BACKGROUND OF THE INVENTION There exists a squeeze casting method for casting metal articles using a vertically acting hydraulic press, on the platen of which one finds a rigidly fixed immovable mould half with a punch on the press plunger. A precisely metered portion of molten metal is poured into the immovable mould half. As moving, the punch closes the orifice of the immovable mould half with a minimum gap while exerting pressure on the molten metal on the entire section of the casting thus, shaping it in the process until its complete solidification.

DISADVANTAGES OF THE ABOVE DESCRIBED METHOD: 1. The dosage inaccuracy of the molten metal portion has an effect on the height of the cast article.

2. The cast article engulfs all the scooped up deleterious solid inclusions from the melt in crucible.

3. In spite of all precautions taken with regards to the velocity and place of the pouring of the molten metal, part of the molten metal is transformed into splashes which get oxidized and engulfed in the cast article.

4. The quick penetration of the punch into the molten metal results in the formation of splashes as well.

5. An extended stroke of the hydraulic press plunger is required for the filling of the immovable mould half with molten metal.

6. Because of the prolonged time gap from the beginning of the mould half filling with the molten metal till the action of the punch, part of the molten metal being already solidified, it is not possible to affect on its structure. This solidified metal layer and the punch construction which compresses the cast article on the entire section, absorbs the greater part of the press force, the remaining force being insufficient for the compressing of the part of the cast article, which is in process of solidification.. It even occurs, when there is no enough pressure for the last portion of the solidifying molten metal, where the shrinkage is formed.

7. It is impossible to cast thin section castings because of the prolonged time gap between the metal filling and the initial action of the punch, part of the molten metal has already been solidified before the punch touches it.

8. The method under discussion does not allow group casting.

There are other methods available (WO 98/43761, WO 96/22851) wherein the molten metal is transffered pneumatically in a closed mould, consisting of two mould halves, from a crucible by means of a vertical conduit. After the mould has been filled with molten metal, one of the mould halves is driven hydraulically, i. e. by means of a hydraulic cylinder; the driven half of the mould compresses the molten metal on its entire section and keeps that position until complete solidification has been effected.

One major disadvantage of the above described method is that, the only movable mould half compresses the molten metal over the entire section of the cast article. This leads, early in the process of solidification, to a great expenditure of casting machine force, which is being absorbed for compressing the thinnest section regions of the cast article, which solidified most quickly and at the very beginning of the process. The regions of the cast article with a greater section keep its liquid state of the molten metal for a longer period of time, and, having been left not compressed, the resultant structure within the regions concerned, differs considerably from the structure of the regions solidified first under the compression exerted by the movable mould half.

Still another disadvantage of the method is the means by which the molten metal is transferred from a gas filled furnace into the mould, which, having been closed in atmosphere conditions, contains a certain amount of atmosphere oxygen; the latter fact results in the oxidization of the molten metal during the filling of the mould and in the increasing of the non-metallic inclusions in the cast article as compared to the initial molten metal.

SUMMARY OF THE INVENTION The goal, set before the invention described below, is to find a method and apparatus for casting metal articles without any limitations as to their shape and with a homogenous structure regardless of their section, whereby the molten metal is not in a relationship with the atmosphere oxygen during its solidification.

The goal of the invention is achieved by method for casting, whereby the preliminarily closed mould, consisting of two mould halves, is filled pneumatically with molten metal by means of a gate running system from a furnace. In compliance with the invention, a variable inert gas pressure is permanently maintained in the furnace and the mould cavity initially, before casting, is filled with inert protective gas. While the mould is being filled with inert protective gas, the punches, forming the part of the cast article, are positioned according to the displacement transducer signals. After the filling of the mould with molten metal, the punches, consecutively and depending on their respective sections, on the shape and on the solidification temperature in particular regions of the future cast article, are moved with a definite stroke, with a definite force and with definite frequency in a definite direction, thereby exerting regulated pressure on these regions of the cast article, where they have already been positioned, so that the solidification period of the cast article is divided into two definite stages- first, initial solidification in conditions of inert protective gas pressure in the furnace, and second, consecutive final regional solidification of the cast article in conditions of partial mechanical pressure.

The goal of the invention is achieved by apparatus, comprising a gas tightened furnace with a crucible in it, in which a conduit is located in a gas- tighten manner. The furnace, set on rails, can be moved outside the apparatus for receiving molten metal or for changing the conduit. The furnace being in the axis of the apparatus, is placed on lifts. The furnace being in upper position, the lifts press it against a plate, whereon the immovable mould half is mounted, wherein the casting cavity, part of the runner, part of the gate running system, being a continuation of the conduit, are formed. The gate running system is situated on one side of the mould cavity of the cast article. The rod of the hydraulic cylinder carries a second plate, where the movable mould half set is mounted, which comprises the mould half itself, sliding bars, springs and a stop for the movable mould half. The second plate is guided on columns, which are part of the apparatus body. There is a filter for molten metal in the gate running system.

According to the invention, the filter is placed on the separation surface between the two mould halves. In the area of the separation surface of the two moul halves, on the side surface of the filter for molten metal, there is a fitting connected with a vessel for protective inert gas by means of a tubing, a valve and a check valve.

The plates are connected by a displacement transducer. To above mentioned second plate another hydraulic cylinder is mounted, whose piston rod carries a third plate, to which the bars being fastened, carry the first pressing punches, which penetrate into the movable mould half cavity and shape part of the future cast article. The second and the third plates are connected by another displacement transducer. To the second plate a subsidiary plate is mounted with bars fixed on it, which, by means of thread connections, carry the second pressing punches.

Temperature gauges are provided in the first and second pressing punches. In another embodiment of the invention the bars carrying the first and the second pressing punches can be constructed as separate actuators, for instance hydraulic cylinders with their own displacement transducers and the first and second pressing punches can penetrate also through the immovable mould half to the casting cavity and to shape part of the future cast article. There are plurality of mould cavities in the mould, each one having its own pressing punch set, in case of a group casting variant.

THE INVENTION ADVANTAGES ARE: 1. The precise metering with clean molten metal, when filling a preliminarily closed mould, with controlled velocity without splashes. The inert gas in the mould cavity leaves it freely through the gaps between the punches and the mould half.

2. The feeding of the cast article with molten metal is performed upwardly by the gate running system which is at the side of the mould cavity, being formed in the immovable and in the movable mould halves and when the mould is open, it is possible to place, before each casting, a new filter on the immovable mould half in the area of the gate running system. The cast article is shaped in the immovable and in the movable mould halves as well as by the action of the pressing punches which are placed in one of the two or in both mould halves.

The punches remain in the mould halves during all their relative movement.

3. The effect on the solidifying metal is divided, for example, in three controlled stages. The first stage begins after mould filling and is exerted by the gas pressure in the furnace. During the second stage, after interrupting of the runner by some of the punches, the first pressing punches, displaced by a separate hydraulic cylinder, perform the first compressing of this part of the cast article with larger relative surface (thin section part of the cast article), where the cast article firstly solidifies. During the third stage, when the second compressing has been accomplished by the second pressing punches, the remained liquid part of the cast article is compressed, until its complete solidification. The second pressing punches are displaced by the main hydraulic cylinder of the apparatus.

4. A cycle is possible, when the second compressing begins during the second stage, simultaneously with the first compressing and continues until the end of the third stage. In this case, during the second stage the first compressing force changes cyclically, with controlled frequency and number of cycles, from 0 to the maximum value. During this stage the compressing force of the second pressing punches, shaping the thick section part of the cast article, changes cyclically with the same frequency and number of the cycles, from the maximum to the minimum value.

5. In case of several first or second pressing punches displaced by one hydraulic cylinder, it is possible to control the compressing pressure on the cast article, not only by the acting punches area but also by controlling the punch force by the dimensions and/or material of the bar displacing the respective punch.

6. In case of several first or second pressing punches displaced by their own hydraulic cylinders, it is possible to control the pressure on the cast article in an absolutely free cycle, which leads to the universality of the apparatus, facilitation of the work on the casting technology optimization after modifying the casting shape or changing the casting alloy.

BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 shows sectional view of the apparatus; Figure 2 shows the cast article; Figure 3 and Figure 4 show enlarged sectional views of the apparatus in different states during the casting: Figure 3 after filling with molten metal; Figure 4 after the final compressing and solidification of the cast article and part of the gate running system.

DETAILED DESCRIPTION OF THE INVENTION The apparatus consists of a gas-tighten furnace 1 with crucible 2, in which a conduit 3 is located in a gas-tighten manner. The furnace 1, set on rails 5, can be moved outside the apparatus for receiving molten metal or for changing the conduit 3. The furnace 1 is placed on lifts 4. The furnace 1 being in upper position, the lifts 4 press it against a plate 6, whereon the immovable mould half 7 is mounted, wherein the casting cavity for the cast article 8, part of the runner 10, part of the gate running system 9, being a continuation of the conduit 3, are formed. The gate running system 9 is situated on one side of the mould cavity for the cast article 8. The rod of the hydraulic cylinder 14 carries a second plate 12, where the movable mould half set 20 is mounted, which comprises the mould half itself 20, sliding bars 21, springs 22 and stop 23 (for the movable mould half 20).

The second plate 12 is guided on columns 13, which are part of the body 28 of the apparatus. There is a filter for molten metal 11 in the gate running system 9. The filter 11 is placed on the separation surface between the movable mould half 20 and the immovable mould half 7. In the area of the separation surface between the movable mould half 20 and the immovable mould half 7, on the side surface of the filter for molten metal 11 there is a fitting 29 connected with a vessel for protective inert gas 32 by means of a tubing 30, a valve 31 and a check valve 34.

The plates 6 and 12 are connected by a displacement transducer 15. To above mentioned second plate 12 another hydraulic cylinder 16 is mounted, whose piston rod carries a third plate 17, to which the bars 26.2 being fastened, which by means of thread connections 27.1 carry the first pressing punches 18, which penetrate into the movable mould half 20 to the mould cavity and shape part of the future cast article 8. The second 12 and the third 17 plates are connected by another displacement transducer 15.1. To the second plate 12 a subsidiary plate 24 is mounted with bars 26.1 fixed on it, which carry the second pressing punches 25.

Temperature gauges 33 are provided in the first and second pressing punches 18 and 25. In another embodiment of the invention the bars 26.1 and 26.2 carrying the first and the second pressing punches 18 and 25, can be constructed as separate actuators, for example hydraulic cylinder with their own displacement transducers and the first and second pressing punches 18 and 25 can penetrate also through the immovable mould half 7 to the mould cavity and to shape there a part of the future cast article 8.

THE APPARATUS OPERATES IN THE FOLLOWING MANNER: In the initial state of the apparatus, the piston of the hydraulic cylinder 16 is hydraulically fixed at a defined, by the displacement transducer 15.1, distance from its upper position and defines the mutual position of the first 18 and second 25 pressing punches at the beginning of the cycle. The piston rod of the cylinder 14 is in such a position, being controlled by the displacement transducer 15, that the distance between the immovable mould half 7 and the movable mould half 20 allows the removing of the cast article 8. The casting cycle begins when the hydraulic cylinder 14 displaces the plate 12 to a preliminarily determined position, being controlled by the displacement transducer 15, where the movable mould half 20 is pressed against the immovable mould half 7 and the first pressing punches 18 and the second pressing punches 25 ensure an open runner 10. By opening the valve 31 an inert gas is supplied into the mould cavity, which pushes out and replaces the atmosphere containing oxygen. The valve 31 is closed partially and by opening the valve 35 a gas pressure is created in the furnace 1 according to a defined low, ensuring the smoothly mould filling with molten metal (fig. 3). The valve 31 is closed. After a definite time t, sec., when dendritic structure in the thin-section part of the cast article appears, the hydraulic cylinder 14, by means of the plate 12 displaces downwards the first pressing punches 18 and the second pressing punches 25, which squeeze out the excess of semi-liquid metal through the runner 10 back in the crucible 2 and the dendrites breaking ensures fine grain structure of the future cast article. After the plugging up of the runner 10 by one of the pressing punches 25, the cast article compressing begins by the first pressing punches 18 and the second pressing punches 25 simultaneously. After a time t2 sec. starts the cyclic pressure change on both piston sides of the hydraulic cylinder 16 with controlled frequency. In this way the compressing pressures in the first pressing punches 18 change from maximum to 0, and in the second pressing punches 25 from minimum to maximum. After a time t3 sec. the gas (from the furnace 1 exhausts to a value, for example 0,0015Mpa, where the remaining liquid metal from the gate running system 9 returns in the crucible 2 and an inert atmosphere is maintained in the furnace 1. After a time t4 sec., starting at the end of the t2 or at the moment when the first pressing punches 18 reach a definite position, or when establishing a lack of relative displacement of the first pressing punches 18 towards the cast article 8 by the indications of the two displacement transducers 15 and 15.1, i. e. when the cast article in the action region of the first pressing punches 18 is completely solidified, the pressure on both sides of the piston of the hydraulic cylinder 16 is dropped, draining them. After time t5 sec., or after establishing the complete solidification of the entire cast article (fig. 4) by the indications of the displacement transducer 15 or of the temperature gauges 33, the hydraulic cylinder 14 lifts the plate 12 to the initial state, opening enough distance between the two mould halves for removing the cast article. The hydraulic cylinder 16 is actuated downwards and the first pressing punches 18 and the ejectors 19 eject the cast article and the solidified part of the gate running system respectively. The piston of the hydraulic cylinder 16 returns to initial position. In the another embodiment of the invention, bars 26.1 and 26.2 carrying the first and second pressing punches 18 and 25, are performed as separate actuators, for instance hydraulic cylinders with their own displacement transducers, and the first and the second pressing punches 18 and 25 can penetrate also through the immovable mould half 7 to the mould cavity. In this case the first and second pressing punches 18 and 25 shape the cast article, being driven consecutively and depending on their respective sections, on the solidification temperature in the particular regions of the future cast article, on the displacement with a definite force, with a definite stroke, in a definite sequence, with a definite frequency and in a definite direction, exerting regulated pressure on these regions of the cast article, where they are positioned.