Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
  • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
  • B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
  • B22C1/183—Sols, colloids or hydroxide gels

Definitions

• the invention relates to a method for producing a filler-containing airgel with a particularly short gelation and drying time.
• Casting in ceramic molds and molds from bonded sands is a standard casting technique for producing precision parts from a wide variety of alloys, especially aluminum, magnesium, titanium or gray cast iron alloys.
• Using modern casting processes it is possible to cast according to the shape and shape (J. Sprunk, W. Blank, W. Grossmann, E. Hauschild, H. Rieksmeier, HG Rosseinbruch; investment casting for all industrial sectors, 2nd edition, headquarters for casting use, Düsseldorf 1987; KA Krekeler, investment casting, in: Handbuch der Fabrication Technology Vol. 1, publisher: G. Speer, Hanser Verlag, Kunststoff 1981; W. Tilch, E. Flemming, Molding Materials and Molding Processes, German publishing house for basic material industry, Leipzig / Stuttgart 1993).
• the state of the core production is characterized by a large number of core molding processes, including various types of binder systems and equipment. Foundries that have a large range of models often have to process several processes side by side apply.
• the focus in the development of new core binders or core molding materials is on improving the specific strength of molding materials while reducing the proportion of binder in order to conserve economic and ecological resources.
• Thin-walled castings which can warp or bend during the gutting test, as well as a high sand-metal ratio, represent a problem that must be solved.
• the emptying behavior of organically bound cores in aluminum casting depends on their mechanical strength. As the strength of the cores increased, the emptying behavior deteriorated. The usual way to achieve good core removal results in lower binder proportions. The cores are then easier to remove. At the same time, however, the stability of the core deteriorates, so that problems such as core breakage or core distortion can occur even before casting.
• the grains of sand can only be sufficiently covered by the binder up to a certain minimum limit, so that complete hardening is difficult.
• Cores are now manufactured on so-called core shooters, i.e. a flowable mass of sand and binder is shot into the core mold under high pressure and then hardened by adding a hardener (the hardening can be carried out by adding a chemical or by shooting into a preheated mold or a microwave).
• the production times for cores today are in the seconds range when it comes to mass products, otherwise in the minutes range.
• plastic and carbon aerogels 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 (RW Pekala, CT. Alviso, FM Kong, SS Hulsey; 3. Non-Cryst. Solids 145 (1992) 90; RW Pekala, CT. Alviso, Mat Res. Soc. Symp. Proc. 270 (1992) 3; R. Petricevic, G. Reichenauer, V. Bock, A. Emmeriing, 3. Fricke; 3. on-Cryst. Solids (1998)). They can be varied within wide limits by the starting materials, their mixture and the manufacturing process.
• the aforementioned object is achieved by a method for producing an airgel containing filler, characterized in that it comprises the following steps: a. Aging of a sol over the course of 1 to 14 days, b. Mixing 1 to 6 parts by weight of the aged sol with 25 parts by weight of the filler, which has a temperature in the range of 40 to 80 ° C, and c. Gelation and drying of the mixture.
• the aging of the sol at room temperature continues until the color of the sol changes from clear to white to white-pink or even brown. Aging of the sol can generally be viewed as pre-gelling of the sol.
• the upper limit of the temperature range of the fillers is 80 ° C, since from this temperature the usual solvent water evaporates too quickly.
• the gelation and drying of the mixture covers the period of time until a dimensionally stable gel filler body has formed. This can then be removed from its shape and, upon further drying, transferred to an airgel containing filler.
• Today's foundry has to meet many requirements. This includes the type and quantity of castings to be manufactured, the level of quality required for the component, and energy-economic and ecological requirements. Compared to other shaping processes, casting has the greatest design freedom.
• a castable mold must be produced at the start of the casting production. In its dimensions and shape, it is a negative form of the later component. Cavities in the casting are created by cores.
• the mold and core production uses various processes. A basic distinction is made between the permanent forms with a service life of several hundred castings and the lost forms, which are only to be used once.
• the quality of a shape is influenced by various parameters. In addition to the full contour rendering, it must be especially in the The moment of the casting must be stable in shape and contour, ie it must not warp or expand. It must meet casting technology requirements such as gas permeability and thermal stability, and the molding material used should be easy to recover after the casting has been completed. These properties must be coordinated in the interaction between the quality of the molding material flowing into the molding process, the processing technology and the production task in such a way that the mold can withstand the mechanical and thermal stresses when manipulating the mold, during casting and solidification. This so-called processing technology behavior of the molding materials generally combines material, technological and equipment-related influencing factors, which are each effective in a specific technological area.
• the forms for single use include the sand forms. They have natural or synthetic sands with grain diameters of 0.02 to 2 mm as the raw material for the mold and form the framework of the mold. There are different types of sands. Besides that for cost reasons Mainly used quartz sand, which can be used for almost all binder systems with certain restrictions, corundum, olivine, chromite and synthetic sands are used. The main requirements for sands are generally high thermal stability, chemically inert behavior against the melt, low thermal expansion, good granulatometric properties and sufficient resistance to temperature changes. In combination with the binder used, the grain pile is responsible for the behavior of the molding material. The gas permeability and porosity as well as strengths and chemical properties depend exclusively on the amount of binder used and the grain size of the sand.
• the binder is applied to the sand grain surface.
• the surface properties of the sands and granules used are particularly important for the adhesive forces between the binder casing and the grain surface.
• a more splintered shape has higher adhesive forces than a round sand shape.
• the binder envelops the grain of sand.
• binder bridges are created which, in connection with the contact surface formed, lead to a complete binding of the grains.
• the wettability of the sand grain surfaces and the surface tension have a decisive influence on the hardening reaction. A good wettability of the surfaces requires a low surface tension of the binder.
• synthetic resin binders make up the most important proportion of organic binders for the production of aerogels.
• the synthetic resin binders include the condensation resins, the most important representatives of which are phenolic, urea and furan resins. They are mostly used in liquid form. Solidification takes place via polycondensation. Here, the same or different types of monomers are converted into crosslinked macromolecules with the elimination of smaller molecules, such as water. During the polycondensation, the by-products formed have to be removed for complete conversion. In contrast to polymerization, polycondensation is slower and gradual, ie it can be interrupted at any point.
• a certain degree of crosslinking can be stopped at any point, for example by changing the thermal boundary conditions, and can then be continued, for example by the action of strong acids, until the formation of highly crosslinked macromolecules.
• Polycondensation is an equilibrium reaction. If the cleavage product is removed, the reaction is shifted in favor of the polymer. In practice it can this will lead to negative effects. Higher water contents in the molding material mixture (condensation water, water content of the hardener) and high air humidity make it difficult to release water from the molded part and delay the hardening process. As soon as the equilibrium reaction is disturbed by such a weakening, this can lead to differences in hardening in the core cross section. Core breakage, gas porosity and other casting defects are the logical consequence.
• Phenolic resin binders are phenols or cresols that form linear or spatially cross-linked macromolecules with formaldehyde with a continuous increase in temperature. As with many other chemical reactions, the course of the reaction can be interrupted at any time by lowering the temperature. Phenol and formaldehyde are reacted with each other in a ratio of 1.2: 1. The reactions that take place are the addition of formaldehyde (HCHO) to phenol (C 6 H 5 OH) and the condensation of the addition product with another phenol molecule with elimination of water. The resulting product (novolaks) consists of predominantly linear macromolecules. The subsequent curing by adding hexamethylenetetraamine leads to a release of formaldehyde.
• reaction taking place is characterized by the addition of formaldehyde at several points in the phenol molecule and the chain growth by further reaction with phenol with elimination of water and formation of methylene bridges until spatial crosslinking.
• Resole resins which are cured by thermal and chemical treatment, form the basis for the cold-curing molding materials and molding processes.
• Urea resins are formed by a reaction of formaldehyde and urea, initially in an alkaline medium and later in an acid medium. Monomethylolurea is initially formed and, in the further course of the condensation, linearly crosslinked intermediates in various ways, some of which are still liquid and soluble. Further hardening takes place by adding an excess of formaldehyde, the hydrogen atoms bonded to the nitrogen atoms of the amine reacting with formaldehyde, thus forming spatially cross-linked macromolecules. Urea resins are mostly used as binders in combination with phenolic or furan resins.
• Furan resins are based on furfural (furan aldehyde). Furfural can be caused to form a resin by a chemical reaction with phenol or urea via condensation reactions. Another possibility for resin formation is the hydrogenation and conversion of furfurol into furfuryl alcohol. It arises Furan polymer with cross-linked macromolecules. Depending on the process, furan resins are cured in different ways. In the case of thermosetting resins, a catalyst is added with simultaneous supply of heat, in the case of cold-curing resins, an acid is sufficient. In both cases, the curing process is characterized by condensation and polymerization reactions, in which fission products are formed.
• furan resin binders are mostly a combination of different resins, for example furan-urea-formaldehyde.
• P-Toluenesulfonic acid and, with a weaker effect, phosphoric acid are used as catalysts.
• Urethane resins for short-term curing are cured with a catalyst additive, which is a low-volatility pyridine derivative (pep set method).
• a catalyst additive which is a low-volatility pyridine derivative (pep set method).
• pep set method a low-volatility pyridine derivative
• the hardening starts suddenly after a few minutes.
• the solidification can be controlled via the addition of catalyst.
• Resorcinol and formaldehyde or a solution of these components are particularly suitable as sol. This is particularly advantageous since plastic aerogels based on resorcinol / formaldehyde with a suitable composition and a suitable content of basic catalyst can be converted into a microstructured plastic airgel at temperatures between 20 and 50 ° C. without supercritical drying.
• the gelling reaction can be adjusted so that, for example, a highly viscous liquid is initially created, which becomes more solid with time / temperature.
• Foundry sands are advantageous fillers because they have high temperature stability and are readily available for this application.
• the gelling and drying time is set in the course of up to one hour. Due to the very short gelling and drying time compared to the prior art, the process described here is accessible for production processes with high throughput. The short gelling and drying time was surprisingly achieved primarily by using preheated filler and aged sol. Furthermore, the process is particularly advantageous if the airgel obtained by the above process is pyrolyzed. This transforms the plastic airgel into a carbon airgel, which is extremely temperature-resistant.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Dispersion Chemistry (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Mold Materials And Core Materials (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Processes Of Treating Macromolecular Substances (AREA)

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• 2003
  • 2003-11-11 DE DE10352574A patent/DE10352574A1/de not_active Withdrawn
• 2004
  • 2004-11-03 DE DE502004003801T patent/DE502004003801D1/de active Active
  • 2004-11-03 WO PCT/EP2004/012401 patent/WO2005046909A1/fr active IP Right Grant
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