.

At the upper end of the body 11 of the piston is fixed the head 12 of the piston. The piston head has on the external face of its front wall 16 a peripheral chamfer 17 forming an acute angle of 25 ° with the normal to this face. The front wall 16 is hollowed out on its internal face with a peripheral groove 19. The chamfer 17 and the groove 19 define a preferred peripheral cooling zone 18.

The rod 10 of the piston has a coaxial channel 20 in which is disposed a central tube 21 of annular section allowing the supply of coolant. The diameter of the tube 21 is sufficiently smaller than the diameter of the coaxial channel of the piston so as to provide an annular slot 28 between the internal wall of the piston rod and the external wall of the tube 21.

This tube 21 is provided at its end with a lateral opening 22 of diameter equal to the diameter of the internal section of the tube 21.

An extension 23 is supported on the internal central part of the head of the piston 12 and has, on the one hand an opening 24 of the same diameter as the opening 22 and on the other hand a spiral rib 25 of trapezoidal section so as to that the openings 22 and 24 are facing each other. All of the internal parts of the piston are held in place by means of the body of the rod 13 which is screwed into the body of the piston 11.

The walls of the body 11 of the piston and of the extension 23 delimit a helical chamber 26. This chamber is extended by another annular chamber 27 defined by the peripheral groove 19 formed in the internal wall of the piston head and is also in communication with the annular slot 28 defined by the walls of the rod 10 of the piston and by the tube 21.

According to Figure 3 the chamfer 17 and the plate 29 constitute the front wall 16 of the head 12 of the piston. The plate 29 is coated with a heat shield 32 over its entire surface. The chamfer 17 has recessed surfaces 30 covered with a thermal protection of the same kind as the coating of the plate and ensure continuity with the coating of the ferrule.

The number of spans that the chamfer 17 has is four but this number can vary from four to eight.

Referring to Figure 4, the head 12 of the piston constructed according to a preferred variant of the invention comprises a ferrule 31 which is provided with a partial thermal protection arranged in wide pitch spirals and having partial continuity with the thermal coating 32 of the stage.

Description of the operating mode of the molding apparatus under pressure according to the invention is produced with reference to all of the figures.

Before starting the molding operation, a composition to be molded is, for example, on standby in a so-called holding oven which makes it possible to keep it at a given temperature, generally comprised around 650 ° C.

The bottomless container 5 is placed in the loading position (in thin lines in FIG. 1). An automatic ladle allows the container to be fed into the waiting position by a predetermined volume of the molten composition.

The ladle allows the volume of metal to be deposited in the container, possibly through a gaseous protection to prevent the formation of oxides.

The filled container 5 is animated by a horizontal sliding movement at variable speed on the heating floor 2. The molten composition is thus translated above the injection cylinder 3 in which the piston 4 is arranged in the low position. The low position of the piston is such that the plate 29 thereof is in the same plane as the upper surface of the sole 2. The front wall 16 of the piston 4, that is to say the plate 29 and the chamfer 17 thus form the base of the injection chamber 6.

After switching on the coolant supply system, it is brought under pressure by the tube 21. It then invades the annular chamber 27 thanks to the connection formed by the openings 22 and 24. The small thickness of the wall of the piston head at the chamfer 17 makes it possible to create a privileged cooling zone 18 on the periphery of the piston head. The coolant then circulates in the helical chamber 26 and then in the slot annular 28 which communicates with a pump not shown.

During the horizontal sliding of the bottomless container 5 on the heating silk 2 towards the molding position, as soon as the opening of the container 5 reaches the edge of the injection cylinder 3, a very rapid natural distribution of a volume occurs. predetermined molten composition in the frustoconical cavity delimited by the chamfer 17 and the lower surface of the injection chamber 6. The contacting of the metal with the cooled zone then causes local crystallization of the metal to be molded and thus joins it formed persists for the duration of the vertical transfer.

During the stroke of the piston, the seal in the form of a frustoconical ring thus produced seals and resists pressure and therefore makes it possible to maintain this pressure during solidification. Any unwanted flow is thus avoided at the level of the clearance between the piston and the supply system containing the metal to be discharged, and any risk of seizure between the piston and the bottomless container is thus eliminated.

Claims (12)

1. Device for transferring a molten metal composition in a vertical moulding press, consisting of an upper module in which a mould (7) can be placed, and of a lower module comprising a feeding device and a cooled plunger (4) moving vertically to ensure the delivery of the metal into the mould (7), characterised in that the plunger (4) comprises a preferential cooling zone (18) situated at the periphery of its frontal wall (16), it being possible for this plunger to slide with a predetermined play inside the feeding device containing the metal to be delivered.

2. Device for transferring a molten metal composition in a vertical moulding press according to Claim 1, characterised in that the frontal wall (16) of the plunger (4) has on its outer face a peripheral bevel (17) and on its inner face an annular peripheral groove (19) defining a preferential cooling zone (18).

3. Device for transferring a molten metal composition in a vertical moulding press according to Claim 2, characterised in that the peripheral bevel (17) forms with the frontal wall (16) of the plunger an acute angle of less than 45° and preferably between 25° and 35°.

4. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 3, characterised in that the thickness of the frontal wall (16) of the plunger has a thickness which is at least two times greater in its central part than in its peripheral part.

5. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 4, characterised in that the central plate (29) of the plunger is covered with a heat protection (32) which has, on the one hand, a heat conductivity coefficient lower than that of the head of the plunger and, on the other hand, a hardness greater than that of the head of the plunger.

6. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 5, characterised in that the hoop (31) of the plunger is completely or partially covered with a heat protection (32) of the same kind as the heat protection of the plate (29) of the plunger.

7. Device for transferring a molten metal composition in a vertical moulding press according to either of Claims 5 and 6, characterised in that the heat coating of the plate (29) and of the hoop (31) consists of a layer of chromium carbide or of tungsten carbide.

8. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 7, characterised in that the preferential cooling zone (18) is connected to an internal cooling circuit in which the flow rate can be modified and is checked.

9. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 8, characterised in that the feeding device consists of a bottomless container (5) sliding horizontally without play on a heating sole plate (2).

10. Device for transferring a molten metal composition in a vertical moulding press according to Claim 9, characterised in that the means for heating (33) the heating sole plate is situated below the zone in contact with the liquid metal when the bottomless container (5) is in a charging position.

11. Device for transferring a molten metal composition in a vertical moulding press according to one of Claims 1 to 10, characterised in that the heating sole plate (2) and the walls of the injection chamber (6) are coated with a heat protection which has, on the one hand, a heat conductivity coefficient lower than that of the head of the plunger and, on the other hand, a hardness greater than that of the head of the plunger.

12. Device for transferring a molten metal composition in a vertical moulding press according to Claim 1, characterised in that the heating sole plate (2) and the walls of the bottomless container (5) are coated with a layer of chromium carbide or of tungsten carbide.

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