EP0072329A1 - Process for making a ceramic shell mould element, and mould element obtained - Google Patents

Abstract

1. A process for producing a mould element comprising effecting a succession of layers by means of a barbotine comprising a refractory powder and a binder having hot and cold agglomerating properties, each deposit of a layer being followed by a powdering with a refractory powder, and effecting a firing, characterised in that it comprises forming a box (1) in which are mounted added parts (2) of a material which disintegrates upon firing, coating the box with a thin film of a mould-stripping agent, then effecting directly the deposit of the succession of layers so as to obtain a shell (3) having walls (31, 32), partitions (33) defining cavities (34) corresponding to the added parts and internal cavities, extracting said shell from the box and leaving the added parts embedded in the shell, these added parts being destroyed upon the firing.

Description

The present invention relates to a method of making a mold element (mold or core) constituted by a ceramic shell and to the mold element thus obtained. This process is intended to be used in precision foundry.

The ceramic shell molding process derived from the lost wax process is known from French patent 1,204,586. This process consists in producing a thin refractory shell by deposition of successive refractory layers around a wax model. Each refractory layer is deposited using a slip comprising an acid reaction binder or a basic reaction and a refractory filler. Each coating of the slip is dusted (stuccoing) using a refractory powder. The binder must be refractory and have cold and hot agglomerating properties. Either ethyl silicate (organic silicate with an acid reaction) or sodium silicate (inorganic silicate with an alkaline reaction) is used. The shell is often formed in accordance with French patent 1,327,999 by depositing a layer with an alkaline binder followed by a layer with an acid binder in order to promote the setting of the binders. The wax model coated with its refractory shell is dewaxed in a heating oven.

In another ceramic shell molding process described in French patent 1,384,487, the refractory shell is made on rigid model plates. However, this process requires that the model plates be covered with an elastic material which affects dimensional accuracy.

The aforementioned methods are poorly suited to the production of cores with large ceramic shells such as the cores providing curved and left-hand recesses such as those used for molding the impellers of turbomachines. A core of this type can be produced in several parts, but mounting the entire core then poses delicate positioning problems.

The subject of the present invention is a method for making a mold element with a refractory shell, of large dimensions, the various refractory layers of which, in particular the outer layer which will come into contact with the metal, have a binder based on silicate of ethyl which gives good refractoriness and great imprint fidelity. This process makes it possible to produce a one-piece mold element of large dimensions having undercut recesses. It allows in particular to core having undercut recesses such as a core used for molding a wheel or a rotor with turbine engine blades. The method makes it possible to produce a one-piece core with a refractory shell having internally partitions provided with recesses and joining the outer walls of the shell leaving internal cavities which provide a lighter structure having advantages from the point of view of rigidity and trimming. solidification.

The method according to the invention consists in forming a shell by depositing, in contact with a rigid box, a succession of refractory layers by a slip comprising a binder and a refractory powder, each deposition of a layer being followed by dusting with a refractory powder, and it is characterized in that the first layer is deposited directly in contact with the box, the surface of which is coated with a release agent.

According to one characteristic of the process, the different layers, the first layer of which comes directly into contact with the box, are deposited using an ethyl silicate slip.

In the case of a molding core from a view of a turbomachine, the method consists in mounting, inside the box, blades added in plastic material to form the refractory shell and in dismantling the box, leaving the blades inside the shell.

The core-type mold element obtained by the process has undercut recesses which correspond to blades and it is characterized by the fact that it forms two walls joined by partitions comprising recesses corresponding to the blades and forms between said walls and the said partitions of the internal cavities which open outwards.

• La figure 1 représente, en élévation, une boite à noyau servant à la confection d'un noyau pour le moulage d'une roue à aubes courbes et gauches.
• La figure 2 est une vue de dessus de la boite représentée sur la figure 1.
• La figure 3 est une coupe partielle selon III-III des figures 1 et 2.
• La figure 4 représente une coupe axiale de la boite dans-laquelle la carapace réfractaire a été constituée.
• La figure 5 est une vue en perspective du noyau à carapace réfractaire, après démoulage et avant cuisson.
• La figure 6 est une vue en perspective du noyau à carapace réfractaire après cuisson.
• La figure 7 représente une coupe axiale d'une boite dans laquelle on réalise la carapace réfractaire du noyau monobloc d'une rouedouble de turbomachine.
• La figure 8 est une coupe selon VIII-VIII de la figure 7.

The invention will now be described in more detail with reference to the accompanying drawings.

• Figure 1 shows, in elevation, a core box used for making a core for molding a curved and left impeller.
• Figure 2 is a top view of the box shown in Figure 1.
• Figure 3 is a partial section on III-III of Figures 1 and 2.
• Figure 4 shows an axial section of the box in which the refractory shell was formed.
• FIG. 5 is a perspective view of the core with a refractory shell, after demolding and before baking.
• Figure 6 is a perspective view of the refractory shell core after firing.
• FIG. 7 represents an axial section of a box in which the refractory shell of the one-piece core of a turbomachine wheel is produced.
• Figure 8 is a section along VIII-VIII of Figure 7.

In the method according to the invention, a box 1 is used which matches the external shape of the ceramic shell mold element which it is desired to obtain. This multi-part box is made of a rigid material, preferably laminated resin, by overmolding a wooden core.

Separate parts are produced corresponding to the undercut recesses provided in the core. These parts are made of plastic material which destroys when the ceramic shell is fired. This plastic material has suitable thermal and mechanical properties, in particular negligible or zero shrinkage and expansion and great mechanical resistance. Then mounted inside the box these parts made of plastic. The internal walls of the box and the attached parts against which the layers will be deposited are coated with a release agent. The release agent is a non-stick product such as a product based on silicone or colloidal graphite.

The refractory shell is then made up of several layers deposited successively on the internal faces of the box and on the attached parts which are eliminated during cooking. To deposit each refractory layer, a slip is used which is introduced into the box. This slip has a refractory filler and a binder having cold and hot agglomerating properties. This binder is formed by organic silicate with an acid reaction (ethyl silicate) of alcohol and acid. The hydrolysis of ethyl silicate in an alcoholic medium gives colloidal silica. The dep8t by the slip is followed by a dusting (stuccoing) of the layer using a refractory powder. Gelation is obtained by hot air or ammonia vapor. The operation is repeated several times until a shell of sufficient thickness is obtained. The first layer is deposited directly in contact with the box. It is carried out using a slip with a binder of ethyl silicate.

The monobloc shell is extracted from the box. At this stage the plastic parts remain embedded in the shell.

The shell is then cooked in a heating oven. During this cooking, the attached plastic parts disintegrate, the residue being removed after cooking. Likewise the binder alcohol is eliminated during cooking. At the end of cooking, the mold element has recesses corresponding to the plastic parts that cooking has made it possible to eliminate.

The method according to the invention applies to the production of mold elements having a shape of revolution such as the cores of single or double wheels of turbomachinery but also to mold elements of parts not having a shape of revolution (Pelton wheel augst for example).

In the case of a turbomachine wheel core, the rigid gearbox is represented by FIGS. 1, 2, 4,7. This box 1 corresponds to the outside of the wheel that we want to obtain. It consists of several separable parts. The box shown in the drawings has two flanges 11 and 12.

In the case of the turbomachine wheel core shown in the figures, the recesses of the core corresponding to the blades of the wheel and the inserts therefore have substantially the profile of the blades of the wheel. A set of plastic blades is produced. These blades, identified 21 - are then positioned and fixed in the box 1. Each blade is held between the flanges 11 and 12 by a fixing system allowing the separation of the flanges 11 and 12; for example to produce the fixing of the plastic blades, the two flanges 11 and 12 comprise, on the sides corresponding sponding grooves on the outside of the core, the side _n ons (or position corresponding to the radial profile of the blades and the blades have raimnres (or tenons) which fit onto the tenons (or in the grooves) of the flanges.

The parts of the box and the added parts against which the refractory layers of the shell are deposited are coated with a release agent. Thus on the case represented by FIGS. 1 and 2, or 7, the faces 111 and 112 are coated as well as the surfaces of the blades 21.

The refractory shell 3 is then produced in accordance with the method explained above, the first layer which comes into contact with the internal walls of the box and the added blades being produced with an ethyl silicate slip. The gelation is carried out as explained above. The box is then dismantled so as to extract the shell from the box. The blades 2 are then embedded in the shell (Figure 5). Cooking is carried out as explained above.

The invention relates to a refractory shell core of the turbomachine rotor core type (pump, etc.). FIG. 6 represents the embodiment which corresponds to FIGS. 1 to 4. This monobloc core is composed walls 31 and 32 joined by intermediate partitions 33 in which are formed recesses 34 which open on the outer faces of the two flanges. These recesses 34 correspond to the blades of the rotor. Between the walls 31 and 32 and the partitions 33 are formed internal cavities, in a scroll, which communicate with each other and open outwards through at least one peripheral opening 35 which opens onto the large diameter of the core between the walls 31 and 32. opening debt 35

allows filling and emptying of the slip during the formation of the shell.

It is understood that it is possible without departing from the scope of the invention to imagine variants and improvements of details / even to envisage the use of equivalent means.

Claims (5)

1.- Method of making a refractory shell mold element, consisting of forming a refractory shell by depositing in a rigid box a succession of refractory layers by a slip comprising a binder and a refractory powder, each dep8t of a layer being followed by dusting with a refractory powder, characterized in that the first layer is deposited directly in contact with the box, the surface of which is coated with a thin film of a release agent. 2.- Method according to claim 1 characterized in that said first layer is deposited using a slip with a binder of ethyl silicate. 3.- Method according to any one of the preceding claims, characterized in that the release agent is a non-stick product based on silicone or colloidal graphite. 4.- Method according to any one of the preceding claims, applied to a core of a turbomachine wheel mold characterized in that it consists in mounting, inside the box, blades made of plastic material, form the refractory shell and dismantle the box, leaving the blades inside the shell. 5.- mold element obtained by implementing the method according to any one of the preceding claims, of the one-piece core type comprising undercut recesses, usable for molding a turbomachine impeller, characterized by the fact that said core forms walls (31-32) joined by partitions (33) which have recesses corresponding to the blades and forms, between the walls and the partitions, cavities which open outwards.

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