Patent Publication Number: US-2015060005-A1

Title: Salt-based cores, method for the production thereof and use thereof

Description:
The invention relates to salt-based cores, to methods for producing salt-based cores, and to the use of such cores as cavity placeholders in the production of metal cast parts, preferably in permanent mold casting technology, which can be completely and easily removed from the workpieces using solvents, without any solid residue remaining. 
    
    
     Cores that are inserted into the molds when casting workpieces made of metal so as to keep the cavities provided in the workpieces free when filling the molds with the melt are subject to demanding requirements. The cores must be easy to produce, be dimensionally stable and have precise contours, and the materials used for the production of the same and the solvents dissolving them should not adversely affect the casting quality or the environment, and should not cause any health hazards. 
     If special demands are imposed on the surface and the contour accuracy of the cavities of the workpieces, the surface of the cores must be particularly smooth and have precise contours, and the cores must be dissolved completely in a suitable solvent and be easy to remove from the cavities of the workpieces, without any solid residue remaining. Residues of cores containing insoluble components, such as silica sand, can result in damage to the surfaces to be finished or cause units to fail, for example when core residue results in the clogging of an injection nozzle in the common rail system of a diesel engine. 
     Salt-based cores that withstand both the extreme thermal stresses and the mechanical stresses that occur during overcasting with fusible metals, and cores that not only have high strength, but can also be easily removed from the cast part after the casting process and leave the best possible smooth surface finish on the cast part, can now be produced using the so-called dry pressing process, without a subsequent sintering or recrystallization process. 
     It is the object of the present invention to produce salt-based cores that have low porosity, a good surface quality and the highest possible strength, and can be easily and completely removed from the workpieces after the workpieces have been cast. 
     It is a further object of the present invention to produce such cores in a shaping method that is as simple and cost-effective as possible, preferably using the so-called dry pressing method. 
     It is a further object of the present invention to improve this dry pressing method and to provide cores that have considerably improved strength and are nonetheless easily removed from the cast part after casting, and that leave a good, smooth surface finish on the cast part. 
     According to the invention, these objects are achieved by the main claim and by the method according to claim  31 . Advantageous embodiments of the invention are characterized in the dependent claims. 
     The cores according to the invention are composed of a salt, with which a special binder system, and optionally auxiliary substances such as fillers, additives, wetting agents and catalysts, can be admixed. These cores are preferably intended for workpieces that are cast from nonferrous metals, such as aluminum, brass or copper, using the permanent mold casting method. The cores according to the invention are composed of substances that can be removed from the cavities of the workpieces without leaving any residue, using water, which is the preferred solvent for environmental protection reasons. 
     The cores according to the invention have the advantage that they are composed of substances (core materials) that, when handled properly, have no gas-liberating reactions which would harm the environment, neither during the production thereof nor during the casting process. In addition, since no cracking products of an organic binder develop during casting, the quality of the cast parts is improved due to the fact that casting defects such as blowholes, gas pores or the like resulting from developing core gases can be prevented. No residue that would require special disposal is created during removal of the cores from the workpieces. Depending on the composition, the substances can be recovered from the liquid phase by using suitable methods; for example, the salt can be recovered by spray drying or concentration. 
     All compositions of the core materials can be processed in conventional mechanical or hydraulic presses by compaction. The complexity of the geometry of the cores determines the manufacturing parameters as well as the configuration and design of the tool used to produce the cores, and of the press. 
     Suitable materials for the cores according to the invention are the salts of alkali elements and alkaline earth elements such as in particular sodium chloride, potassium chloride and magnesium chloride, the sulfates and nitrates of the alkali elements and alkaline earth elements such as in particular potassium sulfate, magnesium sulfate, as well as ammonium salts such as in particular ammonium sulfate. The water-soluble compounds of these core materials are preferred. These substances can be used individually or as mixtures, provided that they do not react with each other and thereby adversely affect the desired properties, since the core material should not undergo any substance transformation during production of the core which would adversely affect the residue-free removal of the core material. In general, all readily soluble salts having a decomposition or melting point above the temperature of the liquid molten metal are suitable. Similarly to sand, the core materials can be easily and simply divided into the desired particle sizes or grain size classes. The selected grain size distribution and the selected degree of compaction influence in particular the surface finish of the cores. The smaller the grain size, the smoother the surface will be. In general, as high a degree of compaction as possible is desirable, which can be achieved by mixing different salts, and optionally the additional substances, having different distribution curves, for example by a bimodal or trimodal grain size distribution in the mixture. 
     According to the invention, grain sizes in the range from 0.01 mm to 2 mm are preferred, depending on the core material, the desired surface quality, and the contour accuracy of the workpiece to be cast. Depending on the desired degree of compaction, grain size fractions of 0.01 mm to 0.29 mm, 0.3 mm to 1.3 mm and/or 1.31 mm to 2.0 mm are mixed in different proportions. 
     Fillers, which can likewise be completely removed without leaving any residue when using water as the solvent, can optionally replace a portion of the salt to the extent that the density and strength are not adversely affected. According to the invention, it has been shown that as much as 30% by weight of the salt can be replaced with appropriate fillers. The grain size of the filler is advantageously matched to the grain size or the grain size distribution of the salt. 
     So as to ensure the necessary stability of the cores after the shaping process, at least one suitable binder system is added to the salt prior to compaction. 
     The approach according to the invention for achieving the object underlying the invention provides for the use of a special binder system which comprises a binder/binding agent and a drying agent tailored thereto. 
     Essentially all binders/binding agents which after the curing process can be removed without leaving any residue, using water as the solvent, and which wet the salt and optionally the additional substances, can be used in this binder system according to the invention, wherein the mixture of these substances must be shapeable into lost cores by compaction. In general, inorganic phosphates, inorganic borates, silicate compounds or mixtures of these binding agents are suitable as binders/binding agents, if they can be removed without leaving any residue, using water as the solvent. For example, alkali phosphate or ammonium phosphate, monoaluminum phosphate, boron phosphate, trisodium phosphate, tetrapotassium pyrophosphate or sodium polyphosphate can be used as the inorganic phosphate. 
     Binders/binding agents made of water-soluble silicates, such as water-soluble water glass having a water glass module of 1 to 5, are preferably used, wherein water glasses having differing water glass modules can also be present as a mixture. The amount added is dependent on the water glass module that is used and, depending on the wetting behavior, is between 0.5% by weight and 15% by weight, preferably between 5% by weight and 8% by weight. So as to achieve the properties necessary for the subsequent casting process, such as strength and dimensional stability, it is also possible to use special mixtures of binders. 
     For the further processing of the core material to form the usable core, the form in which the core material is present is of essential importance. If solid core materials are required, as is the case with the present invention, it is crucially important whether the core materials are present in agglomerated or deagglomerated form, and whether they are present in free-flowing form. Only free-flowing core material mixtures are able to independently and fully fill so-called filling shoes used in the dry pressing method, the preferred shaping method according to the invention. Only free-flowing mixtures comprising the salt, the binder system that is used, and the other admixed substances are therefore usable as the core material for use in the dry pressing method. 
     However, so as to substantially improve the usable core materials for the dry pressing method in the manner according to the invention, it is important to further improve the free flowability of the core materials, in particular when using the above-mentioned binders/binding agents. 
     This is surprisingly achieved according to the invention by adding suitable drying agents in appropriate amounts as a function of the selected binders/binding agents. The combination of binders/binding agents and the drying agent forms the special binder system provided according to the invention. This binder system surprisingly allows the object underlying the invention to be achieved. 
     All hydrophilic substances that are able to reversibly bind water, which is to say that are able to release the absorbed water through suitable treatment, are suitable as drying agents that can be used according to the invention. According to the invention, for example, finely dispersed silicic acids, such as Aerosil, silica gel, zeolites, anhydrous sodium sulfate and/or magnesium sulfate can be used. Due to the chemical and structural properties of these drying agents, they trap water molecules and subsequently change the physical molecular structure thereof due to intermolecular forces. Water molecules can thus no longer escape from the structure and remain bound during the preparation of the core materials. The bound water can be released again by the application of heat. 
     According to the invention, the amount of the drying agents added is always dependent on the type and amount of binders/binding agents used, and can be easily determined by simple experimentation. Slight overdosing of the drying agent can be tolerated. 
     For example, when using 1 to 5% by weight water glass as the binder/binding agent, based on the amount of salt that is used, 0.3 to 1.5% by weight Aerosil, based on the amount of salt that is used, is sufficient as the drying agent that can be used according to the invention to not only ensure the free flowability of the core material, but also to enable an improvement in the free flowability of the core material, compared to core materials that comprise 1 to 5% by weight water glass as the binder/binding agent, based on the amount of salt that is used, but that comprise no drying agent. 
     By using the special binder system according to the invention, comprising the combination of binder/binding agent and drying agent, it is also possible for the first time to use binders/binding agents in liquid form in the preparation of the core materials. Wetting of the core material constituents with the binder/binding agent is considerably improved due to the use of liquid binders/binding agents. The core material constituents, in particular the salt grains which are enveloped by the binder/binding agent, are thus coated. The result is a finished salt core having considerably improved strength. According to the invention, for example, an aqueous 60% tetrapotassium pyrophosphate solution can be used as the liquid binding agent. Based on the amount of salt that is used, 1 to 5% by weight, preferably 2 to 4% by weight, and particularly preferably 2.5% by weight, of aqueous 60% tetrapotassium pyrophosphate solution is added in this variant. So as to still ensure the desired free flowability of the core material, the drying agent that is provided according to the invention is added in a sufficient amount to the core material coated in this way with the binding agent. Slight overdosing of the drying agent can be tolerated in this case as well. 
     It is particularly advantageous when tetrapotassium pyrophosphate in solid form is additionally added to the aqueous 60% tetrapotassium pyrophosphate solution in the same amount, or also in a larger amount. 
     The properties of the mixture according to the invention comprising salt, optionally auxiliary substances such as additives, fillers, wetting agents and/or catalysts, and the special binder system according to the invention can be influenced by the targeted addition of additives. Here as well, the prerequisite is that these additives, or the reaction products of these additives, can be completely removed from the cavity of a workpiece without difficulty and without leaving any residue, using water as the solvent, and that during casting, no gases that impair the casting process are released which can result in casting defects. Depending on the composition of the core materials, these additives can be selected from: wetting agents, for example surfactants, additives that influence the consistency of the mixture, lubricants, deagglomeration additives, gelling agents, additives that modify the thermophysical properties of the core, for example the thermal conductivity, additives that prevent the metal from adhering to the cores, additives that result in improved homogenization and miscibility, additives that increase the shelf life, additives that prevent premature curing, additives that prevent the formation of smoke and condensate during casting, and additives that result in accelerated curing. These additives are known to a person skilled in the art from the production of conventional cores. The amount added depends on the type and composition of the core material. 
     So as to further improve the strength that is required after dry pressing, it may be necessary, depending on the composition of the core material, to employ catalysts that are tailored to the core material and that initiate and accelerate curing of the core material. 
     It has surprisingly been shown that the addition of in particular fine-grained salt, and preferably the addition of powdered salt having a particle size of less than 100 nm, acts as a catalyst for curing. 
     If gaseous catalysts are employed according to the invention, the gas influencing the core material, preferably CO 2  or air, can be blown into the mold while the same is still closed after dry pressing, in particular for curing and drying the cores. The pressure can be as high as 5 bar. 
     It is also possible to carry out thermal post-treatment of the cores at temperatures up to 600° C., preferably at temperatures between 500° C. and 600° C., and particularly preferably at temperatures of 580° C. 
     The core material is composed of the salt and the binder system and, if necessary, the additional substances such as fillers, additives and catalysts, wherein the fillers and the binder system are inorganic. All substances can be homogeneously mixed using known mixing units. The added amounts of the binder system and of the additional substances are to be selected as a function of the intended purpose of the cores, and determine the surface quality as well as the density and strength of the cores. 
     Preparation of the core materials takes place separately from the manufacturing process, wherein optionally suitable protective measures may have to be provided to prevent agglomeration and premature curing. For example, depending on the composition of the core material, the preparation, transport and storage can also take place under a protective gas or a vacuum. 
     The composition and the properties of a core decisively influence the quality of the cast part. 
     The sodium chloride-based salt cores produced according to the invention typically have a density of 1.5 g/cm 3  to 1.9 g/cm 3 , and preferably of 1.2 g/cm 3  to 1.8 g/cm 3 , determined according to the buoyancy method. This corresponds to a porosity of 10% to 35%, and preferably of 5% to 25%. The flexural strength, measured according to VDG (Association of German Foundry Experts) Technical Bulletin P73, is between 400 N/cm 2  and 1500 N/cm 2 . 
     The most important properties are therefore discussed below, based on one exemplary embodiment. The described properties refer to the cores that are not coated with a black wash. 
     A core made of NaCl is used, comprising additional substances such as water glass binder, Aerosil as the drying agent, and further added substances such as release agents, retarding agents, wetting agents and the like. The core was shaped at a pressure of 50 to 120 bar on a hydraulic press. The core was subjected to thermal post-treatment for 60 minutes at 580° C. for curing. The present core is particularly suitable for use in aluminum permanent mold casting. The core must be dimensionally stable in order to be able to withstand the temperatures and forces that occur during casting. The mechanical properties of the core were determined using a sample 180 mm long, 22 mm wide and 22 mm high. Flexural strength, measured according to VDG Technical Bulletin P73 (February 1996), was between 400 N/cm 2  and 1500 N/cm 2 . 
     The surface of the core must not be flushed out or become damaged when the metal flows in. For this reason, the core must have appropriate surface strength. Porosity also plays a vital role. The porosity in the present exemplary embodiment is 30%. 
     After the cast part has completely solidified, the core must be removed. It is important that the core immediately dissolves completely and easily without leaving any solid residue. (Note: If, within the scope of the present invention, the terms “water-soluble,” “dissolve” or “completely dissolve” are mentioned, this does not necessarily refer to the chemical concept of dissolving. The decisive factor is that the constituents of the cores according to the invention can be removed from the cavity of a workpiece easily, completely and without leaving any residue, using water as the solvent.) By nature, the dissolution rate of the core is dependent on the core materials and the pretreatment of the core, as well as the core size. For pure salt, the dissolution rate can deviate from that of a composition comprising binders and fillers. Experiments conducted with a test part have shown that a core having the dimensions 22 mm×22 mm×180 mm can be completely flushed out of the cast part with hot water within 1 minute to 2 minutes. 
     Based on the above discussion, the teaching according to the invention relates to salt-based cores
         which can be produced by shaping and compacting a core material mixture, the core materials of which are selected from at least one salt, at least one binder system comprising a binder/binding agent and a drying agent, and optionally auxiliary substances such as additives, fillers, wetting agents and catalysts, wherein the salt, the binder system and the optionally used auxiliary substances are inorganic, these core materials can be dissolved using water as the solvent, and the core material mixture is shaped into cores and compacted in a dry pressing method.       

     Preferred cores are those made of core material mixtures
         in which salts are used which have a decomposition or melting point above the temperature of the liquid metal that is poured around the cores;   in which the salts used are chlorides of the alkali elements and alkaline earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, sulfates and nitrates of the alkali elements and alkaline earth elements, in particular potassium sulfate and/or magnesium sulfate, ammonium salts, in particular ammonium sulfate, or mixtures of these salts;   in which the salt is sodium chloride;   in which the grain sizes of the salt that is used range from 0.01 mm to 2 mm;   in which the salt that is used is present in a bimodal or trimodal grain size distribution;   in which the salt that is used is present in a grain size distribution ranging from 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or 1.31 to 2.0 mm;   in which the binders/binding agents used in the binder system are inorganic phosphates, inorganic borates or silicate compounds which can be removed without leaving any residue, using water, or mixtures of these binders/binding agents;   in which the binders/binding agents used in the binder system are alkali phosphate or ammonium phosphate, monoaluminum phosphate, boron phosphate, trisodium phosphate, tetrapotassium pyrophosphate or sodium polyphosphate, which can be removed without leaving any residue, using water, or mixtures of these binders/binding agents;   in which the binders/binding agents used in the binder system are water-soluble silicate compounds, and preferably water glasses;   in which the binding agent in the binder system is a water glass having a water glass module of 1 to 5, and/or a mixture of water glasses having differing water glass modules;   in which the content of binders/binding agents is between 0.5% by weight and 15% by weight, based on the salt that is used;   in which the content of the binder/binding agent is between 0.5% by weight and 15% by weight, based on the salt that is used, as a function of the wetting behavior and the water glass module;   in which water glass is present as the binder/binding agent in a content of 0.5% by weight to 15% by weight, based on the salt that is used, as a function of the grain size distribution and tailored to the water glass module;   in which the binding agent in the binder system is tetrapotassium pyrophosphate;   in which the binding agent used in the binder system is tetrapotassium pyrophosphate in liquid form;   in which the binding agent used in the binder system is tetrapotassium pyrophosphate in an aqueous 60% solution;   in which the binding agent used in the binder system is tetrapotassium pyrophosphate in an aqueous 60% solution and in amounts of 1 to 5% by weight, preferably in amounts of 2 to 4% by weight, and particularly preferably in amounts of 2.5% by weight, based on the amount of salt that is used;   in which the binding agent used in the binder system is tetrapotassium pyrophosphate in an aqueous 60% solution and in amounts of 1 to 5% by weight, preferably in amounts of 2 to 4% by weight, and particularly preferably in amounts of 2.5% by weight, based on the amount of salt that is used, and additionally tetrapotassium pyrophosphate in solid form is used in the same amount or also in a larger amount;   in which the binding agent is present in the binder system in a content of 1 to 15% by weight, based on the amount of salt that is used, and the drying agent is present in a content of 0.3 to 4.5% by weight, based on the amount of salt that is used;   in which hydrophilic substances which are able to reversibly bind water are used in the binder system as drying agents;   in which finely dispersed silicic acids, such as Aerosil, silica gel, zeolites, anhydrous sodium sulfate and/or magnesium sulfate are used in the binder system as drying agents;   in which a catalyst is added as an auxiliary substance;   in which the catalyst is particularly fine-grained salt, and preferably powdered salt having a particle size of less than 100 nm;   in which the salt is sodium chloride, which is preferably present in a bimodal or trimodal grain size distribution, particularly preferably in a grain size distribution of 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or 1.31 to 2.0 mm, the binder system is composed of the combination of water glass as the binding agent and Aerosil as the drying agent, the catalyst is particularly fine-grained salt, and preferably powdered salt having a particle size of less than 100 nm, optionally further auxiliary substances such as additives, fillers, wetting agents and/or further catalysts are present, and the mixture of the core materials is free-flowing;   in which the cores are heat-treated after shaping;   in which, after shaping, the cores are heat-treated at a temperature up to 600° C., preferably at temperatures of 500 to 600° C., and preferably at a temperature of 580° C.;   in which the shaped cores have a density of 1.5 g/cm 3  to 2.1 g/cm 3 , and preferably of 1.2 g/cm 3  to 1.8 g/cm 3 ;   in which the shaped cores have a porosity of 10% to 40%, and preferably of 5% to 25%;   in which the shaped cores have a flexural strength between 400 N/cm 2  and 1500 N/cm 2 .       

     The teaching according to the invention further relates to:
         methods for producing salt-based cores, wherein a core material mixture, the core materials of which are selected from at least one salt, at least one binder system comprising a combination of binder/binding agent and drying agent, and optionally auxiliary substances such as additives, fillers, wetting agents and/or catalysts, is homogeneously mixed, shaped into a core, compacted in a dry pressing method, and optionally heat-treated.       

     Preferred methods are those in which
         salt having grain sizes with differing distribution curves, preferably in a bimodal or trimodal grain size distribution, is used and mixed;   the salts selected are chlorides of the alkali elements and alkaline earth elements, in particular sodium chloride, potassium chloride and/or magnesium chloride, sulfates and nitrates of the alkali elements and alkaline earth elements, in particular potassium sulfate and/or magnesium sulfate, and ammonium salts, in particular ammonium sulfate, or mixtures of these salts;   the binders/binding agents used in the binder system are inorganic phosphates, inorganic borates or silicate compounds which can be removed without leaving any residue, using water, or mixtures of these binders/binding agents;   the binders/binding agents used in the binder system are alkali phosphate or ammonium phosphate, monoaluminum phosphate, boron phosphate, trisodium phosphate, tetrapotassium pyrophosphate or sodium polyphosphate which can be removed without leaving any residue, using water, or mixtures of these binders/binding agents;   the binders/binding agents used in the binder system are water-soluble silicate compounds, and preferably water glasses;   the binding agent in the binder system is a water glass having a water glass module of 1 to 5, and/or a mixture of water glasses having differing water glass modules;   the content of binders/binding agents is between 0.5% by weight and 15% by weight, based on the salt that is used;   the content of binder/binding agent is between 0.5% by weight and 15% by weight, based on the salt that is used, as a function of the wetting behavior and the water glass module;   water glass is present as the binder/binding agent in a content of 0.5% by weight to 15% by weight, based on the salt that is used, as a function of the grain size distribution and tailored to the water glass module;   hydrophilic substances which are able to reversibly bind water are used in the binder system as drying agents;   the drying agents used in the binder system are finely dispersed silicic acids, such as Aerosil, silica gel, zeolites, anhydrous sodium sulfate and/or magnesium sulfate;   a catalyst is added as an auxiliary substance;   the catalyst is particularly fine-grained salt, and preferably powdered salt having a particle size of less than 100 nm;   the salt is sodium chloride, which is preferably present in a bimodal or trimodal grain size distribution, particularly preferably in a grain size distribution of 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or 1.31 to 2.0 mm, the binder system is composed of the combination of water glass as the binding agent and Aerosil as the drying agent, the catalyst is particularly fine-grained salt, and preferably powdered salt having a particle size of less than 100 nm, optionally further auxiliary substances such as additives, fillers, wetting agents and/or further catalysts are present, and the mixture of the core materials is free-flowing;   the core materials are homogeneously mixed, shaped into the core, and compacted in a dry pressing method;   the core materials, depending on the material, desired surface quality and contour accuracy of the workpiece to be cast from metal, are used in grain sizes ranging from 0.01 mm to 2 mm, shaped into the core, and compacted in the dry pressing method;   the cores are heat-treated after shaping;   after shaping, the cores are heat-treated at a temperature up to 600° C., preferably at temperatures of 500 to 600° C., and preferably at a temperature of 580° C. The cores according to the invention can be used, for example, as cavity placeholders in the production of metal cast parts, preferably in permanent mold casting technology.