EP4363135A1

EP4363135A1 - Method for the layered construction of molds and cores with a binder containing water glass

Info

Publication number: EP4363135A1
Application number: EP22740283.1A
Authority: EP
Inventors: Christian Appelt; Heinz DETERS; Dominik Polsakiewicz; Katharina Schotten-Welter; Henning ZUPAN
Original assignee: ASK Chemicals GmbH
Current assignee: ASK Chemicals GmbH
Priority date: 2021-06-30
Filing date: 2022-06-21
Publication date: 2024-05-08
Also published as: DE102021116930A1; WO2023274450A1; KR20240027711A; CN117897240A; JP2024525442A

Abstract

The invention relates to a method for the layered construction of molds and cores comprising fire-resistant base molding material, a hydrophobicized metal oxide and a binder containing at least water glass in the form of an aqueous alkali silicate solution. The invention also relates to molds or cores produced in this manner.

Description

Process for the layered construction of molds and cores with a binder containing water glass

The invention relates to a method for building up molds and cores in layers, comprising a refractory basic mold material, a hydrophobic metal oxide as a solid and a binder containing at least water glass in the form of an aqueous alkali silicate solution. For the layered production of molds and cores using 3D printing, it is necessary to apply a refractory base material in layers and print it selectively using the binder. The invention further relates to molds or cores produced in this way.

State of the art

Casting molds essentially consist of cores and moulds, which represent the negative molds of the casting to be produced. These cores and molds consist of a refractory material, such as quartz sand, and a suitable binder that gives the mold sufficient mechanical strength after removal from the mold. For the sake of simplicity, cores and molds are collectively referred to below as a mold or molds. For the production of casting molds, a refractory base material is used, which is provided with a suitable binding agent. The refractory basic molding material is preferably in a free-flowing form, so that it can be filled into a suitable hollow mold. The binding agent creates a solid bond between the particles/grains of the mold base material, so that the casting mold has the required mechanical stability.

Casting molds have to meet various requirements. During the casting process itself, they must first have sufficient strength and temperature resistance to be able to accommodate the liquid metal in the cavity formed from one or more casting (partial) molds. After the start of the solidification process, the mechanical stability of the casting is ensured by a solidified metal layer that forms along the walls of the casting mold. The material of the casting mold must now decompose under the influence of the heat given off by the metal in such a way that it loses its mechanical strength, i.e. the cohesion between individual particles/grains of the refractory material is broken. Ideally, the casting mold crumbles back into fine sand that can be easily removed from the casting.

Various methods for the production of three-dimensional bodies by building them up in layers are known under the name “rapid prototyping”. An advantage of this process is the possibility of producing complex, one-piece bodies with undercuts and cavities. With conventional methods, these bodies would have to be assembled from several individually manufactured parts. Another advantage is that the methods are able to produce the bodies directly from the CAD data without the use of molds.

The 3-dimensional printing process results in new requirements for binders that hold the mold together when the binder or a binder component is to be applied through the nozzles of a print head. Then the binders must not only lead to a sufficient level of strength and good disintegration properties after the metal casting and have sufficient thermal and storage stability, but also be “printable”, i.e. the nozzles of the print head must not become blocked by the binder , on the other hand, the binder should not be able to flow directly out of the print head, but form individual droplets.

In addition, there is an increasing demand for as few emissions as possible in the form of CO2 or hydrocarbons to occur during the manufacture of the casting molds, as well as during casting and cooling, in order to protect the environment and reduce the odor nuisance in the environment caused by hydrocarbons, mainly aromatic hydrocarbons , restrict. In order to meet these requirements, inorganic binding systems have been developed or further developed in recent years, the use of which means that emissions of CO2 and hydrocarbons in the production of metal molds can be avoided or at least significantly reduced. EP 1802409 B1 discloses an inorganic binder system with which it is possible to produce casting molds with sufficient stability. However, the binder system is particularly suitable for thermal curing in a core shooter, in which a previously mixed mold material mixture (mixture of at least refractory material and binder) is conveyed into the heated mold by means of pressure.

WO 2012/175072 A1 discloses a method for the layered construction of models, in which an inorganic binder system is used. The particulate material applied in layers comprises a particulate building material and a spray-dried alkali silicate solution. The flattening is selectively activated using a water-comprising solution that is added via the printhead. Both pure water and modified water containing theological additives are disclosed here. Thickeners such as glycerol, glycol or layered silicates are mentioned as examples of rheological additives, with layered silicates being particularly emphasized. WO 2012/175072 A1 does not disclose the use of aqueous alkali metal silicate solutions. The binder or the water glass solution is not dosed via the print head, but is already contained in solid form as alkali silicate in the layered, particulate material. According to WO 2012/175072 A1, the selective wetting or setting of a material applied in layers with the aid of a binder is therefore only possible indirectly and not directly with the aid of an aqueous alkali silicate solution. Due to the process described in WO 2012/175072 A1, the binder, the spray-dried alkali metal silicate solution, is not only at the intended destination, but also in areas where it is not required. Thus, the binder is unnecessarily consumed.

DE 102011053205 A1 discloses a method for producing a component using deposition technology, in which, among other things, water glass is used as the pressure fluid in addition to many other possibilities. The water glass can therefore be dosed by means of a print head and applied to a predetermined portion of each Weil top layer. However, DE 102011053205 A1 contains no information about which waterglass compositions can be used. The person skilled in the art also does not receive any information about the physical properties of the water glasses used, which could have suggested a chemical composition. Only the prior art described speaks very generally of inorganic binders (such as, for example, free-flowing water glass), which generally contain large amounts of moisture—only up to 60% by weight of water is given as an example. The large amounts of water (eg up to 60% by weight of water) are judged to be disadvantageous because they are difficult to handle.

WO 2013/017134A1 discloses an aqueous alkali metal silicate solution with a viscosity at 20° C. of 45 mPas or less, which has a solids content of 39% by weight in relation to the alkali metal silicate. The ratio between S1O2 and M2O (M2O is Na20 or K2O) is given as a weight ratio. The narrowest limits of this weight ratio are between 1.58 and 3.30. In the examples of WO2013/017134A1, a method is disclosed with which it seems possible to reduce the viscosity of water glass binders using a ball mill. However, such a method is very complex and expensive.

DE 102014118577 A1 describes a method for the layered construction of casting molds comprising refractory base molding material and a binder containing at least one aqueous alkali silicate solution and also a phosphate or a borate or both. The disclosed binding agent should be very “printable”, i.e. the nozzles of the print head should not quickly clog due to the disclosed binding agent. At the same time, the binder can be applied very finely. A clogging of the nozzles of the print head would lead to poor printing results. There are no indications as to how fluid migration of the binder on the substrate could be prevented in order to be able to better comply with the geometric specifications.

DE 102018200607 A1 describes a method for producing casting molds suitable for the production of fiber composite bodies or castings made of metal or plastic from a particulate mold base material and a multi-component binder by means of 3D printing, the particulate mold base material being pretreated with at least one silicon-organic compound, which has a polar hydrophilic end and a non-polar hydrophobic end. After a layer has been formed from the pretreated particulate molding base material, the binder or at least one component of the binder is applied in liquid form to the layer. The basic molding material and the organosilicon compound can be part of a set designed to carry out the method. The task of DE 102018200607 A1 is to minimize the fluid migration of the binder on the substrate in order to be able to better comply with the geometric specifications and to avoid the "running" of the binder as far as possible. According to the application, it is essential that the set comprises at least one organosilicon compound which has a polar hydrophilic end. Liquid components that have to be mixed with the mold base material are given as examples of this. This leads to the risk that the building material mixture clumps together - especially when fine powder additives are also used (as specified in the application) and thus leads to difficulties when building up in layers. As an alternative to the set, it is disclosed that the basic mold material can be pretreated with the organosilicon compound which has a polar hydrophilic end. However, the use of such a pretreated basic molding material appears to be an expensive affair, since the basic molding material accounts for a very high proportion of the building material mixture and the basic molding material must be wetted homogeneously with the organosilicon compound. In the case of water glass-bonded cores, the moisture stability of the casting molds produced must also be considered. Furthermore, any organic compounds lead to undesired emissions during the metal casting - therefore the use is rather undesirable and, if at all necessary, to be minimized.

object of the invention

The inventors have therefore set themselves the task of developing a method for the 3-dimensional printing of casting molds, in which water glass binding agent is selectively dosed directly via a print head onto the spread building material mixture, with the spread building material mixture containing a preferably powdered additive includes, which on the one hand minimizes the fluid migration of the dosed water glass binder and on the other hand leads to the later obtained and hardened casting mold having an extremely good moisture resistance. Summary of the Invention

This object is achieved with a method having the features of the independent claims. Advantageous further developments of the method according to the invention are the subject matter of the dependent patent claims or are described below.

The method for building up bodies in layers comprises at least the following steps: a) Provision of a refractory basic mold material and a hydrophobic metal oxide as components of a building material mixture, the hydrophobic metal oxide being hydrophobic with organosilicon compounds and the proportion of the hydrophobic metal oxide being 0.0001% by weight .% to less than 0.4% by weight, based on the refractory mold base material; b) Spreading out a thin layer of the building material mixture with a layer thickness of 0.05 mm to 3 mm, preferably 0.1 mm to 2 mm and particularly preferably 0.1 mm to 1 mm of the building material mixture, the building material mixture comprising the hydrophobic metal oxide ; c) printing selected areas of the thin layer with a binder comprising water glass and d) repeating steps b) and c) multiple times.

The body may be a core or a mold (collectively referred to herein as a mold(s)). If the building material mixture is used to create casting molds, it can also be referred to as a molding material mixture.

The hydrophobic metal oxide is used as a particulate solid and is present as a particulate solid in the building material mixture. The hydrophobic metal oxide can be used as a powder, suspension (dispersion) or gel. The hydrophobic metal oxide is evenly distributed in particulate form in the building material mixture, especially before the building material mixture is spread out in thin layers. The flydrophobized metal oxide is composed in particular of a substrate comprising the metal oxide, with the surface of the substrate being hydrophobicized with organosilicon compounds. When using the hydrophobic metal oxide as part of the building material mixture, the bodies produced with it have the following properties:

1 . Geometric specifications can be met very well

2. Good strength, especially after thermal curing

3. Very good storage stability

4. Good decay properties after metal casting

5. A minimal emission of CO2 or other organic pyrolysis products during the casting process and cooling, as only a minimal amount of organic components is used.

Surprisingly, it was found that the hydrophobicized metal oxide both minimizes the fluid migration of the applied liquid binder and significantly increases the moisture resistance of the hardened casting mold, with the absolute strengths being hardly negatively affected at the same time. Furthermore, the emission of organic pyrolysis products is reduced to a minimum, since the hydrophobic metal oxide can be dosed in very small quantities.

Furthermore, when the building material mixture is applied in layers, there is no clumping or a "blunt" building material mixture, since the good flowability or pourability of the building material mixture is retained through the use of the hydrophobic metal oxide.

Detailed description of the invention

The binder according to the invention is intended for the 3-dimensional printing of casting molds. The binder serves as a printing fluid with which a material applied in layers, such as a refractory base material (e.g. quartz sand) and, if necessary, one or more additives, collectively referred to as a building material mixture, is selectively printed. The building material mixture does not yet contain the binder. Usually, after the layer-by-layer application of the building material mixture of one or more layers (e.g. two or three), in particular one layer, a selective printing process follows - this process is repeated until the entire printing process is complete and the casting mold can be obtained. The binder can be cured in the usual ways. On the one hand, it is possible to add one or more water glass hardeners to the building material mixture applied in layers, which cause the immediate chemical hardening of the printed water glass-containing binder.

It is also possible to harden the applied water glass using acid gases such as CO2 - although this variant is less preferred.

On the other hand, thermal curing can also take place. It is possible, for example, that after the completion of one or every second or third printing process (immediately before, during or after the next layer of the building material mixture is applied), thermal curing takes place by mixing the building material mixture and binder, for example, with the help of an infra is irradiated with red light. In this layered curing, the infrared light can follow the print head in the form of a spot, for example. Of course, it is also possible to carry out this type of thermal curing in stages only after several layers have been applied. It is also possible to carry out the thermal hardening only after the last printing process has been completed - the steps "application of a layer of the building material mixture" and subsequent "printing process" alternate until the last layer has been printed, which is necessary to complete the casting mold. For this purpose, the applied and partially printed layers remain, for example, in a so-called “job box”, which can then be transferred, for example, to a microwave oven or a convection oven in order to carry out thermal curing. The thermal curing is preferably carried out by microwaves and preferably after the end of the entire printing process in the microwave oven.

Customary and known materials can be used as the refractory base material for the production of casting molds. Suitable are, for example, quartz sand, zircon sand or chrome ore sand, olivine, vermiculite, bauxite, fireclay and artificial basic molding materials such as glass beads, glass granules and/or aluminum silicate hollow microspheres, in particular more than 50% by weight quartz sand based on the refractory basic molding material. In order to keep costs low, the proportion of quartz sand in the refractory basic molding material is advantageously greater than 70% by weight, preferably greater than 80% by weight and particularly preferably greater than 90% by weight. It is not necessary to use only new sand. In order to conserve resources and avoid landfill costs, it is even advantageous to use the highest possible proportion of regenerated used sand, as can be obtained from used molds through recycling.

A refractory basic molding material is understood to mean substances that have a high melting point (melting point). The melting point of the refractory basic mold material is preferably greater than 600°C, preferably greater than 900°C, particularly preferably greater than 1200°C and particularly preferably greater than 1500°C.

The refractory basic molding material preferably makes up more than 80% by weight, in particular more than 90% by weight, particularly preferably more than 95% by weight, of the building material mixture.

A suitable refractory basic molding material, which can also be used as a component of the building material mixture, is described, for example, in WO 2008/101668 A1 (=US 2010/173767 A1). It is also possible to use regenerated materials which are obtainable by washing and then drying comminuted used forms. As a rule, the regenerated material can make up at least about 70% by weight of the refractory basic molding material, preferably at least about 80% by weight and particularly preferably more than 90% by weight.

In one embodiment of the invention, it can be advantageous for certain applications to use regenerated materials that have been obtained by purely mechanical treatment. Mechanical treatment means that at least part of the binder remaining in the used sand is removed from the sand grain by means of a grinding or impact principle. These regenerated materials can be used as required. The proportion of these regenerates can, for example, be greater than 5% by weight, preferably greater than 20% by weight, more preferably greater than 50% by weight, particularly preferably greater than 70% by weight and particularly preferably greater than 80% by weight of the refractory base molding material. Such regenerated materials are used, for example, to effect a (pre- or partial) flattening of the applied binder. The mean particle size of the refractory basic molding material is generally between 50 gm and 600 gm, preferably between 70 gm and 400 gm, preferably between 80 gm and 300 gm and particularly preferably between 100 gm and 200 gm. The particle size can be determined, for example, by sieving according to DIN 66165 Part 2. Particular preference is given to particle shapes/grains with the greatest linear expansion to the smallest linear expansion (at right angles to one another and for all spatial directions) of 1:1 to 1:5 or 1:1 to 1:3, ie those which are not fibrous, for example.

The refractory base material is free-flowing.

Surprisingly, it has now been shown that the hydrophobicized metal oxide both minimizes the fluid migration of the applied water glass binder and significantly increases the moisture resistance of the hardened casting mold, with the absolute strengths being hardly negatively influenced at the same time. Furthermore, the emission of organic pyrolysis products is reduced to a minimum, since the hydrophobic metal oxide can be dosed in very small quantities.

In the case of the hydrophobic metal oxide, the metal oxide is the substrate whose surface is provided with the organosilicon substance. The hydrophobic metal oxide is in particulate form. The metal oxide is preferably amorphous and synthetic in origin.

The metal oxide is preferably selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide or mixed oxides from this group (eg aluminum-silicon mixed oxides), in particular containing or consisting of silicon dioxide. The metal oxide is preferably composed of synthetic amorphous silicon dioxide, and the metal oxide is particularly preferably pyrogenic silica or precipitated silica. The metal oxide includes hydroxy oxides such as boehmite (AlO(OH)) and oxides or hydroxy oxides of semimetals such as silicon. The following organosilicon substances can be used to make the surface of the metal oxide and thus the inorganic substrate hydrophobic: silanes, siloxanes, silazanes. Particular preference is given to the use of C1 to C6 alkylsilazanes, poly(C1 to C6) alkylsiloxanes and C1 to C6 alkylsilanes, particular preference being given to hexamethyldisilazane, polydimethylsiloxanes and chloro(C1 to C6) alkylsilanes. In one configuration, the metal oxide, in particular amorphous silicon dioxide, is first hydrophobicized with at least one organosilicon compound. The hydrophobic metal oxide obtained in this way and the refractory base molding material are combined and then result in the building material mixture, which can also include other components.

The surface modification particularly preferably comprises alkylsiloxy groups such as C1 to C6 alkylsiloxy groups, particularly preferably trimethylsiloxy and dimethylsiloxy groups.

The organosilicon substances preferably form a covalent bond with the metal oxide by reacting OH functionalities of the inorganic substrate with the organosilicon substance.

In particular, the organosilicon compound has no substituents with a hydrophilic end, particularly in the case of physiotherapy, but possibly also in principle, particularly not desired are organosilicon compounds containing a hydroxy (—OH), ethoxy— (—CH2CH2—O -), a hydroxyate (-0-), an amino (-NH2), an ammonium (-NH4+), a carboxyl (-COOH) or a carboxylate group.

The BET surface area according to DIN EN ISO 9277 (nitrogen) of the hydrophobic metal oxide can vary over a wide range and is preferably in the range between 2 and 500 m ² /g. However, it has been shown that the specific surface area should not be too high, otherwise miscibility with the base material causes difficulties, since the material produces too much dust. The BET surface area is preferably less than 300 m ² /g, preferably less than 250 m ² /g and particularly preferably less than 220 m ² /g. The BET surface area is preferably greater than 5 m ² /g, preferably greater than 7 m ² /g.

The proportion of hydrophobic metal oxide in the building material mixture should be very low, also in order to reduce the emission of organic pyrolysis products to a minimum. Preferably, the proportion of the hydrophobic Metal oxide based on the refractory basic molding material less than 0.2% by weight and particularly preferably less than 0.1% by weight and on the other hand greater than 0.001% by weight and preferably greater than 0.005% by weight.

The loss on drying of the hydrophobicized metal oxide at the time of use in the building material mixture is typically below 10% by weight, preferably below 5% by weight, preferably below 2% by weight and particularly preferably below 1% by weight, measured according to DIN EN ISO 787- 2 (percentage of substances that are volatile at 105°C in % by weight).

The carbon content (total) of the hydrophobic metal oxide determined according to DIN ISO 10694 is preferably greater than 0% by weight to 15% by weight, preferably 0.1% by weight to 8% by weight, particularly preferably 0.25% by weight to 7% by weight .% and most preferably 0.5% to 6% by weight.

According to a preferred embodiment, the pH of the hydrophobic metal oxide according to DIN EN ISO 787-9 (measured as a 4% by weight dispersion in one part water and one part methanol (by volume)) is 3 to 11, preferably 3.5 to 10 and more preferably 4 to 9 at 25°C.

According to a preferred embodiment, the sieve residue (>40 μm) of the hydrophobicized metal oxide according to DIN EN ISO 787-18 is 0 to 2.5% by weight, preferably 0 to 1.5% by weight and particularly preferably 0 to 1% by weight.

According to one embodiment, the metal oxide content, in particular Si02 content of the hydrophobicized metal oxide according to DIN EN ISO 3262-19, is typically over 75% by weight, preferably over 80% by weight, particularly preferably over 90% by weight and very particularly preferably over 95 %.

The relative residual silanol (Si-OH) content of the hydrophobic metal oxide describes the remaining OH groups after hydrophobic treatment. This results from the number of OH groups of the hydrophilic silica used (with approx. 2 SiOH/nm ² ), which was processed using the above-mentioned hydrophobic metal oxides, and the number of OH groups of the resulting hydrophobic silica. Residual silanol contents of 5 to 75% are preferred, particularly preferably 15 to 60% and very particularly preferably 22 to 55%. The binder contains water glasses, which are produced, for example, by dissolving vitrified lithium, sodium and/or potassium silicates in water. Be preferred are water glasses that contain at least sodium, the content of which is given as Na20.

The Na 2 O/M 2 O ratio (with M=Na, K and Li in each case, this also applies below) in the binder is preferably greater than 0.4, preferably greater than 0.5 and more preferably greater than 0.6 and particularly preferably greater as 0.7, where M2O is the sum of the moles of lithium, sodium and potassium calculated as the oxide in the binder. In a preferred embodiment, M2O is Na ₂ O.

According to one embodiment, the binder has a molar modulus S1O2/M2O of greater than 1.4, preferably greater than 1.6, preferably greater than 1.8, more preferably greater than 1.9. The water glass preferably has a molar modulus of less than 2.8, preferably less than 2.6, preferably less than 2.5, particularly preferably less than 2.4.

According to one embodiment, the binder has a solids content of less than 42% by weight, preferably less than 40% by weight, preferably less than 38% by weight, particularly preferably less than 37% by weight. The remainder of the binder preferably consists of water.

According to one embodiment, the binder has a solids content of greater than 20 to less than 42% by weight, preferably greater than 24 to less than 38% by weight, preferably greater than 27 to less than 37% by weight. The solids content is determined by carefully evaporating the liquid, thus drying the binder and then heating it at 600° C. for 1 hour in an air atmosphere. The remaining oxidic material is weighed to determine the solids content.

Irrespective of this, the amount of S1O2 and M2O (calculated as mol%) in the binder is generally less than 16 mol%, preferably less than 15 mol%, preferably less than 14 mol%, particularly preferably less than 13.5 mol%. Furthermore, this amount of substance is generally greater than 7 mol%, preferably greater than 8 mol%, preferably greater than 9 mol%, particularly preferably greater than 10 mol% and in particular preferably greater than 10.5 mol%. The binder must not be too thin, but not too thick either. The dynamic viscosity is measured using a Brookfield rotational viscometer. At a temperature of 25° C., according to a preferred embodiment, the binder according to the invention has a viscosity of less than 20 mPas, preferably less than 18 mPas, preferably less than 16 mPas and particularly preferably less than 14 mPas. Also independently of this, at a temperature of 25° C., according to one embodiment, the binder has a viscosity of greater than 3 mPas, preferably greater than 5 mPas, preferably greater than 7 mPas and particularly preferably greater than 8 mPas. The viscosity is measured on a Brookfield rotational viscometer with the spindle 18 measuring geometry at a viscosity of up to 16 mPas and a speed of 200 rpm and at a viscosity of less than 16 mPas with the UL adapter spindle measuring geometry at a speed of 50 rpm

According to a preferred embodiment, the density of the binder is less than

2.5 g/cm ³ , preferably less than 2.0 g/cm ³ and particularly preferably less than

^1.5g /cm3 . Also independently of this, according to a preferred embodiment, the density is greater than 1.0 g/cm ³ , preferably greater than 1.05 g/cm ³ and particularly preferably greater than 1.1 g/cm ³ . The density is measured using the oscillating U-tube method.

According to a preferred embodiment, the surface tension of the binder is less than 60 mN/m, preferably less than 50 mN/m and particularly preferably less than 45 mN/m. Regardless of this, the surface tension of the binding agent is, according to one embodiment, greater than 15 mN/m, preferably greater than 20 mN/m and in particular greater than 25 mN/m. The surface tension is measured using the DeNoüy ring method (at 25°C).

The binder should be a clear solution and as far as possible free from coarser particles which, at their largest extent, have a size between several micrometers and several millimeters and can originate, for example, from impurities. Commercially available water glass solutions usually have these coarser particles. The particle sizes in the water glass and also in the binder are determined using dynamic light scattering in accordance with DIN/ISO 13320 (eg Horiba LA 950, Fraunhofer method). The determined Dgo value (based on the volume in each case) is used as a measure for the larger particles - it means that 90% of the particles are smaller than the specified value. In particular, the water glass according to the invention has a deodorant value (determined by dynamic light scattering) of less than 20 gm, preferably less than 10 gm and particularly preferably less than 5 gm.

Irrespective of this, the water glass according to the invention has a Dioo value of in particular less than 25 gm, preferably less than 20 gm and particularly preferably less than 10 gm in relation to the solid contained therein.

Water glasses described above or the binder containing water glasses can be obtained, for example, by suitable filtration - for example, eig nen are filters with a sieve diameter of 25 gm, preferably 10 gm and be particularly preferably 5 gm A maximum size of 1 gm preferably contains no particles at all.

In one embodiment, the binder according to the invention can have proportions of lithium ions. The molar ratio of L12O / M2O can vary over a wide range, for example between 0.01 and 0.3. The ratio is preferably in the range between 0.03 and 0.17, preferably between 0.035 and 0.16 and particularly preferably between 0.04 and 0.14.

In one embodiment, the binder according to the invention can contain proportions of potassium ions. The molar ratio of K2O / M2O can vary over a wide range, for example between 0.01 and 0.3. The ratio is preferably in the range between 0.01 and 0.17, preferably between 0.02 and 0.16 and particularly preferably between 0.03 and 0.14.

The addition of network formers other than silicate can both increase thermal stability and reduce reactivity. A network former from the group of phosphates can therefore be added to the binder and dissolved in the binder; alkali metal phosphates in particular (eg sodium hexametaphosphate or sodium polyphosphate) have proven to be positive. Among the alkali phosphates, alkali orthophosphates such as trisodium phosphate (Na 3 PO 4 ) are not preferred. Sodium polyphosphates and/or sodium metaphosphates are particularly preferred.

Other network formers that can be added to the binder as an alternative or in addition are borates, in particular alkali metal borates, e.g. disodium tetraborate decahydrate. These are also dissolved in the binder.

The amounts of alkali metals resulting from the proportions of alkali borates and/or alkali phosphates in the total amount of the binder (including diluent) are calculated as oxides and contribute to the total amount (i.e. the sum of the individual amounts) of lithium, sodium and potassium oxide in the entire aqueous solution. Consequently - according to this stipulation - the addition of alkali metal borates and/or alkali metal phosphates also influences the molar modulus S1O2/M2O.

The borates content in the binder, in particular the alkaline borates content, is calculated as B2O3. The molar ratio of B2O3/S1O2 can vary over a wide range, for example from 0 to 0.5. This ratio is preferably less than 0.3, preferably less than 0.2, particularly preferably less than 0.1, particularly preferably less than 0.08 and very particularly preferably less than 0.06. This ratio is preferably greater than or equal to 0. In a further embodiment, this ratio is preferably greater than 0.01, particularly preferably greater than 0.02. Borates within the meaning of the invention are boron compounds in oxidation stage III which are only directly bound to oxygen, i.e. oxygen atoms are the direct binding partners of the boron in the compound.

The phosphate content in the binder, in particular the alkaline phosphate content, is calculated as P2O5. The molar ratio of P2O5/S1O2 can vary over a wide range, for example from 0 to 0.5. This ratio is preferably less than 0.4, preferably less than 0.3, more preferably less than 0.25, particularly preferably less than 0.2 and very particularly preferably less than 0.15. This ratio is preferably greater than 0, preferably greater than 0.01, particularly preferably greater than 0.02. In the context of the invention, phosphates are phosphorus compounds in the oxidation state V, which are directly bound only to oxygen, ie oxygen atoms are the direct binding partners of the phosphorus in the compound.

In a further embodiment, the binder can also contain aluminum, in which case the proportion of aluminum is calculated as Al2O3. The proportion of Al2O3 is then usually less than 2% by weight, based on the total mass of the binder.

In a preferred embodiment, surface-active substances can be added to the binder in order to influence the surface tension of the binder. The proportion of these surface-active substances is generally between 0.01 and 4.0% by weight, preferably between 0.1 and 3.0% by weight.

Suitable surface-active substances in the binder are, for example, in

DE 102007051850 A1, including preferably anionic surfactants which carry a sulfate and/or sulfonate group, in particular C8 alkyl sulfates. Other suitable surface-active substances are, for example, polyacrylate salts (e.g. of sodium - for example Dispex N40 - Ciba) or silicone surfactants for aqueous systems (e.g. Byk 348, Altana). Surface-active substances based on trisioxane or glycol (e.g. polyethylene glycol) can also be used.

In a further embodiment, glycols can be added to the binder in order to make the binder somewhat more "good-natured" or easier to apply. These glycols are preferably polyethylene glycol, low molecular weight polyethylene glycol such as PEG 200 being particularly preferred. The polyethylene glycol used has an average molecular weight of less than 1000 g/mol, preferably less than 500 g/mol and particularly preferably less than 400 g/mol.

The addition of the glycol, based on the binder, is in the range from 0.01% by weight to 2% by weight, preferably from 0.1% by weight to 1% by weight and particularly preferably from 0.2% by weight. -% to 0.7% by weight. In a further embodiment, alcohols can also be added to the binder in order to make the binder easier to apply. They are preferably trihydric alcohols, with glycerol being particularly preferred. The addition of the glycol, based on the binder, is in the range from 0.01% by weight to 2% by weight, preferably from 0.1% by weight to 1% by weight and particularly preferably from 0.2% by weight. % to 0.7% by weight.

Depending on the application and the desired strength level, preferably between 0.5% by weight and 7% by weight of the binder based on water glass are used, preferably between 0.75% by weight and 6% by weight, particularly preferably between 1% by weight and 5 0.0% by weight and particularly preferably between 1% by weight and 4.0% by weight, based in each case on the basic molding material. The information relates to the total amount of the water glass binder, including the (particularly aqueous) solvent or diluent and the (possible) solid material content (total = 100% by weight).

In a preferred embodiment, the building material mixture can contain a proportion of particulate amorphous silicon dioxide in order to increase the strength level of the casting molds. An increase in the strength of the casting moulds, in particular the increase in the tear strength, can be advantageous in the automated production process. Synthetically produced amorphous silica is particularly preferred.

The mean particle size (including any agglomerates) of the amorphous silicon dioxide is preferably less than 300 μm, preferably less than 200 μm, particularly preferably less than 100 μm. The sieve residue of the particulate amorphous S1O2 when passing through a 125 μm sieve (120 mesh) is preferably not more than 10% by weight, more preferably not more than 5% by weight and most preferably not more than 2% by weight. . Irrespective of this, the sieve residue on a sieve with a mesh size of 63 μm is less than 10% by weight, preferably less than 8% 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. The particulate amorphous silicon dioxide preferably used according to the present invention has a water content of less than 15% by weight, in particular less than 5% by weight and particularly preferably less than 1% by weight (drying to constant mass at 105° C.).

The particulate amorphous S1O2 is used as a powder (including dust).

Both synthetically produced and naturally occurring silicic acids can be used as amorphous S1O2. The latter are known, for example, from DE 102007045649, but are not preferred since they usually contain not inconsiderable crystalline fractions and are therefore classified as carcinogenic. By synthetic is meant non-naturally occurring amorphous S1O2, i.e. H. the production of which includes a deliberately carried out chemical reaction, as initiated by a human being, e.g. the production of silica sols by ion exchange processes from alkali silicate solutions, precipitation from alkali silicate solutions, flame hydrolysis of silicon tetrachloride, reduction of quartz sand with coke in an electric arc furnace in the Production of ferrosilicon and silicon. The amorphous S1O2 produced by the last two methods is also referred to as pyrogenic S1O2.

Occasionally, synthetic amorphous silicon dioxide is understood to mean only precipitated silica (CAS No. 112926-00-8) and S1O2 produced by flame hydrolysis (pyrogenic silica, fumed silica, CAS No. 112945-52-5), while that used in ferrosilicon or silicon production is simply referred to as amorphous silicon dioxide (Silica Fume, Microsilica, CAS No. 69012-64-12). For the purposes of the present invention, the product formed during the production of ferrosilicon or silicon is also understood to be amorphous SiO2.

Preference is given to using precipitated silicas and pyrogenic silicon dioxide, ie silicon dioxide produced by flame hydrolysis or by arcing. Particular preference is given to using amorphous silicon dioxide produced by thermal decomposition of ZrSiO4 (described in DE 102012020509) and S1O2 produced by oxidation of metallic Si using an oxygen-containing gas (described in DE 102012020510). Quartz glass powder (mainly amorphous silicon dioxide), which was produced from crystalline quartz by melting and rapid cooling again, is also preferred, so that the particles are spherical and not splintered (described in DE 102012020511). The mean primary particle size of the particulate amorphous silicon dioxide can be between 0.05 μm and 10 μm, in particular between 0.1 μm and 5 μm, particularly preferably between 0.1 μm and 2 μm. The primary particle size can be determined, for example, with the aid of dynamic light scattering (eg Horiba LA 950) and checked by means of scanning electron micrographs (REM images with, for example, Nova NanoSEIVI 230 from FEI). Furthermore, with the help of the REM images, details of the primary particle shape down to the order of 0.01 pm could be made visible. For the SEM measurements, the silicon dioxide samples were dispersed in distilled water and then placed on an aluminum holder covered with copper tape before the water was evaporated.

Furthermore, the specific surface area of the particulate amorphous silicon dioxide was determined using gas adsorption measurements (BET method, nitrogen) in accordance with DIN 66131. The specific surface area of the particulate amorphous S1O2 is between 1 and 200 m ² /g, in particular between 1 and 50 m ² /g, particularly preferably between 1 and 30 m ² /g. If necessary, the products can also be mixed, eg in order to obtain mixtures with specific particle size distributions.

Depending on the type of production and producer, the purity of the amorphous S1O2 can vary greatly. Types with a silicon dioxide content of at least 85% by weight, preferably at least 90% by weight and particularly preferably at least 95% by weight, have proven to be suitable. Depending on the application and the desired level of strength, between 0.1% by weight and 2% by weight of the particulate amorphous S1O2 are used, preferably between 0.1% by weight and 1.8% by weight, particularly preferably between 0.1% by weight % and 1.5% by weight, based in each case on the refractory base molding material.

Based on the total weight of the binder (including diluent or solvent), the amorphous S1O2 is preferably present in a proportion of 1 to 80% by weight, preferably 2 to 60% by weight, particularly preferably 3 to 55% by weight and particularly preferably between 4 to 50% by weight. Or independently of this, preferred, based on the ratio of solids content of the water-glass-based binder (based on the oxides, ie total mass of alkali metal oxides M2O and silicon dioxide) to amorphous S1O2 of 10:1 to 1:1.2 (parts by weight). The amorphous S1O2 is added to the refractory material or the building material mixture before the binder is added. The method according to the invention is therefore further characterized by one or more of the following features when using amorphous S1O2:

(a) The amorphous silicon dioxide is only added to the building material mixture.

(b) The amorphous silicon dioxide has a BET surface area between 1 and 200 m ² /g, preferably greater than or equal to 1 m ² /g and less than or equal to 30 m ² /g, particularly preferably less than or equal to 15 m ² /g .

(c) The amorphous silicon dioxide is selected from the group consisting of: precipitated silicon dioxide, pyrogenic silicon dioxide produced by flame hydrolysis or by electric arc, amorphous silicon dioxide produced by thermal decomposition of ZrSiO4, silicon dioxide produced by oxidation of metallic silicon using an oxygen-containing gas, quartz glass powder with spherical particles prepared by melting and rapidly recooling crystalline quartz, and mixtures thereof, and is preferably amorphous silica prepared by thermal decomposition of ZrSiO 4 .

(d) The amorphous silicon dioxide is preferably used in amounts of 0.1 to 2% by weight, particularly preferably 0.1 to 1.5% by weight, based in each case on the refractory base molding material.

(e) The amorphous silicon dioxide has a water content of less than 5% by weight and particularly preferably less than 1% by weight.

(f) The amorphous silicon dioxide is particulate amorphous silicon dioxide, preferably with an average primary particle diameter determined by dynamic light scattering 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 pm.

In a further embodiment, an inorganic hardener for binders based on water glass is optionally added to the building material mixture before the binder is added. Such inorganic hardeners are, for example, phosphates such as Li thopix P26 (an aluminum phosphate from Zschimmer und Schwarz GmbH & Co KG Chemische Fabriken) or Fabutit 748 (an aluminum phosphate from Chemische Fabrik Budenheim KG). Other inorganic hardeners for binders based on water glass are, for example, calcium silicates and their hydrates, calcium aluminates and their hydrates, aluminum sulfate, magnesium and calcium carbonate. The ratio of hardener to binder can vary depending on the desired property, eg processing time and/or stripping time of the building material 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 contained in the solvent) 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, in each case 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.

Irrespective of this, between 0.05% by weight and 2% by weight of the inorganic hardener are used, preferably between 0.1% by weight and 1% by weight, and particularly preferably between 0.1% by weight and 0.6% by weight .%, in each case based on the mold base material.

As soon as the strength permits, the unbound building material mixture can then be removed from the mold and the mold can be used for further treatment, e.g. preparation for metal casting. The unbound building material mixture can be removed from the bound building material mixture by means of an outlet, for example, so that the unbound building material mixture can trickle out.

The bound building material mixture (casting mold) can be freed from residues of the unbound building material mixture, for example using compressed air or by brushing.

The unbound building material mixture can be reused for a new printing process.

The printing takes place, for example, with a print head having a large number of nozzles, the nozzles preferably being individually selectively controllable. According to a further embodiment, the print head is moved under the control of a computer at least in one plane and the nozzles apply the liquid binding agent in layers. The print head can be, for example, a drop-on-demand print head with bubble jet or, preferably, piezo technology. EXAMPLES

The invention is explained below on the basis of test examples, but without being restricted to these.

Unless otherwise indicated, all ratios and percentages are by weight.

Example 1: Examination of dimensional accuracy

In order to evaluate the dimensional accuracy of components when building three-dimensional bodies, a basic molding material mixture consisting of sand GS 14 (mean grain diameter 0.14 mm) with an additional 0.8% by weight of a powdered additive (amorphous S1O2) (primary molding material mixture 1 ) homogenized in a paddle mixer.

Molding material mixture 1 was further mixed with additives to prepare the molding material mixtures 2 to 6. The following molding material mixtures were produced in a blade mixer:

Molding material mixture 2: Molding material mixture 1 + 0.05% by weight of additive 1 (FIDK Fl 30 from Wacker, dimethylsiloxy-substituted on the surface)

Molding material mixture 3: Molding material mixture 1 + 0.1% by weight of additive 2 (FIDK Fl 13 L from Wacker, dimethylsiloxy substituted on the surface) Molding material mixture 4: Molding material mixture 1 + 0.05% by weight of additive 3 (FIDK Fl 2000 der Wacker company, trimethylsiloxy substituted on the surface) Molding material mixture 5: Molding material mixture 1 + 0.02% by weight of additive 4 (surface covered with silicone oil AP 100 from Wacker, comparison)

Molding material mixture 6: Molding material mixture 1 + 0.02% by weight of additive 5 (surface coating with polyether-modified trisiloxane according to DE 102018200607 A1, comparison)

Molding material mixture 7: Molding material mixture 1 + 0.1% by weight of additive 6 (SIPERNAT D10/1 from EVONIK, polydimethylsiloxy-substituted on the surface)

The specimens were produced on a commercial printing system (VX 200 from Voxeljet AG). An alkaline silicate solution (water glass with a viscosity of about 11 mPas at 25° C., molar modulus S1O2/M2O of 2.0, solids content of 29% by weight, from ASK Chemicals GmbH) was used as the binder in the printing process. The amount of binder was at all Experiments fixed at 3.5 parts by weight based on 100 parts by weight Formgrundstoffmi research. Bending bars with the dimensions (22.36 mm x 22.36 mm x 170.00 mm) were produced as test specimens. After printing, the specimens were cured in a microwave for 4.5 minutes at a power of 1000 watts.

To assess the dimensional accuracy, the width and height of the bending bars produced were measured and compared with the original dimensions of the print file. The evaluation was carried out by averaging 6 specimens per molding material mixture. The dimensional deviations in width and height for each molding compound are shown in Table 1.

Table 1: Dimensional deviations of different molding material mixtures with 3.5% binder INOTEC EP 5061 after curing in the microwave (V = comparison)

Average dev Average dev

Molding mix in width [mm] in height [mm]

Molding material mixture 1 (V) 1.34 1.36 Molding material mixture 2 0.69 0.78 Molding material mixture 3 0.44 0.43 Molding material mixture 4 0.37 0.28 Molding material mixture 5 (V) kAkA Molding material mixture 6 (V) 0.23 0 .20 Molding mix 7 0.18 0.06

No values for the dimensional deviation could be recorded for molding material mixture 5. No test specimens could be obtained during processing on the printing system, since insufficient layer bonding of the individual layers prevented the production of test specimens. The tests show that the use of the additives according to the invention results in significantly lower dimensional deviations in the test specimens. Thus, the fluid migration in the printing process is significantly reduced by the additives according to the invention, which enables the production of components with high dimensional accuracy. Example 2: Investigation of moisture stability

In a further test series, bending bars were produced analogously to example 1. The strength of each tested molding material mixture (molding material mixtures 1 to 4 as well as 6 and 7) was measured on two bending bars 1 hour after opening, 24 hours after storage under room conditions and 24 hours after storage in a controlled climate of 30 °C and 49 % relative humidity determined.

The strengths obtained are shown in Table 2.

Table 2: Strengths of different molding material mixtures with 3.5% binder INOTEC EP 5061 after curing in the microwave after 1 hour, 24 hours under room conditions and 24 hours at 30 °C and 49% relative humidity (V = comparison)

Strength Strength after 24

Strength after 24 1 h after h storage at 30 h storage at

Molding material mixture production °C and 49 % relative room conditions tive humidity gen [N/cm ² ] [N/cm ² ] [N/cm ² ]

Molding mix 1 (V) 422 409 285 (-33%) Molding mix 2 407 424 380 (-7%) Molding mix 3 377 395 358 (-5%) Molding mix 4 392 394 370 (-6%) Molding mix 5 (V) kAkAkA Molding mix 6 (V) 425 408 336 (-21%) Molding mix 7 431 420 366 (-15%) Molding mixture 1 shows a slight drop in strength when stored under room conditions, but a drastic drop in strength when stored at 30° C. and 49% relative humidity.

The molding material mixture 2 according to the invention has a similar initial strength and a significantly higher strength after storage at 30° C. and 49% relative humidity. The molding material mixtures 2 to 4 according to the invention exhibit analogous behavior, albeit with slightly reduced initial strengths. Similar to the determination of the dimensional accuracy, no strength could be determined for molding material mixture 5. The reason for this is insufficient layer bonding when producing the samples on the commercial VX 200 printing system. Molding material mixture 6 shows high strength 1 hour after sample production, but a greater relative strength drop compared to molding material mixtures 2 to 4 (79 % remaining strength for molding material mixture 6, on average 94% for molding material mixtures 2 to 4 according to the invention. Molding mixture 7 shows high strength after 1 h and a moderate drop in strength after storage at 30° C. and 49% relative humidity for 24 h.

Thus, through the use of the additives according to the invention, a significantly improved storage stability, in particular under more severe climatic conditions, can be achieved than without their use or with the use of a formulation according to DE 102018200607 A1. This enables the user to store manufactured components even under tougher external conditions without having to accept the loss of strength that has occurred up until now. In addition, the molding material mixtures 2 to 4 according to the invention with an organic carbon content (without graphite) of at most 0.00015% (addition of up to 0.1% with a maximum carbon content of 15%) have a very low emission potential, in particular compared to Molding material mixture 6 according to DE 102018200607 A1 with a carbon content of 0.01% (addition to the molding material mixture from 0.01% with a carbon content of 100%). This enables a drastic reduction in emissions when using the molding mixtures according to the invention.

Claims

patent claims

1 . Method for building up bodies in layers, comprising at least the following steps: a) providing a refractory basic molding material and a hydrophobic metal oxide as components of a building material mixture, the hydrophobic metal oxide being hydrophobic with organosilicon compounds and the proportion of the hydrophobic metal oxide being 0.0001% by weight % to less than 0.4% by weight, based on the refractory mold base material; b) Spreading out a thin layer of the building material mixture with a layer thickness of 0.05 mm to 3 mm, preferably 0.1 mm to 2 mm and particularly preferably 0.1 mm to 1 mm of the building material mixture, the building material mixture comprising the hydrophobic metal oxide ; c) printing selected areas of the thin layer with a binder comprising water glass, and d) repeating steps b) and c) multiple times.

2. The method according to claim 1, wherein the hydrophobicized metal oxide is made up of a substrate comprising the metal oxide, wherein the surface of the substrate is hydrophobicized with the organosilicon compound.

3. The method according to claim 1 or 2, wherein the metal oxide of the hydrophobic metal oxide selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide or mixed oxides from this group and in particular amorphous silicon dioxide is or comprises.

4. The method according to at least one of the preceding claims, wherein the sili ziumorganische compound chemically reacts with OFI groups on the surface of the Metalloxi.

5. The method according to at least one of the preceding claims, wherein the silicon-organic compound is selected from silanes, siloxanes, silazanes, in particular C1- to C6-alkylsilazanes, C1- to C6-alkylsilanes and more preferably flexamethyldisilazane or a chlorine(C1- to C6- ) is alkylsilane.

6. The method according to at least one of the preceding claims, wherein the hydrophobic metal oxide on the surface has C1 to C6 alkylsiloxy groups, particularly preferably trimethylsiloxy and/or dimethylsiloxy groups.

7. The method according to at least one of the preceding claims, wherein the organosili ziumorganic compound has no substituent with a hydrophilic end.

8. The method according to at least one of the preceding claims, wherein the hydrophobic metal oxide is admixed as a powder, slurry or gel of the building material mixture, in particular powder, before the building material mixture is spread out in layers.

9. The method according to at least one of the preceding claims, wherein the metal oxide or the substrate of the hydrophobicized metal oxide comprises or consists of synthetic amorphous silicon dioxide, in particular comprises or consists of pyrogenic silica or precipitated silica.

10. The method according to at least one of the preceding claims, wherein the hydrophobicized metal oxide is characterized by one or more of the following features: a) it has a BET surface area of 2 to 500 m ² /g, preferably greater than 5 m ² /g to less than 300 m ² /g and more preferably from greater than 7 m ² /g to less than 220 m ² /g; b) the carbon content of the hydrophobicized metal oxide is greater than 0% by weight to 15% by weight, preferably 0.1% by weight to 8% by weight, particularly preferably 0.25% by weight to 7% by weight and very particularly preferably 0 .5% to 6% by weight. c) the pH of the hydrophobicized metal oxide is 3 to 11, preferably 3.5 to 10 and particularly preferably 4 to 9; d) the metal oxide content, in particular the SiO 2 content, of the hydrophobicized metal oxide is more than 75% by weight, preferably more than 80% by weight, particularly preferably more than 90% by weight and very particularly preferably more than 95% by weight; e) the relative residual silanol (Si—OH) content of the hydrophobicized metal oxide is from 5 to 75%, particularly preferably from 15 to 60% and very particularly preferably from 22 to 55%.

11. The method according to at least one of the preceding claims, wherein the hydrophobicized metal oxide is greater than 0.001% by weight to less than 0.2% by weight and particularly preferably greater than 0.005% by weight to less than 0.1% by weight, based on the refractory base molding material. % is used.

12. The method according to at least one of the preceding claims further comprising the step of curing the printed areas, in particular by increasing the temperature, preferably effected by microwaves and/or infrared light.

13. The method according to at least one of the preceding claims further comprising the following steps: i) curing of the body after completion of the layered structure in an oven or by means of a microwave and ii) subsequent removal of the unbound building material mixture from the at least partially cured printed selected areas.

14. The method according to at least one of the preceding claims, wherein in each case also independently of one another: a) the refractory basic molding material comprises quartz sand, zircon sand or chrome ore sand, olivin, vermiculite, bauxite, fireclay, glass beads, glass granules, aluminum silicate hollow microspheres and mixtures thereof and preferably more as 50% by weight of quartz sand based on the refractory mold base material; and/or b) the building material mixture comprises more than 80% by weight, preferably more than 90% by weight, and particularly preferably more than 95% by weight, of refractory basic molding material; and/or c) the refractory basic molding material has an average particle size of 50 μm to 600 μm, preferably between 80 μm and 300 μm, determined by sieve analysis.

15. The method according to at least one of the preceding claims, wherein the water glass, including solvent A / diluent in an amount between 0.5 wt.% And 7 wt.%, Preferably between 0.75 wt.% And 6 wt.% And be particularly preferably between 1 wt.% And 5.0 wt.%, Based on the mold base material is used.

16. The method according to at least one of the preceding claims, wherein the printing takes place with a print head having a plurality of nozzles; where at a) the print head is preferably movable at least in one plane controlled by a computer and the nozzles apply the liquid binder in layers; and/or b) the print head is preferably a drop-on-demand print head with bubble jet or piezo technology.

17. The method according to at least one of the preceding claims, wherein the building material mixture also contains particulate amorphous silicon dioxide, preferably between 0.1% by weight and 2% by weight and particularly preferably between 0.1% by weight and 1.5% by weight. , in each case based on the refractory molding base material and preferably with a particle size of less than 300 μm, preferably less than 200 μm, particularly preferably less than 100 μm.

18. The method according to at least one of the preceding claims, wherein the binder is characterized by one or more of the following features:

the water glass has a molar modulus S1O2/M2O of greater than 1.4 to less than 2.8, preferably greater than 1.6 to less than 2.6, preferably greater than 1.8 to less than 2.5 and more preferably greater than 1.9 and less than 2.4, where M2O is the sum of the molar amounts of lithium, sodium and potassium, each calculated as the oxide;

- the binder has a dynamic viscosity at a temperature of in each case at 25° C. of less than 20 mPas, preferably less than 14 mPas;

any particulate components in the binder have a D90 value of less than 20 μm, preferably less than 10 μm and particularly preferably less than 5 μm;

- the binder contains at least one phosphate or at least one borate or one phosphate and one borate.

19. The method according to at least one of the preceding claims, wherein the binder has a solids content of greater than 20 to less than 42% by weight, preferably greater than 24 to less than 38% by weight, preferably greater than 27 to less than 37% by weight.

20. The method according to at least one of the preceding claims, wherein approximately ige particles in the binder

- have a D90 value of less than 20 pm, preferably less than 10 pm, particularly preferably less than 5 pm and/or - have a Dioo value of less than 25 pm, preferably less than 15 pm, preferably less than 10 pm having.

21. The method according to at least one of the preceding claims, wherein the binder also contains surface-active substances, preferably surfactants, in particular between 0.01 and 4.0% by weight, preferably between 0.1 and 3.0% by weight.

22 mold or core producible according to at least one of claims 1 to 21 for metal casting, in particular iron, steel, copper or aluminum casting.