EP2308614B1 - Green aerosand - Google Patents

Description

The present invention relates to aerogelsands which have green strength by addition of phyllosilicate and xerogel, their use for the production of molds / cores for the casting of metals and a process for the production of such molds / cores.

Many metallic products can be molded in one casting process. In order to meet the requirements of the final product, the composition of the metal must be chosen correctly. But high demands are also placed on the casting mold. So this is on the one hand during the metal casting thermally stable, but at the same time be removed without residue after the metal casting. There is a fundamental distinction between permanent and lost molds. Permanent molds, for example made of ceramic or metal, can be used multiple times. In contrast, lost molds made of, for example, sand can only be used once. Casting in bonded sand molds is a standard casting technique used to make workpieces of a variety of metals or alloys.

Shapes and cores are usually sand-cast from quartz sand, for special applications but also from other sands (alumina, zirconia, olivine, chrome ore, and others), in which the Sand grains are glued together by polymeric or water glass binder and form a dimensionally stable bond for the duration of mold filling with liquid metal. This should as simple as possible be dissolved again after the solidification of the melt, which is especially true for cores, the complex shaped cavities in the casting negative image. For coring or dissolution of the mold mechanical (shaking, shaking, knocking) or thermal aids and pressurized water can be used.

Today's binders will be complex chemically modified (chemical additives) to meet the requirements of foundries, such as high thermal stability with low outgassing and low binder use and yet easy gutting and high surface quality of the resulting metal casting.

Binders for molding sands may be inorganic or organic in nature, with inorganic binders being subdivided into, for example, natural or synthetic inorganic binders. Organic binders include, for example, synthetic resins such as phenolic, urea and furan resins. Inorganic binders include, for example, clays, cement or gypsum. For example, oils, carbohydrate binders, water-soluble liquid binders, dextrose or pitch binders can also be used ( A. Carrier "Foundry Technology" in Ullmann's Encyclopedia of Industrial Chemistry, Weinheim VCH, 5th Edition, Volume A 12, pp. 35-46 ).

However, the binders as described in the prior art can not be considered optimal. The casting cores are often difficult to remove, so that remains of the molding sand stick to the metal casting. This is a complex surface treatment and / or cleaning the metal piece necessary. In addition, it can lead to deformation or cracking of the workpiece. As an alternative binder aerogels have opened up in recent years. Aerogels are highly porous, open-pore solids, which are usually obtained via sol-gel processes via the gelation of colloid-disperse solutions and subsequent mostly supercritical drying.

Aerogels can be mixed with different fillers. So, for example, deals EP 1 697 273 B1 with aerogels which are mixed with inorganic hollow spheres. This leads to a low thermal conductivity of the resulting airgel molding.

EP 1 682 291 B1 also deals with a filler-containing airgel. The gelation time of the filler-containing airgel has been shortened by the fact that the sol is already pre-treated and only then is the filler added.

DE 10216403 B4 is concerned with molding materials for the casting of metals or metal alloys and their use for the production of mold and core materials with particularly good thermal conductivity, which are also fully recyclable. The molding materials described herein contain open-pored plastic aerogels and inorganic SiC fillers in an amount of at least 70% by volume.

In EP 1 852 197 A1 describes a core material made of clay-containing sand containing Aerogelsand containing swellable phyllosilicates. Such molding sand has a higher ultimate strength than Aerogelsande, which contain no phyllosilicates.

The aerogels binders can be cored easily by thermal means, since the nanostructured binder bridges are easily oxidized at temperatures around 300 ° C and the airgel thereby decomposes. Their strength is good to sufficient, but the use of the aerosol binder solution, which contains a lot of water, causes the sand-binder solution mixture to have no green strength but needs to build up only after gelation. Green strength is within the meaning of the invention, the strength of moist mold sands with uniform material and temperature distribution.

The object of the present invention is therefore to provide a molding sand for the production of foundry molds / cores, which reduces or even eliminates the problems known from the prior art, such as adhesion of the sand to the metal. Another object of the present invention is to provide a sand having a sufficient green strength, while at the same time easily removing the core / mold after metal casting.

In a first embodiment, the object underlying the invention is achieved by a molding sand which contains sand, RF aerogel binder, phyllosilicate and xerogel. Surprisingly, such a composition exhibits both sufficient green strength and easy removability of the core after metal casting.

A molding sand according to the invention preferably comprises Minelco sands, quartz and / or aluminum oxide sands as sand component. The sands preferably have an average particle size of about 30 μm. Dust components of the sands are preferably screened to obtain a better surface finish of the metal casting.

RF aerogels according to the invention comprise colloidal substances which are gelled and dried under critical conditions. RF stands for resorcinol-formaldehyde and thus for a class of known aerogels. They have a low density and high open porosity. Up to 95% of the volume of the aerogels consists of pores. Aerogels are considered to be one of the lightest materials and have a high thermal insulation capacity. RF aerogels can be obtained by sol-gel polymerization of resorcinol with formaldehyde.

As a layered silicate, in particular as a swellable layered silicate, bentonite is preferably used for the purposes of the present invention. As Xerogel in particular hydrophilic silica and / or water glass are used, which is commercially available.

The particle size distribution of the RF Aerogelbindemittels, the phyllosilicate and the xerogel are preferably adapted to the sand. This allows a uniform mixing of the individual components.

According to the invention includes an airgel sand a) 80 to 96% by weight sand b) 1 to 1.7, in particular 1.4 to 1.6 wt .-% phyllosilicate c) 0.9 to 1.1% by weight Xerogel and d) 1.2 to 3.5, in particular 1.5 to 2.5 wt .-% RF Aerogelbindemittel.

The percentages by weight are in each case based on the weight of a dry, fully hardened core or a corresponding shape.

By adding the phyllosilicate one achieves an increased stability of the core material / the casting mold. Even a brief preheating of the core or the mold to temperatures up to 500 ° C, for example over a period of about 30 minutes is possible. After the actual metal casting, however, the mold must ultimately decay again at similar temperatures. If you give too high a proportion of phyllosilicate to the molding sand, this hardens too much. The mold core can now only be removed very difficult after the metal casting. Often remnants of the metal workpiece are left behind, so that a time-consuming and expensive cleaning is necessary. However, too low a proportion of phyllosilicate leads to an insufficient green strength of the molding sand. Only after gelation, the mold can then be used in metal casting.

In order to keep the proportion of phyllosilicate in the molding sand as low as possible, a xerogel is added in addition. Surprisingly, this combination shows a high green strength and at the same time easy destemmability. The xerogel acts as a siccative to collect any excess binder. In the present case, water serves as a binder. If a core is made of sand, the molding sand is shot by a core shooter with high pressure in a corresponding shape. Due to the high pressure not only the sand-binder gas mixture is strongly and evenly compacted, but as a side effect, the binder is pressed from the sand interstices to the edge of the mold. On the outer walls of the form of the sand is extremely wet, which he no longer has any stability there. The xerogel absorbs this excess water; it stays evenly distributed in the sand. A reduction of the liquid binder content is not a solution to the problem since only a sufficiently high amount of binder will yield a solid core or solid sand mold.

In a second embodiment, the object underlying the present invention is described by the production of molds / cores for the casting of metals from such a molding sand. The method comprises producing a casting mold / casting core with a molding sand mixture by filling a negative mold of a core / mold with it. In a further step, the wet core / wet form is removed from the (core) mold and dried at temperatures ranging from room temperature up to 80 ° C in air. If the casting cores are to be stored for a long time, it has proven advantageous to dehydrate them. In particular, it has been shown that dehydrogenation in vacuo at temperatures of 200 to 300 ° C increases the storage stability of the cores.

EXAMPLES 1st example

A sand mixture of 88% by weight of MinSand (fine, 230), 1.5% by weight of commercially available bentonite and 1.00% by weight of xerogel (particle size <280 μm) was mixed with 9.5% by weight of an aerosol solution of the following composition: resorcinol : H ₂ O (deion.) (0.044: 1), resorcinol: formaldehyde solution (0.72: 1), resorcinol: Na ₂ CO ₃ (1512: 1), processed into foundry sand in a mixer. The quantities refer to the wet mixture during production.

The molding sand mixture was filled into a core shooter and benders (20mm x 20mm x 150mm) were shot at a pressure of 5.5 bar. After that, the still wet cores became taken from the core mold and gelled at 40 ° C for 30 minutes in air and dried.

2nd example

.. A sand mixture of 80% by volume of quartz sand (Haltern H32) and 8.8% by volume silica xerogel (MultiFit ^® NATURE Cristal Pearls Diamond, particle size <280μm) was mixed with 11.2% by volume of a Aerogellösung the following composition:. Resorcinol: H ₂ O (deion.) (0.044: 1), resorcinol: formaldehyde solution (0.72: 1), resorcinol: Na ₂ CO ₃ (15 12: 1), processed in a mixer to form sand. The quantities refer to the wet mixture during production.

The molding sand mixture was filled into a core shooter and benders (20mm x 20mm x 150mm) were shot at a pressure of 5.5 bar. Subsequently, the still wet cores were taken out of the core mold and gelled at 40 ° C for 30 minutes in air and dried.

The green strength was not determined by mechanical characteristics but by the possibility of removing the cores from the mold by hand or tool immediately after production. The handleability immediately shows a strength necessary for industrial use.

The cores thus produced are storable for a period of at least 6 months without loss of strength.

Claims (8)

1. A molding sand containing sand, RF aerogel binder, sheet silicate and xerogel.
2. The molding sand according to claim 1, characterized in that said sand includes Minelco sands, silica and/or alumina sands.
3. The molding sand according to claim 1, characterized in that said sheet silicate includes bentonite.
4. The molding sand according to claim 1, characterized in that said xerogel includes hydrophilic silica and/or water glass.
5. The molding sand according to any of claims 1 to 4, characterized in that it contains, based on the dry weight of the final product: a) 80 to 96% by weight sand b) 1 to 1.7, especially 1.4 to 1.6, % by weight sheet silicate c) 0.9 to 1.1% by weight xerogel; and d) 1.2 to 3.5, especially 1.5 to 2.5, % by weight RF aerogel binder.
6. Use of a molding sand according to any of claims 1 to 5 for preparing molds and casting cores for the mold casting of metals.
7. A process for producing molds and casting cores from molding sand according to any of claims 1 to 6, characterized in that

   a) a negative mold of a core is filled with a molding sand mixture; and

   b) the wet core is withdrawn from the core mold; and

   c) the latter is dried in air at temperatures within a range of from room temperature to 80 °C.
8. A process for producing molds and casting cores from molding sand according to claim 7, characterized in that said molds/cores are dehydrated under vacuum at 200 °C to 300 °C.