Salt-based cores and process for their production

The invention relates to cores and also to a process for producing cores from salt by means of core shooting for use as cavity place-holders in the production of metallic castings, preferably with the aid of die-casting technology, said cores dissolving completely in a solvent without solid residues remaining and therefore being capable of being removed from the workpieces completely and without difficulty.

Cores that in the case of the die-casting of workpieces from metal are inserted into the dies in order to keep the cavities provided in the workpieces free when the dies are being filled with the melt are subject to distinctly more stringent demands than, for example, in the case of sand casting or in the case of low-pressure chill casting. The cores have to be capable of being produced easily, have to be dimensionally stable and precise in their contour, and the materials used for their production as well as the solvents dissolving them should place no burden either on the quality of casting or on the environment and should not give rise to any health hazards.

If special demands are made of the surface and of the contour precision of the cavities of the workpieces, the surface of the cores has to be particularly smooth and precise in its contour, and the cores have to dissolve completely in a suitable solvent and have to be capable of being removed easily from the cavities of the workpieces without solid residues remaining. Residues of cores that contain insoluble components - such as, for example, quartz sand - may lead to damage on surfaces to be modified or may bring about the failure of an assembly, for example if core residues lead to the blockage of an injection nozzle in the common-rail system of a diesel-electric generating set.

The object of the present invention is to produce cores from salt that in the course of die-casting of the workpieces exhibit the necessary strength and are capable of being easily and completely removed from the workpieces.

In accordance with the state of the art it has not been possible hitherto to produce salt cores, also called closed salt cores, by the so-called core-shooting process that withstand the extreme stresses that arise, for example, in the course of aluminium die-casting. That is to say, on the one hand the cores have to exhibit high strength and, on the other hand, they have to be capable of being easily dissolved out of the casting after casting. According to the state of the art it is possible for sand cores with water glass as binder to be produced that exhibit a maximum strength of 500 N/cm2. In the case of the cores according to the invention, distinctly higher values are obtained, and nevertheless the cores are capable of being removed easily and in residue-free manner after casting.

The object is achieved with cores corresponding to the first claim and also with a process for producing these cores according to Claim 12. Advantageous configurations of the invention are claimed in the dependent claims.

The cores according to the invention consist of a salt to which binders and optionally fillers, additives and catalysts may be admixed. These cores are provided for workpieces that are cast from non-ferrous metals - for example, aluminium, brass or copper - by the die-casting process. The cores according to the invention are composed of substances that dissolve completely in water by -way of solvent that is preferred for reasons of environmental protection, and in this way are capable of being removed from the cavities of the workpieces in residue-free manner.

The cores according to the invention have the advantage that they are composed of substances that, given appropriate handling, display no gas-evolving reactions that burden the environment either in the course of their production or in the course of the casting process. By virtue of the fact that no gases arise in the course of casting, the quality of the castings is improved, inasmuch as casting defects such as shrinkage cavities, gas pores or similar due to core gases arising can be avoided. In the course of removal of the cores from the workpieces no residues arise that require special disposal. Depending on their composition, the substances can be recovered from the liquid phase by suitable processes; for example, the salt can be recovered by spray drying or evaporating.

All compositions according to the invention of the core materials can be processed in conventional core-shooting machines by core shooting by way of forming process. The complexity of the geometry of the cores determines the core- shooting parameters as well as the configuration and constructional design of the tool for producing the cores and of the shooting head of the core-shooting machine. In comparison with forming by pressing, in which the core materials are charged into a forming tool and are then compacted under pressure, core shooting enables the production of cores of geometrically very complicated structure with great contour precision on the surface and also with homogeneous grain structure with uniform density and strength.

Suitable by way of material for the cores according to the invention are the water-soluble salts of the alkali elements and alkaline-earth elements, such as, in particular, sodium chloride, potassium chloride and magnesium chloride, the water-soluble sulfates and nitrates of the alkali elements and alkaline-earth elements, such as, in particular, potassium sulfate, magnesium sulfate, as well as water- soluble ammonium salts such as, in particular, ammonium sulfate. These substances may be employed individually or also as a mixture, so long as they do not react with one another and in this way exert a negative influence on the desired properties, because during the production of the core the core material is not to undergo any transmutation that has a negative influence on its solubility. Generally suitable are all readily soluble salts having a decomposition-point or melting-point that lies above the temperature of the liquid molten metal. The core materials can, in a manner comparable to sand, be divided up readily and simply into the desired grain sizes or grain-size classes. In particular, the surface condition of the cores is influenced by the chosen grain-size distribution. The smaller the grain size, the smoother the surface. Generally, a degree of filling that is as high as possible is striven for, which can be obtained by mixing various salts and, where appropriate, the additional substances with differing distribution curves, for example by virtue of a bimodal or trimodal grain-size distribution of the mixture.

In accordance with the invention, grain sizes within the range from 0.01 mm up to 2 mm are chosen, depending on the material, the desired surface quality and the contour precision of the workpiece to be cast.

Water-soluble fillers may, where appropriate, replace a fraction of the salt, up to 30 vol.%, so long as the density and strength are not negatively influenced thereby. The grain size of the filler is expediently matched to the grain size or grain-size distribution of the salt.

In order to guarantee the requisite stability of the cores after core shooting, a suitable binder or a suitable binder system is added to the salt prior to core shooting. All binders are possible that are completely water-soluble after the curing process and that effectively wet the salt and, where appropriate, the fillers, in which case the mixture of these substances is capable of being formed into cores by means of core shooting. Generally, silicate-like binders are suitable if they are water-soluble. Also employable are the water-soluble alkali phosphates and ammonium phosphates, or binders consisting of monoaluminium phosphate. Preferred are binders consisting of soluble water glass. The added quantity is dependent on the water-glass modulus, 1 to 5, and lies, depending on the wetting behaviour, between 0.5 wt.% and 15 wt.%, preferably 5 wt.% to 8 wt.%. In order to obtain the properties necessary for the die-casting process, such as strength and dimensional stability, special mixtures of binders may also be employed.
The properties of a mixture of salt, optionally filler and binder or binder system can be influenced by the targeted addition of additives. Here too, a prerequisite is that these additives or the reaction products of these additives are capable of being easily removed completely and in residue-free manner from the cavity of a workpiece by dissolution in water, and that in the course of casting no gases impairing the casting operation are released which may lead to casting defects. Depending on the composition of the core materials, these additives may be: wetting agents, admixtures influencing the consistency of the mixture, lubricants, de-agglomeration admixtures, gelling agents, admixtures that change the thermophysical properties of the core, for example the thermal conductivity, admixtures that prevent adhesion of the metal to the cores, admixtures that lead to better homogenisation and miscibility, admixtures that increase the shelf life, admixtures that prevent premature curing, admixtures that prevent formation of smoke and condensate in the course of casting, as well as admixtures that result in acceleration of curing. These additives are known to a person skilled in the art from the production of conventional cores. Their added quantity depends upon the type and composition of the core material.

In order that the cores exhibit the requisite strength after core shooting, it may be necessary, depending on the composition of the core material, to employ catalysts, matched thereto, which initiate and accelerate the curing.

In the case of gaseous catalysts, the gas influencing the core material, preferably CO2 or air, can be blown into the still closed die, in particular for the purpose of curing and drying the cores after shooting. The pressure may be less than in the course of shooting the cores and may amount 'o approximately up to 5 bar.

Also possible is a thermal after treatment of the cores at temperatures up to 500 °C. As a rule, a thermal treatment is undertaken already during the forming in the die, by the heating thereof to a temperature matched to the core material.

The core material is composed of the salt and the binder and also of the added substances such as fillers, additives and catalysts, to the extent that they are required, the fillers and binder being inorganic. All substances can be mixed homogeneously using known mixing units. The added quantity of binder and added substances is to be chosen in a manner depending on the intended purpose of the cores, and determines the surface quality as well as the density and strength of the cores.

Processing of the core materials is undertaken separately from the core-shooting process, whereby suitable preventive measures for preventing agglomeration and premature curing have to be provided where appropriate. For example, processing, transportation and storage may also be undertaken under protective gas, depending on the composition of the core material.

Substances that change the properties of the other substances of the core material, particularly those which are required for curing, are advantageously input directly into the core-shooting machine. Intermixing is then effected in the stream of gas that transports the other substances into the die. The core material is blown into the die with pressures between 1 bar and 10 bar, matched to the composition of the core material and to the filling power and fluidity of the mass. In this connection the filling pressure is dependent on the grain-size distribution or on the grain size and grain shape. Fine-grained salts generally require higher shooting pressures.

The composition and the properties of a core have significant influence on the quality of die-casting. On the basis of an exemplary embodiment, therefore, the most important properties will be listed here. The stated properties relate to cores that have not been covered with a coating.

A core is employed consisting of NaC1 with the following additional substances such as water-glass binder and with further admixtures such as release agent, setting retarder, wetting agent and others. The core was formed in a core- shooting machine with a pressure of 6 bar. It was subjected to a thermal treatment of 1 min duration at 200ºc for the purpose of curing. The present core is particularly suitable for use in aluminium die-casting. In the case of aluminium die-casting, liquid aluminium with a pressure from 10 MPa to 200 MPa is pressed into the die. It flows into the die at a speed of up to 120 m/sec. In order to be able to resist the forces arising in the course of casting, the core has to be dimensionally stable. The mechanical properties of the material were determined in respect of a test piece having the dimensions 180 mm length, 22 mm width and 22 mm height. The flexural strength, measured in accordance with VDG instruction leaflet P73 (February 1996), amounts to 1400 5/cm2-

In the course of the influx of the metal the surface of the core must not be finished out or damaged. For this reason the core has- to exhibit a corresponding surface strength. The porosity also plays a crucial role. The proportion of pores in this exemplary embodiment amounts to 30 %.

As a rule, salt cores based on sodium chloride have a density from 1.2 g/cm3 to 1.8 g/cm3, determined by the buoyancy method. This corresponds to a porosity of 10 % to 35 %. The flexural strength, measured in accordance with VDG instruction leaflet P73, lies between 400 N/cm2 and 1500 N/cm2.

After the die-casting has cured, the core has to be removed. In this connection it is importance that the core dissolves completely and easily, at once and without solid residues. The rate of dissolution of the core material is naturally dependent on the core material and on its pretreatment and also on the size of the core; in the case of pure salt, said rate may differ from that in the case of a composition with binder and fillers. Experiments with a die-casting test piece have shown that a core having the dimensions 22 mm X 22 mm X 180 mm can be washed out of the casting completely within 1 min to 2 min with hot water.

The present invention consequently relates to:

> Water-soluble salt cores that can be produced by compacting a mixture consisting of water-soluble salts, at least one binding agent and, where appropriate, additional auxiliaries such as fillers, additives and catalysts (= core material) under pressure, wherein the binding agent and the, where appropriate, additional auxiliaries are inorganic and the salt cores are formed by means of core-shooting processes;

• wherein forming is undertaken at pressures from 1 bar to 10 bar;

• wherein the formed cores exhibit a density from 1.2 g/cm3 to 1.8 g/cm3;

• wherein they exhibit a porosity from 10 % to 40 %;

• wherein they exhibit a flexural strength between 400 N/cm2 and 1500 N/cm2;

• wherein by way of water-soluble salts those are employed having a decomposition-point or melting- point that lies above the temperature of the liquid metal;

• wherein chlorides of the alkali elements and alkaline-earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, water-soluble sulfates and nitrates of the alkali elements and alkaline-earth elements, in particular potassium sulfate and/or magnesium sulfate, water-soluble ammonium salts, in particular ammonium sulfate, or mixtures of these salts, are employed by way of water-soluble salts;

• wherein the grain sizes of the core materials lie within the range from 0.01 mm to 2 mm;

• wherein a fraction of the core material contains a water-soluble filler, the grain size of the filler has been matched to the grain size of the core material, and the proportion of the filler in the core material may amount to up to 30 vol.%;

• wherein water-soluble silicate compounds, preferably water glasses, alkali phosphates, ammonium phosphates and/or monoaluminium phosphate, or mixtures of these compounds, are employed by way of binding agents;

• wherein the proportion of binding agents lies between 0.5 wt.% and 15 wt.%;

• wherein the binding agent is a water glass and the proportion of the binding agent lies between
0.5 wt.% and 15 wt.%, depending on the wetting behaviour and the water-glass modulus;

• wherein the water-soluble salt is sodium chloride with a grain size between 0.01 mm and 2 mm and the binding agent is water glass;

• wherein the binding agent water glass is contained with a proportion from 0.5 wt.% to 15 wt.%, depending on the grain-size distribution and matched to the water-glass modulus;

• wherein the water-soluble salt is sodium chloride with a grain-size range from 0.04 mm to 0.6 mm, the binding agent is water glass with a proportion of
6 wt.%, and wherein forming is undertaken at room temperature in a core-shooting machine with a shooting pressure of 6 bar, and curing is effected with hot air;

• wherein the density amounts to 1.35 g/cm3, the open porosity amounts to 30 %, and the flexural strength is 1400 N/cm2;

• wherein after the forming the salt cores are heat- treated at a temperature of 500 ºC.

The enumerated features labelled by • signify optional, preferred configurations of the water-soluble salt cores according to the invention.

The present invention consequently further relates to:

> Processes for producing water-soluble salt cores from a mixture consisting of water-soluble salts, at least one binding agent and, where appropriate, additional auxiliaries such as fillers, additives and catalysts, characterised in that the mixture which is completely soluble in water and capable of being removed from the workpieces with water in residue-free manner is mixed homogeneously in non-liquid form and after the core- shooting process is formed into the salt core with pressures matched to the composition of the core material, to the grain-size distribution or to the grain size and grain shape;

° wherein the salt cores are formed at pressures from 1 bar to 10 bar;

° wherein the constituents are mixed with grain sizes of differing distribution curves, preferably by virtue of a bimodal or trimodal grain-size distribution of the constituents, in order to obtain a high degree of filling of the dies by the mixture (the core material);

° wherein chlorides of the alkali elements and alkaline-earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, water-soluble sulfates and nitrates of the alkali elements and alkaline-earth elements, in particular potassium sulfate and/or magnesium sulfate, as well as water-soluble ammonium salts, in particular ammonium sulfate, or mixtures of these salts, are chosen by way of water-soluble salts which, where appropriate with the additional auxiliaries, are mixed homogeneously and formed into the core;

° wherein the core materials are used with grain sizes within the range from 0.01 mm to 2 mm, depending on the material, the desired surface quality and the contour precision of the workpiece to be cast from metal;

° wherein the additional filler or fillers is/are added to the core material with a proportion of up to 30 vol.% and the grain size of the filler is matched to the grain size of the salt or salts;

° wherein one or more binders is/are added, with a proportion depending on the specific surface area, the wetting behaviour and the grain-size distribution, and these binders are preferably water-soluble silicate compounds, in particular water glasses, alkali phosphates, ammonium phosphates and monoaluminium phosphate;

° wherein by way of binder a water glass is added, with a proportion from 5 wt.% to 2 0 wt.%;

° wherein water-soluble additives matched to the core material are added;

° wherein water-soluble catalysts matched to the core material are added;

° wherein after the shooting the cores are gassed with gases matched to the core material for the purpose of curing;

° wherein gassing is effected with hot air;

° wherein gassing is effected with CO2;

° wherein the pressure in the course of gassing amounts to up to 10 bar;

° wherein after the shooting the cores are cured at temperatures up to 500 °C by a heat treatment matched to the core material.

The enumerated features labelled by o signify optional, preferred configurations of the process according to the invention for producing water-soluble salt cores.

Claims
1. Water-soluble salt cores that can be produced by compacting a mixture consisting of water-soluble salts, at least one binding agent and, where appropriate, additional auxiliaries such as fillers, additives and catalysts (= core material) under pressure, characterised in that the binding agent and the, where appropriate, additional auxiliaries are inorganic and the salt cores are formed by means of core-shooting processes.

2. Water-soluble salt cores according to Claim 1, characterised in that forming is undertaken at pressures from 1 bar to 10 bar.

3. Water-soluble salt cores according to Claim 1 or 2, characterised in that the formed cores exhibit a density from 1.2 g/cm3 to 1.8 g/cm3.

4. Water-soluble salt cores according to one or more of Claims 1 to 3, characterised in that they exhibit a porosity from 10 % to 40 %.

5. Water-soluble salt cores according to one or more of Claims 1 to 4, characterised in they exhibit a flexural strength between 400 N/cm2 and 1500 N/cm2.

6. Water-soluble salt cores according to one or more of Claims 1 to 5, characterised in that by way of water- soluble salts those are employed having a decomposition- point or melting-point that lies above the temperature of the liquid metal.

7. Water-soluble salt cores according to one or more of Claims 1 to 6, characterised in that chlorides of the alkali elements and alkaline-earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, water-soluble sulfates and nitrates of the alkali elements and alkaline-earth elements, in particular potassium sulfate and/or magnesium sulfate, water-soluble ammonium salts, in particular ammonium sulfate, or mixtures of these salts, are employed by way of water-soluble salts.

8. Water-soluble salt cores according to one or more of
Claims 1 to 7, characterised in that the grain sizes of the core materials lie within the range from 0.01 mm to 2 mm.

9• Water-soluble salt cores according to one or more of Claims 1 to 8, characterised in that a fraction of the core material contains a water-soluble filler, in that the grain size of the filler has been matched to the grain size of the core material, and in that the proportion of the filler in the core material may amount to up to 30 vol.%.

10. Water-soluble salt cores according to one or more of Claims 1 to 9, characterised in that water-soluble silicate compounds, preferably water glasses, alkali phosphates, ammonium phosphates and/or monoaluminium phosphate, or mixtures of these compounds, are employed by way of binding agents.

11. Water-soluble sail cores according to one or more of Claims 1 to 10, characterised in that the proportion of binding agents lies between 0.5 wt.% and 15 wt.%.

12. Water-soluble salt cores according to one or more of Claims 1 to 11, characterised in that the binding agent is a water glass and the proportion of the binding agent lies between 0.5 wt.% and 15 wt.%, depending on the wetting behaviour and the water-glass modulus.

13. Water-soluble salt cores according to one or more of Claims 1 to 12, characterised in that the water-soluble salt is sodium chloride with a grain size between
0.01 mm and 2 mm and the binding agent is water glass.

14. Water-soluble salt cores according to Claim 13, characterised in that the binding agent water glass is contained with a proportion from 0.5 wt.% to 15 wt.%, depending on the grain-size distribution and matched to the water-glass modulus.

15. Water-soluble salt cores according to one or more of Claims 1 to 14, characterised in that the water-soluble salt is sodium chloride with a grain-size range from 0.04 mm to 0.6 mm, the binding agent is water glass with a proportion of 6 wt.%, and wherein forming is undertaken at room temperature in a core-shooting machine with a shooting pressure of 6 bar, and curing is effected with hot air.

16. Water-soluble salt cores according to one or more of Claims 1 to 15, characterised in that the density amounts to 1.35 g/cm3, the open porosity amounts to 30 %, and the flexural strength is 1400 N/cm2.

17. Water-soluble salt cores according to one or more of Claims 1 to 16, characterised in that after the forming the salt cores are heat-treated at a temperature of
500 ºC.

18. Process for producing water-soluble salt cores from a mixture consisting of water-soluble salts, at least one binding agent and, where appropriate, additional auxiliaries such as fillers, additives and catalysts, characterised in that the mixture which is completely soluble in water and capable of being removed from the workpieces with water in residue-free manner is mixed homogeneously in non-liquid form and after the core- shooting process is formed into the salt core with pressures matched to the composition of the core material, to the grain-size distribution or to the grain size and grain shape.

19. Process according to Claim 18, characterised in that the salt cores are formed with pressures from 1 bar to 10 bar.

20. Process according to Claim 18 or 19, characterised in that the constituents are mixed with grain sizes of differing distribution curves, preferably by virtue of a bimodal or trimodal grain-size distribution of the constituents, in order to obtain a high degree of filling of the dies by the mixture (the core material).

21. Process according to one or more of Claims 18 to 20, characterised in that chlorides of the alkali elements and alkaline-earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, water-soluble sulfates and nitrates of the alkali elements and alkaline-earth elements, in particular potassium sulfate and/or magnesium sulfate, as well as water-soluble ammonium salts, in particular ammonium sulfate, or mixtures of these salts, are chosen by way of water-soluble salts which, where appropriate with the additional auxiliaries, are mixed homogeneously and formed into the core.

22. Process according to one or more of Claims 18 to 21, characterised in that the core materials are used with grain sizes within the range from 0.01 mm to 2 mm, depending on the material, the desired surface quality and the contour precision of the workpiece to be cast from metal.