WO2002064285A2

WO2002064285A2 - Casting mould

Info

Publication number: WO2002064285A2
Application number: PCT/EP2002/001119
Authority: WO
Inventors: Thomas Imwinkelried
Original assignee: Alcan Technology & Management Ag
Priority date: 2001-02-15
Filing date: 2002-02-04
Publication date: 2002-08-22
Also published as: EP1236525A3; WO2002064285A3; EP1236525A2

Abstract

The invention relates to a casting mould (1) comprising a die cavity (10) and a casting mould body which dissipates heat and forms said die cavity (10). Said casting mould is used for casting parts of metal according to a mould casting method and contains a coating (20) in the form of a coating body on the surface forming the die cavity (10). Said coating has a thickness of less than 5 mm. The coating body is made of a skeleton-shaped, highly porous solid body made of a heat-proof material exhibiting more than 50 % porosity and a thermal conductivity k to k < 1 W/mK.

Description

mold

The present invention relates to a casting mold with a mold cavity and a heat-dissipating and forming the mold cavity for casting metal castings, and the use of a casting mold and a method for producing a casting mold.

In the molding process, for example, cast or molded parts are cast directly from so-called casting alloys, with what are known as permanent molds, i.e. reusable molds.

The casting processes are divided into different categories according to the type of casting process. A distinction is made between the so-called gravity casting processes, e.g. Chill casting, and the die casting process. While in the gravity casting process the molten metal flows into the casting mold solely by gravity, the melt is pressed into the casting mold in the die casting process with pressurization. The pressures used to fill the mold can be very high, for example up to 200 bar, so that the molten metal is properly shot into the mold. The molten metal provided for a molded part is generally provided in a metered manner in the so-called filling sleeve or casting chamber and pressed into the mold cavity by means of a piston via the sprue channel. After the molten metal has solidified, the casting mold can be opened and the casting can be removed. Die casting is particularly suitable for the rational production of large series and for the production of thin-walled castings.

In the so-called modified die-casting processes, the mold cavity of the casting mold is evacuated or a negative pressure is generated before the molten metal is shot in. Such methods are known, for example, under the name MFT method (minimum filling time) or vacuum method.

A division into casting processes with slow filling, such as Chill casting, tilting casting, vacuum casting or low pressure casting and in those with short filling times, such as die casting, thixo casting or vacural.

The casting processes all have in common that the heat flow from the molten metal or the casting takes place essentially via the casting mold. The thermal conductivity of the parts of the mold responsible for the heat flow thus significantly influences the course of a casting process. In addition to the thermal conductivity of the casting However, the so-called separating surface between the casting mold and the melt also has a significant influence on the heat exchange. However, the influence of the separating surface makes it particularly difficult to control the heat exchange between the molten metal and the casting mold. The control of the heat exchange is particularly aimed at ensuring that the casting mold extracts as little heat as possible from the melt during the mold filling process, so that the flowability of the melt is preserved and there is no premature solidification and thus incomplete mold filling. After completing the filling process, on the other hand, in order to guarantee short cycle times, as much heat as possible should be dissipated through the casting mold in a short time so that the molten metal can be rapidly solidified and removed from the casting. The casting mold should therefore on the one hand have an insulating effect against the melt and on the other hand have good thermal conductivity.

In more complex casting processes, e.g. die casting, the mold is usually multi-part. The actual molding tool part is called the mold insert. The mold insert of a casting mold is characterized in that it forms the mold cavity of the casting mold and must dissipate the heat from the mold cavity. The mold insert usually also contains a so-called sprue channel, which leads into the mold cavity. The mold insert is usually in several parts. As a rule, the mold insert is in two parts and consists of two mold insert halves.

In addition to the mold insert, the mold can e.g. also contain a casting chamber, piston devices, hydraulic devices, ejectors and other parts. In other casting processes, such as die casting, the casting mold in turn usually only consists of the part that forms the mold cavity. For the sake of simplicity, the part of the casting mold which forms the mold cavity and which dissipates heat is referred to below as the casting mold body. The casting mold body corresponds, for example, to the use of a die casting mold.

In the case of casting processes with slow mold filling, casting molds or casting molds made of gray cast iron are generally used. The casting molds are usually covered with an insulating size at the interface to the mold cavity, which is intended to prevent premature solidification of the melt. The sizes are inorganic, fire-resistant materials with a very fine grain. For example, so-called white sizes made of sludge chalk, clay or kaolin or black sizes made of carbon black or graphite are known. The In addition to regulating the heat transfer, finishing also serves to prevent chemical reactions between the molten metal and the casting mold. The sizes can also have a lubricating function.

The sizes can be thermally conductive or insulating. Furthermore, heat-conducting and insulating sizes can be used in the same mold. This enables a limited control of the heat exchange.

With a sufficient thickness of the insulating coating, for example more than 0.5 mm, the heat exchange is no longer influenced by the separating surface but only by the thermal conductivity of the coating, which allows better control of the heat exchange. However, the sizes should not exceed the layer thickness of 0.8 mm, otherwise the tendency to flake off increases significantly. The control of heat dissipation is therefore only possible in practice to a limited extent, since layer thicknesses of any size cannot be used. Flaking sizing can also get into the melt during mold filling and impair the mechanical properties of the cast part. In the case of casting processes with a short filling time, the casting molds or the casting mold bodies generally consist of steel, in particular of nitrided hot-work steel. In these processes, however, so-called release agents are not applied to the separating surface from the mold cavity. The release agents are intended to prevent the molten metal injected under high pressure from sticking to the casting mold. The separating surface to the mold cavity must be sprayed again with separating agent after each casting process. Remnants of release agents can pass into the melt during the mold filling process and reduce the quality of the cast parts. Since such release agents are generally applied on the basis of an aqueous solution, the rapid injection of the melt due to evaporating solvent residues when the metal melt comes into contact with the release agent can result in gas inclusions which noticeably reduce the quality of the molded part. Furthermore, the release agent, in particular an organic release agent, can react with the melt to form gas. These gases carried in the melt in turn impair the mechanical properties and weldability of the cast parts. In the case of casting processes with a short filling time, the heat exchange between the casting mold and the molten metal can only be insufficiently controlled, since this is largely determined by the heat transfer at the interface. The heat transfer at the separating surface is characterized by the so-called heat transfer coefficient, which in turn depends on many factors that are difficult to control, such as the melt flow rate, the nature of the contact surface, gap formation, local pressure or washout of the separating agent. The control of the heat exchange in known casting molds is therefore very limited. To make matters worse, the heat flow fluctuates greatly over time and reaches very high peak values immediately after the mold is filled. The temperatures of the mold surfaces therefore usually fluctuate between 20 ° C and 500 ° C during the individual casting cycles. The heat balance of the casting molds is usually regulated on the one hand by heating and / or cooling channels in the interior of the casting mold and on the other hand by spraying and blowing out the release agent after removal from the mold. Because of the poor thermal diffusivity of the steel compared to aluminum, a 2-3 cm thick zone at the interface to the mold cavity is usually subject to particularly large temperature fluctuations. For the reasons mentioned above, the selection of suitable materials for the production of casting molds is particularly important for die casting processes, limited. Up to now, aluminum molds have not been suitable for molds due to the greatly reduced mechanical strength at temperatures of over 200 ° C. Furthermore, aluminum has a coefficient of thermal expansion that is comparatively high compared to steel, which is why aluminum casting molds cannot be exposed to high temperature fluctuations.

The difficult to control heat exchange between molten metal and casting mold in conventional casting molds has a negative impact on the process control and reproducibility of the processes. It is usually necessary to produce a few casts as rejects when starting a casting process with continuous casting cycles until the steady-state thermal conditions in the mold are set. Stationary thermal conditions mean that the changing thermal conditions remain essentially the same over a casting cycle in the subsequent casting cycles. The invention is therefore based on the object of proposing a casting mold which allows better control of the heat exchange between molten metal and casting mold and the use of more heat-sensitive materials for casting molds or mold bodies, the use of release agents or sizes being avoided. According to the invention, the object is achieved in that the molded body has a coating in the form of a laminated body with a thickness of less than 5 mm in part or over the entire surface of the surface forming the mold cavity, and the laminated body consists of a thermally insulating, heat-resistant shock-resistant material with a thermal conductivity k of k <1 W / mK.

Partial surface means that the laminate is applied to individual surface sections of the mold cavity surface. In contrast, a full-surface coating covers the entire surface of the mold cavity.

The subclaims describe further design variants of a casting mold according to the invention.

The layered body preferably consists of a porous material or solid, preferably of a highly porous material or solid with a porosity of greater than 50% by volume.

The coating is expediently permanently fixed on said surface, i.e. the coating remains functional over several casting cycles. However, it is within the scope of the invention that the coating may have to be renewed or repaired after a few casting cycles, but not like the release agent has to be completely applied again.

The layered body forming the coating preferably measures a thickness of less than 2000 μm, particularly preferably less than 1000 μm, in particular less than 500 μm, advantageously less than 200 μm, and greater than 0.1 μm, preferably greater than 1 μm, in particular larger than 5 μm.

The laminate advantageously consists of a three-dimensional, skeletal or framework-like network of interlinked particles or particles of a size of preferably 1-50 nm, advantageously 1-20 nm, and in particular 2-10 nm. The laminate is caused by the said skeletal structure, more than 50%, preferably more than 75%, and in particular more than 95%, open-pore, based on the total pore volume of the laminate.

The pore sizes, ie the pore diameters, of the layered body can be up to 10 μm, for example. The pore diameter is more than 0 nm, preferably more than 0.5 nm, ^t vorteilhaf more than 5 nm, in particular more than 10 nm and preferably less than 1000 nm, advantageously less than 500 nm, and especially less than 100 nm. The porosity of the laminated body is preferably over 60 vol .-% (volume percent), advantageously over 85 vol .-%, and in particular over 90 vol .-%, the volume percentages relating to the total volume of the laminated body. The porosity can even be between 95 and 99% by volume. The pores are preferably filled with a gaseous medium such as air. Furthermore, the pores can also be evacuated. The density of the laminate is expediently between 10 and 1,000 kg / m ³ , preferably between 10 and 600 kg / m ³ and in particular between 50 and 400 kg / m ³ .

Due to the structure described above, the thermal conductivity k of the layer body is preferably less than 0.5 W / mK, particularly preferably less than 0.1 W / mK, in particular less than 0.05 W / mK, and advantageously less than 0, 02 W / mK, where "W" stands for watts and "K" for Kelvin and "m" for meters. If the pores of the layer material are evacuated, the thermal conductivity can be 0.01 W / mK or less. Due to the low thermal conductivity of the laminate, the heat transfer at the parting surface plays a subordinate role in the casting molds according to the invention.

The heat flow through the laminate is preferably determined significantly by heat conduction. The heat exchange via convection is of little importance, since the heat is only transported through the gas in the pores by means of the less efficient diffusion. The heat conduction over the skeletal body plays an increasingly smaller role in a transparent layered body in comparison to the heat radiation with increasing layer thickness. However, since the casting cycles do not take too long and the solidification of the cast part must not take any amount of time, the heat conduction should not fall below a certain level and the layer thickness of the laminated body should therefore not exceed a certain thickness.

For example, if the temperature Ti on the melt-facing surface of the laminate is 600 ° C and the temperature T ₂ on the mold-facing surface of the laminate is 20 ° C, the layer thickness of the laminate should have a thermal conductivity k = 0.02 W / mK do not exceed 1 mm in thickness. The surface roughness of the laminated body essentially corresponds, for example, to that of the casting body and is, for example, 10-100 μm for steel molds. The layered body preferably consists of a material produced in a sol-gel process in which a colloidal solution, the so-called sol, for example by means of hydrolysis / polycondensation, in a gel and then by means of a drying process with partial or complete retention of the gel structure in a porous solid with the above-mentioned properties is transferred.

Layer materials which have the structure and properties mentioned above are the so-called aerogels and xerogels.

According to a narrower definition, aerogels are solids made from a high-porosity, framework-like or skeleton-like structure of low density from particles which are linked together to form an open-pore network, the solvent while maintaining the original gel structure by means of a suitable drying process, such as eg supercritical drying from which gel has been removed. An airgel is characterized in particular by the fact that the framework structure remains essentially undamaged after the drying process and the volume of the airgel is at least 50%, and in particular more than 85% of the original volume of the gel before the drying process.

Xerogels are manufactured in the same way as aerogels, except that they are dried while maintaining the original framework structure under subcritical conditions, the framework usually showing cracks and being partially broken. The xerogel is characterized in particular by the fact that the gel body shrinks by around 90% during the drying process.

In addition, so-called cryogels are known as possible laminate materials which are produced analogously to aerogels or xerogels, the removal of the liquid, i.e. the drying process from which the frozen state occurs by sublimation of the solvent. Due to the volume expansion of the frozen solvent, the original gel structure is usually destroyed and the pore size and geometry of the cryogel is essentially based on the crystallization pattern of the solvent.

The manufacturing process of the airgel as well as the xerogel mainly comprises two steps, namely the sol-gel process and the subsequent drying process. In the sol-gel process, an aqueous or alcoholic sol solution is converted, for example via hydrolysis and polycondensation, into a gel in which monomers link to form a three-dimensional, open-pore network or framework. The pH of the solution plays a decisive role in converting the sol into a gel. By adding basic or acidic catalysts, the speed of the sol-gel process and also various properties of the gel body, such as the connection of the individual chains, which in turn is decisive for the thermal conductivity and elasticity of the gel body, can be influenced. The porosity of the gel body is determined by the concentration of polymerizable monomers in the starting solution.

Since the three-dimensional network generally does not contain closed pores, the solvent enclosed by the network can be completely extracted from the gel with the formation of an open-pore structure in a drying process. The conditions under which the drying process is carried out are decisive for whether an airgel, i.e. a gel body made of a stable, three-dimensional open-pored framework, a xerogel, a cryogel, a powder, or a solid that has shrunk to a compact end product remains.

The drying process must be carried out in such a way that when the solvent is extracted from the pore spaces, the gel framework does not collapse and break under the influence and interaction of the capillary forces and surface tensions that occur.

The drying process of an airgel can take place under subcritical conditions, e.g. Environmental conditions, or take place under so-called supercritical conditions. If the drying process takes place under subcritical conditions, special treatment and modification of the gel body, for example by using surface-modifying substances and / or by changing the solvent, must ensure that the gel body is retained and that there is little or no shrinkage.

Under supercritical or hypercritical conditions, the phase transition from liquid to gaseous, in which the greatest capillary forces occur, is avoided by converting the gel into a critical state with a critical pressure P _c and a critical temperature T _c . In the critical state (P _c , T _c ) there is no longer a distinction between the liquid and gas phases, and accordingly there is no longer any phase transition. The density of the liquid phase corresponds to that of the gas phase. In such a state there are no more capillary forces.

There are also known aerogels, in particular silica aerogels (silica aerogels) which, through specific modification of the solid framework, during the drying process under subcritical conditions, ie with a phase transition from liquid to gaseous, by force the action collapses into itself and after the drying process has been completed, it is returned to the original structure by a so-called spring back effect. This category of aerogels is particularly suitable for molds according to the invention. The resilient aerogels can be designed such that they are fully opened, for example before and at the start of the mold filling process, and have maximum porosity and minimum thermal conductivity. Due to the increasing mold filling pressure, the framework structure is gradually or continuously collapsed in the course or towards the end of the mold filling process. Due to the decreasing porosity, the thermal conductivity is increased and the heat flow into the casting mold is promoted, which accelerates the solidification of the casting after the mold filling process and the casting cycle is shortened. After removal from the mold, the airgel structure folds back into the original highly porous skeletal structure by relieving pressure and a new mold filling process can begin. The resilience properties require a certain elasticity of the airgel framework, which means that its brittleness is much lower compared to other materials.

In principle, aerogels as well as xerogels can be produced from any monomer which can be converted into a gel by means of interlinking processes, carbon, metals, metal oxides and polymerizable, organic substances being particularly suitable for this purpose. So-called oxidic aerogels are known, for example, which are produced, for example, from metal oxides or metal alkoxides using a sol-gel process. The sol can be converted into a gel, for example, by hydrolysis and condensation of metal alkoxides, such as Si or Al alkoxides, by gelling particulate or colloidal oxide, such as Si or Al oxide, or a combination of these methods with the participation of basic or acidic catalysts. The liquid is preferably removed from the gel by means of supercritical drying. The oxidic aerogels can be, for example, ZnO-, Fe ₂ O ₃ -, SiO ₂ -, Al ₂ O ₃ -, TiO ₂ -, ZrO ₂ -, SnO ₂ -, Li ₂ O-, CeO ₂ - or V ₂ O ₅ - Be aerogels or mixtures thereof. The oxidic aerogels are preferably in the form of mixed-oxidic aerogels. Examples of binary mixed oxide aerogels are ZnO / Al ₂ O ₃ aerogels, Fe ^ s / SiO ^ airgel and in particular Al ₂ θ ₃ / SiO ₂ aerogels.

Silica aerogels, which consist, for example, of Si oxides or alkoxides, alkoxysilanes such as tetramethoxysilanes (TMOS), tetraethoxysilanes (TEOS) or alkali silicates (water glass), which form a sol with an alcoholic solvent, for example, occupy a special position , getting produced. Silica aerogels can be used at low temperatures below 100 ° C, which can help to avoid, for example, residual stress problems in the airgel layer. The production of gels containing Si is described, for example, in WO 93/06044.

The oxidic aerogels and in particular the silica aerogels are preferably dried under supercritical conditions.

Furthermore, aerogels are also known as organic solids, which are usually made from sol-gel processes, e.g. organic polymers dissolved in an aqueous solution, e.g. Melamine formaldehyde resins, resorcinol formaldehyde resins or phenol furfural (phenolic furfural), with the participation of basic or acidic catalysts by polymerization, polycondensation and subsequent drying. Such aerogels are also called plastic aerogels. The production of organic gels is e.g. in US-A-5,086,085 and in US-A4,873,218.

In further steps, the plastic airgel can be converted into carbon airgel by means of pyrolysis. The pyrolysis usually takes place under protective gas, such as argon or nitrogen, or in vacuo and at temperatures above 600 ° C., in particular above 1000 ° C.

To create, for example, a structure that can withstand higher pressures during the mold filling process, the aerogels can contain fillers. Such fillers can be, for example, inorganic filler materials such as SiO ₂ , Al ₂ O ₃ , TiO ₂ or mixtures thereof, which are introduced into the airgel, for example in a volume fraction of 5 to 30% by volume, based on the total volume of the airgel body. Organic fillers, such as thermoplastic or thermosetting plastic particles, for example polystyrene, can also be used. It should be noted here that during the pyrolysis of the plastic gels, these materials are melted out or burned. With the help of such materials, however, it is possible to control the shrinkage during pyrolysis.

If an IR opacifier such as carbon black, titanium dioxide, iron oxides or zirconium oxide is added to the sol before the gel is produced, the thermal conductivity can also be reduced by the radiation contribution. Furthermore, dyes and / or color pigments can be added to achieve a color.

Possible plastic aerogels can be based, for example, on resorcinol / formaldehyde, which, with a suitable composition and suitable content of basic polymerization catalyst, such as ammonium hydroxide and / or sodium carbonate, at temperatures between 20 and 50 ° C without supercritical drying in a micro-structured plastic airgel can be produced.

The chemical and physical properties of the aerogels can generally be varied and modified within wide limits by the starting materials, their mixture and the manufacturing process, so that it is possible to manufacture customized aerogels according to the task.

Silicic acid or mixed oxide airgel are preferably used as layered body materials. Silica aerogels are characterized in particular by the fact that they are not wetted by metals and metal alloys and are not chemically attacked, for example, by the castable metals and metal alloys listed below.

Thin airgel layers of e.g. e.g. less than 50 µm as a flowable sol by means of brushing, spraying, rolling, spinning or knife coating, dipping, flooding, pouring, or equivalent processes, applied to the corresponding mold surfaces and then cured or dried. Furthermore, the airgel, in particular in the case of airgel layers of more than 50 μm, can be produced in the form of foils and glued to the corresponding mold surfaces, the foil being able to be preformed in accordance with the surface topography. The adhesive, preferably a conventional adhesive, expediently has a heat resistance which is oriented to the temperatures and temperature fluctuations to be expected on the adhesive surfaces during the casting process. If the casting mold or the casting mold body is not heated to above 100 ° C, an Epxoy adhesive system may already be suitable. Carbon aerogels in particular are preferably prefabricated as foils, while e.g. Silicic acid aerogels are preferably applied directly to the surface of the casting mold and cured or dried by one of the methods mentioned at the outset.

If the airgel is applied as a film to the casting mold, the layer thickness can be in a range of 50-200 μm. If the airgel is applied as a sol to the casting mold and then cured or dried, the layer thickness can be in a range of 5-20 μm.

A typical silica airgel consists, for example, of crosslinked particles with a diameter of 2-5 nm, the pore volume being 95% or more of the IUII LI g *> * ι

- 12 -

Accounts for the total volume of the layer material. The pore diameter is around 15-25 nm.

The casting mold or casting mold body expediently consists of a material with good thermal conductivity, the material being intended to be heat-resistant at temperatures of up to 300 ° C, preferably up to 200 ° C and in particular up to 150 ° C, i.e. up to the temperature values mentioned, the material should not experience any significant impairment in its mechanical properties. The casting mold or the casting mold body preferably consists of a metal. The casting mold or the casting mold body can e.g. consist of an iron metal such as steel or a non-ferrous metal such as copper, magnesium or an alloy containing at least one of the metals mentioned. However, the casting mold or the casting mold body advantageously consists of aluminum or an aluminum alloy. Good thermal conductivity in the above sense means thermal conductivities equal to or more than 30 W / mK, preferably more than 100 W / mK, and in particular more than 150 W / mK. The surface, in particular the surface of the casting mold provided with a coating according to the invention, can be subjected to a pretreatment before application of the laminate or the airgel and can be provided, for example, with an adhesion promoter or primer. Said surface of a casting mold or a casting body made of aluminum or an aluminum alloy can have, for example, a pretreatment layer produced by chromating, phosphating or by anodic oxidation.

The pretreatment layer can e.g. an anodic oxide layer built up in a redissolving or non-redissolving electrolyte. Furthermore, the pretreatment layer can be a yellow chromating layer, a green chromating layer, a phosphate layer or a chromium-free pretreatment layer which has been grown in an electrolyte containing at least one of the elements Ti, Zr, F, Mo or Mn. The pretreatment layer can also be an oxide layer made by natural oxidation.

The pretreatment layer can have a thickness of more than 20 nm, preferably more than 50 nm, in particular more than 100 nm, and advantageously more than 150 nm (nanometers) and less than 1500 nm, and preferably less than 200 nm.

Degreasing or mechanical surface removal by abrasive agents can also be used as pretreatment, alone or in combination with the aforementioned pretreatments be applied. Mechanical surface removal can be done by grinding, blasting, brushing or polishing.

The coating according to the invention, optionally applied to a pretreated surface mentioned above, can also be provided on the cavity surface of the casting mold and / or the casting chamber of a casting mold, in particular a die casting mold, in whole or in part.

The casting mold or the casting mold body can contain means for cooling and / or heating the same. The casting mold can in particular contain heating and / or cooling channels in which a gaseous or liquid heating and / or cooling medium, such as e.g. Oil circulated. A damaged or worn coating can be renewed or repaired, for example, by applying an airgel layer in liquid to gel-like form using one of the abovementioned methods in situ and curing or drying under below-critical conditions at temperatures below 200 ° C., in particular below 100 ° C. The new airgel layer can also be glued to the surface in the form of a film. If necessary, existing airgel layers can be acidified, e.g. Hydrofluoric acid, etched away and then renewed.

The casting mold according to the invention is particularly suitable for casting aluminum, magnesium, titanium or alloys containing at least one of the metals mentioned. Particularly suitable aluminum alloys are e.g. AlSi, AISiMg and AICu. Furthermore, the casting mold according to the invention, coated with an airgel, in particular with a silica airgel, is also suitable for casting tin and tin alloys, copper and copper alloys or bronze alloys, lead and lead alloys, zinc and zinc alloys, silver and silver alloys as well as gallium and gallium alloys. Other metals which can be cast using the casting mold according to the invention are not intended to be excluded by the list at hand.

With the casting mold according to the invention, alloys which are difficult to cast, such as e.g. Wrought alloys can be cast in a molding process.

The thermal insulation of the casting mold delays the heat flow from the melt and the metal melt cools down less quickly, especially during the mold filling process. The metal melt therefore has an improved flowability compared to conventional casting molds during the mold filling process. The active heating of the mold to temperatures of over 150 ° C is no longer necessary, which can save energy. The casting mold or the casting mold body, for example made of Aluminum, for example, can have minimum temperatures of 10 to 30 ° C during the casting cycle.

Depending on its thermal conductivity, the dimensioning of the laminate is preferably designed such that the temperature in the mold at the interface to the laminate during the casting process does not exceed 200 ° C., preferably not above 150 ° C., and in particular not above 120 ° C. increases.

Depending on its thermal conductivity, the dimensioning of the layered body is preferably designed such that the setting time of the cast part after the mold filling process has ended is less than 6 seconds, preferably less than 4 seconds, in particular less than 3 seconds.

Since the maximum temperatures of the casting mold or of the casting mold body are lower during the casting cycle, stress relief annealing of the casting mold is no longer necessary in all cases.

The thermal equilibrium is also already present when a casting process is started, which means that the production of rejects at the start of the casting process can be reduced. The changing times of the casting molds or the casting mold bodies are shortened, since they do not have to be cooled or heated when changing.

The better flowability is also partly due to the lower friction of the molten metal on the surface of the laminate. Due to the high porosity in the nanometer range, the aluminum melt only comes into contact with the surface of the laminate at certain points. The surface tension of the melt ensures that the melt does not penetrate into the pores of the laminate. The mechanical loads, particularly in the case of die-casting processes, are significantly lower thanks to the excellent flowability of the metal melts until the mold filling is complete, i.e. Due to the good flowability, the mold can be filled with less pressure than before.

Furthermore, due to the point-to-point contact, the adhesive surface between the casting and the casting mold is small, the demolding forces are correspondingly low and there is no wetting of the casting mold surface by the molten metal, which means that there is no need to use release agents or sizes.

Thanks to the better and longer flowability of the molten metal and the delayed solidification, castings with complicated geometric and thin-walled structures can be manufactured. Furthermore, the heat flow can be controlled locally by varying the thickness of the coating within the same casting mold. For example, in the case of thin-walled areas of the cast part, larger layer thicknesses can be provided in the casting mold in order to delay the outflow of heat here. In the case of thick-walled areas of the casting, smaller layer thicknesses can be provided in the casting mold in order to accelerate the heat dissipation here and to shorten the cycle time. In general, by varying the layer thickness, the time window for the mold filling can be determined as desired within the specified boundary conditions. Boundary conditions are, for example, the prevention of pre-solidification of the melt before the mold filling is completed, as well as maximum mold filling times which still allow an economical production process.

The casting mold according to the invention is preferably used in a casting device for producing castings from metal in a molding process. The casting mold according to the invention can be used in casting devices for all the casting processes mentioned at the outset in the prior art, but in particular for vacuum casting, low-pressure casting and die-casting processes. The casting mold according to the invention is preferably a permanent mold. The casting mold according to the invention can also be used in format casting processes. Other casting processes not mentioned should not be excluded from the use of casting molds according to the invention.

The use of aluminum casting molds coated according to the invention allows easier handling because of the reduced weight.

The above statements regarding the production and use of aerogels as laminated bodies, with the exception of the airgel-specific drying processes, are also intended to apply to xerogels.

An example of the production of a silica, a plastic and a carbon airgel is given below.

Example 1 :

Tetramethoxysilane (TMOS) is dissolved in methanol and stirred into a sol with the addition of water. An acidic or basic catalyst, such as NH ₄ OH, NaOH, KOH, Al (OH) ₃ and / or colloidal silica, is then continuously added to the solution, as a result of which the hydrolysis and polycondensation begin:

Si (OCH ₃ ) ₄ + 4H ₂ O → Si (OH) ₄ + 4CH ₃ OH hydrolysis Si (OH) ₄ -> SiO ₂ + 2H ₂ O polycondensation

The ratio of TMOS to methanol determines the density of the finished airgel, while the amount of water and catalyst added, as well as the choice of catalyst, determine the morphology of the airgel skeleton. The still low-viscosity sol can be applied to the surface of the mold cavity of the casting mold. The subsequent transition from sol to gel can take a few seconds, e.g. at high concentrations of TMOS and strongly basic conditions, up to several minutes, e.g. for lower concentrations of TMOS and less basic conditions. The gel is then dried to a silica airgel. To increase the gel strength, for example, the gel can be subjected to aging in an intermediate step before the drying process; the aging of the gel usually takes place at a temperature of around 20 ° C. to the boiling point of the solvent. Aging can last from a few minutes to several hours. Example 2:

Resorcinol, e.g. in the form of granules, is dissolved in formaldehyde in a molar ratio of 1: 2. Water and sodium carbonate as catalyst are added to the solution with continuous stirring. The present solution is applied to the surface of the mold cavity. The gel can be subjected to aging at a constant temperature before the drying process. Before the drying process, the water in the pores of the aged gel is replaced by an organic solvent such as acetone.

The density of the plastic airgel is determined by the concentration of resorcinol formaldehyde (RF) in the starting solution. Furthermore, the molar ratio R K of resorcinol (R) to catalyst (K) determines the reaction rate and thus the morphology of the airgel skeleton.

In order to avoid shrinking the gel body, RF gels with a low R / K ratio have to be dried under supercritical conditions, while organic gels with a low concentration of catalyst, i.e. with a high R / K ratio, have to be dried under subcritical conditions , e.g. at ambient pressure of 1 bar and a temperature of 50 ° C, the shrinkage is only a few percent.

Example 3: By pyrolysis of the plastic airgel obtained in Example 2 at over 600 ° C and a maximum temperature of around 1050 ° C with continuous aeration with a non-oxidative gas such as argon or nitrogen, the organic airgel framework is retained while maintaining its morphology converted to a carbon scaffold. The porosity of the carbon airgel formed can be influenced by varying the temperature. The porosity can also be influenced by the inclusion of an oxidative gas or by the circulation of a hydrocarbon compound.

The casting mold according to the invention can be produced by applying the coating as a flowable sol on the surface of the mold cavity of the casting mold and converting it into a gel by means of condensation and converting it to a solid laminate by drying, or the coating as a film on the surface of the Mold cavity of the mold is applied and fixed, preferably by gluing.

In a preferred embodiment of the method, the coating is applied to the surface of the mold cavity of the mold by means of brushing, spraying, rolling, spinning, knife coating, dipping, flooding or pouring.

If the molded body is made of aluminum or an aluminum alloy, a pretreatment layer produced by means of anodization, chromating or phosphating can be applied to the surface of aluminum or an aluminum alloy carrying the coating. The casting mold according to the invention is explained in more detail below by way of example and with reference to the accompanying drawings. Show it:

1a: a casting mold of a die casting device;

1b: a section of the mold cavity according to FIG. 1a;

2: a graphical representation of the heat flow from the molten metal into the casting mold for various casting molds.

Fig. 1a shows the schematic representation of the mold 1 of a die casting device. The casting mold 1 contains a fixed first mold half 12 with a first mold insert 2a attached to a mold plate 8 and a movable second mold half 9 with a second mold insert 2b. The second, movable mold half 9 with the second mold insert 2b fits in the closed state to form a mold cavity 10 on the first mold half 12. The casting mold 1 also contains a casting chamber 4 with a piston 6 guided therein. The casting chamber 4 contains feed openings 7a, 7b through which the molten metal is fed into the casting chamber 4. The casting chamber 4 is connected to the mold cavity 10 via the sprue channel 3. The casting chamber 4 can be closed with respect to the pouring channel 3 by means of a valve 5. The valve 5 can be controlled via a hydraulic device 11 with a piston rod. The mold cavity 10 also has one or more vent channels (not shown). The ventilation channel or channels can also be equipped with a device for generating a vacuum or vacuum with corresponding vacuum shut-off valves (not shown). This allows the mold cavity 10 to be evacuated before the molten metal is injected.

The schematic, circular section Q of the mold cavity 10 shown in FIG. 1b is indicated by dashed lines in FIG. 1a. The mold cavity 10 is formed by the first mold insert 2a and the second mold insert 2b. The two mold inserts 2a, 2b each contain a base body 21, 22 made of aluminum or an aluminum alloy, which has a permanently fixed coating 20 made of an airgel, preferably a silica airgel, with a thickness of 10-100 on the surface facing the foam cavity 10 μm. In a modified version of the present invention example, the sprue 3 and / or the pouring chamber 4 can also contain a coating of an airgel of the specified thickness. 2 shows the course over time of a modeled heat flow from an aluminum melt at a temperature of 660 ° C. into a casting mold. Y stands for the heat flow [W / m ² ] and X for the time [s] in seconds. A cast plate made of aluminum with a thickness of 5 mm is cast.

Curve A represents the modeled course of the heat flow into a casting mold made of aluminum, which was preheated to an initial temperature of 20 ° C. Curve B shows the modeled heat flow through a steel mold, which was preheated to an initial temperature of 200 ° C. Curve C shows the modeled course of the heat flow into a casting mold made of aluminum, which was preheated to an initial temperature of 20 ° C., the casting mold facing the mold cavity a coating of a silica airgel with a thermal conductivity of k = 0.02 W / mK and a thickness of 10 microns contains. Curve D shows the modeled course of the heat flow into a casting mold made of aluminum, which has been preheated to an initial temperature of 20 ° C., the casting mold coating the mold cavity contains a carbon airgel with a thermal conductivity of k = 0.02 W / mK and a thickness of 100 μm.

The course of the individual curves A-D shows how the airgel coating of the casting mold significantly reduces the maximum heat flow through the casting mold at the beginning of the casting process compared to both the pure aluminum and the pure steel casting mold. This means that the mold coated with an airgel heats up to much lower maximum temperatures and the metal melt cools down less quickly. In the case of curve C, however, the heat flow decreases less quickly over time compared to curves A and B and, after a certain time, is even above the heat flow of curves A and B. A distribution of the heat flow with a flat course takes place over time , The heat flow in the casting mold with a thicker 100 μm airgel layer (curve D) remains comparatively low compared to the other three curves A, B and C over the entire period.

The following Table 1 contains values for the solidification times in seconds and the maximum mold temperatures reached when casting an aluminum plate with a thickness of 2 mm or. 5 mm when using a steel mold preheated to 200 ° C, an aluminum mold preheated to 20 ° C, an aluminum mold preheated to 20 ° C with a 10 μm thick airgel coating and an aluminum mold preheated to 20 ° C with a 100 μm thick airgel coating. Both Aerogel coatings have a thermal conductivity of k = 0.02 W / mK.

The table values show that by using an airgel coating, the mold is exposed to smaller thermal loads through controlled heat dissipation, but the solidification times and therefore the cycle times increase.

2 and Table 1 further show that by varying the airgel layer thickness on the one hand the maximum thermal load on the casting mold body and on the other hand the time course of the heat flow from the molten metal into the casting mold can be controlled in a targeted and controlled manner. When choosing the layer thickness of the airgel, i.e. when determining a specific heat flow curve, the thermal load capacity of the casting mold materials, the economy with regard to longer cycle times, the geometry of the filling times and the maintenance of the flowability of the melt during mold filling must be taken into account become.

Table 1

Claims

Expectations

1. casting mold (1) with a mold cavity (10) and a heat-dissipating and the mold cavity (10) forming mold body for casting metal castings, characterized in that the mold body on the surface forming the mold cavity (10) partially or has a coating (20) in the form of a laminate with a thickness of less than 5 mm, which is resistant to liquid molten metal, and the laminate consists of a thermally insulating, heat shock-resistant material with a thermal conductivity k of k <1 W / mK.

2. Casting mold according to claim 1, characterized in that the laminated body is a solid made of a skeletal or framework-like, three-dimensional network of interlinked particles, particles or colloids of a size of preferably 1-50 nm, advantageously 1-20 nm, and in particular 2-10 nm.

3. Casting mold according to one of claims 1 to 2, characterized in that the laminated body consists of a porous material, preferably of a highly porous material with a porosity of greater than 50% based on the total volume of the laminated body.

4. Casting mold according to one of claims 1 to 3, characterized in that the coating has a thickness of less than 2000 μm, preferably less than 1000 μm, in particular less than 500 μm, and advantageously less than 200 μm and greater than 0, 1 μm, preferably larger than 1 μm, in particular larger than 5 μm.

5. Casting mold according to one of claims 1 to 4, characterized in that the laminated body has a thermal conductivity k of k <0.5 W / mK, preferably k <0.1 W / mK, in particular k <0.05 W / mK has.

6. Casting mold according to one of claims 1 to 5, characterized in that more than 50%, preferably more than 75% and in particular more than 95% of the total pore volume of the laminate is open-pore and the pore volume is more than 60%, preferably more than 85 %, and in particular more than 90% of the total volume of the laminate.

7. Casting mold according to one of claims 1 to 6, characterized in that the laminated body consists of an airgel or xerogel, and preferably of a silica airgel, carbon airgel, mixed oxide airgel or plastic airgel.

8. Casting mold according to one of claims 1 to 7, characterized in that the casting mold body, in particular the mold insert, consists of a metal, in particular of a metal with high thermal conductivity.

9. Casting mold according to claim 8, characterized in that the casting mold body, in particular the mold insert, consists of an iron metal, such as steel or a non-ferrous metal, such as aluminum, copper, magnesium or an alloy thereof.

10. Casting mold according to claim 9, characterized in that the casting mold body, in particular the mold insert of the casting mold, consists of aluminum or an aluminum alloy and the surface carrying the coating of aluminum or an aluminum alloy is a pretreatment layer, preferably an anodically produced oxide layer or a chromate coating or Pretreatment layer produced by phosphating.

11. Casting mold according to one of claims 1 to 10, characterized in that the casting mold is a multi-part casting mold, preferably with a mold insert (2a, 2b) forming the mold cavity (10, 2b), a device for a molding process, preferably for a vacuum casting, low pressure casting - or die casting process.

12. Casting mold according to one of claims 1 to 11, characterized in that the casting mold, in particular the casting mold body, contains means for cooling and / or heating it.

13. Casting mold according to claim 12, characterized in that the casting mold, in particular the casting mold body, contains heating and / or cooling channels for the circulation of a heating and / or cooling medium, preferably an oil.

14. Casting mold according to one of claims 1 to 13, characterized in that the laminated body of a casting mold has different layer thicknesses and, in the case of thin-walled regions of the casting in the casting mold, the laminated body has a greater thickness than a mean thickness within a casting mold to delay the heat flow in this zone and in thick-walled areas of the Casting of the laminate has a smaller thickness to accelerate the heat flow in this zone.

15. Casting mold according to one of claims 1 to 14, characterized in that the dimensioning of the thickness of the laminated body as a function of its thermal conductivity

5 is designed in such a way that the temperature in the casting mold body at the interface with the layer body during the casting process does not rise above 200 ° C., preferably not above 150 ° C. and in particular not above 120 ° C.

16. Casting mold according to one of claims 1 to 15, characterized in that the dimensioning of the thickness of the laminated body as a function of its thermal conductivity

10 is designed in such a way that the solidification time of the casting after completion of the

Mold filling process is less than 6 sec, preferably less than 4 sec, in particular less than 3 sec.

17. Casting mold (1) according to one of claims 1 to 16, wherein the casting mold (1) contains a sprue channel (3) and / or casting chamber (4), characterized in that the

15 casting mold (1) on the cavity surface forming the pouring channel (3) and / or the casting chamber (4) has a coating in the form of a laminated body with a thickness of less than 5 mm, in part or over the entire surface, which is resistant to molten metal, and the Laminate consists of a thermally insulating, heat shock resistant material with a thermal conductivity k of k <1 W / mK.

20 18. Use of a casting mold (1) according to one of claims 1 to 17 in a casting device for producing castings from metal in a molding process.

19. Use of a casting mold (1) according to claim 18 in a die-casting device for producing metal castings in a die-casting process.

20. A method for producing a casting mold (1) according to claim 1,

25 characterized in that the coating (20) as a flowable sol is applied to the surface of the mold cavity (10) of the casting mold and converted into a gel by means of condensation and converted to a solid laminate by drying, or the coating (20) as a film applied to the surface of the mold cavity (10) of the casting mold (1) and fixed, preferably by means of gluing.

21. The method according to claim 20, characterized in that the coating is applied by means of brushing, spraying, rolling, spinning, knife coating, dipping, flooding or pouring onto the surface of the mold cavity of the casting mold.

22. The method according to any one of claims 20 to 21, characterized in that the mold body consists of aluminum or an aluminum alloy and on which

Coating-bearing surface made of aluminum or an aluminum alloy, a pre-treatment layer produced by means of anodization, chromating or phosphating is applied before the coating is applied.