GB2050220A - Method and installation for low pressure casting of metal parts in a thin-walled sand impression - Google Patents

Abstract

The impression (21) is wedged within a mass of magnetisable particles (22), which are packed before casting by vibrating the mould, e.g. by vibrators (15) disposed below a casting table (14), and then made rigid by a magnetic field produced by an electromagnet (17). <IMAGE>

Description

SPECIFICATION Method and installation for low pressure casting of metal parts in a thin-walled sand impression The present invention relates to low pressure casting of metal parts, for example from pig iron, in a mould comprising a thin-walled impression constituted by a hardened mixture of sand and resin.

Low pressure casting, in which foundry moulds are supplied from the bottom at a pressure greater than atmospheric pressure, but substantially less than 2 bars, is known.

The advantage of the latter is to facilitate the filling of impressions of complicated shape which would be poorly filled by simple gravity casting and to lend itself to automation in the control of successive casting operations, whilst eliminating troublesome and dangerous operations of handling tilting casting ladles or casting ladles comprising a stopper-rod. Furthermore, by using suitable means for supplying the mould and controlling the pressure (see for example French Patent No.

7442 71 3 in the name of the Applicant published under No. 2 295 808), the advantage of low pressure casting is to eliminate shrinkage-heads and certain casting appendages.

It is known that low pressure moulding or casting are frequently carried out in sand moulds, in particular in order to obtain certain metallic structures in the cast parts. This is the case for example when one wishes to produce rough-cast parts from spheroidal graphite cast iron, which have no carbides in the rough-cast state, without necessitating any subsequent thermal treatment.

This result is due essentially to the refractory nature of the sand forming a thermal barrier, which eliminates a hardening phenomenon of the cast metal.

In addition, one thus benefits from the permeability to gases of the sand.

Thus, moulds with walls consisting of sand prevent casting faults and reduce rejects.

However, the use in this method of conventional sand moulds, which are solid, causes difficulties: on account of the fact that the filling pressure is maintained for a prolonged period of time, the moulds must have relatively high mechanical strength. It is therefore necessary to use sand comprising a greater proportion of binding agent and/or moulds which are thicker than in the case of simple gravity casting. But this increases the cost price of the parts since on the one hand the moulds, which can be used only once, are more expensive to produce and on the other hand, cooling of the parts becomes too slow, which considerably reduces the production rate, if only on account of the delay necessary for the solidification of the inlet point of the metal into the moulding cavity.

This is why an attempt has been made to limit the quantity of sand used. In a known solution (see the Applicant's French Patent No. 77 08 364 published under No.

2 384 568), the impression is enclosed by a metal shell. The shell must thus be changed for each shape of cast part. In addition, the shell frequently requires machining, even though rough machining.

The object of the present invention is to provide another solution which is more flexible and more economical, for casting of this type, thus ensuring an effective support for the impression, enabling the latter to withstand the force of the metal under pressure.

To this end, the invention relates to a method for the low pressure casting of metal parts, in particular from cast iron, in a thinwalled sand impression, characterised in that the impression is packed within a mass of magnetisable particles which are compressed and made rigid before casting, by means of a magnetic field.

The invention also relates to a mould for carrying out such a method. This mould, of the type comprising a thin-walled sand impression, is characterised in that it also comprises a closed box provided in its upper part with a closable opening for filling with magnetisable particles and, in its base, a casting orifice, the impression comprising a casting orifice and being able to rest on the base of the box with its casting orifice in communication with that of this box.

The invention also relates to a low pressure casting installation for carrying out the method described above. This installation, of the type comprising a frame which surrounds a casting ladle connected to a source of compressed propellant gas and provided with a roughly vertical supply pipe emerging from this ladle, the frame comprising means for supporting successive moulds and applying the casting orifice thereof against the upper end of the supply pipe, is characterised in that each mould is such as defined above and in that the installation comprises an electro-magnet able to be placed around each mould during casting.

Further features and advantages of the invention will become apparent from the ensuring description, given as a non-limiting example and with reference to the accompanying drawings in which: Figure 1 is a partial diagrammatic view in vertical section of an installation according to the invention; Figure 2 is a partial plan view of this installation.

The low pressure casting installation illustrated comprises a casting ladle 1, a frame 2 and a certain number of moulds 3 which are all identical, of which only one has been shown. The dimensions of the mould 3 have been exaggerated with respect to those of the ladle 1 for the sake of clarity of the drawings.

The ladle 1 comprises a container 4 of large dimensions containing liquid metal 5 and comprising an oblique filling spout 6 closed by a lid 7. A vertical supply pipe 8 of refractory material passes through the upper wall of this container. It descends virtually to the bottom of the latter and projects thereabove over a short length, where it is surrounded by a reinforcing nozzle 9 of general frustoconical shape. The upper end of the pipe 8 is contained in the same horizontal plane P as the flat upper side of the nozzle 9.

A pipe 10 communicating with the inside of the container 4 may be connected, under the control of a valve 11, either to a source of compressed gas, for example compressed air, or to the outlet.

The frame 2 comprises four telescopic uprights 1 2 provided with helical springs 1 3.

The uprights 1 2 support a horizontal table 14 pushed permanently upwards by the springs 1 3. Fixed below the table 1 4 are vibrating devices 15, whereas an electro-magnet 1 6 of U shape comprising two parallel sides or arms 1 7 connected by a coil 1 7a is positioned on this table.

The table 14 comprises a frustoconical orifice 1 8 mating with the outer surface of the nozzle 9. In the lower position of the table, the orifice 18 bears on this surface and the upper side of the table is contained in the plane P described above.

The frame also comprises a retractable vertical jack 1 9 located at a slightly higher level, straight above the ladle 1.

The mould 3 is constituted by a box 20, a shell or casting screen 21 and a mass of particles 22. The box and mass 22 are common to all the moulds, in contrast to the shell 21.

The box 20 is a parallelepipedal metal box whereof the length is slightly less than the distance separating the two sides 1 7 of the electro-magnet 16. The base of this box has a frustoconical opening 23, which is convergent in the downwards direction, whereas its upper wall constitutes a removable lid or pressing plate 24. The opening 23 opens to the outside in a rebate 25 of slight depth.

The shell 21 is a thin casing or impression consisting of a hardened mixture of sand and thermosetting resin such as for example an isocyanate resin or a phenol-formaldehyde resin. This shell, divided into two halves along a horizontal interface Q, defines with an inner core 26 a moulding cavity 27 in the shape of a part to be cast, which is for example a gatevalve body.

The lower half-shell comprises a tubular projection or appendage 28 which defines a main conduit or inlet 29 of the same diameter as the pipe 8. Externally, this projection is frustoconical and fits the opening 23 in the box. When it is fitted in this opening, its end face is flush with the bottom of the rebate 25.

The conduit 29 communicates with the moulding cavity solely through a small horizontal conduit or secondary inlet 30.

Externally, the shell 21 has annular stiffening or strengthening projections, one of which, bearing the reference numeral 31, is at a distance from the appendage 28. When the shell is placed in the box 20, which leaves a considerable clearance on all sides, the projection 31 rests on the bottom of this box, possibly through the intermediary of a wedge 32 as illustrated. The shell is thus perfectly positioned, in a stable manner, without the sand being subjected to excessive stress. The space remaining free in the box 20 is filled with the mass 22, constituted by compressed magnetisable iron shot, the grain size of which is 17/10 mm for example.

The operation of this installation is as follows.

At the beginning, the casting ladle 1 containing liquid metal 5, for example pig iron, is kept under a moderate air pressure, for example of the order of 1 bar, through the conduit 10. Under the effect of this pressure, the pig iron is located in the supply pipe 8 at a level N substantially lower than the level of the outlet opening of this pipe. The table 14 is in the upper position under the action of the springs 13, the jack 1 9 being retracted as is the lid 24. The latter may moreover be integral with the piston rod of the jack 19.

An adhesive sealing washer 33, for example of the type described in the aforementioned French Patent No. 7442 713, is placed in the rebate 25 of the box 20, which is then placed on the table 14 between the arms of the electro-magnet 1 6 and positioned so that its orifice 23 is centered on the orifice 1 8 of this table. A slight clearance, for example of 0.5 mm, not visible in the drawing, then exists between each arm 1 7 and the adjacent side of the box 20. A shell 21 containing a core 26 is placed lengthwise on the base of the box 7 in order to be supported by its projection 31 and positioned and supported by its appendage 28.

Despite the weight of the mould and of the electromagnet, the springs 1 3 are not completely compressed and thus lift the table 14 slightly, so that the washer 33 is still not compressed.

Magnetisable iron shot is then poured or blown into the box 20 in order to fill its lower part, between the shell 21 and the bottom of the box, as well as its upper part, in order that the shell is covered completely. The outer shape of the shell 21, constituted almost exclusively by rounded surfaces, promotes the introduction of shot below the shell.

Then, the vibrators 15 are set in action, in order to cause the table 14 to vibrate on its springs 13 and to pack the mass of shot 22 in a suitable manner.

During this packing operation, the level of shot in the box 20 drops. The latter is then topped-up with shot and packing and refilling are recommenced until the level of shot is flush with the upper part of the box 20. When it has been packed in a suitable manner, the mass of shot 22 has the minimum of gaps therebetween, in particular in the vicinity of the sl'ell and the maximum density.

After vibration and packing, the electromagnet 1 6 is excited, which subjects the mass of shot 22 to a powerful magnetic field which renders this mass 22 rigid, the vibrations being stopped immediately after the shot-filling operation and before the excitation of the electro-magnet.

The mass 22 packed in this way and hardened provides an excellent rigid support for the shell 21, the arrangement being virtually equivalent, from the mechanical point of view, to the metal shell lined internally with sand and described in the aforesaid French Patent No. 7708364.

The pressure plate 24 is then put in position and lowered by the jack 1 9 in order to press the mould 3 with force against the vibrating table 1 4. This compresses the springs 1 3 and presses the opening 1 8 in the table against the nozzle 9 and the sealing washer 33 is compressed between the latter and the bottom of its housing.

For reasons of safety, at least one clamp 34 can be placed around the mould 3 and the table 1 4 in order to complete the inter-locking of these two parts of the installation, which is now ready for casting.

In order to carry out the latter, the valve 11 is opened and air, for example at a pressure of the order of 1.2 bars, is introduced into the container 4 above the liquid pig iron 5. The pig iron rises, through the pipe 8, the main inlet 29 and the secondary inlet 30 into the casting impression 27, which it fills completely. The force which the metal introduced under pressure exerts on the walls of the shell 21 is counter-balanced by the resistance offered by the means of shot 22 which is packed and made rigid and by the pressure plate 24, which prevents rising of the mould 3 above the table 14.

During casting, the magnetic mass 22 remains relatively cold, in particular in the vicinity of the shell 21, where its temperature scarcely exceeds 200"C owing to the refractory nature of the shell, which does not transmit the heat from the cast iron. In the upper part of the mould, the mass of shot is quite cold. This mass thus remains magnetic and rigid, since it is far from the Curie point of 750"C at which iron loses its magnetic properties.

It will be noted that during casting, which is very favourable, no gas is liberated outside the mould 3, although the gases produced are able to leave the casting impression 27 through the porous shell 21 and the permeable mass of shot 22. It is considered that since the mould 3 is completely filled with shot in its upper part and the mass 22 remains cold, the gases spread out jn this mass with a tendency towards condensation which prevents them from rising to the upper end of the mould.

After filling the impression 27 with liquid pig iron and after the pressure is maintained for a certain relatively short period of time, corresponding to the solidification of the secondary inlet 30 after an adequate supply of pig iron to the impression, as described in the said French Patent No. 7442713, the pressure is released in the ladle 1 by supplying through the conduit 10 a pressure which is less than the casting pressure but sufficient to restore the pig iron solely to a level N1 illustrated in broken line, close to the upper part of the nozzle 9.

The supply of electric current to the electromagnet 1 6 is then cut. The pressure plate 24 is raised and retracted with the jack 1 9 and the mass of shot 22 is removed at least partly from the mould 3 which has just been filled with cast iron. This operation takes place under a hood which is not shown, since it is at this time that the gases formed during casting and which have been retained up till then in the upper part of the mass of shot 22, outside the shell 21, are liberated, probably by evaporation. When this shell has been exposed sufficiently from the mass of shot, it is removed with the casting, stripping of which is then carried out.

For the subsequent casting operation, it is sufficient to introduce a new shell 21 provided with its core 26, into the remaining shot, in the same box 20 once more placed on the table 14, then to complete the operation of filling with shot, using the shot which has just been removed from the preceding mould 3 and to carry out packing and the other successive operations which have been described. In practice, it is unnecessary to remove all the shot after each casting, since the latter retains its configuration to a certain extent, when the shell 21 is extracted therefrom.

As a numerical example, in order to cast a valve body cast iron having a weight of 60 kg with a tubular diameter of 200 mm, the box 20 measuring 850 X 950 X 500 mm, the mass 22 is subjected to a magnetic field of 4000 gauss, this field depending on the section of shot to be made rigid and thus on the distance between the arms 1 7 of the electromagnet. The gran size of the shot is chosen to be sufficiently fine to leave the minimum number of gaps, since gaps lead to an increase in the magnetic field necessary, without however loading the mould excessively.

The cast part obtained has a skin, i.e. an outer surface condition, of very fine appear ance and has very accurate dimensions.

Since a low pressure casting process is used, liquid metal which is always hot is introduced into the casting impression 27 and this liquid metal is never apparent externally, which guarantees the comfort and safety of the operators who may be in the vicinity of the installation. Furthermore, the rising and falling of the liquid metal in the pipe 8 takes place simply by operating the valve 11 and connection to a source of suitable pressure or to a discharge, which can be readily automated. Thus, the supply of liquid metal to the mould has no troublesome nor dangerous features.Owing to the vibrating means 15, the density of the mass 22 is sufficient to guarantee that there are no local gaps in contact with the shell causing locally defective external support for the latter and consequently a risk of deformation and rupture under the force of liquid metal, despite the fact that the mass 22 is made rigid by the magnetic field.

The mass of shot 22 is inter-changeable and completely reusable whatever the shape of casting impression and the complementary equipment constituted by the vibrators 1 5 and electro-magnet 1 6 comprise solely parts which are easily available. The method of the invention is thus very economical.

The casting method described is well suited to producing parts of complicated shapes. The only limitations as regards volume and weight of the castings which are able to be cast are dependent on the power of the magnetic field which can be produced by means of the electro-magnet 16. Thus, with a magnetic field of 4000 gauss, one can produce parts weighing up to 500 kg.

As a variation, the mould 3 could be filled with shot and vibrated away from the casting station, on another vibrating table. In this case, the table 14, serving solely to receive the mould at the casting station and to support the electro-magnet 16, would no longer be provided with vibrating means 1 5.

In all cases, one attempts to give the shell external shape which facilitates good distribution of the shot, even if the moulding cavity does not have corresponding shapes. The latter may be rounded surfaces, as in the example illustrated, or any other suitable deflecting shape according to the technique of filling the box.

Claims (14)

1. A method for the low pressure casting of metal parts in a thin-walled sand impression, characterised in that the impression is packed within a mass of magnetisable particles which are vibrated, and the packed mass is made rigid before casting by means of a magnetic field.

2. A method according to Claim 1, characterised in that the magnetisable particles are in the form of iron shot.

3. A method according to Claim 2, characterised in that the magnetisable means is packed by causing a mould support to vibrate.

4. A mould for carrying out a method according to any one of Claims 1 to 3, of the type comprising a thin-walled sand impression, characterised in that it also comprises a closed box provided in its upper part with a closable opening through which the box may be filled with magnetisable particles and, in its base, a casting orifice, the impression comprising a casting orifice and being adapted to rest on the bottom of the box with its casting orifice in communication with that of the box.

5. A mould according to Claim 4, characterised in that the casting orifice of the impression is provided in a tubular appendage of the impression which appendage fits in the casting orifice of the box.

6. A mould according to Claim 5, characterised in that the lower part of the impression has at least one supporting projection spaced apart from said tubular appendage.

7. A mould according to Claim 4, characterised in that when the impression is positioned in the box, its free surfaces are essentially rounded.

8. An installation for low pressure casting for carrying out a method according to any one of Claims 1 to 3, of the type comprising a frame which surrounds a casting ladle connected to a source of. compressed propellant gas and provided with a vertically extending supply pipe which emerges from the ladle, the frame comprising means for supporting successive moulds and for pressing a casting orifice thereof against an upper end of the supply pipe, the installation being characterised in that each mould is in accordance with any one of Claims 3 to 5 and in that the installation comprises an electro-magnet which can be placed around each mould during the casting operation.

9. An installation according to Claim 8, characterised in that the electro-magnet is supported by said means for supporting successive moulds.

10. An installation according to any one of Claims 8 and 9, characterised in that the means for supporting the moulds are constituted by a table supported by springs and in that a jack is provided for pushing the table supporting a mould downwards. /

11. An installation according to Claim 10, characterised in that the jack acts on a lid of the opening through which the box may be filled.

1 2. An installation according to Claim 1 0 or 11, characterised in that the table is provided with vibrating devices.

1 3. A mould for carrying out a method according to any one of claims 1 to 3, substantially as hereinbefore described with reference to the accompanying drawings.

14. An installation for low pressure casting for carrying out a method according to any one of Claims 1 to 3, substantially as hereinbefore described with reference to the accompanying drawings.