EP1036610B1 - Investment and mould casting in carbon and organic aerogels - Google Patents

Abstract

Open-pored aerogel mold material is obtained by sol-gel polymerization e.g., resorcinol/formaldehyde with a polymerization catalyst such as ammonium or sodium carbonate, followed by complete or partial pyrolysis of the resultant aerogel. Fillers, e.g. aluminum oxide, titanium oxide, quartz, polystyrene particles, carbon or silicon carbide fibers can be added. Independent claims are also included for the following precision casting processes for metals or alloys: (a) a wax mold is coated with a sol of suitable composition and a catalyst and converted to a gel at a temperature below the melting point of the wax. The process is repeated to build up the mold, after which the gel is dried. The temperature is then raised to melt or burn out the wax; (b) a wax pattern is placed in a container which is then filled with the synthetic sol and heated to below the wax melting point. The resultant gel is dried and the temperature raised to melt or burn out the wax.

Description

The invention relates to a molding material for the fine and casting of metals or metal alloys comprising plastic and / or carbon aerogels and a process for the preparation of corresponding molding materials.

Investment casting in ceramic shell molds is a standard casting technique for precision parts made of various alloys. The forms are in usually made by the lost wax process; d. H. a wax body of the part to be cast is wetted with a silica sol, in several steps sanded, dried and then the shell mold is fired, wherein the wax is melted out in an autoclave or burned. through Modern casting process, it is possible to cast in the correct shape and close to the final form (J.Sprunk, W. Blank, W. Grossmann, E. Hauschild, H. Rieksmeier, H.G. Rosselnbruch; Investment casting for all industrial sectors, 2nd edition, headquarters for casting use, Dusseldorf 1987; K. A. Krekeler, investment casting, in: Handbuch der Fertigungstechnik Vol. 1, publisher G. Speer, Hanser Verlag, Munich 1981).

Aerogels are highly porous, open-pored oxidic solids, which are usually via sol-gel processes of metal alkoxides by polymerization, polycondensation to gels and subsequent supercritical drying are obtained. For some years, it has also been possible to use plastics via sol-gel processes gelled and by supercritical drying in a highly porous organic Transform solid state. Pyrolysis of such plastic aerogels under inert gas or in vacuum at temperatures above 1000 ° C, this converts into carbon aerogels around. As the oxidic aerogels have plastic and carbon aerogels extremely low effective thermal conductivities (order of magnitude some mW / K / m) and are considerably lighter. The physical and mechanical Properties of plastic and carbon aerogels are in the literature Documented (R.W. Pekala, C.T. Alviso, F.M. Kong, S.S. Hulsey, J. Non-Cryst. Solids 145 (1992) 90; R.W. Pekala, C.T. Alviso, Mat. Res. Soc. Symp. Proc. 270 (1992) 3; R. Petricevic, G. Reichenauer, V. Bock, A. Emmerling, J. Fricke; J. Non-Cryst. Solids (1998)). They let themselves by the starting materials, their mixture and vary the manufacturing process within wide limits.

DE 197384 66 C1 describes a continuous casting apparatus with the bands of a any cross-section by means of strand or strip casting in one Aerogetformwerkzeug can be produced. The airgel mold may consist of silica aerogels or not working under reducing atmosphere of carbon aerogels.

K. E. Höner "Foundry Industry", Ullmanns Encyklopädie der technischen Chemie, P. 271-287, Bd 12, 4th edition, Verlag Chemie Weinheim, 1976 in a review of the various foundry processes between those with lost form what the molding and casting belongs and those with Permanent form, to which the continuous casting belongs.

It is therefore an object of the present invention to simplify the known in the prior art process for the production of moldings for the fine and cast molding of metals and metal alloys, in particular to reduce the duration of the drying process.

The above object is achieved in a first embodiment by a Forming material for the fine and casting of metals or metal alloys comprising highly porous, open-pored plastic and / or carbon aerogels, available by sol-gel polymerization of organic plastic materials optionally followed by partial or complete pyrolysis of the obtained Kunststoffaerogels.

The molding material according to the invention is particularly suitable for use in lost wax casting processes and does not have, as in the prior art in oxidic Gels, applied in several steps.

The forms thus obtained are filled with melt by conventional techniques and the melt solidifies. In the usual casting techniques, the heat dissipation takes place over the mold shell or the molding sand. Pour and solidify in On the other hand, aerogels, since carbon aerogels are quasi adiabatic, that the heat dissipation is unique to feeders and risers respectively attached heat sink is done, what cleverly the riser and feeder can be used by yourself, but do not have to. That way is one Fully controlled solidification possible and the structure can be adjusted according to the required property spectrum are adjusted.

The airgel forms produced according to the invention are particularly suitable for the casting of aluminum alloys (with the mold practically unheated must be because there is no heat dissipation by themselves). This increases the economy, because energy costs can be reduced. Magnesium- and titanium alloys also do not react with carbon, so that these carbon aerogels also for these alloys under inert gas or Offer vacuum as molding material.

A particular advantage of the molding materials according to the invention is that the Sol-gel formation at room temperature, that is, especially at temperatures completed within a few hours below the pour point of the wax can be. A supercritical drying, as with the purely inorganic Gels are not required. Nevertheless, it is possible to increase the pore size in the micrometer range adjust. For drying in the supercritical temperature range In addition, pore sizes in the nanometer range possible.

In addition, the molding materials according to the invention can also be inorganic or organic filler materials. These are essentially Under solidification conditions inert inert materials understood. inorganic Filler materials are, for example, selected from alumina, titania and / or quartz, each in an amount of 5 to 30% by volume. used can be. Fillers in the context of the present invention further include Fiber materials containing a fiber reinforcement with organic, inorganic or allow carbon and / or SiC fibers at about equal volume fractions.

In the same way, it is also possible to use organic fillers, for example thermoplastic or thermosetting plastic particles, for example polystyrene and / or organic (polyacrylonitrile) fibers. However, this is too Note that during pyrolysis of the plastic gels these materials are also melted out or burned. However, with the help of such materials is one Control of shrinkage during pyrolysis possible.

For the purposes of the present invention, particular preference is given to the molding material Plastic aerogels based on resorcinol / formaldehyde used in the suitable composition and suitable content of basic catalyst at temperatures between 20 and 50 ° C without supercritical drying in one microstructured plastic airgel can be transferred. By selection the composition of the sol-gel polymerization is adjustable so that, for example First, a highly viscous liquid is formed, which is on a wax mold can be applied. This is also possible in several operations, so that the layer thickness adapted to the needs of the applications in the foundry can be.

a) eine Wachsform mit einem Kunststoffsol geeigneter Zusammensetzung und einem geeigneten Katalysator benetzt,

b) bei einer Temperatur unterhalb der Fließtemperatur des Wachses das Sol in ein Gel überführt,

b') gegebenenfalls eine oder weitere Schichten des Sols aufbringt und jeweils teilweise oder vollständig in die Gelform überführt,

c) das Gel bei einer Temperatur unterhalb des Fließpunktes des Wachses trocknet und

d) bei einer Temperatur oberhalb der Fließtemperatur des Wachses dieses aus dem erstarrten Gel ausschmilzt oder ausbrennt.

Thus, another embodiment of the present invention is a process for making molds for the casting and casting of metals or metal alloys and the use of highly porous, open-pored plastic and / or carbon aerogels, wherein:

a) wetting a wax mold with a plastic sol of suitable composition and a suitable catalyst,

b) at a temperature below the flow temperature of the wax, the sol is gelated,

b ') optionally applying one or more layers of the sol and in each case partially or completely converted into the gel form,

c) the gel dries at a temperature below the pour point of the wax and

d) at a temperature above the flow temperature of the wax this melts or burns out of the solidified gel.

a) einen Wachsformkörper in einen Behälter einbringt,

b) den Behälter teilweise oder vollständig mit einem Kunststoffsol auffüllt,

c) bei einer Temperatur unterhalb der Fließtemperatur des Wachses das Sol in die Gelform überführt,

d) das Gel bei einer Temperatur unterhalb der Fließtemperatur des Wachses trocknet und

e) bei einer Temperatur oberhalb der Fließtemperatur des Wachses dieses aus dem erstarrten Gel ausschmilzt oder ausbrennt.

An alternative procedure for the production of the mold is that

a) introducing a wax molding into a container,

b) partially or completely filling the container with a plastic sol,

c) at a temperature below the flow temperature of the wax, the sol is converted into the gel form,

d) the gel dries at a temperature below the flow temperature of the wax and

e) at a temperature above the flow temperature of the wax this melts or burns out of the solidified gel.

Thus, it is possible to simply place the wax molding in a suitable container to fill up with the starting solution for the plastic aerogels and then carry out the airgel production process.

In this way can be analogous to the known block-mold process (in the essential gypsum used) massive but light quasi-adiabatic forms produce.

The temperature of the conversion of the solution into a plastic airgel must be Melting point of the wax can be adjusted. After conversion into one Plastic airgel, the wax can be melted out and at the same time Conversion into a carbon airgel takes place under exclusion of air. Dependent from the composition of the starting solution, the gelation temperature, The density of the resulting porous body can be used to produce molds both as a plastic and as a carbon airgel, on a micrometer scale are superficially smooth and form sharp contours. According to the invention requires the production of molds up to the plastic airgel usually 1 to 3 days, often only up to 24 hours. The pyrolysis time is determined by the thickness the mold shell; for example, with a wall thickness of 1 cm, the time is less than 24 hours, usually 10 hours. Compared to the production of typical investment casting shells using oxidic sol-gel processes are the Manufacturing times short and therefore economical. The shrinkage takes place in the both process steps is always isotropic and varies from a few percent to 20% and is therefore manageable. It can be explained by the composition of the sol, reduce and influence the drying conditions, the molding material and fillers and is therefore manageable.

a) Block-Mold-Verfahren:

1. Herstellung der Ausgangslösung (Resorcin, Formaldehyd, Wasser und basischer Katalysator);

2. Lagerung des Wachsmodells in einer PTFE oder Glasform;

3. Auffüllung des Behälters in 2. mit der Ausgangslösung (da das spezifische Gewicht der Wachsmodelle im allgemeinen geringer ist, als das der Lösung, muß die Form entsprechend beschwert werden (am besten an den Steigern und Speisem);

4. Gelierung im Wasserbadthermostaten (hierbei sollte die Form dicht verschlossen sein, damit die Lösung ihre Zusammensetzung nicht verändert) oder in einem Luftumwälzer im Temperaturbereich von 20 bis 50 °C;

5. Nach erfolgter Gelierung wird das noch nasse Gel in der geschlossenen Form bei der gleichen Temperatur getrocknet. Hierbei entsteht das mikrostrukturierte Kunststoffaerogel;

6. Einbringen des Kunststoffaerogelblockes mit eingeschlossenem Wachsmodell in einem Pyrolyseofen, der ausreichend mit Schutzgas gespült wird. Aufheizen über ca. 3 Stunden auf 1050 °C und ca. 4 bis 24 Stunden halten bei dieser Temperatur. Die Form wird dabei so gestellt, dass das Wachs auslaufen kann.

b) Feingussformschalen:

1. Identisch zu a) 1;

2. Identisch zu Schritt a) 4. Hier kann die Gelierung gestoppt werden, um eine hochviskose Flüssigkeit zu behalten;

3. Eintauchen des Wachsformkörpers in die teilgelierte Ausgangslösung und

4. Endgelierung und Trocknung in einem Luftumwälzer bei ca. 40 °C;

5. werden die Schritte 3. und 4. wiederholt (ohne vollständige Trocknung) lassen sich verschieden dicke Schichten aufbringen, denen die endgültige Trocknung und Überführung in ein Kunststoffaerogel im Luftumwälzer folgt;

6. Identisch zu a) 6.

By way of example, the respective process steps for the production of plastic airgel forms are characterized as follows:

a) Block-mold process:

1. Preparation of the starting solution (resorcinol, formaldehyde, water and basic catalyst);

2. Storage of the wax model in a PTFE or glass mold;

3. filling of the container in 2. with the starting solution (since the specific weight of the wax models is generally lower than that of the solution, the shape must be weighted accordingly (best on the risers and edible);

4. Gelation in the water bath thermostat (in this case the mold should be tightly closed, so that the solution does not change its composition) or in an air circulator in the temperature range of 20 to 50 ° C;

5. After gelation, the still wet gel is dried in the closed mold at the same temperature. This results in the microstructured plastic airgel;

6. introducing the Kunststoffaerogelblockes with enclosed wax model in a pyrolysis furnace, which is sufficiently purged with inert gas. Heat up to 1050 ° C for about 3 hours and keep at this temperature for about 4 to 24 hours. The shape is set so that the wax can leak.

b) investment casting shells:

1. Identical to a) 1;

2. Identical to step a) 4. Here the gelation can be stopped to keep a highly viscous liquid;

3. Immerse the wax molding in the partially gelled starting solution and

4. Final gelling and drying in an air circulator at approx. 40 ° C;

5. If steps 3. and 4. are repeated (without complete drying) layers of different thicknesses can be applied, followed by final drying and transfer to a plastic airgel in the air circulator;

6. Identical to a) 6.

Embodiment:

A solution of 110 g of resorcinol (Merck), 162 g of 37% formaldehyde solution (Merck), 0.075 g of Na ₂ CO ₃ and 750 ml of water was mechanically stirred at room temperature.

A glass container in which a wax model (weighted with steel plates) of the Shaped body was filled with the solution until the model completely was covered. The container was sealed. Gelled within two hours the solution in an air circulator (Heraeus) at 40 ° C. It was a color change the clear solution after ocher yellow / light brown observed. The drying of the gel was obtained in the air circulator over 24 hours. Subsequently At a temperature of 60 ° C, the wax was melted out.

In a further step, the Kunststöffaerogel was in a cold muffle furnace brought in. The oven was heated slowly (3 hours) to 1050 ° C, whereby continuous nitrogen (argon or another inert gas is possible analogously) was blown to avoid oxidation. The temperature of 1050 ° C was maintained for 24 hours.

It was then cooled under steady gas flow and the Kohlestoffaerogelform taken.

Claims (10)

1. A molding material for the precision casting and dead-mold casting of metals or metal alloys comprising highly porous, open-pore plastic and/or carbon aerogels obtainable by sol-gel polymerization of organic plastic materials, optionally followed by the partial or complete pyrolysis of the obtained plastic aerogel.
2. The molding material according to claim 1, containing inorganic or organic filler materials.
3. The molding material according to claim 2, characterized in that said inorganic filler materials are selected from alumina, titanium dioxide and/or quartz, especially in an amount of from 5 to 30% by volume.
4. The molding material according to claim 2, characterized in that said filler materials are selected from thermoplastic or thermosetting plastic particles, especially polystyrene.
5. The molding material according to claim 2, characterized in that said filler materials comprise organic, inorganic carbon and/or SiC fibers.
6. The molding material according to any of claims 1 to 5, comprising a resorcinol/formaldehyde sol-gel and a basic polymerization catalyst, especially ammonium hydroxide and/or sodium carbonate.
7. A process for the preparation of casting molds for the precision casting and dead-mold casting of metals or metal alloys using highly porous, open-pore plastic and/or carbon aerogels, wherein:

   a) a wax mold is wetted with a plastic sol of appropriate composition and a suitable catalyst;

   b) the sol is transferred to a gel at a temperature below the flow temperature of the wax;

   b') optionally one or more layers of the sol are applied, which are each transferred to the gel form partially or completely;

   c) the gel is dried at a temperature below the flow point of the wax; and

   d) the wax is melted or burned from the solidified gel at a temperature above the flow temperature of the wax.
8. A process for the preparation of casting molds for the precision casting and dead-mold casting of metals or metal alloys using highly porous, open-pore plastic and/or carbon aerogels, wherein:

   a) a wax molded part is inserted into a container;

   b) the container is filled partially or completely with a plastic sol;

   c) the sol is transferred to the gel form at a temperature below the flow temperature of the wax;

   d) the gel is dried at a temperature below the flow temperature of the wax; and

   e) the wax is melted or burned from the solidified gel at a temperature above the flow temperature of the wax.
9. The process according to claim 7 or 8, characterized in that said drying of the gel is performed at a temperature within a range of from 20 to 50°C in the course of less than 24 hours.
10. The process according to any of claims 7 to 8, characterized in that the pyrolysis of the solidified gel is performed at a temperature of at least 600 °C, especially at least 1000 °C, within 4 to 24 hours.