EP0479672B1 - Soluble shell mould for foundry and method for disposal - Google Patents

Description

The present invention relates to a shell mold for foundries, the composition of which provides properties facilitating the elimination after casting of the parts, in particular in foundry processes of the lost wax type.

It also relates to a process for eliminating said shell mold.

• réalisation d'un modèle, généralement en cire, de la pièce à réaliser,
• enrobage du modèle par couches successives à partir de barbotines de compositions déterminées avec séchages intermédiaires, de manière à former ledit moule-carapace,
• traitement thermique ou cuisson selon des cycles déterminés de températures et durées de manière à obtenir une consolidation ou "cuisson" du moule et l'élimination du modèle,
• réalisation d'une opération de coulée de pièce dans le moule obtenu qui peut par exemple avoir la forme d'une "grappe" dans le cas d'une coulée simultanée de plusieurs pièces, notamment pour des applications aéronautiques telles que des aubes de moteur, particulièrement visées par l'invention,
• élimination du moule-carapace.

As is already well known, foundry processes using ceramic shell molds include the following steps:

• production of a model, generally in wax, of the part to be produced,
• coating of the model in successive layers from slips of determined compositions with intermediate drying, so as to form said shell mold,
• heat treatment or cooking according to determined cycles of temperatures and times so as to obtain a consolidation or "cooking" of the mold and the elimination of the model,
• carrying out a part casting operation in the mold obtained, which may for example have the form of a "cluster" in the case of a simultaneous casting of several parts, in particular for aeronautical applications such as engine blades, particularly targeted by the invention,
• elimination of the shell mold.

This last operation usually uses known techniques of mechanical release then shot peening and sandblasting. However, some poorly resolved difficulties were encountered. For certain pieces of complex shape, in particular having cavities or elbows, these operations are insufficient to eliminate the shell in the enclosed areas. In addition, certain parts, such as compressor wheels of a monobloc design having blades with a very small fabric thickness, have very fragile parts which risk being damaged or broken during a mechanical release. Finally, the parts with a monocrystalline structure must not be subjected to shocks during all the finishing operations in order to avoid the appearance of recrystallization phenomena.

Furthermore, examples of slip compositions used for the manufacture of foundry shell molds have in particular been described by EP-A-0 251 847, US-A-3 249 972, FR-A-2 348 772.

These known compositions do not, however, entirely resolve all of the problems of elimination of the shell mold which have been noted above.

A shell mold for foundry meeting these conditions and allowing in particular improved conditions for removing the mold after casting the parts is obtained according to the invention from slips based on a binder product chosen from colloidal silica or silicate. of ethyl and of a ceramic mineral filler leading to a mold composition in percentages of dry mass comprising between 70 and 95% of fused silica and between 5 and 30% of zircon.

• (a) élimination manuelle partielle du moule-carapace ;
• (b) découpe des pièces coulées ;
• (c) dissolution complète du moule-carapace en bain basique à base de soude ou de potasse.

A remarkable process for eliminating said shell mold after casting the parts is characterized in that it comprises the following stages:

• (a) partial manual elimination of the shell mold;
• (b) cutting the castings;
• (c) complete dissolution of the shell mold in a basic soda or potash bath.

Other characteristics and advantages of the invention will be better understood on reading the description which follows of nonlimiting exemplary embodiments illustrating the invention.

EXAMPLE 1

• première barbotine de contact comprenant :
  20 % de silice colloïdale,
  79 % de farine de zircon,
  1 % d'un agent complémentaire à propriétés mouillantes et anti-mousse.

• puis une deuxième barbotine de contact, comprenant :
  25 % de silice colloïdale,
  49 % de farine de zircon,
  25 % de farine de silice fondue,
  1 % d'un agent complémentaire à propriétés mouillantes et anti mousse.
• et enfin une barbotine de renfort, comprenant :
  20 % de silice colloïdale,
  33 % de farine de zircon,
  42 % de silice fondue
  3 % de farine de cristobalite,
  2 % d'un agent complémentaire de suspension.

A shell mold for lost wax foundry according to the invention and having a composition specially adapted to allow elimination by chemical dissolution after casting of parts and consequently having remarkable solubility properties can be obtained by successive and repeated operations a number of times suitable for the desired result, according to techniques known per se, of dipping the wax model and then drying, in slips of which the remarkable compositions and in accordance with the invention are the following in the present example:

• first contact slip comprising:
  20% colloidal silica,
  79% zircon flour,
  1% of an additional agent with wetting and anti-foaming properties.

• then a second contact slip, comprising:
  25% colloidal silica,
  49% zircon flour,
  25% melted silica flour,
  1% of an additional agent with wetting and anti-foaming properties.
• and finally a reinforcing slip, comprising:
  20% colloidal silica,
  33% zircon flour,
  42% fused silica
  3% cristobalite flour,
  2% of an additional suspension agent.

This method of obtaining the shell mold results in a final composition, in percentage of dry mass, of 70% of fused silica and 30% of zircon.

OTHER EXAMPLES

Instead of a binder product based on colloidal silica, the reinforcing slip used may include a binder product based on ethyl silicate.
A reinforcing slip usable in such an embodiment and in accordance with the invention may include:
35% ethyl silicate,
23% zircon flour,
37% fused silica,
3% cristobalite,
2% of an additional suspension agent.
Furthermore, the percentages of dry mass can vary in the proportions between fused silica which remains the majority element and zircon. In particular, in another exemplary embodiment, the final composition, in percentages of dry mass, is 95% of fused silica and 5% of zircon. It should therefore be noted that these percentages can vary between 70 and 95% of fused silica and between 5 and 30% of zircon.

• (a) élimination manuelle partielle du moule-carapace ;
• (b) découpe des pièces coulées ;
• (c) dissolution complète du moule-carapace en bain basique à base de soude ou de potasse.

The shell molds according to the invention, the compositions of which have just been described above, have the essential advantage, in addition to the usual properties required by their destination, of providing advantageous facilities during the disposal operations which follow. necessarily in the foundry stage of casting and solidification by cooling of the parts. Indeed, these compositions make it possible to apply a remarkable process for eliminating the shell mold which comprises the following steps:

• (a) partial manual elimination of the shell mold;
• (b) cutting the castings;
• (c) complete dissolution of the shell mold in a basic soda or potash bath.

This last step (c) of dissolution which characterizes the invention can in particular be implemented using an autoclave and a soda bath, under average conditions of temperature and pressure of 200 ° C. and 1.5 MPa.

An alternative solution allows the use of an air potash bath.

• une amélioration des qualités mécaniques de la carapace à la coulée, due à une formation de cristobalite à hautes températures ;
• des propriétés de faible dilatation amenant une amélioration de la résistance aux chocs thermiques.

It will further be noted that the shell-mold compositions in accordance with the invention thanks to the presence of molten silica provides additional advantages and in particular:

• an improvement in the mechanical qualities of the shell during casting, due to the formation of cristobalite at high temperatures;
• properties of low expansion leading to an improvement in resistance to thermal shocks.

Claims (6)

1. Shell mould for founding, especially using a lost-wax method, obtained from slurries based on a binder substance chosen from colloidal silica and ethyl silicate and a ceramic inorganic filler based on fused silica and on zircon, leading to a mould composition in percentages of dry matter comprising between 70 and 95% of fused silica and between 5 and 30% of zircon.
2. Shell mould for founding according to Claim 1, obtained by successive operations of dipping and drying, in the following slurries:

   - (a) a first contacting slurry comprising:

   - 20% of colloidal silica,

   - 79% of zircon flour,

   - 1% of a complementary agent having wetting and anti-foaming properties;

   - (b) a second contacting slurry comprising:

   - 25% of colloidal silica,

   - 49% of zircon flour,

   - 25% of fused-silica flour,

   - 1% of a complementary agent having wetting and anti-foaming properties,

   - (c) a reinforcing slurry comprising:

   - 20% of colloidal silica,

   - 33% of zircon flour,

   - 42% of fused silica,

   - 3% of cristobalite flour,

   - 2% of a complementary suspension agent

   and comprising, in the final stage, as percentages of dry matter, 70% of fused silica and 30% of zircon.
3. Shell mould for founding according to Claim 1, obtained by successive operations of dipping and drying, in the following slurries:

   - (a) a first contacting slurry comprising:
   20% of colloidal silica,
   79% of zircon flour,
   1% of a complementary agent having wetting and anti-foaming properties;

   - (b) a second contacting slurry comprising:
   25% of colloidal silica,
   49% of zircon flour,
   25% of fused-silica flour,
   1% of a complementary agent having wetting and anti-foaming properties,

   - (c) a reinforcing slurry comprising:
   35% of ethyl silicate,
   23% of zircon flour,
   37% of fused silica,
   3% of cristobalite,
   2% of a complementary suspension agent.
4. Method of disposing, after the castings have been cast, of a shell mould in accordance with any one of Claims 1 to 3, characterized in that it includes the following steps:

   - (a) partial manual removal of the shell mould,

   - (b) cutting out the castings,

   - (c) complete dissolution of the shell mould in a base bath.
5. Method of disposing of a shell mould according to Claim 4, in which the dissolution step (c) is carried out in a bath of sodium hydroxide within an autoclave, under mean temperature and pressure conditions of 200°C and 1.5 MPa.
6. Method of disposing of a shell mould according to Claim 4, in which step (c) for dissolving a shell mould is carried out in a bath of potassium hydroxide in air.