EP1820582A1 - Aerogel containing core for light alloy and/or lost wax casting - Google Patents

Abstract

A mechanically and thermally stable core for the light metal- and/or investment casting, comprises hydrophilic aerogel granulate, sand and binding agent. An independent claim is included for the production of a core comprising: mixing the aerogel granulates with sand and binding agent; bringing the mixture into a negative form of the core; condensing the mixture in the negative form; drying the condensed mixture; and drafting the core in the negative form.

Description

The invention relates to a water-soluble core which can be used in the field of light metal casting and / or investment casting.

Castings of different metals or metallic alloys are produced by different casting methods. Equal to all is the use of permanent models (wood, ceramic, plastic) or lost models of sand, or polystyrene. If cavities are to be realized in a casting, a lost core is mainly made of sand. Because of the high thermal and mechanical stress prevailing in the foundry process, they generally consist of chemically bonded molding materials, such as plastic-bonded ceramic powders or synthetic resin-bonded sands. Here, the molding materials consist of sands, which are connected by binders in the desired shape of the core with the aim after the casting production to destroy the binder and thus to remove the core. Disadvantage of the known processes for core production is that usually the removal of the cores from the casting is possible only with great effort, the distribution of the sands in the core is inhomogeneous or cracking germs exist, which can lead to fracture under thermal-mechanical stress among others , Binders for core sands used today mean that cores are difficult to remove from the workpieces (because they are thermally very stable). Especially in aluminum casting, low casting temperatures require only lower temperatures for the thermal Decomposition of the binder. If the binder bridges are insufficiently destroyed, this will cause the cores to have higher strength even after casting, and be difficult to remove by mechanical vibration or high pressure water jets. High-pressure water jets can not always lead to the complete release of the core to damage the workpiece, or in geometrically complicated cores with undercuts. This mechanical removal of the cores is especially critical in light metal casting, since the desired filigree cast structures can be easily destroyed here. Partially polluting (organic) substances are used in the common resin bound cores.

The molding material is a suitable sand (molding sand), which is mixed with a chemical binder or a molding material binder, which is then hardened, for example, again by a liquid or solid catalyst or hardener or by additional heat. As sand predominantly quartz sand is used. Chromite, zircon and olivine sand are also used for special applications. Likewise, mold bases based on fireclay, magnesite, silimanite and corundum are used.

Binders for the molding sands may be inorganic or organic in nature, the inorganic binders being divided into natural and synthetic inorganic binders. Natural inorganic binders include clays such as montmorillonite, glauconite, kaolinite, illite or attapulgite. Synthetic inorganic binders include, among others, water glass, cement and gypsum. Organic binders include synthetic resins such as the phenolic, urea, Furan resins and ethyl silicate. Oils, carbohydrate binders, water-soluble liquid binders based on sulfite waste liquors, molasses, dextrose processes, alkanolamines and pitch binders are still used ( KE Höner "foundry", Ullmann's Encyclopedia of Industrial Chemistry, pp. 271-287, vol. 12, 4th edition, Verlag Chemie Weinheim, 1976 ).

The production methods of the sand-bonded casting molds and cores are sufficiently documented in the literature ( C. Henry, R. Showman, G. Wandtke, Foundry Practice No. 12, 1999 ; P. Carey, M. Swartzlander, Sand Binder Systems, Part II - Resin / Sand Interactions, Foundry Management and Technology 97, 1995 ; W. Tilch, E. Flemming Forming and Forming, German publisher for basic industry Leipzig / Stuttgart 1993 ; Foundry Yearbook, Giesserei-Verlag GmbH Düsseldorf, Volume 1, 2000 ; P. Carey, J. Archibald, Sand Binder Systems, Part X - The Phenolic Urethane Amine Cold Box System, Foundry Management and Technology 98, 1995 ; GS Cole, RM Nowicki, Sand Cores and Their Removal From Aluminum Semipermanent Molded Castings, Trans. Amer. Foundrym. Soc. 87, 1979 ; I. Bindernagel, molding materials and molding processes in foundry technology, VDG-Tschenbuch 12, foundry - publishing house, Dusseldorf 1983 ; D. Boenisch, J. Nitsche, W. Patterson, Properties of Resin-Bonded Shapes, Aluminum 46, (3), 1970 ).

1. 1. Die keramischen Formen sind regelmäßig aus einem Stück, selten mehrfach geteilt;
2. 2. das Wachsmodell wird im Heißdampfautoklaven entfernt;
3. 3. die keramische Formschale wird bei 800 bis 950 °C gebrannt.

Especially for aluminum investment casting there is a need for mechanically and thermally stable cores that can be easily removed from the casting. The method of investment casting of aluminum alloys involves special problems that must be considered in the development of a core material:

1. 1. The ceramic forms are regularly one piece, rarely divided several times;
2. 2. the wax model is removed in a hot steam autoclave;
3. 3. The ceramic shell mold is fired at 800 to 950 ° C.

Subject of the EP 1 077 097 B1 describes the use of highly porous, open-pored plastic / carbon aerogels obtainable by sol-gel polymerization as a core material in casting.

DE102 16 464 B4 describes core materials for the fine and casting of metals and metal alloys, a process for their production and their use as a core in fine and cast molding. In particular, new binder components for the sands are used here.

1. 1. thermisch stabil sind, das heißt Temperaturen bis ca. 900°C aushalten,
2. 2. mechanisch stabil sind, das heißt sich bei den üblichen Gießereitemperaturen nicht verändern,
3. 3. chemisch inert gegenüber den eingesetzten Metallen oder Metalllegierungen wie beispielsweise Aluminium, Magnesium und Titan sind,
4. 4. umweltfreundliche Materialen während des gesamten Herstellprozesses verwenden und
5. 5. aufgrund ihrer Eigenschaften sich möglichst rückstandsfrei aus dem Gussteil entfernen lassen.

Object of the present invention is to produce cores for fine and / or cast molding, the

1. 1. are thermally stable, that is temperatures up to about 900 ° C endure,
2. 2. are mechanically stable, that is do not change at the usual Gießereitemperaturen
3. 3. are chemically inert to the metals or metal alloys used, such as aluminum, magnesium and titanium,
4. 4. use environmentally friendly materials throughout the manufacturing process and
5. 5. due to their properties can be removed as far as possible without residue from the casting.

The present object is achieved in a first embodiment by a mechanically and thermally stable core for light metal and / or precision casting, the core containing hydrophilic airgel granules, sand and binder. According to the invention, the aerogels are used as a partial replacement of the sand.

The invention is based in particular on a combination of classic foundry sands and hydrophilic airgel granules. The inorganic mixture of sand and airgel granules is bound, for example, with different types of waterglass. After drying, the core is poured over with boiling water. After a period of seconds to minutes, the originally solid composite decays completely. The core remnants can be removed from the casting without further mechanical effort. It should be noted, however, that a higher water glass binder content results in an exponential increase in disintegration time.

Aerogels according to the invention include colloidal substances which are gelled and dried. They have a lower density and high, open porosity. They consist of only about one to fifteen percent of a solid, while the rest of their volume is filled by the surrounding gas or vacuum, that is, they have a high surface area (up to 1000m ² / g). Inorganic aerogels are usually inherently hydrophilic. Aerogels are considered to be one of the lightest materials and best thermal insulators.

According to the invention, a hydrophilic airgel in the form of a granulate is to be used as a further molding material. Airgel granules are obtained in particular by grinding airgel monoliths won. Hydrophilic means water-loving, ie the airgel granules used show a pronounced interaction with polar solvents such as water. Thus, the hydrophilic airgel granules used have a wetting angle with water less than 10 °.

Most preferably, the hydrophilic airgel granules comprise silica aerogels since they are not wetted or chemically attacked by liquid metals. Silica aerogels are molten metals to chemically inert. Its sintering point is around 1050 ° C. In addition, they are non-flammable and non-toxic.

There are various types of aerogels, with the silicate-based (silica airgel) being commercially available. Plastic or carbon based aerogels are important for special applications. Also, metal oxide based aerogels are known. According to the invention, these aerogels can also be used if the surface is hydrophilic in the meaning of the invention by appropriate treatment.

Advantageously, the airgel granules should have a particle size in the order of magnitude of the sand, since both are used as mold bases and optimum mixing of equal particles is easier to carry out.

• Dorsten 0,84 mm (Sorte D020), 0,56 mm (D030), 0,39 mm (D040), 0,13 mm (DO110) ;
• Frechen 0,32 mm (Sorte F31), 0,23 mm (F32), 0,22 mm (F33), 0,20 mm (F34), 0,18 mm (F35), 0,16 mm (F36)
• Gambach 0,37 mm (Sorte G30), 0,29 mm (G31), 0,23 mm (G32), 0,21 mm (G33), 0,19 mm (G34),
• Haltern 0,36 mm (Sorte H31), 0,32 mm (H32), 0,26 mm (H33), 0,21 mm (H34) und 0,19 mm (H35)

The sands which may be used include, inter alia, commercially available quartz new sands of the following origin in Germany with the following average grain size in mm:

• Dorsten 0.84 mm (grade D020), 0.56 mm (D030), 0.39 mm (D040), 0.13 mm (DO110);
• Frechen 0.32 mm (grade F31), 0.23 mm (F32), 0.22 mm (F33), 0.20 mm (F34), 0.18 mm (F35), 0.16 mm (F36)
• Gambach 0.37 mm (grade G30), 0.29 mm (G31), 0.23 mm (G32), 0.21 mm (G33), 0.19 mm (G34),
• Holders 0.36 mm (grade H31), 0.32 mm (H32), 0.26 mm (H33), 0.21 mm (H34) and 0.19 mm (H35)

As an alternative to the quartz sands used, corundum sands of similar size (0.1 to 0.9 mm) can also be used.

The sand content may be 83 to 95 wt .-%, the binder content and the Airogelgranulatanteil adds up to 100 wt .-%. In particular, the binder content is 2 to 10 wt .-%. The proportion of airgel granules is preferably 2 to 6 wt .-%.

The quartz sands shown above are new sands, in fact these are only added to the "old sands" in foundries to a limited extent. Used sand is the sand that accumulates when the castings are emptied out of the molds, which, after appropriate cooling and reconditioning, is returned to the molding shop. The reprocessing has two tasks to accomplish: cleaning the quartz grain from adhering binders and removing dusty constituents.

In this process, any remaining agglomerates are mechanically comminuted and thus the binder coats partially removed from the quartz grains. In this process, the originally rather rounded surface of the grain of sand undergoes a change. From around she is too fragmented. This grain shape is important for the process of forming, thus ensuring that only one comparatively low binder content is needed.

In one embodiment, waterglass and / or airgel precursor are used as binders for binding airgel granules and sand. The use of viscous aqueous water glass has the advantage that it is available as a mass product and this is suitable as a molding material binder. It is also possible to use precursors for airgel production around a core by means of the process DE 102 16 464 B4 manufacture. Here, in particular silica airgel precursors such as tetraethoxysilane (TEOS) or TEOS formamide mixtures with solvents such as water and / or ethanol should be used as a binder, since the resulting silica aerogels can be easily destroyed by well-wetting fluids.

In another embodiment, the blend contains a sand content of 83 to 95 weight percent, using here 1 to 20 weight percent virgin sand and 80 to 99 weight percent reclaim (circuit molding, i.e., purified recycled sand).

1. 1. Mischung des Aerogelgranulats mit Sand und Bindemittel,
2. 2. Einbringung der Mischung in eine Negativform des Kerns,
3. 3. Verdichtung der eingebrachten Mischung in der Negativform,
4. 4. Trocknung der verdichteten Mischung und
5. 5. Kernentnahme aus der Negativform.

According to the invention, it has proven useful to carry out the following steps:

1. 1. mixture of airgel granules with sand and binder,
2. 2. introduction of the mixture into a negative mold of the core,
3. 3. compaction of the introduced mixture in the negative mold,
4. 4. Drying of the compressed mixture and
5. 5. Core removal from the negative mold.

The compaction is done for example by core shooting, shaking, tapping and stomping. For drying have become Temperatures from 20 ° C to 80 ° C proved to be particularly suitable. The duration of the drying is preferably a few seconds to minutes.

The inorganic cores according to the invention can be used in the foundry industry, in particular in light metal and / or investment casting, where cores which are easy to dislodge, especially in filigree cast structures, are required.

After the casting process, the core can be removed by a fluid which wets it. This is particularly advantageous since here the core decomposes without residue through the fluid which wets it.

In particular, these are well-wetting fluids such as water. Wettability refers to the ability of liquids to spread on a surface; the better the wettability, the smaller is the contact angle that occurs during wetting. Surfaces are also referred to as (incompletely) wettable when the contact angle with the surface is up to 90 °. The higher the temperature of the wetting fluid, the better the cores can be removed. Particular preference is therefore given to fluids having a temperature of from 30 to 100.degree. Here is exploited that silica aerogels can easily be destroyed by well wetting liquids (for example, boiling water).

In another embodiment, the core may be destroyed by alcoholic fluids or short chain alcohols having a chain length of up to six carbon atoms. To avoid the risk of fire, non-flammable alcohol mixtures should be used.

The object according to the invention is achieved, for example, by mixing quartz sands or corundum sands with hydrophilic airgel granules (for example Airglass in Sweden) and bound with water glass in different proportions. After drying, the sample can be used as a core for light metal and / or investment casting. After cooling the sample, the core can be removed by a hot fluid (eg, boiling water). After a period of seconds or minutes, the nucleus decays completely. The core remnants can be removed from the casting without further mechanical effort.

The embodiments shown here aim at a complete decay of the core with the addition of boiling water, a thermal stability (> 1000 ° C) and the lowest possible binder content.

The flexural strengths of the obtained airgel granulate quartz sand-waterglass-bonded molding sands are between 1 and 3 MPa, at a density of about 1.5 g / cm ³ , ie the density is determined by the quartz sand and a specific surface in the range of 4 to 8, in particular 6m ² / g.

EXAMPLES Example 1:

• 3,5 Gew.% hydrophiles Aerogelgranulat ca. Korngröße Mittelwert 220 µm (Firma Airglass Schweden)
• 87,7 Gew.% Quarzsand (Quarzwerke Frechen Deutschland), Korngröße 0,16-0,32 mm
• 8,8% Gew.% Carsil^™ (Wasserglasbindemittel, Firma Foseco Deutschland).

An airgel granulate quartz sand mixture prepared with the following composition:

• 3.5% by weight of hydrophilic airgel granules approx. Grain size mean value 220 μm (Airglass Sweden)
• 87.7% by weight of quartz sand (Quarzwerke Frechen Germany), grain size 0.16-0.32 mm
• 8.8% by weight of Carsil ^™ (water glass binder, Foseco Germany).

Airgel granules and quartz sand were first mixed. The complete mixture was then treated with the waterglass binder. Once sufficient mixing had occurred, the mixture was placed in a core negative and compacted by pounding. The final drying took place in a drying oven at 60 to 70 ° C for about one day. After complete drying, the core could be inserted into the cavity and cast. After cooling the casting, the core was doused with boiling water. After a period of a few seconds, the sample completely disintegrated in the water. The core remnants could be removed from the casting without further mechanical effort.

Example 2:

• 3,5 Gew.-% hydrophiles Aerogelgranulat ca. Korngröße Mittelwert 0,22 mm (Firma Airglass Schweden)
• 4 % Gew.% Quarzsand (Quarzwerke Frechen Deutschland), Korngröße 0,16 bis 0,32 mm
• 83,7 Gew.% Regenerat (gereinigter wiederverwendeter Sand)
• 8,8% Gew.% Carsil (Wasserglasbinder, Firma Foseco).

Es wurden die gleichen Beobachtungen wie in Beispiel 1 gemacht.Analogously to Example 1, a mixture with a modified sand content was used:

• 3.5% by weight of hydrophilic airgel granules approx. Grain size average 0.22 mm (Airglass Sweden)
• 4% wt.% Quartz sand (Quarzwerke Frechen Germany), grain size 0.16 to 0.32 mm
• 83.7% by weight regrind (purified reused sand)
• 8.8% wt.% Carsil (waterglass binder, Foseco company).

The same observations were made as in Example 1.

Example 3:

As in Example 1, a core was prepared. After drying, the core was fired in the oven for one hour at 850 ° C to simulate the investment casting conditions. Thereafter, the core was placed in the cavity and poured (cast alloy A357 (AlSi7Mg 0.6)). After cooling the casting, the core was doused with boiling water. In contrast to Example 1, the resolution of the core now took about 10 minutes.

Claims (9)

Mechanically and thermally stable core for light metal and / or precision casting, characterized in that the core contains hydrophilic airgel granules, sand and binder. Core according to claim 1, characterized in that the airgel granulate comprises silica airgel. Core according to claim 1 or 2, characterized in that the airgel granules have a particle size in the order of magnitude of the sand. Core according to one of claims 1 to 3, characterized in that the binder comprises water glass and / or an airgel precursor. Process for producing a core according to one of Claims 1 to 4, characterized in that the following steps are carried out: a. Mixture of airgel granules with sand and binder, b. Introduction of the mixture into a negative mold of the core, c. Compaction of the introduced mixture in the negative mold, d. Drying of the compressed mixture and e. Core removal from the negative mold. Use of a core according to one of claims 1 to 4 in Leichtmetallformguss- or in Leichtmetallfeinguss. Use according to claim 6, wherein the core is removed from the mold by a fluid which wets it. Use according to claim 7, wherein the core is removed by water at a temperature of at least 95 ° C. Use according to claim 7, wherein the core of alcohol contained by fluids is removed from the mold.