EP0055947B1 - Process and means for the automation of a low pressure casting cycle - Google Patents

Abstract

1. A process for automating a low pressure-type casting cycle of a metal in a closed mould, consisting of : - acting on an intensive single action variable, that is the pressure of the backflow fluid of the molten metal contained in the furnace in order to subject the metal to a cycle of variations of level and pressure, which cycle is divided into phases, is started when the metal passes the bottom of the mould by means of a presence sensor which serves to determine the reference pressure in the furnace at the start of the cycle which ends on completion of the decompression of the furnace, the said process being characterised in that : - after a preliminary preparation stage consisting of : a) dividing the said cycle into a particular number of successive phases, each phase corresponding, as a function of the shape of the part to be obtained and the desired metallurgical quality, to the determined conditions of time and value of the speed and/or pressure of the metal, b) and determining, for each of these phases, from the conditions to be applied to the metal, those to be applied to the pressure of the backflow fluid, - the pressure of the backflow fluid is continuously controlled during casting with a view to obtaining the characteristics desired for the metal in each phase, by dividing the duration of each phase into successive elementary steps, by effecting, at each step, a comparison between the real value of the said fluid pressure and the corresponding predetermined value in the said preliminary stage and by raising or reducing the real pressure of the fluid accordingly.

Description

Various processes are known for manufacturing molded parts and in particular parts made of alloys cast under low pressure.

• - on remplit par le bas un moule, métallique ou non, avec un métal ou alliage liquide, contenu dans un four hermétiquement clos et susceptible de se solidifier. Ce métal peut remonter dans le moule par l'intermédiaire d'un tube d'injection ;
• - on effectue ce remplissage à l'aide d'un fluide de refoulement introduit dans le four sous une pression de quelques décibars ;
• - après remplissage du moule, on maintient une surpression de masselottage au cours de la solidification du matériau ;
• - on récupère le matériau non solidifié, situé dans le bas du moule dans les canaux d'injection, dès solidification de la pièce et après cessation de la pression de refoulement.

Low pressure casting is a very well-known foundry technique in which:

• - A mold, metallic or not, is filled from below with a metal or liquid alloy, contained in a hermetically sealed oven and capable of solidifying. This metal can go back into the mold via an injection tube;
• - This filling is carried out using a discharge fluid introduced into the oven under a pressure of a few decibars;
• - After filling the mold, a weighting overpressure is maintained during the solidification of the material;
• - the non-solidified material is recovered, located at the bottom of the mold in the injection channels, as soon as the part solidifies and after the discharge pressure has ceased.

• - soit des moules métalliques,
• - soit des moules réalisés en sable, ou en matériaux divers (graphite, zircon, carborundum) dont les grains sont réunis par un liant (généralement, ce liant est une résine synthétique),
• - soit quelquefois aussi des moules construits en céramique ou en plâtre.

We use in this technique:

• - either metal molds,
• - either molds made of sand, or of various materials (graphite, zircon, carborundum), the grains of which are joined by a binder (generally, this binder is a synthetic resin),
• - or sometimes also molds constructed of ceramic or plaster.

The metal molds are robust but expensive and therefore reserved for large series.

Non-metallic molds have a comparatively low cost. They also have the advantage of having adjustable permeability and allow satisfactory filling of the imprint to be obtained.

This low pressure casting technique in inexpensive sand molds is particularly suited to the new needs of the industry, especially in the aeronautical field, which require the production of medium series of molded parts of high mechanical quality alloy, with fine and defined tolerances.

• - la maîtrise de la turbulence du métal lors de son élévation dans le moule, turbulence du métal lors de son élévation dans le moule, turbulence liée à la vitesse d'évolution du métal et qui détermine son oxydation,
• - la protection contre les coups de bélier pouvant intervenir lors de l'établissement des surpressions de masselottage des pièces (c'est-à-dire de compensation de leur retrait) et pouvant conduire à une incrustation du métal entre les grains du moule,
• - un déroulement non prématuré de la solidification,
• - une évolution du métal (en structure, en déplacement, en refroidissement, etc...) conforme aux nécessités thermiques de la coulée,
• - une reproductibilité des opérations permettant d'uniformiser la qualité des pièces produites,
• - une rationalisation du déroulement des tâches.

The technical casting problems affecting the quality of the products concerned are mainly:

• - control of the turbulence of the metal during its elevation in the mold, turbulence of the metal during its elevation in the mold, turbulence linked to the rate of evolution of the metal and which determines its oxidation,
• protection against water hammer which may occur during the establishment of overpressures for massaging the parts (that is to say compensation for their shrinkage) and which may lead to metal encrustation between the grains of the mold,
• - a not premature course of solidification,
• - an evolution of the metal (in structure, displacement, cooling, etc.) in accordance with the thermal requirements of the casting,
• - reproducibility of operations to standardize the quality of the parts produced,
• - rationalization of the progress of tasks.

In order to better understand the operating principle of the method according to the invention, it should be noted that when the "metal pouring front is almost static at a level H above the level of the metal in the crucible, the discharge pressure in the crucible is P = Hpg (p being the density of the metal considered and g representing the coefficient of acceleration of gravity). As soon as there is movement of the liquid, braking forces develop between the metal and the walls.

(V est la vitesse verticale de montée du front de coulée et K, 1 est un coefficient tenant compte des frottements qui dépend de la géométrie du moule et de V).Experience and calculations show that we then obtain a differential law of variation of the discharge pressure governed by the formula:

(V is the vertical speed of rise of the casting front and K, 1 is a coefficient taking into account friction which depends on the geometry of the mold and V).

For low values of the speed V, and therefore of dP / dt, K = 1.

For the strong values of V, K and therefore V tend towards an asymptotic value.

• - en phase de remplissage : P = pgH et dP/dt = pgV
• - en phase de surpression : P = pgH + ΔP, ΔP étant la surpression subie par le métal à la partie supérieure du moule.

In the following, we will place ourselves in the most common case where V is weak, we then have:

• - during the filling phase: P = pgH and dP / dt = pgV
• - in the overpressure phase: P = pgH + ΔP, ΔP being the overpressure undergone by the metal at the top of the mold.

It can be noted that, when the metal is in the overpressure phase, this overpressure depends on the level of this metal in the crucible, by means of the term H, the latter varying with the succession of flows.

• - d'une part, d'agir sur la pression de refoulement du métal, pendant les phases dynamiques, de telle sorte que le front de coulée progresse régulièrement et suivant des caractéristiques de vitesse précises. Quelle que soit la forme ou la finesse des empreintes à remplir, cette progression doit s'effectuer sans ralentissement brusque provoquant un figeage trop rapide de la masse liquide et interrompant brusquement cette dernière avant qu'elle ait pu être achevée, mais aussi sans turbulences susceptibles de provoquer des oxydations déterminant des faiblesses ou des discontinuités locales dans les pièces à couler,
• - d'autre part, d'appliquer au métal, après qu'il a rempli l'empreinte, des surpressions assez rapides et importantes pour compenser le retrait en cours de solidification, mais dans des conditions de variation telles qu'elles ne provoquent pas de pénétration du métal entre les grains du moule,
• - enfin, d'effectuer ces actions en tenant compte des perturbations aléatoires telles que la baisse de niveau du métal dans le creuset et les fuites de gaz.

The taking into account of the theoretical conditions concerning the casting recalled previously requires:

• - on the one hand, to act on the discharge pressure of the metal, during the dynamic phases, so that the casting front progresses regularly and according to precise speed characteristics. Whatever the shape or fineness of the imprints to be filled, this progression must be carried out without abrupt slowing down causing a too rapid freezing of the liquid mass and abruptly interrupting the latter before it could be completed, but also without likely turbulence to cause oxidations determining local weaknesses or discontinuities in the parts to be cast,
• - on the other hand, to apply to the metal, after it has filled the imprint, overpressures fast enough and significant to compensate for the shrinkage during solidification, but under conditions of variation such that they do not cause metal penetration between the mold grains,
• - finally, carry out these actions taking into account random disturbances such as the drop in level of the metal in the crucible and gas leaks.

• - dans certains systèmes connus à ce jour tels que ceux décrits dans les documents FR-A-2 189 150 et FR-A-2 146 148, le cycle de coulée suit des phases limitées par des repères situés dans l'empreinte. Plus précisément, dans le document FR-A-2 146 148, on compare la valeur d'une grandeur intensive (la pression dans le four) à une pression de référence pour déclencher et contrôler les moyens d'alimentation du four en gaz sous-pression, ce qui permet de tenir compte des phénomènes aléatoires non déterminés, bien que l'on n'établisse pas une relation entre les caractéristiques associées à l'état du métal et le niveau du métal dans le four. Ce document a servi de base pour l'élaboration du préambule de la revendication 1.

The prior art has vainly endeavored to solve these problems globally:

• - In some systems known to date such as those described in documents FR-A-2 189 150 and FR-A-2 146 148, the casting cycle follows phases limited by marks located in the imprint. More precisely, in the document FR-A-2 146 148, the value of an intensive quantity (the pressure in the oven) is compared with a reference pressure for triggering and controlling the means of supplying the oven with under-gas pressure, which makes it possible to take account of undetermined random phenomena, although a relationship is not established between the characteristics associated with the state of the metal and the level of the metal in the furnace. This document served as the basis for the preparation of the preamble of claim 1.

In other words, at a real pressure which corresponds to a forecast pressure a gas flow is introduced and it is controlled. We are masters of random phenomena by the correspondence of the two pressures, but not of the characteristics of the metal.

In fact, if there was a gas leak in the oven, the time for matching the two pressures is delayed, the speed of the metal was therefore reduced in the corresponding phase.

This method therefore relates to the fixing of the pressure difference between the initial state and the final state and not the characteristics of the path to pass from one to the other and those of evolution of the metal.

• - d'autres systèmes imposent une vitesse de variation de pression constante pour tout le cycle, ou encore effectuent un réglage en plusieurs paliers de pression jusqu'à obtention d'une pression finale déterminée.

It neither makes it possible to modify the speed of the metal in the mold during filling as a function of the geometry of the part, nor to constantly bring the real characteristics and the planned characteristics into agreement.

• - other systems impose a constant pressure variation speed for the entire cycle, or even adjust in several pressure stages until a final final pressure is obtained.

• - d'autres systèmes (FR-A-2 276 125) effectuent une correction à partir d'indications données au début de la séquence, notamment à un calculateur analogique. Mais ceci exige un réglage préalable et exclut la possibilité de couler des pièces différentes à chaque cycle comme cela est souvent le cas dans le domaine aéronautique. Par ailleurs, les corrections effectuées souffrent d'imprécision dans leur évaluation et les erreurs commises ne font en général que croître quand se succèdent les coulées.

These systems do not correct pressure to account for the drop in metal level. This prohibits any reproducibility of the flows.

• - other systems (FR-A-2 276 125) carry out a correction on the basis of indications given at the start of the sequence, in particular to an analog computer. However, this requires prior adjustment and excludes the possibility of pouring different parts in each cycle, as is often the case in the aeronautical field. In addition, the corrections made suffer from imprecision in their evaluation and the errors made generally only increase when successive flows are made.

Finally, another process is known (FR-A-2 394 347) in which the duration of the flow and maintenance phases of the metal is adjusted as a function of the temperature of the mold and that of the metal. For this purpose, the duration of the step of each increase in potential difference applied to the electromagnetic pump which sets the metal in motion is modified as a function of these temperatures, but this potential difference is not the sole action variable on the system. Indeed, the force which sets the metal in motion is the consequence of many factors, the most important, but not the only one, is the said difference in potential. Furthermore, in this process, the changes planned for the metal in the imprint are not brought into agreement with the real changes.

The purpose of the present invention is to globally solve all of the previously mentioned problems posed by low pressure casting by proposing a process making it possible to impose on the metal, during the dynamic phases of filling the imprint, a cycle having precise speed and acceleration characteristics adapted to its evolution and defined in advance, and, after filling the imprint, before solidification, overpressure phases at a suitable predetermined level.

The method according to the invention takes care, in order to impose these characteristics, to take direct account of random disturbances such as the drop in the level of the metal in the mold and gas leaks.

• - à agir sur une variable intensive d'action unique, à savoir la pression du fluide de refoulement du métal en fusion contenu dans le four, afin de faire subir au métal un cycle de variations de niveau et de pression, divisé en phases, initialisé au passage du métal au pied du moule à l'aide d'un capteur de présence servant à définir la pression de référence dans le four au départ du cycle, ce dernier se terminant à la fin de la décompression du four,
  ledit procédé étant caractérisé en ce que :
• - après un stade préliminaire de mise au point consistant :
  • a) à diviser ledit cycle en un certain nombre de phases successives correspondant chacune, en fonction de la géométrie de la pièce à obtenir et de la qualité métallurgique recherchée, à des conditions déterminées de temps et de valeur de la vitesse et/ou de la pression du métal,
  • b) et à déduire, pour chacune de ces phases, à partir des conditions à appliquer au métal, celles qui sont à appliquer à la pression du fluide de refoulement,
• - on asservit en permanence, durant la coulée, la pression du fluide de refoulement en vue d'obtenir les caractéristiques recherchées pour le métal dans chaque phase, en divisant la durée de chaque phase en pas élémentaires successifs, en effectant à chaque pas la comparaison entre la valeur réelle de ladite pression du fluide et la valeur correspondante prédéterminée dans ledit stade préliminaire et en augmentant ou diminuant la pression réelle du fluide en conséquence.

To this end, the subject of the invention is a method of automating a low-pressure type casting cycle of a metal in a closed mold which consists of:

• - to act on a single intensive action variable, namely the pressure of the discharge fluid of the molten metal contained in the furnace, in order to subject the metal to a cycle of level and pressure variations, divided into phases, initialized at the passage of the metal at the foot of the mold using a presence sensor used to define the reference pressure in the oven at the start of the cycle, the latter ending at the end of the decompression of the oven,
  said process being characterized in that:
• - after a preliminary stage of development consisting:
  • a) dividing said cycle into a number of successive phases, each corresponding, depending on the geometry of the part to be obtained and the metallurgical quality sought, under determined conditions of time and value of the speed and / or of the metal pressure,
  • b) and to deduce, for each of these phases, from the conditions to be applied to the metal, those which are to be applied to the pressure of the discharge fluid,
• - the pressure of the discharge fluid is permanently controlled during casting in order to obtain the desired characteristics for the metal in each phase, by dividing the duration of each phase into successive elementary steps, by performing at each step the comparison between the actual value of said fluid pressure and the corresponding value predetermined in said preliminary stage and by increasing or decreasing the actual fluid pressure accordingly.

• - un ensemble entrées-sorties prenant en charge les indications sur les différentes variables d'action, aussi bien réelles et délivrées par des organes de mesure situés sur le dispositif de coulée, que souhaitées et transmises par l'opérateur, de façon à les délivrer sous forme numérique et les transmettre à un calculateur,
• - un ensemble calculateur reliant les caractéristiques de phases aux évolutions nécessaires à imposer à la variable d'action et effectuant les calculs annexes nécessaires au déroulement du procédé, ce calculateur étant relié, d'une part, à un ensemble mémoire et, d'autre part, à un organe de décision,
• - un ensemble mémoire stockant de façon transitoire les informations à utiliser en cours de coulée et éventuellement sur une longue durée des informations générales sur la coulée,
• - un système d'horloges relié à un ensemble programmé rythmant les activités de chacun des organes,
• - un organe de décision activant des organes d'action sur le système, notamment des vannes, et recevant ces informations, notamment de la part de l'ensemble mémoire et de l'ensemble entrées-sorties de l'ensemble programmé et du système d'horloges.

The invention also relates to a device connected to a low pressure casting system and its accessories, intended to control the operating phases so as to optimize the quality and speed of its production by implementing the above method , said device being characterized in that it comprises:

• - an input-output set taking charge of the indications on the various action variables, both real and delivered by measuring members located on the casting device, as desired and transmitted by the operator, so as to deliver them in digital form and transmit them to a computer,
• a computer unit connecting the phase characteristics to the changes necessary to impose on the action variable and carrying out the additional calculations necessary for the process to proceed, this computer being connected, on the one hand, to a memory unit and, on the other hand part, to a decision-making body,
• a memory assembly temporarily storing the information to be used during casting and possibly over a long period of general information on casting,
• - a system of clocks linked to a programmed set that punctuates the activities of each of the organs,
• - a decision-making body activating action bodies on the system, in particular valves, and receiving this information, in particular from the memory unit and the input-output unit of the programmed unit and of the system 'clocks.

• la figure 1 montre une coupe schématique d'une machine de coulée basse pression adaptée à la mise en oeuvre du procédé selon l'invention, aihsi que le pilote qui en commande automatiquement le fonctionnement ;
• la figure 2 représente le cycle de coulée jugé idéal selon l'invention ;
• la figure 3 est un schéma du dispositif d'asservissement de la vanne et du dispositif d'automatisation du cycle ;
• la figure 4 représente un capteur de pression à ultra-sons utilisé suivant le procédé de l'invention à la partie supérieure du tube de montée du métal dans le moule ;
• la figure 5 représente un schéma d'un dispositif selon l'invention, utilisé dans le cas du moulage de pièces, possédant une zone de faible épaisseur transversale ;
• la figure 6 représente une forme de calage des moules adaptée au stade de mise au point ;
• la figure 7 représente une solution au problème de calage dans le stade de production de pièces différentes.

Other characteristics and advantages will emerge from the description which follows, given by way of example only and with reference to the appended drawings in which:

• Figure 1 shows a schematic section of a low pressure casting machine adapted to the implementation of the method according to the invention, aihsi that the pilot who automatically controls the operation;
• FIG. 2 represents the casting cycle deemed ideal according to the invention;
• Figure 3 is a diagram of the valve control device and the cycle automation device;
• FIG. 4 represents an ultrasonic pressure sensor used according to the method of the invention at the upper part of the tube for raising the metal in the mold;
• FIG. 5 represents a diagram of a device according to the invention, used in the case of molding parts, having an area of small transverse thickness;
• FIG. 6 represents a form of wedging of the molds adapted to the stage of development;
• FIG. 7 represents a solution to the stalling problem in the stage of production of different parts.

The invention consists in regulating the evolution of the casting cycle according to precise characteristics by means of an automatic pilot acting on the discharge pressure of the metal by means of a valve controlled by this pilot.

• - une machine de coulée basse pression classique ;
• - un pilote automatique,
• - une vanne asservie par le pilote,
• - un capteur de la pression dans le four contenant le creuset, transmettant ses informations au pilote,

The proposed system essentially comprises:

• - a classic low pressure casting machine;
• - an autopilot,
• - a valve controlled by the pilot,
• - a pressure sensor in the furnace containing the crucible, transmitting its information to the pilot,

• - un capteur de température du métal dans le creuset lié au pilote. Suivant le procédé de l'invention; la fabrication d'une série de pièces s'effectue en deux stades :
• - le premier stade est un stade de mise au point.

- a certain number of sensory organs detecting the presence of metal, located on the elevation path of the metal in the mold at the point of step change, and also transmitting their information to the pilot,

• - a metal temperature sensor in the crucible linked to the pilot. According to the method of the invention; the production of a series of parts takes place in two stages:
• - the first stage is a stage of development.

After drawing the theoretically established casting system, a pressure variation curve is drawn, leading to rising rates of the metal and to overpressures liable to generate a part of satisfactory metallurgical quality. According to a preferred characteristic of the invention, a cycle divided into eight phases is then chosen.

The first three phases correspond to the filling of the mold. One imposes on the metal, during each of them, a constant speed of elevation adapted to the geometry of the part. To do this, we establishes a constant rate of change of discharge pressure during these phases.

The first phase corresponds to the step of raising the metal from its rest level in the crucible towards the mold and takes place inside a tube opening into the mold.

During this phase, the speed can be quite fast and depends only on the casting machine.

During the second phase, the entry cone in the mold is filled.

The third phase corresponds to the entry of the metal into the mold casting system, that is to say the part joining the tube to the mold. This phase is carried out at a variable speed depending on the type of parts.

During the fourth phase, the metal fills the imprint. This phase is possibly divided into sub-phases. The optimal speed is then linked to the geometric shape of the part: thickness and height in particular.

At the end of the fourth phase, the metal must have filled the imprint.

In order to coordinate during this dynamic part of the casting the action phases on the discharge pressure at the different dynamic stages of the metal, four presence detectors (notably internal electrodes crossing the walls of the mold or ultrasound transmitter-receiver system located at the part upper part of the feed tube) are located at the point where the geometrical steps change and transmit the phase change orders to the pilot.

A particular sensor, in particular the first encountered by the metal, makes it possible to establish the link between the discharge pressure and the subsequent hammer pressures. To this end, the sensor instructs the pilot, when passing the metal at his height, to record the level of the discharge pressure. Thereafter, the pilot considers only relative pressures by taking, as pressure zero, the value of the measurement recorded during this operation triggered by the sensor.

Thus, the problem caused by the drop in the level of the metal in the crucible is resolved.

Additional sensors can optionally be used to divide each phase into sub-phases.

The following three steps are carried out after filling the imprint.

During phase 5, an overpressure ΔP1 is established relative to the pressure level at the end of the filling of the mold. It is carried out for a time AT1. The speed and acceleration of the discharge pressure are chosen so as to avoid water hammer, in order to be able to use fine sand molds.

During phase 6, an overpressure at P2 is established for a very short time AT2.

The sum AP1 + toP2 represents the presser pressure and must be exerted before the part begins to solidify.

AP1 and AP2, ΔT1 and ΔT2 depend on the characteristics of the part and in particular on the nature of the alloy, the thickness, the length and the height.

Phase 7 corresponds to maintaining the overpressure.

This phase is interrupted by the pilot after information transmitted by a thermocouple of the end of solidification at the base of the part. This thermocouple is located in the hottest part of the casting system.

• la température,
• les vitesses de montée du métal au cours des phases 2, 3 et 4. Ces vitesses sont proportionnelles aux vitesses de variation de pression au cours de ces phases et imposent donc la valeur de ces dernières
• la valeur de la surpression ΔP1 et le temps ΔT1
• la valeur de la surpression ΔP2 et le temps ΔT2.
• Toutes ces grandeurs peuvent donc être imposées par l'intermédiaire de la pression de refoulement.

Phase 8 is the relaxation phase. The parameters imposed on casting, during a test, are therefore 8:

• temperature,
• the rates of rise of the metal during phases 2, 3 and 4. These speeds are proportional to the rates of pressure variation during these phases and therefore impose the value of the latter
• the value of the overpressure ΔP1 and the time ΔT1
• the value of the overpressure ΔP2 and the time ΔT2.
• All these quantities can therefore be imposed by means of the discharge pressure.

These parameters greatly influence the metallurgical qualities of the parts by governing the different levels of oxide, blowing, non-arrival, watering, microporosities, shrinkage and microshrinkage.

In general, several tests of this type are carried out by iteration and used in particular statistically by varying the parameters of the casting cycle and by acting in addition on the temperature of the furnace. These tests are continued until satisfactory metallurgical quality is obtained. The pilot records the values of the preceding characteristics actually obtained during the flows and delivers them. In addition, it records and delivers the durations of the mold filling phases. After each casting, the quality of the parts obtained is examined.

Following this series of tests, the eight values of the optimal characteristics of the casting are isolated. The times of phases 5, 6 and 7 are associated with them.

The correlation existing between the type of part (or its reference), the eight characteristics of the cycle and the three durations of the corresponding phases 5, 6, and 7 are introduced into the memory of the pilot.

The second stage or mass production stage can then begin.

The mold being placed on the low pressure machine, the only manual operations to be performed are the display of the part reference and possibly the start of the cycle. On these indications alone, the pilot regulates the pouring and the temperature according to the optimal characteristics which he has in memory.

According to a preferred form of the invention, the device used in the productive phase can be simplified so as to have only one presence sensor which will be described later. This sensor can be, for example, located at the outlet of the metal riser. The molds are then devoid of sensor. In this case, it is advisable to use this sensor both to interrupt the first phase and to define the reference pressure level of the casting. This reference will take into account the drop in the level of the metal.

In this type of casting and in the series stage, the time information, corresponding to the changes in phases 2, 3 and 4, is no longer given by the mold presence sensors but imposed by the pilot himself, according to their values optimal.

Still according to the method of the invention, means are provided in order to be able to produce parts having parts of very small thickness. In this case, a depression is created at the end of the interior cavity of the mold intended to form the fine parts of the part. During casting, the metal traps a gas bubble in these cavities. According to the invention, the establishment of a vacuum in this cavity is programmed. This action is carried out by means of a channel passing through the walls of the mold in the zone considered. This vacuum is provided according to parameters of the same type as those for establishing the overpressure in the furnace. In this case, the flow of the metal is not disturbed by the fineness of the cavities concerned and it is thus possible to obtain in these zones a complete filling with very satisfactory surface finish. This technique therefore consists in establishing, automatically and regulated according to the shapes of the parts, a pressure vacuum in areas of small cross section.

Furthermore, according to the invention, means are provided to prevent leakage of liquid metal at the base of the mold. It is indeed necessary to keep the mold in place despite the action of the metal thrust directed from bottom to top.

All of the above will now be illustrated by the description of the accompanying drawings.

In FIG. 1, if one is interested in the various organs which constitute the casting machine, one sees a crucible 1 situated inside a sealed oven 2. This oven is closed by a fixed cover 3.

Inside the crucible is the metal 4. The mold cavity 5 is supplied with liquid metal via the injection tube 6 and the casting system 7. A flow of discharge gas (air or neutral gas) is introduced into the mold via the conduit 8. The mold shown is suitable for the development stage, it is provided with three metal presence sensors E2, E3 and E4. These presence sensors are electrodes grounded by the passage of metal. A fourth sensor E1 is fixedly located at the upper part of the pipe 6. To avoid fouling of an immersed member due to the succession of casting operations, a system composed of a transmitter, a receiver, a generator and a wave beam analyzer. The preferred form of this system will be specified in more detail below.

Note the presence of an assisted valve 9 controlling the arrival of the delivery fluid in the mold, a thermocouple 10 located 20 mm below the part in the hottest attack of the casting system and a thermocouple 11 located inside the metal crucible. A pressure sensor 12 is placed inside the oven enclosure. The oven is heated by a resistor 13.

As for the pilot's control panel, there are ten coding wheels 14 to 23 at the upper part. The central part of the table is equipped at its upper part by twelve dials 24a and 241 and at its lower part by a display dial 25 on which a broken line is materialized intersected by nine small lamps 26a to 26i. At the base of the table are on the left a coding wheel 27, then a three-position switch 28, a switch 29 and a pusher 30 with light display.

The four presence sensors E1, E2, E3 and E4, the thermocouples 10 and 11 and the pressure sensor 12 transmit their information to the pilot via cables 31 to 37. The pilot, in turn, controls the opening and closing the assisted valve 9 via the cable 38 and energizing the resistor 13 via the cable 39.

We will now describe the procedure for regulating the casting system by the pilot during a development test of a part of a given type.

This control consists in imposing on the discharge pressure P the monitoring of the phases of variation whose curve is shown in FIG. 2.

In FIG. 2, the first four phases numbered 1, 2, 3 and 4 correspond to the stages of dynamic evolution of the metal in the mold. Phases 5 and 6 correspond to the establishment of overpressures after filling the cavity with the metal. Phase 7 maintains the overweight of the flyweight during solidification. Phase 8 effects the relaxation of the system; during this phase the metal falls back into the crucible.

One test consists in imposing precise rates of pressure variation during phases 2, 3 and 4 at levels such that the rates of rise of metal in the mold (which are, as we have seen, proportional to them) are established to selected values V2, V3 and V4. During a test, the duration ΔT1 and the overpressure AP1 of phase 5 are also imposed, as well as the duration AT2 and the overpressure AP2 of phase 6.

Before any test of this type, the values chosen for this test of V2, V3, V4, ΔP1, AT1 are adjusted using the encoder wheels 14 to 20. AP2, AT2. The temperature T of the metal is also fixed during casting by means of the coding wheel 21. All the fixed values are displayed on the front face of the coding wheels.

The pilot takes into account and stores these eight values.

The flow of the test casting will continue as follows.

First of all, the mold concerned is put in place.

The device is started by pressing the switch 30.

After a stabilization phase of the system which ends with a red light display of the switch, the casting begins.

During the first phase, the assisted valve 9, initially closed, is opened by the pilot.

The pressure rises and the metal initially at rest at its level in the crucible, rises in the tube 6 at a speed fixed during the construction of the machine. It reaches the presence sensor E1. This transmits to the pilot the information of the passage of the metal at its level. The pilot then interrogates the pressure sensor 12. The latter transmits the pressure level indication in the oven. The pilot memorizes this value and will later consider it as reference pressure.

From this moment, the pilot takes charge of all the evolution of the system and controls the pressure variations according to a principle which will be explained below, so as to establish during the subsequent phases, the characteristics which have been specified to him and that this one has memorized.

Phase 2 then opens.

The metal fills the inlet channel into the mold. During this phase, the pilot will act on the assisted valve 9 so as to effectively establish the speed of variation of discharge pressure which will impose the rate of rise of the metal V2. Most often this speed V2 is lower than the speed V1 of the metal rising in the tube. This phase 2 is interrupted when the metal passes in front of the presence sensor E2. The information is transmitted to the pilot who changes phase.

During phase 3, the metal fills the casting system. The pilot then imposes, via the discharge pressure, an elevation speed V3.

During phase 4, the metal fills the imprint. The pilot adapts the variations in the discharge pressure so as to raise the metal at speed V4, the metal finally meets the electrode E4 which indicates to the pilot that the metal has completely filled the imprint.

The following phases are the overpressure phases.

During phase 5, the pilot imposes the pressure increase AP1 during the time AT1.

During phase 6, the pilot imposes the pressure increase AP2 during the time at T2.

During phase 7, the pilot stabilizes the overpressure. The solidification of the metal occurs during this phase, it is generally carried out from top to bottom. The thermocouple 10 analyzes the temperature level in the casting system at the base of the cavity.

As soon as the temperature reaches the end of the solidification stage, that is to say as soon as the metal is completely solidified in the cavity, the information is transmitted to the pilot. Phase 7 is complete, phase 8 begins, the pilot decompresses the enclosure. The liquid metal goes back down into the crucible.

During the test, the operator is informed of the rise in the flow rate via the dial 25. In fact, the lamps 26a, 26b, ... 26i light up successively after each phase change.

At the end of each stage, the pilot assesses and stores the characteristics which have actually been obtained. At the end of the casting, the cycle characteristics V2, V3, V4, ΔP1, ΔT1, ΔP2, ΔT2, the time characteristics Δt2, Δt3, Δt4 of phases 2, 3 and 4 and the cycle temperature actually obtained are displayed in the dials 24a, 24b, 24c ... 24k.

The operator can use them for verification.

• - une fonction entrées-sorties qui relie le pilote d'une part aux organes de mesure et, d'autre part aux indications données sur son tableau d'affichage,
• - une fonction calcul-comparaison-décision,
• - une fonction mémoire.

We will now describe the operating principle of the pilot. It has three main functions:

• - an input-output function which connects the pilot on the one hand to the measuring devices and, on the other hand to the indications given on his display board,
• - a calculation-comparison-decision function,
• - a memory function.

• Elle est initiée par le capteur de présence E1. A partir de cet instant, le pilote prend en mains la destinée de la coulée.

We consider, for example, the course of the second phase:

• It is initiated by the presence sensor E1. From this moment, the pilot takes in hand the destiny of the casting.

The pilot's operating rhythm is sequenced by a system of clocks dividing the timescale into successive elementary steps.

From the characteristics of the cycle he has memorized, the pilot knows that he must impose an elevation speed V2 during this phase. Thanks to its calculation set, it deduces that during each time interval of this phase, it will have to increase the pressure of a theoretical increase ΔP _t = pgV2Δt. However, the pressure sensor 12, plugged into the enclosure, transmits to the pilot during each time interval the value of the actual pressure increase ΔP _r . The pilot then performs the comparison described in FIG. 3 between ΔP _t and ΔP _r . If ΔP _t is greater than ΔP _r , that is to say if during the time interval the actual pressure increase has been smaller than the theoretical pressure increase, the pilot opens the assisted valve 9 through its input-output set.

Similarly if ΔP _t is less than or equal to ΔP _r , the pilot closes the assisted valve 9 and this is repeated successively step by step during the course of the time scale before each time step.

Depending on whether the pilot has been connected using the switch 28 in the focused position or in the serial position, the phase ends are either communicated to him from the outside using presence sensors, or communicated from the 'interior by the durations of phases put in memory and imposing the number of time steps of each phase.

The actual curve resulting from the overall control of a casting can be viewed using a table tracing. These curves include, as can be seen in Figure 2, a continuous series of small staircases framing the theoretical curve. Each small staircase corresponds to a time interval t and an action of the pilot on the assisted valve 9.

• ― le calcul de ΔP_t ;
• ― la mesure de ΔP_r ;
• ― la comparaison entre ΔP_t et ΔP_r ;
• - l'action sur l'électrovanne.

The four functions of the pilot during these time intervals are therefore:

• - the calculation of ΔP _t ;
• - the measurement of ΔP _r ;
• - the comparison between ΔP _t and ΔP _r ;
• - action on the solenoid valve.

The system includes a microprocessor allowing it to perform these four functions and thus achieve complete control of the casting.

The device can adapt its pressure control characteristics so as to cast parts from a few centimeters to more than 2.50 m with satisfactory and constant precision for each of them. To do this, the range within which the pressure will evolve is indicated at the start of each casting using the encoder wheel 22. The pilot divides this pressure range into 2 ¹² = 4,096 steps. However, the precision of piloting, that is to say the finesse with which the pilot follows his theoretical curve, is expressed by the ratio ΔP _t / Δt of the jump in the increase in discharge pressure at the duration Δt of the step corresponding time.

Also, when choosing the range, the pilot chooses the duration of each step so as to maintain constant precision. These durations vary from 50/1000 of a second for the lowest range to around 200/1000 of a second for the highest range.

Six ranges are made accessible in the device via the encoder wheel 22. For each of these ranges, the increase in each elementary pressure level and the duration of the time step are stored in the microprocessor during building the pilot.

Generally, at the end of such a test, the parts are observed and their mechanical characteristics evaluated. These tests are repeated several times taking into account the previous tests. At the end of the development series, the optimal characteristics of the cycle, according to which the part must be cast, are established statistically. They are materialized by the eleven values displayed in 24a, 24b, 24c, ... 24k which were obtained following the casting of the part having presented the best mechanical qualities. The operator then displays, thanks to the encoder wheel 27, the reference of the part concerned and places the multi-position switch 28 in the recording state. The eleven characteristic values of the casting are then displayed at 27, memorized by the pilot in correlation with the reference of the part displayed at 27.

The succession of the preceding test operations has been described in the case where the switch 29 is in the "automatic" position, that is to say that as we have seen, phase 7 is interrupted automatically by order of the thermocouple 10. According to another option, when the switch 29 is in the "manual" position, the duration D of phase 7 is previously imposed on the casting among the characteristics of the cycle. It is displayed on the coding wheel 23.

Still in this case, when the optimal characteristics are recorded, the value found D is displayed at 241 and memorized among the characteristics to be imposed by the pilot for the series phase.

To start in series a part of a given type, the development of which has been previously carried out and the optimal characteristics of which are memorized, it suffices to display the reference of the part using the coding wheel 27, to place the multi-position switch in serial state and press the operating button 30. The pilot then calls the eleven values V2, V3, V4, AP1, ΔT1, AP2, AT2, att2, att3, att4 and T pertaining to the test stage , the type of the range G and possibly the duration D. These values are stored in memory, the flows are carried out and the parameters obtained are displayed in 24.

To carry out a casting of the series stage, it is no longer necessary to use molds comprising presence sensors. We only keep the sensor E1. In fact, the time indications transmitted by the sensors E2, E3 and E4 during the test phase will be replaced by the stored data at t2, Δt3 and Δt4.

Apart from these simplifications, serial stage casting takes place in the same way as trial stage casting.

FIG. 4 shows the preferred E1 presence sensor according to the invention. This is of the ultrasonic type. It is composed of a generator-decoder assembly 40 outside the system and a probe 41 located inside the fixed plate 42 opposite and outside the connecting nozzle 43 and shown on the left side of that -this.

The generator-decoder assembly 40 emits a signal in the ultrasonic band, this is transmitted to the probe 41 by the conductor 44 and transmitted by the probe. The reflected beam of ultrasound resulting is recovered by the probe 41, transmitted to the assembly 40 via the conductor 45 and analyzed by the decoder.

In the case where the metal casting front 46a is located at a level lower than the probe 41, the operation of the apparatus can be shown diagrammatically by means of the curve of FIG. 4a. The probe emits a beam of ultrasound, the action of which can be shown diagrammatically by the peak E. This beam is first reflected on the left internal part 43a of the connecting nozzle, then crosses the internal channel of the nozzle. by weakening slightly and then is reflected on the right internal face 43b of the same connecting nozzle 43.

These successive reflections are characterized, with regard to the reflection on the left face of the nozzle, by the peak R1 and on the right face of the nozzle by the peak R2. It can be noted that the peaks E, R1 and R2 are decreasing respectively but the peaks R1 and R2 are of the same order of magnitude. The two peaks R1 and R2 represent both the phase shift and the energy of the beams reflected and received by the probe 41 and led to the decoder part of the assembly 40.

In the case where the casting front 46b is at a level higher than that of the probe 41, the operation of the system is represented by the curve of FIG. 4b. The reflections respectively on the left and right face of the connecting nozzle are concretized by the peaks R'1 and R'2, the emission peak being represented by E '. It is noted that, in this case, the peak R'2 is very weakened compared to the peak R'1. This information is, as before, transmitted to the decoder part of the assembly 40.

During operation, the role of this decoder is to distinguish the arrangements of the type 46a and type 46b casting front. To do this, this decoder has organs capable of distinguishing the resulting peaks of type R2 and of type R'2.

The decoder transmits to the pilot 47, via the cable 48, the information concerning the position of the metal relative to the position of the probe.

If we now refer to Figure 5, we can see the thin part of a part being cast. This part is the trailing edge of a turbo-machine blade during casting. The metal 49 progresses inside the cavity 50 left inside the mold 51. At the end of this cavity is a small channel 52 1 mm high x 2 mm wide. This channel opens into a pipe 53 connected to a vacuum source 54 via an assisted valve 55.

Means are provided and in particular the cable 56 for transmitting pressure indications to the pilot and for enabling him to control the depression in the cavity as the metal advances. These means are of the same type as those described above and used to control the discharge pressure. The pilot slaves in this case a pressure vacuum so as to suck up the gas bubble trapped by the metal in the cavity 50 during its evolution and thus allow a good penetration of the metal in all the points of the imprint and leading to a satisfactory surface finish.

An electrode 57 is installed in certain cases to fulfill the role of presence sensor and to initiate the vacuum-pressure phase directed by the pilot. In the serial phase, 56 and 57 are deleted and the trips are made by times memorized in the pilot.

Referring to Figure 6, there is a device for wedging the molds, used in the development phase. This device essentially comprises a metal box 58 inside which the cores of a sand mold 59 are positioned. Under the action of the thrust of the metal 60 rising in the mold footprint, the latter supports constraints which tend to raise it with respect to the fixed plate 61. Means are provided for holding it in place. For this purpose, rules 62 are fixed by keying across the upper surface 63 of the trunk 58. Screws 64, integral with the previous rules, apply the cores of the mold 59 frontally to the base of the trunk by means of shims 65 The mold and the trunk are then secured. To apply them against the fixed plate 61, bars 67 and 68 transmit a vertical force from top to bottom exerted by the movable plate 69. Different types of shims 70 and 71 are provided to adapt this system to the different dimensions of molds and chests .

FIG. 7 represents a wedging system used in the production stage. It is suitable for successive positioning of molds of different dimensions. To do this, the different molds are held in place in boxes 72 or 73 by means of rules-screws-blocks system of the type described in FIG. 6. A pair of jacks 74 is secured to the movable plate 69. Means are provided to move these two cylinders symmetrically on either side of the axis of the casting machine. The arrows f1 and f'1 symbolize these movements. In addition, the rods 75 are movable vertically relative to each of the jacks and terminate in a shoulder 76. The arrows f2 and f'2 account for these movements. During each installation of molds, the type of the corresponding part is taken into account by the pilot 40. The latter has in memory the position of the jacks corresponding to the type of the part. It automatically controls, via the servo-motor 77, the following movement f1 of the axis of the two jacks so as to bring them opposite the upper reach of the two metal boxes. The pilot then orders the deployment of the two jacks 74. The two shoulders 76 press the trunk 73 against the fixed plate 61. Once the casting is finished, the pilot commands the re-entry of the two rods of jacks 75. The mold and the trunk containing the freshly cast part can be removed from the system.

We realize that the methods and devices described above make it possible to completely control the dynamic, static and thermal conditions of each casting, according to adjustable predetermined characteristics. The conditions imposed during casting take into account the different types of random variations that can occur during molding. In this case, the flow of the flows is completely independent of the drop in the level of the metal in the crucible, of leakage of discharge gas and of heat losses. The casting conditions are fully reproducible and lead to completely identical series of parts in quality.

We also realize that the process described rationalizes the successive sequences of a series of parts of a given type. It offers solutions suited to development stages and mass production stages. Each series start-up then requires only very limited human operations.

Furthermore, we realize that the materials described are simple but nevertheless precise and effective in their actions. The ultrasonic sensor system eliminates fouling problems. The process of regulated vacuum pressure allows the production of very fine parts which, until now, were very difficult to obtain by molding. Finally, the proposed wedging systems considerably simplify the installation of the molds.

It will be noted that the methods described can be adapted to all moldable materials such as magnesium, steel or plastics and that the devices considered can be applied to any pressure casting apparatus. The origin of the movement of the metal caused by a gas flow can completely be replaced by a liquid, a rotating field or an electromagnetic pump. It suffices, in fact, to know the correlation which exists between the height of the metal and the factor which caused its movement. This correlation can in any case be established mathematically or experimentally.

Claims (7)

1. A process for automating a low pressure-type casting cycle of a metal in a closed mould, consisting of :

- acting on an intensive single action variable, that is the pressure of the backflow fluid of the molten metal contained in the furnace in order to subject the metal to a cycle of variations of level and pressure, which cycle is divided into phases, is started when the metal passes the bottom of the mould by means of a presence sensor which serves to determine the reference pressure in the furnace at the start of the cycle which ends on completion of the decompression of the furnace, the said process being characterised in that :

- after a preliminary preparation stage consisting of :

a) dividing the said cycle into a particular number of successive phases, each phase corresponding, as a function of the shape of the part to be obtained and the desired metallurgical quality, to the determined conditions of time and value of the speed and/or pressure of the metal,

b) and determining, for each of these phases, from the conditions to be applied to the metal, those to be applied to the pressure of the backflow fluid,

- the pressure of the backflow fluid is continuously controlled during casting with a view to obtaining the characteristics desired for the metal in each phase, by dividing the duration of each phase into successive elementary steps, by effecting, at each step, a comparison between the real value of the said fluid pressure and the corresponding predetermined value in the said preliminary stage and by raising or reducing the real pressure of the fluid accordingly.

2. A process according to claim 1, characterised in that, in the said preliminary stage, several sensors for detecting the presence of metal are used which are located on the walls of the mould in the region of the phase changes of the cycle, the durations of the said phases are measured by recording the curves and the values to be retained for these durations during mass-production operations, for which only the sensor for determining the start of the cycle is maintained, are determined statistically from these.

3. A process according to claim 2, characterised in that the cycle characteristics which are judged to be optimum are stored in correlation with each type of part, which characteristics are statistically obtained following the preparation tests of the preliminary stage, and the casting of a part is started up by imposing, by the control of adapted means, the production of the stored characteristics before casting.

4. A process according to one of claims 1 to 3, characterised in that the said cycle has, after the mould has been filled, a phase during which excess pressure is applied at a speed and an acceleration to avoid water hammers, then a phase during which excess pressure is applied, adding to the former in a very short time, the total of these two excess pressures representing the gating pressure and being applied before the part begins to harden and these excess pressures being a function of the characteristics of the part to be produced.

5. A process according to one of claims 1 to 4, characterised in that the said presence sensor for detecting the start of the cycle is an outer sensor on the wall of the mould, delimiting the course of the metal and consisting of a generator, an emitter, a receiver and a wave group analyser and in that a group of waves is emitted towards a point on the path of the metal during casting, the groups caused by the successive reflections on the different layers of material facing the emitter are received and the presence or absence of the metal facing the emitter is thereby determined using the analyser.

6. A process according to one of claims 1 to 5, more particularly applied to moulds having a pocket cavity of small thickness, characterised in that the said cycle has an auxiliary phase for depressurising, during casting, the said cavity in order to evacuate the bubble of gas trapped by the metal developping in the said cavity, the said depressurisation being effected by linking the cavity to the outside and, depending on the predetermined speeds for predetermined periods.

7. A device linked to a low-pressure casting system and the attachment parts thereof intended for monitoring the operating phases in order to maximise the quality and speed of production when carrying out the process according to claims 1 to 6, characterised in that it consists of :

- an inlet-outlet system responsible for information on the different action variables, both real variables and variables supplied by the measuring members positioned on the casting device and desired variables and those transmitted by the operator, so as to supply them in numerical form and transmit them to a computer,

- a computer system linking the phase characteristics to the necessary developments to be imposed on the action variable and carrying out secondary calculations necessary for the progress of the process, this computer being linked, on the one hand, to a memory unit and, on the other hand, to a decision member,

- a memory unit for storing, temporarily, information to be used during casting and possibly, on a long term basis, general information about casting,

- a clock system linked to a programmed system giving rythm to the activity of each of the members,

- a decision member activating the action members in the system, more particularly the valves, and receiving information, particularly from the memory system and the inlet-outlet system of the programmed system and the clock system.

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