EP1077097A1 - Use of plastic and/or carbon aerogels as core material - Google Patents

Abstract

Use of highly porous open-pored plastic and/or carbon aerogels obtained by sol-gel polymerization of organic materials, as core material for a casting mold is new.

Description

The invention relates to the use of plastic / carbon aerogels as the core material in molding.

Pouring in ceramic molds and molds from bonded sands are standard casting techniques to make precision parts from various Alloys, in particular of aluminum, magnesium, titanium or Manufacture gray cast iron alloys. The shapes are usually over the lost wax process is established; d. H. a wax body the part to be cast is wetted with a silica sol, in several Sanded steps, dried and then the molded shell fired, the wax is melted in an autoclave or burns. Using modern casting processes, it is possible to conform to the contour and casting close to the final shape (J. Sprunk, W. Blank, W. Grossmann, E. Hauschild, H. Rieksmeier, H.G. Rosselnbruch; Investment casting for all industrial areas, 2nd edition, headquarters for casting use, Düsseldorf 1987; K.A. Krekeler, investment casting, in: manual of manufacturing technology Vol. 1, publisher: G. Speer, Hanser Verlag, Munich 1981).

Cavities within the mold must be stable using a core be preformed. Such kernels are usually because of there prevailing high thermal and mechanical stress plastic-bonded ceramic powders. Disadvantage of Today's core manufacturing process is that removal the cores from the casting are only possible with extremely great effort is (e.g. combustion in an autoclave), the distribution of the sands in the core is inhomogeneous, crack germs exist, which among other things break under can lead to thermal-mechanical stress.

Aerogels are highly porous, open-pore oxidic solids that are found in the Rule about sol-gel processes from metal alkoxides by polymerization, Polycondensation to gels and subsequent supercritical drying be won. For a few years now, too Gelling plastics using sol-gel processes and by supercritical Convert drying into a highly porous organic solid (see for example DE 195 23 382 A1, DE 694 09 161 T2 and US-A-5,086,085). Pyrolysis of such plastic aerogels under protective gas or in a vacuum at temperatures above 1000 ° C this converts to carbon aerogels around. Like the oxidic aerogels, plastic and Carbon aerogels have extremely low effective thermal conductivities (In the order of a few mW / K / m) and are considerably lighter. 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 admired by the raw materials, their mixture and the manufacturing process vary widely.

It is therefore an object of the present invention cores that are quasi-adiabatic whose specific weight can be adjusted which have extremely smooth surfaces (roughness in the range of one Micrometers), are non-reactive with Al, Mg and Ti alloys and above everything with a simple high pressure jet or suitable, have the airgel wetting and decomposing fluids removed, to provide.

The above object is achieved in a first embodiment through the use of highly porous, open-pore plastic and / or Carbon aerogels, obtainable by sol-gel polymerization of organic plastic materials as the core material for molding.

Cores of any shape can be produced because the starting solution in a corresponding negative form is inserted and gelled (as material PTFE is particularly suitable for these shapes). It can also through professional adjustment of the composition and gelling conditions the transition from sol to solid gel can be delayed so that A highly viscous, flowable mass is created, which is introduced into every shape can be. It is also possible to use ceramic powder and fiber add to the sol if this is due to the expected mechanical stress appears necessary.

The aerogels produced according to the invention are particularly suitable as cores for the formation of cavities when casting aluminum alloys (whereby the mold is practically not heated must, since there is no heat dissipation by them). This increases the economy because energy costs can be reduced. Magnesium and titanium alloys also do not react with carbon, so these carbon aerogels are also suitable for these alloys Offer protective gas or vacuum as the core.

A particular advantage of aerogels is that the sol-gel formation can be completed at room temperature. A supercritical Drying, as with the purely inorganic gels, is not necessary. Nevertheless, it is possible to determine the pore size in the micrometer range adjust. When drying in the supercritical temperature range pore sizes in the nanometer range are also possible.

The aerogels can also be inorganic or organic Contain filler materials, especially fiber materials. Below become essentially stable materials which are inert under solidification conditions Roger that. Inorganic filler materials of any grain size are selected, for example, from aluminum oxide, titanium dioxide, Zirconia and quartz and their mixtures, each in an amount from 5 to 30% by volume, in particular up to 60% by volume can.

In the same way, however, it is also possible to use organic fillers, for example thermoplastic or thermosetting plastic particles, for example to use polystyrene. However, it should be noted here that these materials also melted out during the pyrolysis of the plastic gels or be burned. With the help of such materials, however the shrinkage can be checked during pyrolysis.

Are particularly preferred in the sense of the present invention Plastic aerogels based on resorcinol / formaldehyde are used with a suitable composition and a suitable content of baischen Catalyst at temperatures between 20 and 50 ° C without supercritical Dried transferred to a microstructured plastic airgel can be. By choosing the composition is the gelling reaction adjustable so that, for example, initially a highly viscous Liquid is formed which becomes firmer over time / temperature.

a) eine Negativform eines Kerns mit einem Kunststoffsol geelgneter Zusammensetzung und einem geeigneten Katalysator füllt,

b) das Sol in ein Gel überführt,

c) das erstarrte Gel in an sich bekannter Weise in üblichen Wachsmodellen des Fein und Formgusses einsetzt und

d) das Gel entfernt.

A further embodiment of the present invention thus consists in the use of highly porous, open-pore plastic and / or carbon aerogels, where

a) fills a negative form of a core with a plastic sol of a suitable composition and a suitable catalyst,

b) the sol is converted into a gel,

c) the solidified gel is used in a manner known per se in customary wax models of fine and molded casting and

d) the gel is removed.

The cores used according to the invention are particularly suitable for Use in lost wax processes.

The desired shapes are made using the usual techniques with the cores and the melt filled and the melt solidified. With the usual Casting techniques, the heat is dissipated via the molded shell or the molding sand.

The cores obtained in this way are produced using conventional techniques Wax models used. In contrast to those after today Core materials customary in the prior art do not absorb heat through the airgel cores because of their effective thermal conductivity is typically only a few mW / km. Thermal loads and thus thermal stresses do not occur in the core body on. The airgel cores can be removed by pyrolysis or high pressure water jet, but also through wetting fluids such as silicone oil, which Fluidize airgel, remove easily from the casting.

Depending on the composition of the starting solution, the gelation temperature and the density of the resulting porous body produce cores for molds, both as plastic and also as carbon airgel, which is superficial on a micrometer scale are smooth and show sharp contours. According to the invention Production of molds up to the plastic airgel a maximum of 24 hours. Pyrolysis in the absence of air takes place in short times (that of the thickness of the mold is determined; with a 1 cm core the time is for example 24 hours). The shrinkage takes place in the two process steps always isotropic and is only a few percent (the Shrinkage can be reduced by the appropriate choice of composition of the sol, as well as the drying conditions) and is therefore manageable.

Design example:

1. Preparation of the airgel solution:
22 g resorcinol + 20 ml formaldehyde solution (37%) + 0.013 g Na ₂ CO ₃ + 82 ml H ₂ O and stirring at room temperature

2. Mixing the airgel solution with molding sand:
Example: 10 cm ³ Alodur® sand with a grain size of 0.0633 µm to 0.125 µm absorbs 45 ml of solution. The sand is added to the airgel solution while stirring.

3. Filling the core mold:
Filling the core mold with vibration and knock compression

4. Drying:
Drying the closed mold in a drying cabinet at 40 ° C for 24 hours

5. Final shaping

6. Final drying at room temperature

7. Installation of the airgel-bound sand core in a standard casting mold

Claims (8)

Use of highly porous, open-pore plastic and / or Carbon aerogels, obtainable by sol-gel polymerization of organic Plastic materials as the core material for molding. Use according to claim 1, containing inorganic or organic Filler materials, especially in powder or fiber form. Use according to claim 2, characterized in that the inorganic filler materials are selected from alumina, Titanium dioxide, zinc oxide and quartz and mixtures thereof, in particular in an amount of 5 to 60% by volume. Use according to claim 2, characterized in that the Fillers are selected from thermoplastic or thermosetting Plastic particles, especially polystyrene. Use according to any one of claims 1 to 4, comprising a Resorcinol / formaldehyde sol gel and a basic polymerization catalyst, in particular ammonium hydroxide and / or sodium carbonate. Use according to one of claims 1 to 5, wherein one a) fills a negative form of a core with a plastic sol of a suitable composition and a suitable catalyst, b) the sol is converted into a gel, c) the solidified gel is used in a manner known per se in customary wax models of fine and molded casting and d) the gel is removed. Use according to one of claims 1 to 6, wherein the Gel removed by high pressure water jet. Use according to one of claims 1 to 6, wherein the Gel by pyrolysis at a temperature of at least 1000 ° C in Removed over 24 hours.