EP2204246B1 - Foundry core with improved gutting properties I - Google Patents

Description

The present invention relates to foundry cores with improved gutting properties, a process for their preparation and their use.

Shapes and cores are usually sand cast from quartz sand, for special applications but also from other sands (alumina, zirconia, olivine, chrome ore) produced in which the grains of sand are bonded together by polymeric binder and a dimensionally stable for the duration of mold filling with liquid metal Forming a composite. 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 aids (shaking, shaking, tapping), thermal aids or pressurized water can be used. Today, phenolic resins, polyurethanes, ureas and furan resins, which are complexly chemically modified (chemical additives), are used as binders to meet the requirements of the foundries. Likewise aerogels are known. The binders are optimized to the applications in their chemical composition to meet the conflicting requirements, such as

An example of high thermal stability with low outgassing and low binder use and yet easy gutting and high surface quality.

The binders of the prior art, however, can not yet be regarded as optimal at the present time. So affect Adhesion of molten metal to the mold and core material as well as the mineralization of the casting surface, the casting quality, the demoulding and the coring as well as the subsequent use of the casting. Almost all binders are difficult to remove on filigree or complex shaped castings, leaving buildup and mineralization, and a coarse, rough casting surface.

DE 10 2006 003 198 A1 ( EP 1820582 A1 ) describes a water-soluble core, which can be used in the field of light metal casting and / or precision casting. The inorganic mixture of sand and hydrophilic airgel granules is bound with various binders. Airgel granules comprising silica aerogels are described as being particularly preferred because they are not wetted or chemically attacked by liquid metals. Examples of hydrophilic organic aerogels / airgel granules, in particular resorcinol-formaldehyde airgel / airgel granules, are not mentioned in this application. Field of application of the described inorganic cores is the foundry industry.

It is thus an object of the present invention to provide foundry cores which alleviate or even solve the specific foundry technical problems of the prior art, i.e., adhesion, mineralization and surface finish.

With the present invention, a new route is now taken: Instead of chemically or physically modifying the binder or binders as before, additives are added to the sand which, with their special properties, solve these specific problems.

In a first embodiment, the problem underlying the present invention is solved by a foundry core comprising sand, binder and resorcinol-formaldehyde airgel granules (RF airgel granules), wherein the proportion of airgel granules ranges from 0.18 to 0.93 wt .-% and add the proportions of sand, binder, airgel granules and optionally the proportions of other ingredients to 100 wt .-%.

Binders for the sands (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 phenolic, urea and furan resins as well as ethyl silicate. Oils, carbohydrate binders, water-soluble liquid binders based on sulfite blue, molasses, dextrose, alkanolamines and pitch binders are still used ( KE Höner "Founding", Ullmann's Encyclopedia of Industrial Chemistry, pp. 271-287, Vol. 12, 4th edition, Verlag Chemie Weinheim, 1976 ).

RF aerogels according to the invention comprise colloidal substances which are gelled and subcritically dried. They have a low density and high, open porosity. They consist only of about 10 to 30 vol .-% of a solid, while the rest of their volume through the
surrounding gas or vacuum is filled, that is, they have a high surface area (up to 50 - 300 m ² / g).

Inorganic aerogels but also, for example, the RF airgel of the present invention as an organic airgel are usually inherently hydrophilic. Aerogels are considered one of the lightest materials and the best heat insulators.

Airgel granules are obtained in particular by the milling of airgel monoliths. Hydrophilic means water-loving, that is, the used RF-Aerogelgranulat shows a pronounced interaction with polar solvents like water. Thus, the hydrophilic RF aerogels used have a wetting angle with water of <10 °.

Apart from the addition of the hydrophilic RF-Aerogelgranulats (or the replacement of a certain proportion of the molding material by the RF-Aerogelgranulat) remains the other process of molding, core or Kernpakserstellung unchanged; So there are still all possible combinations of sands and binding materials used.

A possible reason for the improvements observed by the foundry core according to the invention could be related to the fact that the RF aerogels used have macroscopic dimensions but are nanostructured (like all aerogels). The use of a sufficient proportion of this airgel granules could now cause the melt, the mold can no longer sufficiently reactive wet the mold, since the nanostructure of the airgel granules allows only punctiform contacts. In this way, attachments and mineralization would be suppressed. It is surprising, however, that such an effect can be achieved with an organic airgel, since it inevitably at least partially decomposes under the thermal stresses during the casting, that is charred to a carbon airgel granules.

Overall, the castings obtained via the use of the foundry cores according to the invention prove to be very smooth (exact casting quality), adhesions and mineralization are significantly suppressed compared to castings of the prior art.

The sand preferably comprises quartz sand, an Al ₂ O ₃ -based and / or a mullite-based sand.

• Dorsten 0,84 mm (Sorte DO20), 0,56 mm (DO30), 0,39 mm (DO40), 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 DO20), 0.56 mm (DO30), 0.39 mm (DO40), 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 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 material, thus ensuring that only a comparatively small amount of binder is needed.

Preferably, the mixture of which the foundry core is produced, a sand content of 83 to 95 wt .-% wherein here 1 to 20 wt .-% virgin sand and 80 to 99 wt .-% regenerate (circuit molding material, that is purified reused sand) are preferred. The addition of regenerated sand can be dispensed with, especially in red, brass and bronze castings. The proportion of binder is preferably 1 to 10 wt .-%. Sand, binder and Airogelgranulatanteil (and optionally the proportions of other ingredients) add up to 100 wt .-% or vol .-%.

The airgel granules preferably have a particle size distribution in the order of magnitude of the sand.

Preferably, the airgel granules and / or the sand have / have a particle size distribution in a range from 0.1 to 0.9 mm.

The airgel granules preferably have a particle size / particle size distribution in a range of ≦ 0.5 mm.

The advantage of the grain size distributions / grain sizes just described can be seen in the fact that both the RF airgel granules and the sand are used as mold bases and optimum mixing of particles of the same size is easier to carry out. In addition, it has been shown that in the grain size distributions / grain sizes according to the invention, the observed effects, that is to say a reduced extent of adhesion and mineralization as well as a more precise casting surface, are greater than in the case of other particle size distributions / grain sizes.

The proportion of airgel granules is in a range from 0.18 to 0.93, preferably from 0.5 to 0.75 wt .-%. Preferably, the proportion of airgel granules in the core in a range of 5 to 15, in particular from 8 to 12 vol .-%.

The binder is preferably an organic binder, in particular a binder or a binder mixture, which comprises at least one member selected from phenolic resins, urea resins, furan resins, polyurethane resins and resorcinol-formaldehyde resins, and RF airgel binders.

Organic binders have been found to be preferred, as charring of the binder occurs during casting and this contributes to further ease of gutting and reduces mineralization.

1. a. Mischung eines Aerogelgranulats mit Sand und Bindemittel,
2. b. Einbringung der Mischung in eine Negativform des Kerns, gegebenenfalls gefolgt von einer Verdichtung der Mischung,
3. c. Härtung des Bindemittels und
4. d. Kernentnahme aus der Negativform.

In a second embodiment, the object underlying the invention is achieved by a method for producing a foundry core according to the invention, which is characterized in that the following steps are carried out:

1. a. Mixture of an airgel granulate with sand and binder,
2. b. Introducing the mixture into a negative mold of the core, optionally followed by a compression of the mixture,
3. c. Hardening of the binder and
4. d. Core removal from the negative mold.

The compaction is done for example by core shooting, shaking, tapping and / or pounding. For the curing of the binder, temperatures of 20 to 300 ° C have been found to be particularly suitable, in particular 80 to 250 ° C. The duration of the curing is preferably a few seconds to minutes.

Drying of foundry cores is either completed after curing or by storage of the cores at room temperature or at temperatures above room temperature to 300 ° C from 1 to 24 hours or in the microwave.

In a third embodiment, the object underlying the invention is achieved by the use of the foundry core according to the invention in metal casting, in particular in non-ferrous metal, light metal or cast iron.

In particular, the core is removed after the solidification of the melt by a thermal treatment at elevated temperature, in particular a temperature of ≥ 350 ° C, or mechanically (shaking, ultrasound, knocking).

The removal with a thermal treatment is advantageous because here the core decomposes without leaving any residue.

Errors of the dimensional stability which occur due to the core expansion during the quartz jump using quartz sand during casting can be compensated for by the elasticity of the granules used as a function of the granulate content and binder content.

Embodiment:

RF airgel granules were used. First, monoliths were prepared via the sol-gel process to produce RF airgel granules. The following molar ratios were used: resorcinol (MERCK, very pure) to water (deionized): 0.044, formaldehyde (VWR, 37%, aqueous solution) to resorcinol: 1.38 and resorcinol to catalyst (sodium carbonate anhydrous for analysis, Merck): 1,512.63. The airgel undercritically dried after gelation at 40 ° C. was comminuted in a beater mill (SK100, Retsch), the maximum particle size being 0.5 mm.

• Phenolharzbinder mit gasförmigem Amin-Katalysator
• Harnstoffbinder
• Polyurethanbinder
• RF-Aerogelbinder

As binders were used:

• Phenol resin binder with gaseous amine catalyst
• urea binder
• polyurethane binder
• RF Aero Yellow Indians

In all cases, solid molded materials or cores, as well as bending bars, were produced according to the VDG leaflet P73. Castings with brass, bronze and aluminum alloys showed castings free from buildup or mineralization and clean, sometimes smooth surfaces. Cores made from the composite sand with airgel granulate and polymeric binder could easily and easily be removed from sample casts (model plate for bending bars, but also technical castings), either mechanically or thermally (oxidation at approx. 350 ° C). The castings were also pore and void-free, that is, the cores produced, although they contained organic substances, no additional gas evolution, since the aerogels additive in the sand acts as a siccative or absorbent for casting gases.

Of the above-mentioned binders, RF Aerogel binder proved to be particularly advantageous:
For the production of bending bars, 500 g of sand mixture (MinSand 230 / natural sand, mixing ratio 1: 0.208), 10% by volume of RF airgel granules (3.4 g, grain size <0.5 mm) and 62 g of RF binder (own production , Sol aqueous solution, composition according to the molar ratios: resorcinol (MERCK, purest) to water (deionized): 0.044, formaldehyde (VWR, 37%, aqueous solution) to resorcinol: 1.38 and resorcinol to catalyst (sodium carbonate anhydrous to Analysis, Merck): 1512.63). The bending bars were hand-molded and then dried at 40 ° C in a warming cabinet. The Entkernbarkeit was significantly improved, the cores decomposed thermally. The castings had a smooth surface.

Claims (10)

1. A casting core containing sand, binder and resorcinol-formaldehyde aerogel granules, wherein the proportion of the aerogel granules is within a range of from 0.18 to 0.93% by weight, and the proportions of sand, binder, aerogel granules and optionally the proportions of further ingredients add up to 100% by weight.
2. The core according to claim 1, characterized in that said sand comprises silica sand, a sand based on Al₂O₃, and/or a sand based on mullite.
3. The core according to claim 1 or 2, characterized in that said aerogel granules and/or said sand have a grain size distribution within a range of from 0.1 to 0.9 mm.
4. The core according to any of claims 1 to 3, characterized in that said aerogel granules have a grain size within a range of ≤ 0.5 mm.
5. The core according to any of claims 1 to 4, characterized in that the proportion of the aerogel granules is within a range of from 5 to 15%, especially from 8 to 12%, by volume.
6. The core according to any of claims 1 to 5, characterized in that the proportion of the aerogel granules in the core is within a range of from 0.5 to 0.75% by weight.
7. The core according to any of claims 1 to 6, characterized in that said binder is an organic binder, especially a binder or mixture of binders comprising at least one member selected from phenol resins, urea resins, furan resins, polyurethane resins, resorcinol-formaldehyde resins, and RF aerogel binders.
8. A process for producing a casting core according to any of claims 1 to 7, characterized in that the following steps are performed:

   a. mixing said aerogel granules with sand and binder;

   b. introducing the mixture into a negative mold of the core, optionally followed by compaction of the mixture;

   c. curing the binder; and

   d. removing the core from the negative mold.
9. Use of a casting core according to any of claims 1 to 7 in metal casting, especially in non-ferrous metal, light metal or iron casting.
10. The use according to claim 9, characterized in that said core is removed by thermal treatment at an elevated temperature, especially a temperature of ≥ 300 °C.