EP3010669B1 - Method for producing a lithium-containing molding material mixture based on an inorganic binder for producing molds and cores for metal casting - Google Patents

Description

The invention relates to a process for the preparation of molding compositions based on inorganic binders for the production of molds and cores for metal casting, comprising at least one refractory molding material, one or more lithium compounds, at least water glass as an inorganic binder and amorphous silica as an additive. Furthermore, the invention relates to a lithium-containing inorganic binder and a method for producing molds and cores using the molding material mixtures prepared by the above method.

State of the art

Casting molds are essentially composed of molds and molds and cores which represent the negative molds of the casting to be produced. These cores and forms consist of a refractory material, such as quartz sand, and a suitable binder, which gives the mold after removal from the mold sufficient mechanical strength. 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. In the casting process itself, they must first have sufficient strength and temperature resistance in order to be able to absorb the liquid metal in the cavity formed from one or more casting molds. After the start of the solidification process, the mechanical stability of the casting is ensured by a solidified metal layer which forms along the walls of the casting mold.

The material of the casting mold must now decompose under the influence of the heat given off by the metal in such a way that it loses its mechanical strength, that is to say the cohesion between individual particles of the refractory material is removed. Ideally, the mold decays back to a fine sand that can be easily removed from the casting.

Since casting molds are subjected to very high thermal and mechanical stresses during the casting process, defects can occur at the contact surface between liquid metal and casting mold, for example in that the casting mold breaks or liquid metal penetrates into the structure of the casting mold. In most cases, therefore, those surfaces of the mold, which come into contact with the liquid metal, provided with a protective coating, which is also referred to as sizing.

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 can be used to 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. In iron and steel casting, occasionally defects develop on the surface of the casting, such as a scarred, rough or mineralized surface, chipping, pits, holes or pinholes, or white or black deposits are formed.

If the errors described above occur, elaborate reworking of the surface of the casting is required to achieve the desired surface properties. This requires additional work steps and thus a drop in productivity or an increase in costs. If the defects occur on surfaces of the casting which are poorly or not at all accessible, this can also lead to a loss of the casting.

Further, the sizing may metallurgically affect 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 selectively 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.

A size usually consists of an inorganic refractory and a binder which are dissolved or slurried in a suitable solvent, for example water or alcohol. If possible, one would dispense with the use of alcohol-containing sizes and instead use aqueous systems, since in the course of the drying process, the organic solvents cause emissions.

For the production of molds both organic and inorganic binders can be used, the curing of which can be effected in each case by cold or hot processes. Cold processes are those processes which are carried out essentially without heating the mold used for core production, i.d.R. at room temperature or at a temperature caused by any reaction. The curing takes place, for example, in that a gas is passed through the molding mixture to be cured and thereby initiates a chemical reaction. In hot processes, the molding material mixture after shaping is e.g. heated by the heated mold to a sufficiently high temperature to expel the solvent contained in the binder and / or to initiate a chemical reaction by which the binder is cured.

Due to their technical properties, organic binders have currently been economically viable. the greater importance in the market. Regardless of their composition, however, they have the disadvantage that they decompose during casting and thereby sometimes emit considerable amounts of pollutants such as benzene, toluene and xylene. In addition, the casting of organic binder usually leads to odor and smoke pollution. In some systems, undesirable emissions even occur during the production and / or storage of the molds. Although emissions could be reduced over the years due to binder development, they can not be completely avoided with organic binders.

For this reason, in recent years, R & D has reverted to the inorganic binders to further improve these and the product properties of the molds and cores thus produced.

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

Inorganic binders have been known for a long time, especially those based on water glasses. They were widely used in the 1950s and 60s, but with the advent of modern organic binders, they quickly lost their importance. There are three different methods of curing the water glasses:

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

The thermal curing of water glass, for example, deals with the US 5,474,606 in which a binder system consisting of alkali water glass and aluminum silicate is described.

However, the use of inorganic binder systems is often associated with other disadvantages, which are described in detail in the following.

A disadvantage of inorganic binders is that the molds made from them have relatively low strengths. This is especially evident immediately after the removal of the mold from the tool. The strengths at this time, which are also referred to as hot strengths, but are particularly important for the production of complicated and / or thin-walled moldings and their safe handling. But also the cold strength, which is the strength after complete curing of the mold, is an important criterion, so that the desired casting can be produced with the required dimensional accuracy.

Furthermore, the comparatively high viscosity of inorganic compared to organic binders adversely affects their application in the automated serial production of castings.

Since a higher viscosity is accompanied by a reduced flowability of the molding material mixture, filigree hollow molds, as described, for example, in US Pat. are required for the production of complicated and / or thin-walled moldings, not sufficiently compacted.

Another significant disadvantage of inorganic binders is their comparatively low storage stability at elevated air humidity. The moisture content of the air is given as a percentage for a specific temperature by the relative humidity or in g / m ³ by the absolute humidity. The storage stability of molds, which were prepared by hot curing and using inorganic binder decreases significantly, especially at an absolute humidity of 10 g / m ³ , which is characterized by an increased decrease in the strengths of, in particular produced by hot curing, casting molds during storage makes noticeable. This effect is attributable, in particular in the case of hot curing, to a reverse reaction of the polycondensation with the water of the air, which leads to a softening of the binder bridges.

The decrease in strength under such storage conditions is sometimes associated with the occurrence of so-called storage cracks. Due to the decrease in strength, the structure of the casting mold is weakened, which in places can lead to slight tearing of the casting mold in areas of high mechanical stress.

In addition to storage stability at elevated humidity, cores which have been hot-cured using an inorganic binder have low resistance to water-based resin coatings, e.g., as compared to organic binders. Simple. That is, their strengths are enhanced by the coating e.g. fall sharply with an aqueous sizing and this method is difficult to implement in practice.

The EP 1802409 B1 discloses that higher strengths and improved storage stability can be realized through the use of a refractory mold base, a water glass based binder and a proportion of particulate amorphous silica. As a curing method, in particular the hot curing is described in detail. Another possibility for increasing the storage stability is the use of organosilicon compounds, such as in US 6017978 is set forth.

As Owusu reports, the storage stability of inorganic binders is a problem especially in hot curing, while molds cured by CO _{2 are} significantly more resistant to increased humidity ( Owusu, AFS Transactions, Vol. 88, 1980, pp. 601-608 ). Owusu discloses that storage stability can be increased by the addition of inorganic additives such as Li ₂ CO ₃ or ZnCO ₃ . Owusu assumes that the poor solubility of these additives and the high hydration of the cations contained have a positive influence on the stability of the silicate gel and thus on the storage stability of the water glass binder. However, storage stability by altering the composition of the liquid inorganic binder is not investigated in this publication.

The improvement in the moisture resistance of water glass binders is described in the DE 2652421 A1 and the US 4347890 described. The DE 2652421 A1 deals in particular with various processes for the preparation of lithium-containing binders based on aqueous alkali silicate solutions. The in the DE 2652421 A1 Binders described are characterized by a weight ratio Na ₂ O and / or K ₂ O: Li ₂ O: SiO ₂ in the range of 0.80 - 0.99: 0.01 - 0.20: 2.5 - 4.5, which corresponds to a molar ratio of Li ₂ O / M ₂ O of 0.02-0.44 and a molar ratio SiO ₂ / M ₂ O of 1.8-8.5. Here, [M ₂ O] denotes the sum of the amounts of alkali metal oxides. The binders described therein have improved water resistance, that is, they are less prone to absorbing water from the atmosphere, as demonstrated by gravimetric studies. Although the production of foundry molds is given as a possible application, no information is given on the strength of the molds produced, let alone their storage stability.

The US 4347890 describes a method for producing an inorganic binder, consisting of an aqueous sodium silicate solution and a solution of a lithium compound, in which case in particular lithium hydroxide and lithium silicate are preferred. The lithium compound is added to increase the moisture stability of the binder. The alkali silicate binder after the US 4347890 has a molar ratio of Li ₂ O / M ₂ O (M ₂ O = Li ₂ O + Na ₂ O) of 0.05 to 0.44.

Problems of the prior art and task

1. a) die Herstellung von Gießformen ermöglicht, die auch bei erhöhter Luftfeuchtigkeit lagerstabil sind. Eine ausreichende Lagerstabilität ist insbesondere wünschenswert, um Gießformen nach ihrer Herstellung für längere Zeit lagern zu können und so das Prozessfenster des Fertigungsprozesses zu verlängern.
2. b) ein entsprechendes Festigkeitsniveau erreicht, welches im automatisierten Fertigungsprozess nötig ist, insbesondere eine ausreichende Heißfestigkeit bzw. Kaltfestigkeit.
3. c) mit einem Formgrundstoff eine gute fließfähige Formstoffmischung ergibt, so dass auch Gießformen mit komplexer Geometrie ermöglicht werden können. Da die Fließfähigkeit der Formstoffmischung unmittelbar von der Viskosität des Bindemittels abhängt, muss diese weitestgehend verringert werden.
4. d) die Herstellung von Gießformen mit einer verbesserten Beständigkeit der hergestellten Kerne gegenüber Formstoffüberzügen mit einem Wassergehalt an der Trägerflüssigkeit von mindestens 50 Gew%. Dabei ist die Trägerflüssigkeit der Bestandteil des Formstoffüberzugs, der bei 160°C und Normaldruck (1013 mbar) verdampfbar ist. Da derartige Formstoffüberzüge auf Wasserbasis aus ökologischer Sicht und aus Gründen der Arbeitssicherheit zu bevorzugen sind, ist es erstrebenswert sie auch für Gießformen zu verwenden, die mit anorganischen Bindemitteln hergestellt wurden.
5. e) mit geringen Kosten für die Gießereien verbunden ist, da das Bindemittel lediglich für den einmaligen Gebrauch bestimmt ist. Insbesondere der Lithiumanteil am Bindemittel muss gering gewählt werden, da sich Lithium-Verbindungen wegen der erhöhten Nachfrage in jüngerer Zeit deutlich verteuert haben.

The previously known inorganic binder systems for foundry purposes still have room for improvement. Above all, it is desirable to develop an inorganic binder system which:

1. a) allows the production of molds that are stable even at elevated humidity. Sufficient storage stability is particularly desirable in order to be able to store casting molds for a longer time after their production and thus to extend the process window of the production process.
2. b) reaches a corresponding strength level, which is necessary in the automated manufacturing process, in particular a sufficient hot strength or cold strength.
3. c) results in a good flowable molding material mixture with a molding material, so that casting molds with complex geometry can be made possible. Since the flowability of the molding material mixture depends directly on the viscosity of the binder, it must be reduced as much as possible.
4. d) the production of molds with an improved resistance of the cores produced over molding compounds having a water content of the carrier liquid of at least 50% by weight. In this case, the carrier liquid is the constituent of the molding material coating, which is vaporizable at 160 ° C and atmospheric pressure (1013 mbar). Since such water-based moldings are to be preferred from an ecological point of view and for reasons of occupational safety, it is desirable to use them also for molds made with inorganic binders.
5. e) is associated with low costs for the foundries, since the binder is intended for single use only. In particular, the lithium content of the binder must be chosen low because lithium compounds have become more expensive due to the increased demand in recent times.

The invention therefore an object of the invention to provide a molding material mixture or a binder for the production of molds for metal processing available, which meet the requirements described above (a) to (e).

Summary of the invention

This object is achieved by a process for the production of molding material mixtures, by binders or the process for the production of casting molds and cores having the features of the respective independent patent claims. Advantageous developments are the subject matter of the dependent claims or will be described below.

Surprisingly, it has been found that by using a lithium-containing molding material mixture based on an inorganic binder which has a defined molar ratio [Li ₂ O _active ] / [M ₂ O] (M = alkali metal) and a defined molar ratio [SiO ₂ ] / [M ₂ O] in each case according to the following definition, the problems described above are solved much better.

In particular, the molding material mixture is characterized by the fact that the casting molds produced from it have an increased storage stability with a simultaneously high level of strength. At the same time, the casting molds produced with the molding material mixture are more stable than water-based molding coatings, ie molding coatings with a water content of at least 50% by weight of the carrier liquid. These positive properties are associated with a lower viscosity of the binder and thus an improved flowability of the molding material mixture. Surprisingly, these advantages can only be achieved if the molar ratio [Li ₂ O _active ] / [M ₂ O] and the molar ratio [SiO ₂ ] / [M ₂ O] are within certain well-defined limits and simultaneously amorphous particulate silica is added to the molding material mixture.

Compared to the prior art, the molding mixtures allow the foundries to produce casting molds with a sufficient storage stability and increased stability to water-based molding coatings, without compromising their strength or the flowability of the molding material mixture.

• einen feuerfesten Formgrundstoff; und
• partikuläres amorphes SiO₂ und
• Wasserglas als anorganisches Bindemittel
• eine oder mehrere Lithiumverbindungen,

The molding material mixture has:

• a refractory molding base; and
• particulate amorphous SiO ₂ and
• Water glass as an inorganic binder
• one or more lithium compounds,

wherein the molar ratio [Li ₂ O _active ] / [M ₂ O] in the molding material mixture is 0.030 to 0.17, preferably 0.035 to 0.16 and particularly preferably 0.040 to 0.14, and the molar ratio [SiO ₂ ] / [ M ₂ O] is 1.9 to 2.47, preferably 1.95 to 2.40 and more preferably 2 to 2.30.

[M₂O]

die Stoffmenge in Mol an Alkalimetall M, berechnet als M₂O, wobei abschließend nur folgende Verbindungen in die Berechnung eingehen: amorphe Alkalisilikate, Alkalimetalloxide und Alkalimetallhydroxide, einschließlich deren Hydrate, wobei Li als Teil von M ohne einen Wirksamkeitsfaktor eingeht.

[Li₂O_aktiv]

die Stoffmenge in Mol an Li, berechnet als Li₂O, wobei für [Li₂O] abschließend nur folgende Verbindungen eingehen: amorphe Lithiumsilikate, Lithiumoxide und Lithiumhydroxid, einschließlich deren Hydrate, nach nachfolgendem Schema unter Berücksichtigung von Wirksamkeitsfaktoren.

[SiO₂]

die Stoffmenge in Mol an Si, berechnet als SiO₂, wobei abschließend nur folgende Verbindungen in die Berechnung eingehen: amorphe Alkalisilikate.

According to the present invention, [SiO ₂ ], [M ₂ O] and [Li ₂ O _active ] always have the following meaning:

[M ₂ O]

the amount of substance in moles of alkali metal M, calculated as M ₂ O, wherein finally only the following compounds are included in the calculation: amorphous alkali metal silicates, alkali metal oxides and alkali metal hydroxides, including their hydrates, where Li forms part of M without an efficacy factor.

[Li ₂ O _active ]

the amount of substance in moles of Li, calculated as Li ₂ O, concluding for [Li ₂ O] finally only the following compounds: amorphous lithium silicates, lithium oxides and lithium hydroxide, including their hydrates, according to the following scheme, taking into account efficacy factors.

[SiO ₂ ]

the amount of substance in moles of Si, calculated as SiO ₂ , in which case only the following compounds are included in the calculation: amorphous alkali metal silicates.

• Komponente (F) umfasst einem feuerfesten Formgrundstoff und kein Wasserglas;
• Komponente (B) umfasst ein Wasserglas als anorganisches Bindemittel und kein zugesetztes partikuläres amorphes SiO₂;
• Komponente (A) umfasst partikuläres amorphes SiO₂ als Additiv-Komponente sowie ggfs. eine oder mehrere Lithiumverbindungen als Feststoff und kein Wasserglas.

The molding material mixture for producing casting molds for metal processing is preferably producible according to one embodiment by bringing together at least the following three components which are present separately from each other:

• Component (F) comprises a refractory base stock and not a water glass;
• Component (B) comprises a water glass as the inorganic binder and no added particulate amorphous SiO ₂ ;
• Component (A) comprises particulate amorphous SiO ₂ as an additive component and optionally one or more lithium compounds as a solid and no water glass.

Component (A) is called additive. According to this embodiment of the invention, component (B) including component (A) has a molar ratio [Li ₂ O _active ] / [M ₂ O] of 0.030 to 0.17, preferably 0.035 to 0.16 and more preferably 0.040 to 0.14 and a molar ratio [SiO ₂ ] / [M ₂ O] of 1.9 to 2.47, preferably 1.95 to 2.40 and more preferably from 2 to 2.30.

[Li₂O_aktiv] = 1 *

amorphe Lithiumsilikate, die über die Komponente anorganisches Bindemittel (B) hinzugegeben werden, berechnet als Mole Li₂O, +

1 *

Lithiumoxid, das über die Komponente anorganisches Bindemittel (B) hinzugegeben wird, berechnet als Mole Li₂O, +

1 *

Lithiumhydroxid, das über die Komponente anorganisches Bindemittel (B) hinzugegeben wird, berechnet als Mol Li₂O, +

0,33 *

amorphe Lithiumsilikate, die nicht über das Bindemittel (B) hinzugegeben werden, berechnet als Mole Li₂O, +

0,33 *

Lithiumoxid, das nicht über das Bindemittel (B) hinzugegeben wird, berechnet als Mol Li₂O, +

0,33 *

Lithiumhydroxid, das nicht über das Bindemittel (B) hinzugegeben wird, berechnet als Mole Li₂O (* = multipliziert),

einschließlich deren Hydraten. 0,33 bzw. 1 ist jeweils der (molare) Wirksamkeitsfaktor.Surprisingly, it has been found that the activity of the lithium compounds according to the invention depends on the manner in which the lithium compounds used are used, the above compounds in this respect having a different activity. That fact is addressed by defining an active content [Li ₂ O _active ] which defines the lithium content beyond the definition of the active compounds by the following efficacy factors as follows (Scheme):

[Li ₂ O _active ] = 1 *

amorphous lithium silicates added via the component inorganic binder (B), calculated as moles of Li ₂ O, +

1 *

Lithium oxide added via the inorganic binder component (B) is calculated as moles Li ₂ O, +

1 *

Lithium hydroxide, which is added via the component inorganic binder (B), calculated as mol Li ₂ O, +

0.33 *

amorphous lithium silicates which are not added via the binder (B), calculated as moles of Li ₂ O, +

0.33 *

Lithium oxide, which is not added via the binder (B), calculated as moles of Li ₂ O, +

0.33 *

Lithium hydroxide not added via the binder (B), calculated as moles of Li ₂ O (* = multiplied),

including their hydrates. 0.33 and 1, respectively, is the (molar) efficacy factor.

The above definitions for [M ₂ O], [SiO ₂ ] and [Li ₂ O _active ] apply to all embodiments and categories of the present invention including, for example, the definition of [K ₂ O] / [M ₂ O].

Surprisingly, it was found that based on the calculated molar [Li ₂ O] content, three times as much (molar) amorphous lithium silicates, lithium oxide or lithium hydroxide must be used when these compounds are added via the additive component, compared with the molar amount of substance amorphous lithium silicate, lithium oxide or lithium hydroxide, which are added via the component inorganic binder (B), in which they usually / preferably dissolve.

• ein molares Verhältnis [SiO₂] / [M₂O] von 1,9 bis 2,47, vorzugsweise 1,95 bis 2,40 und besonders bevorzugt von 2 bis 2,30 auf und
• ein molares Verhältnis [Li₂O_aktiv] / [M₂O] von 0,030 bis 0,17, vorzugsweise 0,035 bis 0,16 und besonders bevorzugt 0,040 bis 0,14, auf.

Particularly preferably, the lithium compound (s) is / are completely dissolved in the component inorganic binder (B). Such a component (B) contains water glass as an inorganic binder and has

• a molar ratio [SiO ₂ ] / [M ₂ O] of 1.9 to 2.47, preferably 1.95 to 2.40, and more preferably from 2 to 2.30, and
• a molar ratio [Li ₂ O _active ] / [M ₂ O] of 0.030 to 0.17, preferably 0.035 to 0.16 and particularly preferably 0.040 to 0.14.

The component additive consists of one or more solids, in particular in the form of a free-flowing powder. Preferably, all lithium compounds contributing to the [Li ₂ O _active ] content are present in component B.

Detailed description of the invention

As a refractory molding material (hereinafter abbreviated molding material (s)), the usual materials for the production of molds can be used. Suitable examples are quartz, zircon or chrome ore sand, olivine, vermiculite, bauxite and chamotte. It is not necessary to use only new sands. In terms of resource conservation and to avoid landfill costs, it is even advantageous to use the highest possible proportion of regenerated used sand.

A suitable sand is eg in the WO 2008/101668 A1 (= US 2010/173767 A1 ). Also suitable are regenerates, which are obtained by washing and subsequent drying. It is also possible to use regenerates obtained by purely mechanical treatment. As a rule, the regenerates can replace at least about 70% by weight of the virgin sand, preferably at least about 80% by weight and more preferably at least about 90% by weight.

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

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

In casting experiments with aluminum, it has been found that, when using artificial mold raw materials, especially glass beads, glass granules or microspheres, less molding sand adheres to the metal surface after casting than when using pure quartz sand. The use of artificial mold base materials therefore makes it possible to produce smoother cast surfaces, wherein a complicated post-treatment by blasting is not required or at least to a considerably lesser extent.

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

As a further constituent, the molding material mixture comprises an inorganic binder based on alkali silicate solutions. Aqueous solutions of alkali metal silicates, in particular lithium, sodium and potassium silicates, which are also referred to as water glass, find application as binders in other fields, such as in construction.

The preparation of water glass is e.g. large-scale by melting quartz sand and alkali carbonates at temperatures of 1350 ° C to 1500 ° C. The water glass is initially in the form of a piece of solid glass, which is dissolved by the application of temperature and pressure in water. Another method for the production of water glasses is the direct dissolution of quartz sand with caustic soda.

The resulting alkali metal silicate solution can then be adjusted to the desired molar ratio [SiO _2] / [M ₂ O] by addition of alkali metal hydroxides and / or alkali oxides and their hydrates. Furthermore, the composition of the alkali silicate solution can be adjusted by dissolving alkali silicates having a different composition. In addition to alkali metal silicate solutions, hydrous alkali metal silicates present in solid form, such as, for example, the product groups Kasolv, Britesil or Pyramid from PQ Corporation, are also suitable.

The binders may also be based on water glasses containing more than one of said alkali ions. Furthermore, the water glasses may also contain polyvalent ions, such as boron or aluminum (corresponding water glasses are eg in the EP 2305603 A1 (= US 2012/196736 A1 ).

The lithium-containing binder or the lithium-containing molding material mixture is prepared by adding a lithium compound, namely amorphous lithium silicate, Li ₂ O and / or LiOH to an inorganic binder. Amorphous lithium silicate, Li ₂ O and LiOH also include their hydrates. The lithium compound can be added both in powder form and in an aqueous solution or suspension. In a preferred embodiment, the lithium-containing binder is a homogeneous solution of the lithium compounds described above in the binder according to the invention.

Furthermore, the addition of the lithium compound can also take place exclusively via the component (A), additive, to the molding material mixture, but it is preferred to add the lithium compound at least partially, preferably exclusively, via the component (B), inorganic binder.

Surprisingly, it has been found that casting molds having a markedly improved storage stability and increased stability to water-based molded coatings and still high instantaneous and cold strengths, as required for automated series production, can be produced using the molding material mixture. Furthermore, the component (B) of inorganic binder according to the invention is distinguished from the prior art by a low viscosity and thus high flowability of the molding material mixture produced therewith.

However, the effect according to the invention could only be observed if both the molar ratio [Li ₂ O _active ] / [M ₂ O] and the molar ratio [SiO ₂ ] / [M ₂ O] are within certain limits and the abovementioned lithium compounds be used. The positive effect of lithium on the moisture stability of molds made from the molding material mixtures, even at low concentrations, has not been clarified. Without being bound by theory, the inventors assume that the small ion radius of Li ^{+ has} a stabilizing effect on the silicate framework while the charge remains constant.

As is customary for inorganic binders based on alkali metal silicates, the composition of the inorganic binder component according to the invention is given by the proportion of SiO ₂ , K ₂ O, Na ₂ O, Li ₂ O and H ₂ O.

The molar ratio [Li ₂ O _active ] / [M ₂ O] of the molding material mixture, the inorganic binder component and the additive or the inorganic binder alone is greater than or equal to 0.030, preferably greater than or equal to 0.035 and particularly preferably greater than or equal to 0.040. The upper limits are less than or equal to 0.17, preferably less than or equal to 0.16, and most preferably less than or equal to 0.14. The aforementioned upper and lower limits can be combined as desired.

At the same time, the molar ratio [SiO ₂ ] / [M ₂ O] of the molding material mixture, the components inorganic binder and additive or the inorganic binder alone is greater than or equal to 1.9, preferably greater than or equal to 1.95, and particularly preferably greater than or equal to 2.

The upper limit for the molar ratio [SiO ₂ ] / [M ₂ O] is less than or equal to 2.47, preferably less than or equal to 2.40, and particularly preferably less than or equal to 2.30. The aforementioned upper and lower limits can be combined as desired.

The inorganic binders preferably have a solids content of greater than or equal to 20% by weight, preferably greater than or equal to 25% by weight, more preferably greater than or equal to 30% by weight and particularly preferably greater than or equal to 33% by weight. The upper limits for the solids content of the preferred water glasses are less than or equal to 55% by weight, preferably less than or equal to 50% by weight, more preferably less than or equal to 45% by weight and particularly preferably less than or equal to 42% by weight. The solids content is defined as the weight fraction of M ₂ O and SiO ₂ .

In a preferred embodiment, the inorganic binder according to the invention contains both amorphous lithium and sodium and potassium silicates. Potassium-containing water glasses have compared to pure sodium or. mixed lithium sodium water glasses have a lower viscosity. The mixed according to the invention particularly preferred, mixed lithium-sodium-potassium water glasses thus combine the advantage of increased moisture stability at the same time a high level of strength and a further reduction in viscosity. Low viscosity values are indispensable for automated mass production in order to ensure a good flowability of the molding material mixture, thus enabling complex core geometries. However, the potassium content of the inorganic binder according to the invention must not be too high, since an excessively high potassium content has a negative effect on the storage stability of the casting molds produced.

The molar [K ₂ O] / [M ₂ O] ratio in the inorganic binder, in particular in component B, is preferably greater than 0.03, particularly preferably greater than 0.06 and particularly preferably greater than 0.1. The upper limit of the molar ratio [K ₂ O] / [M ₂ O] results in a value of less than or equal to 0.25, preferably less than or equal to 0.2, and particularly preferably less than or equal to 0.15. The aforementioned upper and lower limits can be combined as desired. In the calculation of [K ₂ O], the following compounds are finally included: amorphous potassium silicates, potassium oxides and potassium hydroxides, including their hydrates.

Depending on the application and the desired strength level, greater than 0.5% by weight, preferably greater than 0.75% by weight and particularly preferably greater than 1% by weight, of the binder according to the invention are used. The upper limits here are less than 5% by weight, preferably less than 4% by weight and particularly preferably less than 3.5% by weight. These data refer to the molding material in each case. The% by weight refers to the inorganic binders with a solids content as indicated above, i. the% by weight includes the diluent.

Based on the amount of alkali silicates, calculated as M ₂ O and SiO ₂ , which are added to the mold base with the inorganic binder according to the invention, without consideration of the diluent, the amount of binder used is 0.2 to 2.5% by weight, preferably 0.3 to 2 wt.% Relative to the molding material, wherein M ₂ O has the meaning given above.

In a further embodiment, the binder according to the invention may additionally contain alkali borates. Alkaline borates as constituents of water glass binders are used, for example, in GB 1566417 discloses where they serve to complex carbohydrates. Typical addition levels of the alkali borates are from 0.5% to 5% by weight, preferably from 1% to 4% by weight and more preferably from 1% to 3% by weight, based on the weight of the binder.

A proportion of a particulate amorphous SiO ₂ in the form of the additive component is added to the molding material mixture in order to increase the strength level of the casting molds produced with such molding material mixtures. An increase in the strengths of the casting molds, in particular the increase in hot strengths, can be advantageous in the automated production process. The particulate amorphous silica has a particle size of preferably less than 300 microns, preferably less than 200 microns, more preferably less than 100 microns. The particle size can be determined by sieve analysis. The sieve residue of the particulate amorphous SiO ₂ when passing through a 125 μm mesh (120 mesh) sieve is preferably not more than 10% by weight, more preferably not more than 5% by weight, and most preferably not more than 2%. %.

The sieve residue is determined according to the machine screen method described in DIN 66165 (Part 2), wherein additionally a chain ring is used as screen aid.

The amorphous SiO ₂ preferably used according to the present invention has a water content of less than 15% by weight, in particular less than 5% by weight and particularly preferably less than 1% by weight. In particular, the amorphous SiO _{2 is used} as a pourable powder.

As amorphous SiO ₂ , both synthetically produced and naturally occurring silicas can be used. The latter, known for example from DE 102007045649 , but are not preferred because they usually contain not inconsiderable crystalline components and are therefore classified as carcinogenic.

Synthetically , non-naturally occurring amorphous SiO _{2 is} understood, but the preparation comprises a (man-induced) chemical reaction, for example the production of silica sols by ion exchange processes from alkali silicate solutions, the precipitation of alkali metal silicate solutions, the flame hydrolysis of silicon tetrachloride or the reduction of silica sand with coke in the electric arc furnace in the production of ferrosilicon and silicon. The amorphous SiO ₂ produced by the latter two methods is also referred to as pyrogenic SiO ₂ .

Occasionally, synthetic amorphous SiO _{2 is} only precipitated silica ( CAS-No. 112926-00-8 ) and flame-hydrolysed SiO ₂ (pyrogenic silica, fumed silica, CAS-No. 112945-52-5 ), while the product resulting from the production of ferrosilicon or silicon is merely understood as amorphous SiO ₂ (silica fume, microsilica, CAS-No. 69012-64-12 ) referred to as. For the purposes of the present invention, the product formed in the production of ferrosilicon or silicon is also understood as a synthetic amorphous SiO ₂ .

Preference is given to using precipitated silicas and pyrogenic SiO ₂ , ie flame-hydrolysed or arc-produced SiO ₂ . Particular preference is given to using amorphous SiO ₂ produced by thermal decomposition of ZrSiO ₄ (see DE 102012020509 ) and by oxidation of metallic Si by means of an oxygen-containing gas produced SiO ₂ (see DE 102012020510 ).

Also preferred is fused quartz powder (mainly amorphous SiO ₂ ), which has been prepared by melting and rapid re-cooling from crystalline quartz, so that the particles are spherical and not splintered (see DE 102012020511 ). The average primary particle size of the synthetic amorphous silicon dioxide may be between 0.05 μm and 10 μm, in particular between 0.1 μm and 5 μm and particularly preferably between 0.1 μm and 2 μm.

The primary particle size can be determined, for example, by means of dynamic light scattering (eg Horiba LA 950) and checked by scanning electron microscope images (SEM images with, for example, Nova NanoSEM 230 from FEI). In order to avoid particle agglomeration, the samples are dispersed in water in an ultrasonic bath prior to particle size measurements. In addition, details of the primary particle shape up to the order of 0.01 μm could be visualized with the aid of SEM images. The SiO ₂ samples were dispersed in distilled water for SEM measurements and then coated on a copper tape-covered aluminum holder before the water was evaporated.

The mean primary particle size is preferably between 0.05 μm and 10 μm, measured with dynamic light scattering (for example Horiba LA 950) and, if necessary, checked by scanning electron microscope images.

Furthermore, the specific surface area of the synthetic amorphous silicon dioxide was determined by means of gas adsorption measurements (BET method) according to DIN 66131. The specific surface area of the synthetic amorphous SiO ₂ is preferably between 1 and 35 m ² / g, preferably between 1 and 17 m ² / g and particularly preferably between 1 and 15 m ² / g. If necessary. The products can also be mixed, for example to obtain specific mixtures with certain particle size distributions.

Depending on the production method and producer, the purity of the amorphous SiO _{2 can} vary greatly. Types having a content of at least 85% by weight of SiO ₂ , preferably of at least 90% by weight and more preferably of at least 95% by weight, have proven to be suitable.

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

The ratio of water glass to particulate metal oxide, and particularly amorphous SiO _2, can be varied within wide limits. This offers the advantage of greatly improving the initial strengths of the cores, ie, the strength immediately after removal from the tool, without significantly affecting the ultimate strengths. 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 the cores after their production or to assemble them into whole core packages, on the other hand, the final strengths should not be too high to avoid difficulties in core decay after casting, ie the molding base should be easily removed after casting from cavities of the mold.

Based on the weight of the binder (including diluent or solvent), the particulate amorphous SiO ₂ in the molding material mixture is preferably in a proportion of 2 to 60 wt.%, Particularly preferably 3 to 55 wt.% And particularly preferably 4 to 50% by weight.

The addition of the amorphous SiO ₂ can according to EP 1802409 B1 both before and after the binder is added directly to the refractory, but it can also, as in EP 1884300 A1 (= US 2008/029240 A1 ) first prepared a premix of the SiO ₂ with at least a portion of the binder or caustic soda and then admixed the refractory. Any remaining binder (s) not used in the premix may be added to the refractory material before or after the addition of the premix or together with it.

In a further embodiment, the additive component barium sulfate may be added to further improve the surface of the casting, especially in light metal casting, such as aluminum casting. The barium sulfate can be synthetically produced as well as natural barium sulfate, ie added in the form of minerals containing barium sulfate, such as barite or barite.

This as well as other features of the suitable barium sulfate and the molding material mixture produced with it are described in the DE 102012104934 described in more detail and its disclosure content is in so far made by reference to the disclosure of the present patent.

In a further embodiment, the additive component of the molding material mixture may further comprise at least aluminum oxides and / or aluminum / silicon mixed oxides in particulate form or metal oxides of aluminum and zirconium in particulate form, as described in US Pat DE 102012113073 or the DE 102012113074 to that extent, the additives disclosed therein are also considered part of the disclosure of the present patent. Through such additions castings, in particular made of iron or steel with very high surface quality can be obtained after the casting of metal, so that after the removal of the casting mold little or no post-processing of the surface of the casting is required.

In a further embodiment, the additive component of the molding material mixture may comprise a phosphorus-containing compound. Such an addition is preferred in very thin-walled sections of a casting mold and in particular in cores, since in this way the thermal stability of the cores or of the thin-walled section of the casting mold can be increased. This is of particular importance when the liquid metal encounters an inclined surface during casting and exerts a strong erosive effect there due to the high metallostatic pressure or can lead to deformations of thin-walled sections of the casting mold in particular. Suitable phosphorus compounds do not or not significantly affect the processing time of the novel molding material mixtures. Suitable representatives and their addition levels are in the WO 2008/046653 A1 described in detail and this is so far as the disclosure of the present protective right asserted.

The preferred proportion of the phosphorus-containing compound, based on the mold base, is between 0.05 and 1.0 wt .-%, and preferably between 0.1 and 0.5 wt .-%.

In a further embodiment, the molding material mixture with the additive component organic compounds (according to EP 1802409B1 and WO2008 / 046651 ) may be added. A slight addition of organic compounds may be advantageous for specific applications - for example, to regulate the thermal expansion of the cured molding material mixture. However, such is not preferred, as this in turn is associated with emissions of CO ₂ and other pyrolysis products.

Binders containing water have a poorer flowability compared to organic solvent-based binders. This means that molds with narrow passages and several deflections can be filled worse. As a result, the cores may have portions with insufficient compaction, which in turn may lead to casting defects during casting. According to an advantageous embodiment, the additive component of the molding material mixture contains a proportion of platelet-shaped lubricants, in particular graphite or MoS ₂ . Surprisingly, it has been found that with the addition of such lubricants, in particular graphite, even complex shapes can be produced with thin-walled sections, wherein the casting molds consistently have a uniformly high density and strength, so that substantially no casting defects were observed during casting. The amount of added platelet-shaped lubricant, in particular graphite, is preferably 0.05 to 1 wt.%, Particularly preferably 0.05 to 0.5 wt.%, Based on the molding material.

Instead of or in addition to the platelet-shaped lubricant, it is also possible to use surface-active substances, in particular surfactants, in the inorganic binder component, which likewise further improve the flowability of the molding material mixture. Suitable representatives of these compounds are, for example in the WO 2009/056320 A1 (= US 2010/0326620 A1 ). Mentioned here are in particular surfactants with sulfuric acid or sulfonic acid groups. Further suitable representatives as well as the respective addition quantities are in the WO 2009/056320 A1 described in detail and this is so far as the disclosure of the present protective right asserted.

In addition to the constituents mentioned, the molding material mixture may also comprise further additives. For example, release agents can be added which facilitate the detachment of the cores from the mold. Suitable release agents are e.g. Calcium stearate, fatty acid esters, waxes, natural resins or special alkyd resins. If these release agents are soluble in the binder and do not separate from it even after prolonged storage, especially at low temperatures, they may already be present in the binder component, but they may also form part of the additive.

Furthermore, silanes can also be added to the molding material mixture, for example in order to further increase the storage stability of the cores and / or their resistance to water-based molding coatings. According to a further preferred embodiment, the molding material mixture therefore contains a proportion of at least one silane. As silanes, for example, aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes and ureidosilanes can be used. Examples of such silanes are γ-aminopropyltrimethoxysilane, γ-hydroxypropyltrimethoxysilane, 3-ureidopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β- (3,4-epoxycycloherxyl) trimethoxysilane, N-β- ( Aminoethyl) -γ-aminopropyltrimethoxysilane and their triethoxyanalogous compounds. The silanes mentioned, in particular the aminosilanes, can also be prehydrolyzed. About 0.1% by weight to 2% by weight of silane, based on the binder, is typically used, preferably about 0.1% by weight to 1% by weight.

Contains the molding material silanes, they are usually added in the form that they are incorporated in advance in the binder. But they can also be added to the molding material.

In the preparation of the molding material mixture, the refractory molding base material is placed in a mixer and then first added the liquid component and mixed with the refractory molding material until a uniform layer of the binder has formed on the grains of the refractory molding material.

The mixing time is chosen so that an intimate mixing of refractory base molding material and liquid component takes place. The mixing time depends on the amount of the molding mixture to be produced and on the mixing unit used. Preferably, the mixing time is selected between 1 and 5 minutes. With preferably further agitation of the mixture, the solid component (s) in the form of the amorphous silicon dioxide and, if appropriate, further pulverulent solids is / are then added, and the mixture is then further mixed. Again, the mixing time depends on the amount of the molding mixture to be produced and on the mixing unit used. Preferably, the mixing time is selected between 1 and 5 minutes. A liquid component is understood to mean both a mixture of different liquid components and the entirety of all individual liquid components, the latter being able to be added to the molding material mixture jointly or else successively. In practice, it has proven useful to first add the (other) solid components to the refractory base molding material, to mix and only then to supply the liquid component (s) of the mixture, and then to mix again.

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. Another possibility is to free-flow the molding material mixture from the mixer into the mold and to compact it there by shaking, stamping or pressing.

The molding material mixture can in principle be cured by all curing methods known for water glasses, such as hot curing or by the CO ₂ process. A further development of the CO ₂ process, which involves a combination of CO ₂ and air fumigation, is described in the DE 102012103705.1 described and also represents a suitable method for curing the molding material mixture.

In order to accelerate the curing, the CO ₂ or the air or both gases can also be heated in this process, for example, to temperatures of up to 100 ° C.

Another method for curing the molding material mixture is curing by means of liquid (e.g., organic esters, triacetin, etc.) or solid catalysts (e.g., suitable aluminum phosphates).

Another method for producing the casting molds is the so-called rapid prototyping. This technology differs in particular in that the molding material mixture is not compacted by pressure into the desired shape, but first the solid components such as the mold base material and possible additives are applied in layers. In the next step, the liquid component of the molding material mixture is specifically printed on the sand / additive mixture. The mold is then made by curing the "printed" areas. For inorganic binders, hardening in the area of rapid prototyping technology takes place, inter alia, by microwave curing, by hardening by means of a liquid or solid catalyst or by drying in an oven or in air. Further details on rapid prototyping technology can be found in the EP 0431924 B1 and US 6610429 B2 ,

Preference is given to the hot curing, in which the molding material mixture is exposed to a temperature of 100 to 300 ° C, preferably 120 to 250 ° C. In the hot curing of the molding material mixture water is removed. As a result, condensation reactions between silanol groups are presumably also initiated so that crosslinking of the water glass occurs.

The heating can be carried out, for example, in a mold, which preferably has a temperature of 100 to 300 ° C, particularly preferably from 120 ° C to 250 ° C. In this case, a gas (for example air) is preferably passed through the molding material mixture, this gas preferably having a temperature of from 100 to 180.degree. C., particularly preferably from 120 to 150.degree. Further details of the curing of the mold are in the EP 1802409 B1 described in detail and this is also considered as part of the disclosure of the present patent.

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.

For example, the irradiation of the microwaves can be made after the mold has been removed from the mold. In this case, 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. For the purposes of the above-described rapid prototyping technology, 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. It is e.g. it is possible to mix the molding base material with the solid, powdery component (s), apply this mixture in layers on a surface and print the individual layers using a liquid binder component, in particular with the aid of a waterglass, wherein the layered application of the solid mixture in each case one Printing process using the liquid binder follows. At the end of this process, i. after completion of the last printing operation, the entire mixture can be heated in a microwave oven

The processes according to the invention are in themselves suitable for the production of all casting molds customary for metal casting, that is to say, for example, of cores and molds.

Despite the high strengths which can be achieved with the molding material mixtures, the cores produced from these molding material mixtures show good disintegration after casting, so that the molding material mixture can be removed again after casting, even from narrow and angular sections of the casting. The moldings produced from the molding material mixtures are generally suitable for casting metals, such as light metals, non-ferrous metals or ferrous metals.

As a further advantage, the casting mold has a very high stability under mechanical stress, so that even thin-walled sections of the casting mold can be realized without 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.

Reference to the following examples, the invention will be explained in more detail without being limited to these.

Examples: 1. Preparation of the waterglass binder from a lithium hydroxide solution

Tables 1, 2, 3 and 4 give an overview of the composition of the different inventive or non-inventive waterglass binders, which were tested in the present study. The preparation of the waterglass binder is carried out by mixing the chemicals indicated in Table 1 or 2, so that a homogeneous solution is present. Their use was only one day after their preparation to ensure their homogeneity. The concentration of the alkali oxides and of [SiO ₂ ] in the waterglass binder used and their molar ratio and molar ratio [Li ₂ O _active ] / [M ₂ O] are summarized in Tables 4 and 5. Table 3 gives an overview of the molding mixtures in which the lithium compound was added via the additive component. The addition of the solid lithium compound was carried out together with the amorphous SiO ₂ (see 2.1).

2. Studies on storage stability 2.1 Production of the molding material mixtures

100 parts by weight (GT) of quartz sand (quartz sand H32 from Quarzwerke GmbH) were introduced into the bowl of a mixer from Hobart (model HSM 10). With stirring, 2 pbw of the binder were then added and each intensively mixed with the sand for 1 minute. Following the binder, 0.5 pb of amorphous SiO _{2 was} added and this was also mixed in for 1 minute. The amorphous SiO ₂ was amorphous silicon oxide POS BW 90 LD from Possehl Erzkontor GmbH.

2.2 Preparation of the test specimens

For the testing of the molding material mixtures cuboidal test bars with dimensions of 150 mm x 22.36 mm x 22.36 mm were prepared (so-called Georg-Fischer-Riegel). Part of a 3.1. prepared molding material mixture was transferred to the storage bunker of a H 2.5 hot box core shooter Röperwerk foundry machines GmbH, Viersen, DE, whose mold was heated to 180 ° C. <b> Table 1 </ b> Composition of the binders used # Sodium waterglass binder ^a) [GT] NaOH ^b) [GT] LiOH.H ₂ O ^c) [GT] VE water (additional) [GT] 1.1 81.63 3.12 0.40 14.85 1.2 81.75 2.74 0.81 14.70 1.3 81.88 2.36 1.21 14.55 1.4 82.07 1.79 1.82 14.32 1.5 82.26 1.21 2.44 14.09 1.6 82.42 0.74 2.94 13,90 1.7 75.02 6.29 1.44 17.25 1.8 77.34 4.96 1.36 16.34 1.9 79.82 3.54 1.28 15.36 1.10 81.88 2.36 1.21 14.55 1.11 83.35 1.52 1.16 13.97 1.12 84.79 0.69 1.12 13.40 1.13 85.98 0 1.08 12.94 a) sodium waterglass 48/50 from BASF SE; molar ratio [SiO ₂ ] / [M ₂ O] ca. 2.82; Solids content approx. 45.5%
b) Sodium Hydroxide Cookies (Sigma-Aldrich)
c) lithium hydroxide monohydrate (solid, supplier: Lomberg GmbH)
VE = fully desalted, GT = parts by weight (100 GT = total binder, including diluent water) Composition of the binders used # Sodium waterglass binder ^a) [GT] Potassium-water glass binder ^b) [GT] NaOH ^c) [GT] LiOH.H ₂ O ^d) [GT] VE water (additional) [GT] 2.1 64.4 16.1 3.1 0 16.4 2.2 64.4 16.1 2.0 1.2 16.3 2.3 64.4 16.1 0.9 2.3 16.3 a) sodium silicate glass 47/48 from BASF SE; molar ratio [SiO ₂ ] / [M ₂ O] about 2.68; Solids content approx. 43.5%
b) potash waterglass 35 from BASF SE; molar ratio [SiO ₂ ] / [M ₂ O] approx. 3.45, solids content approx. 34.8%
c) Sodium Hydroxide Cookies (Sigma-Aldrich)
d) lithium hydroxide monohydrate (solid, supplier: Lomberg GmbH) Composition of the binder used, and additive components ^a) Composition of the water glass binder, which was already prepared in advance of the experiment solid sodium or. Lithium compound added as additives to the molding material mixture # Sodium waterglass binder ^b) [GT] NaOH ^c) [GT] VE water (additional) [GT] NaOH ^d) [GT] lithium compound 3.1 70.8 3.1 26.1 0 0 3.2 70.8 3.1 26.1 5 0 3.3 70.8 3.1 26.1 0 5 GT LiOH · H ₂ O ^e) a) Examples 3.1 to 3.3 each contain 25 parts by weight particulate amorphous silica, POS BW 90 LD manufacturer Possehl Erzkontor GmbH
b) sodium waterglass 48/50 from BASF SE; molar ratio [SiO ₂ ] / [M ₂ O] ca. 2.82; Solids content approx. 45.5%
c) Sodium hydroxide cookie portion (Sigma-Aldrich) dissolved in the vehicle
d) Sodium hydroxide cookie portion (Sigma-Aldrich) added to the molding compound via the additive component.
e) lithium hydroxide monohydrate (solid, supplier: Lomberg GmbH) Composition of the binders used # Molar concentration in mol / kg based on the binder molar ratio [SiO ₂ ] / [M ₂ O] (M = Li, Na, K) Molar ratio [Li ₂ O _active ] / [M ₂ O] [SiO ₂ ] [Na ₂ O] [K ₂ O] [Li ₂ O] ^a) 1.1 4.52 2.01 0 0.05 2.20 0.023 not according to the invention 1.2 4.53 1.97 0 0.10 2.20 0.047 inventively 1.3 4.54 1.92 0 0.14 2.20 0,070 inventively 1.4 4.55 1.85 0 0.22 2.20 0.105 inventively 1.5 4.56 1.78 0 0.29 2.20 0.140 inventively 1.6 4.57 1.73 0 0.35 2.20 0.169 inventively 1.7 4.16 2.27 0 0.17 1.70 0,070 not according to the invention 1.8 4.29 2.15 0 0.16 1.85 0,070 not according to the invention 1.9 4.42 2.03 0 0.15 2.03 0,070 inventively 1.10 4.54 1.92 0 0.14 2.20 0,070 inventively 1.11 4.62 1.84 0 0.14 2.33 0,070 inventively 1.12 4.70 1.77 0 0.13 2.47 0,070 inventively 1.13 4.77 1.71 0 0.13 2.60 0,070 not according to the invention 2.1 4.06 1.65 0.19 0 2.21 0 not according to the invention 2.2 4.06 1.51 0.19 0.14 2.21 0,076 inventively 2.3 4.06 1.38 0.19 0.27 2.21 0,147 inventively a) for Examples 1.1 to 2.3 [Li ₂ O] is equal to [Li ₂ O _active ], since the LiOH.H ₂ O added together with the component inorganic binder contributes to one hundred percent to [Li ₂ O _active ]. Composition of the components used Binder and additive # Molar concentration in mol / kg molar ratio [SiO ₂ ] / [M ₂ O] (M = Li, Na) ^a) Molar ratio [Li ₂ O _active ] / [M ₂ O] ^b) [SiO ₂ ] ^a) [Na ₂ O] ^a) [Na ₂ O] ^b) [Li ₂ O] ^b) [Li ₂ O _active ] ^b) 3.1 3.93 1.78 1.78 0 0 2.21 0 not according to the invention 3.2 3.93 1.78 2.41 0 0 2.21 0 not according to the invention 3.3 3.93 1.78 1.78 0.60 0.20 2.21 0.08 inventively a) molar concentration, calculated for the component inorganic binder.
b) molar concentration, calculated for the components inorganic binder and additive together.

The remainder of the respective molding material mixture was stored in a carefully sealed vessel until the core shooter was refilled to prevent it from drying out and to prevent premature reaction with the CO ₂ present in the air.

The molding material mixtures were introduced from the storage bunker into the mold by means of compressed air (5 bar). The residence time in the hot mold for curing the mixtures was 35 seconds. To accelerate the curing process, hot air (2 bar, 100 ° C on entering the tool) was passed through the mold during the last 20 seconds. The mold was opened and the test bars removed.

2.3 Strength investigations of the test specimens produced

To determine the flexural strengths, the test bars were placed in a Georg Fischer Strength Tester equipped with a 3-point flexure, and the force was measured which resulted in the breakage of the test bars. The flexural strengths were determined both immediately, ie not more than 10 seconds after removal (hot strengths) and also approx. 24 hours after preparation (cold strengths). The storage stability was investigated by the cores then for a further 24 hours in a climatic chamber (Rubarth Apparate GmbH) at 30 ° C and a relative humidity of 60%, which corresponds to an absolute humidity of 18.2 g / m ³ , were stored and again their flexural strength was measured. The accuracy with which the predefined values for temperature and humidity were generated by the climate chamber was checked regularly with a calibrated testo 635 humidity / temperature / pressure dew point measuring instrument from testo.

The results of the strength tests are shown in Table 6. The values given here are averages of multiple determinations on at least 4 cores.

2.4 Results

While the binders of Examples 1.1 to 1.6 differ only in terms of their molar ratio [Li ₂ O _active ] / [M ₂ O], the binders of Examples 1.7 to 1.12 have a different molar ratio at a constant value for the molar ratio [Li ₂ O _active ] / [M ₂ O]. The comparison of Examples 1.1 to 1.6 thus clarifies the influence of the molar ratio [Li ₂ O _active ] / [M ₂ O] on the strength values, while Examples 1.7 to 1.12 illustrate the influence of the molar ratio [SiO ₂ ] / [M ₂ O] reflect. <b> Table 6 </ b> Bending strength of the test bars produced # Hot Strengths [N / cm ² ] Cold Strength ^a) [N / cm ² ] after storage in a climate chamber ^b) [N / cm ² ] after storage in a climatic chamber ^c) [%] 1.1 100 398 123 30.9 not according to the invention 1.2 100 398 248 62.3 inventively 1.3 100 393 280 71.2 inventively 1.4 100 375 303 80.8 inventively 1.5 100 363 323 89.0 inventively 1.6 100 355 335 94.4 inventively 1.7 95 445 100 22.5 not according to the invention 1.8 95 440 155 35.2 not according to the invention 1.9 105 430 240 55.8 inventively 1.10 100 385 243 63.1 inventively 1.11 110 365 283 77.5 inventively 1.12 120 355 265 74.6 inventively 1.13 125 305 287 94.1 not according to the invention 2.1 150 425 147 34.6 not according to the invention 2.2 130 378 268 70.9 inventively 2.3 140 313 310 99.0 inventively 3.1 140 378 88 23.3 not according to the invention 3.2 65 340 15 4.4 not according to the invention 3.3 130 380 305 80.3 inventively a) The determination of the strengths was carried out after 24 hours storage at room temperature
b) The determination of the strengths was carried out after storage for 24 hours in a climatic chamber at 30 ° C and 60% relative humidity, following storage at room temperature.
c) Remaining strength after storage in the climatic chamber with respect to the cold strength.

Influence of the molar ratio [Li ₂ O _active ] / [M ₂ O] of the binder:
The flexural strengths summarized in Table 6 clearly demonstrate the positive effect that can be achieved by the addition of lithium to the storage stability of the binder.

While the strengths of cores made with the binder of Example 1.1 fall after storage for one day at elevated humidity up to 71%, the decrease in strength of the cores produced with the other lithium-rich binders is much lower. This effect already occurs in binders with a relatively low [Li ₂ O _active ] / [M ₂ O] ratio of 0.047. The comparison of Examples 1.2 to 1.6 clearly shows that with increasing molar ratio [Li ₂ O _active ] / [M ₂ O] increases the storage stability of the binder, with a residual strength of 94% after storage in the climatic chamber, based on the cold strength, can be achieved can.

With respect to the hot strengths, Examples 1.1 to 1.6 show no difference, while the cold strengths with increasing molar ratio [Li ₂ O _active ] / [M ₂ O] a significant deterioration of the values by up to 40 N / cm ² is recorded.

Examples 1.1 to 1.6 illustrate that the sand cores produced with these binders have a high storage stability with a simultaneously high cold strength. A further increase in the molar ratio does not lead to a significant improvement in storage stability, while the cold strengths decrease.

These observations can be made for mixed Li-Na waterglasses as well as for mixed Li-Na-K waterglasses, as shown in examples 2.1 to 2.3.

Example 3.3 illustrates the effect of the invention for molding material mixtures in which the lithium compound was added as an additive. Compared with the non-inventive examples 3.1 and 3.2, which contain no lithium, the storage stability of the cores produced with these binders is significantly increased, while the cold strengths are still at a good level.

Influence of the molar ratio [SiO ₂ ] / [M ₂ O] of the binder:
As can be seen from Examples 1.7 to 1.13, as the molar ratio increases, the hot strengths increase while the cold strengths decrease.

Furthermore, it can also be observed that the increasing molar ratio of the binders has a clearly positive effect on the storage stability of the sand cores produced. While for examples 1.11 to 1.13, the strength of the cores after storage in the climatic chamber increase with increasing molar ratio, but due to the opposite trend of decreasing cold strengths no absolute improvement can be found. Thus, for the molar ratio [SiO ₂ ] / [M ₂ O], there is an optimum that the binders of composition 1.9 to 1.12 have. A lower molar ratio leads to a significantly reduced storage stability, while a further increase in the molar ratio has a negative influence on the cold strengths.

3. Studies on the viscosity of the binder 3.1 Viscosity measurements

The viscosity was measured on a Brookfield viscometer equipped with a small sample adapter. In each case about 15 g of the binder to be tested were transferred to the viscometer and measured their viscosity with the spindle 21 at a temperature of 25 ° C and a speed of 100 revolutions per minute. The results of the measurements are summarized in Table 7. <b> Table 7 </ b> Viscosity of the binders used # Viscosity [mPa · s] 1.1 63 not according to the invention 1.2 64 inventively 1.3 66 inventively 1.4 66 inventively 1.5 71 inventively 1.6 79 inventively 1.7 78 not according to the invention 1.8 70 not according to the invention 1.9 66 inventively 1.10 66 inventively 1.11 63 inventively 1.12 68 inventively 1.13 73 not according to the invention 2.1 24 not according to the invention 2.2 25 inventively 2.3 27 inventively

3.2 Results

While the binders of Examples 1.1 to 1.6 differ only in terms of their molar ratio [Li ₂ O _active] / [M ₂ O], the binders of Examples 1.7 to 1.12 have a different molar ratio [SiO ₂ ] / [M ₂ O] a constant value for the molar ratio [Li ₂ O _active ] / [M ₂ O]. The comparison of Examples 1.1 to 1.6 thus illustrates the influence of the molar ratio [Li ₂ O _active ] / [M ₂ O] on the viscosity, while Examples 1.7 to 1.12 reflect the influence of the molar ratio.

Influence of the molar ratio [Li ₂ O _active ] / [M ₂ O] of the binder:
The values summarized in Table 7 for the viscosity make it clear that with increasing molar ratio [Li ₂ O _active ] / [M ₂ O], the viscosity of the binder increases.

Influence of the molar ratio [SiO ₂ ] / [M ₂ O] of the binder:
With regard to the molar ratio of the binder, the viscosity passes through a minimum in the range of the inventive binders of Examples 1.9 to 1.11.

Influence of the K ₂ O content of the binder:
The viscosity of Examples 2.1 to 2.3 is well below the viscosity of the other examples, which is due to the lower solids content of these binders. Nevertheless, the K ₂ O dissolved in the binder exerts a positive influence on the viscosity, which, however, is not apparent from the comparison of the viscosity of Examples 2.1 to 2.3 with Examples 1.1, 1.3 and 1.5 because of the lower solids content of Examples 2.1 to 2.3 ,

In summary, it can be stated that the binders according to the invention of Examples 1.2 to 1.6, 1.9 to 1.12 and 2.2 to 2.3 represent an improvement over the prior art, since the sand cores produced with them have good storage stability with simultaneously high cold strengths. In addition, the binders according to the invention are distinguished by low viscosity values and, owing to their comparatively low lithium content, by low production costs.

4. Studies on sizing stability 4.1. Production and strength tests on the sized specimens

For the investigation of the sizing stability, the waterglass binders 2.1. and 1.3, the preparation of which was described in 1. used. The preparation of the molding material mixture or the test bars used is under 2.1. and 2.2. described. The addition amounts are identical to those in 2.2. and particulate amorphous silica POS BW 90 LD (supplier: Possehl Erzkontor GmbH) was also used. As a further additive together with the amorphous SiO ₂ 0.1 GT glossy powder graphite (manufacturer: Luh) were added to the molding material mixture.

After preparation, the cores were stored at room temperature for 24 hours for complete cure and then immersed in a sizing for 1 to 4 seconds.

The sizing agent was an aqueous, slightly alkaline sizing (pH = 6.5-8.5) having a water content of about 51% and a viscosity of about 0.3-0.6 Pa · s at 25 ° C (product MIRATEC W 8 from ASK Chemicals GmbH). The sized, ie coated with a thin film of size, cores were immediately dried in a drying oven (Model FED 115, Binder) at 100 ° C. An air change of 10 m ³ / h was achieved via an air supply pipe.

The flexural strengths of the sized test bars were determined after 2, 6, 12 and 24 minutes, respectively, after the start of the drying process. Table 8 summarizes the results of the strength tests. The values given here are averages of 10 cores each. For comparison, the flexural strength of untreated test bars was determined. <b> Table 8 </ b> Bending strengths [N / cm ² ] of the test bars produced Dwell time [min] in a drying oven at 100 ° C / after removal from the sizing bath Water glass binder 2.1 not according to the invention Water glass binder 1.3 according to the invention 0 / unsatisfactory 415 385 2 / finished 280 260 6 / settled 90 230 12 / settled 150 235 24 / finished 255 250

4.2 Results

The flexural strengths clearly demonstrate that the cores made with the molding compound blend are significantly more stable to the aqueous sizing. Both the cores made with the binder of the present invention and the cores made with the non-inventive binder undergo a minimum strength at about six minutes after being taken out of the sizing bath before their strength again increases significantly , At this point in time, when the minimum strength occurs, the increased stability of the cores produced with the binder 1.3 according to the invention becomes clear. While the cores produced with binder not according to the invention 2.1 drop to a strength of 90 N / cm ² , the cores produced with the binder 1.3 have a strength of 235 N / cm ² .

In particular for automated series production, such a drop in strength is extremely disadvantageous, as in the example with the binder 2.1, since the casting molds produced are not sufficiently stable against mechanical stress at such low strength values.

Claims (22)

1. Method for preparing a molding material mixture, wherein the molding material mixture is prepared by bringing together at least three of the following components, each of which is provided separate from each another:

   • component (F) comprising at least a refractory basic molding material and no water glass in the form of an aqueous solution of alkali silicates;

   • component (B) comprising at least a water glass in the form of an aqueous solution of alkali silicates as inorganic binder wherein the water glass has a [SiO₂] / [M₂O] molar ratio of 1.90 to 2.47 and does not comprise particulate amorphous SiO₂ and

   • component (A) comprising at least particulate amorphous SiO₂ as an additive component and no water glass in the form of an aqueous solution of alkali silicates,

   wherein the components (A) and (B) together have a [Li₂O_active] / [M₂O] molar ratio of 0.03 to 0.17 wherein

   [M₂O] is the amount of substance in mol of alkali metal M, calculated as M₂O, wherein ultimately only the following compounds enter into the calculation: amorphous alkali silicates, alkali metal oxides and alkali metal hydroxides, including the hydrates thereof, wherein Li is included as part of M without an activity factor,

   [Li₂O_active] is the amount of substance in mol Li, calculated as Li₂O, wherein ultimately only the following compounds enter into the calculation: amorphous lithium silicates, lithium oxides and lithium hydroxide, including the hydrates thereof,

   [SiO₂] is the amount of substance in mol Si, calculated as SiO₂, wherein ultimately only the following compounds enter into the calculation: amorphous alkali silicates,

   wherein an activity factor enters into the calculation of the molar amount of [Li₂O_active] as follows:

   [Li₂O_active] = 1 * amorphous lithium silicates, which are added as constituents of the inorganic binder component (B), calculated as mol Li₂O, +

   1 * lithium oxide, which is added as a constituent of the inorganic binder component (B), calculated as mol Li₂O, +

   1 * lithium hydroxide, which is added as a constituent of the inorganic binder component (B), calculated as mol Li₂O +

   0.33 * amorphous lithium silicates, which are not added as a constituent of the inorganic binder component (B), calculated as mol Li₂O, +

   0.33 * lithium oxide, which is not added as a constituent of the inorganic binder component (B), calculated as mol Li₂O, +

   0.33 * lithium hydroxide, which is not added as a constituent of the inorganic binder component (B), calculated as mol Li₂O, in each case including the hydrates thereof,

   wherein the component (B) comprises LiO₂, LiOH and/or amorphous lithium silicate.
2. Method according to Claim 1, wherein the particulate amorphous SiO₂ has a BET of greater than or equal to 1 m²/g and less than or equal to 35 m²/g, preferably less than or equal to 17 m²/g and particularly preferably of less than or equal to 15 m²/g.
3. Method according to one or more of the preceding claims, wherein the mean particle diameter, determined by dynamic light scattering, of the particulate amorphous SiO₂ in the molding material mixture is between 0.05 µm and 10 µm, especially between 0.1 µm and 5 µm and particularly preferably between 0.1 µm and 2 µm.
4. Method according to one or more of the preceding claims, wherein the molding material mixture contains the particulate amorphous SiO₂

   • in quantities of 0.1 to 2 wt.%, preferably 0.1 to 1.5 wt.%, in each case based on the basic molding material,
   and independently thereof

   • 2 to 60 wt.%, particularly preferably 4 to 50 wt.% based on the weight of the binder, wherein the solids fraction of the binder amounts to 20 to 55 wt.%, preferably from 25 to 50 wt.%.
5. Method according to one or more of the preceding claims, wherein the amorphous SiO₂ used has a water content of less than 15 wt.%, especially less than 5 wt.% and particularly preferably less than 1 wt.% and independently thereof is used especially as a flowable powder.
6. Method according to any one or more of the preceding claims, wherein the molding material mixture contains a maximum of 1 wt.%, preferably a maximum of 0.2 wt.%, organic compounds.
7. Method according to one or more of the preceding claims, wherein the inorganic binder component (B) has a [K₂O]/[M₂O] molar ratio of 0.03 to 0.25, preferably 0.06 to 0.2, particularly preferably from 0.1 to 0.15 in the inorganic binder.
8. Method according to one or more of the preceding claims, wherein the water glass is present in the molding material
   in an amount of 0.2 to 2.5 wt.%, preferably 0.3 to 2 wt.% soluble alkali silicates relative to the basic molding material and calculated as the oxides thereof,
   and/or
   the binder has a solids fraction of greater than or equal to 20 wt.% and less than or equal to 55 wt.%, preferably of greater than or equal to 25 wt.% and less than or equal to 50 wt.%, particularly preferably of greater than or equal to 30 wt.% and less than or equal to 45 wt.%, and especially preferably of greater than or equal to 33 wt.% and less than or equal to 42 wt.%, based on the binder.
9. Method according to one or more of the preceding claims, wherein the lithium compound is added exclusively as a constituent of the inorganic binder and independently thereof, optionally also in addition, [Li₂O_active] is defined as follows:
   the amount of substance in mol of Li, calculated as Li₂O, exclusive of the following compounds: amorphous lithium silicates and/or lithium hydroxide, including the hydrates thereof.
10. Method according to one or more of the preceding claims, wherein the molding material mixture furthermore contains surfactants, preferably selected from the group of anionic surfactants, especially those with a sulfonic acid group or sulfonate group.
11. Method according to Claim 10, wherein the surfactant is present in the molding material mixture in a fraction of 0.001 to 1 wt.%, particularly preferably 0.01 to 0.2 wt.% based on the weight of the refractory basic molding material.
12. Method according to one or more of the preceding claims, wherein the [SiO₂] / [M₂O] molar ratio is from 1.95 to 2.40, preferably from 2 to 2.30.
13. Molding material mixture according to one or more of the preceding claims, wherein the [Li₂O_active] / [M₂O] molar ratio is 0.035 to 0.16, preferably 0.04 to 0.14.
14. Method according to one or more of the preceding claims, wherein the lithium silicate, Li₂O and LiOH, including the hydrates thereof, are present in homogeneous solution in the binder or in homogeneous solution in component (B) and are homogeneously and completely dissolved, without a precipitate, in the aqueous solvent as a constituent of the binder or of component (B).
15. Lithium-containing inorganic binder (B) comprising at least water glass in the form of an aqueous solution of alkali silicates as the inorganic binder and having

    • a [SiO₂] / [M₂O] molar ratio of 1.9 to 2.47 in the inorganic binder (B) and

    • a [Li₂O_active]/[M₂O] molar ratio of 0.04 to 0.14 in the inorganic binder (B),

    wherein

    [M₂O] is the amount of substance in mol of alkali metal M, calculated as [M₂O], wherein ultimately only the following compounds enter into the calculation: amorphous alkali silicates, alkali metal oxides and alkali metal hydroxides, including the hydrates thereof, wherein Li is included as part of M without an activity factor,

    [Li2O_active] is the amount of substance in mol Li, calculated as [Li₂O], wherein ultimately only the following compounds enter into the calculation: amorphous lithium silicates, lithium oxides and lithium hydroxide, including the hydrates thereof,

    [SiO₂] is the amount of substance in mol Si, calculated as [SiO₂], wherein ultimately only the following compounds enter into the calculation: amorphous alkali silicates,

    and an activity factor is included in [Li₂O_active] as follows:

    [Li₂O_active] = 1 * amorphous lithium silicates, which are added as a constituent of the inorganic binder component (B), calculated as mol [Li₂O], +

    1 * lithium oxide, which is added as a constituent of the inorganic binder component (B), calculated as mol [Li₂O] +

    1 * lithium hydroxide, which is added as a constituent of the inorganic binder component (B), calculated as mol [Li₂O],

    in each case including the hydrates thereof, and Li₂O, LiOH and/or amorphous lithium silicate including the hydrates thereof are present in a homogenous solution in the lithium containing binder and are compeletly, without precipitate homogeneously dissolved in the aqueous solvent as a constituent of the lithium containing binder.
16. Lithium-containing inorganic binder according to Claim 15, wherein the lithium-containing inorganic binder has a [SiO₂] / [M₂O] molar ratio of 1.95 to 2.40, preferably of 2 to 2.30.
17. Lithium-containing binder according to Claim 15 or 16, wherein the binder furthermore comprises surfactants, preferably selected from the group of anionic surfactants, especially those with a sulfonic acid- or a sulfonate group.
18. Lithium-containing binder according to one or more of Claims 15 or 16, wherein the binder has a [K₂O]/[M₂O] molar ratio of 0.03 to 0.25, preferably 0.06 to 0.2 and particularly preferably of 0.1 to 0.15.
19. Method for preparing casting molds or cores comprising:

    • the method for preparing the molding material mixture according to at least one of Claims 1 to 14,

    • introducing the molding material mixture into a mold, and

    • curing the molding material mixture.
20. Method according to Claim 19, wherein the molding material mixture is introduced into the mold by using a core shooting machine operated by compressed air and the mold is a molding tool and the molding tool has one or more gases passing through, especially CO₂ or gases containing CO₂, preferably CO₂ heated to above 60°C and/or air heated to above 60°C.
21. Method according to Claim 19 or 20, wherein the molding material mixture is exposed to a temperature of at least 100°C for less than 5 min for curing.
22. Method according to Claim 19, wherein a gas, preferably air, is passed through the molding material mixture for curing it and said gas has a temperature of 100 to 180°C, particularly preferred of 120 to 150°C.