EP2104580A1

EP2104580A1 - Moulding material mixture containing carbohydrates

Info

Publication number: EP2104580A1
Application number: EP07819173A
Authority: EP
Inventors: Jens Müller; Diether Koch; Marcus Frohn; Jörg KÖRSCHGEN; Stefan Schreckenberg
Original assignee: Ashland Suedchemie Kernfest GmbH
Current assignee: ASK Chemicals GmbH
Priority date: 2006-10-19
Filing date: 2007-10-19
Publication date: 2009-09-30
Anticipated expiration: 2027-10-19
Also published as: AU2007312540B2; BRPI0718281A2; WO2008046651A1; JP2010506730A; EP2104580B2; ES2593078T5; ES2593078T3; BRPI0718281B1; EP2104580B1; CA2666760C; KR20090076979A; US20100224756A1; EA015239B1; AU2007312540A1; JP5170813B2; PL2104580T3; PL2104580T5; CA2666760A1; EA200970391A1; HUE029506T2

Abstract

The invention relates to a moulding material mixture for producing casting moulds for machining metal, to a method for producing casting moulds, to casting moulds obtained according to said method and to the use thereof. A fire-resistant moulding base material and a binding agent based on water glass are used in the production of casting moulds. A fraction of the particulate metal oxide is added to the binding agent, said metal oxide being selected from the group comprising silicon dioxide, aluminium oxide, titanium oxide and zinc oxide. Synthetic amorphous silicon dioxide is preferably used. Said moulding material mixture contains a carbohydrate as an additional component. The mechanical resistance of casting moulds and the quality of the surface of the casting piece can be improved by adding carbohydrates.

Description

CARBOHYDRATHALT I GE MIXTURE OF FORM

DESCRIPTION

The invention relates to a molding material mixture for the production of casting molds for metal processing, which comprises at least one pourable refractory molding base, a water glass based binder, and a proportion of a particulate metal oxide, which is selected from the group of silica, alumina, titania and zinc oxide, includes. Furthermore, the invention relates to a process for the production of casting molds for metal processing using the molding material mixture as well as a casting mold obtained by the process.

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

Casting molds are made of a refractory material, such as quartz sand, whose grains are connected after molding of the mold by a suitable binder to ensure sufficient mechanical strength of the mold. For the production of molds so you use a refractory molding material, which has been treated with a suitable binder. 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.

Molds must meet different requirements. During the casting process itself, they must first of all have sufficient stability and temperature resistance in order to receive the liquid metal in the mold formed from one or more casting molds. After the start of the solidification process, the mechanical stability of the mold is ensured by a solidified metal layer, which forms along the walls of the mold. The material of the casting mold then has to be under the influence of the votes from the metal heat in the manner decompose that it loses its mechanical strength, so ^"en ^~ of the ^~ ^'lt ^~" related to Albert zwrsch ^~ various ^~ Pa ^~ rt ^~ i ^~ keϊn ^{~ "} d ^" it is ^" molten material" repealed. This is achieved, for example, by decomposing the binder under heat. After cooling, the solidified casting is shaken, in the - -

Ideally, the material of the molds again decomposes into a fine sand, which can pour out of the cavities of the metal mold.

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 material 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.

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

An example of the production of molds using organic binders is the so-called Ashland cold box process. It is a two-component system. The first component consists of the solution of a polyol, usually a phenolic resin. The second component is the solution -in-nes-Pθ-lyi-soya-n-tes-ϊ -So-according to the US 3, 4-0-9> -S-7-9-A the - Reacted two components of the polyurethane binder by a gaseous tertiary amine is passed through the mixture of molding material and binder after shaping. The curing reaction of polyurethane binders is - -

to a polyaddition, i. a reaction without cleavage of by-products, e.g. Water. Other advantages of this cold-box process include good productivity, dimensional accuracy of the molds, and good engineering properties such as the strength of the molds, the processing time of the mixture of mold base and binder, etc.

Hot-curing organic processes include the hot-box process based on phenolic or furan resins, the warm box process based on furan resins, and the croning process based on phenolic novolac resins. In the hot-box and warm-box processes, liquid resins are processed with a latent curing agent which is only effective at elevated temperatures to form a molding material mixture. In the croning process mold base materials, such as quartz, chrome ore, zirconium, etc., are coated at a temperature of about 100 to 160 ⁰ C with a liquid at this temperature phenol novolac resin. Hexamethylenetetramine is added as a reaction partner for the subsequent curing. In the above-mentioned hot-curing technologies shaping and curing takes place in heated tools, which are heated to a temperature of up to 300 ⁰ C.

Regardless of the curing mechanism, all organic systems have in common that they thermally decompose when the liquid metal is poured into the mold, releasing pollutants such as benzene, toluene, xylenes, phenol, formaldehyde and higher, sometimes unidentified cracking products. Although various measures have succeeded in minimizing these emissions, they can not be completely avoided with organic binders. ^{^"Organic"} even with inorganic-s ^"Chren ^~ ^Hybrid" systemen7 ^"" the, ^~ as ^{~ "di} ^~ e ^~ eg ^* b ^~ e ^~ irrr ^~ Re ^~ ^s" σl ^{~~ ""} -CC> ₂ - ^{~ "} Method used binders containing a proportion of organic compounds, such unwanted emissions occur when casting the metals. In order to avoid the emission of decomposition products during the casting process, it is necessary to use binders which are based on inorganic materials or contain at most a very small proportion of organic compounds. Such binder systems have been known for some time. Binder systems have been developed which can be cured by the introduction of gases. Such a system is described for example in GB 782 205, in which an alkali water glass is used as a binder, which can be cured by introduction of CO ₂ . DE 199 25 167 describes an exothermic feeder composition which contains an alkali metal silicate as binder. Furthermore, binder systems have been developed which are self-curing at room temperature. Such a system based on phosphoric acid and metal oxides is described, for example, in US Pat. No. 5,582,232. Finally, inorganic binder systems are known which are cured at higher temperatures, for example in a hot tool. Such hot-curing binder systems are known, for example, from US Pat. No. 5,474,606, in which a binder system consisting of alkali water glass and aluminum silicate is described.

However, inorganic binders also have disadvantages compared to organic binders. For example, the casting molds made with water glass as a binder have a relatively low strength. This results in particular in the removal of the mold from the tool to problems because the mold can break. Good strength at this time is particularly important for the production of complicated, thin-walled moldings and their safe handling. The reason for the low strength is primarily that the molds still contain residual water from the binder. Longer dwell times in the hot, closed tool only help to a limited degree because the steam does not escape sufficiently - -

can. In order to achieve the most complete possible drying of the casting molds, it is proposed in WO 98/06522 to leave the molding material mixture in a tempered core box only after shaping, so that a dimensionally stable and stable edge shell is formed. After opening the core box, the mold is removed and then completely dried under the action of microwaves. However, the additional drying is complex, prolongs the production time of the molds and contributes, not least by the energy costs, significantly to the increase in the cost of the manufacturing process.

Another weak point of the previously known inorganic binders is the low stability of the casting molds produced therewith against high humidity. For a storage of the molded body over a longer period, as usual with organic binders, not secured possible.

Molds made with water glass as a binder often show poor disintegration after metal casting. In particular, when the water glass has been cured by treatment with carbon dioxide, the binder can be vitrified under the influence of the hot metal, so that the mold is very hard and can be removed only with great effort from the casting. Attempts have therefore been made to add organic components to the molding material mixture which burn under the influence of the hot metal and, as a result of pore formation, facilitate disintegration of the casting mold after casting.

In DE 2 059 538 core and molding sand mixtures are described which contain sodium silicate as a binder. To obtain improved disintegration of the mold after casting, glucose syrup is added to the mixture. The molding sand mixture processed into a casting mold is set by passing carbon dioxide gas through it. The molding sand mixture contains 1 to 3 wt .-% glucose syrup, 2 to 7 wt .-% of an alkali metal silicate and a sufficient amount of a core or molding sand. In the examples it has been found that forms and nuclei containing glucose syrup have much better disintegration properties than forms and nuclei containing sucrose or pure dextrose.

EP 0 150 745 A2 describes a binder mixture for solidifying molding sand, which consists of an alkali metal silicate, preferably sodium silicate, a polyhydric alcohol and further additives, wherein the additives provided are modified carbohydrates, non-hygroscopic starch, a metal oxide and a filler. As the modified carbohydrate, a non-hygroscopic starch hydrolyzate having a reducing power of 6 to 15% is used, which can be added as a powder. The non-hygroscopic starch and the metal oxide, preferably iron oxide, are added in an amount of 0.25 to 1% by weight of the amount of sand. Possibly. may be added to the binder mixture, a lubricant in powder form or as an oil. The binder mixture is preferably cured by the use of CO ₂ or a chemical catalyst.

GB 847,477 describes a binder composition for the production of casting molds comprising an alkali metal silicate having a modulus SiO ₂ / M ₂ O of 2.0 to 3.22 and a polyhydroxy compound. The binder is mixed with a refractory base molding material for the production of molds and cured after forming the mold by gassing with carbon dioxide. As polyhydroxy compounds, for example, mono-, di-, tri- or tetrasaccharides are used, wherein provided no great demands on the purity of this compound ^{^"duhgeri" ^""} ^'. ^'""

In GB 902,199 a molding material mixture for the production of molds is described, which in addition to a refractory molding material comprises a binder composition, which - -

a mixture of 100 parts of a cereal-derived glue, 2 to 20 parts of sugar and 2 to 20 parts of a halogen acid or a salt of a halogen acid. A suitable salt is, for example, ammonium chloride. The glue is made by partially hydrolyzing starch. For making a mold, the molding mixture is first brought into the desired shape and then heated to a temperature of at least 175 - 180 ⁰ C heated.

In GB 1 240 877 a molding material mixture for the production of molds is described, which comprises a water-containing binder in addition to a refractory molding material, which in addition to an alkali metal silicate compatible with the alkali metal silicate oxidizing agent and, based on the solution, 9 to 40 wt .-% a readily oxidizable organic material. As the oxidizing agent, for example, nitrates, chromates, dichromates, permanganates or chlorates of the alkali metals can be used. For example, starch, dextrins, cellulose, hydrocarbons, synthetic polymers, such as polyethers or polystyrene, and hydrocarbons, such as tar, can be used as easily oxidisable material. The molding material mixture can be cured by heating or by gassing with carbon dioxide.

No. 4,162,238 describes a molding material mixture for the production of casting molds which, in addition to a refractory molding base material, comprises a binder based on an alkali metal silicate, in particular water glass. The binder amorphous silica is added in a proportion corresponding to the solution of the binder 2 to 75%. The amor- ^{^"phe" ^"Si"} ri ^~ zϊümä ^{^"iöxi"} d ^{^"indicates"} a ^{^"par"} ti ^~ kelg ^{^"r"} OLS ^~ e ^'~ in the ^"V ^ott' etwä to 2 to 500 nm. Further, the binder comprises a modulus SiC> ₂ : M ₂ O of 3.5 to 10, where M is an alkali metal. - -

Because of the above-discussed problem of harmful emissions occurring during casting, efforts are being made to replace the organic binders with inorganic binders in the production of casting molds, even with complicated geometries. However, even with complicated casting molds, sufficient strength of the casting mold must be ensured even in thin-walled sections, both immediately after production during removal from the tool and during metal casting. The strength of the mold should not deteriorate significantly during storage. The mold must therefore have sufficient stability against atmospheric moisture. Further, after production, the casting should not require excessive finishing of the surface. The post-processing of castings requires a great deal of time, labor and material, and therefore represents a significant cost factor in the production. Already immediately after removal from the mold, the casting should therefore already have a high surface quality.

It is an object of the present invention to provide a molding material mixture for the production of metal working molds which comprises at least one refractory molding base material and a water glass based binder system, the molding material mixture containing a proportion of a particulate metal oxide which is selected from the group of silica, alumina, titania and zinc oxide, which allows the production of casting molds with complex geometry and which may include, for example, thin-walled sections, after casting the casting obtained should already have a high surface quality

This object is achieved with a molding material mixture having the features of patent claim 1. Advantageous developments of - -

Molding material mixture according to the invention are the subject of the dependent claims.

Surprisingly, it has been found that casting molds based on inorganic binders can be prepared by the addition of carbohydrates to the molding material mixture, which have high strength both immediately after production and during prolonged storage. Further, after the casting of the metal, a casting having a very high surface quality is obtained, so that after the removal of the casting mold, only a slight finishing of the surface of the casting is required. This is a significant advantage because it can significantly reduce the cost of producing a casting in this way. In the case of casting, compared to other organic additives such as acrylic resins, polystyrene, polyvinyl esters or polyalkyl compounds, significantly less smoke is observed, so that the workload for the employees there can be significantly reduced.

The molding material mixture according to the invention for the production of casting molds for metalworking comprises at least:

a refractory molding base; a water glass based binder; and a proportion of a particulate metal oxide selected from the group consisting of silica, alumina, titania and zinc oxide.

According to the invention, the molding material mixture contains a carbohydrate as further constituent.

As a refractory molding base material can be used for the production of molds usual materials. The refractory base molding material must have sufficient dimensional stability at the temperatures prevailing during metal casting. A - -

suitable refractory molding material is therefore characterized by a high melting point. The melting point of the refractory mold raw material is preferably higher than 700 ° C, preferably higher than 800 ⁰ C, particularly preferably higher than 900 ⁰ C and most preferably higher than 1000 ⁰ C. As refractory mold raw materials silica sand or zircon example, are suitable. Furthermore, fibrous refractory mold bases are suitable, such as chamotte fibers. Other suitable refractory mold bases are, for example, olivine, chrome ore sand, vermiculite.

Next artificial refractory mold raw materials, glass beads, glass granules or known under the name "Cerabeads ^®" or "Carboaccucast ^®" spherical ceramic mold raw materials can be used as refractory mold raw materials such as aluminum silicate hollow spheres (microspheres called.). These artificial refractory mold bases are synthetically manufactured or, for example, fall as waste in industrial processes. These . spherical ceramic mold base materials contain as minerals, for example mullite, corundum, ß-cristobalite in different proportions. They contain as essential proportions alumina and silica. Typical compositions contain, for example, Al ₂ O ₃ and SiO ₂ in approximately equal proportions. In addition, other constituents may be present in proportions of <10%, such as TiO ₂ , Fe ₂ O ₃ . The diameter of the spherical refractory mold bases is preferably less than 1000 microns, especially less than 600 microns. Are also suitable refractory mold raw materials synthetically produced, such as mullite (x AI2O ₃ ^• y SiO _2, where x = 2 to 3, y = 1 to 2; ideal formula: Al ₂ SiO _5). These synthetic mold raw materials ^"will not go back to a natural source and can also a special molding process have been subjected to, such as in the production of aluminosilicates katmikrohohlkugeln, glass beads or spherical ceramic Mold raw materials. Aluminum silicate microbubbles, for example, result from the combustion of fossil fuels or other combustible materials and are separated from the ashes produced during combustion. Hollow microspheres as an artificial refractory base molding material are characterized by a low specific weight. This is due to the structure of these artificial refractory mold bases which comprise gas-filled pores. These pores can be open or closed. Preference is given to using closed-cell artificial refractory molding base materials. When using open-pored artificial refractory mold raw materials, a part of the water glass-based binder is absorbed in the pores and can then develop no binding effect.

According to one embodiment, glass materials are used as artificial molding bases. These are used in particular either as glass beads or as glass granules. Conventional glasses can be used as the glass, with glasses showing a high melting point being preferred. Suitable examples are glass beads and / or glass granules, which is made of glass breakage. Also suitable are borate glasses. The composition of such glasses is exemplified in the table below.

- 1 -

Table: Composition of glasses

ΛII alkaline earth metal, - e.g. Mg, Ca, Ba

M ¹ : alkali metal, eg Na, K

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

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

Preferably, only part of the refractory base molding material is formed by glass materials. The proportion of the glass material on the refractory molding base material is preferably less than 35 wt .-%, more preferably less than 25 wt .-%, particularly preferably less than 15 wt .-% selected.

In casting experiments with aluminum, it has been found that, when using artificial mold raw materials, especially glass beads, glass granules or glass microspheres, less molding sand adheres to the metal surface after casting than when using pure quartz sand. The use of such artificial mold bases based on glass materials therefore allows - -

the production of smooth cast surfaces, wherein a complex post-treatment by blasting is not required or at least to a much lesser extent.

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

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

For cost reasons, the proportion of artificial refractory mold raw materials is kept low. Preferably, the proportion of the artificial refractory mold raw materials in the refractory molding base material less than 80 wt .-%, preferably less than 75 wt .-%, more preferably less than 65 wt .-%.

As a further component, the molding material mixture according to the invention comprises a water glass-based binder. Conventional water glasses can be used as the water glass, as they are already used as binders in molding material mixtures. These water glasses contain dissolved sodium or potassium silicates and can be prepared by dissolving glassy potassium and sodium silicates in water. The water glass preferably has a modulus of SiO ₂ / M ₂ θ in the range of 1.6 to 4.0, in particular 2.0 to 3.5, wherein M is sodium and / or potassium. The water glasses preferably have a solids content in the range of 30 to 60 wt .-%. The solids content refers to the amount of SiO ₂ and M ₂ O contained in the water glass.

Furthermore, the molding material mixture contains a proportion of a particulate metal oxide, which is selected from the group of silica, alumina, titania and zinc oxide. The average primary particle size of the particulate metal oxide may be between 0.10 μm and 1 μm. However, because of the agglomeration of the primary particles, the particle size of the metal oxides is preferably less than 300 μm, preferably less than 200 μm, more preferably less than 100 μm. It is preferably in the range of 5 to 90 .mu.m, more preferably 10 to 80 .mu.m, and most preferably in the range of 15 to 50 microns. The particle size can be determined, for example, by sieve analysis. Particularly preferably, the sieve residue on a sieve with a mesh width of 63 μm is less than 10% by weight, preferably less than 8% by weight.

Particularly preferred as the particulate metal oxide silica is used, in which case synthetically produced amorphous silica is particularly preferred.

As the particulate silica, precipitated silica and / or fumed silica is preferably used. Precipitated silica is obtained by reaction of an aqueous alkali metal silicate solution with mineral acids. The resulting precipitate is then separated, dried and ground. Fumed silicas are understood to mean silicic acids which are obtained by coagulation from the gas phase at high temperatures. The production of fumed silica, for example, by flame hydrolysis of silicon tetrachloride or - -

in the electric arc furnace by reduction of silica sand with coke or anthracite to silicon monoxide gas followed by oxidation to silica. The pyrogenic silicas produced by the arc furnace process may still contain carbon. Precipitated silica and fumed silica are equally well suited for the molding material mixture according to the invention. These silicas are hereinafter referred to as "synthetic amorphous silica".

The inventors believe that the strong alkaline water glass can react with the silanol groups located on the surface of the synthetic amorphous silica and that upon evaporation of the water, an intense bond between the silica and the then solid water glass is produced.

As essential further component, the molding material mixture according to the invention contains a carbohydrate. Both mono- or disaccharides and relatively high molecular weight oligosaccharides or polysaccharides can be used. The carbohydrates can be used both as a single compound and as a mixture of different carbohydrates. The purity of the carbohydrates used are not excessive requirements. It is sufficient if the carbohydrates, based on the dry weight, in a purity of more than 80 wt .-%, more preferably more than 90 wt .-%, more preferably more than 95 wt .-%, in each case based on the dry weight. The monosaccharide units of the carbohydrates can be linked as desired. The carbohydrates preferably have a linear structure, for example an α- or β-glycosidic 1,4-linkage. The carbohydrates may also be wholly or partially 1, 6-linked, such as. For example, the amylopectin, which has up to 6% α-1, 6 bonds. The amount of carbohydrate is preferably chosen to be relatively low. In itself, the aim is to keep the proportion of organic components in the molding material mixture as low as possible so that the smoke development caused by the thermal decomposition of these organic compounds is suppressed as far as possible. Therefore, relatively small amounts of carbohydrate are added to the molding material mixture, wherein a significant improvement in the strength of the casting molds before casting or a significant improvement in the quality of the surface of the casting can already be observed. Preferably, the proportion of the carbohydrate, based on the refractory molding material, greater than 0.01 wt .-%, preferably greater than 0.02 wt .-%, more preferably greater than 0.05 wt .-% selected. A high proportion of carbohydrate causes no further improvement in the strength of the casting mold or the surface quality of the casting. The amount of carbohydrate, based on the refractory molding base material, is preferably less than 5% by weight, preferably less than 2.5% by weight, more preferably less than 0.5% by weight, particularly preferably less than 0, 4 wt .-% selected. For a technical application, low levels of carbohydrates in the range of more than 0.1 wt .-% lead to significant effects. For industrial application, the proportion of carbohydrate in the molding material mixture, based on the refractory molding material, preferably in the range of 0.1 to 0.5 wt .-%, preferably 0.2 to 0.4 wt .-%. At levels of more than 0.5% by weight of carbohydrate, no significant improvement in properties is achieved, so amounts of more than 0.5% by weight of carbohydrate are not required per se.

According to another embodiment of the invention, the carbohydrate is used in underivatized form. Such carbohydrates can be favorably obtained from natural sources, such as plants, for example, cereals or potatoes. The molecular weight of such derived from natural sources - -

Carbohydrates can be lowered for example by chemical or enzymatic hydrolysis, for example to improve the solubility in water. In addition to underivatized carbohydrates, which are thus composed only of carbon, oxygen and hydrogen, but also derivatized carbohydrates can be used, in which, for example, a part or all hydroxy groups with e.g. Alkyl groups are etherified. Suitable derivatized carbohydrates are, for example, ethylcellulose or carboxymethylcellulose.

In itself, even low molecular weight hydrocarbons, such as mono- or disaccharides can be used. Examples are glucose or sucrose. However, the advantageous effects are observed in particular when using oligosaccharides or polysaccharides. It is therefore particularly preferred to use an oligosaccharide or polysaccharide as the carbohydrate.

In this case, it is preferred that the oligosaccharide or polysaccharide have a molecular weight in the range from 1000 to 100,000 g / mol, preferably 2,000 and 30,000 g / mol. In particular, when the carbohydrate has a molecular weight in the range of 5,000 to 20,000 g / mol, a significant increase in the strength of the mold is observed, so that the mold can be easily removed from the mold during manufacture and transported. Even with prolonged storage, the mold shows a very good strength, so that even for a series production of castings required storage of the molds, even over several days in the event of access of humidity, readily possible. Also, the stability upon exposure to water, such as un- to the casting mold, for example, when applying a sizing ve ^~ rme ^~ i ^~ dϊrch ^"i ^~ ^st," is ^~ s ^~ ^~~ ore good. ^"~

The polysaccharide is preferably composed of glucose units, these being particularly preferably linked to α- or β-glycosidic 1,4. However, it is also possible to combine carbohydrate fertilize, which contain other monosaccharides besides glucose, such as galactose or fructose to use as an inventive additive. Examples of suitable carbohydrates are lactose (α- or β-1, 4-linked disaccharide of galactose and glucose) and sucrose (disaccharide of α-glucose and β-fructose).

The carbohydrate is particularly preferably selected from the group of cellulose, starch and dextrins and derivatives of these carbohydrates. Suitable derivatives are, for example, derivatives completely or partially etherified with alkyl groups. However, it is also possible to carry out other derivatizations, for example esterifications with inorganic or organic acids.

A further optimization of the stability of the casting mold and the surface of the casting can be achieved if special carbohydrates and in this case particularly preferred starches, dextrins (hydrolyzate product of the starches) and their derivatives are used as additive for the molding material mixture. As starches, especially the naturally occurring starches, such as potato, corn, rice, peas, banana, horse chestnut or wheat starch can be used. However, it is also possible to use modified starches, such as, for example, swelling starch, low-boiling starch, oxidized starch, citrate starch, acetate starch, starch ethers, starch esters or starch phosphates. There is no limit to the choice of strength per se. The starch may, for example, be low-viscosity, medium-viscosity or high-viscosity, cationic or anionic, cold-water-soluble or hot-water-soluble. The dextrin is particularly preferably selected from the group consisting of potato dextrin, corn dextrin, Gelbdextriri, ^~ white dextrin, ^"borax, and Cyclödextrin ^{'maltodextrin.}

Particularly in the production of casting molds with very thin-walled sections, the molding material mixture preferably comprises additionally a phosphorus-containing compound. In this case, both organic and inorganic phosphorus compounds can be used per se. In order to trigger any unwanted side reactions in metal casting is also preferred that the phosphorus in the phosphorus-containing compounds preferably in the oxidation state V is present. The addition of phosphorus-containing compounds, the stability of the mold can be further increased. This is particularly important when metal casting, the liquid metal strikes a sloping surface and there because of the high metallostatic pressure exerts a high erosion effect or can lead to deformation in particular thin-walled portions of the mold.

The phosphorus-containing compound is preferably present in the form of a phosphate or phosphorus oxide. The phosphate can be present as alkali metal or as alkaline earth metal phosphate, with alkali metal phosphates and especially the sodium salts being particularly preferred. As such, ammonium phosphates or phosphates of other metal ions can also be used. However, the alkali metal or alkaline earth metal phosphates mentioned as being preferred are readily available and are available inexpensively in amounts which are in themselves arbitrary. Phosphates of polyvalent metal ions, especially trivalent metal ions, are not preferred. It has been observed that when using such phosphates of polyvalent metal ions, in particular trivalent metal ions, the processing time of the molding material mixture is shortened.

If the phosphorus-containing compound is added to the molding material mixture in the form of a phosphorus oxide, the phosphorus oxide is preferably present in the form of phosphorus pentoxide. However, also Phθsphθrtri- and phosphoric tetroxide can be used.

According to a further embodiment, the phosphorus-containing compound in the form of the salts of the fluorophosphoric acids may be added to the molding material mixture. Particularly preferred are in the case of the salts of monofluorophosphoric acid. Especially preferred is the sodium salt.

According to a preferred embodiment, organic phosphates are added to the molding material mixture as the phosphorus-containing compound. Preference is given here to alkyl or aryl phosphates. The alkyl groups preferably comprise 1 to 10 carbon atoms and may be straight-chain or branched. The aryl groups preferably comprise 6 to 18 carbon atoms, wherein the aryl groups may also be substituted by alkyl groups. Particularly preferred are phosphate compounds derived from monomeric or polymeric carbohydrates such as glucose, cellulose or starch. The use of a phosphorus-containing organic component as an additive is advantageous in two respects. On the one hand can be achieved by the phosphorus content, the necessary thermal stability of the mold and on the other hand, the surface quality of the corresponding casting is positively influenced by the organic content.

Both orthophosphates and polyphosphates, pyrophosphates or metaphosphates can be used as phosphates. The phosphates can be prepared, for example, by neutralization of the corresponding acids with a corresponding base, for example an alkali metal base, such as NaOH, or optionally also an alkaline earth, limetal base, wherein not necessarily all negative charges of the phosphate ion must be saturated by metal ions. It is possible to use both the metal phosphates and the metal hydrogen phosphates and the metal dihydrogen phosphates, for example Na ₃ PO ₄ , Na ₂ HPO ₄ and NaH ₂ PO ₄ . Likewise, the anhydrous phosphates as well as hydrates of the phosphates can be used. The phosphates can be introduced into the molding material mixture both in crystalline and in amorphous form. Polyphosphates are understood in particular to be linear phosphates which comprise more than one phosphorus atom, the phosphorus atoms being connected in each case via oxygen bridges. Polyphosphates are obtained by condensation of orthophosphate ions with elimination of water, so that a linear chain of PO ₄ tetrahedra is attached, which are each connected via corners. Polyphosphates have the general formula (0 (PO ₃ ) _n ) ^{<n + 2)} where n is the chain length A polyphosphate may comprise up to several hundred PO ₄ tetrahedrons However, polyphosphates with shorter chain lengths are preferred n has values of from 2 to 100, in particular preferably from 5 to 50. It is also possible to use more highly condensed polyphosphates, ie polyphosphates in which the PO ₄ tetrahedra are connected to one another via more than two corners and therefore polymerize into two or more. show three dimensions.

Metaphosphates are understood to mean cyclic structures composed of PO ₄ tetrahedra connected by vertices. Metaphosphates have the general formula ((PCb) _n ) ^{n -} where n is at least 3. Preferably, n has values of 3 to 10.

Both individual phosphates and mixtures of different phosphates and / or phosphorus oxides can be used.

The preferred proportion of the phosphorus-containing compound, based on the refractory molding material, is between 0.05 and 1.0 wt .-%. With a proportion of less than 0.05 wt .-%, no significant influence on the dimensional stability of the mold to determine. If the proportion of the phosphate exceeds 1.0% by weight, the hot strength of the casting mold greatly decreases. Preferably, the proportion of the phosphorus-containing compound is selected to be between 0.10 and 0.5% by weight. The phosphorus-containing compound preferably contains between 0.5 and 90% by weight of phosphorus, calculated as P ₂ O ₅ . If inorganic phosphorus compounds are used, they contain preferably 40 to 90% by weight, particularly preferably 50 to 80% by weight of phosphorus, calculated as P ₂ O ₅ . If organic phosphorus compounds are used, these preferably contain from 0.5 to 30% by weight, particularly preferably from 1 to 20% by weight, of phosphorus, calculated as P ₂ O ₅ .

The phosphorus-containing compound may be added per se in solid or dissolved form of the molding material mixture. The phosphorus-containing compound is preferably added to the molding material mixture as a solid. If the phosphorus-containing compound is added in dissolved form, water is preferred as the solvent.

As a further advantage of adding phosphorus-containing compounds to molding material mixtures for the production of casting molds, it has been found that the molds after metal casting show a very good disintegration. This is true for metals that require lower casting temperatures, such as light metals, especially aluminum. However, a better disintegration of the casting mold during iron casting was also found. In iron casting higher temperatures of more than 1200 ⁰ C act on the mold, so there is an increased risk of vitrification of the mold and thus a deterioration of the decay properties.

Iron oxide as a possible additive was also considered in the inventors' study of the stability and disintegration of molds. When iron oxide is added to the molding material mixture, an increase in the stability of the casting mold during metal casting is likewise observed. The addition of iron oxide can thus also potentially improve the stability of thin-walled sections of the casting mold. However, the addition of iron oxide does not effect the improvement in the disintegration properties of the casting mold after casting, in particular iron casting, observed during the addition of phosphorus-containing compounds. - A -

The molding material mixture according to the invention represents an intensive mixture of at least the constituents mentioned. In this case, the particles of the refractory molding material are preferably coated with a layer of the binder. By evaporation of the water present in the binder (about 40-70 wt .-%, based on the weight of the binder), a firm cohesion between the particles of the refractory base molding material can then be achieved.

The binder, i. the water glass as well as the particulate metal oxide, in particular synthetic amorphous silicon dioxide, and the carbohydrate are contained in the molding material mixture preferably in a proportion of less than 20 wt .-%, particularly preferably in a range of 1 to 15 wt .-%. The proportion of the binder refers to the solids content of the binder. If massive refractory mold bases are used, such as quartz sand, the binder is preferably present in a proportion of less than 10% by weight, preferably less than 8% by weight, more preferably less than 5% by weight. If refractory mold raw materials are used which have a low density, such as the micro hollow balls described above, the proportion of the binder increases accordingly.

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

The Ve-rhäitn-is-of-Wa-s-se-rgia-s -te-iiehenfö-rm-i-gem-Metail-oxi-d, in particular synthetic amorphous silica, can be varied within wide ranges , This offers the advantage of the initial strength of the casting mold, ie the strength immediately after removal from the hot mold, and the moisture content. keitsbeständigkeit without affecting the ultimate strengths, ie the strengths after cooling of the mold, compared to a water glass binder without amorphous silica significantly affect. This is of great interest especially in light metal casting. On the one hand, high initial strengths are desired in order to be able to easily transport these after the production of the casting mold or to assemble them with other casting molds. On the other hand, the final strength after curing should not be too high to avoid difficulties in binder decay after casting, ie the molding base should be easily removed from mold cavities after casting.

The molding material contained in the molding material mixture according to the invention may contain at least a proportion of hollow microspheres in one embodiment of the invention. The diameter of the hollow microspheres is usually in the range of 5 to 500 μm, preferably in the range of 10 to 350 μm, and the thickness of the shell is usually in the range of 5 to 15% of the diameter of the microspheres. These microspheres have a very low specific gravity, so that the molds produced using hollow microspheres have a low weight. Particularly advantageous is the insulating effect of the hollow microspheres. The hollow microspheres are therefore used in particular for the production of molds, if they are to have an increased insulating effect. Such casting molds are, for example, the feeders already described in the introduction, which act as a compensation reservoir and contain liquid metal, wherein the metal should be kept in a liquid state until the metal filled into the mold has solidified. Another application of casting molds containing hollow microspheres are, for example, sections of a casting mold which correspond to particularly thin-walled sections of the finished casting mold. Due to the insulating effect of - -

Hollow microspheres ensure that the metal in the thin-walled sections does not prematurely solidify and thus clog the paths within the mold.

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

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

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

According to a preferred embodiment, the hollow microspheres consist of a borosilicate glass. The borosilicate glass has a proportion of boron, calculated as B ₂ O ₃ , of more than 3% by weight. The proportion of hollow microspheres is preferably chosen to be less than 20% by weight, based on the molding material mixture. at Use of borosilicate glass microballoons preferably a small proportion is selected. This is preferably less than 5 wt .-%, preferably less than 3 wt .-%, and is more preferably in the range of 0.01 to 2 wt .-%.

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

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

Binders containing water result in poorer flowability of the molding material mixture as compared to organic solvent based binders. The flowability of the molding material mixture can be further deteriorated by the addition of the particulate metal oxide. This means that molds with narrow passages and several deflections can be filled worse. As a result, the molds have portions with insufficient compaction, which in turn can lead to casting defects during casting. According to an advantageous embodiment, the molding material mixture according to the invention contains a proportion of a lubricant, preferably a platelet-shaped lubricant, in particular graphite, MoS ₂ , talc and / or pyrophillite. Surprisingly, it has been shown that when - -

Set such lubricants, especially graphite, even complex shapes can be made with thin-walled sections, the molds consistently have a consistently high density and strength, so that during casting substantially no casting defects are observed. The amount of added platelet-shaped lubricant, in particular graphite, is preferably 0.05 wt .-% to 1 wt .-%, based on the refractory molding material.

In addition to the constituents mentioned, the molding material mixture according to the invention may also comprise further additives. For example, internal release agents can be added which facilitate the separation of the molds from the mold. Suitable internal release agents are e.g. Calcium stearate, fatty acid esters, waxes, natural resins or special alkyd resins. Furthermore, silanes can also be added to the molding material mixture according to the invention.

Thus, the molding material mixture according to the invention contains in a preferred embodiment, an organic additive which has a melting point in the range of 40 to 180 ⁰ C, preferably 50 to 175 ⁰ C, that is fixed at room temperature. Organic additives are understood to be compounds whose molecular skeleton is composed predominantly of carbon atoms, that is, for example, organic polymers. By adding the organic additives, the quality of the surface of the casting can be further improved. The mechanism of action of the organic additives has not been clarified. Without wishing to be bound by this theory, however, the inventors assume that at least some of the organic additives are burnt during the casting process, thereby creating a thin gas cushion between liquid metal and the molding base material forming the wall of the casting mold and thus a reaction between the liquid metal and the molding base material is prevented. Further, the inventors believe that some of the organic additives are lower than the reducing ones of the casting - -

Atmosphere forms a thin layer of so-called lustrous carbon, which also prevents a reaction between metal and mold base material. As a further advantageous effect, an increase in the strength of the casting mold after curing can be achieved by adding the organic additives.

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

Surprisingly, it has been found that an improvement of the surface of the casting with very different organic additives can be achieved. Suitable organic additives are, for example, phenol-formaldehyde resins, e.g. Novolacs, epoxy resins such as bisphenol A epoxy resins, bisphenol F epoxy resins or epoxidized novolaks, polyols such as polyethylene glycols or polypropylene glycols, polyolefins such as polyethylene or polypropylene, copolymers of olefins such as ethylene or propylene, and other comonomers, such as vinyl acetate, polyamides such as polyamide-6, polyamide-12 or polyamide-6, 6, natural resins such as gum rosin, fatty acids such as stearic acid, fatty acid esters such as cetyl palmitate, fatty acid amides such as ethylenediamine bisstearamide and metal soaps , such as stearates or oleates of monovalent to trivalent metals. The organic additives can be contained both as a pure substance, as well as a mixture of different organic compounds.

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

About 5 to 50% by weight of silane, based on the particulate metal oxide, are typically used, preferably about 7 to 45% by weight, more preferably about 10 to 40% by weight.

Despite the high strengths which can be achieved with the binder according to the invention, the casting molds produced with the molding compound according to the invention, in particular cores and molds, surprisingly show good disintegration after casting, in particular during aluminum casting. As already explained, it has also been found that casting molds can be produced with the molding material mixture according to the invention, which also show a very good disintegration during iron casting, so that the molding material mixture can easily be poured out again from narrow and twisted sections of the casting mold after casting. The use of the molded articles produced from the molding material mixture according to the invention is therefore not limited to light metal casting. The molds are generally suitable for casting metals. Such metals are, for example, non-ferrous metals, such as brass or bronze, and ferrous metals.

The invention further relates to a method for the production of molds for metal processing, wherein the molding material mixture according to the invention is used. The method according to the invention comprises the steps:

Producing the above-described molding material mixture; - -

Forms of the molding material mixture;

Curing the molded molding material mixture by heating the molding compound to obtain the cured mold.

In the production of the molding material mixture according to the invention, the procedure is generally such that initially the refractory molding base material is introduced and then the binder is added with stirring. In this case, the water glass and the particulate metal oxide, in particular the synthetic amorphous silicon dioxide, and the carbohydrate can be added per se in any order. The carbohydrate may be added in dry form, for example in the form of starch powder. But it is also possible to add the carbohydrate in dissolved form. Preference is given to aqueous solutions of the carbohydrate. The use of aqueous solutions is particularly advantageous if, as in the case of glucose syrup, they are already available as a solution due to the production process. The solution of carbohydrate may also be mixed with the water glass prior to addition to the refractory base stock. Preferably, the carbohydrate is added in solid form to the refractory base molding material.

According to a further embodiment, the carbohydrate can be introduced into the molding material mixture by enveloping a suitable carrier, for example other additives or the refractory molding material with a solution of the corresponding carbohydrate. As the solvent, water or an organic solvent can be used. However, water is preferably used as the solvent. For a better bond between carbohydrate shell and carrier and to remove the solvent, a drying step may be performed after coating. This can be done, for example, in a drying oven or under the action of microwave radiation. - -

The additives described above may be added per se in any form of the molding material mixture. They can be added individually or as a mixture. They can be added in the form of a solid, but also in the form of solutions, pastes or dispersions. If added as a solution, paste or dispersion, water is preferred as the solvent. It is also possible to use the water glass used as a binder as a solvent or dispersion medium for the additives.

According to a preferred embodiment, the binder is provided as a two-component system, wherein a first liquid component contains the water glass and a second solid component contains the particulate metal oxide. The solid component may further comprise, for example, the phosphate and optionally a, preferably platelet, lubricant. If the carbohydrate is added in solid form to the molding material mixture, this can also be added to the solid component.

In the preparation of the molding material mixture, the refractory molding base material is placed in a mixer and then preferably first the solid component (s) of the binder is added and mixed with the refractory molding base material. The mixing time is chosen so that an intimate mixing of refractory base molding material and solid binder component takes place. The mixing time depends on the amount of the molding compound to be produced and on the mixing unit used. Preferably, the mixing time is selected between 1 and 5 minutes. The liquid component of the binder is under preferably further agitating the mixture then added and then the mixture is further mixed until has formed on the grains of the refractory -Formg-rundsto-ffs a- uniform layer of the binder ^". Again, the mixing time Depending on the amount of molding material mixture to be produced and on the mixing unit used, the duration for the mixing process is preferred - -

between 1 and 5 minutes. A liquid component is understood to mean both a mixture of different liquid components and the totality of all liquid individual components, the latter also being able to be added individually. Likewise, a solid component is understood as meaning both the mixture of individual or all of the solid components described above and the entirety of all solid individual components, the latter being able to be added to the molding compound jointly or else successively.

According to another embodiment, the liquid component of the binder may also first be added to the refractory base molding material and only then be fed to the solid component of the mixture. According to a further embodiment, first 0.05 to 0.3% of water, based on the weight of the molding material, is added to the refractory molding material and only then the solid and liquid components of the binder are added. In this embodiment, a surprising positive effect on the processing time of the molding material mixture can be achieved. The inventors believe that the dehydrating effect of the solid components of the binder is thus reduced and the curing process is thereby delayed.

The molding material mixture is then brought into the desired shape. In this case, customary methods are used for the shaping. For example, the molding material mixture can be shot by means of a core shooting machine with the aid of compressed air into the mold. The molding material mixture is then cured by supplying heat in order to evaporate the water contained in the binder. Upon heating, the molding material mixture is deprived of water. Due to the removal of water, condensation reactions between silanol groups are presumably also initiated, so that a cross-linking of the water glass occurs. In the prior art described cold oxidation process, for example, by introducing carbon dioxide or by polyvalent metal cations precipitation of poorly soluble compounds and thus solidification of the mold causes.

The heating of the molding material mixture can be done for example in the mold. It is possible to fully cure the mold already in the mold. But it is also possible to cure the mold only in its edge region, so that it has sufficient strength to be removed from the mold can. The casting mold can then be completely cured by removing further water. This can be done for example in an oven. The dehydration can for example also be done by the water is evaporated at reduced pressure.

The curing of the molds can be accelerated by blowing heated air into the mold. In this embodiment of the method, a rapid removal of the water contained in the binder is achieved, whereby the mold is solidified in suitable periods for industrial use. The temperature of the injected air is preferably 100 ^° C. to 180 ^° C., particularly preferably 120 ^° C. to 150 ^° C. The flow rate of the heated air is preferably set so that the casting mold is cured in periods suitable for industrial use. The periods depend on the size of the molds produced. It is desirable to cure in less than 5 minutes, preferably less than 2 minutes. For very large molds but longer periods may be erforderlieh.

The removal of the water from the molding material mixture can also be carried out in such a way that the heating of the molding material mixture is effected by irradiation of microwaves. The radiation However, the microwaves is preferably made after the mold has been removed from the mold. For this purpose, however, the casting mold must already have sufficient strength. As already explained, this can be achieved, for example, by curing at least one outer shell of the casting mold already in the molding tool.

Due to the thermal curing of the molding material mixture with removal of water, the problem of post-consolidation of the casting mold during metal casting is avoided. In the cold curing process described in the prior art, in which carbon dioxide is passed through the molding material mixture, carbonates are precipitated from the water glass. In the cured mold, however, relatively much water remains bound, which is then expelled during metal casting and leads to a very high solidification of the casting mold. Further, molds solidified by the introduction of carbon dioxide do not achieve the stability of molds thermally cured by dehydration. Due to the formation of carbonates, the structure of the binder is disturbed, so this loses strength. With cold-cured molds based on water glass, therefore, can thin sections of a mold, which may also have a complex geometry, do not produce. Casting molds that are cold-cured by the introduction of carbon dioxide are therefore not suitable for displaying castings of very complicated geometry and narrow passages with multiple baffles, such as oil passages in internal combustion engines, since the casting mold does not achieve the required stability and the casting mold after Metal casting can only be completely removed from the casting with great effort. In the thermal curing DAS water wird- largely from the casting ^'form when removed and metal cast a significantly lower post-curing of the mold is observed. After the metal casting, the mold shows a much better disintegration than casting molds by introducing - -

Carbon dioxide were cured. Thermal curing also makes it possible to produce molds that are suitable for the production of castings with very complex geometry and narrow passages.

As already explained above, the flowability of the molding material mixture according to the invention can be improved by the addition of, preferably platelet-shaped, lubricants, in particular graphite and / or M0S ₂ and / or talc. Also talc-like minerals, such as pyrophyllite, can improve the flowability of the molding material mixture. In the production, the platelet-shaped lubricant, in particular graphite and / or talc, can be added separately from the two binder components of the molding material mixture. However, it is just as possible to premix the platelet-shaped lubricant, in particular graphite, with the particulate metal oxide, in particular the synthetic amorphous silicon dioxide, and then to mix it with the water glass and the refractory molding base material.

In addition to the carbohydrate, the molding material mixture may also comprise other organic additives as already described. The addition of these other organic additives can be done per se at any time during the preparation of the molding material mixture. The addition of the organic additive can be carried out in bulk or in the form of a solution. However, the amount of organic additives is preferably chosen to be low, in particular preferably less than 0.5% by weight, based on the refractory molding base material. Preferably, the total amount of organic additives, including the carbohydrate, is chosen to be less than 0.5% by weight, based on the refractory base molding material.

Water-soluble organic additives can be used in the form of an aqueous solution. If the organic additives are soluble in the binder and are stable in storage over several months in the binder, they can also be dissolved in the binder and so on be added together with this the molding material. Water-insoluble additives may be used in the form of a dispersion or a paste. The dispersions or pastes preferably contain water as the dispersing medium. As such, solutions or pastes of the organic additives can also be prepared in organic solvents. However, if a solvent is used for the addition of the organic additives, water is preferably used.

Preferably, the addition of the organic additives is carried out as a powder or as a short fiber, wherein the average particle size or the fiber length is preferably selected so that it does not exceed the size of the refractory molding base particles. Particularly preferably, the organic additives can be sieved through a sieve with the mesh size of about 0.3 mm. In order to reduce the number of components added to the refractory molding base, the particulate metal oxide and the organic additive (s) are preferably not added separately to the molding sand but are premixed.

Contains the molding material silanes or siloxanes, they are usually added in the form that they are incorporated in advance in the binder. The silanes or siloxanes may also be added to the molding base as a separate component. However, it is particularly advantageous to silanize the particulate metal oxide, ie to mix the metal oxide with the silane or siloxane, so that its surface is provided with a thin silane or siloxane layer. Substituting the thus pretreated particulate metal oxide is a, it is found compared to the untreated metal oxide increased strengths sowi ^~ ^~ e ^~~ e ^"in ^~ ^e" improved ^{^"resistance"} against ^{^"height", ^"humidity".} If, as described, an organic additive is added to the molding material mixture or the particulate metal oxide, it is expedient to do so before the silanization. - -

The inventive method is in itself suitable for the production of all casting molds customary for metal casting, that is to say, for example, of cores and molds. It is also particularly advantageous to produce casting molds which comprise very thin-walled sections. In particular, when adding insulating refractory molding base material or when adding exothermic materials to the molding material mixture according to the invention, the inventive method for the production of feeders is.

The molds produced from the molding material mixture according to the invention or with the inventive method have a high strength immediately after the production, without the strength of the molds after curing is so high that difficulties after the production of the casting occur during removal of the mold. Here it was found that the casting mold has very good disintegration properties both in light metal casting, in particular aluminum casting, and in iron casting. Furthermore, these molds have a high stability at elevated humidity, i. Surprisingly, the casting molds can also be stored without problems for a long time. As a particular advantage, the mold has a very high stability under mechanical stress, so that even thin-walled portions of the mold can be realized without these being deformed by the metallostatic pressure during the casting process. Another object of the invention is therefore a mold, which was obtained by the inventive method described above.

The casting mold according to the invention is generally suitable for metal casting, in particular light metal casting. Particularly advantageous -rgebnrsse "are obtained in the aluminum casting ^" . - ^~ - ^""

The invention will be explained in more detail with reference to examples. - - •

example 1

Influence of synthetically produced amorphous silica and various carbohydrates on the strength of shaped articles with quartz sand as molding material.

1. Preparation and testing of the molding material mixture

For testing the molding material mixture, so-called Georg Fischer test bars were produced. Georg Fischer test bars are cuboid test bars measuring 150 mm x 22.36 mm x ?? - 36 mm.

The composition of the molding material mixture is given in Table 1. The Georg Fischer test bars were prepared as follows:

The components listed in Table 1 were mixed in a laboratory blade mixer (Vogel & Schemann AG, Hagen, DE). For this purpose, initially the quartz sand was introduced and added with stirring the water glass. As a water glass, a sodium water glass was used, which had proportions of potassium. In the tables below, the modulus is therefore given as SiO ₂ : M ₂ O, where M is the sum of sodium and potassium. After the mixture was stirred for one minute, the amorphous silica and / or the carbohydrate were added, if necessary, with further stirring. The mixture was then stirred for an additional 1 minute;

The molding material mixtures were in the stock hopper of an H 2.5 hot box core shooting machine from Röperwerk - transferred foundry Maschinen GmbH, Viersen, DE, whose molding tool had been heated to 200 ⁰ C; The molding material mixtures were 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 mixtures was during the last 20 seconds hot air (2 bar, 120 ⁰ C when entering the tool) passed through the mold;

The mold was opened and the test bars removed.

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

The flexural strengths were measured according to the following scheme:

- 10 seconds after removal (hot strength)

- 1 hour after removal (cold strength)

- 3 hours storage of the cooled cores in a climate chamber at 30 ⁰ C and 75% relative humidity.

- -

Table 1 Composition of the molding material mixtures

a) alkali water glass having an SiO ₂ IN ₂ O of about 2.3 ^b) Elkem Microsilica 971 (pyrogenic silica;. Preparation in an arc furnace) c) yellow potato dextrin (Cerestar), as a solid added d) ethyl cellulose (Ethocel ^®, Fa Dow), added as solid e) Potato starch derivative (Emdex GDH 43, Emsland-Stärke GmbH), added as a solid - 4 -

Table 2 Bending strengths

Result

Influence of added carbohydrate

Example 1.1 shows that sufficient hot strengths can not be achieved without the addition of amorphous silica or a carbohydrate. The shelf life of the cores produced with molding material mixture 1.1 shows that with this no reliable core production is possible. By adding amorphous silica, the hot strengths can be increased (Example 1.2 and 1.3), so that the cores have sufficient strength to process them directly after the core production. The addition of amorphous silica improves the shelf life of the cores, especially at high relative humidity. The addition of carbohydrate compounds, in particular dextrin compounds (Example 1.4), surprisingly similar to the case of the amorphous silica to improve the hot strength. In addition, it shows up in comparison to molding material mixture - -

1.1 improved storage stability of the cores produced. The combined addition of amorphous silica and dextrin (Example 1.5) shows particularly high immediate strengths and a further optimized storage stability. The final strengths are also significantly higher than the other mixtures. The use of ethylcellulose (Example 1.6) or a potato starch derivative (Example 1.7) in combination with amorphous silica also enables process-safe core production. An addition of only 0.1% potato dextrin (mixture 1.8) has a positive effect on the immediate strength and the shelf life of the cores (compared to mixture 1.3).

Example 2

Influence of synthetically produced amorphous silica and various carbohydrates on the casting surface of the castings prepared with moldings of the above-mentioned molding material mixture (Table 1).

Georg Fischer test bars of the molding material mixtures 1.1 to 1.8 were installed in a sand casting mold such that three of the four longitudinal sides come into contact with the casting metal during the casting process. Casting was done with an aluminum alloy type 226 at a casting temperature of 735 ° C. After cooling the casting mold, the casting was freed from the sand by means of high-frequency hammer blows. The castings were assessed for remaining sand buildup.

The cast cut of the mixture 1.1 shows as well as the mixtures 1.2 and 1.3 very strong sand buildup. The carbohydrate-containing molding material mixture (mixture 1.4) has. a positive influence on the quality of the cast surface. The Ausgusschnchnitte mixtures 1.5, 1.6 and 1.7 also have hardly any Sandanhaftungen, whereby in these cases, the positive influence of carbohydrates (here in the form of dextrin and ethylcellulose) on the cast surface quality is confirmed. Even the addition of only 0.1% dextrin (mixture 1.8) produces a significant improvement in the surface quality compared to the carbohydrate-free comparison (mixture 1.3).

Claims

1. molding material mixture for the production of casting molds for metal processing, comprising at least

a refractory molding base; a water glass based binder; a proportion of a particulate metal oxide which is from ewählt ^g of the GRU ^^ e of silicon dioxide, aluminum oxide, titanium oxide and zinc oxide;

characterized in that the molding material mixture is a carbohydrate added.

2. Molding material mixture according to claim 1, characterized in that the proportion of the carbohydrate, based on the refractory molding material, is selected in the range of 0.01 to 5 wt .-%, preferably in the range of 0.02 to 2.5 wt. -%, more preferably in the range of 0.05 to 2.5%, particularly preferably in the range of 0.1 to 0.4 wt .-%.

3. Molding material mixture according to one of the preceding claims, characterized in that the carbohydrate is an oligo- or polysaccharide.

4. molding material mixture according to claim 4, characterized in that the oligosaccharide or polysaccharide has a molecular weight in the range of 1,000 to 100,000 g / mol, preferably 2,000 and 30,000 g / mol.

5. molding material mixture according to claim 4 or 5, characterized in that the polysaccharide is composed of glucose units.

6. molding material mixture according to any one of the preceding claims, characterized in that the carbohydrate is selected from the group of cellulose, starch and dextrins and derivatives of these carbohydrates.

7. Molding material mixture according to one of the preceding claims, characterized in that the carbohydrate is an underivatized carbohydrate.

8. molding mixture according to claim 6, characterized in that the dextrin is selected from the group of potato dextrin, corn dextrin, yellow dextrin, white dextrin, Boraxdextrin, cyclodextrin and maltodextrin.

9. molding material mixture according to claim 6, characterized in that the starch is selected from the group of potato, corn, rice, peas, banana, horse chestnuts or wheat starch.

10. Molding material mixture according to one of the preceding claims, characterized in that the molding material mixture is added a phosphate.

11. Molding material mixture according to claim 10, that the phosphorus-containing compound is an orthophosphate, metaphosphate or poly-phosphate.

12. Molding material mixture according to claim 10, characterized in that the phosphate is an organic phosphate, which is preferably derived from the group of alkyl, A- ryl-, or carbohydrate-containing phosphates.

13. molding material mixture according to one of claims 10 to 12, characterized in that the proportion of phosphorus-containing Compound, based on the refractory molding material, is selected between 0.05 and 1.0 wt .-%.

14. Molding material mixture according to one of claims 10 to 13, characterized in that the phosphorus-containing compound has a phosphorus content of 0.5 to 90 wt .-%, calculated as P ₂ O ₅ .

15. Molding material mixture according to one of the preceding claims, characterized in that the particulate metal oxide is selected from the group of precipitated silica and fumed silica.

16. Molding material mixture according to one of the preceding claims, characterized in that the water glass has a modulus SiO ₂ / M ₂ O in the range of 1.6 to 4.0, in particular 2.0 to 3.5, wherein M sodium ions and / or Potassium ions means.

17. Molding material mixture according to one of the preceding claims, characterized in that the water glass has a solids content of SiO ₂ and M ₂ O in the range of 30 to 60 wt .-%.

18. Molding material mixture according to one of the preceding claims, characterized in that the binder is contained in a proportion of less than 20 wt .-% in the molding material mixture.

19. Molding material mixture according to one of the preceding claims, characterized in that the particulate metal oxide is present in a proportion of 2 to 80 wt .-% based on the binder. - -

20. Molding material mixture according to one of the preceding claims, characterized in that the mold base material contains at least a proportion of hollow microspheres.

21. Molding material mixture according to claim 20, characterized in that the hollow microspheres are aluminum silicate microbubbles and / or glass microbubbles.

22. Molding material mixture according to one of the preceding claims, characterized in that the mold base material contains at least a proportion of glass granules, glass beads and / or spherical ceramic moldings.

23. Molding material mixture according to one of the preceding claims, characterized in that the mold base material contains at least a proportion of mullite, chrome ore and / or olivine.

24. Molding material mixture according to one of the preceding claims, characterized in that the molding material mixture is added an oxidizable metal and an oxidizing agent.

25. Molding material mixture according to one of the preceding claims, characterized in that the molding material mixture contains a proportion of a platelet-shaped lubricant.

26. Molding material mixture according to claim 24, characterized in that the platelet-shaped lubricant is selected from graphite, molybdenum sulfide, talc and / or pyrophyllite.

27. Molding material mixture according to one of the preceding claims, characterized in that the molding material mixture contains a proportion of at least one solid at room temperature organic additive. - -

28. Molding material mixture according to one of the preceding claims, characterized in that the molding material mixture contains at least one silane or siloxane.

29. Method for producing casting molds for metalworking, comprising the steps of:

Production of a molding material mixture according to one of claims 1 to 28;

Forms of the molding material mixture;

Curing the shaped molding material mixture by heating the molded molding material mixture to obtain the cured mold.

30. The method according to claim 29, characterized in that the molding material mixture is heated to a temperature in the range of 100 to 300 ⁰ C.

31. The method according to any one of claims 29 or 30, characterized in that for curing heated air is injected into the molded molding material mixture.

32. The method according to any one of claims 29 to 31, characterized in that the heating of the molding material mixture is effected by the action of microwaves.

33. The method according to any one of claims 29 to 32, characterized in that the casting mold is a feeder.

34. Mold, obtained by a method according to any one of claims 29 -bis- 33rd

35. Use of the casting mold according to claim 34 for cast metal, in particular light metal casting.