EP2853320A1 - Casting mould or a casting mould core made from coated mould sand for metal casting - Google Patents

Abstract

Casting mold and / or mold core made of molding sand for metal casting, wherein a first layer is arranged on the surface of molding sand grains of the molding sand, wherein the first layer is cured and consists of water glass and / or phosphate glass.

Description

The present invention relates to a casting mold and / or a casting core of coated molding sand for metal casting.

When molding hollow molds, the mold cavity is formed from a mixture of sand and binder, the mold ideally being broken down by the action of temperature during casting and easily removed from the cooled casting. As a rule, this sand, together with the sand of the external form, goes back into the sand loop of the foundry and is reused in the outer, usually bentonite-bound, mold mass.

As a binder for core molds, a variety of different organic and inorganic components is used. However, the largest market share in the production of core molds is the so-called cold-box process. In this case, the core sand, which was coated with a polyurethane resin portion (PU resin) of 0.5 to 2 wt .-% resin by mixing, shot in the core mold and fumigated to cure with amine. This mixing process also adds other additives and lustrous carbons. By gassing it comes to the curing of the PU resin. The grains of sand are connected to each other after curing via resin bridges. In order to keep the amounts of resin low, the grain distribution of the sand is limited downwards, so as to keep the specific surface area (or the surface / volume ratio) of the individual particles small and "save surfaces". It is important in the core production as dense as possible grain package to obtain a good green strength and a good mold surface. At the same time, however, there is a certain open porosity of the core mold necessary to be able to dissipate water vapor and other gases that arise during casting.

The green strength of a core mold is according to the foundry encyclopedia the strength of damp molding sands of uniform material and temperature distribution at room temperature (15 - 30 ° C).

In many cases, the core forms are coated after curing with a size. A sizing is a suspension of refractory material which again improves the mold surface. In part, this size is also used to reduce casting errors. The most frequently occurring casting defects in molds with sand cores are the so-called leaf ribs. An example of such leaf ribs in a casting is in the one taken from the foundry dictionary Fig. 1 shown.

Leaf ribs are according to the foundry dictionary by cores caused by irregular, filigree, thin metallic outgrowths at angles or corners and edges of the castings. Due to the expansion of the quartz, because the projections resulting from shell formation grow into the mold cavity and do not break up as in the Schülpe, another casting defect, the casting metal backfills the gap and forms the typical rib-like projection. Leaf ribs lead to an increased reworking effort and sometimes to rejects.

The frequency of this leaf rib formation depends on various factors. Decisive is also the binder. Especially PU binders (cold box method) cut off only mediocre, but have the advantage of comparatively easy and fast production of core shapes and are dominant on the market. The tendency to leaf rib formation depending on the binder is in Fig. 2 shown.

Blade ribs can occur in all metal castings, but especially frequently in gray cast iron, nodular cast iron, malleable cast iron and cast steel. The reason for this is the temperature of about 1400 ° C - 1600 ° C used in the casting of these metal alloys. Especially with quartz sand as the most commonly used foundry sand occurs at 573 ° C to a phase transformation. From the so-called Tiefquarz forms spontaneously from this temperature, the modification of high quartz. This reversible phase transformation is accompanied by an increase in the volume of the quartz grain by 0.8%. Since this leads to an increase in volume in the core form, the general opinion is that this increase in volume leads to the rupture of the Surface of the core shape leads, and thus the liquid metal allows access into the core interior and thus forms the leaf ribs.

Since this casting defect occurs very frequently and is associated with high reworking costs or even the casting cast-off, some strategies have already been developed for avoiding the leaf ribs.

Obviously, the replacement of quartz sand by other granular minerals, which have no phase transition in the usual temperature range of the melt. A comparison of the expansion behavior of different materials is in the also taken from the foundry dictionary Fig. 3 shown. The behavior of quartz sand, feldspar, chromite sand, andalusite, chamotte, cerabeads and zircon sand are shown.

In addition to the minerals already mentioned above but also olivine, alumina spheres and mixtures of feldspar and quartz sand and ceramic balls are known. Pure feldspar is not suitable for gray cast iron, since it can start to melt at the temperatures prevailing there and thus can no longer be removed from the workpiece.

A common disadvantage of all of these quartz alternatives is economy and availability. Many of the minerals are available only in a few places and therefore burdened with high transport costs or energy-intensive in the production (such as alumina or Cerabeads). Another disadvantage is partly the grain shape. Not all minerals occur as round or rounded natural sands, but are produced in part by crushing and milling followed by classifying. Such splinter grain is disadvantageous in achieving a dense particle packing in the core form and therefore leads to a lower green strength and a higher binder requirement.

The use of such expensive quartz scandal alternatives is also burdened with high costs. If you do not want to mix the expensive alternative sand when "shaking out" of the casting with the rest of quartz sand and thus lose, special facilities are required to separate the alternative sands and recycle.

All in all, therefore, the use of quartz radiation alternatives on special areas remains limited. Quartz sand is indispensable for the mass production of castings.

A historically grown method to reduce the volume growth of the core mold during casting, is the addition of organic material, which is to burn / pyrolyze during casting and thereby create free volume for the volume growth of quartz jump. Traditionally, sawdust has been used for this purpose, for example. However, this solution has drawbacks: on the one hand decreases by the addition of such soft material, the green strength; a part of the binder is absorbed by the sawdust and is thus no longer available for the binding of the sand. The biggest disadvantage, however, is that in the thermal decomposition of sawdust and similar complex organic compounds under exclusion of air so-called VOC (Volatile Organic Compounds) in the exhaust gases. This is on the one hand an odor nuisance for residents near foundries, on the other hand, VOC exhaust gases are suspected of being carcinogenic. Therefore, the maintenance of such simple additives is associated with high costs for the exhaust gas purification.

Modern additives, which also derive their effect from the decomposition of organic material and the associated creation of free volume, consist of plastics which decompose ideally in pyrolysis in water vapor and carbon dioxide, but in no case form VOC. Such additives are used at about 0.5-3%, e.g. about 1%, added to the molding sand and are very expensive.

Basically, however, a problem remains with this concept. The additive can only work if it is either in the binder bridges between the sand particles (situation 1, see also Fig. 4a ), or is greater than the average pore volume of the sand mixture (situation 2, see also Fig. 4b ).

Thus, the addition of additive always requires an increase in the required, expensive binder. In addition, in situation 1 significant overdoses of additive are unavoidable, since the additive accumulates evenly on the sand grain during mixing with the binder, but is only needed at the contact points between the grains.

In situation 2, the additive can disadvantageously clog the pores in the molding body until it decomposes and thus close the way for the released gases. In addition, the larger additive particles required for this model will not only accumulate in the pores, but will also adhere uniformly over the binder and / or on the sand grains, thus leading to even greater consumption of binder.

In addition to providing free volume in the sand core by thermal destruction of additives during casting, another approach is known: this is commonly described as increasing high temperature plasticity. To understand this approach, it is necessary to have a thorough knowledge of the processes affecting the core shape during casting.

The starting point for casting is a mold which, before casting, has a temperature of approximately 20-30 ° C everywhere. In this form (for example in gray cast iron), the liquid metal is filled at about 1400 ° C. Due to the high density of the molten metal and the high temperature so a very large amount of heat in a very short time is available. This amount of heat must be dissipated until the metal has solidified. The molding sand serves as a heat sink. However, it should be remembered that the heat transfer in the core sand is limited despite the high modulus of elasticity (modulus of elasticity) of the sand (high thermal conductivity).

For the description of the heat transport one can set the root time law (thus: twice penetration depth of the heat into the body means 4-times time), with which a body heats up from the outer shell to the core. This means that the sand on the surface of the core mold and thus in direct contact with the molten metal within a very short time already reaches the necessary for the quartz jump 580 ° C, while the temperature inside the core mold is still below this temperature. Since the phase transformation from deep quartz to high quartz is associated with an increase in volume, but the volume of the core form can not change due to the hydrostatic pressure of the melt, this increase in volume in the outer region of the mold leads to compressive stresses. From fracture mechanics, however, it is known that solids do not break under compressive stress, but only under tensile stresses. Thus, the quartz jump is indeed the cause of leaf ribbing, but not the trigger.

The tensile stresses that cause cracking in the core form, rather than below the interface between the liquid metal and the core form, arise from the shear stresses between the sand layer, which has a temperature below 580 ° C (deep quartz), and the sand layer, which is at a temperature above 580 ° C (high quartz), since a shear stress can always be divided into a tensile and compressive stress component. Leaf ribs can only form when this crack below the surface of the core mold reaches the surface of the core mold, and liquid metal penetrates into this crack. Therefore, also sizing is attributed a positive effect on leaf rib formation, as these seal a possible crack on the surface and thus prevent the penetration of liquid metal into the core shape.

It is also known from fracture mechanics that a crack only forms when the stress at a defect is large enough to cause the crack to spread (Griffith criterion). According to Griffith, crack propagation occurs when the elastic strain energy W _e released by elongation of the crack is equal to or greater than the surface energy W _o (Formula 1) required to form the fracture surfaces. $$W_e\ge W_O$$

The criterion of unstable crack propagation is described according to formula 2 (with crack length a). $$\frac{d{W}_e}{da}\ge \frac{d{W}_O}{da}$$

$$W_o=4a{\gamma }_o$$

Formula 3 describes the elastic strain energy W _e and formula 4 the surface energy W _o for both fracture surfaces (with elastic modulus E, surface tension γ _o and stress σ). $$W_e=\frac{{\mathit{<figure-callout\; id="2"\; label="\pi \sigma "\; filenames="imgf0004.png"\; state="\{\{state\}\}">\pi \sigma </figure-callout>}}^2{a}^2}{e}$$

$$W_O=4a{\gamma }_O$$

The criterion of crack propagation then leads to formula 5. $$\frac{2{\mathit{a\pi \sigma }}^2}{e}\ge 4{\gamma }_O$$

Under these conditions, an existing crack spreads unstable, ie without external energy supply. The elastically stored energy in the body (eg from thermal expansion) is converted into surface energy during crack propagation (see Fig. 4 for the energy balance of unstable crack propagation).

For the tension required for crack propagation, formula 6 is obtained. $${\sigma }_c=\sqrt{\frac{2e{\gamma }_O}{\mathit{\pi a}}}$$

Formula 7 describes the associated critical crack length (Griffith length) a ₀ . $$a_0=\frac{2e{\gamma }_{\mathrm{<figure-callout\; id="2"\; label="O\; \pi \sigma "\; filenames="imgf0004.png"\; state="\{\{state\}\}">O\; </figure-callout>}}}{{\mathit{\pi \sigma }}^{}}$$

Cracks shorter than the critical Griffith length a ₀ are stable for energetic reasons. As soon as the critical length is exceeded, cracks are unstable and grow faster and faster as the length increases.

The critical crack length a ₀ can not be increased indefinitely in the case of a core shape, since the crack concentrators, ie factors favoring the formation of cracks, are for the most part present in the pores between the sand grains and can not be reduced in size indefinitely because they remove the gases in the core form. However, lowering the modulus of elasticity of the material results in lower stress, less elastically stored energy and thus larger crack concentrators with the same deformation. This means that at the same deformation of the workpiece (eg by quartz jump) results in a lower tensile stress, and thus lower tendency to breakage.

In the following, measures for lowering the modulus of elasticity of the core form are explained.

In the uppermost layer of the core form, the binding of the sand by the polymer no longer plays a role, since this already pyrolyzed at temperatures around 580 ° C and above. Thus, the "modulus of elasticity" of the core shape in this area depends only on the friction of the grains of sand with each other. The stronger the friction between the grains, the higher are the stresses that can be built up locally as volume changes.

Thus, it makes sense to reduce the friction of the grains of sand with each other, so that during the quartz jump, a rearrangement of the individual grains can be done and thus tensions that can lead to cracking, reduce or not build up.

A historically grown method to reduce leaf ribbing by reducing friction between the sand grains is to incorporate iron oxide in the form of hematite or magnetite powder having a surface area of> 10m ² / g to the core sand or using ferrous sand. In this case, the iron oxide is mainly as a thin layer on the quartz grain and can optionally be removed by treatment in acid with subsequent flotation (eg for the treatment of glass sands).

After casting, the mineral fayalite, an island silicate of the olivine group, is found in the core sand. Fayalite is a melting point-lowering iron silicate with the composition 2 FeO • SiO ₂ .

The quartz sand reacts at higher temperatures, for example the casting temperatures in iron and steel casting, with metal oxides (for example FeO) to form low-melting orthosilicates according to the following reaction scheme:

First, the hematite or magnetite is reduced by carbon monoxide, which forms, for example, from the decomposition of the binder or the lustrous carbon under exclusion of air to wüstite or 2-valent FeO (see here the blast furnace process). This FeO can react with SiO ₂ at temperatures of molten iron or steel to form fayalite. The resulting melt phase facilitates the rearrangement of grains of sand and therefore leads to a lower stress build-up, thus larger crack concentrators and ultimately less cracks in the core shape and thus less leaf ribs. However, fayalite also has the disadvantage that it reduces the contact angle between liquid iron and the quartz sand and thus liquid iron can penetrate at least in the uppermost layers of the core form, which is known as mineralization. This is disadvantageous for the quality of the casting surface.

Iron shares can consequently reach the molding sand both due to natural causes and through recycling of molding sands. Irrespective of the formation, iron-containing sands in particular have an iron oxide layer arranged in the surface region of the respective sand grain.

Another disadvantage is the high temperature at which it comes only to form a melt of fayalite. This is above 1400 ° C and thus far above the quartz crack. From the point of view of glass chemistry, the formation of this melt can be explained by the fact that iron is a flux for the quartz. Boric acid in borosilicate glasses, for example, has a similar effect. Surprisingly, iron oxide is even more effective as a flux in silicate melts than boric acid. Since iron oxide reacts very well with silicate melts and builds firmly into the glass network, it can be seen that almost all business glasses have a slight green cast by Fe ²⁺ ions in the glass. As a result of this melt, which subsequently crystallizes into fayalite, it is only later that it is possible to break down stresses from the increase in volume of the quartz crack and not even in the temperature range in which the quartz jump takes place.

For this reason, the patent literature also mentions additives in grain or powder form which melt at the contact points between sand grain and additive at lower temperatures, soften or form a melt in contact with the quartz sand, which facilitates the rearrangement of sand particles after the quartz crack and thus reduce the local stresses that lead to crack formation in the molded body.

It is known e.g. Iron mica, cryolite, spodumene (form a flux with quartz sand even at low temperatures), borosilicate glass granules, expanded perlite, tempered perlite (melting or softening in the area of the quartz cracking temperature).

Organic coated core sands are also known. This process is called a croning process. The sand is coated with a (very thick) layer of organic material. Background of this idea is the absence of binder. The core sand becomes brought into shape and heated. The thick layers of organic material sinter to form binder bridges between the grains and give the mold the required strength. It is coated the entire molding sand. During casting, the organic component then burns. In such systems neither a melting point depression is expected nor known.

However, all of these solutions have the disadvantages of all other leaf rib avoidance additives. Namely, the effect occurs only punctiform at the contact points between sand and additive. Furthermore, a higher binder consumption or a lower green strength is assumed. In addition, the additives are difficult to dose or there is a risk of melting sand with the additive, whereby the additive goes into the sand cycle.

It is therefore an object of the invention to provide a way that does not have these disadvantages.

Surprisingly, it has been found that a practicable solution is generally to coat the foundry sand, or a certain proportion and / or grain band of the molding sand so that this layer softens or melts at temperatures around the quartz crack and thus facilitates the rearrangement of the sand grains.

A molding sand in the sense of this invention preferably contains a proportion of quartz sand. This quartz sand content is preferably> 2 wt .-%, preferably> 50 wt .-% and particularly preferably> 70 wt .-%.

Grain belt is to be understood as meaning granules of specific grain sizes (or of a grain size range). The term grain size describes the size of individual particles (also called grains), for example, sand particles or quartz sand grains. For particles of various shapes, instead of the diameter, as in the case of spherical particles, an equivalent diameter is used instead. This means that another measurable property is determined and the measurements are related to equal (equivalent) spheres.

From glass chemistry, it is known that 3 - 5 valent elements function as network formers, whereas 1 and 2 valent elements act as network transformers lowering the melting point:

Thus, the melting point of pure quartz at 1710 ° C (very high-viscosity melt), commercial soda lime glass with the composition 75% SiO ₂ , 15% Na ₂ O, 10% CaO is processed between 780 and 1000 ° C and up to 1400 ° C melted (Läuterbereich, very thin liquid melt). An exact melting point can not be specified for glasses, but only a melting range, since the glass is a supercooled liquid and the viscosity decreases as a function of the temperature by several orders of magnitude.

Basically, however, you can assign processing areas to certain viscosities and temperatures. At the so-called transformation temperature or glass transition temperature Tg of the glass, the glass is already a highly viscous liquid, which can no longer remove shear stress and is deformable, which is utilized, for example, when bending glass for vehicle windows. With soda-lime glass the value for the Tg is approx. 600 ° C and therefore already in the range of the quartz jump.

The lowest melting glasses are called alkali metasilicate glasses, the alkalis being technically lithium (Li), sodium (Na), and potassium (K). The reduction in viscosity with increasing temperature decreases from Li via Na to K. Melting temperatures increase from Li over Na to K. A guideline for the melting range of sodium metasilicate is 1080 ° C. Thus, the transformation temperature of these glasses is well below the 600 ° C of soda lime glass. Thus, when reaching the quartz cracking temperature, a glass layer on the grain of sand already favors the repositioning of grains against each other.

Alkali Metasilikatgläser are often referred to as so-called "water glasses" and sold and used primarily as a binder. Water glasses come as aqueous solutions of 30 - 70% solids in the market. The exact ratio of alkalis to SiO ₂ (modulus) varies depending on the application, as well as the doping with other components.

An alternative to the water glasses are phosphate glasses, borate glasses and chalcogenide glasses for certain applications. These too can be partially used as aqueous solutions are used and are also used as a binder. Since the bond between phosphorus and oxygen is weaker than with silicates, glasses constructed with phosphorus as a network former have a low melting point and transformation point even without alkalis, and a lower pH than water glass based binders due to the absence of alkalis.

The mixture of both binder systems is known. Accordingly, alkali-water glasses, phosphate glasses or mixtures thereof or mixtures of melt-point-reducing materials bound to the grain with these binders are particularly suitable.

The object of the invention is thus achieved by a casting mold and / or a mold core made of molding sand for metal casting, wherein a first layer is arranged on the surface of molding sand grains of the molding sand, wherein the first layer is cured and made of alkali-water glass and / or phosphate glass and Boratglas and optionally one or more additional additives. As can be seen from the above reference to the suitability of the molding sand for use as foundry sand for a casting mold and / or a casting core, such a molding sand can be used as foundry sand for the shell of a foundry mold or as so-called "core sand" (ie for a casting core) become. In the following, therefore, should always be included in the use of the term molding sand "core sand" of this formulation. In particular, the use of such a coated foundry sand for foundry cores is convenient, but it may also be advantageous (e.g., more geometrically elaborate) to produce casings of a foundry mold (at least in part) from such a coated foundry sand. In particular, the recycling of the core sand and the subsequent use as foundry sand for casings of a foundry mold lends itself, which also passes coated core sand into the foundry sand for casings of a foundry mold.

A "hardened" layer is characterized in particular by the fact that it has undergone a process step which is used in particular for changing its chemical-physical properties supported by heat and / or catalysts, with or without pressure, and consequently no longer change its physicochemical properties , Suitable process steps for forming a cured layer are described below.

Water glass based binders are known in the manufacture of core molds. The molding sand is mixed with water glass and brought the liquid water glass either by increasing the temperature or gassing with carbon dioxide for polymerization. This creates binder bridges of solid alkali metasilicate between all sand grains of the molding sand. However, this type of binding is used only to a very small extent, because the curing of the water glass either takes a long time or is associated with high energy consumption. Another disadvantage is the fact that the nuclear decay after casting is bad. The binder bridges made of water glass are retained after casting and do not decompose like the organic binders (coldbox furan resin, croning sand) but can even solidify at the temperatures of the casting. The demoulding of the core sand after casting is associated with great expense. This solidification of grain bonding during casting is the reason why such binder systems are considered to have a positive influence on leaf rib formation.

Preferably, a sand is coated with 5% by weight or less of water glass, preferably below 2% by weight of water glass, more preferably below 1.5% by weight of water glass, more preferably below 1% by weight of water glass. These proportions refer to the wet sand before a curing process, ie to an aqueous solution of the waterglass used for coating. Higher levels of water glass cause the sand to lump when wet. Unless otherwise indicated, all percentages (also referred to above) relate to percent by weight (wt .-%). In connection with aqueous solutions such as e.g. For water glass, there is the problem that they are usually offered as 30-70% solutions. After firing or drying, the weight of the alkali silicate bound on the sand is correspondingly reduced. The above quantities are based on the proportion of the applied amount of waterglass solution.

As described above, phosphate glasses or borate glasses are possible alternatives to water glasses (alkali metasilicate glasses). Preferably, therefore, a portion of the water glass can be replaced by a corresponding proportion of a phosphate glass or another alternative glass with similar properties (eg, a borate glass) or mixtures thereof. Chalcogenide glasses can be used as additives. Therefore, the abovementioned water glass content can preferably also be completely or partially replaced by such alternatives. Accordingly, mixtures of water glass (or Water glasses with different alkali metals) with one or more of the alternatives mentioned above.

Of course, an aqueous (water glass) solution is ideally suited to producing thin layers on granular material, for example, thin (water glass) layers on foundry sand grains. This can be done by mechanically mixing the aqueous solution with the granular material or by spraying e.g. in a fluidized bed.

A preferred embodiment of the present invention provides that the first layer has an average layer thickness of 0.01-50 μm, preferably 0.1-20 μm, more preferably 0.5-10 μm, particularly preferably 1-5 μm.

A further preferred embodiment of the present invention provides that the water glass is at least one of soda water glass, potassium water glass or lithium water glass.

In addition, as mentioned above, an embodiment in which an additional additive is present in the coating is preferred. It is preferred that the at least one additive is selected from a group which contains alkalis, preferably alkali salts, alkali oxides, alkali hydroxides, particularly preferably soda and / or caustic soda, alkaline earths, preferably alkaline earth salts, alkaline earth oxides, alkaline earth hydroxides, fluorides, preferably cryolite, borates, Chalcogenide glass, iron oxide, spodumene, iron-containing silicates, viscosity-lowering solids, and combinations thereof.

The coating material is preferably fixed at temperatures of 0-1400 ° C., preferably 0-1000 ° C., particularly preferably 0-300 ° C., to the carrier particles. The fixing can be carried out (at lower temperatures) by removing the solvent, polymerization or (at higher temperatures) by baking in the surface of the carrier particles. As an alternative to the temperature range of 0-300 ° C. specified above, a further temperature range is therefore particularly preferred, namely a temperature range of 500-700 ° C., more preferably about 600 ° C., since this temperature range depends on the carrier material above the temperature of the quartz jump lies. As mentioned above, depending on the temperature range, a different fixation of the coating material takes place. In the latter preferred range of 500-700 ° C, a condensation reaction takes place and, with the elimination of water, a mechanically loadable glass layer is formed on the substrate.

At these and even higher temperatures, there is a very strong adhesion of the coating to the foundry sand (eg quartz grain), since the boundary surface foundry sand (eg quartz grain) / water glass (eg during firing) is also already melted by the alkalis and a solid compound of layer and shaped sand grain (eg quartz grain) is formed.

The layer preferably has melting point-lowering properties.

More preferably, the coating may also contain an additive in addition to the binder (e.g., waterglass). An additive in the coating material is preferably an additive selected from a group containing pigments, alkali elements, alkali salts (containing alkali ions), alkali compounds and fluoride. In particular, additions of alkali elements, salts and / or compounds are suitable for lowering the melting point of the carrier particle material (the molding sand grains). By such additives (eg soda, caustic soda (and / or their Li and / or K equivalents), other alkali or alkaline earth salts and / or oxides), it is possible that at higher temperatures, when the surface of the carrier particles have increased viscosity, diffuse into the carrier particle surface and thus form layers of a comparatively viscous material, which promotes the sliding or sliding along of contacting grains to each other. Thus, it can be achieved by such a coating that the waterglass layer does not have to be completely or completely applied to the carrier particles, but the surface of the (or some) carrier particles is itself converted into a glass or converted into a glassy state. This transformation can be done by casting or by prior (temperature) treatment.

It is further preferred that one or more additional layers can be applied to all, one selection (for example a grain band) or a portion of the molding sand particles, or that these molding sand particles have one or more additional layers. Such an additional layer could serve, for example, for the hydrophobization of the molding sand particles.

Preferably, only part of the molding sand has at least one coating as described above. Particularly preferred is only a selected grain band (ie a Selection of particles of selected particle sizes or equivalence sizes or equivalent diameters) surface-coated. Particularly preferably, the coated grain band is one of the particle sizes with large particle sizes. In a preferred embodiment, the particles of an upper (ie larger) Konbandes are surface-coated, preferably the largest 50%, preferably the largest 30%, more preferably the largest 15% and most preferably comprises the largest 5% of the particles. However, molding sands are also possible in which the largest particle fraction is not surface-coated but, for example, the particles of a grain band are surface-coated, which covers the size range from the largest 50% to the largest 1% of the particles. Another preferred size range of coated particles is that which ranges in size from the largest 20% to the largest 2% of the particles.

The coating of the sand preferably does not itself act as a binder. In particular, no bonds between grains of sand are preferably caused by the coating.

In the system according to the invention or else the mold and / or mold core according to the invention, it is furthermore preferred to maintain known organic binder systems, e.g. the cold box method with the advantages of rapid curing of the core shape and the decay of the bond during casting by thermal decomposition of the binder provided or possible. Therefore, preference is given to the use of the systems according to the invention with known organic and inorganic binders or in clay-bound (for example bentonite-bonded) ramming compositions.

From the prior art, no core sand is known, which is thus coated with a melting point-lowering layer. In particular, no systems are known which have a melting point reduction (or softening, for which the Tg of the coating is decisive), preferably above 300 ° but below 1400 ° C. Particularly preferred is a coating of the sand with a material which softens itself in this temperature range, or in contact with the core sand forms a layer which softens from or above 300 ° C. Suitable coatings include, for example, coatings of sand with soda, caustic soda or other alkali carrier, or the burning of sand with such coatings to form a layer of glass on the grain. Particular preference is given to coatings of inorganic glass mixtures having a Tg of less than or equal to 1400 ° C., particularly preferably less than or equal to the quartz transition temperature.

A further preferred embodiment of the present invention accordingly provides that the glass transition temperature of the first layer is at most 1400 ° C. and is particularly preferably below the quartz transition temperature of 573 ° C.

Depending on the grain size, grain surface and area of application, between 0.1 and 5% water glass solution based on the weight of the sand is used in the coating of foundry sand. Preferably, a core sand 2 to 20% (wt .-%) coated sand in the core sand. The proportion of hardened water glass is preferably about 0.045-5% (wt .-%), preferably less than 2%, more preferably less than 1%, more preferably less than 0.5% based on the total mass of the molding sand.

The total mass of the molding sand here describes the mass of all molding sand grains that are part of the mold and / or the mold core.

It is therefore advantageous according to a further preferred embodiment, if the first layer consists of water glass and bound in the first layer total mass (mwb) of the water glass (7) a maximum of 0.045 - 5 wt .-%, preferably less than 2 wt .-% , particularly preferably less than 1 wt .-%, more preferably less than 0.5 wt .-% of the total mass (mQ) of the molding sand (4).

All grain-shaped materials and material mixtures which are suitable for casting metals can be coated, but quartz sand is particularly preferred.

Depending on the application and manufacturer, the grain sizes of the foundry sand are between 50 μm and 2000 μm in diameter, preferably between 100 and 500 μm in diameter.

A further preferred embodiment of the present invention accordingly provides that the grain size of the molding sand is between 50 μm and 2000 μm, preferably between 63 and 500 μm.

The entire grain spectrum of the foundry sand (ie 100%) can be coated. Preference is given to coating 1 to 25% (% by weight), more preferably 2 to 20%, more preferably 5 to 15%, of the total weight of the foundry sand. This portion is preferably removed from the coarser portion of the molding sand and coated. Preferred is only then mix the coarser coated grain spectrum with the remaining (uncoated) sand.

Accordingly, it is advantageous according to a further preferred embodiment, when the molding sand composed of a first coated and a second uncoated portion, wherein the mass of the coated portion of the molding sand 1 to 25 wt .-%, preferably 2 to 20 wt .-%, particularly preferably 5 to 15 wt .-% of the total mass of the molding sand.

Furthermore, it is advantageous according to a further preferred embodiment, when the grain size of the first portion of the molding sand is greater than the grain size of the second portion of the molding sand.

The sand can either be premixed with a coated and an uncoated portion used as core sand, or the coated sand content can be added later as an additive to an uncoated sand content, so that the desired particle size distribution (PPS) of the foundry sand results after the addition of the coated sand ,

The sand may preferably be coated by mixing with the meltdown agent, by dipping or spraying. Curing may preferably be accomplished by drying, firing or other mechanisms of fixation (e.g., gassing with waterglass).

The coating material can also be used to bond further additives to the grain of sand.

Optionally, it is possible to hydrophobicize the coated sand grain after its production with a further coating in order to achieve a better binding of the grain with an organic binder, for example, to achieve a higher green strength. Siloxanes are sometimes added to the organic binder.

A further advantageous embodiment therefore provides that a second hydrophobic layer is arranged on the surface of the first layer.

The coated sand can be reused after casting and preferably fed into the normal sand cycle and treated accordingly or reused separately after treatment.

It is not known from the prior art that recycled sand after its use as a casting using inorganic binders would have a positive effect on leaf rib formation. This is u.a. on the inhomogeneous on the surface of the sand particles distributed remnants of the inorganic binder, the distribution of the residues over all grain fractions (particle sizes) or on the fact that the sand has already passed once by the quartz crack returned. In a preferred embodiment of the present invention, therefore, the sand is present in a modification which has not yet undergone a quartz jump.

Preferably, a core sand 10 to 15% (wt .-%) coated sand in the core sand. The proportion of hardened water glass is preferably about 0.045-5 wt .-%, preferably less than 2 wt .-%, more preferably less than 1 wt .-%, more preferably less than 0.5 wt .-% of the total mass of molding.

1. a) Aufbringen einer ersten Schicht aus Wasserglas und/oder Phosphatglas in Form einer wässrigen Lösung auf die Oberfläche eines ersten Anteils der für die Gießform und/oder den Gießformkerns verwendeten Formsandkörner mittels Mischen, Tauchen oder Besprühen, insbesondere in einer Wirbelschichtanlage,
2. b) Aushärten der ersten Schicht durch Trocknen, Brennen oder Begasen des beschichteten ersten Anteils, sowie
3. c) Mischen des ersten beschichteten Anteils mit einem unbeschichteten zweiten Anteil der für die Gießform und/oder den Gießformkerns verwendeten Formsandkörner, wobei die Korngröße des ersten Anteils größer ist als die des zweiten Anteils.

The object is also achieved, as described above, by a method for producing a molding sand mixture suitable for a casting mold and / or a casting core according to claim 1, characterized by the following method steps:

1. a) applying a first layer of water glass and / or phosphate glass in the form of an aqueous solution to the surface of a first portion of the molding sand grains used for the casting mold and / or the casting core by means of mixing, dipping or spraying, in particular in a fluidized bed plant,
2. b) curing the first layer by drying, firing or gassing the coated first portion, and
3. c) mixing the first coated portion with an uncoated second portion of the molding sand grains used for the casting mold and / or the casting core, wherein the grain size of the first portion is greater than that of the second portion.

Depending on the grain size, grain surface and area of application, between 0.1 and 5% water glass solution based on the weight of the sand is used in the coating of foundry sand. The sand is preferably coated with 5% by weight or less of water glass, preferably below 2% by weight of water glass, more preferably below 1.5% by weight of water glass, more preferably below 1% by weight of water glass.

Based on the weight of the coated material, it is preferred to add between 0.5-10% as coating material to a mixture of an aqueous solution of water glass and / or phosphate glass and / or borate glass. With particular preference, between 1 and 5%, more preferably between 1 and 5%, of the coating material is applied, based on the weight of the coated material. This excludes that the coating of the grains with water glass already builds up a firm bond of the core sand, as is known in the known use of water glass as a binder.

A further preferred embodiment of the present invention accordingly provides that the mass of the aqueous solution of the waterglass used for the application of the first layer is between 0.15 and 5% by weight, preferably less than 2% by weight, more preferably less than 1.5 wt .-%, more preferably less than 1 wt .-% of the mass of the coated molding sand or used for the application of the first layer mass of the aqueous solution of the waterglass and the phosphate glass and / or borate glass between 0.15 and 10 wt .-%, preferably between 1 and 5 wt .-%, particularly preferably between 1 and 2 wt .-% of the mass of the first portion of the molding sand.

Furthermore, a method for producing a suitable for a mold and / or mold core molding, subject of the present invention, in which suitable for melting point depression of the material of the molding sand grains material and / or an alkali water glass and / or phosphate glass and / or an additional binder is applied to a surface of molding sand grains and then by thermal treatment, the surface of the molding sand grains or a selection of the molding sand grains is at least partially converted into a glass or transferred into a glassy state.

Examples

For the described exemplary experiments, a commercial foundry sand, which is also used in a hut as a standard product, based (see KGV in Fig. 6 ).

From this sand, the grain fraction> 0.355 mm was separated by sieving. The sand greater than 0.355 mm was subsequently coated with sodium waterglass solution (2% by weight, based on the mass of the sand) in a mixing unit and subsequently fired at 600 ° C. for about 20 minutes. A batch was coated, in addition to coating with water glass, also with 2% by weight of hematite powder, as is commonly used in core molds. This batch was also fired at 600 ° C.

Subsequently, the KGV of the original molding sand was restored by adding respectively 5, 10 and 15% by weight of the coated sand to the total sand to the previously screened sand <0.355 mm and filling the remainder with uncoated sand> 0.355 mm.

The coarse grain size of the foundry sand was chosen for four reasons: Firstly, the surface-to-mass ratio of this KGV is significantly more favorable with respect to the use of the water glass, on the other hand it can be assumed that the coated larger grains are always in contact in the molding sand to smaller uncoated grains and thus the "sliding effect" of the glass coating always comes into play, and ultimately to avoid that too many coated grains have contact with each other, which could lead to a caking of the molding sand. In such a "caking" of the molding sand, the surface coating would act as a binder, which is expressly not the subject of the present invention and contrary to the inventive concept. In addition, the volume change in the quartz jump or grain jump in the largest grains is also the largest. Thus, the smaller grains will be easier to relocate in contact with the large grain.

Example 2

In each case 40 kg of mixtures of 5, 10 and 15 wt .-% of the coated sand with pure water glass coating and water glass + hematite powder were prepared. The compaction of these mixtures was compared to the untreated core sand with 1% organic additive. As a result, no difference could be detected. investigations the green strength also in comparison to an untreated molding sand also showed no difference.

For the casting trials, three identical core molds were made in a core shot machine: Sample 1 consisted of the core sand mix with additive used in production; Sample 2 contained 15% by weight of the waterglass-coated sand, but no additive; Sample 3 contained 15% by weight of the waterglass-coated sand with hematite, but no additive.

The samples were poured in normal production: no difference was found in the nuclear decay between the three samples. The two sandblasted coated sand bodies were identical. However, leaf vein formation was reduced by about 50% compared to the conventional sample.

The fact that there was no difference in leaf rib formation in both waterglass-coated samples can be seen as evidence for the assumption that the incorporation of iron oxide has a positive effect only at much higher temperatures. The origins of leaf rib formation, cracks in the core sand form, occur even at significantly lower temperatures. In this temperature range around 600 ° C, the water glass is already effective and facilitates the rearrangement of the sand grains, thereby preventing the build-up of stress peaks in the core shape and thus reduces the crack and leaf rib formation.

Figur 1

Blattrippen in einem Gussstück;

Figur 2

Neigung zur Blattrippenbildung in Abhängigkeit vom Bindemittel;

Figur 3

Vergleich des Ausdehnungsverhaltens verschiedener Materialien in Abhängigkeit von der Temperatur;

Figuren 4a und 4b

schematisch die Wechselwirkung zwischen Additiv und Formsandkorn;

Figur 5

Energiebilanz bei der instabilen Rissausbreitung;

Figur 6

Korngrößenverteilung des für die Versuche verwendeten Formsandes;

Figur 7

schematisch eine Gießform mit Gießformkern;

Figur 8

schematisch einen Ausschnitt aus einer Korngrößenverteilung von Formsandkörnern;

Figur 9

schematisch ein Formsandkorn mit erster und zweiter Schicht;

Fig. 10

exemplarisch ein Formsandkorn, auf dessen Oberfläche eine erste Schicht angeordnet ist.

Further advantages, objects and characteristics of the present invention will be explained with reference to the appended drawing and the following description. In the drawing show:

FIG. 1

Leaf ribs in a casting;

FIG. 2

Tendency to leaf rib formation depending on the binder;

FIG. 3

Comparison of the expansion behavior of different materials as a function of the temperature;

FIGS. 4a and 4b

schematically the interaction between additive and molding sand grain;

FIG. 5

Energy balance in unstable crack propagation;

FIG. 6

Particle size distribution of the molding sand used for the experiments;

FIG. 7

schematically a mold with Gießformkern;

FIG. 8

schematically a section of a particle size distribution of sand molding grains;

FIG. 9

schematically a molding sand grain with first and second layer;

Fig. 10

an example of a molding sand grain, on the surface of a first layer is arranged.

The most common casting defect in hollow molds with sand cores, the so-called leaf ribs, is exemplary in the Fig. 1 shown.

The tendency to leaf rib formation f as a function of the binder is in Fig. 2 shown. In the process, the leaf fin formation f of waterglass binder c1, phenol resin binder novolak type (mask molding material) c2, oil binder c3, urethane binder (Ashland Cold Box) c4, urethane binder (pep set) c5, phenolic resin binder resole type (hot box) c6 to furan resin binder (cold resin method ) c7.

A comparison of the expansion behavior ε (T) of different materials with respect to the strain ε as a function of the temperature T is shown in the also taken from the foundry dictionary Fig. 3 shown. The graphs show the expansion behavior b1 of quartz sand, b2 of feldspar, b3 of chromite sand, b4 of andalusite, b5 of chamotte, b6 of cerabeads and b7 of zircon sand. It is clear that among all other materials shown, quartz sand has the highest strain ε at all temperatures T shown.

Fig. 4a and Fig. 4b schematically show possible interactions between an additive 12 and the quartz sand grains 5, wherein the additive 12 is used to compensate for the increase in volume of the quartz sand or quartz sand grains 5 during casting by burning it during casting. The additive 12 can act only if it is located either in the binder bridges between the quartz sand grains 5 or sand particles ( Fig. 4a ) or greater than the average pore volume of the sand mixture of quartz sand grains 5 (FIG. Fig. 4b ).

In Fig. 5 the energy balance is shown in the unstable crack propagation, wherein the energy E is plotted over the crack length a. The graphs show the surface energy g1, the elastic strain energy g2 and the total energy g3.

The particle size distribution of a commercially available quartz sand 4 as foundry sand shows Fig. 6 , Here is the sieve size y and the mass my shown per sieve. The numbers assigned to the sieve size y each mean that the sieve size y is above the given numerical value. The numerical value 0 represents the sieve bottom. Thus, for example, in the grain size distribution shown, a total mass of quartz sand grains 5 of 7.35 g can be assigned a particle size of greater than 0.090 mm, but less than or equal to 0.125 mm.

In FIG. 7 an example of a mold 1 with mold core 2 is shown, which in the present example allow the production of a rectangular workpiece 3 with rounded corners. It should be noted that only one half of the mold is shown. Other elements that are necessary for casting, such as gates, risers, risers and vents, are not shown here. The casting mold 1 and / or the casting core 2 can consist of a coated molding sand 4, as described above, completely or proportionally.

FIG. 8 schematically shows a section through a grain packing K, as it is present for example in a molding sand core or mold core 2 made of a coated molding sand 4 as described above. It should be noted that this arrangement, for reasons of clarity, although shown in two dimensions, but of course is present in three dimensions, that is, viewed in particular from the image plane in front of and behind the quartz grains shown 5 further quartz grains 5 are not shown.

The molding sand 4, whose grain packing K is shown in this case, is composed of the molding sand grains 5 and has the total mass mQ. In this example, it consists of a first portion 9 with a mass mQ1 and a second portion 10 with a mass mQ2.

With regard to their particle size distribution, the molding sand grains 5 illustrated in this example can be used, for example, as the quartz sand 4, which can be found in FIG Fig. 5 is described correspond.

It can be seen particles in different grain sizes d1, d2, d3, d4, d5, d6, or the particles sand grains 5, wherein the molding sand grains 5 are shown as ideal spheres. The particle sizes d1, d2 are larger than the particle sizes d3, d4, d5 and d6. Of course, it can be assumed that further particle sizes are present, so that the particle sizes d1 to d6 shown only show an exemplary section of the particle size distribution.

It can be seen that preferably on the surface 5a of the first portion 9 of the molding sand grains 5, in this example on the surface of the molding sand grains 5 with larger grain sizes d1, d2, a first layer 6 is arranged, which preferably consists of water glass 7. But it can also consist of a mixture of water glass 7 and 15 phosphate glass. In contrast, the second portion 10 of the molding sand grains 5 is uncoated. In this example, the second portion 10 of the molding sand grains 5 is formed by the molding sand grains 5 having smaller grain sizes d3, d4, d5, d6. As can be seen, both parts 9, 10 are mixed.

The first layer 6 is cured before, wherein based on the mass of all first layers 6 on the quartz grains 5, a total mass mWb results when a pure water glass coating is present. This layer is shown here ideally with respect to the homogeneity of the layer thickness.

Clearly visible is the grain packing which is ideally dense for the casting process, i. There is almost no cavity 8, which is not filled with molding sand grains 5.

Furthermore, it becomes clear that each uncoated shaped sand grain 5 is in contact with a coated molding sand grain 5, so that an optimal friction reduction between the molding sand grains 5 can be assumed. This results not least from the selection of the particle sizes of the first portion 9 and the second portion 10.

Fig. 9 shows by way of example a molding sand grain 5, on the surface 5a of which a first layer 6 having a layer thickness 13 is arranged, which, as in FIG Fig. 8 described, for example, consists of water glass 7. The first layer 6 is cured here before. In addition to this is on the surface 6a of the first layer 6, a second layer 11 with a layer thickness 14 is arranged. This second layer 11 may preferably be hydrophobic.

Fig. 10 shows by way of example a molding sand grain 5, on the surface 5a, a first layer 6b is arranged, which is analogous to the first layer 6 as in Fig. 8 described, for example, consists of water glass 7. In contrast to the first layer 6 according to FIGS. 8 and 9 However, this first layer 6b is not yet cured, but is in the form of an aqueous solution 7b, for example of water glass 7, before. In addition to this molding sand grain 5 shown, it is of course possible for there to be a plurality of molding sand grains 5 which are present with a first layer 6b in the form of the aqueous solution 7b. Based on the mass of all first layers 6b in the form of the aqueous solution 7b on the quartz grains 5, this results in a total mass mWa for pure water glass 7.

All disclosed in the application documents features are claimed as essential to the invention, provided they are new individually or in combination over the prior art.

LIST OF REFERENCE NUMBERS

1

mold

2

mold core

3

workpiece

4

molding sand

5

Molding sand grain

5a

surface

6

first layer, cured

6a

surface

6b

first layer, not cured

6c

first layer, not cured, water glass and phosphate glass

7

water glass

7b

aqueous solution

8th

cavity

9

first share

10

second share

11

second layer

12

additive

13

layer thickness

14

layer thickness

15

phosphate glass

16

aqueous solution

a

crack length

e

energy

ε

strain

b1-b7

Expansion behavior, ε (T)

c1-c7

binder

d1-d6

grain size

g1

surface energy

g2

elastic distortion energy

g3

total power

f

Flash formation

K

grain packing

mWA

Mass of water glass, not hardened

MET

Mass of water glass, hardened

mQ

Total mass molding sand

Mq1

Mass first share

mq2

Mass second share

my

Mass per sieve

T

temperature

T _g

Glass transition temperature

y

screen size

Claims (12)

Mold (1) and / or mold core (2) made of molding sand (4) for metal casting,
characterized in that
a first layer (6) is arranged on the surface (5a) of molding sand grains (5) of the molding sand (4), the first layer (6) being hardened and made of alkali water glass (7) and / or phosphate glass (15) and Boratglas and optionally one or more additional additives. Mold (1) and / or mold core (2) according to claim 1,
characterized in that
the first layer has an average layer thickness of 0.01-50 μm, preferably 0.1-20 μm, more preferably 0.5-10 μm, particularly preferably 1-5 μm. Mold (1) and / or mold core (2) according to claim 1 or 2,
characterized in that
the at least one additive is selected from a group which comprises alkalis, preferably alkali salts, alkali oxides, alkali hydroxides, more preferably soda and / or caustic soda, alkaline earths, preferably alkaline earth salts, alkaline earth oxides, alkaline earth hydroxides, fluorides, preferably cryolite, borates, chalcogenide glass, iron oxide, spodumene , iron-containing silicates, viscosity-lowering solids, and combinations thereof. Mold (1) and / or mold core (2) according to one of the preceding claims,
characterized in that
the glass transition temperature (Tg) of the first layer (6) is at most 1400 ° C and is more preferably below the quartz cracking temperature of 573 ° C. Mold (1) and / or mold core (2) according to one of the preceding claims,
characterized in that
the first layer (6) consists of water glass (7) and a total mass (mwb) of the water glass (7) bound in the first layer (6) is at most 0.045-5 wt is less than 2 wt .-%, more preferably less than 1 wt .-%, more preferably less than 0.5 wt .-% of the total mass (mQ) of the molding sand (4). Mold (1) and / or mold core (2) according to one of the preceding claims,
characterized in that
the grain size (d1, d2, d3, d4, d5, d6) of the molding sand (4) is between 50 μm and 2000 μm, preferably between 63 and 500 μm. Mold (1) and / or mold core (2) according to one of the preceding claims,
characterized in that
the molding sand (4) consists of a first coated (9) and a second uncoated portion (10), wherein the mass (mQ1) of the first portion (9) of the molding sand (4) 1 to 25 wt .-%, preferably 2 to 20 wt .-%, particularly preferably 5 to 15 wt .-% of the total mass (mQ) of the molding sand (4). Mold (1) and / or mold core (2) according to claim 7,
characterized in that
the grain size (d1, d2) of the first portion (9) of the molding sand (4) is greater than the grain size (d3, d4, d5, d6) of the second portion (10) of the molding sand (4). Mold (1) and / or mold core (2) according to one of the preceding claims,
characterized in that
on the surface (6a) of the first layer (6) of the molding sand (4) one or more further layers (11) is / are arranged, wherein the / of the further layer / layers is preferably a hydrophobic layer (11). Process for producing a molding sand (4) suitable for a casting mold (1) and / or casting core (2) according to Claim 1, characterized by a) application of a first layer (6b; 6c) of alkali-water glass (7) and / or phosphate glass (15) and / or boratate glass in the form of an aqueous solution (7b; 16) and optionally an additional additive to the surface (5a) a first portion (9) of the molding sand grains (5) used for the casting mold (1) and / or the casting core (2) by means of mixing, dipping or spraying, in particular in a fluidized bed plant, b) curing the first layer (6b, 6c) by drying, firing or gassing the coated first portion (9), and c) mixing the first coated portion (9) with an uncoated second portion (10) of the molding sand grains (5) used for the casting mold (1) and / or the casting core (2), wherein the particle size (d1, d2) of the first portion (9) is greater than the grain size (d3, d4, d5, d6) of the second portion (10). Method according to claim 10,
characterized in that
the mass (mWa) of the aqueous solution (7b) of the water glass (7) and / or phosphate glass (15) and / or boratate glass used for applying the first layer (6b) is between 0.15 and 10% by weight, preferably between 1 and 5 wt .-%, particularly preferably between 1 and 2 wt .-% of the mass (mQ1) of the first portion (9) of the molding sand (4) and preferably the mass used for the application of the first layer (6b) (mWa) of the aqueous solution (7b) of the water glass (7) between 0.15 and 5 wt .-%, preferably less than 2 wt .-%, more preferably less than 1.5 wt .-%, more preferably less than 1 wt .-% of the mass (mQ1) of the first portion (9) of the molding sand (4). Method for producing a molding sand (4) suitable for a casting mold (1) and / or casting core (2), characterized in that
a material suitable for lowering the melting point of the material of the molding sand grains (5) and / or an alkali water glass (7) and / or phosphate glass (15) and / or an additional binder is applied to a surface (5a) of molding sand grains (5) and then by thermal treatment, the surface of the molding sand grains (5) or a selection of the molding sand grains (5) is at least partially converted into a glass or converted into a glassy state.

Images