EP3950168A1 - Mould material mixtures on the basis of inorganic binders, and method for producing moulds and cores for metal casting - Google Patents

Abstract

The invention relates to molding material mixtures based on inorganic binders for the production of molds and cores for metal casting, consisting of at least one refractory basic molding material, an inorganic binder and amorphous silicon dioxide as an additive. The invention also relates to a method for producing molds and cores using the mold material mixtures.

Description

The invention relates to molding material mixtures based on inorganic binders for the production of molds and cores for metal casting, consisting of at least one refractory basic mold material, an inorganic binder and particulate amorphous silicon dioxide as an additive. Furthermore, the invention relates to a method for producing molds and cores using the mold material mixtures.

State of the art

Casting molds are essentially made up of molds or molds and cores, which represent the negative molds of the casting to be produced. These cores and molds consist of a refractory material, for example quartz sand, and a suitable binder that gives the casting mold sufficient mechanical strength after it has been removed from the mold. 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.

When casting, molds form the outer wall for the casting, cores are used to form cavities within the casting. It is not absolutely necessary that molds and cores are made of the same material. In chill casting, for example, the external shaping of the castings is carried out with the help of permanent metal moulds. It is also possible to combine molds and cores that have been produced from mold material mixtures with different compositions and using different processes. If, for the sake of simplicity, only molds are mentioned below, the statements also apply to cores that are based on the same mold material mixture and were manufactured using the same process.

Both organic and inorganic binders can be used to produce molds and can be hardened by cold or hot processes.

Cold processes are processes that are carried out essentially without heating the mold used to produce the core, usually at room temperature or at a temperature caused by a possible reaction. The curing takes place, for example, in that a gas is passed through the mold material mixture to be cured, thereby triggering a chemical reaction. In the case of hot processes, the mold material mixture is heated after shaping, e.g. by the heated mold, to a sufficiently high temperature to expel the solvent contained in the binder and/or to initiate a chemical reaction that hardens the binder.

Due to their technical properties, organic binders currently have greater importance in the market. Irrespective of their composition, however, they have the disadvantage that they decompose during pouring and sometimes emit considerable amounts of pollutants such as benzene, toluene and xylenes. In addition, the casting of organic binders usually leads to odor and smoke nuisance. In some systems, undesirable emissions even occur during core manufacture and/or storage. Even if the emissions have been reduced over the years through the development of binders, they cannot be completely avoided with organic binders. For this reason, research and development work has turned back to inorganic binders in recent years in order to further improve these and the product properties of the molds and cores produced in this way.

• Durchleiten eines Gases, z.B. CO₂, Luft oder eine Kombination aus beiden,
• Zugabe von flüssigen oder festen Härtern, z.B. Ester
• thermische Aushärtung, z.B. im Hot Box-Verfahren oder durch Mikrowellen-Behandlung.

Inorganic binders have been known for a long time, in particular those based on water glasses. They were most widespread in the 1950s and 1960s, but quickly lost importance with the advent of modern organic binders. There are three different methods available for hardening the water glasses:

• Passing through a gas, e.g. CO ₂ , air or a combination of both,
• Addition of liquid or solid hardeners, eg esters
• thermal curing, eg in the hot box process or by microwave treatment.

CO ₂ hardening is used, for example, in GB634817 described, curing by means of hot air without the addition of CO ₂ , for example in H. Polzin, W. Tilch and T. Kooyers, Gießerei-Praxis 6/2006, p. 171. A further development of CO ₂ curing by subsequent rinsing with air is in DE 102012103705.1 disclosed. Ester hardening is off, for example GB1029057 known (so-called no-bake method).

With the thermal hardening of water glass deal eg US4226277 and EP1802409 In the latter case, particulate synthetic amorphous SiO ₂ is added to the mold material mixture to increase the strength.

Other known inorganic binders are based on phosphates and/or a combination of silicates and phosphates, with curing also taking place using the above-mentioned methods. Mention should be made here, for example U.S. 5,641,015 (phosphate binder, thermal curing), U.S. 6,139,619 (silicate/phosphate binder, thermal curing), U.S. 2,895,838 (silicate/phosphate binder, CO ₂ hardening) and U.S. 6,299,677 (Silicate/phosphate binder, ester hardening).

In the cited patents or applications EP1802409 and DE 102012103705.1 it is proposed to add amorphous silicon dioxide to the mold material mixtures. The task of the SiO ₂ is to improve the decomposition of the cores after thermal stress, for example after casting. In EP 1802409 B1 and DE 102012103705.1 it is explained in detail that the addition of synthetic particulate amorphous SiO ₂ causes a significant increase in strength.

In EP 2014392 B1 it is proposed to add a suspension of amorphous, spherical SiO ₂ to the mold material mixture, consisting of mold material, caustic soda, binder based on alkali silicate and additives, with the SiO ₂ being present in two grain size classifications. With this measure, good flowability, high flexural strength and a high curing speed should be obtained.

task

The object of the present invention is to further improve the properties of inorganic binders, also in order to make them even more universally applicable and to make them an even better alternative to the currently dominant organic binders.

It is particularly desirable to provide molding material mixtures that allow cores with complex geometries to be produced due to further improved strength and/or improved compaction or, in the case of simpler core geometries, to reduce the amount of binder and/or shorten curing times.

Summary of the Invention

This problem is solved by molding mixtures with the features of the independent claims. Advantageous developments are the subject matter of the dependent patent claims and are described below.

Surprisingly, it was found that among the amorphous silicon dioxides there are types which differ significantly from the others in their effect as an additive to the binder. If particulate amorphous SiO ₂ is used as an additive, which was produced by thermal decomposition of ZrSiO ₄ to form ZrO ₂ and SiO ₂ and essentially complete or partial removal of the ZrO ₂ , it is found that with identical amounts added and under identical reaction conditions Surprisingly significantly improved strength and / or that the core weight is higher than when using in the EP 1802409 B1 mentioned particulate amorphous SiO ₂ from other production processes. The increase in core weight with the same outer dimensions of the core is accompanied by a reduction in gas permeability, which indicates denser packing of the molding material particles.

The particulate amorphous SiO ₂ produced by the above method is also identified by the term "artificially produced amorphous SiO 2". The particulate amorphous SiO ₂ can also be described cumulatively or as an alternative to production by the following parameters.

• einen feuerfesten Formgrundstoff,
• ein anorganisches Bindemittel, vorzugsweise basierend auf Wasserglas, Phosphat oder einer Mischung von beiden,
• ein Additiv bestehend aus partikuläre amorphem SiO₂, wobei dieses durch thermische Zersetzung von ZrSiO₄ zu ZrO₂ und SiO₂ erhalten wird.

The molding material mixture according to the invention comprises at least:

• a refractory molding material,
• an inorganic binder, preferably based on water glass, phosphate or a mixture of both,
• an additive consisting of particulate amorphous SiO ₂ , this being obtained by thermal decomposition of ZrSiO ₄ to form ZrO ₂ and SiO ₂ .

Detailed description of the invention

In the production of a mold material mixture, the procedure is generally such that the refractory mold base material is introduced and then the binder and the additive are added together or one after the other while stirring. It is of course also possible to first add all or part of the components and then and/or during this time to stir. The binder is preferably charged before the additive. It is stirred until an even distribution of the binder and the additive in the basic mold material is guaranteed.

The mold material mixture is then brought into the desired shape. Standard 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. Another option is to let the mold material mixture flow freely from the mixer into the mold and compact it there by shaking, tamping or pressing.

According to one embodiment of the invention, the molding material mixture is cured using the hot box process, ie it is cured with the aid of hot tools. The hot tools preferably have a temperature of 100 to 300°C, particularly preferably 120°C to 250°C. A gas (eg, CO ₂ or CO ₂ -enriched air) is preferably passed through the mold material mixture, this gas preferably having a temperature of 100 to 180° C., particularly preferably 120 to 150° C., as in FIG EP 1802409 B1 described. The above process (hot box process) is preferably carried out in a core shooter.

Irrespective of this, curing can also take place in that CO ₂ , a CO ₂ / gas mixture (e.g. with air) or CO ₂ and a gas / gas mixture (e.g. air) one after the other (as in detail in UK 102012103705 described) is passed through the cold mold or through the mold material mixture contained therein, the term "cold" meaning temperatures below 100° C., preferably below 50° C. and in particular at room temperature (eg 23° C.). The gas or gas mixture passed through the mold or through the molding material mixture can preferably be slightly heated, ie up to a temperature of 120°C, preferably up to 100°C, particularly preferably up to 80°C.

Last but not least, as an alternative to the two above methods, it is also possible to add a liquid or solid hardener to the mold material mixture before shaping, which then causes the hardening reaction.

Materials customary for the manufacture of casting molds can be used as the refractory base material (hereinafter referred to as base material(s) for short). For example, quartz, zirconium or chrome ore sand, olivine, vermiculite, bauxite and fireclay are suitable. It is not necessary to use only new sand. In order to conserve resources and avoid landfill costs, it is even advantageous to use as high a proportion of regenerated used sand as possible.

A suitable sand is, for example, in the WO 2008/101668 (= U.S. 2010/173767 A1 ) described. Regenerates obtained by washing and subsequent drying are also suitable. Regenerates obtained by purely mechanical treatment can also be used. As a rule, the regenerated material can make up at least about 70% by weight of the basic molding material, preferably at least about 80% by weight and particularly preferably at least about 90% by weight.

The average diameter of the basic mold materials is generally between 100 μm and 600 μm, preferably between 120 μm and 550 μm and particularly preferably between 150 μm and 500 μm. The particle size can be determined, for example, by sieving according to DIN 66165 (Part 2).

Furthermore, artificial molding materials can also be used as basic molding materials, in particular as an additive to the above basic molding materials, but also as an exclusive basic molding material, such as glass beads, glass granules, the spherical ceramic basic molding materials known under the name "Cerabeads" or "Carboaccucast" or hollow aluminum silicate microspheres (so-called microspheres ). Such hollow aluminum silicate microspheres are marketed, for example, by Omega Minerals Germany GmbH, Norderstedt, under the name "Omega Spheres". Corresponding products are also available from PQ Corporation (USA) under the name "Extendospheres".

In casting tests with aluminum, it was found that when using artificial mold base materials, especially glass beads, glass granules or microspheres, less molding sand sticks to the metal surface after casting than when using pure quartz sand. The use of artificial mold raw materials therefore enables the production of smoother casting surfaces, with complex post-treatment by blasting not being necessary, or at least to a much lesser extent.

It is not necessary to form the entire molding base from the artificial molding bases. The preferred proportion of the artificial basic molding materials is at least about 3% by weight, particularly preferably at least about 5% by weight, particularly preferably at least about 10% by weight, preferably at least about 15% by weight, particularly preferably at least about 20% % by weight, in each case based on the total amount of the refractory basic molding material.

As a further component, the molding material mixture according to the invention comprises an inorganic binder , for example based on water glass. Conventional water glasses such as those previously used as binders in molding material mixtures can be used as the water glass.

These water glasses contain dissolved alkali silicates and can be produced by dissolving vitreous lithium, sodium and potassium silicates in water.

The water glasses preferably have a molar SiO ₂ /M ₂ O modulus in the range from 1.6 to 4.0, in particular from 2.0 to less than 3.5, where M is lithium, sodium or potassium. The binders can also be based on water glasses that contain more than one of the alkali metal ions mentioned, such as those from DE 2652421 A1 (= GB1532847 ) known lithium-modified water glasses. Furthermore, the water glasses can also contain polyvalent ions such as boron or aluminum (corresponding are, for example, in EP 2305603 A1 (= WO2011/042132 A1 ) described).

The water glasses have a solids content in the range from 25 to 65% by weight, preferably 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.

Depending on the application and the desired level of strength, between 0.5% by weight and 5% by weight of the binder based on water glass are used, preferably between 0.75% by weight and 4% by weight, particularly preferably between 1% by weight and 3% by weight. 5% by weight, in each case based on the basic molding material. The percentage by weight relates to water glasses with a solids content as indicated above, i.e. includes the diluent.

Instead of water glass binders, those based on water-soluble phosphate glasses and/or borates can also be used, such as those described in U.S. 5,641,015 to be discribed.

The preferred phosphate glasses have a solubility in water of at least 200 g/L, preferably at least 800 g/L and contain between 30 and 80 mol% P ₂ O ₅ , between 20 and 70 mol% Li ₂ O, Na ₂ O or K ₂ O, between 0 and 30 mol% CaO, MgO or ZnO and between 0 and 15 mol% Al ₂ O ₃ , Fe ₂ O ₃ or B ₂ O ₃ . The particularly preferred composition is 58 to 72% by weight P ₂ O ₅ , 28 to 42% by weight Na ₂ O and 0 to 16% by weight CaO. The phosphate anions are preferably present as chains in the phosphate glasses.

The phosphate glasses are usually used as approximately 15 to 65% by weight, preferably as approximately 25 to 60% by weight, aqueous solutions. However, it is also possible to add the phosphate glass and the water to the basic molding material separately, with at least part of the phosphate glass dissolving in the water during the production of the molding material mixture.

Typical added amounts of the phosphate glass solutions are 0.5% by weight to 15% by weight, preferably between 0.75% by weight and 12% by weight, particularly preferably between 1% by weight and 10% by weight, in each case based on the basic mold material . The specification relates to phosphate glass solutions with a solids content as specified above, i.e. includes the diluent.

In the case of curing according to the so-called no-bake method, the mold material mixtures preferably also contain hardeners which cause the mixtures to harden without heat being supplied or a gas having to be passed through the mixture. These hardeners can be liquid or solid, organic or inorganic in nature.

Suitable organic hardeners include esters of carbonic acid such as propylene carbonate, esters of monocarboxylic acids having 1 to 8 carbon atoms with mono-, di- or trifunctional alcohols such as ethylene glycol diacetate, glycerol mono-, di- and -triacetic acid ester, and cyclic esters of hydroxycarboxylic acids such as for example γ-butyrolactone. The esters can also be used mixed with one another.

Suitable inorganic hardeners for binders based on water glass are, for example, phosphates such as Lithopix P26 (an aluminum phosphate from Zschimmer und Schwarz GmbH & Co KG Chemische Fabriken) or Fabutit 748 (an aluminum phosphate from Chemische Fabrik Budenheim KG).

The ratio of hardener to binder can vary depending on the desired property, e.g. processing time and/or stripping time of the molding mixture. The proportion of hardener (weight ratio of hardener to binder and, in the case of water glass, the total mass of the silicate solution or other binders taken up in the middle of the solution) is advantageously greater than or equal to 5% by weight, preferably greater than or equal to 8% by weight, particularly preferably greater than or equal to 10% by weight .%, based on the binder. The upper limits are less than or equal to 25% by weight, based on the binder, preferably less than or equal to 20% by weight, particularly preferably less than or equal to 15% by weight.

The mold material mixtures contain a proportion of an artificially produced particulate amorphous SiO ₂ , this originating from the process of thermal decomposition of ZrSiO ₄ to form ZrO ₂ and SiO ₂ .

Corresponding products are made, for example, by the companies Possehl Erzkontor GmbH, Doral Fused Materials Pty. Ltd., Cofermin Rohstoffe GmbH & Co.KG and TAM Ceramics LLC (ZrSiO ₄ process) on the market.

Surprisingly, it has been found that artificially produced particulate amorphous SiO ₂ with identical addition amounts and reaction conditions gives the cores higher strength and/or a higher core weight than amorphous SiO ₂ from other production processes, e.g. silicon or ferrosilicon production, flame hydrolysis of SiCl ₄ or a precipitation reaction. The molding material mixtures according to the invention therefore have improved flowability and can therefore be compressed to a greater extent at the same pressure.

Both have a positive effect on the usage properties of the molding material mixtures, since cores with more complex geometries and/or thinner walls can be produced than before. Conversely, in the case of simple cores without high demands on strength, it is possible to lower the binder content and thus increase the cost-effectiveness of the process. The improved compaction of the mold material mixture brings with it a further advantage in that the particles of the mold material mixture are present in a closer bond than in the prior art, so that the core surface is free of pores, which leads to a reduced roughness depth in the casting.

Without wanting to be bound by this theory, the inventors assume that the improved flowability is based on the fact that the particulate amorphous SiO ₂ used according to the invention has less tendency to agglomerate than the amorphous SiO ₂ from the other production processes and therefore already without the effect of strong shearing forces more primary particles are present. In 1 one can see that the SiO ₂ according to the invention has more isolated particles than in the comparison ( 2 ). One recognizes in 2 in addition, a stronger intergrowth of individual spheres into larger associations, which can no longer be broken up into the primary particles. In addition, the two figures indicate that the primary particles of the SiO ₂ according to the invention have a broader grain size distribution than in the prior art, which can also contribute to improved flowability.

The particle size was determined using dynamic light scattering on a Horiba LA 950, the scanning electron microscope images using an ultra-high-resolution Nova NanoSem 230 scanning electron microscope from FEI, which was equipped with a through-the-lens detector (TLD). For the REM measurements, the samples were dispersed in distilled water and then applied to an aluminum holder covered with copper tape before the water was evaporated. In this way, details of the primary particle shape down to the order of 0.01 µm could be made visible.

Due to its production, the amorphous SiO ₂ originating from the ZrSiO ₄ process can still contain zirconium compounds, especially ZrO ₂ . The zirconium content, calculated as ZrO ₂ , is usually below approx. 12% by weight, preferably below approx. 10% by weight, particularly preferably below approx. 8% by weight and particularly preferably below approx. 5% by weight .% and on the other hand greater than 0.01% by weight, greater than 0.1% by weight, or even greater than 0.2% by weight.

Furthermore, Fe ₂ O ₃ , Al ₂ O ₃ P ₂ O ₅ , HfO ₂ , TiO ₂ , CaO, Na ₂ O and K ₂ O with a total content of less than approx. 8% by weight, preferably less than approx. 5% by weight and more preferably less than about 3% by weight.

The water content of the particulate amorphous SiO ₂ used according to the invention is less than 10% by weight, preferably less than 5% by weight and particularly preferably less than 2% by weight. In particular, the amorphous SiO ₂ is used as a pourable dry powder. The powder is free-flowing and pourable under its own weight.

The average particle size of the particulate amorphous SiO ₂ is preferably between 0.05 µm and 10 µm, in particular between 0.1 µm and 5 µm and particularly preferably between 0.1 µm and 2 µm, primary particles with diameters between approx. 0.01 µm and approx. 5 µm were found. The determination was made using dynamic light scattering on a Horiba LA 950.

The particulate amorphous silicon dioxide has an average particle size of preferably less than 300 μm, preferably less than 200 μm, particularly preferably less than 100 μm. The particle size can be determined by sieve analysis. The sieve residue of the particulate amorphous SiO ₂ when passing through a sieve with a mesh size of 125 µm (120 mesh) is preferably not more than 10% by weight, particularly preferably not more than 5% by weight and very particularly preferably not more than 2% by weight. %.

The sieving residue is determined according to the machine sieving method described in DIN 66165 (Part 2), with a chain ring also being used as a sieving aid.

It has also proven to be advantageous if the residue of particulate amorphous SiO ₂ used according to the invention is not more than approx. 10% by weight, particularly preferably not more than approx 5% by weight and very particularly preferably not more than about 2% by weight (sieving according to DIN ISO 3310).

The ratio of primary particles (non-agglomerated, non-fused and non-fused particles) to the secondary particles (agglomerated, non-fused and/or fused particles including particles which (clearly) do not have a spherical shape) of the particulate amorphous SiO ₂ can be determined by means of scanning electron micrographs. These recordings were made using an ultra-high-resolution Nova NanoSem 230 scanning electron microscope from FEI, which was equipped with a through-the-lens detector (TLD).

For this purpose, the samples were dispersed in distilled water and then placed on an aluminum holder covered with copper tape before the water was evaporated. In this way, details of the primary particle shape down to 0.01 µm could be made visible.

1. a) Die Partikel liegen zu mehr als 20%, bevorzugt zu mehr als 40% insbesondere bevorzugt zu mehr als 60% und ganz besonders bevorzugt zu mehr als 80%, bezogen auf die Gesamtanzahl der Partikel, in Form im Wesentlichen sphärischer Primärpartikel, jeweils insbesondere mit obigen Grenzwerten in Form von sphärischen Primärpartikeln mit Durchmessern kleiner 4 µm, und besonders bevorzugt kleiner 2 µm.
2. b) Die Partikel liegen zu mehr als 20 Vol.%, bevorzugt zu mehr als 40 Vol.%, insbesondere bevorzugt zu mehr als 60 Vol.% und ganz besonders bevorzugt zu mehr als 80 Vol%, bezogen auf das kumulierte Volumen der Partikel, in Form von im Wesentlichen sphärischen Primärpartikeln vor, jeweils insbesondere mit obigen Grenzwerten in Form von sphärischen Primärpartikeln mit Durchmessern kleiner 4 µm, und besonders bevorzugt kleiner 2 µm. Die Berechnung der jeweiligen Volumina der einzelnen Partikel sowie das kumulierte Volumen aller Partikel erfolgte unter der Annahme einer jeweils für einzelne Partikel vorliegenden Kugelsymmetrie und unter Zuhilfenahme der mittels REM-Aufnahmen bestimmten Durchmesser für die jeweiligen Partikel.
3. c) die Partikel liegen zu mehr als 20 Flächen%, bevorzugt zu mehr als 40 Flächen%, insbesondere bevorzugt zu mehr als 60 Flächen% und ganz besonders bevorzugt zu mehr als 80 Flächen%, bezogen auf die kumulierte Fläche der Partikel, in Form von im Wesentlichen sphärischen Primärpartikeln vor, jeweils insbesondere mit obigen Grenzwerten in Form von sphärischen Primärpartikeln mit Durchmessern kleiner 4 µm, und besonders bevorzugt kleiner 2 µm.

The ratio of the primary particles to the secondary particles of the particulate amorphous SiO ₂ is advantageously and independently characterized as follows:

1. a) The particles are more than 20%, preferably more than 40%, particularly preferably more than 60% and very particularly preferably more than 80%, based on the total number of particles, in the form of essentially spherical primary particles, in each case in particular with the above limit values in the form of spherical primary particles with diameters of less than 4 μm, and particularly preferably less than 2 μm.
2. b) The particles are more than 20% by volume, preferably more than 40% by volume, particularly preferably more than 60% by volume and very particularly preferably more than 80% by volume, based on the cumulative volume of the particles, in the form of essentially spherical primary particles, in each case in particular with the above limit values in the form of spherical primary particles with diameters of less than 4 μm, and particularly preferably less than 2 μm. The respective volumes of the individual particles and the cumulative volume of all particles were calculated assuming spherical symmetry for each individual particle and with the help of the diameters for the respective particles determined by means of REM images.
3. c) the particles are more than 20% by area, preferably more than 40% by area, particularly preferably more than 60% by area and very particularly preferably more than 80% by area, based on the cumulative area of the particles, in the form of essentially spherical primary particles, in each case in particular with the above limit values in the form of spherical primary particles with diameters of less than 4 μm, and particularly preferably less than 2 μm.

The percentage recording is based on a statistical evaluation of a large number of SEM images, such as those in Fig.1 and Fig.2 are shown, whereby agglomeration / intergrowth / fusion can only be classified as such if the respective contours of individual adjacent spherical (converging) primary particles can no longer be recognized. In the case of particles lying on top of each other, where the respective contours of the spherical geometries can (otherwise) be recognized, the classification is made as primary particles, even if the view does not allow an actual classification due to the two-dimensional nature of the recordings. When determining the area, only the visible particle areas are evaluated and contribute to the total.

Furthermore, the specific surface area of the particulate amorphous SiO ₂ used according to the invention was determined using gas adsorption measurements (BET method, nitrogen) in accordance with DIN 66131. It was found that there appears to be a relationship between BET and compressibility. Suitable particulate amorphous SiO ₂ used according to the invention has a BET of less than or equal to 35 m ² /g, preferably less than or equal to 20 m ² /g, particularly preferably less than or equal to 17 m ² /g and particularly preferably less than or equal to 15 m ² /g. The lower limits are greater than or equal to 1 m ² /g, preferably greater than or equal to 2 m ² /g, particularly preferably greater than or equal to 3 m ² /g and particularly preferably greater than or equal to 4 m ² /g.

Depending on the application and the desired level of strength, between 0.1% by weight and 2% by weight of the particulate amorphous SiO ₂ are used, preferably between 0.1% by weight and 1.8% by weight and particularly preferably between 0.1% by weight % and 1.5% by weight, based in each case on the basic molding material.

The ratio of inorganic binder to particulate amorphous SiO ₂ used according to the invention can be varied within wide limits. This offers the possibility of greatly varying the initial strength of the cores, ie the strength immediately after removal from the mold, without significantly influencing the final strength. This is of great interest, especially in light metal casting. On the one hand, high initial strengths are desired here so that the cores can be transported without problems after production or assembled into whole core packages, on the other hand the final strengths should not be too high in order to avoid problems when the core disintegrates after casting.

Based on the weight of the binder (including any diluents or solvents), the particulate amorphous SiO ₂ is preferably contained in a proportion of 2% by weight to 60% by weight, particularly preferably from 3% by weight to 55% by weight and entirely more preferably from 4% to 50% by weight. The artificially produced (particulate) amorphous SiO ₂ corresponds to the particulate amorphous SiO ₂ according to the terminology of the claims and is used in particular as a powder, in particular with a water content of less than 5% by weight, preferably less than 3% by weight, in particular less than 2% by weight. %, (water content determined according to Karl Fischer). Irrespective of this, the ignition loss (at 400° C.) is preferably less than 6, less than 5 or even less than 4% by weight.

The particulate amorphous SiO ₂ used according to the invention can be added both before and after or mixed together with the addition of the binder directly to the refractory material. The particulate amorphous SiO ₂ used according to the invention is preferably added directly to the refractory material in dry and powder form after the addition of the binder.

According to a further embodiment of the invention, a premix of the SiO ₂ is first prepared with an aqueous alkaline solution, such as caustic soda, and optionally the binder or a part of the binder, and this is then mixed into the refractory basic molding material. The binder or proportion of binder that may still be present but not used for the premix can be added to the basic molding material before or after the addition of the premix or together with it.

According to a further embodiment, in addition to the particulate amorphous SiO ₂ a non-inventive synthetic particulate amorphous SiO ₂ according to EP 1802409 B1 be used in a ratio of 1 to less than 1, for example.

Mixtures of inventive and non-inventive SiO ₂ can be advantageous when the effect of the particulate amorphous SiO ₂ is to be “weakened”. The additions of inventive and non-inventive amorphous SiO ₂ to the mold material mixture allow the strength and/or compaction of the casting molds to be adjusted in a targeted manner.

In the case of an inorganic binder based on water glass, the molding material mixture according to the invention can, in a further embodiment, comprise a phosphorus-containing compound. Such an addition is preferred in the case of very thin-walled sections of a casting mold and in particular in the case of cores, since in this way the thermal stability of the cores or the thin-walled section of the casting mold can be increased. This is particularly important when the liquid metal hits a sloping surface during casting and exerts a strong erosive effect there due to the high metallostatic pressure or can lead to deformations, particularly of thin-walled sections of the casting mold.

Suitable phosphorus compounds do not or not significantly affect the processing time of the molding mixtures according to the invention. An example of this is sodium hexametaphosphate. Other suitable representatives and their addition amounts are in the WO 2008/046653 described in detail and this is also made to the disclosure of the present property rights.

Although the molding material mixtures according to the invention already have improved flowability compared to the prior art, if desired, this can be increased even further by adding flake-form lubricants, for example to completely fill molds with particularly narrow passages. According to an advantageous embodiment, the mold material mixture according to the invention contains a proportion of flake-form lubricants, in particular graphite or MoS ₂ . The amount of the added flake-form lubricant, in particular graphite, is preferably 0.05% by weight to 1% by weight, based on the basic mold material.

Instead of the flake-form lubricant, it is also possible to use surface-active substances, in particular surfactants, which also further improve the flowability of the molding material mixture according to the invention.

Suitable representatives of these compounds are, for example, in WO 2009/056320 (= US2010/0326620A1 ) described. Mention may be made here in particular of surfactants with sulfuric acid or sulfonic acid groups. Other suitable representatives and the respective amounts added are in the WO 2009/056320 described in detail and this is also made to the disclosure of the present property right.

In addition to the components mentioned, the molding material mixture according to the invention can also include other additives. For example, release agents can be added, which facilitate the detachment of the cores from the mold. Examples of suitable release agents are calcium stearate, fatty acid esters, waxes, natural resins or special alkyd resins. If these release agents are soluble in the binder and do not separate from it even after prolonged storage, especially at low temperatures, they can already be contained in the binder component, but they can also represent part of the additive or be added to the molding mixture as a separate component .

Organic additives can be added to improve the cast surface. 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 or polypropylene glycols, glycerol or polyglycerol, polyolefins such as polyethylene or polypropylene, copolymers Olefins such as ethylene and/or propylene with other comonomers such as vinyl acetate or styrene and/or diene monomers such as butadiene, polyamides such as polyamide 6, polyamide 12 or polyamide 6,6, natural resins such as gum resin, fatty acid esters such as cetyl palmitate, fatty acid amides such as ethylenediamine bisstearamide, metal soaps such as stearates or oleates of divalent or trivalent metals, and carbohydrates such as dextrins. Carbohydrates, in particular dextrins, are particularly suitable. Suitable carbohydrates are in the WO 2008/046651 A1 described. The organic additives can be used either as a pure substance or in a mixture with various other organic and/or inorganic compounds.

The organic additives are preferably used in an amount of 0.01% by weight to 1.5% by weight, more preferably 0.05% by weight to 1.3% by weight and most preferably 0.1% by weight to 1 % by weight added, in each case based on the molding material.

Furthermore, silanes can also be added to the mold material mixture according to the invention in order to increase the resistance of the cores to high atmospheric humidity and/or to water-based mold material coatings. According to a further preferred embodiment, the molding material mixture according to the invention therefore contains a proportion of at least one silane. Examples of suitable silanes are aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes and ureidosilanes. Examples of suitable silanes are γ-aminopropyl-trimethoxysilane, γ-hydroxypropyl-trimethoxysilane, 3-ureidopropyl-trimethoxysilane, γ-mercaptopropyl-trimethoxysilane, γ-glycidoxypropyl-trimethoxysilane, β-(3,4-epoxycyclohexyl)-trimethoxysilane, N-β -(Aminoethyl)-γ-aminopropyl-trimethoxysilane and their triethoxy-analogous compounds. The silanes mentioned, in particular the aminosilanes, can also be prehydrolyzed. Based on the binder, typically about 0.1% by weight to 2% by weight of silane is used, preferably about 0.1% by weight to 1% by weight.

Other suitable additives are alkali metal siliconates, for example potassium methyl siliconate, of which about 0.5% by weight to about 15% by weight, preferably about 1% by weight to about 10% by weight and particularly preferably about 1% by weight up to approx. 5% by weight, based on the binder, can be used.

If the molding material mixture includes an organic additive, it can be added at any time during the production of the molding material mixture. The addition can take place in bulk or else 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 in it for several months without decomposing, they can also be dissolved in the binder and thus added to the 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 liquid medium.

If the mold material mixture contains silanes and/or alkali metal siliconates, they are usually added in the form of being worked into the binder beforehand. However, they can also be added to the mold material as a separate component.

Inorganic additives can also have a positive effect on the properties of the molding material mixtures according to the invention. For example, the in AFS Transactions, Vol. 88, pp 601 - 608 (1980) or . Vol 89, pp 47 - 54 (1981 ) carbonates mentioned the moisture resistance of the cores during storage, while the out WO 2008/046653 (= CA2666760A1 ) known phosphorus compounds increase the thermal stability of the cores, provided that it is a binder based on water glass.

Alkali borates as components of water glass binders are used, for example, in EP0111398 disclosed.

Suitable inorganic additives to improve the casting surface based on BaSO ₄ are in DE 102012104934.3 described and can be added to the mold material mixture as a complete or at least partial replacement of the organic additives mentioned above.

Further details such as the respective amounts to be added are in the DE 102012104934.3 described in detail and this is also made to the disclosure of the present property right.

Despite the high strength that can be achieved with the molding material mixtures according to the invention, the cores produced from these molding material mixtures disintegrate well after casting, particularly in aluminum casting. However, the use of the cores produced from the molding material mixtures according to the invention is not limited to light metal casting. The casting molds are generally suitable for casting metals. Such metals are, for example, also non-ferrous metals such as brass or bronze, as well as ferrous metals.

• Fig. 1 Rasterelektronenmiskroskop-Aufnahme des erfindungsgemäß eingesetztem partikulären amorphen SiO₂.
• Fig. 2 Rasterelektronenmiskroskop-Aufnahme eines amorphen nicht-erfindungsgemäßes amorphen SiO₂ hergestellt bei der Produktion von Silicium/Ferrosilicium
• Fig. 3 einen Prüfkörper in Form eines Einlasskanalkerns

The figures show

• 1 Scanning electron micrograph of the particulate amorphous SiO ₂ used according to the invention.
• 2 Scanning electron micrograph of an amorphous non-inventive amorphous SiO ₂ produced during the production of silicon/ferrosilicon
• 3 a test body in the form of an intake port core

The invention is to be explained in more detail on the basis of the following examples, without being restricted to these.

Examples: 1. Heat curing 1.1. Experiment 1: Strengths and core weights depending on the type of particulate amorphous SiO ₂ added 1.1.1. Production of the mold material mixtures 1.1.1.1 Without adding SiO ₂

Silica sand was placed in the bowl of a Hobart mixer (model HSM 10). The binder was then added with stirring and mixed intensively with the sand for 1 minute each time. The sand used, the type of binder and the amounts added are listed in Table 1.

1.1.1.2. With the addition of SiO ₂

It was as in 1.1.1.1. procedure with the difference that after the addition of the binder to the mold material mixture, particulate amorphous SiO ₂ was also added and this was also mixed in for 1 minute. The type of particulate amorphous SiO ₂ and the amounts added are listed in Table 1. <b>Table 1</b> (Trial 1) Composition of the molding material mixtures Quartz sand H 32 binder Amorphous SiO ₂ separately added ZrO ₂ [GT] [GT] [GT] [GT] 1.1 100 2.0 ^a) not according to the invention 1.2 100 2.0 ^a) 0.5d ⁾ not according to the invention 1.3 100 2.0 ^a) 0.5e ⁾ not according to the invention 1.4 100 2.0 ^a) 0.475e ⁾ 0.025n ⁾ not according to the invention 1.5 100 2.0 ^a) 0.475e ⁾ 0.025o ⁾ not according to the invention 1.6 100 2.0 ^a) 0.5 ^f) according to the invention 1.7 100 2.0 ^a) 0.5g ⁾ according to the invention 1.8 100 2.0 ^a) 0.5 ^hours) according to the invention 1.9 100 2.0 ^a) 0.5 ⁱ⁾ according to the invention 1.10 100 2.0 ^b) not according to the invention 1.11 100 2.0 ^b) 0.5e ⁾ not according to the invention 1.12 100 2.0 ^b) 0.5 ^f) according to the invention 1.13 100 2.0 ^c) not according to the invention 1.14 100 2.0 ^c) 0.5e ⁾ not according to the invention 1.15 100 2.0 ^c) 0.5 ^f) according to the invention a) alkali water glass; molar modulus about 2.1; Solids approx. 35% by weight
b) sodium polyphosphate solution; 52% by weight (NaPO ₃ )n with n = ca. 25; 48% by weight water
c) Mixture of 83% by weight a) and 17% by weight b)
d) Microsilica 971 U (Elkem AS; manufacturing process: production of silicon/ferrosilicon)
e) Microsilica white GHL DL 971 W (RW Silicon GmbH; manufacturing process: see d)
f) Microsilica POS BW 90 LD (Possehl Erzkontor GmbH; manufacturing process: production of ZrO ₂ and SiO ₂ from ZrSiO ₄ )
g) Silica Fume (Doral Fused Materials Pty.,Ltd.; manufacturing process: see f)
h) Silica Fume SIF-B white (Cofermin Rohstoffe GmbH & Co. KG; manufacturing process: see f)
i) Fume Silica 605 MID (TAM Ceramics LLC.; manufacturing process: production of Ca-stabilized ZrO ₂ and SiO ₂ from ZrSiO ₄ )
n) Fused Monoclinic Zirconia - 45 µm (Cofermin Rohstoffe GmbH & Co. KG)
o) Calcia Stabilized Fused Zirconia - 45 µm (Cofermin Rohstoffe GmbH & Co. KG)

1.1.2. Production of the test specimens

To test the molding mixtures, cuboid test bars measuring 150 mm x 22.36 mm x 22.36 mm were produced (so-called Georg Fischer bars). A part of a after 1.1.1. The mold material mixture produced was 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 180.degree. The remainder of the respective mold material mixture was kept in a carefully closed container until the core shooter was refilled to protect it from drying out and to avoid a premature reaction with the CO ₂ present in the air.

The mold material mixtures were introduced into the mold from the storage bunker using compressed air (5 bar). The residence time in the hot tool for curing the mixtures was 35 seconds. In order to accelerate the hardening process, hot air (2 bar, 100°C when entering the tool) was passed through the mold during the last 20 seconds. The mold was opened and the test bar was removed. The test specimens for determining the core weights are produced using this method.

1.1.3. Examination of the test bodies 1.1.3.1. strength test

To determine the flexural strength, the test bars were placed in a Georg Fischer strength tester equipped with a 3-point bending device, and the force which caused the test bars to break was measured.

• 10 Sekunden nach der Entnahme (Heißfestigkeiten)
• ca. 1 Std. nach der Entnahme (Kaltfestigkeiten)

The flexural strengths were determined according to the following scheme:

• 10 seconds after removal (hot strengths)
• approx. 1 hour after removal (cold strength)

The results are listed in Table 2 1.1.3.2. Determination of core weight

Before the cold strength was determined, the Georg Fischer bars were weighed on a laboratory balance with an accuracy of 0.1 g. The results are listed in Table 2. <b>Table 2</b> (Trial 1) Flexural strengths and core weights # Hot strength [N/cm ² ] Cold strengths [N/cm ² ] core weight [g] 1.1 90 380 123.2 not according to the invention 1.2 150 480 123.1 not according to the invention 1.3 155 500 123.6 not according to the invention 1.4 150 485 123.7 not according to the invention 1.5 150 485 123.5 not according to the invention 1.6 180 575 127.2 according to the invention 1.7 185 600 127.1 according to the invention 1.8 180 580 128.2 according to the invention 1.9 155 530 126.2 according to the invention 1.10 10 145 119.7 not according to the invention 1.11 45 160 121.7 not according to the invention 1.12 50 175 125.9 according to the invention 1.13 95 405 122.7 not according to the invention 1.14 145 500 121.1 not according to the invention 1.15 160 550 125.3 according to the invention

Result:

Table 2 shows that the production method of the artificially produced particulate amorphous SiO ₂ has a significant influence on the properties of the cores. The cores that were produced with an inorganic binder and the SiO ₂ according to the invention have higher strengths and higher core weights than the cores that contain the SiO ₂ not according to the invention.

Examples 1.5 and 1.6 show that the positive effects are not based on the presence of ZrO ₂ in the amorphous SiO ₂ according to the invention originating from the ZrSO ₄ process.

1.2. Experiment 2 Flowability of the molding material mixtures as a function of the type of artificially produced particulate amorphous SiO ₂ , the sand and the shooting pressure. 1.2.1. Production of the mold material mixtures

The molding material mixtures were analogous to 1.1.1. manufactured. Their compositions are listed in Table 3. <b>Table 3</b> (experiment 2) Composition of the molding material mixtures # basic molding material [GT] Binder [GT] amorphous SiO ₂ [GT] Surfactant [GT] 2.1 100 ^a) 2.0d ⁾ 0.5f ⁾ not according to the invention 2.2 100 ^a) 2.0e ⁾ 1.0g ⁾ not according to the invention 2.3 100 ^a) 2.0d ⁾ 0.5 ^hours) according to the invention 2.4 100 ^b) 2.0d ⁾ 0.5 ^f) not according to the invention 2.5 100 ^b) 2.0d ⁾ 0.5 ^hours) according to the invention 2.6 ^100c) 2.0d ⁾ 0.5f ⁾ not according to the invention 2.7 100 ^c) 2.0d ⁾ 0.5 ^hours) according to the invention 2.8 100 ^a) 2.0d ⁾ 0.5f ⁾ 0.04 ⁱ⁾ not according to the invention 2.9 100 ^a) 2.0d ⁾ 0.5 ^hours) 0.04 ⁱ⁾ according to the invention a) Halterner quartz sand H 32 (Quartzwerke Frechen)
b) Frechen quartz sand F 32 (Quartzwerke Frechen)
c) Quartz sand Sajdikove Humence SH 32 (Quarzwerke Frechen)
d) alkali water glass; molar modulus about 2.1; Solids approx. 40% by weight
e) 1.8 pbw alkali water glass d) + 0.2 pbw NaOH (33% by weight) accordingly EP2014392
f) Microsilica white GHL DL 971 W (RW Silicon GmbH; manufacturing process: production of silicon/ferrosilicon
g) Suspension of 25% NanoSiO ₂ , 25% Micro SiO ₂ and 50% water accordingly EP2014392
h) Microsilica POS BW 90 LD (Possehl Erzkontor GmbH; manufacturing process: production of ZrO ₂ and SiO ₂ from ZrSiO ₄ )
i) Texapon EHS (Cognis)

1.2.2. Production of the test specimens

In order to investigate the influence of the artificially produced particulate amorphous SiO ₂ on the flowability of the mold material mixtures in more detail, cores from foundry practice, so-called inlet channel cores, were produced which are larger and have a more complex geometry than the Georg Fischer bars ( 3 ).

Preliminary tests had also shown that the informative value of this test is greater when using a complex built practice core as a test specimen than when using the Georg Fischer flowability test with its simple geometry (S.Hasse, Gießerei-Lexikon, Fachverlag Schiele and Schön). Three different sands with different grain shapes were used as basic molding materials.

The mold material mixtures were transferred to the storage bunker of an L 6.5 core shooter from Röperwerk-Gießereimaschinen GmbH, Viersen, DE, whose mold was heated to 180° C., and from there introduced into the mold by means of compressed air. The pressures used are listed in Table 4.

The residence time in the hot tool for curing the mixtures was 35 seconds. In order to accelerate the hardening process, hot air (2 bar, 150°C when entering the tool) was passed through the mold during the last 20 seconds.

The mold was opened and the test bar was removed.

1.2.3. Determination of core weights

After cooling, the cores were weighed on a laboratory balance with an accuracy of 0.1 g. The results are listed in Table 4. <b>Table 4</b> (experiment 2) Core weights of different mold material mixtures with variation of the shooting pressure # core weight [g] 5 bars 3 bars 2 bars 2.1 1297.7 1280.7 1238.0 not according to the invention 2.2 1290.1 1270.4 1225.7 not according to the invention 2.3 1357.0 1350.7 1314.0 according to the invention 2.4 1244.3 1232.3 1205.0 not according to the invention 2.5 1295.3 1274.0 1248.3 according to the invention 2.6 1354.8 1335.9 1290.0 not according to the invention 2.7 1393.7 1388.5 1356.0 according to the invention 2.8 1323.0 1319.3 1298.0 not according to the invention 2.9 1373.7 1367.7 1335.3 according to the invention

Result:

Table 4 confirms the improved flowability of the molding material mixtures according to the invention compared to the prior art on the basis of a core from foundry practice. The positive effect is independent of the type of sand and the shooting pressure.

The addition of a surfactant to the SiO ₂ according to the invention brings about a further improvement in the flowability, albeit not as markedly as when using amorphous SiO ₂ from other production processes.

2. Curing using a gas in non-heated tools. 2.1. Experiment 3: Strengths and core weights depending on the type of particulate amorphous SiO ₂ added. 2.1.1. Production of the mold material mixtures

The molding material mixtures were produced analogously to 1.1.1. Their compositions are given in Table 5. <b>Table 5</b> (Trial 3) Composition of the molding material mixtures # Quartz sand H 32 ^a) [GT] Binder ^b) [GT] amorphous SiO ₂ [GT] separately added ZrO ₂ [GT] 3.1 100 2.0 not according to the invention 3.2 100 2.0 0.5 ^c) not according to the invention 3.3 100 2.0 0.475 ^c) 0.025g ⁾ not according to the invention 3.4 100 2.0 0.475 ^c) 0.025h ⁾ not according to the invention 3.5 100 2.0 0.5d ⁾ according to the invention 3.6 100 2.0 0.5e ⁾ according to the invention 3.7 100 2.0 0.5f ⁾ according to the invention a) Quartz Works Frechen GmbH
b) alkali water glass; molar modulus about 2.33; Solids content approx. 40% by weight
c) Microsilica 971 U (Elkem AS; manufacturing process: production of silicon/ferrosilicon)
d) Microsilica POS BW 90 LD (Possehl Erzkontor GmbH; manufacturing process: production of ZrO ₂ and SiO ₂ from ZrSiO ₄ )
e) Silica Fume (Doral Fused Materials Pty.,Ltd.; manufacturing process: see d)
f) Fume Silica 605 MID (TAM Ceramics, LLC.; manufacturing process: production of Ca-stabilized ZrO ₂ and SiO ₂ from ZrSiO ₄ )
g) Fused Monoclinic Zirconia - 45 µm (Cofermin Rohstoffe GmbH & Co. KG)
h) Calcia Stabilized Fused Zirconia - 45 µm (Cofermin Rohstoffe GmbH & Co. KG)

2.1.2. Production of the test specimens

A part of a according to 2.1.1. The mold material mixture produced was transferred to the pantry of a H1 core shooter from Röperwerk-Gießereimaschinen GmbH, Viersen, DE. The remainder of the mold material mixture was kept in a carefully sealed container until the core shooter was refilled to protect it from drying out and to avoid a premature reaction with the CO ₂ present in the air.

The mold material mixtures were shot from the storage chamber using compressed air (4 bar) into a non-tempered mold with two engravings for round cores with a diameter of 50 mm and a height of 50 mm.

2.1.2.1. Curing with a combination of CO ₂ and air

For curing, CO ₂ was first injected for 6 seconds at a CO ₂ flow of 2 L/min. and then compressed air at a pressure of 4 bar is passed through the mold filled with the mold material mixture for 45 seconds. The temperatures of both gases were about 23°C when they entered the mold.

2.1.2.2. Curing with CO ₂

For curing, CO ₂ was injected at a CO ₂ flow of 4 L/min. passed through the mold filled with the mold material mixture. The temperature of the CO ₂ when it entered the mold was about 23°. <b>Table 6</b> (Trial 3) Compressive strengths and core weights when cured with a combination of CO ₂ and air instant strengths Strength after 24 hours core weight # [N/ ^cm2 ] [N/cm ² ] [G] 3.1 56 238 141.1 not according to the invention 3.2 173 289 143.3 not according to the invention 3.3 193 280 143.1 not according to the invention 3.4 189 300 143.4 not according to the invention 3.5 214 383 151.1 according to the invention 3.6 197 371 149.3 according to the invention 3.7 195 333 148.4 according to the invention Compressive strengths after storage at elevated temperature and humidity, curing with a combination of CO ₂ and air # Instant strength [N/cm ² ] Strength after 24 hours ^a) [N/cm ² ] Strength after 4 days ^b) [N/cm ² ] Strength after 6 days ^b) [N/cm ² ] 3.1 63 248 215 188 not according to the invention 3.2 166 298 256 221 not according to the invention 3.5 205 396 384 373 according to the invention a) Storage at 23°C/50% rel. humidity
b) Storage 24 hours at 23°C/50% rel. humidity, then at 30°C/80% rel. humidity

2.1.2.3. curing with air

For curing, air at a pressure of 2 bar was passed through the mold filled with the mold material mixture. The temperature of the air as it entered the mold was between about 22 and about 25°C.

Tab. 8 lists the gassing times with air. <b>Table 8</b> (Trial 3) Compressive strengths when cured by CO ₂ # gassing time [sec] Instant strength [N/cm ² ] Strength after 24 hours [N/cm ² ] 10 12 64 15 20 57 20 24 51 3.1 30 35 44 not according to the invention 45 40 46 60 42 45 90 43 38 10 33 67 15 42 65 20 46 66 3.2 30 49 57 not according to the invention 45 51 54 60 56 52 90 57 48 10 40 93 15 48 94 20 48 95 3.5 30 54 88 according to the invention 45 60 83 60 63 78 90 67 67

2.1.3. Examination of the test bodies

After curing, the test specimens were removed from the mold and their compressive strengths were determined using a Zwick universal testing machine (model Z 010) immediately, ie a maximum of 15 seconds, after removal. In addition, the compressive strength of the test specimens was tested after 24 hours, and in some cases also after 3 and 6 days of storage in a climate chamber. Constant storage conditions could be ensured with the aid of a climate chamber (Rubarth Apparate GmbH).

Unless otherwise stated, a temperature of 23°C and a relative humidity of 50% were set. The values given in the tables are mean values from 8 cores each. In order to check the compaction of the mold material mixtures during core production, the core weights were also determined 24 hours after removal from the core box in the case of combination curing with CO ₂ and air. The weighing was carried out on a laboratory balance with an accuracy of 0.1 g.

The results of the strength tests and the core weights, if the latter were carried out, are listed in Tables 6 and 7 (cure with CO ₂ and air), Table 8 (cure with CO ₂ ) and Table 9 (cure with air). <b>Table 9</b> (Trial 3) Compressive strengths when cured in air # gassing time [sec] Instant strength [N/cm ² ] Strength after 24 hours [N/cm ² ] 3.1 30 27 75 not according to the invention 45 71 93 60 101 104 3.2 30 41 143 not according to the invention 45 88 222 60 123 273 3.5 30 32 282 according to the invention 45 108 307 60 131 335

Result:

Tables 6 - 9 show that the positive properties of the particulate amorphous SiO ₂ used compared to the prior art are not limited to hot curing (Table 2), but also when the molding material mixtures are cured using a combination of CO ₂ and air, by means of CO ₂ and by means of air can be observed.

3. Cold curing q 3.1. Experiment 4: Strengths and core weights depending on the type of particulate amorphous SiO ₂ added 3.1.1. Production of the mold material mixtures 3.1.1.1. Without adding SiO ₂

Quartz sand from Quartzwerke Frechen GmbH was filled into the bowl of a mixer from Hobart (model HSM 10). First the hardener and then the binder were then added with stirring and mixed intensively with the sand for 1 minute each time.

The amounts added and the type of hardener and binder are listed with the individual tests.

3.1.1.2. With the addition of SiO ₂

It was as in 3.1.1.1. procedure with the difference that after the addition of the binder to the mold material mixture, the particulate amorphous SiO ₂ was also added and this was also mixed in for 1 minute. The amount added and the type of particulate amorphous SiO ₂ are listed for the individual experiments.

3.1.2. Production of the test specimens

The composition of the mold material mixtures used to produce the test specimens is listed in Table 10 in parts by weight (pbw).

To test the molding mixtures, cuboid test bars measuring 220 mm x 22.36 mm x 22.36 mm were produced (so-called Georg Fischer bars).
A part of a according to 3.1.1. The mold material mixture produced was introduced by hand into a mold with 8 engravings and compacted by pressing with a hand plate.

The processing time (VZ), ie the time within which a molding material mixture can be compacted without any problems, was determined visually. Exceeding the processing time can be recognized by the fact that a molding material mixture no longer flows freely, but rolls off like clods. The processing times of the individual mixtures are given in Table 10.

To determine the stripping time (AZ), i.e. the time after which a mold material mixture has hardened to such an extent that it can be removed from the mold, a second part of the respective mixture was poured into a round mold 100 mm high and 100 mm in diameter by hand and also compacted with a hand plate. The surface hardness of the compacted mold material mixture was then tested at specific time intervals using the Georg Fischer surface hardness tester. As soon as a mold material mixture is so hard that the test ball no longer penetrates the core surfaces, the stripping time has been reached. The stripping times for the individual mixtures are given in Table 10. <b>Table 10</b> (Trial 4) Composition of the molding material mixtures Quartz sand H 32 ^a) [GT] Binder ^b) [GT] Catalyst [GT] amorphous SiO ₂ [GT] 4.1 100 2.5 0.35 ^c) not according to the invention 4.2 100 3.0 0.35 ^c) not according to the invention 4.3 100 2.5 0.35 ^c) 0.5e ⁾ not according to the invention 4.4 100 2.5 0.35 ^c) 0.5 ^f) according to the invention 4.5 100 2.5 0.35 ^c) 0.5g ⁾ according to the invention 4.6 100 2.5 0.35 ^c) 0.5 ^hours) according to the invention 4.7 100 2.5 0.35d ⁾ not according to the invention 4.8 100 2.5 0.35d ⁾ not according to the invention 4.9 100 2.5 0.35d ⁾ 0.5e ⁾ not according to the invention 4.10 100 2.5 0.35d ⁾ 0.5 ^f) according to the invention 4.11 100 2.5 0.35d ⁾ 0.5g ⁾ according to the invention a) Quartz Works Frechen GmbH
b) Nuclesil 50 (Cognis)
c) Catalyst 5090 (ASK Chemicals GmbH), ester mixture
d) Lithopix P26 (Zschimmer & Schwarz)
e) Microsilica 971 U (Elkem SA; manufacturing process: production of silicon/ferrosilicon)
f) Microsilica POS BW 90 LD (Possehl Erzkontor GmbH; manufacturing process: production of ZrO ₂ and SiO ₂ from ZrSiO ₄ )
g) Silica Fume (Doral Fused Materials Pty.,Ltd.; manufacturing process: see f)
h) Fume Silica 605 MID (TAM Ceramics, LLC.; manufacturing process: production of Ca-stabilized ZrO ₂ and SiO ₂ from ZrSiO ₄ )

3.1.3. Examination of the test bodies 3.1.3.1. strength test

To determine the flexural strength, the test bars were placed in a Georg Fischer strength tester equipped with a 3-point bending device, and the force which caused the test bars to break was measured.

• 4 Stunden nach der Kernherstellung
• 24 Stunden nach der Kernherstellung
• Die Ergebnisse sind in Tab. 10 aufgeführt

The flexural strengths were determined according to the following scheme:

• 4 hours after core making
• 24 hours after core making
• The results are listed in Table 10

3.1.3.2. Determination of core weight

Before determining the firmness, the Georg Fischer bars were weighed on a laboratory balance with an accuracy of 0.1 g. The results are listed in Table 10.

Result:

Table 11 shows the positive effects of the particulate amorphous SiO ₂ used with regard to strength and core weight during cold hardening using an ester mixture (Ex. 4.1-4.6) or a phosphate hardener (Ex. 4.7-4.11) compared to the prior art. <b>Table 11</b> (Trial 4) Flexural strengths and core weights VZ ^a) /AZ ^b) Strength after 4 hours Strength after 24 hours core weight [minutes] [N/ ^cm2 ] [N/ ^cm2 ] [G] 4.1 15/80 145 250 119.5 not according to the invention 4.2 17/85 125 265 117.0 not according to the invention 4.3 4/75 185 290 119.7 not according to the invention 4.4 3/70 215 425 125.5 according to the invention 4.5 5/70 250 475 124.9 according to the invention 4.6 7/80 210 385 123.8 according to the invention 4.7 3/80 175 270 115.8 not according to the invention 4.8 4/85 160 290 115.0 not according to the invention 4.9 3/65 195 335 116.0 not according to the invention 4.10 4/60 210 415 121.3 according to the invention 4.11 4/60 215 415 120.1 according to the invention a) Processing Time
b) switch-off time

• Gegenstand 1: Formstoffmischung zur Herstellung von Gießformen und Kernen für die Metallverarbeitung, umfassend mindestens:
  • einen feuerfesten Formgrundstoff;
  • ein anorganisches Bindemittel und
  • partikuläres amorphes SiO₂ herstellbar durch thermische Zersetzung von ZrSiO₄zu ZrO₂ und SiO₂.
• Gegenstand 2: Formstoffmischung nach Gegenstand 1, wobei das partikuläre amorphe SiO₂ eine BET von größer gleich 1 m²/g und kleiner gleich 35 m²/g, bevorzugt kleiner gleich 17 m²/g und besonders bevorzugt von kleiner gleich 15 m²/g. aufweist.
• Gegenstand 3: Formstoffmischung nach einem der vorherigen Gegenstände, wobei die mittlere durch dynamische Lichtstreuung bestimmte Partikelgröße (Durchmesser) des partikulären amorphen SiO₂ in der Formstoffmischung zwischen 0,05 µm und 10 µm, insbesondere zwischen 0,1 µm und 5 µm und besonders bevorzugt zwischen 0,1 µm und 2 µm beträgt.
• Gegenstand 4: Formstoffmischung nach einem der vorherigen Gegenstände, wobei die Formstoffmischung das partikuläre amorphe SiO₂ in Mengen von 0,1 bis 2 Gew.%, vorzugsweise 0,1 bis 1,5 Gew.%, jeweils bezogen auf den Formgrundstoff enthält und unabhängig hiervon 2 bis 60 Gew.%, besonders bevorzugt 4 bis 50 Gew.% bezogen auf das Gewicht des Bindemittels, wobei der Feststoffanteil des Bindemittels 25 bis 65 Gew.%, vorzugsweise von 30 bis 60 Gew.%, beträgt.
• Gegenstand 5: Formstoffmischung nach einem der vorherigen Gegenstände, wobei das eingesetzte partikuläre amorphe SiO₂ einen Wassergehalt von kleiner 10 Gew.%, insbesondere kleiner 5 Gew.% und besonders bevorzugt kleiner 2 Gew.% aufweist und unabhängig insbesondere als Pulver eingesetzt wird.
• Gegenstand 6: Formstoffmischung nach einem der vorherigen Gegenstände, wobei die Formstoffmischung maximal 1 Gew.%, vorzugsweise maximal 0,2 Gew.%, organische Verbindungen enthält.
• Gegenstand 7: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei
  das anorganische Bindemittel zumindest ein wasserlösliches Phosphatglas, ein wasserlösliches Borat und/oder Wasserglas ist und insbesondere ein Wasserglas mit einem molaren Modul SiO₂/M₂O von 1,6 bis 4,0, vorzugsweise 2,0 bis kleiner 3,5, mit M gleich Lithium, Natrium und/oder Kalium.
• Gegenstand 8: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei die Formstoffmischung 0,5 bis 5 Gew.% Wasserglas, vorzugsweise 1 bis 3,5 Gew.% Wasserglas enthält, bezogen auf den Formgrundstoff, wobei der Feststoffanteil des Wasserglases 25 bis 65 Gew.%, vorzugsweise von 30 bis 60 Gew.%, beträgt.
• Gegenstand 9: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei die Formstoffmischung weiterhin Tenside enthält, vorzugsweise ausgewählt aus einem oder mehreren Mitgliedem der Gruppe der anionischen Tenside, insbesondere solche mit einer Sulfonsäure- oder Sulfonatgruppe, oder insbesondere umfassend Oleylsulfat, Stearylsulfat, Palmitylsulfat, Myristylsulfat, Laurylsulfat, Decylsulfat, Octylsulfat, 2-Ethylhexylsulfat, 2-Ethyloctylsulfat, 2-Ethyldecylsulfat, Palmitoleylsulfat, Linolylsulfat, Laurylsulfonat, 2-Ethyldecylsulfonat, Palmitylsulfonat, Stearylsulfonat, 2-Ethylstearylsulfonat, Linolylsulfonat, Hexylphosphat, 2-Ethylhexylphosphat, Caprylphosphat, Laurylphosphat, Myristylphosphat, Palmitylphosphat, Palmitoleylphosphat, Oleylphosphat, Stearylphosphat, Poly-(1,2-ethandiyl-)-Phenolhydroxiphosphat, Poly-(1,2-ethandiyl-)-Stearylphosphat, sowie Poly-(1,2-ethandiyl-)-Oleylphosphat.
• Gegenstand 10: Formstoffmischung nach Gegenstand 9, wobei das Tensid bezogen auf das Gewicht des feuerfesten Formgrundstoffs in einem Anteil von 0,001 bis 1 Gew. %, besonders bevorzugt 0,01 bis 0,2 Gew. % in der Formstoffmischung enthalten ist.
• Gegenstand 11: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei die Formstoffmischung weiterhin Graphit enthält, vorzugsweise von 0,05 bis 1 Gew.%, insbesondere 0,05 bis 0,5 Gew.%, bezogen auf das Gewicht des feuerfesten Formgrundstoffs.
• Gegenstand 12: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei die Formstoffmischung weiterhin zumindest eine phosphorhaltige Verbindung enthält, vorzugsweise von 0,05 und 1,0 Gew.%, besonders bevorzugt 0,1 und 0,5 Gew.%, bezogen auf das Gewicht des feuerfesten Formgrundstoffs.
• Gegenstand 13: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei das partikuläre amorphe SiO₂ als Pulver eingesetzt wird, vorzugsweise wasserfrei, abgesehen ggf. von einer etwaigen Feuchte verursacht durch Raumluft.
• Gegenstand 14: Formstoffmischung nach zumindest einem der vorherigen Gegenstände, wobei der Formstoffmischung ein Härter zugesetzt ist, insbesondere zumindest eine Ester- oder Phosphat-Verbindung.
• Gegenstand 15: Verfahren zur Herstellung von Gießformen oder Kernen umfassend:
  • Bereitstellen der Formstoffmischung nach zumindest einem der Gegenstände 1 bis 14,
  • Einbringen der Formstoffmischung in eine Form, und
  • Aushärten der Formstoffmischung.
• Gegenstand 16: Verfahren nach Gegenstand 15, wobei die Formstoffmischung mittels einer Kernschießmaschine mit Hilfe von Druckluft in die Form eingebracht wird und die Form ein Formwerkzeug ist und das Formwerkzeug mit einem oder mehreren Gasen durchströmt wird, insbesondere CO₂.
• Gegenstand 17: Verfahren nach Gegenstand 15 oder 16, wobei die Formstoffmischung zum Aushärten einer Temperatur von zumindest 100°C für unter 5 min ausgesetzt wird.
• Gegenstand 18: Verfahren nach zumindest einem der Gegenstände 15 bis 17, wobei die heißausgehärtete Formstoffmischung, insbesondere bei 180°C, in Form eines bei 5 bar geschossenen Georg-Fischer-Prüfriegels von 220 mm x 22,36 x 22,36 mm, der unter Verwendung des partikulären amorphen SiO₂, ein um 1%, bevorzugt 1,5%, besonders bevorzugt 2,0%, insbesondere bevorzugt 2,5% und ganz besonders bevorzugt 3,0% vergrößertes Kerngewicht aufweist, relativ zu einem Georg-Fischer-Testriegel ebenfalls von 220 mm x 22,36 x 22,36 mm, hergestellt unter den gleichen Bedingungen und mit der gleichen Formstoffmischung, aber unter Verwendung von Microsilica 971 U der Firma Elkem anstelle des partikulären amorphen SiO₂ nach einem der Gegenstände 1 bis 14.
• Gegenstand 19: Form oder Kern herstellbar nach zumindest einem der Gegenstände 15 bis 18.
• Gegenstand 20: Verwendung der Formstoffmischung nach zumindest einem der Gegenstände 1 bis 14 für das Gießen von Aluminium, vorzugsweise weiterhin enthaltend Mikrohohlkugeln insbesondere Aluminiumsilikatmikrohohlkugeln und/oder Borsilikatmikrohohlkugeln.

The invention also includes the following objects:

• Subject 1: mold material mixture for the production of casting molds and cores for metal processing, comprising at least:
  • a refractory mold base;
  • an inorganic binder and
  • particulate amorphous SiO ₂ can be produced by thermal decomposition of ZrSiO ₄ to ZrO ₂ and SiO ₂ .
• Subject 2: Molding material mixture according to subject 1, wherein the particulate amorphous SiO ₂ has a BET of greater than or equal to 1 m ² /g and less than or equal to 35 m ² /g, preferably less than or equal to 17 m ² /g and particularly preferably less than or equal to 15 m ² /G. having.
• Object 3: Molding mixture according to one of the preceding objects, wherein the average particle size (diameter) of the particulate amorphous SiO ₂ in the molding mixture, determined by dynamic light scattering, is between 0.05 μm and 10 μm, in particular between 0.1 μm and 5 μm and particularly preferred is between 0.1 µm and 2 µm.
• Subject 4: Molding material mixture according to one of the preceding subjects, wherein the molding material mixture contains the particulate amorphous SiO ₂ in amounts of 0.1 to 2% by weight, preferably 0.1 to 1.5% by weight, based in each case on the basic molding material and independently 2 to 60% by weight thereof, particularly preferably 4 to 50% by weight, based on the weight of the binder, the solids content of the binder being 25 to 65% by weight, preferably 30 to 60% by weight.
• Subject 5: Molding mixture according to one of the preceding subjects, wherein the particulate amorphous SiO ₂ used has a water content of less than 10% by weight, in particular less than 5% by weight and particularly preferably less than 2% by weight and is used independently in particular as a powder.
• Subject 6: Molding material mixture according to one of the preceding subjects, wherein the molding material mixture contains a maximum of 1% by weight, preferably a maximum of 0.2% by weight, of organic compounds.
• Item 7: molding mixture according to at least one of the previous items, wherein
  the inorganic binder is at least a water-soluble phosphate glass, a water-soluble borate and/or water glass and in particular a water glass with a molar modulus SiO ₂ /M ₂ O of 1.6 to 4.0, preferably 2.0 to less than 3.5 M is lithium, sodium and/or potassium.
• Item 8: Molding material mixture according to at least one of the above items, wherein the molding material mixture contains 0.5 to 5% by weight water glass, preferably 1 to 3.5% by weight water glass, based on the basic mold material, the solids content of the waterglass being 25 to 65% by weight .%, preferably from 30 to 60% by weight.
• Subject 9: Molding material mixture according to at least one of the preceding subjects, wherein the molding material mixture also contains surfactants, preferably selected from one or more members of the group of anionic surfactants, in particular those with a sulfonic acid or sulfonate group, or in particular comprising oleyl sulfate, stearyl sulfate, palmityl sulfate, myristyl sulfate , lauryl sulfate, decyl sulfate, octyl sulfate, 2-ethylhexyl sulfate, 2-ethyloctyl sulfate, 2-ethyldecyl sulfate, palmitoleyl sulfate, linolyl sulfate, lauryl sulfonate, 2-ethyldecyl sulfonate, palmityl sulfonate, stearyl sulfonate, 2-ethylstearyl sulfonate, linolyl sulfonate, hexyl phosphate, 2-ethylhexyl phosphate, caprylic phosphate, lauryl phosphate, myristyl phosphate , palmityl phosphate, palmitoleyl phosphate, oleyl phosphate, stearyl phosphate, poly(1,2-ethanediyl) phenol hydroxyphosphate, poly(1,2-ethanediyl) stearyl phosphate, and poly(1,2-ethanediyl) oleyl phosphate.
• Item 10: Molding material mixture according to item 9, wherein the tenside is contained in the molding material mixture in a proportion of 0.001 to 1% by weight, particularly preferably 0.01 to 0.2% by weight, based on the weight of the refractory base molding material.
• Subject 11: Molding material mixture according to at least one of the preceding subjects, wherein the molding material mixture also contains graphite, preferably from 0.05 to 1% by weight, in particular 0.05 to 0.5% by weight, based on the weight of the refractory base molding material.
• Item 12: Molding material mixture according to at least one of the preceding items, wherein the molding material mixture also contains at least one phosphorus-containing compound, preferably from 0.05 and 1.0% by weight, particularly preferably 0.1 and 0.5% by weight, based on the Weight of refractory mold base.
• Subject 13: Molding mixture according to at least one of the preceding subjects, wherein the particulate amorphous SiO ₂ is used as a powder, preferably anhydrous, apart from any moisture that may be caused by ambient air.
• Subject 14: Molding material mixture according to at least one of the previous subjects, wherein a hardener is added to the molding material mixture, in particular at least one ester or phosphate compound.
• Item 15: Process for the production of casting molds or cores comprising:
  • Providing the molding material mixture according to at least one of items 1 to 14,
  • introducing the mold material mixture into a mold, and
  • Hardening of the mold material mixture.
• Subject 16: Method according to subject 15, wherein the mold material mixture is introduced into the mold by means of a core shooter with the aid of compressed air and the mold is a mold and one or more gases, in particular CO ₂ , flow through the mold.
• Item 17: Method according to item 15 or 16, wherein the mold material mixture is exposed to a temperature of at least 100°C for less than 5 minutes for curing.
• Subject 18: Method according to at least one of subjects 15 to 17, wherein the heat-cured molding material mixture, in particular at 180° C., in the form of a Georg Fischer test bar of 220 mm x 22.36 x 22.36 mm shot at 5 bar, the using the particulate amorphous SiO ₂ , has a core weight increased by 1%, preferably 1.5%, particularly preferably 2.0%, particularly preferably 2.5% and very particularly preferably 3.0%, relative to a Georg Fischer - Test bars also of 220 mm x 22.36 x 22.36 mm, produced under the same conditions and with the same mold material mixture, but using Microsilica 971 U from Elkem instead of the particulate amorphous SiO ₂ according to one of items 1 to 14 .
• Item 19: Mold or core producible according to at least one of items 15 to 18.
• Subject 20: Use of the mold material mixture according to at least one of subjects 1 to 14 for the casting of aluminum, preferably also containing hollow microspheres, in particular hollow aluminum silicate microspheres and/or hollow borosilicate microspheres.

Claims (16)

Molding mixture for the production of casting molds and cores for metal processing, comprising at least: • a refractory mold base; • an inorganic binder and • particulate amorphous SiO ₂ can be produced by thermal decomposition of ZrSiO ₄ to form ZrO ₂ and SiO ₂ , the particulate amorphous SiO ₂ having a BET of greater than or equal to 1 m ² /g and less than or equal to 35 m ² /g. Molding mixture according to Claim 1, in which the particulate amorphous SiO ₂ has a BET of greater than or equal to 1 m ² /g and less than or equal to 17 m ² /g and preferably of less than or equal to 15 m ² /g. Molding material mixture according to one of the preceding claims, wherein the average particle size (diameter) of the particulate amorphous SiO ₂ in the molding material mixture, determined by dynamic light scattering, is between 0.05 µm and 10 µm, in particular between 0.1 µm and 5 µm and particularly preferably between 0 is 1 µm and 2 µm. Molding material mixture according to one of the preceding claims, wherein the molding material mixture contains the particulate amorphous SiO ₂ in amounts of 0.1 to 2% by weight, preferably 0.1 to 1.5% by weight, based in each case on the basic molding material, and independently of this 2 to 60% by weight, particularly preferably 4 to 50% by weight, based on the weight of the binder, the solids content of the binder being 25 to 65% by weight, preferably from 30 to 60% by weight. Molding mixture according to one of the preceding claims, wherein the particulate amorphous SiO ₂ used has a water content of less than 10% by weight, in particular less than 5% by weight and particularly preferably less than 2% by weight and is used independently in particular as a powder. Molding material mixture according to at least one of the preceding claims, wherein the inorganic binder is at least one water-soluble phosphate glass, a water-soluble borate and/or water glass and in particular a water glass with a molar modulus SiO ₂ /M ₂ O of 1.6 to 4.0, preferably 2 .0 to less than 3.5, with M equal to lithium, sodium and/or potassium. Molding material mixture according to at least one of the preceding claims, wherein the molding material mixture contains 0.5 to 5% by weight water glass, preferably 1 to 3.5% by weight water glass, based on the basic mold material, the solids content of the waterglass being 25 to 65% by weight, preferably from 30 to 60% by weight. Molding material mixture according to at least one of the preceding claims, wherein the molding material mixture also contains surfactants, preferably selected from one or more members of the group of anionic surfactants, in particular those with a sulfonic acid or sulfonate group, or in particular comprising oleyl sulfate, stearyl sulfate, palmityl sulfate, myristyl sulfate, lauryl sulfate, Decyl sulfate, octyl sulfate, 2-ethylhexyl sulfate, 2-ethyloctyl sulfate, 2-ethyldecyl sulfate, palmitoleyl sulfate, linolyl sulfate, lauryl sulfonate, 2-ethyldecyl sulfonate, palmityl sulfonate, stearyl sulfonate, 2-ethylstearyl sulfonate, linolyl sulfonate, hexyl phosphate, 2-ethylhexyl phosphate, caprylic phosphate, lauryl phosphate, myristyl phosphate, palmityl phosphate, palmitoleyl phosphate, oleyl phosphate, stearyl phosphate, poly(1,2-ethanediyl) phenolic hydroxyphosphate, poly(1,2-ethanediyl) stearyl phosphate, and poly(1,2-ethanediyl) oleyl phosphate, and also independently thereof the surfactant based on the weight of the refractory mold base in e in a proportion of 0.001 to 1% by weight, particularly preferably 0.01 to 0.2% by weight, is contained in the mold material mixture. Molding mixture according to at least one of the preceding claims, characterized by one or more of the following features: a) the molding material mixture also contains graphite, preferably from 0.05 to 1% by weight, in particular 0.05 to 0.5% by weight, based on the weight of the refractory basic molding material; b) the molding material mixture also contains at least one phosphorus-containing compound, preferably from 0.05 to 1.0% by weight, particularly preferably 0.1 to 0.5% by weight, based on the weight of the refractory basic molding material; c) a hardener is also added to the mold material mixture, in particular at least one ester or phosphate compound; d) the mold material mixture contains a maximum of 1% by weight, preferably a maximum of 0.2% by weight, of organic compounds; e) the zirconium content of the particulate amorphous SiO ₂ , calculated as ZrO ₂ , is greater than 0.01% by weight. and below 12% by weight. Molding mixture according to at least one of the preceding claims, wherein the particulate amorphous SiO ₂ is used as a powder, preferably anhydrous, apart from any moisture that may be caused by ambient air. Process for the production of casting molds or cores comprising: • providing the mold material mixture according to at least one of claims 1 to 10, • introducing the mold material mixture into a mold, and • Hardening of the mold material mixture. Method according to Claim 11, in which the mold material mixture is introduced into the mold by means of a core shooter with the aid of compressed air and the mold is a mold and one or more gases, in particular CO ₂ , flow through the mold. Method according to claim 11 or 12, wherein the mold material mixture is exposed to a temperature of at least 100°C for less than 5 minutes for curing. The method according to at least one of claims 11 to 13, wherein the hot-cured molding material mixture, in particular at 180°C, in the form of a Georg Fischer test bar of 220 mm x 22.36 x 22.36 mm, shot at 5 bar, which is particulate amorphous SiO ₂ , has a core weight increased by 1%, preferably 1.5%, particularly preferably 2.0%, particularly preferably 2.5% and very particularly preferably 3.0%, relative to a Georg Fischer test bar as well of 220 mm x 22.36 x 22.36 mm, produced under the same conditions and with the same mold material mixture, but using Microsilica 971 U from Elkem instead of the particulate amorphous SiO ₂ according to one of Claims 1 to 14. Mold or core producible according to at least one of Claims 11 to 14. Use of the mold material mixture according to at least one of Claims 1 to 10 for the casting of aluminium, preferably also containing hollow microspheres, in particular hollow aluminum silicate microspheres and/or hollow borosilicate microspheres.

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