Production of shaped bodies made of sintered quartz glass, as well as molds for production and manufactured shaped bodies.

Abstract

Production of a molded body by means of slip casting, comprising the following process steps: - Preparation of a slip mass (20) wherein the slip mass comprises quartz glass in the form of substantially amorphous SiO2 grains with the addition of water, - Pouring the slurry (20) into a mold (10), wherein the mold (10) comprises, at least in part of the contact area (11) with the poured slurry (20), a mold material whose maximum micropore distribution is at most 1 µm, wherein the mold material comprises sintered quartz glass, - Drying the slip mass (20) to form a pre-body, - wherein the produced pre-body does not experience any enrichment with impurities compared to the original slurry mass (20), - Sintering of the pre-body at a predetermined sintering temperature to obtain the shaped body (30), - Cooling of the sintered molded body (30).

Description

[0001] The present invention generally relates to the production of shaped bodies made of ceramic material by means of slip casting. Shaped bodies, for example, made of quartz glass, can be produced by this method.

[0002] Ceramic molded bodies, such as crucibles and other containers, have found their way into many industrial manufacturing processes. Slip casting is a widely used manufacturing process for such ceramic molded bodies.

[0003] In this process, a ceramic slip is produced and placed in a suitable casting mold. The slip is then dewatered and dried to form a preform, which is then sintered at a specified sintering temperature to obtain a sintered, solid, and temperature-stable molded body. A ceramic material can include, for example, sintered quartz glass.

[0004] For example, the publication DE 10 2006 060 561 A1 mentions quartz glass containers manufactured from quartz glass by slip casting. These quartz glass containers can be used, for example, in the production of polycrystalline silicon for the solar industry. To produce the preform, the slip mass is poured into a specially designed casting mold.

[0005] German patent application DE 196 09 126 A1 relates to a porous mold for dewatering porcelain masses, consisting essentially of sintered alumina (Al2O3) and having an average pore diameter between approximately 0.05 and 5.0 µm. It is produced through the following steps: A pore-forming agent and a green binder are added to a moldable mass formed from powdered Al2O3 and a fluidizing agent. The mass containing these components is then formed into a mold, which is then dried. The dried mold is sintered at a temperature between 800 and 1800°C, so that the pore-forming agent and the green binder evaporate, leaving a porous mold.

[0006] European patent application EP 0440874 A2 relates to a mould for casting ceramic products, which mould consists of an aggregate with a very fine and fine grain size and a binder based on a thermosetting resin and comprises at least one inner layer with a structure with very fine porosity and at least one outer layer with a structure with greater porosity than that of the inner layer, wherein the inner layer with the structure with very fine porosity serves as a filter for thickening the slip on its outer surface and the outer layer with the structure with greater porosity serves as a support layer for the inner layer and enables easy countercurrent rinsing of the mould.

[0007] The document DE 68910 898 T2 discloses a process for producing porous molds, in which a source of fine-grained silicon dioxide with an average particle size of less than 10 µm, a trialkaline earth metal silicate and optionally a hydraulic binder are mixed with a liquid, in particular water, and this mixture is transferred into a mold in which it is bound until it reaches a strength sufficient for demolding, after which the green mold thus obtained is demolded and subjected to a treatment in an autoclave under saturated liquid vapor pressure to form a mold consisting essentially of a mass of porous alkaline earth metal silicate hydrate.

[0008] The American patent specification US 6180034 B1 relates to a method for producing a mold, comprising the following steps: (a) immersing a piece of investment material at least once in a slurry containing a colloidal silica-based binder; (b) coating the slurry-coated investment pattern at least once with a dry powder mixture containing calcium sulfate; (c) drying the dry, powder-coated investment pattern to form a layered shell; (d) curing the layered shell to harden the shell; (e) pyrolyzing the hardened shell to vaporize the pattern and form the mild material; and (f) cooling the mold.

[0009] European patent application EP 1 516 864 A2 describes a process for producing a casting from a composite material by providing a composite slurry containing material particles in the size range below 10 µm and material grain in the size range above 10 µm, homogenising the composite slurry, pouring the composite slurry into a mould to form a blue body, drying the blue body to form a porous green body, and thermally solidifying the green body, characterised in that the composite slurry has a solids content of at least 80 wt.-% is poured into a mold which has a mold wall impermeable to the liquid, wherein the composite slurry is cooled after pouring to form a frozen blue body to a cooling temperature below the freezing point of the liquid, and wherein the drying comprises heating the frozen blue body to a heating temperature below the boiling point of the liquid.

[0010] The German patent application DE 10 2006 060 561 A1 discloses a method for producing a quartz glass molded body comprising the steps of: introducing a quartz glass-water mixture into a casting mold, drying the quartz glass-water mixture in the casting mold, and demolding the green body thus produced, which method further comprises at least one of the following steps: at least partially covering or replacing the shaping surfaces of the casting mold with or from a waterproof substance before introducing a quartz glass-water mixture; controlling the removal of water from the slip during drying by applying a pressure difference across the casting mold or across parts thereof; controlling demolding by applying a pressure difference across the casting mold or across parts thereof, setting the casting mold into vibration when introducing the quartz glass-water mixture;Casting a quartz glass crucible in a two-step process, whereby a crucible inner shell or a crucible outer shell is cast, the crucible inner shell or crucible outer shell is placed in a closed casting mold and the quartz glass crucible is finally cast.;

[0011] These molds are typically made of plaster. Clay molds are also known.

[0012] Slip casting in plaster molds has been a recognized manufacturing process for many years. Gypsum is suitable for many reasons. Firstly, the raw material for making plaster molds is relatively easy and inexpensive to obtain. The production of plaster molds in mold foundries, sometimes using a so-called master piece, is also common. The plaster molds can be coreless, cored, or constructed from several individual parts with different division levels.

[0013] When the slip mass is poured into the plaster mold during slip casting, a process begins in which the gypsum absorbs water from the slip mass, causing the mineral components to settle on the plaster mold, compact, and increasingly solidify. This process is known as body formation. The plaster mold must therefore be carefully dried beforehand. Once sufficient water has been removed and the pre-form is sufficiently stable, it can be removed from the plaster mold. The plaster mold can be cleaned, dried, and made available for subsequent casting cycles.

[0014] However, slip casting using plaster molds also has some disadvantages.

[0015] On the one hand, every casting mold is subject to a certain amount of wear during each casting cycle, for example due to material removal on the mold's forming contact surface, so that the mold must be replaced sooner or later, since otherwise the specified tolerances in the dimensions of the cast molded bodies can no longer be maintained.

[0016] Making a plaster mold is time-consuming due to the long setting times of the plaster. This leads to problems, especially when parts of a plaster mold collide and break, for example, during assembly, rendering the mold unusable. Replenishment times are usually several days, making a quick replacement unthinkable.

[0017] In this context, the low mechanical strength of plaster should also be considered, which determines the short lifespan of the molds. This risk is particularly high with more complex casting molds consisting of multiple sub-moles, since the molds must be disassembled, cleaned, and reassembled after each casting. Collisions between sub-moles can quickly lead to chipping of corners or edges or other damage, which usually precludes further use of the mold. Producing replacements for sub-moles is equally difficult, as the entire casting mold, with all its sub-moles, often has to be adapted to each other.

[0018] Due to the low mechanical stability of plaster, casting molds are often made with very thick walls. This requires the installation of reinforcements or support frames in large casting molds, for example. This can lead to contamination of the plaster and ultimately the poured slip with iron compounds, which can then enter the preform and ultimately the final product. In this context, the undesirable contamination of products from a release agent, which is often applied in the contact area with the slip and which aids demolding, should also be considered.

[0019] A further disadvantage is that the body formation and drying process, i.e., the time required for water removal, is quite long. For larger molded bodies made of ceramic material, which can easily weigh 100 kg or even 1,000 kg or more, several days or even weeks are required for the molding process.

[0020] Heating to promote drying is unfavorable for the plaster molds once it reaches about 40°C or more.

[0021] Finally, it is also unfortunate that molded parts with delicate contours or small radii are hardly feasible to produce.

[0022] It would therefore be desirable to provide a process for the production of shaped bodies from ceramic material which at least reduces some of these disadvantages or, at best, does not have them at all.

[0023] In particular, such a process should enable faster body formation and drying of the filled slip mass without compromising the stability and quality of the produced pre-body.

[0024] Furthermore, the durability of the molds should be improved, allowing a higher number of castings from a single mold. Thus, mold wear should be reduced by improving the mold's mechanical strength.

[0025] Finally, contamination of the manufactured molded parts with foreign components from the mold should also be counteracted.

[0026] The inventors have addressed these problems.

[0027] This problem is solved surprisingly simply by a method for producing a molded body by slip casting, a casting mold, and a molded body according to one of the independent claims. Preferred embodiments and further developments of the invention can be found in the respective subclaims.

[0028] The invention therefore relates to the production of a shaped body by slip casting, comprising the following process steps: - Creating a slip mass, wherein the slip mass comprises quartz glass in the form of essentially amorphous SiO2 grains with the addition of water, - pouring the slip mass into a casting mold, wherein the casting mold comprises, at least in a part of the contact area with the poured slip mass, a molding material whose maximum in the micropore distribution is at most 1 µm, wherein the molding material comprises sintered quartz glass, - Drying the slip mass to a pre-body, - the produced pre-body does not experience any enrichment with impurities compared to the original slip mass, - Sintering the pre-body at a given sintering temperature to obtain the shaped body, - Cooling of the sintered molded body.

[0029] The invention further relates to a shaped body produced by means of this process.

[0030] Quite surprisingly, it has been found that a casting mold that is not based on plaster enables at least comparable, if not better, results in the shaping of ceramic materials.

[0031] The inventors have discovered that the size and distribution of the pores in the molding material, at least in the contact area with the slurry to be introduced, are of great importance with regard to water extraction from the slurry and thus to the molding time. This now makes it possible to consider molding materials other than gypsum, provided the micropore distribution remains below a maximum limit of 1 µm.

[0032] Finer pores in the mold material allow for a greater absorption effect, thus accelerating the water extraction from the slip poured into the mold. This is due to the capillary absorption forces of the mold material, which increase with decreasing capillary diameter, i.e., smaller pores. This results in a better dewatering effect of the casting molds according to the invention, especially compared to mold materials such as plaster or clay. This significantly reduces the throughput time, i.e., the time the molds remain in the mold with the poured slip.

[0033] According to the invention, the casting mold can therefore comprise a molding material whose maximum micropore distribution is at most 1 µm, at least in a portion of the contact area with the slip mass to be poured or cast in, i.e., at least a portion of the shaping surfaces.

[0034] A micropore distribution with a maximum of 1 µm describes the presence of sufficiently small pores in the material, which release the higher capillary forces with respect to water.

[0035] It has been shown that an even smaller maximum in the micropore distribution can further enhance these capillary forces. In a preferred embodiment, the maximum in the micropore distribution is therefore at most 0.5 µm, preferably at most 0.4 µm, and particularly preferably at most 0.3 µm. However, to release capillary forces at all, the fundamental presence of micropores is required. A completely closed surface in the contact area would not be able to release capillary forces.

[0036] Therefore, a material with an open porosity in the range between 4 and 30%, preferably between 6 and 20%, particularly preferably between 7 and 15%, is used for the fine-pored contact area of ​​the casting mold according to the invention. Since very fine micropores increase the capillary suction effect but simultaneously severely limit the amount of water transported away, the maximum in the micropore distribution should be at least 0.01 µm, preferably at least 0.03 µm, and particularly preferably at least 0.05 µm. The micropore distribution can be determined based on the DIN-EN 993-1 standard.

[0037] It is also very important that the pores are distributed as consistently and evenly as possible in terms of their size and number over the volume of the molding material, especially for the production of molded bodies with a constant wall thickness.

[0038] For the production of molded bodies with different or very strongly fluctuating wall thicknesses, it is advantageous if the total volume of the open porosity can be varied in a targeted manner, and thus the rate of water removal can be adapted to the corresponding wall thickness.

[0039] Advantageously, the entire contact area of ​​the casting mold according to the invention with the poured slip material can comprise the mold material according to the invention. This can be achieved, for example, by appropriately lining the contact surface with the mold material or by replacing the shaping surfaces with the mold material. When lining the contact surface of the casting mold, care must be taken to ensure that the lining is sufficiently thick, i.e., that it extends sufficiently in the direction of the surface normal on the shaping surface. This should be at least 1 cm, preferably 2 cm. If the entire contact area of ​​the casting mold is covered with the fine-pored mold material, uniform, homogeneous water removal can be achieved.

[0040] Of course, the mold can also be made entirely of the mold material, which may be suitable for smaller, less complex molds, for example. If the mold is multi-part or equipped with a core, the same applies here, meaning that these shaping surfaces also advantageously encompass the mold material.

[0041] In another embodiment, only selected parts of the contact area of ​​the inventive casting mold with the poured-in slip mass can comprise the inventive molding material. The remaining parts of the contact area can be made of water-impermeable and non-porous materials, such as plastic. In this way, the water removal can be directed in specific directions, for example, to dry certain areas of the preform faster than others.

[0042] According to the invention, the molding material can be largely or completely free of gypsum, i.e. of calcium sulfate dihydrate, ie the use of gypsum or clay materials is largely or completely dispensed with for the molding material.

[0043] For larger, multi-part casting molds, however, it is possible to provide supporting parts made of plaster or clay materials, for example, and then cover the contact area with the slip with the molding material. This can be achieved, for example, by cutting plates from the molding material, which can then form a lining for the contact surface.

[0044] In a particularly preferred embodiment, the molding material may comprise sintered quartz glass. Sintered quartz glass in various formats may, for example, be sold under the name “Quarzal R" from Schott AG, Mainz. All types of sintered quartz glass with maximum grain sizes of up to 15 mm are suitable as molding material, as long as the described micropore distribution is present.

[0045] Sintered quartz glass can be formed into corresponding shaped components by slip casting of amorphous silica, as described, for example, in the publication P. Schultz: “Slip casting of amorphous silica of large-format and complex parts”, Keramische Zeitschrift, 38th year, No. 8, 1986.

[0046] Unexpectedly, it has been shown that such a sintered quartz glass shows the desired micropore distribution and can therefore be ideally used for the shaping of ceramic moldings themselves.

[0047] Another major advantage here is that no changes are required regarding the use of the slip, for example, when switching from conventional plaster casting molds to a casting mold according to the invention with the aforementioned micropore distribution and the use of sintered quartz glass in the contact area with the slip. This is all the more surprising given the close interaction between the slip and the material of the forming contact surface. Therefore, it would have been expected that changes would be necessary during the creation of the slip, for example, regarding the viscosity or rheology of the slip.

[0048] The process offers the further major advantage that there is no undesirable accumulation of impurities in the preform due to diffusion processes from the molding material into the preform. Therefore, the degree of impurity preferably does not change from the slip mass to the preform, i.e., a high-purity slip mass leads to a high-purity preform and ultimately to a high-purity ceramic molded body.

[0049] Therefore, the formed pre-body is not enriched with calcium compounds compared to the original slip mass. Likewise, the produced pre-body is not enriched with elements from the reinforcement or supporting structures, such as iron compounds, compared to the slip mass.

[0050] Another advantage is the higher mechanical strength of sintered quartz glass as a mold material compared to plaster or clay, for example. This results in less wear, even on delicate mold details. Wear can occur during mold handling or when filling the molds with the abrasive slip. In both cases, the higher mechanical strength of sintered quartz glass as a mold material has a positive effect. This leads to a significantly longer mold life and a longer durability of the casting molds, thus reducing replacement costs.

[0051] Larger casting molds are also possible, which can provide sufficient stability even without reinforcement or support frames. A modular design with frequent replacement of heavily stressed casting parts is also possible. Finally, it is also possible to produce castings with delicate contours, sharp edges, or small radii.

[0052] A further advantage of using sintered quartz glass as a mold material is the higher shard formation rate compared to plaster molds. This leads to a significant reduction in the process time until the preform is produced. This allows for a higher casting frequency per mold, so that fewer molds are required, especially for larger batch sizes, resulting in lower investment costs and less floor space.

[0053] In this context, it is also advantageous that the use of sintered quartz glass as the mold material also allows for accelerated drying of the slip in the casting mold. Typically, this drying of the slip takes place in a plaster casting mold at room temperature or at a slightly elevated temperature, avoiding temperatures above 40°C, as this would damage the plaster material.

[0054] In a further development of the invention, it is therefore provided to carry out the body formation and drying at an elevated temperature in order to further reduce the process time. Drying at a temperature of at least 40°C, preferably at least 45°C, and particularly preferably at least 50°C, or even higher, can significantly shorten the drying process, particularly towards the end of the process. Preferably, the casting mold with the slip mass is exposed to an elevated temperature for a period of at least 1 hour, preferably at least 2 hours, and particularly preferably at least 3 hours.

[0055] The body formation and drying process is complete when the preform reaches a certain strength level. This is followed by demolding, i.e., the removal of the preform from the mold. The mold can then be cleaned and dried. This is followed by the sintering of the preform at a predetermined sintering temperature, as is familiar to those skilled in the art, and the cooling of the sintered molded body to room temperature. The molded body made of ceramic material produced in this way can then be further processed.

[0056] Furthermore, it has been surprisingly shown that when using sintered quartz glass as a mold material, the use of otherwise conventional release agents in the area of ​​the forming contact surfaces can be largely or completely dispensed with, which on the one hand saves preparation time in production, but on the other hand also means that the pre-body is not contaminated by the release agent, often a graphite suspension.

[0057] The production according to the invention can generally be used for slip masses which comprise a mineral base material with the addition of water.

[0058] In a preferred embodiment, the slip mass may comprise quartz glass in the form of substantially amorphous SiO2 grains with the addition of water.

[0059] The present invention further relates to a casting mold for shaping a ceramic molded body by means of slip casting, wherein the casting mold comprises, at least in a part of the contact area with the poured slip mass, a molding material whose maximum in the micropore distribution is at most 1 µm.

[0060] In a preferred embodiment, the maximum in the micropore distribution of the mold material of the casting mold is at most 0.5 µm, preferably at most 0.4 µm and particularly preferably at most 0.3 µm.

[0061] Preferably, the mold material covers the entire contact area of ​​the casting mold with the slip mass to be poured.

[0062] The molding material is advantageous if it is largely or completely free of calcium sulfate dihydrate or calcium compounds.

[0063] In a particularly preferred embodiment, the mold material of the casting mold comprises sintered quartz glass. Such a casting mold, in particular a casting mold comprising or consisting of sintered quartz glass, can also be produced using a slip casting process.

[0064] In a further aspect, the invention relates to the use of a casting mold as described above for producing a ceramic molded body. The ceramic molded body can comprise or consist of sintered quartz glass.

[0065] Finally, the invention relates to a molded body comprising a ceramic material, which is produced by slip casting using the inventive production method described above. Preferably, a casting mold as described above is used.

[0066] The molded body produced in this way is characterized by only minimal contamination with calcium and / or iron compounds. This also applies to the outer surfaces of the molded body, especially those outer surfaces that are in direct contact with the casting mold during the molding process. Accordingly, the impurities in the material of the molded body preferably do not exceed those already present in the original slip mass.

[0067] In a preferred embodiment, a molded body produced according to the invention thus comprises a near-surface region, which in particular comprises those surfaces that are in direct contact with the casting mold during molding, where, compared to the base material of the molded body, there is no accumulation of impurities, in particular calcium and / or iron compounds. This near-surface region extends, in particular, to a depth of a few millimeters, starting from the surface, into the molded body.

[0068] In a particularly preferred embodiment, the proportion of impurities, in particular with calcium and / or iron compounds, in a near-surface region up to a depth of at least 2 mm, preferably at least 4 mm and particularly preferably at least 6 mm or 10 mm is therefore equal to the proportion of impurities in the slip mass used.

[0069] The shaped body, in particular the shaped body made of sintered quartz glass, can have a wide variety of geometric shapes and can be designed, purely for example, as a block or plate, as a curved plate or base, as a container or as a roll or cylinder.

[0070] The invention is described in more detail below using preferred embodiments and with reference to the attached figures.

[0071] The drawings show: Fig. 1 schematically shows the structure of a casting mold in cross section, Fig. 2 a molded body in a plan view, which is produced by means of the casting mold from Fig. 1 was formed, and Fig. 3 the micropore distribution of selected materials. Detailed description of preferred embodiments

[0072] In the following detailed description of preferred embodiments, for the sake of clarity, like reference numerals designate substantially similar parts in or on these embodiments. However, to better illustrate the invention, the preferred embodiments illustrated in the figures are not always drawn to scale.

[0073] Fig. Figure 1 shows a schematic cross-sectional view of the structure of a casting mold 10 according to the invention. The casting mold 10 is formed as a single piece. Multi-piece casting molds are also possible, for example, with a core to produce molded bodies designed as containers. In the example, the casting mold 10 is filled with a slip mass 20.

[0074] The casting mold 10 is used to form a ceramic molded body using slip casting. Slip casting is a casting process for forming a preform with a certain strength, also called a green body. The preform is then sintered to obtain the solid molded body.

[0075] Without being limited to the illustrated embodiment, the casting mold 10 is made entirely of the mold material according to the invention, i.e., it has a monolithic structure. In the example, the casting mold 10 is made entirely of sintered quartz glass. The casting mold 10 has a maximum micropore distribution of 1 µm at most.

[0076] In the example, the sintered quartz glass includes the material called “Quarzal R" and is available from Schott AG, Mainz. In principle, quartz glass materials from other manufacturers may also be suitable, provided the required micropore distribution is met.

[0077] This means that the molding material according to the invention is present in the contact area 11, i.e., the shaping surface of the casting mold 10 with the poured slip 20, and its maximum micropore distribution is at most 1 µm. Accordingly, the walls and bottom of the casting mold 10 are made of the molding material according to the invention.

[0078] A micropore distribution with a maximum of 1 µm or less describes the presence of sufficiently small pores that release the higher capillary forces with respect to water and thus support the drying process of the slip. In a preferred embodiment, the maximum in the micropore distribution is even smaller, in particular at a maximum of 0.5 µm, preferably at a maximum of 0.4 µm, and particularly preferably at a maximum of 0.3 µm.

[0079] The open porosity of the mold material in the contact area 11 is approximately 12%, without limitation to the exemplary embodiment. A range between 4% and 30%, preferably between 6% and 20%, and particularly preferably between 7% and 15%, is considered suitable.

[0080] In the example shown, the mold material covers the entire contact area 11. However, it is also possible to manufacture the casting mold from a different material and line it only in the contact area 11 with the mold material according to the invention. This can be achieved, for example, by lining the shaping side surfaces of the casting mold 10 with, for example, plate-shaped mold material.

[0081] In the example shown, casting mold 10 is made entirely without plaster and / or clay materials. This means the casting mold is free of calcium sulfate dihydrate. Furthermore, casting mold 10 does not require any reinforcements or support structures, making it free of iron compounds.

[0082] The casting mold 10, comprising sintered quartz glass, is also produced using a slip casting process. Various compositions for producing a slip mass suitable for this purpose are possible. It has been shown that the maximum micropore distribution depends on the grain size distribution of the SiO2 grains included in the slip mass. A sufficient fine grain fraction of the material included in the composition is therefore important. One possible composition, for example, has a fine grain fraction with a grain size of up to 0.3 mm, preferably up to 0.2 mm, in a range of approximately 30% to 70% and a further fine grain fraction with a grain size in a range of 0.3 mm to 1.0 mm in a range of approximately 3% to 10%. Water is also added to produce the slip.

[0083] The casting mold 10 is used to produce a molded body by slip casting, comprising the following steps: - Creating a slip mass - Pouring the slip mass into the casting mold 10 according to the invention, wherein the casting mold comprises, at least in the contact area with the poured slip mass, a molding material whose maximum in the micropore distribution is at most 1 µm and an open porosity of at least 4%, - Drying the slip mass to a pre-body, - Sintering the pre-body at a given sintering temperature to obtain the shaped body, - Cooling of the sintered molded body.

[0084] The finer pore structure in the mold material allows for a higher suction effect, thus accelerating the water extraction from the slip poured into the casting mold 10. This is due to the capillary suction forces of the mold material, which increase with decreasing capillary diameter, i.e., smaller pores. This results in a better drainage effect of the casting molds according to the invention, especially compared to mold materials such as plaster or clay.

[0085] The open porosity of the mold material in the contact area 11 is approximately 12%, without limitation to the exemplary embodiment. A range between 4% and 30%, preferably between 6% and 20%, and particularly preferably between 7% and 15%, is considered suitable.

[0086] No special requirements are imposed beyond those usual for the preparation of the slip mass. This means that known slip masses can be used. In particular, with regard to the viscosity or rheology of the slip, no changed requirements are imposed by the use of the casting mold 10 according to the invention, so that in principle any slip mass that has previously been cast in plaster or clay casting molds can also be used for the process according to the invention. When using a casting mold 10 according to the invention, it is not necessary to provide the contact surfaces with a release agent, as is necessary when using plaster or clay molds.

[0087] The introduction of impurities from the casting mold 10 due to diffusion processes from the mold material into the preform during drying can be effectively prevented. This is particularly important when the casting mold 10 is made of sintered quartz glass and the slip contains amorphous silica. The degree of contamination therefore does not change from the slip to the preform, meaning that preforms and thus also molded bodies can be produced with very high purity after sintering. The material quality of the molded body therefore depends essentially on the material quality and purity of the raw materials used in the slip.

[0088] The higher body formation rate significantly accelerates the removal of water from the slip, resulting in a significant reduction in the process time for producing the preform. To further shorten the process time, the casting mold 10 with the poured slip 20 is exposed to an elevated temperature. This temperature is preferably above room temperature.

[0089] In one embodiment, drying takes place at a temperature of at least 40°C, preferably at least 45°C, and particularly preferably at least 50°C or even higher. To achieve thorough drying, the casting mold 10 is exposed to this elevated temperature for a prolonged period. In one embodiment, therefore, the casting mold 10 with the slip mass 20 is exposed to an elevated temperature for a period of at least 1 hour, preferably at least 2 hours, and particularly preferably at least 3 hours.

[0090] The drying process is complete when a sufficient amount of water has been removed from the slip mass 20. The mineral components of the slip mass are then deposited on the casting mold 10, increasingly compacting and solidifying. As soon as the preform reaches a certain strength, demolding can begin, i.e., the removal of the preform from the casting mold 10. Before sintering, the partially still moist preform can be reworked, if necessary.

[0091] According to the invention, a hollow body can also be produced as a preform using a hollow casting process. It should be noted that the slip mass remains in the casting mold 10 for a shorter period of time. Once the preform has partially reached a certain strength level, in other words, after a predetermined body thickness has been formed, the remaining, still liquid slip mass is poured out, and the body remaining in the casting mold 10 then continues to dry until it reaches the strength required for demolding.

[0092] This is followed by sintering the preform at a specified sintering temperature and cooling the sintered molded body to room temperature. The ceramic molded body produced in this way can then be further processed. Fig. 2 shows, purely by way of example, a shaped body 30 in a plan view, which was formed by means of the casting mold 10.

[0093] The process according to the invention can generally be used for slip masses which comprise a mineral base material with the addition of water.

[0094] In a particularly preferred embodiment, the slip comprises quartz glass in the form of essentially amorphous SiO2 grains with added water. The purity, i.e., the proportion of SiO2 in the starting material, is very high and is typically at least 99%, preferably 99.5%. A possible slip can, for example, be composed as follows: 38 wt.% of the grain size 0.3-1 mm 7 wt.% of the grain size 0-0.3 mm 39 wt.% of the grain size <50 µm 16 wt.% of the grain size <10 µm

[0095] The water content of the slip relative to the glass content is approximately 10%.

[0096] Fig.Figure 3 illustrates the micropore distribution of selected materials. Measurements were conducted to determine the pore volume distribution by mercury intrusion in accordance with DIN 66 133.

[0097] The measurements were based on various molding materials according to the invention based on sintered quartz glass, which differ in their grain size distribution.

[0098] The measurements show that the molding materials based on sintered quartz glass according to the invention exhibit a maximum micropore distribution at a pore diameter of less than 1 µm. For most of the analyzed molding materials based on sintered quartz glass, the maximum micropore distribution is even less than 0.5 µm, in some cases less than 0.4 µm, and in another part less than 0.3 µm.

[0099] For comparison, a gypsum-based molding material was analyzed. Here, the micropore distribution measurements showed a significantly coarser micropore distribution, with the maximum micropore distribution being well over 1 µm, approximately 2 µm or even higher.

[0100] Since the capillary suction forces increase with decreasing capillary diameter, this explains the better dewatering effect of castings made of sintered quartz glass as a mold material compared to castings made of plaster.

Claims

[1] Production of a shaped body by slip casting, comprising the following process steps: - creating a slip mass (20), wherein the slip mass comprises quartz glass in the form of substantially amorphous SiO2 grains with the addition of water, - pouring the slip mass (20) into a casting mold (10), wherein the casting mold (10) comprises, at least in a part of the contact area (11) with the poured slip mass (20), a molding material whose maximum in the micropore distribution is at most 1 µm, wherein the molding material comprises sintered quartz glass, - drying the slip mass (20) to a pre-body, - wherein the produced pre-body does not experience any enrichment with impurities compared to the original slip mass (20), - sintering the pre-body at a predetermined sintering temperature to obtain the shaped body (30), - Cooling the sintered molded body (30). [2] Production according to claim 1, characterized by that the molding material covers the entire contact area (11) with the poured slip mass (20). [3] Manufacture according to one of the preceding claims, characterized by that the maximum in the micropore distribution of the molding material is at most 0.5 µm, preferably at most 0.4 µm and particularly preferably at most 0.3 µm. [4] Manufacture according to any one of the preceding claims, characterized by that the open porosity of the molding material is in a range between 4% and 30%, preferably between 6% and 20%, particularly preferably between 7% and 15%. [5] Manufacture according to one of the preceding claims, characterized by that the molding material is largely or completely free of calcium sulfate dihydrate. [6] Manufacture according to any one of the preceding claims, characterized by that the produced pre-body does not experience any enrichment with calcium or iron compounds. [7] Manufacture according to any one of the preceding claims, characterized by that the body formation and drying takes place at an elevated temperature of at least 40°C, preferably at least 45°C and particularly preferably at least 50°C. [8] Manufacture according to any one of the preceding claims, characterized by that the casting mold (10) is exposed to the elevated temperature for a period of at least 1 hour, preferably at least 2 hours and particularly preferably at least 3 hours. [9] Manufacture according to one of the preceding claims, characterized by that the contact surface of the casting mould (10) is free of release agent when slip mass (20) is filled in. [10] Casting mold for shaping a ceramic molded body by means of slip casting, wherein the casting mold (10) comprises, at least in a part of the contact area (11) with the slip mass (20) to be poured in, a molding material whose maximum in the micropore distribution is at most 1 µm, wherein the molding material comprises sintered quartz glass, and wherein the slip mass (20) comprises quartz glass in the form of substantially amorphous SiO2 grains with the addition of water. [11] Casting mould according to the preceding claim, characterized by that the moulding material covers the entire contact area (11) with the slip mass (20) to be poured. [12] Casting mould according to one of the two preceding claims, characterized by that the maximum in the micropore distribution is at most 0.5 µm, preferably at most 0.4 µm and particularly preferably at most 0.3 µm. [13] Casting mould according to one of the three preceding claims, characterized bythat the molding material is largely or completely free of calcium sulfate dihydrate. [14] Use of a casting mold according to one of the preceding claims 10 to 13 for producing a molded body comprising or consisting of sintered quartz glass. [15] Shaped body comprising or consisting of sintered quartz glass, produced by a method according to one of the preceding claims 1 to 9, using a casting mold according to one of the preceding claims 10 to 13, wherein the pre-body produced has no enrichment with impurities compared to the original slip mass (20).

Images