Abstract:
A mold for producing a ceramic heat shield element having a mold shell is provided. The mold shell comprises a plurality of mold surfaces and a casting aperture for pouring in a ceramic material. The mold shell is embodied as a single-piece mold shell during casting, and the casting aperture is embodied as an aperture in one of the mold surfaces. Separating or retaining elements enable graded or reinforced heat shield elements to be produced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefits of European Patent application No. 05014798.2 filed Jul. 7, 2005 and is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to a mold for producing a ceramic heat shield element. 
   BACKGROUND OF THE INVENTION 
   The walls of hot gas conducting combustion chambers, of gas turbine plants for example, require their supporting structure to be thermally shielded against attack by hot gas. The thermal shielding can be implemented for example by means of a hot gas lining disposed in front of the actual combustion chamber wall, in the form of a ceramic heat shield for example. A hot gas lining of said kind is usually constructed from a number of ceramic heat shield elements with which the combustion chamber wall is lined over its surface area. Ceramic materials are ideally suited for constructing the hot gas lining compared with metallic materials on account of their high temperature resistance, corrosion resistance and low thermal conductivity. A ceramic heat shield is described for example in EP 0 558 540 B1. In particular a heat shield element can comprise a core zone and an edge zone, the thermal conductivity of the material in the edge zone being less than in the core zone. A heat shield element of said kind is described in EP 1 508 761 A1. 
   SUMMARY OF THE INVENTION 
   Currently it is customary to manufacture ceramic heat shield elements by casting or compression molding. In order to illustrate the casting or compression-molding process, a heat shield element and a mold for producing said heat shield element will be described below with reference to  FIGS. 1 to 3 . 
   The ceramic heat shield element  100  shown in  FIG. 1  has a hot side  102  which faces toward the hot gas when the heat shield element  100  is installed in the heat shield of a combustion chamber. Opposite the hot side  102  is the cold side  104 , which faces toward the combustion chamber wall that is to be protected when the heat shield element is installed in a heat shield. Also present are peripheral sides  106 ,  108  which extend between the hot side  102  and the cold side  104 . Two peripheral sides  108  opposite each other are also provided with grooves  110  which serve to fix the heat shield element  100  in place on the supporting wall structure by means of retaining elements. 
     FIG. 2  shows in a perspective representation a compression mold  200  for producing the heat shield element from  FIG. 1 . The mold  200  consists of a number of molded parts  202   a  to  202   e  which are inserted into a molding box  203  and held in position by the latter. The internal surfaces  204 ,  206 ,  208  of the molded parts  202   a  to  202   e  represent the mold surfaces for molding the surface of the heat shield element  100 . Thus, for example, the internal surface  204  serves for molding the cold side  104  of the heat shield element, the internal surfaces  206  for molding the side areas  106  without groove, and the internal surfaces  208  for molding the side areas  108  with groove  110 . The internal surfaces  208  have spring-like projections  210  for shaping the grooves  110 . 
   In order to produce the ceramic heat shield element  100 , a ceramic molding compound  220  is introduced into the mold  200  with the installed molded parts  202   a  to  202   e  and subsequently pressed into shape by means of a die  212 . In this case the surface  214  of the die  212  facing toward the molding compound  220  molds the hot gas surface  102  of the ceramic heat shield element  100 . The compressive pressure necessary for compressing the molding compound  220  requires the mold  200  to be completely closed during the compression-molding process, i.e. the die  212  must be embodied as a precise fit for the mold  200 . Furthermore the compressive pressure can result in a springing back of the molded parts. Variations in the material quantity of the molding compound  220  can also lead to variations in the thickness of the finished ceramic heat shield element. 
   As an alternative to the compression molding, the heat shield element  100  can also be cast using the mold  200 , i.e. without a compression-molding operation taking place. Since the heat shield element is cast in a horizontal position, however, either the hot side  102  or the cold side  104  is not defined by the mold during the casting process. The undefined side requires a time-consuming and laborious reworking after the casting in order to produce the desired shape of the heat shield element  100 . 
   Finally, the molds described are not suitable for producing a heat shield element which has different material zones with different material properties in a single casting or compression-molding step. The production of heat shield elements with reinforcing elements on the inside is also not possible. 
   It is therefore an object of the present invention to provide a mold for producing a ceramic heat shield element that is advantageous compared to the described background art. 
   A further object of the present invention is to provide an advantageous method for producing a ceramic heat shield element. 
   The first object is achieved by a mold as claimed in the claims and the second object is achieved by a method as claimed in the claims. The dependent claims contain advantageous embodiments of the invention. 
   An inventive mold for producing a ceramic heat shield element has a mold shell which comprises a number of mold surfaces and a casting aperture for pouring in a ceramic material. The mold shell is embodied as a single-piece mold shell during casting and the casting aperture is embodied as an opening in one of the mold surfaces. The phrase “single-piece mold shell during casting” is not to be understood in this context as indicating that the mold shell is formed monolithically from a single piece, but rather as meaning that during the pouring in of the casting compound the mold shell does not consist of two elements that are not permanently joined to each other, for example of molded parts simply inserted into a molding box and a die as described with reference to  FIGS. 2 ,  3   a  and  3   b . The mold according to the invention can, however, be composed of a number of individual parts provided these are tightly mated with one another during the pouring in of the molding compound. In contrast to the mold described in the introduction, with the mold according to the invention no molding box is necessary. Such a molding box impedes in particular the fabrication of graded and/or reinforced heat shield elements, since during the production of a heat shield element the molded parts are disposed inaccessibly in the interior of the molding box. 
   If the mold is composed of a plurality of individual parts which need to be tightly joined to one another for the casting process for forming the single-piece mold shell, easy removal of the cured heat shield element is possible by detaching the individual parts from one another. 
   In contrast to the mold according to the prior art, in which one side of the mold is completely missing during the casting process, in the mold according to the invention a mold surface having a casting aperture for feeding the mold is present. In other words the mold surface in which the casting aperture is present defines the corresponding surface of the heat shield element at least in a rudimentary manner. With the mold according to the invention, therefore, all surfaces of the heat shield element can be formed at least rudimentarily without the need for compression molding of the heat shield element. In this case the result of the rudimentarily present mold surface in the area of the casting aperture is that surplus casting material present in the area of the casting aperture can be removed after the curing with the aid of the rudimentary heat shield element surface formed by the mold surface as a reference surface. The removal of superfluous material and the finishing of the heat shield element are therefore possible with relatively little effort. 
   Moreover, when the mold according to the invention is used the dimensions of the cast heat shield element are not dependent on the amount of material poured in, since no compression molding is performed. As the mold is completely closed in the compression-molding process according to the prior art, there is no possibility of the casting material escaping from the mold. Varying amounts of casting material therefore lead to the production of heat shield elements of differing thickness. In the mold according to the invention, on the other hand, surplus casting material can escape through the casting aperture without the dimensions of the heat shield element being compromised as a result. Furthermore, no springing back of the mold under compressive pressure occurs during casting. The mold according to the invention therefore permits the production of heat shield elements with reduced tolerances. 
   The mold according to the invention comprises in particular mold surfaces for molding a large-area first surface and a large-area second surface as well as mold surfaces for molding small-area (compared to said first and second surfaces) peripheral surfaces which extend from the first surface to the second surface. The casting aperture is then embodied in a mold surface for molding one of the peripheral surfaces. 
   In a special embodiment of the mold according to the invention at least one separating element is present by means of which different areas in the interior of the mold shell can be separated from one another. The separating element is embodied and to be disposed in the mold shell in such a way that it can be removed from the interior of the mold shell again before the curing of the poured-in ceramic material, without the mold having to be opened. This embodiment enables in particular the production of graded heat shield elements, i.e. such heat shield elements that comprise at least two zones which consist of materials with different material properties. 
   A graded heat shield element can then be produced, for example, by introducing the inserts into the casting mold before the ceramic material is poured in, then pouring in the ceramic material, and removing the inserts again after the ceramic material has been poured in. After the inserts have been removed the different ceramic materials can come into contact with one another and so form a firmly bonded join during curing. It is also possible that the adjacent materials merge in the threshold region when the separating elements are removed, with the result that on completion of the curing a heat shield element is present in which the two materials exhibit a fluid transition into each other. 
   The at least one separating element can be embodied in particular as an insert for inserting into the mold shell through the casting aperture. In particular an insert can be present which separates the interior of the mold shell into an area facing toward the mold surface for molding the large-area first surface and an area facing toward the mold surface for molding the large-area second surface. This enables the production, for example, of ceramic heat shield elements in which the cold side has different material properties from the hot side, for example a different rigidity or a different coefficient of thermal expansion. 
   A further possibility consists in providing two inserts which separate the interior of the mold shell into a central area and into two areas which face toward opposite mold surfaces for molding peripheral surfaces of the heat shield element. With this embodiment, heat shield elements can be produced which, in the area of two peripheral sides, have different material properties than in the area lying in between, for example different coefficients of thermal expansion or a different rigidity. 
   In a further embodiment of the mold according to the invention, said mold can comprise at least one retaining element that is to be introduced into the interior of the mold shell. The retaining element is embodied and disposed in the mold shell in such a way that it can fix a body, for example a reinforcing element, in position in the interior of the mold shell and that it can be removed from the interior of the mold shell again before the poured-in ceramic material has cured. For example, retaining pins which can be moved from the exterior of the mold shell into the interior of the mold shell and back out again may be present as retaining elements. The retaining pins can be disposed in particular in the mold surface for molding a large-area first surface and/or in the mold surface for molding a large-area second surface. 
   The retaining elements enable bodies such as, for example, reinforcing elements to be held in position in the interior of the mold when the ceramic material is poured in. After the ceramic material has been poured in, the retaining elements can be removed from the interior of the mold, with the result that the body is held in position solely by the surrounding ceramic material. Following the curing the body forms a body cast into the ceramic heat shield element. In this way high-strength reinforcing elements, for example, can be incorporated into a ceramics heat shield element. 
   In the inventive method for producing a ceramic heat shield element, a ceramic heat shield element is produced using a casting process. In this case a mold according to the invention is used for the casting. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features, characteristics and advantages of the present invention will emerge from the following description of exemplary embodiments with reference to the accompanying figures. 
       FIG. 1  shows a ceramic heat shield element in a perspective representation. 
       FIG. 2  shows schematically a compression mold for producing a heat shield element, as shown in  FIG. 1 , in a perspective representation. 
       FIG. 3  shows the compression mold illustrated in  FIG. 2  in a sectional side view. 
       FIG. 4  shows the individual parts of a first exemplary embodiment for the mold according to the invention. 
       FIG. 5  shows the individual parts of a second exemplary embodiment for the mold according to the invention. 
       FIG. 6  shows the individual parts for a third exemplary embodiment of the mold according to the invention. 
       FIG. 7  shows the mold of the third exemplary embodiment in a side view. 
       FIG. 8  shows the mold of the third exemplary embodiment in a plan view. 
       FIG. 9  shows an open mold according to the invention having a ceramic heat shield element disposed therein. 
       FIG. 10  shows a mold shell having inserts disposed therein for separating different areas in the interior of the mold shell. 
       FIG. 11  shows a mold shell having an insert disposed therein for separating different areas in the interior of the mold shell. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A first exemplary embodiment for an inventive mold for producing a ceramic heat shield element, as illustrated schematically in  FIG. 1 , is shown in  FIG. 4 . The figure shows the individual parts of the mold shell which are tightly yet detachably joined to one another before a ceramic material is poured in. In the present exemplary embodiment the parts are joined by means of clamp connections but they can also just as effectively be joined by means of other detachable connections, screw connections for example. Compared with screw connections, however, clamp connections have the advantage that the connection can be produced and released again without tools. 
   The individual parts which can be joined to the mold shell comprise the shell elements  1  and  3  which have mold surfaces  2  and  4  by means of which the hot side  102  and the cold side  104  of the heat shield element  100  are molded. 
   Side pieces  5  and  7  are also present, each of which has a spring-like projection  6 ,  8 . These two individual parts form the mold surfaces for the peripheral sides  108  of the heat shield element  100 , said peripheral sides  108  being provided with the grooves  110 . In this arrangement the spring-like projections  6 ,  8  serve to shape the grooves. 
   The mold shell additionally comprises a bottom element  9  which serves as a mold surface  10  for molding one of the peripheral sides  106  of the heat shield element  100  without grooves. The mold stands on the bottom element  9  when the heat shield element is cast. 
   Finally two shell elements  11 ,  13  are present which are located opposite the bottom element  9  when the mold shell is assembled. The two shell elements  11 ,  13  are provided with set-offs  12 ,  14  which are disposed in such a way that after the two shell elements  11 ,  13  are joined together they form a casting aperture for pouring in the ceramic material. In addition, these two shell parts each have a mold surface  15 ,  17  by means of which edge zones of the second peripheral side  106  are molded without grooves. Also present in the recesses are webs  16 ,  18  by means of which the casting aperture of the assembled mold shell is subdivided into two partial apertures. If the ceramic material is poured only into the one partial aperture, air can escape from the interior of the mold shell through the other partial aperture. 
     FIG. 9  shows the mold shell in the partially assembled state after the casting of a ceramic heat shield element  100 . The shell parts  4 ,  5  and  7  from  FIG. 4  can be seen.  FIG. 9  shows in particular that parts of the peripheral side  106  are molded in the area of the casting aperture during the casting. Casting residues  112  on the ceramic heat shield element  100  are mechanically removed following the curing. The already shaped parts of the peripheral side  106  can be used in this case as a reference surface. 
   To allow the shell elements to be tightly joined to one another, the shell element  1  is equipped with four clamping elements  19  which can be brought into engagement with hooks  20  of the shell element  3  and tensioned. In order to prevent slipping of the side elements  5 ,  7  disposed between the shell elements  1  and  3 , the bottom element  9  and the shell elements  11  and  13  forming the casting aperture during the tensioning process, mold projections  21  are present which interact positively with various mold projections or set-offs of other shell elements. Also present are pins  22  which engage in receptacles  23  of adjacent mold elements and so prevent the mold elements from shifting relative to one another. 
   The mold shown in  FIG. 4  also comprises inserts  24  which can be introduced through the casting aperture into the interior of the mold shell in order to separate different areas in the interior of the mold shell from one another. Guide grooves  25  are present in the shell element  1  for the purpose of guiding the inserts  24  when they are introduced into the assembled mold shell. The shell element  11  also has guide recesses  26  for guiding the inserts  24 . 
   The inserts  24  are introduced into the assembled mold shell before the casting of a heat shield element so that in its interior areas that are adjacent to the shell elements  5 ,  7  with the spring-like projections  6 ,  8  are separated from a central area. A different ceramic material is poured into the areas which are adjacent to the shell elements  5 ,  7  than in the central area of the mold shell. After the materials have been poured in, the inserts  24  are removed from the mold shell so that the two materials can mix with each other in the threshold region and produce a bonded join during curing. In this way graded heat shield elements can be produced. 
   The assembled mold shell with inserts disposed therein is shown in  FIG. 10 . 
   A second exemplary embodiment for the mold according to the invention is shown in  FIG. 5 . Like  FIG. 4 ,  FIG. 5  shows the mold shell of the mold in individual parts. In order to avoid repetitions, only the differences from the mold shown in  FIG. 4  will be dealt with. The reference numerals of the shell elements shown in  FIG. 5  are consistent with the reference numerals of the corresponding shell elements from  FIG. 4 . 
   In contrast to the mold shown in  FIG. 4 , the mold shown in  FIG. 5  comprises only one insert  34 , which is suitable for separating the interior of the assembled mold shell into a hot-side area, i.e. an area which adjoins the shell element  1  with the mold surface  2  for molding the hot side  102 , and a cold-side area, i.e. an area which adjoins the shell element  3  with the mold surface  4  for molding the cold side  104 . Accordingly, no guide grooves are present in the shell elements  1  and  11 . Instead, the spring-like projections  6  and  8  have guide grooves for guiding the insert  34 . 
   The assembled mold shell from  FIG. 5  with insert disposed therein is shown in  FIG. 11 . 
   A third exemplary embodiment for the mold according to the invention is shown in  FIG. 6 . Like  FIGS. 4 and 5 ,  FIG. 6  shows the mold shell broken down into its individual parts. The individual parts are designated by the same reference numerals as the corresponding individual parts from  FIGS. 4 and 5 . In order to avoid unnecessary repetitions, only the differences from the molds shown in  FIGS. 4 and 5  will be referred to at this juncture. 
   The mold shell from  FIG. 6  is not provided for the insertion of inserts. Accordingly the shell elements also have no guide grooves for molded parts of this kind. Present instead in the shell elements  1  and  3  are retaining pins  40  which are movably arranged in such a way that with the mold shell assembled they are to be introduced from the exterior of the shell elements  1 ,  3  into the interior of the mold shell. For this purpose a retaining pin plate  42  carrying the retaining pins  40  is arranged on the outside of the mold shells  1 ,  3 . The distance of said retaining pin plate from the outside of the respective shell element  1 ,  3  can be varied by means of a crank  44  or by means of an automated embodiment of the retaining pins. If the retaining pin plate completely abuts the outside of the shell element  1 , the retaining pins  40  project to a maximum into the interior of the mold shell. This state is shown with reference to the shell element  1  in  FIGS. 6 and 7 . If, on the other hand, the retaining pin plate  42  is at its greatest distance from the outside of the shell element  1 , the retaining pins  40  are completely countersunk in the wall of the shell element, so they no longer project into the interior of the mold shell. This state is shown in  FIG. 8  and also in  FIG. 6  in the case of the shell element  3 . 
   The retaining pins  40  can be used, for example, to hold reinforcing elements in place while the ceramic material is poured into the interior of the mold shell. The holding in place can be effected, for example, simply on account of the retaining pins  40  pressing from two opposite sides against the reinforcing element and fixing the latter in place by means of the resulting friction. Alternatively it is also possible to provide openings in the reinforcing element, into which openings the retaining pins  40  can engage for the purpose of holding the reinforcing element in place. 
   In particular two-dimensional reinforcing elements can be introduced as reinforcing elements into the interior of the mold shell, said reinforcing elements extending for example parallel to the hot side or cold side  102 ,  104  of the heat shield element  100  that is to be molded. However, rod- or bone-shaped reinforcing elements can also be introduced into the interior of the mold shell, said reinforcing elements extending essentially along the length of the shell elements  5 ,  7 ,  9  which shape the peripheral sides  106 ,  108  of the heat shield element  100 . In the finished heat shield element the reinforcing elements extend along the length of the peripheral sides  106 ,  108 . 
   After the reinforcing elements have been fixed in place in the interior of the mold shell, a ceramic material is poured into the mold shell. Subsequently the retaining pins  40  are withdrawn from the interior of the mold shell by means of the crank  44  or, as the case may be, an automatic retraction device. This state is shown in  FIG. 8 . The reinforcing elements are then fixed in their position solely by the introduced ceramic material.