EP0463911B1 - Process, mould and device for low-pressure multi-stage casting of metal - Google Patents

Abstract

At each stage of the mould (4), the sum of the areas of the inlet cross-sections of the intermediate channels (30A, 30B) which feed the casting runners (32A, 32B) is markedly smaller than the area of the cross-section of the casting pool. Application to the multi-stage casting of thin-wall castings. <IMAGE>

Description

The present invention relates to metal casting under low pressure in a multi-stage blind sand mold. It relates firstly to a process for feeding the imprints of a mold in blind sand with several stages, in metal casting under low pressure, the supply taking place via a casting well and, on each stage, at least one intermediate channel starting from this well and the attacks connecting this channel to the or each footprint. A process of this type is described in FR-A-2 295 808.

The low pressure casting technique (see for example FR-A-2 295 808, 2 367 566 and 2 556 996 in the name of the Applicant) is particularly advantageous, compared to gravity casting, for producing metal parts with thin wall and / or of complex shape and / or of large dimensions. Indeed, the pressure exerted by the metal, which results from the injection of a gas into a sealed pocket containing the liquid metal, can be controlled at will so as to push the metal into all the recesses of the imprints.

In the conventional technique of the aforementioned FR-A-2 295 808, on each stage of the mold, two intermediate channels of large section connect the casting well to all of the attacks on the stage.

• (1) Il se crée de fortes turbulences dans le flot de métal, ce qui favorise l'érosion du sable et l'occlusion de bulles d'air, au détriment de la santé des pièces obtenues.
• (2) Il n'est pas possible de faire monter rapidement le métal jusqu'au sommet du puits de coulée, et le remplissage s'effectue en fait étage par étage, ce qui ne permet pas de bénéficier de tous les avantages de la coulée basse pression.
• (3) Lorsque la pression est relâchée, après solidification des attaques qui forment obturateurs (voir le FR-A-2 295 808 précité), le métal contenu dans le canal intermédiaire, représentant un volume relativement grand qui s'est notablement refroidi, retourne dans la poche de coulée. A la coulée suivante, c'est du métal moins chaud qui monte en premier dans le puits de coulée, ce qui nuit à la qualité de certaines pièces moulées. Pour la même raison, un déplacement de métal excessif est nécessaire à chaque coulée.

This way of doing things has the following drawbacks, linked to the large section of the channels.

• (1) Strong turbulence is created in the metal flow, which promotes the erosion of the sand and the occlusion of air bubbles, to the detriment of the health of the parts obtained.
• (2) It is not possible to quickly raise the metal to the top of the pouring well, and the filling is in fact carried out stage by stage, which does not make it possible to benefit from all the advantages of low pressure casting.
• (3) When the pressure is released, after solidification of the attacks which form obturators (see FR-A-2 295 808 cited above), the metal contained in the intermediate channel, representing a relatively large volume which has notably cooled, returns in the ladle. At the next casting, less warm metal rises first in the casting well, which affects the quality of some molded parts. For the same reason, excessive metal displacement is necessary for each casting.

The invention aims to remedy these drawbacks. To this end, the method according to the invention is characterized by the characterizing part of claim 1.

Other features are described in claims 2 and 3.

The invention also relates to a blind sand mold intended for the implementation of such a process. This mold is described in claim 4, and other features are described in claims 5 and 6.

The invention also relates to a low pressure metal casting installation in a blind sand mold on several floors, of the type comprising a casting ladle from which a feed tube opens upwards, a gas source under pressure connected to the ladle, at least one blind sand mold comprising a casting well open downwards and at least two stages on each of which is provided at least one footprint fed via attacks connected to the casting well by at least one intermediate channel, and means for applying the base of the tapping well to the opening of the supply tube, characterized in that the mold conforms to the definition above.

The preambles of claims 4 and 7 are also based on FR-A-2 295 808.

• la Fig. 1 représente schématiquement, en coupe verticale, une installation de coulée conforme à l'invention ;
• la Fig. 2 représente un moule utilisé dans cette installation, en coupe suivant la ligne II-II de la Fig. 3 ; et
• la Fig. 3 est une vue prise en coupe suivant la ligne III-III de la Fig. 2.

A nonlimiting example of implementation of the invention will now be described with reference to the appended drawings in which:

• Fig. 1 schematically represents, in vertical section, a casting installation in accordance with the invention;
• Fig. 2 shows a mold used in this installation, in section along line II-II of FIG. 3; and
• Fig. 3 is a view taken in section along line III-III of FIG. 2.

The installation shown in Fig. 1 includes an enclosure 1 forming a pocket or reserve of liquid metal 2, a frame 3 for mold support and a sand mold 4. It is applied to low pressure casting of cast iron (gray cast iron or spheroidal graphite cast iron), steel or a superalloy in the mold 4. Apart from the internal configuration of this mold, the installation is identical to that described in the aforementioned FR-A-2 295 808.

The pocket 1, fixed, has an upper cover 5 tightly fixed to its side walls and locked by suitable means (not shown). A pouring nozzle 6 passes through an orifice 7 formed in the cover 5. This nozzle 6 comprises a tubular lower part 8 whose outside diameter corresponds to the diameter of the orifice 7 and an upper part 9, of generally frustoconical shape, which s' presses tightly on the periphery of the orifice 7 by its large flat base 10. A seal 11 formed by an asbestos bead is housed in a groove formed in the base 10 of the nozzle. The nozzle 6 is traversed by a conduit or supply tube 12 made of refractory material dipping into the cast iron up to the vicinity of the bottom of the pocket 1; the upper part of the duct 12 opens out at the center of the nozzle 6, at the level of the planar upper face of the latter.

The bag 1 is connected to a source 13 of pressurized gas by a conduit 14, the communication of the bag 1 with the pressure source 13 or with the atmosphere effected by a suitable device 15 outside the bag. A pressure gauge 16 makes it possible to monitor the pressure prevailing inside the pocket during pouring.

The frame 3 comprises posts 17 provided at their base with wheels 18 carried by two rails 19.

The posts 17 are joined at their upper end by a ceiling 20 carrying a jack 21 directed downwards and whose piston rod 22 carries, articulated at its lower end, a support plate 23.

The posts 17 also each carry a flange 24 on which a helical spring 25 rests. A horizontal support plate 26 can slide vertically along a part of the: posts 17 located above the flanges 24; this plate 26 is in constant abutment on the upper end of the springs 25 and is urged by them upwards. When no downward pressure is applied to the plate 26, it is at a level higher than that of the upper face of the nozzle 6. A circular opening 27, of sufficient diameter to allow the nozzle 6 to pass , is provided in the plate 26.

The mold 4 is a solid multi-stage blind mold, for example with three stages, as shown in FIG. 2. It comprises a vertical pouring well 28, of circular section approximately equal to that of the supply tube 12. This well is open at its base, which has a housing 29 having a tapered flared shape combined with that of the nozzle 6. It extends to a certain distance from the upper end face of the mold.

The three stages are identical, and the structure of each stage appears in FIG. 3. From the pouring well 28, two channels 30A and 30B leave horizontally, which extend in the extension of one another, on either side of the well 28. The stage also comprises two indentations 31 of elongated shape , identical. Each footprint, which extends on either side of the well 28, is fed by three attacks. In the example shown, there is for each fingerprint an attack 32A on one side of the well and two attacks 32B on the other side. The three attacks 32A connect the fingerprints to channel 30A, and the three attacks 32B connect them to channel 30B. Thus, each channel 30A or 30B connects the casting well to all: the attacks located on the same side of the well 28 and therefore participates in the supply of the two footprints of the floor.

Channels 30A and 30B have a relatively small section. More precisely, the sum of the areas of the sections of the intermediate channels 30A, 30B of a stage is much less than the area of the section of the taphole 28, for example less than 10% of this area. If the channels are of variable section, it is their input section which fulfills this condition.

In addition, the sum of the areas of the attack sections fed by the same channel is at least equal to the area of the section, or of the entry section, of this channel.

The operation of the installation is as follows: the frame 3 being distant from the pocket 1, an appropriate refractory seal 33 is applied to the bottom of the housing 29 of the mold 4. The mold 4, containing in each cavity a non-core shown, is placed on the plate 26 and centered on the opening 27 thereof, then the frame 3 is brought to the rails 19 above the pocket 1 of liquid iron so that the nozzle 6 is located opposite the housing 29 of the mold. The jack 21 is then extended so as to lower, by means of the plate: 23, the mold 4 and its support 26 against the force of the springs 25. This operation clamps the seal 33 between the bottom of the housing 29 and the nozzle 6 and ensures the tight connection of the pouring well to the supply tube.

The bag 1 is then connected to the pressure source 13 by operating the device 15. The pressure acting on the free surface of the cast iron makes it rise in the tube 12. The cast iron fills the well 28 of the mold, the channels 30A and 30B and the imprints 31. The pressure is maintained for a time determined according to the dimensions and the shapes of the parts to be obtained. The well 28 plays during this time the role of reserve or counterweight by bringing to the imprints the additional liquid cast iron intended to compensate for the withdrawals. Then the attacks and the intermediate channels solidify, the gas pressure is brought back to atmospheric pressure in the pocket 1, by maneuvering of the device 15, and the liquid iron being in the well 28 and in the tube 12 goes back down in the pocket 1 by evacuating these two conduits.

The action of the jack 21 is then eliminated, the mold-support assembly 26 is moved away from the nozzle 6 under the action of the springs 25, and the assembly of the frame 3 is moved away horizontally from the pocket on the rails 19.

Thanks to the dimensioning of the channels 30A, 30B indicated above and by ensuring through the conduit 14 an appropriate gas flow, the liquid metal rises rapidly in well 28, the metallostatic pressure is established in the channels on each stage, and the two cavities of each stage are supplied by the pair of channels 30A and 30B corresponding. This makes it possible to simultaneously fill all the imprints, whatever their shape. In addition, the narrowness of the intermediate channels limits the metal flow which flows there, which leads on the one hand to a better controlled filling, quieter and reproducible for each of the imprints of the different stages, and on the other hand minimal metal displacement with each pour. This is further favored by the relaxation of the liquid metal at its entry into the attacks, resulting from the aforementioned dimensioning of these. This ultimately results in an improvement in the health of the castings.

The relaxing effect of the metal can, as a variant, be favored by an increasing section, from upstream to downstream, of the intermediate channels.

It should also be noted that the use of intermediate channels as plugs prevents any return of significantly cooled metal in the pocket, without reducing the yield of metal. This is very advantageous for the reproducibility of the casting conditions. In addition, the slightly cooled metal contained in the casting well is, on the next casting, distributed among all of the mold cavities.

Claims (7)

1. Process for supplying the impressions (31) of a multi-stage blind sand mould (4) with metal melt under low pressure, the supply being carried out via a casting shaft (28) and, at each stage, at least one intermediate channel (30A, 30B) starting from this shaft and ingates (32A, 32B) connecting this channel to the or each impression, characterised in that the flow of liquid metal is throttled at each stage at the inlet of the intermediate channels, and the liquid metal is relieved after its entry into the intermediate channels (30A, 30B) and at the latest when it enters the ingates (32A, 32B).
2. Process as claimed in Claim 1, characterised in that a flow of liquid metal which is adapted so as to cause this metal to rise above all the intermediate channels (30A, 30B) is sent into the inlet tube.
3. Process as claimed in Claim 1 or 2, characterised in that the mould supply pressure is maintained until solidification has taken place in all the intermediate channels (30A, 30B), then this pressure is released.
4. Blind sand mould for carrying out a process as claimed in any one of Claims 1 to 3, of the type comprising a casting shaft (28) and at least two stages at each of which is provided at least one impression (31) supplied via ingates (32A, 32B) connected to the casting shaft by at least one intermediate channel (30A, 30B), characterised in that the sum of the areas of the inlet sections of the intermediate channels of each stage is clearly less than the area of the section of the casting shaft, and in that the sum of the areas of the sections of the ingates (32A, 32B) supplied by one and the same channel (30A, 30B) is at least equal to the area of the inlet section of this channel.
5. Sand mould as claimed in Claim 4, characterised in that each impression (31) is supplied by at least two intermediate channels (30A, 30B) extending an either side of the casting shaft (28).
6. Sand mould as claimed in Claim 4 or 5, characterised in that the sum of the areas of the inlet sections of the intermediate channels (30A, 30B) of each stage is less than 10% of the area of the section of the casting shaft (28).
7. Installation for casting metal under low pressure in a multi-stage blind sand mould, of the type comprising a casting ladle (1) from which an inlet tube (12) extends which is open towards the top, a source (13) of gas under pressure connected to the ladle, at least one blind sand mould (4) comprising a casting shaft (28) which is open towards the bottom and at least two stages, at each of which is provided at least one impression (31) supplied via ingates (32A, 32B) connected to the casting shaft by at least one intermediate channel (30A, 30B), and means (21) for applying the base of the casting shaft onto the opening of the inlet tube, characterised in that the mould conforms to any one of Claims 4 to 6.

Images