FR2667523A1 - SOLUBLE CARAPACEOUS MOLD FOR FOUNDRY AND METHOD OF REMOVAL. - Google Patents

Abstract

A shell mold that can be used in lost wax casting is obtained from slips based on a binder product of the colloidal silica or ethyl silicate type and a ceramic mineral filler based on fused silica and zircon, resulting in a composition in percentages of dry mass comprising 70 to 95% of fused silica and 5 to 30% of zircon. The removal process involves dissolving in a basic bath.

Description

DESCRIPTION

  The present invention relates to a casting mold-shell whose composition provides properties facilitating the elimination after casting of parts, particularly in foundry processes lost wax type. It also relates to a method of removing said shell mold. As is already well known, the foundry processes using ceramic shell molds comprise in particular the following steps: making a model, generally in wax, of the part to be produced, coating the model in successive layers from slips of determined compositions with drying

  intermediates, so as to form said mold

  carapace, heat treatment or cooking according to determined cycles of temperatures and times so as to obtain a consolidation or "baking" of the mold and the elimination of the model, performing a piece casting operation in the mold obtained which can for example have the shape of a "cluster" in the case of a simultaneous casting of several parts, especially for aeronautical applications such as motor vanes, particularly covered by the invention,

removal of the shell mold.

  This last operation usually uses known techniques of mechanical unblocking and then shot blasting and sandblasting Some difficulties have been solved however have been encountered for some pieces of complex shape, including cavities or elbows, these operations are insufficient to eliminate the carapace In addition, certain parts, such as compressor wheels of monoblock design having blades of very small fabric thickness, have very fragile parts that may be damaged or broken during a mechanical shake-out. 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 been described in particular by EP-A-0 251 847,

US-A-3,249,972, FR-A-2,348,772.

  These known compositions, however, do not solve

  completely all the problems of eliminating the mold-

  shell that have been noted above.

  A mold-shell for a foundry meeting these conditions and allowing in particular improved conditions of removal of the mold after casting of the parts is characterized in that it is obtained from slip based on a binder product of the colloidal silica type or of the ethyl silicate type and of a ceramic mineral filler whose composition in percentages of dry mass comprises between 70 and 95% of silica

  melted and between 5 and 30% zircon.

  A remarkable method of removing said mold

  carapace after casting of the parts is characterized in that it comprises the following steps: (a) partial manual removal of the shell mold; (b) cutting castings; (c) complete dissolution of the shell mold in a bath

  basic to a base of soda or potash.

  Other features and advantages of the invention

  will be better understood by reading the description that

  will follow non-limiting examples of realization

illustrating the invention.

EXAMPLE 1

  A casting mold-shell for lost-wax casting according to the invention and having a composition specially adapted to allow elimination by chemical dissolution after casting of parts and therefore having remarkable properties of solubility can be obtained by successive and repeated operations. a number of times that are suitable for the desired result, according to techniques known per se, of soaking the wax model and then of drying, in slips whose outstanding compositions and according to the invention are the following in the present example: first slip of contact comprising% of colloidal silica, 79% of zircon flour, 1% of a wetting agent

and antifoam.

4 2667523

  then a second contact slip, comprising% of colloidal silica, 49% of zircon flour, 25% of fused silica flour, 1% of a wetting agent complementary agent

and anti foam.

  and finally a reinforcing slip, comprising: 20% of colloidal silica, 33% of zircon flour, 42% of fused silica flour 3% of cristobalite flour,

  2% of a suspending agent.

  This method of obtaining the mold-shell leads to a final composition, as a percentage of dry mass, of

  % fused silica and 30% zircon.

OTHER EXAMPLES

  Instead of a bonding product based on colloidal silica, the reinforcing slips used may comprise a

  binder product based on ethyl silicate.

  A reinforcing slip which can be used in such an embodiment and in accordance with the invention may comprise% of ethyl silicate, 23% of zircon flour, 37% of fused silica, 3% of cristobalite,

  2% of a suspending agent.

  Moreover, the percentages of dry mass may vary in the proportions between fused silica, which remains the major element and zircon. In particular, in another embodiment example, the final composition, in percentages of dry mass, is 95% of fused silica. and 5% zircon. It should therefore be noted that these percentages may vary between 70 and 95% of

  fused silica and between 5 and 30% zircon.

  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 batching facilities for the elimination operations, which follow. necessarily casting in the casting and solidification step by cooling parts Indeed, these compositions allow to apply a remarkable method of removing the mold-shell which comprises the following steps: (a) partial manual removal of the mold-carapace ; (b) cutting castings; (c) complete dissolution of the shell mold in a bath

  basic soda or potash.

  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

2000 C and 1.5 M Pa.

  An alternative solution allows to use a bath of

potash in the air.

  It should further be noted that the mold compositions

  shell according to the invention thanks to the presence of fused silica provides additional advantages and in particular: an improvement in the mechanical properties of the shell to the casting, due to a formation of cristobalite at high temperatures; properties of low expansion leading to

  improvement of thermal shock resistance.

Claims (6)

  Mold casting shell, in particular by a lost wax process, characterized in that it is obtained from slip materials based on a binder product of the colloidal silica or ethyl silicate type and with a mineral filler. ceramic based on fused silica and zircon leading to a composition in percentages of dry mass comprising between 70 and 95% of fused silica and between 5 and 30% zircon.   2 Mold casting shell according to claim 1 obtained by successive operations of quenching and drying in the following slip: - (a) a first contact slip comprising% of colloidal silica, 79% of zircon flour, 1% of a complementary agent with wetting and anti-mouse properties; (b) a second contact slip comprising 25% of colloidal silica, 49% of zircon flour,% of fused silica meal, 1% of a complementary agent with wetting and anti-foaming properties; (c) a reinforcing slip comprising% of colloidal silica, 33% of zircon flour, 42% of fused silica flour, 3% of cristobalite flour, 2% of a suspending agent and comprising at the final, in percentages of mass   70% dry fused silica and 30% zircon.   Casting mold shell according to Claim 1, obtained by successive quenching and drying operations in the following slurries: (a) a first contact slip comprising% of colloidal silica, 79% of zircon flour, 1% of an additional agent with wetting and anti-foaming properties, - (b) a second contact slip comprising% of colloidal silica, 49% of zircon flour,% of molten silicate flour, 1% of a wetting property-enhancing agent and antifoam, (c) a reinforcing slip comprising% ethyl silicate, 23% zircon flour, 37% fused silica, 3% cristobalite,   2% of a suspending agent.   4 Process for the elimination, after casting of the pieces, of a mold-shell conforming to any one of the   Claims 1 to 3, characterized in that it comprises   the following steps: - (a) partial manual removal of the shell mold, - (b) cutting of the castings, - (c) complete dissolution of the shell mold in a bath basic.   A process for removing a shell mold according to claim 4, wherein the dissolving step (c) is carried out in an autoclave in a sodium hydroxide bath under average temperature and pressure conditions. C and 1.5 M Pa.   A method of removing a shell mold according to claim 4 wherein the step (c) of dissolving a shell mold is carried out in a potash bath. looks.