The present invention relates to a molding material for the precision casting and dead-mold casting of metals or metal alloys comprising plastic and/or carbon aerogels, and a process for the preparation of such molding materials.
Precision casting in ceramic shell molds is a standard casting technique for preparing precision moldings from a wide variety of alloys. The molds are usually prepared by the lost-wax process, i.e., a wax molding of the part to be cast is wetted with a silica sol, sand-coated in several steps, dried, and then the shell mold is baked wherein the wax is melted and drained or burned in an autoclave. With modern casting processes, it is possible to achieve conformal casting with high fidelity (J. Sprunk, W. Blank, W. Grossmann, E. Hauschild, H. Rieksmeier, H. G. Rosselnbruch; Feinguβ für alle Industriebereiche, 2nd edition, Zentrale für Guβverwendung, Düsseldorf 1987; K. A. Krekeler, Feingieβen, in: Handbuch der Fertigungstechnik Bd. 1, editor: G. Speer, Hanser Verlag, München 1981).
Aerogels are highly porous open-cell oxidic solids which are usually obtained by sol-gel processes from metal alkoxides by polymerization, polycondensation to gels, followed by supercritical drying. For some years, it has also been possible to prepare plastic gels by a sol-gel process and to convert them to a highly porous organic solid by supercritical drying. Pyrolysis of such plastic aerogels under a protective gas or vacuum at temperatures above 1000° C. converted them to carbon aerogels. Like the oxidic aerogels, plastic and carbon aerogels have extremely low effective thermal conductivities (on the order of some mW/K/m), but they are significantly lighter than the oxidic aerogels. The physical and mechanical properties of plastic and carbon aerogels are documented in the literature (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 can be varied within a wide range by appropriately selecting the starting materials, their mixing ratio and the preparation process.
Therefore, it has been the object of the present invention to simplify the prior art processes for the preparation of molding materials for the precision casting and dead-mold casting of metals and metal alloys, especially to reduce the time required for drying in the process.
In a first embodiment, the above object is achieved by a molding material for the precision casting and dead-mold casting of metals or metal alloys comprising highly porous open-cell plastic and/or carbon aerogels, obtainable by the sol-gel polymerization of organic plastic materials, optionally followed by partial or complete pyrolysis of the plastic aerogel obtained.
The molding material according to the invention is particularly suitable for use in lost-wax processes, eliminating the need for application in multiple steps, as with oxidic gels of the prior art.
According to conventional techniques, the molds thus obtained are filled with a melt, and the melt is solidified. In the usual casting techniques, the heat is dissipated through the shell mold or the molding sand. In contrast, since carbon aerogels are quasi-adiabatic, casting and solidification in aerogels means that the heat is dissipated solely through feeders and risers or through especially provided cooling means; conveniently, but not necessarily, the feeders and risers themselves may be used for this purpose. Thus, a completely controlled solidification is possible, and the assembly can be adjusted in accordance with the range of properties required.
The aerogel molds prepared according to the invention are especially suitable for casting aluminum alloys (the casting mold having to be heated virtually not at all, since there is no heat dissipation through the mold itself). This increases economic efficiency because energy costs can be lowered. Magnesium and titanium alloys do not react with carbon either; thus, the carbon aerogel molds are also a good selection as molding materials for these alloys under protective gas or vacuum.
One particular advantage of the molding materials according to the invention is that the sol-gel formation can be completed within a few hours at room temperature, i.e., in particular, at temperatures below the pour point of the wax. Supercritical drying, as with the purely inorganic gels, is not necessary. Nevertheless, it is possible to adjust the cell size in the micrometer range. In addition, when drying is performed in a supercritical range of temperatures, cell sizes in the nanometer range are also possible.
In addition, the molding materials according to the invention may also contain inorganic or organic filler materials. This essentially means stable materials which are inert under solidification conditions. For example, inorganic filler materials may be selected from alumina, titania and/or quartz each of which may be employed in a proportion of from 5 to 30% by volume. Fillers according to the present invention further include fibers, allowing a fiber reinforcement by organic, inorganic, carbon and/or SiC fibers in appropriately the same proportion.
Similarly, it is also possible to employ organic fillers, for example, thermoplastic or thermosetting plastic particles, such as polystyrene, or organic (such as polyacrylonitrile), inorganic (such as SiC) or carbon fibers. However, it has to be taken care that these materials are also removed by melting or burning off in the pyrolysis of the plastic gels. Using such materials, it is possible, however, to control the shrinkage during the pyrolysis.
It is particularly preferred according to the present invention to employ for the molding material resorcinol/formaldehyde-based plastic aerogels which, when having an appropriate composition and an appropriate content of basic catalyst, can be converted to a microstructured plastic aerogel at temperatures of between 20 and 50° C. without supercritical drying. By selecting the composition, the sol-gel polymerization can be adjusted in such a way, for example, that a highly viscous liquid is first formed which can be applied to a wax mold. This can also be done in several working cycles so that the layer thickness can be adapted to the requirements of the applications in casting.
Thus, another embodiment of the present invention is 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-cell plastic and/or carbon aerogels, comprising the steps:
Thus, it is possible simply to place the wax molding in a suitable container, fill it with the starting solution for the plastic aerogels and then perform the process for preparing the aerogel.
In this way, solid, but light-weight quasi-adiabatic molds can be prepared by analogy with the known block mold process (which essentially uses gypsum).
The conversion temperature of the solution to a plastic aerogel must be adapted to the melting point of the wax. After conversion to a plastic aerogel, the wax can be removed by melting, and at the same time, with the exclusion of air, the conversion to a carbon aerogel can be effected. Depending on the composition of the starting solution, the gelling temperature and the density of the porous body formed, casting molds can be prepared both as a plastic and as a carbon aerogel which have a smooth finish on a micrometer scale and provide conformal molding. According to the invention, the preparation of molds up to the stage of the plastic aerogel usually takes from 1 to 3 days, often only up to 24 hours. The duration of pyrolysis is determined by the thickness of the casting shell mold; for example, a wall thickness of 1 cm requires a time of less than 24 hours, often less than 10 hours. As compared to the preparation of typical precision casting shell molds using oxidic sol-gel processes, the preparation times are short and thus economically efficient. In both process steps, shrinkage is always isotropic and may vary between a few percent and 20%. It can be reduced and influenced by selecting the composition of the sol, the drying conditions, the mold material and fillers, and thus is under control.
By way of example, the respective process steps for the preparation of plastic aerogel molds are characterized as follows:
a) Block mold method:
A solution of 110 g of resorcinol (Merck), 162 g of formaldehyde solution (37%, Merck), 0.075 g of Na2CO3 and 750 ml of water was stirred mechanically at room temperature.
A glass container in which a wax model (weighted with steel plates) of the molding was provided was filled with the solution until the model was completely covered. The container was sealed. Within two hours, the solution gelled within a forced air oven (Heraeus) at 40° C. The color of the clear solution was observed to turn ocher yellow/light brown. Drying of the gel was achieved in the forced air oven within 24 hours. Then, the wax was removed by melting at a temperature of 60° C.
In a further step, the plastic aerogel was placed in a cold muffle furnace. The furnace was slowly (3 hours) heated to 1050° C. with a constant flow of nitrogen (argon or another inert gas is also possible) for avoiding oxidation. The temperature of 1050° C. was maintained for 24 hours.
Subsequently, cooling was effected with a constant gas flow, and the carbon aerogel mold was removed.
Number | Date | Country | Kind |
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199 11 847 | Mar 1999 | DE | national |
This is a division of application No. 09/527,809, filed Mar. 17, 2000, now U.S. Pat. No. 6,599,953.
Number | Name | Date | Kind |
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4873218 | Pekala | Oct 1989 | A |
5242647 | Poco | Sep 1993 | A |
6068882 | Ryu | May 2000 | A |
6099965 | Tennent et al. | Aug 2000 | A |
6599953 | Ratke et al. | Jul 2003 | B1 |
Number | Date | Country |
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19721600 | Nov 1998 | DE |
19738466 | Dec 1998 | DE |
Number | Date | Country | |
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20030212152 A1 | Nov 2003 | US |
Number | Date | Country | |
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Parent | 09527809 | Mar 2000 | US |
Child | 10449794 | US |