WO2013050022A2 - Coating compositions for inorganic casting molds and cores, containing salts, and use thereof - Google Patents

Abstract

The subject matter of the invention relates to sizing compositions as coating compositions for casting molds. The sizing composition is suitable for cores and molds, in particular those produced using water glass as a binder. The sizing composition contains certain salts.

Description

 COATINGS FOR INORGANIC CASTORS AND CEREALS CONTAINING SALT AND ITS USE

description

The invention relates to a sizing composition as a coating composition for casting molds. The sizing composition is suitable for cores and molds, especially those made using water glass as a binder. The sizing composition contains certain salts. State of the art

 Molds are available from a refractory material, such as quartz sand, to form into a casting mold and set by a suitable binder to ensure sufficient mechanical strength of the mold. For the production of casting molds, a fireproof molding base material and a suitable binder are thus used. The refractory molding base material is preferably present in a free-flowing form, so that it can be filled into a suitable mold and compacted there. The binder produces a firm cohesion between the particles of the molding base material, so that the casting mold obtains the required mechanical stability.

For the production of casting molds both organic and inorganic binders can be used, the curing of which can be carried out in each case by cold or hot processes. Cold processes are processes which are carried out essentially at room temperature without heating the casting mold. The curing is usually carried out by a chemical reaction, which is triggered for example by the fact that a gas is passed as a catalyst through the mold to be cured. In hot processes, the molding compound mixture is heated to a sufficiently high temperature after molding to expel, for example, the solvent contained in the binder or to initiate a chemical reaction by which the binder is cured, for example, by crosslinking.

Regardless of the curing mechanism, all organic systems have in common that they thermally decompose when filling the liquid metal into the mold, releasing pollutants such as benzene, toluene, xylenes, phenol, formaldehyde, and other partially unidentified cracking products. Although various measures have succeeded in minimizing these emissions, they can not be completely avoided with organic binders. In order to minimize or avoid the emission of decomposition products during the casting process, it is possible to use binders which are based on inorganic materials or contain a very low proportion of organic compounds. Such binder systems have been known for some time.

Binder systems have been developed which can be cured by the introduction of gases. Such a system is described for example in GB 782205, in which an alkali water glass is used as a binder, which can be cured by the introduction of CO2. In US 6972059 B1 an exothermic feeder composition is described which contains an alkali silicate as a binder. Furthermore, binder systems have been developed which are self-curing at room temperature. Such, based on phosphoric acid and metal oxides system is described for example in US 5582232 A. Finally, inorganic binder systems are known which are cured at higher temperatures, for example in a hot mold. Such hot-curing binder systems are known, for example, from US Pat. No. 5,474,606 A, in which a binder system consisting of alkali water glass and aluminum silicate is described. In US 7022178 B1 a binder system for molding sands for the production of cores is described. The waterglass-based binder system consists of an aqueous alkali silicate solution and a hygroscopic base, such as sodium hydroxide, added in a ratio of 1: 4 to 1: 6. The water glass has a modulus SiO ₂ / M ₂ O of 2.5 to 3.5 and a solids content of 20 to 40%. In order to obtain a free-flowing molding material mixture, which can also be filled into complicated core molds, and to control the hygroscopic properties, the binder system also contains a surface-active substance, such as silicone oil, which has a boiling point> 250 ° C. The binder system is mixed with a suitable refractory material, such as quartz sand, and can then be shot with a core shooter into a core box. The hardening of the molding material mixture takes place by removal of the water or condensation still contained. The drying or hardening of the casting mold can also take place under the action of microwaves. However, inorganic binders also have disadvantages compared to organic binders, for example the known inorganic binders have a low stability of the casting molds produced therewith against high humidity or against water. For a storage of the molded body over a longer period, as usual with organic binders, not secured possible. In order to obtain higher initial strengths, a better resistance of the casting mold to atmospheric moisture and a better result on the casting surface during casting, US Pat. No. 7,770,629 B2 proposes a molding material mixture which contains a water-glass-based binder in addition to a refractory molding base material. A proportion of a particulate metal oxide is added to the molding material mixture. Precipitated silica or fumed silica is preferably used as the particulate metal oxide.

The methods described for the production of molds and cores usually also include the application of a refractory mold coating, which is also called sizing, at least on those surfaces of the basic shape, which come into contact with the cast metal. The purpose of the mold coatings is to influence the molding surface, to improve the casting appearance, to influence the casting metallurgically and / or to avoid casting defects.

The commonly used sizing agents contain as base materials e.g. Clays, quartz, kieselguhr, cristobalite, tridymite, aluminum silicate, zirconium silicate, mica, chamotte and also coke and graphite. These bases are the purportive portion of the sizings which cover the mold surface and close the pores against the penetration of the cast metal.

By means of these coatings, therefore, the surface of the casting mold can be modified and matched to the properties of the metal to be processed. Thus, the size may improve the appearance of the casting by creating a smooth surface because the size compensates for irregularities caused by the size of the grains of the molding material. Further, the sizing may metallurgically influence the casting by, for example, selectively transferring additives to the casting at the surface of the casting via the sizing which enhance the surface properties of the casting. Further, the sizings form a layer which chemically isolates the casting mold from the liquid metal during casting. This prevents any adhesion between the casting and the casting mold so that the casting can be easily removed from the casting mold. However, the sizing can also be used to specifically control the heat transfer between the liquid metal and the casting mold in order, for example, to effect the formation of a specific metal structure by the cooling rate.

The curing of the inorganic binder used is nowadays increasingly via a condensation reaction initiated by elevated temperatures, in which the formation of the binder bridges takes place via the splitting off of water. As with many chemical reactions, this is a reversible reaction, i. by contact and reaction with water, the bonds can be cleaved again, the extent of this back-reaction is highly dependent on the process parameters for core production. Among the process parameters typically used in series production (fast cycle times, high temperatures), the casting molds lose their strength through contact with water and sometimes also through contact with alcohol, the surface softens and the casting mold loses its shape. task

 The invention had the object of proposing a sizing, by which a defect-free coating, especially inorganic cores and molds, can be ensured without negatively affecting the stability of the cores or molds and thus the processing and storage.

Summary of the invention

This object is achieved with a size composition having the features of patent claim 1, advantageous embodiments form the subject of the dependent claims or are described below. The sizing composition according to the invention is provided according to a preferred embodiment in the form of a paste or a suspension. Also in this embodiment, the sizing composition contains a carrier liquid.

Surprisingly, it has been found that the addition of certain salts in a certain concentration range to an aqueous size composition, the quality of the sized inorganic cores and molds can be sustainably improved and, for example, a shelf life of the cores and forms of several days can be achieved easily.

Detailed description of the invention

 It has surprisingly been found that salts of the metals magnesium and / or manganese can be used to achieve the above object. The manganese salts are preferably used in the oxidation state +2 or +4. Particular preference is given to using magnesium salts in the oxidation state +2 and the manganese salts in the oxidation state +2. The concentration of the salts is greater than 1% by weight, preferably greater than 3% by weight and particularly preferably greater than 5% by weight, based on the size composition. According to one embodiment, the concentration is limited to the top by the saturation concentration of the amount used and type of carrier liquid. According to another embodiment, the maximum concentration is less than 10% by weight, based on the size composition. Sulfate ion and / or particularly preferably the monovalent chloride ion are preferably used as counterions as anions.

Depending on the desired use of the sizing composition, e.g. as the basecoat or topcoat, and the desired layer thickness of the coating made from the sizing composition, further characteristic parameters of the sizing composition may be adjusted.

As a further additive component of the sizing composition, it is possible to use esters of formic acid (methanoic acid), the chain length of the alcohol or alcohol mixture used in the esterification in particular being on average less than 6 and particularly preferably less than 3 carbon atoms. Methyl formate (methyl formate) and ethyl formate (ethyl formate) are particularly preferably used. It is also possible for the alcohol group (s) or some of the alcohol groups to carry one or two further groups, such as ether, hydroxy groups, ester groups or carboxyl groups, or the formic acid through the second or third hydroxyl group is crosslinked, eg by condensation.

The total content of the above additives, based on the size composition, is from 1 to 8% by weight, preferably from 2 to 8% by weight and more preferably from 3 to 6% by weight. As an additive, e.g. It has the CAS number 107-31 -3.

The sizing may e.g. include the combination of certain clays as ingredients of the shallows. As clay materials was a combination of

 a) 1 to 10 parts by weight, in particular 1 to 5 parts by weight of palygorskite, b) 1 to 10 parts by weight, in particular 1 to 5 parts by weight, hectorite and c) 1 to 20 parts by weight, in particular 1 to 5 parts by weight,

 sodium bentonite

(in each case relative to one another), in particular in a weight ratio Palygorskit to hectorite of 1 to 0.8 to 1, 2 and a ratio of Palygorskit to hectorite common to sodium bentonite of 1 to 0.8 to 1.2. The total clay content of the size of the above clays is 0.1 to 4.0% by weight, preferably 0.5 to 3.0% by weight and more preferably 1.0 to 2.0% by weight, based on the solids content of size composition. By using these clays, one can further increase the storage stability of the cores and molds.

The carrier liquid can be proportionately or completely formed by water. The carrier liquid is the constituent which is vaporizable at 160 ° C and atmospheric pressure and in this sense, by definition, that which is not solids content. The carrier liquid contains greater than 50% by weight, preferably 75% by weight, in particular greater than 80% by weight, if appropriate greater than 95% by weight of water.

The other ingredients in the carrier liquid may be organic solvents. Suitable solvents are alcohols, including polyhydric alcohols and polyether alcohols. Exemplary alcohols are ethanol, n-propanol, isopropanol, butanol and glycol. The solids content of the ready-to-use size composition is preferably adjusted in the range from 10 to 85% by weight, or in the form of sale (before dilution), in particular from 30 to 70% by weight. Thus, the sizing composition according to the invention comprises at least one powdered refractory material. This refractory material serves to close the pores in a mold against the penetration of the liquid metal. Next is achieved by the refractory thermal insulation between the mold and liquid metal. As refractory material, conventional refractory materials can be used in metal casting. Examples of suitable refractory materials are quartz, aluminum oxide, zirconium oxide, aluminum silicates, such as pyropyllite, kyanite, andalusite or chamotte, zirconium, ciconsilicates, olivine, talc, mica, coke, feldspar, diatomite, calcined kaolins, kaolinite, metakaolinite, iron oxide and / or bauxite , The refractory material is provided in powder form. The grain size is chosen so that a stable structure is produced in the coating and that the sizing can preferably be distributed without problem on the wall of the casting mold with a spray device. Suitably, the refractory material has an average particle size (measured by means of light scattering according to DIN / ISO 13320) in the range from 0.1 to 500 μm, in particular preferably in the range from 1 to 200 μm. In particular materials are suitable as a refractory material which have a melting point which is at least 200 ° C. above the temperature of the liquid metal and, irrespective of this, do not react with the metal. The fraction of the refractory material (in each case contributing only to the solids content), based on the solids content of the sizing composition, is preferably greater than 70% by weight, preferably greater than 80% by weight, particularly preferably greater than 85% by weight. According to one embodiment (in each case contributing only to the solids content), the fraction of the refractory material is chosen to be less than 70% by weight, according to a further embodiment less than 60% by weight and according to a further embodiment less than 50% by weight. According to one embodiment, the size according to the invention may comprise at least one setting agent. The adjusting agent causes an increase in the viscosity of the size, so that the solid components of the size in the suspension do not or only to a small extent decrease. To increase the viscosity, both organic and inorganic materials or mixtures of these materials can be used. Suitable inorganic adjusting agents are, for example, strongly swellable clays, such as sodium bentonite.

As an adjusting agent, organic thickening agents may alternatively or additionally be selected, since these can be dried to the extent that they hardly give off any more water upon contact with the liquid metal after application of the protective coating. Suitable organic adjusting agents are, for example, swellable polymers, such as carboxymethyl, methyl, ethyl, hydroxyethyl and hydroxypropylcellulose, mucilages, polyvinyl alcohols, polyvinylpyrrolidone, pectin, gelatin, agar agar, polypeptides and / or alginates. The proportion of the adjusting agent, based on the total size composition, is preferably selected to be from 0.1 to 5% by weight, preferably from 0.5 to 3% by weight, particularly preferably from 1 to 2% by weight.

According to a preferred embodiment, the size according to the invention comprises as further constituent at least one binder. The binder allows a better fixation of the size or of the protective coating produced from the size on the wall of the mold. In addition, the binder increases the mechanical stability of the protective coating so that less erosion is observed under the action of the liquid metal. Preferably, the binder cures irreversibly, so that an abrasion-resistant coating is obtained. Particular preference is given to binders which do not soften on contact with atmospheric moisture. All binders which are used in sizing can be contained per se. In this case, both inorganic and organic binders can be used. For example, clays, in particular bentonite and / or kaolin, can be used as binders.

The proportion of the binder is preferably selected in the range of 0.1 to 20 wt .-%, particularly preferably 0.5 to 5 wt .-%, based on the solids content of the sizing composition. According to another preferred embodiment, the size contains a proportion of graphite. This supports the formation of lamellar carbon at the interface between casting and mold. The proportion of graphite is preferably selected in the range of 1 to 30 wt .-%, particularly preferably 5 to 15 wt .-%, based on the solids content of the size composition. Graphite has a favorable effect on the surface quality of castings during iron casting.

If appropriate, the size composition according to the invention may also contain further components customary for sizing, for example wetting agents, defoamers, pigments, dyes or biocides. The proportion of these further constituents in the ready-to-use coating composition is preferably less than 10% by weight, preferably less than 5% by weight and more preferably less than 1% by weight. Suitable wetting agents are, for example, anionic and non-anionic surfactants, in particular those having an HSB value of at least 7. An example of such a wetting agent is disodium dioctyl sulfosuccinate. The wetting agent is preferably used in an amount of 0.01 to 1 wt .-%, preferably 0.05 to 0.3 wt .-%, based on the ready-to-use sizing composition.

Defoamers, or anti-foaming agents, can be used to prevent foaming in the preparation of the sizing composition or in applying it. Foaming on application of the sizing composition can result in uneven layer thickness and soldering in the coating. As defoamers, for example, silicone or mineral oil can be used. Preferably, the defoamer in an amount of 0.01 to 1 wt .-%, preferably from 0.05 to 0.3 wt .-%, based on the ready-to-use sizing composition. In the sizing composition of the present invention, if necessary, usual pigments and dyes may be used. These are added in order to achieve a different contrast, for example between different sizes, or to bring about a greater separation effect of the size of the casting. Examples of pigments are red and yellow iron oxide and graphite. Examples of color materials are commercially available dyes such as the Luconyl ^® color range of BASF AG, Ludwigshafen, Germany. The dyes and pigments are preferably contained in an amount of 0.01 to 10 wt .-%, preferably from 0.1 to 5 wt .-%, based on the solids content of the sizing composition. In another embodiment, the sizing composition contains a biocide to prevent bacterial attack, thereby avoiding a negative impact on the rheology and binding power of the binding agents. This is particularly preferred if the carrier liquid present in the sizing composition is formed substantially in relation to the weight of water, ie the sizing composition according to the invention is provided in the form of a so-called water sizing. Examples of suitable biocides are formaldehyde, 2-methyl-4-isothiazolin-3-one (MIT), 5-chloro-2-methyl-4-iosthiazolin-3-one (CIT) and 1, 2-benzisothiazolin-3-one (BIT). Preferably, MIT, BIT or a mixture thereof are used. The biocides are usually used in an amount of from 10 to 1000 ppm, preferably from 50 to 500 ppm, based on the weight of the ready-to-use sizing composition.

In addition to the constituents already mentioned, the size composition according to the invention may contain further constituents customary for sizes.

The size composition of the invention can be prepared by conventional methods. For example, a sizing composition according to the present invention can be prepared by initially charging water and breaking up a clay acting as a sizing agent using a high shear stirrer. Subsequently, the refractory components, pigments and dyes and the metallic additive are stirred until a homogeneous mixture is formed. Finally, wetting agents, antifoams, biocides and binders are added.

The sizing composition according to the invention can be prepared and sold as ready-to-use sizing. However, the size according to the invention can also be prepared and sold in concentrated form. In this case, to provide a ready-to-use size, the amount of carrier liquid necessary to adjust the desired viscosity and density of the size is added. In addition, the sizing composition according to the invention can also be provided and sold in the form of a kit, wherein, for example, the solid components and the solvent component are present side by side in separate containers. The solid components can be provided as a powdery solid mixture in a separate container. Optionally, further liquid components to be used, such as, for example, binders, wetting agents, wetting agents / defoamers, pigments, dyes and biocides, can again be present in a separate container in this kit. The solvent component can either comprise the optionally additionally used components, for example in a common container, or it can be present in a separate container separately from further optional components. To prepare a ready-to-use size, the appropriate amounts of the solid component, the optional further components and the solvent component are mixed together.

The sizing compositions according to the invention are suitable for coating casting molds. As used herein, the term "mold" includes all types of bodies necessary to make a casting, such as cores, molds and molds. The use according to the invention of the size compositions also comprises a partial coating of casting molds. The sizings are used for metal working molds obtainable from inorganic molding material mixtures comprising at least one refractory molding base, a water glass based binder, and preferably a portion of a metal oxide selected from the group of silica, in particular amorphous silica, alumina , Titanium oxide or zinc oxide and mixtures thereof, which is preferably present particulate and in particular particle sizes of less than 300 μιτι (sieve analysis). Amorphous silica is available, for example, via precipitation processes starting from water glass, which is obtainable by digesting quartz sand with sodium carbonate or potassium carbonate. SiO _{2 produced in this way is} called precipitated silica depending on the process conditions. Another important production variant is the production of so-called pyrogenic SiO ₂ in a blast gas flame, starting from liquid chlorosilanes such as silicon tetrachloride. The molding material mixtures and a process for producing casting molds for metal processing using the cured molding material mixtures are described in WO 2006/024540 (= US Pat. No. 7,770,629 B2) and the publication is hereby incorporated by reference into the content of this application. Preferred molding material mixtures are the subject of the claims of WO 2006/024540. Coating molds thus typically include

a) greater than 80% by weight of a refractory base molding material (including additives which behave like refractory molding materials), b) 0.01 to 5 wt.% Tempered water glass as a binder

 c) 0 to 5% by weight, if appropriate, of the above metal oxide or metal oxides.

The invention further relates to a process for the production of sized casting molds for metal processing by applying the sizes to the above partially or fully cured inorganic molding material mixtures. The application or provision of the sizes can be carried out as follows:

When dipping as an application method, the mold, in the mold cavity optionally a primer coating has been applied, immersed for about 2 seconds to 2 minutes in a container which is filled with a ready-to-use, inventive sizing composition. The mold is then removed from the sizing composition and excess sizing composition drained from the mold. The time taken to drain the excess sizing composition after dipping depends on the flow behavior of the sizing composition used.

When spraying as application method commercially available pressure sprayers are used. Here, the sizing composition is filled in a dilute state in a pressure vessel. The sizing can be pressed into a spray gun via the overpressure to be set, where it is sprayed with the aid of separately adjustable atomizing air. When spraying, the conditions are preferably chosen so that the pressure for size composition and atomizing air is adjusted to the gun such that the sprayed size composition still wet on the mold or the core, but gives a uniform application.

In the case of flooding as an application method, the casting mold, in the mold cavity of which optionally a base coat has been applied, is poured over with the aid of a hose, a lance or similar instruments with a ready-to-use size composition according to the invention. The mold is completely covered with the sizing composition, the excess sizing composition drains from the mold. The time taken for the flow of excess sizing composition after flooding depends on the flow behavior of the sizing composition used.

In addition, the sizing can also be applied by brushing. The carrier liquid contained in the size is then evaporated so that a dry sizing layer is obtained. As the drying method, any conventional drying method can be used, such as air drying, dehumidified air drying, microwave or infrared radiation drying, convection oven drying, and similar methods.

In a preferred embodiment of the invention, the coated casting mold is dried at 20 to 250 ° C, preferably at 50 to 180 ° C, in a convection oven.

When using alcohol sizes, the size composition according to the invention is preferably dried by burning off the alcohol or alcohol mixture. In this case, the coated casting mold is additionally heated by the heat of combustion. In a further preferred embodiment, the coated casting mold is dried in the air without further treatment or using microwaves.

The size can be applied in the form of a single layer or in the form of several layers arranged one above the other. The individual layers may be the same or different in their composition. For example, at first a base coat can be produced from a commercial size which does not contain a metallic additive according to the invention. As a primer coating, for example, water-based or alcohol-based sizing can be used. However, it is also possible to prepare all layers from the size composition according to the invention. However, the layer which later comes into contact with the liquid metal is always produced from the sizing according to the invention. When applying several layers, each individual layer can be completely or partially dried after application.

The coating prepared from the sizing composition preferably has a dry film thickness of at least 0.1 mm, preferably at least 0.2 mm, more preferably at least 0.45 mm, most preferably at least 0.55 mm. According to one embodiment, the thickness of the coating is chosen to be less than 1.5 mm. The dry layer thickness here is the layer thickness of the dried coating, which was obtained by drying the sizing composition by substantially complete removal of the solvent component and optionally subsequent curing. The dry layer thickness of the base coat and the top coat are preferably determined by measurement with the wet film thickness comb. If necessary, the casting mold can then be completely assembled. The casting is preferably carried out for the production of iron and steel castings.

Example 1 :

 In this example, the influence of salts in sizings on the strengths of the sized cores was examined. The core sizes 1 to 3 used have the compositions given in Table 1 to 3.

Table 1: Composition of the sizes

The mold casting size was prepared as follows: Water is introduced and the salts are dissolved therein. The clay is then disrupted by using a high shear stirrer for at least 15 minutes. Subsequently, the refractory components, pigments and dyes are stirred for at least 15 minutes until a homogeneous mixture is formed. Finally, additives such as wetting agents, defoamers and preservatives and the binder are stirred.

The sizes were adjusted to a viscosity suitable for the application in the range of 0.6 Pas for the following tests. The adjustment was carried out by adding appropriate amounts of water to the original composition and subsequent homogenization. The main parameters are the viscosity at 20 ° C, measured with a Brookfield viscometer (DIN EN ISO 2555) and with a DIN 4 mm flow cup (DIN EN ISO 2431).

Table 2: Application parameters

The manganese chloride contained in the size 1 or the magnesium sulfate contained in size 2, when used on inorganic cores and molds, significantly reduces the dissolution or softening of the inorganic binder in the surface. Thus, a higher stability of the sized cores compared to conventional sizes can be determined.

 The effect is based on the fact that the water contained in the sizing on contact with the cured inorganic binder must absorb the ions formed during the dissolution process. However, if the water already contains an unusually high ion content, the solubility product increases and the uptake of further ions is hindered or prevented. Thus, the solubilized salts in the sizing contribute to the prevention of destabilization of the inorganic cores and molds during the sizing process.

A comparable behavior can also be achieved with chemically related salts of the metals magnesium and manganese. Production and Testing of the Formstoffprüfkörper

Two different core geometries were tested for experimental purposes. One geometry, the so-called Georg Fischer test strip, illustrates the behavior of the sized test specimens on thicker core geometries, the other geometry, so-called long cores, illustrates the behavior of the sized test specimens on thin geometries. Georg Fischer test bars are cuboid test bars measuring 150 mm x 22.36 mm x 22.36 mm. The long cores have dimensions of 13 mm x 20 mm x 235 mm.

The composition of the molding material mixture is given in Table 3. The Georg Fischer test bars were prepared as follows: The components listed in Table 3 were mixed in a laboratory paddle mixer (Vogel & Schemmann AG, Hagen, DE). For this purpose, the quartz sand was initially introduced and the water glass was added with stirring. As a water glass, a sodium water glass was used, which had proportions of potassium. The modulus Si02: M20 of the water glass was about 2.2, where M is the sum of sodium and potassium. After stirring the mixture for one minute, amorphous silica was added, if necessary, with further stirring. The amorphous silica is pyrogenic silica from RW silicon. The mixture was then stirred for an additional 1 minute.

The molding material mixture was transferred to the storage bunker of an H 2.5 hot-box core shooter from Röperwerk-Gießereimaschinen GmbH, Viersen, DE, whose mold had been heated to 180 ° C. The molding material mixture was introduced into the mold by means of compressed air (5 bar) and remained in the mold for a further 35 seconds. To accelerate the curing of the mixture, hot air (2 bar, 150 ° C on entering the mold) was passed through the mold during the last 20 seconds. The mold tool was opened and the test bars removed.

The coating compositions were applied to the test bars by dipping, the application parameters are listed in Table 2. The test bars were coated either immediately after removal from the mold or after a 30 minute cooling time. The coated test bars were stored after application of the coating in a drying oven for 30 minutes at 150 ° C. To determine the bending strengths, the test bars were placed in a Georg Fischer strength tester equipped with a 3-point bending device (DISA Industrie AG, Schaffhausen, CH), and the force was measured, which leads to the breakage of the test bars.

The flexural strengths of the uncoated test bars were measured according to the following scheme:

 - 10 seconds after removal (hot strength)

- 1 hour after removal (cold strength).

The test bars were coated according to the following scheme:

- 1 minute after removal coated (hot coating)

 - coated 30 minutes after removal (cold coating) The flexural strengths of the coated test bars were measured according to the following scheme

 - 10 seconds after removal from the oven

 (Hot strengths, hot-coated)

 - Coated for 30 minutes after collection

 (Cold strengths, hot-coated, directly after removal of the furnace)

 - 10 seconds after removal from the oven

 (Hot strength, cold-coated, room temperature 20 ° C)

 - Coated for 30 minutes after collection

 (Cold strength, cold coated)

Table 3: Composition of the molding material mixtures

Quartz sand AlkaliAmorphic coating

 H32 silica glass silica

 1 100 GT 2.0 0.5 - Comparison, not according to the invention

2 100 GT 2.0 0.5 sizing 1 according to the invention

3 100 GT 2.0 0.5 sizing 2 according to the invention

4 100 GT 2.0 0.5 sizing 3 Not according to the invention Table 4: Strengths on Georg Fischer test bars

The use of an alcoholic coating is undesirable because of the disadvantage of emissions generation during the casting process. Using standardized water-based coating compositions, the test specimens of water-glass-containing molding material lose much strength (size 4). Surprisingly, it was found in the search for ways to suppress the reaction between the water from the coating with the binder of the molding material that the addition of certain salts in a certain concentration range to an aqueous sizing composition, the quality of the sized inorganic cores and molds can be sustainably improved, and For example, a storage stability of the cores and forms of several days can be achieved easily. For this purpose, salts of the metals magnesium or manganese can be used. Preferably, the manganese salts are used with a valency of +4. Particularly preferred are magnesium salts having a valency of +2 and the manganese salts having a valency of +2.

Sulfate ion is preferably used as anions, particularly preferably the monovalent chloride ion is used as anions.

The concentration of the salts is in particular between 1% and the respective saturation concentration, preferably between 3% and the respective saturation concentration, particularly preferably between 5% and the respective saturation concentration.

Claims

claims 1 . Containing a sizing composition (A) salts of the metals magnesium and / or manganese in a concentration of, if appropriate together, greater than 1% by weight, based on the size composition,  (B) a carrier liquid containing or consisting of water, and  (C) refractories. 2. sizing composition according to claim 1, characterized in that sulfate ions (s) and / or chloride ions are used as anions of the salts. 3. sizing composition according to claim 1 or 2, characterized marked that magnesium in the oxidation state +2 or +4 and / or manganese in the Oxidation level +2 is / are used. 4. sizing composition according to one or more of the preceding claims, wherein the concentration of the salts is greater than 3 wt .-% and in particular greater than 5% by weight, based on the sizing composition. 5. sizing composition according to one or more of the preceding claims, wherein the concentration of the salts corresponds at most to the saturation concentration of the carrier liquid used. 6. The sizing composition according to one or more of claims 1 to 4, wherein the concentration of the salts is less than 10 wt .-%, based on the sizing composition. 7. sizing composition according to one or more of the preceding claims, wherein the carrier liquid to greater than 50 wt.% Water and further preferably contains alcohols, including polyalcohols and polyether alcohols and in particular at up to 160 ° C and 1013 mbar is completely vaporizable. A sizing composition according to one or more of the preceding claims, wherein the solids content of the sizing composition is from 30 to 70% by weight. A sizing composition according to one or more of the preceding claims, wherein the sizing composition contains from 10 to 85% by weight of refractories, based on the solids content of the sizing composition. 10. A sizing composition according to one or more of the preceding claims, wherein the refractories quartz, alumina, zirconium dioxide, aluminum silicates, zircon sands, cicosilicates, olivine, talc, mica, coke, feldspar, diatomite, calcined kaolins, kaolinite, metakaolinite, iron oxide, bauxite and / or mixtures thereof. 11. sizing composition according to one or more of the preceding claims, wherein the refractory particles particle sizes of 0.1 to 500 μητι, in particular 1 to 200 μητι, measured by means of light scattering according to DIN / ISO 13320, have. 12. Sizing composition according to one or more of the preceding claims, wherein the sizing composition 0.1 to 20 wt .-%, in particular 0.5 to 5 wt .-%, at least one binder, based on the solids content of the sizing composition. A sizing composition according to one or more of the preceding claims, wherein the sizing composition comprises a lustrous carbon former or Graphite comprises, in particular to 0.1 to 10 wt .-%, in particular 0.5 to 3 wt .-%. 14. Use of the size composition according to one or more of the preceding claims for coating molding material mixtures which have been cured by water glass as a binder, optionally after dilution with at least water. 15. Use according to claim 14, wherein the molding material mixtures comprise silica, alumina, titania or zinc oxide and / or mixtures thereof and in particular amorphous silica.