EP0968064B1 - Method and installation for low pressure die casting in a mould with ceramic casting die - Google Patents

Description

The invention relates to a method and a low pressure casting installation of a metallic composition in liquid state, in a mold comprising a ceramic shell, one side of which delimits a casting space and a mass of sand without binder in contact with the other face of the shell, the mold being placed in a box having at its base a access connected to the pouring space and connected by a line for supplying liquid metal to a metal source liquid below, for example a casting furnace.

We already know processes and facilities of this type, in which the source of metal liquid such as a casting furnace is set communication with a pressurized gas source of such that is applied to the liquid metal contained in the liquid metal source, a first value of pressure by which the liquid metal is brought in and maintained in the area of the caisson access for this liquid metal.

When you want to cast a piece in introducing liquid metal into the mold, the level of liquid metal is raised above the mold threshold where it was, increasing the pressure to a second value slightly higher than the value necessary to bring the liquid metal to fill the casting space, in order to have a margin of security and to be certain that all the crevices of the mold, even the tallest ones, are filled with metal liquid, and the pressure is maintained at this second value for enough time to leave establish a rest regime, and a metallo-static pressure weak on the shell walls in ceramic. Then the pressure is further increased to a third value to fill the cavities or possible shrinkage, and the pressure is maintained at this third value for a period allowing to obtain the desired solidification of the metal contained in the mold, after which we decrease the pressure of such so that the level of unused liquid metal drops approximately at the initial level, and we maintain this pressure for the time required for example to remove the mold, unmold the piece, and put a mold allowing the manufacture of another part.

However, the ceramic shells of these molds generally have an extremely thick weak and thus a certain flexibility which their confers, when subjected to pressure from the liquid metal, a tendency to undergo deformations which affect their dimensional stability.

In order to remedy dimensional instabilities which occur in the case of the casting process described above, it has been proposed to maintain in depression the mass of sand in contact with the shell, so provide dynamic stability to the grains of sand and thus the dimensional stability of the shell. At this end, the box has a second access connected to the space where the mass of sand is, connected by a suction line to a delivery device depression.

But this depression of the mass of sand has the disadvantage of disturbing the rise and good distribution of liquid metal in the space of casting, through the porosity of the ceramic ("vacuum" filling), which leads to overfilling, generating faults in the castings.

The object of the invention is to remedy these disadvantages and to this end relates to a method of low pressure casting in a mold comprising a ceramic shell, one side of which defines a space of casting and. a space containing a mass of sand without binder in contact with the other face of the shell, the mold being placed in a box having at its base a access connected to the pouring space and connected to a source of liquid metal below, process in which applies to the liquid metal of the source a first pressure value suitable for supplying liquid metal in the region of said access, the pressure is increased up to a second value slightly higher than a value needed to fill the casting space and we maintains this second value for a first predetermined duration, then the pressure is increased to a third value and we maintain this third value for a second predetermined duration, then we decreases the pressure so that the level of the liquid metal descend approximately in the region of said access or below, characterized in that, approximately when the pouring space becomes completely filled with liquid metal, the pressure in the space of the mass of sand below atmospheric pressure up to a predetermined vacuum value, we maintains this depression value at least until let a crust be obtained against the shell solidified, and we go up approximately to the pressure atmospheric pressure in space the mass of sand.

• la durée de la phase au cours de laquelle on diminue la pression dans l'espace où se trouve la masse de sable, au-dessous de la pression atmosphérique, jusqu'à une valeur de dépression prédéterminée, est plus courte que la première durée prédéterminée ;
• la phase au cours de laquelle on augmente la pression dans l'espace où se trouve la masse de sable, approximativement jusqu'à la pression atmosphérique, est menée à son terme avant le terme de la phase au cours de laquelle on diminue la pression à laquelle est soumis le métal liquide contenu dans la source de métal liquide de telle sorte que le niveau du métal liquide descende approximativement dans la région dudit accès ou au-dessous ;
• la valeur de dépression prédéterminée correspond à une pression absolue dans la gamme approximativement de 0,5 à 0,9 bar ;
• la première valeur de pression est d'environ 1,15 bars absolus ;
• la deuxième valeur de pression est d'environ 1,4 bars absolus ;
• la troisième valeur de pression est d'environ 1,7 bars absolus.

The method according to the invention can also have one or more of the following characteristics:

• the duration of the phase during which the pressure is reduced in the space where the mass of sand is located, below atmospheric pressure, up to a predetermined depression value, is shorter than the first predetermined duration ;
• the phase during which the pressure in the space where the mass of sand is increased, approximately up to atmospheric pressure, is completed before the end of the phase during which the pressure is reduced to which is subjected to the liquid metal contained in the source of liquid metal so that the level of the liquid metal drops approximately in the region of said access or below;
• the predetermined vacuum value corresponds to an absolute pressure in the range approximately from 0.5 to 0.9 bar;
• the first pressure value is approximately 1.15 bar absolute;
• the second pressure value is approximately 1.4 bar absolute;
• the third pressure value is approximately 1.7 bar absolute.

Thanks to the fact that it's only when the pouring space is filled with liquid metal which is decreases the pressure in space of the mass of sand, the filling is not disturbed by the vacuum.

The invention also relates to an installation for the implementation of the above process, comprising a mold comprising a ceramic shell, one side of which delimits a casting space, and a space containing a mass of sand without binder in contact with the other face of the shell, the mold being placed in a box presenting at its base an access connected to the space of poured and connected to a source of liquid metal in below, installation characterized in that the box internally has a vacuum chamber provided with less an air permeable wall region having an access opening provided with a vacuum outlet to be connected to a suction device via a solenoid valve controlled by a programmable controller also controlling a device for regulating source pressure of liquid metal.

The installation according to the invention can moreover feature the fact that the room vacuum extends annularly in the box against a base wall and side walls thereof.

Thanks to this installation, the molding process can be implemented very quickly and can be controlled precisely with an investment in remaining material despite everything moderate.

• la figure 1 est un schéma général d'une installation pour la mise en oeuvre du procédé selon l'invention, et
• la figure 2 est un diagramme illustrant la succession des phases du procédé selon l'invention.

Other characteristics and advantages of the invention will emerge from the description which follows of an exemplary embodiment of the invention illustrated by the accompanying drawings in which:

• FIG. 1 is a general diagram of an installation for implementing the method according to the invention, and
• FIG. 2 is a diagram illustrating the succession of the phases of the method according to the invention.

The installation according to the invention comprises a box 1 in which is formed a mold 2 comprising a ceramic shell 21 which may be of the type "Consumable" of which one side delimits a space of casting 22 whose shape corresponds to the outside shape of the part to be cast, and a mass 23 of sand without binder in the space of the box delimited between the walls of this one and the other side of the shell and so in contact with this other side.

Thanks to the invention, the thickness of the shell 21 may be small, for example a thickness of 2 mm can be reached.

In general, the thickness is in the range of 2 to 4 mm, but the process is also suitable for greater thicknesses.

The box 1 has a base wall 11 and side walls 12 having a common edge with the base wall and top edge bent outwards so as to constitute a flange 13 extending parallel to the base wall.

The base wall has an access opening 14 connected to the casting space 22 by a seal 15 tubular with a part fitted into the opening of the base wall and a part serving as worn on the inside, of conjugate shape, of the mouth (lower) of the shell 21.

Case 1 has a vacuum chamber 3 annular extending against the peripheral region of its base wall 11 and the side wall regions 12 which end up at this base wall. This room vacuum has at least one wall 31 in contact with the mass of sand 23; this wall 31 includes at least one region breathable but sandproof. One of the walls sides of the box 1 and of the vacuum chamber 3 comprises a second access opening 16 opening into the vacuum chamber, fitted with a vacuum outlet 17.

The mass of sand 23 fills the space of the box 1 between the walls thereof and the shell 21, at with the exception of the vacuum chamber 3.

The box is placed on a support plate (not shown) having an opening opposite the access opening of its base wall.

With a view to filling the caisson with sand at means for example of a hopper whose base orifice opens into the box, the support plate is everything first placed on a vibrating device (also not shown) comprising for example rollers eccentrics.

Thus, when the shell 21 is positioned on the seal 15, the box 1 can be filled with soft sand, that is to say without binder, while being vibrated to obtain maximum densification. The sand level is then equalized with the rule to obtain a level regular on the upper surface of the box.

The box 1 filled with sand is then ready to be transported near a casting furnace 4, arranged below the box. The box is positioned so that the access opening 14 of its base wall lined with the gasket 15 is opposite an outlet neck 41 of a metal supply pipe 42 liquid from the oven, so that the neck can be secured to the box and that the supply line thus connects the mold casting space to the interior space of the oven.

Also, a movable plate 5 under which is fixed a seal 51 is attached to the box for example by clamping to constitute the cover of the latter, the seal 51 then being interposed between the plate and the flange 13 formed on the side walls of the caisson. The seal, crushed between the plate and the flange, following the shape of these, so lets do the empty inside the box.

Approximately simultaneously with the application of the movable plate 5 on the box, the vacuum outlet 17 is tightly connected to a pipe 61 connecting this socket to a device suction 6 comprising a vacuum manifold 62 setting vacuum by a pump 63 such as a pallets.

A solenoid valve 64 controlled by a PLC programmable 7 is inserted between the vacuum manifold 62 and the vacuum outlet 17 of the box; a pressure gauge 65 measures the pressure at the inlet of the box.

The casting furnace 4 comprises, above the mass 43 of liquid metal, a vault 44 connected by a pressure line 45 to a pressure device pressure regulation 8 also controlled by the programmable controller 7.

Thanks to this installation, and under the command of the programmable logic controller 7, it is possible to implement the process according to the invention which will be described below and which is illustrated in Figure 2.

The liquid metal of the casting furnace 4 is applied, by means of the pressurization line 45, a pressure of value P1 for example of the order of 1.15 absolute bars, which brings the liquid metal by a movement ascending in the region of access opening 14 of the base of the box 1, at the threshold of the mouth of the mold 2.

Then, increase the pressure (phase AB), until a P2 value for example of approximately 1.4 bar absolute, and more generally a value slightly greater than that necessary to fill completely so ascending the casting space with a safety margin Δh sufficient, and we maintain this value to let establish a rest regime (BC phase).

In addition, when the pressure has reached approximately its value allowing the filling of the mold, and preferably its P2 value with the margin of security, (point B), we decrease rapidly, by through the suction line, and therefore the opening of the solenoid valve 64 controlled by PLC 7, the pressure in the mass of sand; the decrease is continued until one reaches a depression value compared to the atmospheric pressure, corresponding to a pressure absolute predetermined preferably in the range approximately 0.5 to 0.9 bar, and this is maintained depression value for a certain time as we will see it later.

Preferably, the reduction of the pressure in space of the mass of sand quickly, because the duration of this phase of decrease below atmospheric pressure up to the vacuum value chosen must be shorter than that of phase BC where the mass of liquid metal is subjected to the pressure of P2 value. According to the invention, therefore, the liquid metal pressure at the P2 value here of 1.4 bars, not only until the cavity 22 is filled, but also as long as the depression in the mass of sand did not reach the chosen level.

After which, the shell 21 being stiffened by the compaction of the mass of sand due to depression, we further increases the pressure applied to the liquid metal (CD phase) up to a value P3 for example of the order 1.7 bars to fill cavities or shrinkage possible, and we maintain this value P3 of overpressure for a period allowing to obtain everything first a solidified crust against the shell, then the solidification of the metal contained in the mold (phase DE).

The depression in the mass of sand is maintained at a value corresponding to the pressure absolute from 0.5 to 0.9 bar here approximately up to point E, and in any case the "vacuum" must not be "Broken" before the metal crust formed against the shell 21.

When the desired solidification of the metal of the casting space 22 is obtained, the pressure is reduced on the liquid metal (phase EF) so that the level of it goes down approximately to the level initial (pressure value P1 here equal to 1.15 bars approximately), and this pressure is maintained for the time necessary for example to remove the mold and hand over another for making another room. As we have seen, we can break the "vacuum" (increase pressure) in the mass of sand from from the moment the solid crust is formed, but usually we do this only immediately before the pressure is released applied to liquid metal or when loosening; however, this phase of increasing the pressure in the space where the mass of sand is located approximately until atmospheric pressure is done early and / or quickly enough for it to be completed before term F of phase EF at during which the pressure on the metal is reduced liquid to ensure its return to pressure P1 or even below. The mass of sand is no longer in depression, the liquid metal can descend freely into the supply line 42, its level then returning approximately in the region of access 14 or below, and you can easily remove the box without fear liquid metal returns.

Thus, thanks to the invention, the depression being applied to the mass of sand after filling of the shell, it does not affect the flow of the metal in the latter, and this flow remains controlled by pressure, while the depression in the mass of sand ensures the stiffening of the shell in the overpressure phase at pressure P3, which is the most critical, at least as long as the shell is not stiffened by a metal crust.

Claims (9)

1. Process for low pressure die casting in a mould (2) comprising a ceramic casting die (21) one side of which delimits a casting space (22), and a space containing a mass of sand (23) without any binder, in contact with the other side of the casting die, the mould being placed in a box (1) having in its base an access point (14) communicating with the casting space and connected to a source (4) of liquid metal below, in which process a first pressure value (P1) is applied to the liquid metal of the source, adapted to bring the liquid metal into the region of said access point, the pressure is increased to a second value (P2) slightly higher than a value needed to fill the casting space and this second value is maintained for a first predetermined length of time (BC), then the pressure is increased to a third value (P3) and this third value is maintained for a second predetermined length of time (DE), then the pressure is reduced so that the level of liquid metal falls to approximately the region of said access point (14) or below it, characterised in that, approximately when the casting space becomes totally filled with liquid metal, the pressure in the space for the mass of sand (23) is reduced below atmospheric pressure to a predetermined underpressure value, this underpressure value is maintained at least until a solidified crust is obtained against the casting die (21), and the pressure in the space for the mass of sand is raised to approximately atmospheric pressure.
2. Die casting process according to claim 1, characterised in that the duration of the phase during which the pressure in the space where the mass of sand (23) is located is reduced below atmospheric pressure to a predetermined underpressure value, is shorter than the first predetermined length of time (BC).
3. Die casting process according to claim 1, characterised in that the phase during which the pressure in the space where the mass of sand (23) is located is raised approximately to atmospheric pressure is brought to an end before the end (F) of the phase (EF) during which the pressure to which the liquid metal contained in the source (4) of liquid metal is reduced so that the level of the liquid metal falls to approximately the region of said access point (14) or below.
4. Die casting process according to any one of claims 1 to 3, characterised in that the predetermined underpressure value corresponds to an absolute pressure approximately in the range from 0.5 to 0.9 bar.
5. Die casting process according to any one of claims 1 to 4, characterised in that the first pressure value (P1) is about 1.15 bars of absolute pressure.
6. Die casting process according to any one of claims 1 to 5, characterised in that the second pressure value (P2) is about 1.4 bars of absolute pressure.
7. Die casting process according to any one of claims 1 to 6, characterised in that the third pressure value (P3) is about 1.7 bars of absolute pressure.
8. Apparatus for carrying out the process according to any one of claims 1 to 7, comprising a mould (2) having a ceramic casting die (21) one side of which delimits a casting space (22), and a space containing a mass of sand (23) without any binder, in contact with the other side of the casting die, the mould being placed in a box (1) having at its base an access point (14) communicating with the casting space and connected to a source (4) of liquid metal below, and comprising on the inside a vacuum chamber (3) provided with at least one wall portion (31) which is air-permeable and having an access opening (16) provided with a vacuum connector (17) for connecting to a suction device (6), the apparatus being characterised in that the vacuum connector (17) is connected to the suction device (6) via an electromagnetic valve (64) controlled by a process controller (7) which also controls a device for regulating the pressure (8) of the source (4) of liquid metal.
9. Apparatus according to claim 8, characterised in that the vacuum chamber (3) extends annularly in the box against a base wall (11) and side walls (12) thereof.

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