EP2097192B2 - Moulding material mixture containing phosphorus for producing casting moulds for machining metal - Google Patents

Description

The invention relates to the use of a casting mold for light metal casting obtained by a method and using a mold material mixture according to claim 1.

Casting molds for the production of metal bodies are essentially produced in two versions. The so-called cores or forms form a first group. From these, the casting mold is assembled, which essentially represents the negative mold of the casting to be produced. A second group consists of hollow bodies, so-called feeders, which act as equalizing reservoirs. These absorb liquid metal, with appropriate measures being taken to ensure that the metal remains in the liquid phase longer than the metal that is in the casting mold that forms the negative mold. If the metal solidifies in the negative mold, liquid metal can flow out of the compensating reservoir to compensate for the volume contraction that occurs when the metal solidifies.

Casting molds consist of a refractory material, for example quartz sand, the grains of which are bonded by a suitable binder after the casting mold has been formed, in order to ensure sufficient mechanical strength of the casting mold. A refractory base material that has been treated with a suitable binder is therefore used for the production of casting moulds. The refractory basic molding material is preferably in a free-flowing form, so that it can be filled into a suitable hollow mold and compacted there. The binder creates a solid bond between the particles of the basic molding material, so that the casting mold has the required mechanical stability.

Casting molds have to meet various requirements. During the casting process itself, they first have to have sufficient stability and temperature resistance to accommodate the liquid metal in the hollow mold formed from one or more (partial) casting molds. After the start of the solidification process, the mechanical stability of the casting mold is ensured by a solidified metal layer that forms along the walls of the cavity. The material of the casting mold must now decompose under the influence of the heat emitted by the metal in such a way that it loses its mechanical strength, i.e. the cohesion between individual particles of the refractory material is broken. This is achieved, for example, by the binder decomposing under the influence of heat. After cooling, the solidified casting is shaken, which ideally causes the material of the casting molds to crumble back into fine sand that can be poured out of the cavities of the metal mold.

Both organic and inorganic binders can be used to produce the casting molds and can be hardened by cold or hot processes. Cold processes are processes which are carried out essentially at room temperature without heating the casting mold. The hardening usually takes place through a chemical reaction, which is triggered, for example, by passing a gas as a catalyst through the mold to be hardened. In the case of hot processes, the mold material mixture is heated to a sufficiently high temperature after shaping in order, for example, to expel the solvent contained in the binder or to initiate a chemical reaction by which the binder is cured, for example by crosslinking.

At present, for the production of casting molds, organic binders are often used in which the curing reaction is accelerated by a gaseous catalyst or which are cured by reaction with a gaseous curing agent. These methods are referred to as "cold box" methods.

An example of the production of casting molds using organic binders is the so-called Ashland cold box process. It is a two-component system. The first component consists of a solution of a polyol, usually a phenolic resin. The second component is the solution of a polyisocyanate. So according to the US 3,409,579A the two components of the polyurethane binder are reacted by passing a gaseous tertiary amine through the mixture of molding base material and binder after molding. The curing reaction of polyurethane binders is a polyaddition, ie a reaction without splitting off by-products such as water. Other advantages of this cold box process include good productivity, dimensional accuracy of the molds, as well as good technical properties such as mold strength, pot life of the mold base and binder mixture, etc.

Heat-curing organic processes include the hot-box process based on phenolic or furan resins, the warm-box process based on furan resins and the Croning process based on phenolic novolak resins. In the hot-box and warm-box processes, liquid resins are processed into a molding mixture with a latent hardener that only becomes effective at elevated temperatures. In the Croning process, basic mold materials such as quartz, chrome ore, zircon sands, etc. are coated at a temperature of approx. 100 to 160°C with a phenol novolak resin that is liquid at this temperature. Hexamethylenetetramine is added as a reaction partner for later curing. With the heat-curing technologies mentioned above, shaping and curing takes place in heatable tools that are heated to a temperature of up to 300°C.

Irrespective of the curing mechanism, all organic systems have in common that they thermally decompose when the liquid metal is poured into the mold, releasing pollutants such as benzene, toluene, xylene, phenol, formaldehyde and higher, partially unidentified cracking products. Various measures have succeeded in reducing these emissions minimised, but they cannot be completely avoided with organic binders. Such undesired emissions also occur when the metals are cast in inorganic-organic hybrid systems which, like the binders used, for example, in the resol-CO ₂ process, contain a proportion of organic compounds.

In order to avoid the emission of decomposition products during the casting process, binders must be used that are based on inorganic materials or that contain at most a very small proportion of organic compounds. Such binder systems have been known for some time. Binder systems have been developed which can be cured by introducing gases. Such a system is for example in GB 782 205 described, in which an alkali water glass is used as a binder, which can be cured by introducing CO ₂ . In the DE 199 25 167 will describe an exothermic feeder mass that contains an alkali silicate as a binder. Furthermore, binder systems have been developed which are self-curing at room temperature. Such a system based on phosphoric acid and metal oxides is, for example, in U.S. 5,582,232 described. Finally, inorganic binder systems are also known which are cured at higher temperatures, for example in a hot tool. Such hot-curing binder systems are, for example, from U.S. 5,474,606 known, in which a binder system consisting of alkali water glass and aluminum silicate is described.

However, inorganic binders also have disadvantages compared to organic binders. For example, the casting molds made with water glass as a binder have relatively low strength. This leads to problems in particular when the casting mold is removed from the tool, since the casting mold can break. Good strength at this point is particularly important for the production of complicated, thin-walled molded parts and their safe handling. The reason for the low strength is primarily that the casting molds still contain residual water from the binder. Longer dwell times in the hot, closed mold are only of limited help, since the water vapor cannot escape to a sufficient extent. In order to achieve the most complete drying of the molds, in the WO 98/06522 suggested leaving the mold material mixture in a temperature-controlled core box after molding only long enough for a dimensionally stable and load-bearing edge shell to form. After opening the core box, the mold is removed and then completely dried under the influence of microwaves. However, the additional drying is complex, lengthens the production time of the casting molds and contributes significantly to the increase in the cost of the manufacturing process, not least because of the energy costs.

Another weak point of the previously known inorganic binders is the low stability of the casting molds produced with them to high atmospheric humidity. This means that storage of the moldings over a longer period of time, as is usual with organic binders, is not possible with certainty.

In the EP 1 122 002 a method is described which is suitable for the production of casting molds for metal casting. To produce the binder, an alkali metal hydroxide, in particular caustic soda, is combined with a particulate metal oxide mixed, which can form a metalate in the presence of the alkali metal hydroxide solution. The particles are dried after a layer of the metalate has formed at the edge of the particles. A portion where the metal oxide has not reacted remains in the core of the particles. A disperse silicon dioxide or also finely divided titanium oxide or zinc oxide is preferably used as the metal oxide.

In the WO 94/14555 a mold material mixture is described which is also suitable for the production of casting molds and which, in addition to a refractory mold base material, contains a binder which consists of a phosphate or borate glass, the mixture also containing a finely divided refractory material. Silicon dioxide, for example, can also be used as a refractory material.

In the EP 1 095 719 A2 a binder system for molding sands for the production of cores is described. The binder system based on water glass consists of an aqueous alkali silicate solution and a hygroscopic base such as sodium hydroxide, which is added in a ratio of 1:4 to 1:6. The water glass has a SiO ₂ /M ₂ O modulus of 2.5 to 3.5 and a solids content of 20 to 40%. In order to obtain a free-flowing mold material mixture that can also be filled into complicated core molds and to control the hygroscopic properties, the binder system also contains a surface-active substance such as silicone oil, which has a boiling point ≥ 250°C. The binder system is mixed with a suitable refractory material, such as quartz sand, and can then be shot into a core box using a core shooter. The molding material mixture hardens by removing the water that is still present. The drying or curing of the casting mold can also take place under the action of microwaves.

The WO 2006/024540 A2 proposed a molding material mixture which, in addition to a refractory molding material, contains a binder based on water glass. A proportion of a particulate metal oxide is added to the mold material mixture. Precipitated silica or pyrogenic silica is preferably used as the particulate metal oxide.

In the EP 0 796 681 A2 describes an inorganic binder for the production of casting molds, which contains a silicate and a phosphate in dissolved form. Polyphosphates of the formula ((PO ₃ ) _n ), where n corresponds to the average chain length and can assume values of 3 to 32, are preferably used as phosphates. The binder is mixed with a refractory mold base and then formed into a casting mold. The casting mold is hardened by heating the mold to temperatures of around 120 °C while blowing air through it. The test molds produced in this way show high hot strength after demolding as well as high cold strength. A disadvantage here, however, is the initial strength, with which process-reliable series core production cannot be guaranteed. The thermal stability is also insufficient for use at temperatures above 500° C., particularly in the case of molds subjected to severe thermal stress.

Because of the above-discussed problem of harmful emissions occurring during casting, efforts are being made in the production of Casting molds to replace the organic binders with inorganic binders, even with complicated geometries. However, when casting molds are made that include very thin-walled segments, deformation of these thin-walled sections is often observed during the casting process. This can lead to deviations in the dimensions of the casting that cannot be compensated for by subsequent processing. The casting thus becomes unusable. Thin-walled sections of the mold are subjected to greater thermal loads during casting than thick-walled sections and are therefore more prone to deformation. This problem already occurs with aluminum casting, although the temperatures here are relatively low at around 650 - 750 °C compared to iron or steel casting. This becomes particularly problematic when the liquid metal hits the thermally highly stressed thin-walled sections at an angle of inclination when it is poured into the casting mold and high mechanical forces act on the thin-walled sections due to the metallostatic pressure.

The invention was therefore based on the object of providing a molding material mixture for the production of casting molds for light metal casting, which comprises at least one refractory basic molding material and a binder system based on water glass, the molding material mixture containing a proportion of a particulate metal oxide which is selected from Group of silica, alumina, titania and zinc oxide, which enables the production of casting molds comprising thin-walled sections, wherein the thin-walled sections show no deformation in metal casting.

This object is achieved by using the features of patent claim 1. Advantageous developments of the use according to the invention are the subject matter of the dependent patent claims.

Surprisingly, it was found that by adding a phosphorus-containing compound according to claim 1, the strength of the casting mold can be increased to such an extent that thin-walled sections can also be realized that do not experience any deformation during metal casting. This also applies when the liquid metal hits the surface of the thin-walled sections of the casting mold at an angle during casting and therefore strong mechanical forces act on the thin-walled section of the casting mold. This means that casting molds with very complex geometries can also be produced using inorganic binders, so that the use of organic binders can also be dispensed with for these applications.

The use according to the invention is characterized by claim 1.

The molding mixture contains as a further Constituent is a phosphorus-containing compound, the proportion of the phosphorus-containing compound, based on the refractory base material, being between 0.05 and 0.5% by weight, and the phosphorus-containing compound being a sodium metaphosphate or a sodium polyphosphate.

Materials customary for the manufacture of casting molds can be used as the refractory mold base material. The refractory mold base material must have sufficient dimensional stability at the temperatures prevailing during metal casting. A suitable refractory base molding material is therefore characterized by a high melting point. The melting point of the refractory basic molding material is preferably higher than 700°C, preferably higher than 800°C, particularly preferably higher than 900°C and particularly preferably higher than 1000°C. Quartz or zircon sand, for example, is suitable as a refractory base molding material. In addition, fibrous refractory molding materials are also suitable, such as fireclay fibers. Other suitable refractory basic molding materials are, for example, olivine, chrome ore sand, vermiculite.

Artificial refractory base materials can also be used as refractory base materials, such as aluminum silicate hollow spheres (so-called microspheres), glass beads, glass granules or spherical ceramic base materials known as "Cerabeads ^® " or "Carboaccucast ^® ". These artificial refractory basic mold materials are produced synthetically or, for example, accumulate as waste in industrial processes. These spherical ceramic mold base materials contain, for example, mullite, corundum and β-cristobalite in different proportions as minerals. They contain aluminum oxide and silicon dioxide as essential components. Typical compositions contain, for example, Al ₂ O ₃ and SiO ₂ in approximately equal proportions. In addition, other components can also be present in proportions of <10%, such as TiO ₂ , Fe ₂ O ₃ . The diameter of the spherical, refractory basic mold materials is preferably less than 1000 μm, in particular less than 600 μm. Also suitable are synthetically produced refractory basic mold materials, such as mullite (x Al ₂ O ₃ .y SiO ₂ , with x=2 to 3, y=1 to 2; ideal formula: Al ₂ SiO ₅ ). These artificial basic molding materials do not go back to a natural origin and can also have been subjected to a special shaping process, such as in the production of hollow aluminum silicate microspheres, glass beads or spherical ceramic basic molding materials. Hollow aluminum silicate microspheres are formed, for example, when fossil fuels or other combustible materials are burned and are separated from the ash produced during combustion. Hollow microspheres as an artificial refractory molding material are characterized by a low specific weight. This is due to the structure of these artificial refractory mold bases, which include gas-filled pores. These pores can be open or closed. Closed-pored artificial refractory basic molding materials are preferably used. When using open-pored artificial refractory basic molding materials, part of the binder based on water glass is absorbed in the pores and can then no longer develop a binding effect.

According to one embodiment, glass materials are used as the artificial mold raw materials. These are in particular either as glass beads or used as glass granules. Conventional glasses can be used as the glass, with glasses having a high melting point being preferred. For example, glass beads and/or glass granules made from broken glass are suitable. Borate glasses are also suitable. The composition of such glasses is given by way of example in the table below. Table: composition of glasses component broken glass borate glass SiO ₂ 50-80% 50-80% _{Al2O3 _} 0-15% 0 - 15% _{Fe2O3 _} < 2% < 2% M ^II O 0 - 25% 0 - 25% M ^I ₂ O 5-25% 1 - 10% _{B2O3 _} < 15% Otherwise. < 10% < 10% M ^II : alkaline earth metal, eg Mg, Ca, Ba M ^I : alkali metal, e.g. Na, K

In addition to the glasses listed in the table, however, other glasses can also be used whose content of the abovementioned compounds is outside the ranges mentioned. Likewise, special glasses can also be used which, in addition to the oxides mentioned, also contain other elements or their oxides.

The diameter of the glass spheres is preferably 1 to 1000 μm, preferably 5 to 500 μm and particularly preferably 10 to 400 μm.

Preferably, only part of the refractory base mold material is formed by glass materials. The proportion of glass material in the refractory basic molding material is preferably chosen to be less than 35% by weight, particularly preferably less than 25% by weight, particularly preferably less than 15% by weight.

In casting tests with aluminium, it was found that when using artificial basic mold materials, especially glass beads, glass granules or glass microspheres, less molding sand sticks to the metal surface after casting than when using pure quartz sand. The use of such artificial basic molding materials based on glass materials therefore makes it possible to produce smooth casting surfaces, with complex post-treatment by blasting not being required, or at least to a considerably lesser extent.

In order to obtain the described effect of producing smooth cast surfaces, the proportion of glass material in the refractory basic molding material is preferably greater than 0.5% by weight, preferably greater than 1% by weight, particularly preferably greater than 1.5% by weight. , particularly preferably greater than 2 wt .-% selected.

It is not necessary to form the entire refractory mold base from the artificial refractory mold bases. The preferred proportion of the artificial basic molding materials is at least about 3% by weight, particularly preferably at least 5% by weight, particularly preferably at least 10% by weight, preferably at least about 15% by weight, particularly preferably at least about 20% by weight % by weight, based on the total amount of the refractory basic molding material. The refractory basic molding material preferably has a free-flowing state, so that the molding material mixture can be processed in conventional core shooting machines.

For reasons of cost, the proportion of artificial refractory basic mold materials is kept low. The proportion of the artificial refractory base molding materials in the refractory base molding material is preferably less than 80% by weight, preferably less than 75% by weight, particularly preferably less than 65% by weight.

As a further component, the mold material mixture includes a binder based on water glass. Conventional water glasses can be used as the water glass, as they are already used as binders in molding material mixtures. These water glasses contain dissolved sodium or potassium silicates and can be made by dissolving vitreous potassium and sodium silicates in water. The water glass preferably has a SiO ₂ /M ₂ O modulus in the range from 1.6 to 4.0, in particular 2.0 to 3.5, where M stands for sodium and/or potassium. The water glasses preferably have a solids content in the range from 30 to 60% by weight. The solids content refers to the amount of SiO ₂ and M ₂ O contained in the water glass.

The molding mixture also contains a proportion of a particulate metal oxide which is synthetically produced amorphous silicon dioxide. The average primary particle size of the particulate metal oxide can be between 0.10 μm and 1 μm. Because of the agglomeration of the primary particles, however, the particle size of the metal oxides is preferably less than 300 μm, preferably less than 200 μm, particularly preferably less than 100 μm. It is preferably in the range from 5 to 90 μm, particularly preferably in the range from 10 to 80 μm and very particularly preferably in the range from 15 to 50 μm. The particle size can be determined, for example, by sieve analysis. The sieve residue on a sieve with a mesh size of 63 μm is particularly preferably less than 10% by weight, preferably less than 8% by weight.

Precipitated silica and/or pyrogenic silica is preferably used as the particulate silicon dioxide. Precipitated silica is obtained by reacting an aqueous alkali silicate solution with mineral acids. The resulting precipitate is then separated off, dried and ground. Pyrogenic silicic acids are understood as meaning silicic acids which are obtained from the gas phase by coagulation at high temperatures. The production of fumed silica can, for example, by flame hydrolysis of silicon tetrachloride or in an arc furnace by reducing quartz sand with coke or anthracite to silicon monoxide gas with subsequent oxidation to silicon dioxide. The pyrogenic silicas produced by the arc furnace process can still contain carbon. Precipitated silica and pyrogenic silica are equally suitable for the molding mixture. These silicas are hereinafter referred to as "synthetic amorphous silicon dioxide".

The inventors assume that the strongly alkaline water glass can react with the silanol groups arranged on the surface of the synthetically produced amorphous silicon dioxide and that when the water evaporates an intensive bond is produced between the silicon dioxide and the then solid water glass.

The mold material mixture contains a phosphorus-containing compound as a further essential component and the proportion of the phosphorus-containing compound, based on the refractory base molding material, is selected between 0.05 and 0.5% by weight, and the phosphorus-containing compound is a sodium metaphosphate or a sodium Polyphosphate is, in the following only briefly, phosphate.

Polyphosphates are understood to mean, in particular, linear phosphates which comprise more than one phosphorus atom, the phosphorus atoms each being connected via oxygen bridges. Polyphosphates are obtained by dehydrating condensation of orthophosphate ions to form a linear chain of PO ₄ tetrahedra, each connected at corners. Polyphosphates have the general formula (O(PO ₃ ) _n ) ^(n+2)- , where n is the chain length. A polyphosphate can contain up to several hundred PO ₄ tetrahedra. However, preference is given to using polyphosphates with shorter chain lengths. n preferably has values from 2 to 100, particularly preferably 5 to 50. Higher condensed polyphosphates can also be used, ie polyphosphates in which the PO ₄ tetrahedra are connected to one another via more than two corners and therefore exhibit polymerization in two or three dimensions.

Metaphosphates are understood as meaning cyclic structures which are made up of PO ₄ tetrahedra which are each connected via corners. Metaphosphates have the general formula ((PO ₃ ) _n ) ^n- where n is at least 3. n preferably has values from 3 to 10.

Both individual phosphates and mixtures of different phosphates can be used.

The proportion of the phosphorus-containing compound, based on the refractory base molding material, is between 0.05 and 0.5% by weight. If the content is less than 0.05% by weight, there is no significant influence on the dimensional stability of the casting mold. When the content of the phosphate exceeds 1.0% by weight, the hot strength of the mold decreases greatly.

The phosphorus-containing compound can be added to the mold material mixture in solid or dissolved form. The phosphorus-containing compound is preferably added to the mold material mixture as a solid. If the phosphorus-containing compound is added in dissolved form, water is the preferred solvent.

A further advantage of adding phosphorus-containing compounds to molding material mixtures for the production of casting molds was found that the molds disintegrate very well after metal casting. This applies to metals that require lower pouring temperatures, such as light metals, especially aluminum. With iron casting, higher temperatures of more than 1200°C act on the casting mold, so that there is an increased risk of the casting mold vitrifying and thus worsening the decay properties.

Iron oxide was also considered as a possible additive as part of the investigations carried out by the inventors on the stability and disintegration of casting molds. When iron oxide is added to the mold material mixture, an increase in the stability of the casting mold is also observed in metal casting. The addition of iron oxide can also potentially improve the stability of thin-walled sections of the casting mold. However, the addition of iron oxide does not bring about the improvement in the disintegration properties of the casting mold after metal casting that is observed when phosphorus-containing compounds are added.

The mold material mixture is an intensive mixture of at least the components mentioned. The particles of the refractory basic mold material are preferably coated with a layer of the binder. By evaporating the water present in the binder (approx. 40-70% by weight, based on the weight of the binder), firm cohesion between the particles of the refractory base mold material can then be achieved.

The binder, i.e. the water glass and the particulate metal oxide, namely synthetic amorphous silicon dioxide, and the phosphate are preferably contained in the mold material mixture in a proportion of less than 20% by weight. The proportion of the binder refers to the solids content of the binder. If solid refractory basic molding materials are used, such as quartz sand, the binder is preferably present in a proportion of less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 5% by weight. If refractory basic molding materials are used which have a low density, such as the hollow microspheres described above, the proportion of binder increases accordingly.

The particulate metal oxide, namely the synthetic amorphous silica, is based on the total weight of the binder, preferably contained in a proportion of 2 to 80% by weight, preferably between 3 and 60% by weight, particularly preferably between 4 and 50% by weight.

The ratio of water glass to particulate metal oxide, namely synthetic amorphous silicon dioxide, can be varied within wide ranges. This offers the advantage of improving the initial strength of the casting mold, i.e. the strength immediately after removal from the hot tool, and the moisture resistance without significantly affecting the final strengths, i.e. the strengths after the casting mold has cooled, compared to a water glass binder without amorphous silicon dioxide. This is of great interest, especially in light metal casting. On the one hand, high initial strengths are desired so that after the casting mold has been produced it can be transported without any problems or assembled with other casting molds. On the other hand, the final strength after curing should not be too high in order to avoid problems with the binder breaking down after casting, i.e. the basic mold material should be able to be easily removed from cavities of the mold after casting.

In one embodiment of the invention, the basic molding material contained in the molding material mixture can contain at least a proportion of hollow microspheres. The diameter of the hollow microspheres is usually in the range of 5 to 500 µm, preferably in the range of 10 to 350 µm, and the thickness of the shell is usually in the range of 5 to 15% of the diameter of the microspheres. These microspheres have a very low specific weight, so that the casting molds produced using hollow microspheres have a low weight. The insulating effect of the hollow microspheres is particularly advantageous. The hollow microspheres are therefore used in particular for the production of casting molds if these are to have an increased insulating effect. Such casting molds are, for example, the feeders already described in the introduction, which act as a compensating reservoir and contain liquid metal, with the metal being kept in a liquid state until the metal filled into the hollow mold has solidified. Another area of application for casting molds that contain hollow microspheres are, for example, sections of a casting mold that correspond to particularly thin-walled sections of the finished casting mold. The insulating effect of the hollow microspheres ensures that the metal in the thin-walled sections does not solidify prematurely and thus block the paths within the casting mold.

If hollow microspheres are used, the binder, due to the low density of these hollow microspheres, is preferably used in a proportion in the range of preferably less than 20% by weight, particularly preferably in the range from 10 to 18% by weight. The values relate to the solids content of the binder.

The hollow microspheres preferably have sufficient temperature stability so that they do not soften prematurely and lose their shape during metal casting. The hollow microspheres preferably consist of an aluminum silicate. These hollow aluminum silicate microspheres preferably have an aluminum oxide content of more than 20% by weight, but can also have a content of more than 40% by weight. Such hollow microspheres are for example from Omega Minerals Germany GmbH, Norderstedt, under the names Omega-Spheres ^® SG with an aluminum oxide content of approx. 28 - 33%, Omega-Spheres ^® WSG with an aluminum oxide content of approx. 35 - 39% and E-Spheres ^® with an aluminum oxide content of approx. 43%. Corresponding products are available from PQ Corporation (USA) under the name “ ^{Extendospheres®} ”.

According to a further embodiment, hollow microspheres which are made of glass are used as the refractory base molding material.

According to a preferred embodiment, the hollow microspheres consist of a borosilicate glass. The borosilicate glass has a boron content, calculated as B ₂ O ₃ , of more than 3% by weight. The proportion of hollow microspheres is preferably chosen to be less than 20% by weight, based on the mold material mixture. When using hollow borosilicate glass microspheres, a small proportion is preferably selected. This is preferably less than 5% by weight, preferably less than 3% by weight, and is particularly preferably in the range from 0.01 to 2% by weight.

As already explained, in a preferred embodiment the mold material mixture contains at least a portion of glass granules and/or glass beads as the refractory base molding material.

It is also possible to form the mold material mixture as an exothermic mold material mixture which is suitable, for example, for the production of exothermic feeders. For this purpose, the mold material mixture contains an oxidizable metal and a suitable oxidizing agent. Based on the total mass of the mold material mixture, the oxidizable metals preferably make up a proportion of 15 to 35% by weight. The oxidizing agent is preferably added in a proportion of 20 to 30% by weight, based on the mold material mixture. Suitable oxidizable metals are, for example, aluminum or magnesium. Suitable oxidizing agents are, for example, iron oxide or potassium nitrate.

Binders that contain water have poorer flowability than binders based on organic solvents. The flowability of the mold material mixture can deteriorate further as a result of the addition of the particulate metal oxide. This means molds with narrow passages and multiple diversions are harder to fill. As a result, the molds have sections of insufficient compaction, which in turn can lead to casting defects during casting. According to an advantageous embodiment, the mold material mixture contains a proportion of a lubricant, preferably a flake-form lubricant, in particular graphite, MoS ₂ , talc and/or pyrophillite. Surprisingly, it has been shown that when such lubricants, in particular graphite, are added, complex shapes with thin-walled sections can also be produced, with the casting molds consistently having a consistently high density and strength, so that essentially no casting defects are observed during casting. The amount of the added flake-form lubricant, in particular graphite, is preferably 0.05% by weight to 1% by weight, based on the refractory base molding material.

In addition to the components mentioned, the mold material mixture can also include other additives. For example, internal release agents can be added, which facilitate the detachment of the casting molds from the mold. Suitable internal release agents are, for example, calcium stearate, fatty acid esters, waxes, natural resins or special alkyd resins. Furthermore, silanes can also be added to the mold material mixture.

Thus, in a preferred embodiment, the mold material mixture contains an organic additive which has a melting point in the range from 40 to 180° C., preferably 50 to 175° C., ie is solid at room temperature. Organic additives are understood as meaning compounds whose molecular structure is made up predominantly of carbon atoms, ie organic polymers, for example. The addition of the organic additives can further improve the quality of the surface of the casting. The mechanism of action of the organic additives has not been clarified. However, without wishing to be bound by this theory, the inventors assume that at least part of the organic additives burns during the casting process, creating a thin gas cushion between liquid metal and the basic mold material forming the wall of the casting mold and thus a reaction between liquid metal and basic mold material is prevented. The inventors also assume that some of the organic additives form a thin layer of so-called lustrous carbon under the reducing atmosphere prevailing during casting, which also prevents a reaction between the metal and the basic mold material. As a further advantageous effect, an increase in the strength of the casting mold after curing can be achieved by adding the organic additives.

The organic additives are preferably used in an amount of 0.01 to 1.5% by weight, more preferably 0.05 to 1.3% by weight, more preferably 0.1 to 1.0% by weight, respectively based on the refractory base molding material. In order to avoid heavy smoke development during metal casting, the proportion of organic additives is usually chosen to be less than 0.5% by weight.

Surprisingly, it was found that the surface of the casting can be improved with very different organic additives. Suitable organic additives are, for example, phenol-formaldehyde resins such as novolaks, epoxy resins such as bisphenol A epoxy resins, bisphenol F epoxy resins or epoxidized novolaks, polyols such as polyethylene glycols or polypropylene glycols, polyolefins such as polyethylene or polypropylene, copolymers Olefins such as ethylene or propylene and other comonomers such as vinyl acetate, polyamides such as polyamide 6, polyamide 12 or polyamide 6,6, natural resins such as gum rosin, fatty acids such as stearic acid, fatty acid esters such as cetyl palmitate , Fatty acid amides such as ethylenediamine bisstearamide, monomeric or polymeric carbohydrate compounds such as glucose or cellulose and derivatives thereof such as methyl, ethyl or carboxymethyl cellulose, and metal soaps such as stearates or oleates of monovalent to trivalent metals. The organic additives can be contained either as a pure substance or as a mixture of various organic compounds.

According to a further preferred embodiment, the mold material mixture contains a proportion of at least one silane. Examples of suitable silanes are aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes, methacrylsilanes, ureidosilanes and polysiloxanes. Examples of suitable silanes are γ-aminopropyltrimethoxysilane, γ-hydroxypropyltrimethoxysilane, 3-ureidopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)-trimethoxysilane, 3-methacryloxypropyltrimethoxysilane and N-β(aminoethyl)-γ- aminopropyltrimethoxysilane.

Based on the particulate metal oxide, typically about 5-50% by weight of silane is used, preferably about 7-45% by weight, particularly preferably about 10-40% by weight.

Despite the high strength that can be achieved with the binder, the casting molds produced with the mold material mixture, in particular cores and molds, surprisingly show good disintegration after casting, in particular in the case of aluminum casting. As already explained, it was also found that casting molds can be produced with the molding material mixture, so that the molding material mixture can be easily poured out again even from narrow and angled sections of the casting mold after casting.

When producing the mold material mixture, the procedure is generally such that the refractory base mold material is initially introduced and then the binder is added with stirring. The water glass and the particulate metal oxide, namely the synthetic amorphous silicon dioxide, and the phosphate itself can be added in any order. According to a preferred embodiment, the binder is provided as a two-component system, with a first liquid component containing the water glass and a second solid component containing the particulate metal oxide, the phosphate and possibly a preferably flake-form lubricant and/or an organic component. In the production of the mold material mixture, the refractory base mold material is placed in a mixer and the solid component of the binder is then preferably added first and mixed with the refractory base mold material. The mixing time is chosen so that an intimate mixing of refractory basic molding material and solid binder component takes place. The mixing time depends on the amount of molding material mixture to be produced and on the mixing unit used. The mixing time is preferably chosen to be between 1 and 5 minutes. The liquid component of the binder is then added, preferably with further agitation of the mixture, and the mixture is then further mixed until a uniform layer of the binder has formed on the grains of the refractory basic molding material. Here, too, the mixing time depends on the quantity of molding material mixture to be produced and on the mixing unit used. The duration for the mixing process is preferably chosen to be between 1 and 5 minutes.

According to another embodiment, however, the liquid component of the binder can first be added to the refractory base molding material and only then the solid component of the mixture can be added. According to a further embodiment, 0.05 to 0.3% water, based on the weight of the basic molding material, is first added to the refractory basic molding material and only then are the solid and liquid components of the binder added. In this embodiment, a surprisingly positive effect on the processing time of the mold mixture can be achieved. The inventors assume that the water-removing effect of the solid components of the binder is reduced in this way and the curing process is delayed as a result.

The mold material mixture is then brought into the desired shape. Conventional methods are used for shaping. For example, the mold material mixture can be shot into the mold using a core shooter with the aid of compressed air. The mold material mixture is then hardened by supplying heat in order to evaporate the water contained in the binder. Water is extracted from the mold material mixture when it is heated. The removal of water presumably also initiates condensation reactions between silanol groups, so that crosslinking of the water glass occurs. In the cold hardening processes described in the prior art, for example by introducing carbon dioxide or by polyvalent metal cations, difficultly soluble compounds are precipitated and the casting mold is thus strengthened.

The molding material mixture can be heated, for example, in the mold. It is possible to fully harden the casting mold in the mold. However, it is also possible to harden the casting mold only in its edge region, so that it has sufficient strength to be able to be removed from the mold. The casting mold can then be fully cured by removing more water from it. This can be done in an oven, for example. The water can also be removed, for example, by evaporating the water at reduced pressure.

The hardening of the casting molds can be accelerated by blowing heated air into the mold. In this embodiment of the method, rapid removal of the water contained in the binder is achieved, as a result of which the casting mold is solidified within periods of time suitable for industrial use. The temperature of the blown air is preferably 100°C to 180°C, particularly preferably 120°C to 150°C. The flow rate of the heated air is preferably adjusted so that the mold hardens in periods of time suitable for industrial use. The periods depend on the size of the molds being made. The aim is curing in less than 5 minutes, preferably less than 2 minutes. However, longer periods of time may be required for very large molds.

The water can also be removed from the mold material mixture by heating the mold material mixture by irradiating it with microwaves. However, the irradiation of the microwaves is preferably carried out after the casting mold has been removed from the mold. For this, however, the casting mold must already have sufficient strength. As already explained, this can be brought about, for example, in that at least one outer shell of the casting mold is already cured in the mold.

The thermal hardening of the mold material mixture with the removal of water avoids the problem of post-solidification of the casting mold during metal casting. In the cold curing process described in the prior art, in which carbon dioxide is passed through the mold material mixture, carbonates are precipitated from the water glass. However, a relatively large amount of water remains bound in the hardened casting mold, which is then expelled during the metal casting and leads to a very high level of hardening of the casting mold. Furthermore, molds that have been solidified by introducing carbon dioxide do not achieve the stability of molds that have been thermally cured by dehydration. The formation of carbonates disturbs the structure of the binder, which is why it loses strength. Thin sections of a casting mold, which may also have a complex geometry, cannot therefore be produced with cold-hardened casting molds based on water glass. Casting molds that are cold-hardened by introducing carbon dioxide are therefore not suitable for the production of castings with very complicated geometry and narrow passages with several deflections, such as oil channels in internal combustion engines, since the casting mold does not achieve the required stability and the casting mold after the Cast metal can only be completely removed from the casting with great effort. During thermal curing, most of the water is removed from the casting mold, and with metal casting, a significantly lower post-curing of the casting mold is observed. After the metal has been cast, the mold disintegrates significantly better than molds that have been hardened by introducing carbon dioxide. Thermal hardening can also be used to produce casting molds that are suitable for the production of castings with very complex geometries and narrow passages.

As already explained above, the flowability of the mold material mixture can be improved by adding lubricants, preferably in the form of flakes, in particular graphite and/or MoS ₂ and/or talc. Talc-like minerals such as pyrophyllite can also improve the flowability of the molding mixture. During production, the flake-form lubricant, in particular graphite and/or talc, can be added to the mold material mixture separately from the two binder components. But it is just as well possible, the platelet-shaped lubricant, especially graphite, with the particulate metal oxide, especially the synthetic amorphous Silicon dioxide, to be premixed and only then to be mixed with the water glass and the refractory basic molding material.

If the molding material mixture includes an organic additive, the organic additive can be added at any time during the production of the molding material mixture. The organic additive can be added in bulk or in the form of a solution.

Water-soluble organic additives can be used in the form of an aqueous solution. If the organic additives are soluble in the binder and can be stored therein undecomposed for several months, they can also be dissolved in the binder and thus added to the basic molding material together with it. Water-insoluble additives can be used in the form of a dispersion or a paste. The dispersions or pastes preferably contain water as the dispersing medium. As such, solutions or pastes of the organic additives can also be produced in organic solvents. However, if a solvent is used for the addition of the organic additives, water is preferably used.

The organic additives are preferably added as a powder or as short fibers, with the mean particle size or the fiber length preferably being chosen such that it does not exceed the size of the refractory particles of the basic molding material. The organic additives can particularly preferably be sieved through a sieve with a mesh size of about 0.3 mm. In order to reduce the number of components added to the refractory mold base, the particulate metal oxide and the organic additive(s) are preferably not added separately to the molding sand but are premixed.

If the mold material mixture contains silanes or siloxanes, the silanes are usually added in such a way that they are worked into the binder beforehand. However, the silanes or siloxanes can also be added to the basic molding material as a separate component. However, it is particularly advantageous to silanize the particulate metal oxide, i.e. to mix the metal oxide with the silane or siloxane so that its surface is provided with a thin layer of silane or siloxane. If the particulate metal oxide pretreated in this way is used, increased strength and improved resistance to high atmospheric humidity are found compared to the untreated metal oxide. If, as described, an organic additive is added to the mold material mixture or the particulate metal oxide, it is expedient to do this before the silanization.

The method is suitable for the production of all the molds customary for metal casting, for example cores and moulds. Casting molds that include very thin-walled sections can also be produced particularly advantageously. The process for the production of feeders is particularly suitable when insulating, refractory basic mold material is added or when exothermic materials are added to the mold material mixture.

The casting molds produced from the mold material mixture or with the method have a high strength immediately after production, without the strength of the casting molds decreasing hardening is so high that after the casting has been made, difficulties arise in removing the mold. It was found here that the casting mold for light metal casting, in particular aluminum casting, has very good disintegration properties. Furthermore, these casting molds have a high stability at elevated humidity, ie, surprisingly, the casting molds can also be stored without problems over a long period of time. As a particular advantage, the casting mold has a very high stability under mechanical stress, so that even thin-walled sections of the casting mold can be realized without being deformed by the metallostatic pressure during the casting process.

The casting mold is suitable for light metal casting. Particularly advantageous results are obtained with aluminum casting.

Fig. 1:

einen schematischen Aufbau einer BCIRA Hot Distortion Apparatur ( G.C. Fountaine, K.B. Horton, "Heißverformung von Cold-Box-Sanden", Giesserei-Praxis, Nr.6, S. 85-93, 1992 )

Fig. 2:

ein Diagramm des BCIRA Hot Distortion Tests eines phosphathaltigen Prüfkörpers und eines Prüfkörpers ohne Phosphatanteil ( Morgan, A.D., Fasham E.W., "The BCIRA Hot Distortion Tester for Quality Control in Production of Chemically Bonded Sands, AFS Transactions, vol. 83, S. 73 - 80 (1975 );

Fig. 3:

eine schematische Wiedergabe eines Gussstückauschnittes, wobei die Gießform einmal ohne (a) und einmal mit (b) Zusatz von Phosphaten hergestellt worden ist.

The invention is explained in more detail below using examples and with reference to the accompanying figures. It shows:

Figure 1:

a schematic structure of a BCIRA Hot Distortion apparatus ( GC Fountaine, KB Horton, "Hot deformation of cold box sands", Giesserei-Praxis, No.6, pp. 85-93, 1992 )

Figure 2:

a diagram of the BCIRA Hot Distortion Test of a phosphate-containing specimen and a specimen without phosphate content ( Morgan, AD, Fasham EW, "The BCIRA Hot Distortion Tester for Quality Control in Production of Chemically Bonded Sands, AFS Transactions, vol. 83, pp. 73-80 (1975 );

Figure 3:

a schematic representation of a casting section, the casting mold having been produced once without (a) and once with (b) the addition of phosphates.

example 1

Influence of synthetically produced amorphous silicon dioxide and phosphorus-containing components on the strength of molded bodies with quartz sand as the basic molding material.

1. Production and testing of the mold material mixture

So-called Georg Fischer test bars were produced to test the molding mixture. Georg Fischer test bars are cuboid test bars with the dimensions 150 mm x 22.36 mm x 22.36 mm.

• Die in Tabelle 1 aufgeführten Komponenten wurden in einem Laborflügelmischer (Firma Vogel & Schemmann AG, Hagen, DE) gemischt.

The composition of the molding mixture is given in Table 1. To produce the Georg Fischer test bars, the procedure was as follows:

• The components listed in Table 1 were mixed in a laboratory blade mixer (Vogel & Schemmann AG, Hagen, DE).

For this purpose, the quartz sand was initially introduced and the water glass was added with stirring. A sodium water glass which contained potassium was used as the water glass. The modulus is therefore given as SiO ₂ : M ₂ O in the following tables, where M indicates the sum of sodium and potassium. After the mixture was stirred for one minute, the amorphous silica and/or phosphorus-containing component, if any, were added with continued stirring. The mixture was then stirred for a further minute;

The molding material mixtures were transferred to the storage bunker of an H 2.5 hot-box core shooter from Röperwerk-Giessereimaschinen GmbH, Viersen, DE, whose mold was heated to 200° C.;
The mold material mixtures were introduced into the mold using compressed air (5 bar) and remained in the mold for a further 35 seconds;
To accelerate the curing of the mixtures, hot air (2 bar, 120° C. on entry into the mold) was passed through the mold for the last 20 seconds;
The mold was opened and the test bar was removed.

To determine the flexural strengths, the test bars were placed in a Georg Fischer strength tester equipped with a 3-point bending device (DISA Industrie AG, Schaffhausen, CH) and the force which caused the test bars to break was measured.

• 10 Sekunden nach der Entnahme (Heißfestigkeiten)
• 1 Stunde nach der Entnahme (Kaltfestigkeiten)
• 3 Stunden Lagerung der erkalteten Kerne im Klimaschrank bei 25 °C und 75 % relativer Luftfeuchte.

Tabelle 1 Zusammensetzung der Formstoffmischungen Quarzsand H32 Alkaliwasserglas Amorphes Siliciumdioxid Phosphat 1.1 100 GT 2,0 ^a) Vergleich, nicht erfindungsgemäß 1.2 100 GT 2,0 ^a) 0,5 ^b) Vergleich, nicht erfingdungsgemäß 1.3 100 GT 2,0 ^a) 0,3 ^c) Vergleich, nicht erfindungsgemäß 1.4 100 GT 2,0 ^a) 0,5 ^b) 0,3 ^c) Erfindungsgemäße Verwendung 1.5 100 GT 2,0 ^a) 0,5 ^b) 0,1 ^c) Erfindungsgemäße Verwendung 1.6 100 GT 2,0 ^a) 0,5 ^b) 0,5 ^c) erfindungsgemäße Verwendung 1.7 100 GT 2,0 ^a) 0,3 ^d) Vergleich, nicht erfindungsgemäß 1.8 100 GT 2,0 ^a) 0,5 ^b) 0,3 ^d) Erfindungsgemäße Verwendung ^a) Alkaliwasserglas mit Modul SiO₂:M₂O von ca. 2,3 ^b) Elkem Microsilica 971 (pyrogene Kieselsäure; Herstellung im Lichtbogenofen) ^c) Natriumhexametaphosphat (Fa. Fluka), als Feststoff zugesetzt ^d) Metakorin^® TWP 15 (Polyphosphatlösung der Fa. Metakorin Wasser-Chemie GmbH) Tabelle 2 Biegefestigkeiten Heißfestigke iten [N/cm²] Kaltfestigk eiten [N/cm²] Nach Lagerung im Klimaschrank [N/cm²] 1.1 70 420 20 Vergleich, nicht erfindungsgemäß 1.2 170 500 400 Vergleich, nicht erfindungsgemäß 1.3 60 410 20 Vergleich, nicht erfindungsgemäß 1.4 160 490 390 Erfindungsgemäße Verwendung 1.5 170 500 400 Erfindungsgemäße Verwendung 1.6 150 460 350 Erfindungsgemäße Verwendung 1.7 80 430 30 Vergleich, nicht erfindungsgemäß 1.8 160 450 380 Erfindungsgemäße Verwendung The flexural strengths were measured according to the following scheme:

• 10 seconds after removal (hot strengths)
• 1 hour after removal (cold strengths)
• 3 hours storage of the cooled cores in the climate cabinet at 25 °C and 75% relative humidity.

<u>Table 1</u> Composition of the molding material mixtures Silica sand H32 alkali water glass Amorphous Silica phosphate 1.1 100 GT 2.0 ^a) Comparison, not according to the invention 1.2 100 GT 2.0 ^a) 0.5 ^b) Comparison, not according to the invention 1.3 100 GT 2.0 ^a) 0.3 ^c) Comparison, not according to the invention 1.4 100 GT 2.0 ^a) 0.5 ^b) 0.3 ^c) Use according to the invention 1.5 100 GT 2.0 ^a) 0.5 ^b) 0.1 ^c) Use according to the invention 1.6 100 GT 2.0 ^a) 0.5 ^b) 0.5 ^c) Use according to the invention 1.7 100 GT 2.0 ^a) 0.3d ⁾ Comparison, not according to the invention 1.8 100 GT 2.0 ^a) 0.5 ^b) 0.3d ⁾ Use according to the invention ^a) Alkaline water glass with a SiO ₂ :M ₂ O modulus of about 2.3 ^b) Elkem Microsilica 971 (fumed silica; production in an electric arc furnace) ^c) sodium hexametaphosphate (from Fluka), added as a solid ^d) Metakorin ^® TWP 15 (polyphosphate solution from Metakorin Wasser-Chemie GmbH) Hot strength [N/cm ² ] Cold strength [N/cm ² ] After storage in the climate cabinet [N/cm ² ] 1.1 70 420 20 Comparison, not according to the invention 1.2 170 500 400 Comparison, not according to the invention 1.3 60 410 20 Comparison, not according to the invention 1.4 160 490 390 Use according to the invention 1.5 170 500 400 Use according to the invention 1.6 150 460 350 Use according to the invention 1.7 80 430 30 Comparison, not according to the invention 1.8 160 450 380 Use according to the invention

2nd result Effect of added amount of amorphous silica and phosphate

All mold material mixtures were produced with a constant amount of mold material and water glass. Examples 1.3 and 1.7 show that storable cores cannot be produced simply by adding phosphate. In Examples 1.2, 1.4, 1.5, 1.6 and 1.8 molding mixtures with amorphous silicon oxide were produced. The hot strengths and strengths after storage in the climatic cabinet are significantly increased compared to the other examples. Examples 1.4, 1.5 and 1.8 show that the hot and cold strengths and the strengths after storage in a climatic cabinet of molding mixtures which contain amorphous silicon dioxide as a component are not adversely affected by the addition of a phosphate-containing component. This means that the test bars produced with the molding mixture essentially retain their strength even after prolonged storage. Example 1.6 indicates that above a certain phosphate content in the molding material mixture, a negative influence on the strength is to be expected.

example 2 1. Deformation measurement

Deformation under thermal stress was determined using the BCIRA Hot Distortion Test ( Morgan, AD, Fasham EW, "The BCIRA Hot Distortion Tester for Quality Control in Production of Chemically Bonded Sands, AFS Transactions, vol. 83, pp. 73-80 (1975 ).

In the BCIRA hot deformation test, which is carried out in 1 is shown, a test specimen made of chemically bound sand measuring 25 x 6 x 114 mm is clamped in as a cantilever and heated from below on the flat side ( GC Fountaine, KB Horton, "Hot deformation of cold box sands", Giesserei-Praxis, No.6, pp. 85-93, 1992 ). This one-sided heating causes the specimen to bend upwards towards the cold side as a result of the thermal expansion of the hot side. This movement of the specimen is referred to as "Maximum Extension" in the curve. As the specimen heats up overall, the binder begins to decompose and transition to the thermoplastic state. Due to the thermoplastic properties of the various binder systems, the load from the load arm pushes the specimen back down. This downward movement along the ordinate in the 0-line until fracture is referred to as "hot deformation". The time elapsed between the onset of maximum expansion on the curve and rupture is referred to as "time to rupture" and is another characteristic. The movement that occurs in this experimental setup can indeed be observed in molds and cores.

The molding material mixtures were produced in accordance with the method described in example 1, with the difference that the test bars had the dimensions 25 mm×6 mm×114 mm. <u>Table 3</u> Composition of the molding material mixtures Silica sand H32 alkali water glass Amorphous Silica phosphate 2.1 100 GT 2.0 ^a) 0.5 ^b) Comparison, not according to the invention 2.2 100 GT 2.0 ^a) 0.5 ^b) 0.3 ^c) Comparison, not according to the invention ^a) Alkaline water glass with a SiO ₂ :M ₂ O modulus of about 2.3 ^b) Elkem Microsilica 971 (fumed silica; production in an electric arc furnace) ^c) sodium hexametaphosphate (from Fluka), added as a solid

2 results

The measured values for the deformation under thermal load are in 2 shown. Without the addition of phosphate (molding mixture 2.1), the test specimen is already deformed after a brief thermal load. Test specimens produced according to molding mixture 2.2, on the other hand, show a significantly improved thermal stability. By adding phosphate, the time until "hot deformation" and thus the "time until fracture" can be delayed.

Example 3 Manufacture of casting molds using phosphate-free and phosphate-containing moldings

To check the improved thermal stability of moldings shown in Example 2, cores were produced according to the molding mixtures 2.1 and 2.2. These cores were tested in a casting process (aluminum alloy, approx. 735°C) with regard to their thermal stability. It turned out that a circular segment of the molded body could only be reproduced correctly in the corresponding mold in the case of the mold material mixture 2.2 ( Figure 3b ). Without the addition of the phosphate component, elliptical deformations could be detected in the casting mold, schematized in Figure 3a shown.

It follows from this that the use of the molding material mixture according to the invention reduces the tendency of moldings to deform during the casting process and thus the casting quality of corresponding casting molds can be improved.

Claims (8)

1. A use of a casting mold for light metal casting obtained by a process comprising the following steps:

   - providing a molding material mixture;

   - molding the molding material mixture;

   - hardening the molded molding material mixture by heating the molded molding material mixture, wherein a hardened casting mold is obtained, wherein the molding material mixture comprises at least:

   - a refractory mold raw material;

   - a binder based on water glass;

   - a proportion of a particulate metal oxide, which is synthetically produced amorphous silicon dioxide;
   wherein a proportion of a phosphorus-containing compound is added to the molding material mixture and the proportion of the phosphorus-containing compound is selected to be between 0.05 and 0.5 wt. %, relative to the refractory mold raw material, and the phosphorus-containing compound is a sodium metaphosphate or a sodium polyphosphate.
2. The use according to claim 1, characterized in that the particulate metal oxide is selected from the group consisting of precipitated silica and pyrogenic silica, and/or the particulate metal oxide is present in a proportion of from 2 to 60% by weight, based on the binder.
3. The use according to one of the preceding claims, characterized in that the water glass has an SiO₂/M₂O ratio in the range from 1. 6 to 4.0 , in particular from 2.0 to 3.5, where M represents sodium ions and/or potassium ions, and/or the water glass has a solids content of SiO₂ and M₂O in the range from 30 to 60% by weight.
4. The use according to one of the preceding claims, characterized in that the binder is present in a proportion of less than 20% by weight in the molding material mixture.
5. The use according to one of the preceding claims, characterized in that the mold raw material contains at least a proportion of hollow microspheres, wherein the hollow microspheres are preferably hollow aluminum silicate microspheres and/or hollow glass microspheres.
6. The use according to one of the preceding claims, characterized in that

   - the mold raw material contains at least a proportion of glass granules, glass beads and/or spherical ceramic bodies; and/or

   - the mold raw material contains at least a proportion of mullite, chromium ore sand and/or olivine; and/or

   - the mold raw material contains an oxidizable metal and an oxidizer is added to the molding material mixture; and/or

   - the molding material mixture contains a proportion of a platelet-like lubricant, wherein the platelet-like lubricant is preferably selected from among graphite, molybdenum sulphide, talc and/or pyrophyllite; and/or

   - the molding material mixture contains a proportion of at least one organic additive which is solid at room temperature, and/or contains at least one silane or siloxane.
7. The use according to claim 1, characterized in that the molding material mixture is produced by

   - providing the refractory raw mold material;

   - adding to the refractory mold raw material solid constituents which comprise at least the particulate metal oxide and also the phosphate, and mixing the components to form a dry mix; and

   - adding the liquid components to the dry mix, the liquid components comprising at least the water glass.
8. A process according to claim 1 or 7, characterized in that

   - the molding material mixture is heated to a temperature in the range from 100 to 300°C, and/or

   - heated air is blown into the molded molding material mixture for curing; and/or

   - the heating of the molding material mixture is effected by the action of microwaves; and/or

   - the casting mold is a feeder.

Images