Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D18/00—Pressure casting; Vacuum casting
  • B22D18/04—Low pressure casting, i.e. making use of pressures up to a few bars to fill the mould

Definitions

• the invention relates to a method and an installation for low pressure casting of a metallic composition in the liquid state, in a mold comprising a ceramic shell, one face of which defines 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 an access connected to the casting space and connected by a liquid metal supply pipe to a source of liquid metal below, for example a casting furnace.
• Casting methods and installations of this type are already known, in which the source of liquid metal such as a casting furnace is placed in communication with a source of pressurized gas so that it is applied to the liquid metal contained in the source of liquid metal, a first pressure value by which the liquid metal is brought and maintained in the region of the access of the box for this liquid metal.
• the level of the liquid metal is raised above the threshold of the mold where it was located, by increasing the pressure to a second value slightly higher than the value required to bring the liquid metal remp the ir the casting space, to have a safety margin and to be certain that all the crevices of the mold, even the highest, are filled with metal li q uid, and the pressure is maintained at this second temperature for a time sufficient to allow the establishment of a system of rest, and a low static metallo pressure on the walls of the shell Marti q level.
• the pressure is further increased to a third value in order to fill any cavities or recesses, and the pressure is maintained at this third value for a period making it possible to obtain the desired solidification of the metal contained in the mold, after which the pressure is reduced so that the level of the unused liquid metal drops approximately to the initial level, and this pressure is maintained for the time necessary for example to remove the mold, unmold the part, and replace a mold allowing the manufacture of another part.
• the ceramic shells of these molds generally have an extremely small thickness and thus a certain flexibility which gives them, when they are subjected to the pressure of the liquid metal, a tendency to undergo deformations which harm their dimensional stability.
• the box is provided with a second access connected to the space where the mass of sand is located, connected by a suction pipe to a device for putting under vacuum.
• this depression of the mass of sand has the disadvantage of disturbing the rise and good distribution of the liquid metal in the casting space, by means of the porosity of the ceramic (filling "by vacuum”), which leads to too rapid filling generating defects in the castings.
• the invention aims to remedy these drawbacks and for this purpose relates to a low-pressure casting process in a mold comprising a ceramic shell, one face of which delimits a space for 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 an access connected to the casting space and connected to a source of liquid metal below , process in which a first pressure value adapted to bring liquid metal into the region of said access is applied to the liquid metal of the source, the pressure is increased to a second value slightly greater than a value necessary to fill the space and this second value is maintained for a first predetermined duration, then the pressure is increased to a third value and this third value is maintained for a second predetermined duration, then the pressure is reduced so that the level of the liquid metal drops approximately in the region of said access or below, characterized in that, approximately when the casting space becomes completely filled with liquid metal, the pressure in the space of the mass of sand is reduced below atmospheric pressure to a
• 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 bars absolute; - the second pressure value is approximately
• the third pressure value is approximately 1.7 bars absolute.
• the invention also relates to an installation for implementing the above method, comprising a mold comprising a ceramic shell, one face of which defines a pouring 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 having at its base an access connected to the casting space and connected to a source of liquid metal below, installation characterized in that the box has an internal vacuum chamber provided at least one wall region permeable to air and having an access opening provided with a vacuum connection to be connected to a suction device via a solenoid valve controlled by a programmable automaton also controlling a device for regulating the pressure of the liquid metal source.
• the installation according to the invention can also have the characteristic that the vacuum chamber extends annularly in the box against a base wall and side walls of the latter.
• the molding process can be implemented very quickly and be precisely controlled with an investment in material remaining moderate despite everything.
• FIG. 1 is a general diagram of an installation for setting work of the process according to the invention
• FIG. 2 is a diagram illustrating the succession of the phases of the process according to the invention.
• the installation according to the invention comprises a box 1 in which a mold 2 is made up comprising a ceramic shell 21 which may be of the “consumable” type, one face of which defines a pouring space 22 whose shape corresponds to the external 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 the latter and the other face of the shell and thus in contact with this other face. Thanks to .'invention, the thickness of the shell
• 21 can be weak, and for example a thickness of 2 mm can be reached.
• the box 1 has a base wall 11 and side walls 12 having an edge common with the base wall and an upper 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 pouring space 22 by a tubular joint 15 having a part fitted into the opening of the base wall and a part serving as a bearing for the inner face, of form conjugate, from the mouth (lower) of the shell 21.
• the box 1 comprises an annular vacuum chamber 3 extending against the peripheral region of its base wall 11 and the regions of the side walls 12 which terminate at this base wall.
• This vacuum chamber has at least one wall 31 in contact with the mass of sand 23; this wall 31 comprises at least one region permeable to air but impermeable to sand.
• One of the side walls of the box 1 and of the vacuum chamber 3 has a second access opening 16 opening into the vacuum chamber, provided with a vacuum outlet 17.
• the mass of sand 23 fills the space of the box 1 between the walls of the latter and the shell 21, with the exception of the vacuum chamber 3.
• the box is placed on a support plate
• the support plate With a view to filling the caisson with sand, for example by means of a hopper, the base opening of which opens into the caisson, the support plate is first of all placed on a vibration device (also not shown) comprising, for example, eccentric rollers.
• the box 1 can be filled with soft sand, that is to say without binder, while being vibrated to obtain maximum densification.
• the level of sand is then equalized with the rule to obtain a regular level at 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 provided with the seal 15 is opposite an outlet neck 41 of a pipe 42 for supplying liquid metal from the oven, so that the neck can be secured to the box and the supply line thus connects the casting space of the mold to the interior space of the oven.
• a movable plate 5 under which is fixed a seal 51 is attached to the box for example by clamping to form the cover thereof, the seal 51 then being interposed between the plate and the flange 13 formed on the side walls of the box.
• the seal crushed between the plate and the flange, matching the shape of the latter, therefore makes it possible to create a vacuum inside the box.
• the vacuum socket 17 is tightly connected to a suction pipe 61 connecting this socket to a suction device 6 comprising a vacuum manifold 62 placed under vacuum by a pump 63 such as a vane pump.
• a solenoid valve 64 controlled by a programmable automaton 7 is inserted between the vacuum feeder 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 pipe 45 for pressurizing a device for pressure regulation 8 also controlled by the programmable controller 7.
• a pressure of value PI for example of the order of 1.15 bar absolute, which brings the liquid metal by an upward movement in the region of the access opening 14 of the base of the box 1, at the threshold of the mouth of the mold 2.
• phase AB the pressure is increased (phase AB), up to a value P2 for example of approximately 1.4 bar absolute, and more generally a value slightly greater than that necessary to fill the ascending space fully upward with a safety margin
• phase BC ⁇ h sufficient, and this value is maintained to allow a rest regime to be established (phase BC).
• point B the pressure in the mass of sand; the reduction is continued until a vacuum value is reached with respect to atmospheric pressure, corresponding to a predetermined absolute pressure preferably in the range approximately from 0.5 to 0.9 bar, and this is maintained depression value for a certain period of time as will be seen below.
• the reduction in the pressure in the space of the mass of sand is carried out quickly, because the duration of this phase of reduction below atmospheric pressure to the selected vacuum value must be shorter than that of phase BC where the mass of liquid metal is subjected to the pressure of value P2.
• the pressure of the liquid metal is therefore maintained at the value P2 here 1.4 bars, not only as long as the cavity 22 is not filled, but also as long as the depression in the mass of sand does not has not reached the chosen level.
• the pressure applied to the liquid metal is further increased to a value P3 for example of the order of 1.7 bars in order to fill in any cavities or shrinkage, and this P3 value of overpressure is maintained for a period making it possible first to obtain a solidified crust against the shell, then the solidification of the metal contained in the mold (DE phase).
• the depression in the mass of sand is maintained at a value corresponding to the absolute pressure of 0.5 to 0.9 bar here approximately up to point E, and in any case the "vacuum” must not be “broken” before the formation of the crust of metal against the shell 21.
• the pressure on the liquid metal is reduced (phase EF) so that the level thereof descends approximately to the initial level (PI pressure value here equal to approximately 1.15 bars), and this pressure is maintained for the time necessary for example to remove the mold and put back another for the production of another part .
• PI pressure value here equal to approximately 1.15 bars
• the vacuum being applied to the mass of sand at the end of the filling of the shell it does not affect the flow of the metal in the latter, and this flow remains controlled by the pressure , while the depression in the mass of sand stiffens 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Casting Devices For Molds (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Other Liquid Machine Or Engine Such As Wave Power Use (AREA)

Applications Claiming Priority (3)

Publications (2)

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Family Applications (1)

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• 1998
  • 1998-01-07 FR FR9800091A patent/FR2773337B1/fr not_active Expired - Fee Related
  • 1998-12-29 EP EP98964545A patent/EP0968064B1/de not_active Expired - Lifetime
  • 1998-12-29 US US09/380,463 patent/US6422293B1/en not_active Expired - Fee Related
  • 1998-12-29 WO PCT/FR1998/002908 patent/WO1999034945A1/fr not_active Ceased
  • 1998-12-29 DE DE69808892T patent/DE69808892T2/de not_active Expired - Lifetime
  • 1998-12-29 ES ES98964545T patent/ES2186248T3/es not_active Expired - Lifetime
  • 1998-12-29 CA CA002282673A patent/CA2282673A1/fr not_active Abandoned

Non-Patent Citations (1)

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