EP2193858B1 - Foundry core with improved gutting properties II - 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 for the applications in terms of their chemical composition to meet the conflicting requirements, such as high thermal stability with low outgassing and low binder use and yet easy gut removal 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.

Out WO 95/06617 A1 hydrophobic silica aerogels are known. These are obtainable by reacting a waterglass solution with an acid at a pH of 7.5 to 11, substantially freeing the formed silicic acid hydrogenation of ionic constituents by washing with water or dilute aqueous solutions of inorganic bases, wherein the pH of the hydrogel in the range from 7.5 to 11, displacement of the hydrogel-containing aqueous phase by an alcohol and subsequent supercritical drying of the resulting alkogel.

DE 10 2006 003 198 A1 describes a water-soluble core that 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. Field of application of these 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 containing sand, organic binder and hydrophobic oxidic airgel granules.

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 ).

Aerogels according to the invention include colloidal substances which are gelled and dried. They have a lower density and high, open porosity. They consist only to about 1 to 15 vol .-% 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 1000 m ² / g). Inorganic aerogels but also, for example, resorcinol-formaldehyde airgel 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. Hydrophobic means water-repellent, that is, the airgel granules used shows a pronounced interaction with polar solvents like water. Thus, the hydrophobic airgel granules used have a wetting angle with water ≥ 160 °.

In many cases, hydrophobic airgel granulates can be prepared starting from hydrophilic airgel granules by subjecting the latter to a hydrophobing treatment. This is often the case with inorganic aerogels such as, for example, SiO ₂ -based aerogels: a treatment with, for example, trimethylsilyl chloride leads to silylation of the free OH groups of the hydrophilic airgel granules and thus to etherification and thus hydrophobization.

Apart from the addition of hydrophobic airgel granulate (or the replacement of a certain proportion of the molding material by the hydrophobic airgel granules) 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 hydrophobic airgel granules used have macroscopic dimensions but are nanostructured (like all aerogels). The use of a sufficient proportion of hydrophobic airgel granules could now lead to the melt, the mold can no longer adequately reactive wet the mold, since the nanostructure of the hydrophobic airgel granules allows only punctiform contacts. In this way, attachments and mineralization would be suppressed.

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 D020), 0,56 mm (D030), 0,39 mm (D040), 0,13 mm (D0110);
• 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 (D0110);
• 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 (cycle molding material, that is purified recycled sand) preferred are. The addition of regenerated sand can be dispensed with, especially in red, brass and bronze casting. 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 .-%.

It is preferred that the oxidic airgel granules comprise SiO ₂ , TiO ₂ and / or ZrO ₂ . For the hydrophobization of correspondingly prepared aerogels, the above-mentioned trimethylsilylation by treatment with TMSCI is particularly suitable.

Furthermore, it is preferred that the airgel granules 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 particle size distributions / grain sizes just described is that both the hydrophobic 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 found that in the grain size distributions / grain sizes according to the invention, the observed effects, that is to say a reduced amount of adhesion and mineralization, as well as a more precise Casting surface, are larger than other grain size distributions / grain sizes.

The proportion of airgel granules is preferably in a range from 3 to 15, particularly preferably from 8 to 12,% by volume. Alternatively or cumulatively, the proportion of airgel granules in the core is in a range from 0.05 to 0.24, in particular from 0.13 to 0.19 wt .-%.

According to the invention , the binder is 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.

The hydrophobic airgel granules are particularly preferably hydrophobized silica or waterglass airgels. Due to the high thermal loads during the casting, the organic groups introduced for the hydrophobization are destroyed and the hydrophobic air is converted into a hydrophilic airgel, which is very easy to react with, for example Remove water.

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, Leichmetall- or iron casting.

In particular, after the solidification of the melt, the core is removed by a thermal treatment at elevated temperature, in particular a temperature of ≥ 300 ° C., or by a fluid which wets it, in particular water.

The removal with a wetting fluid is advantageous, since here the core decomposes without residue by the fluid which wets it.

In particular, these are well-wetting fluids such as water, since the hydrophobization (trimethylsilylation of the inner surfaces of the aerogels) is destroyed by the influence of heat during casting. 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 it is exploited that hydrophilic 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, for example with water.

EXAMPLES

• hydrophobes Silica-Aerogelgranulat
• hydrophobes Titanoxid-Aerogel
• hydrophobes Zirkonoxid-Aerogel

All types of hydrophobized oxidic airgel granules could be used. In particular, the following airgel granules were investigated, which after preparation of the aerogels were brought to the correct particle size (sand-adapted) by grinding:

• hydrophobic silica airgel granules
• hydrophobic titanium oxide airgel
• hydrophobic zirconia airgel

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

As binders were used (in all combinations with the above aerogels):

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

In all cases solid molded materials or cores were produced. 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 polymer binder could easily and easily be removed from sample casts (model plate for bending bars, but also technical castings), either mechanically, thermally (oxidation at approx. 350 ° C) or even with water. since the hydrophobization (trimethylsilylation of the inner surfaces of the aerogels) is destroyed by the heat influence during casting. The castings were also pore and void-free, that is, the cores produced, even if they contained organic substances, no additional gas evolution, since the aerogels additive in the sand acts as a siccative or absorbent for casting gases. 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.

Example 1 Hot-Box Process (Urea-Formaldehyde Binder) (see foundry dictionary, Schiele & Schön, Berlin)

For the production of cores, 500 g sand (H32) and 10 g binder system (Resin, Hardener AT, Konserver from Hüttenes Albertus) were mixed. 30 ml hydrophobic silica airgel (Cabot Nanogel GmbH, Frankfurt, Nanogel ^®, translucent airgel, silica, -Modified [(trimethylsilyl) oxy], 1.8 g, particle size <0.5 mm) was homogeneously mixed with the sand / binder Mixed mixture. The cores were easy to produce (230 - 275 ° C, baking time: 35-28 s). The dried kernels were poured off. The casting had a smooth surface even without sizing and was free of mineralization. The core could be removed from the casting without effort.

Example 2: Hot Box Method (UF Polymer)

For the production of bending bars, 500 g of sand (quartz sand) 8.27 g of binder (hot-box resin HB587 (UF polymer, Borden Chemical UK LTD), hardener AT21 (Hüttenes Albertus), flow oil (Tego emulsion 35, Goldschmidt AG)) were mixed , 10% by volume of hydrophobic silica airgel granules (1.71 g, grain size <0.5 mm) were added to the finished mixture and mixed homogeneously. Bending bars were hand-formed, and then dried at 180 ° C. The bending strength corresponded to the usual values. The Entkernbarkeit could be significantly improved. The castings had a smooth surface.

Example 3 Cold Box Process (Phenolic Resin / Isocyanate with Amine Cure)

For the production of bends 500 g sand (quartz sand, recycled) with 2.23 wt.% Binder (Ecocure 200 EP, Ecocure 100 EP (both: ASK Chemicals)) were mixed. 10% by volume of hydrophobic silica airgel granules (3.36 g, grain size <0.5 mm) were added to the finished mixture and mixed homogeneously. Benders were hand-molded and cured by gassing with ethyldimethylamine (Catalyst 702, ASK Chemicals). The cores were stable and could be poured easily. The gutting was simplified, the quality of the casting surface improved.

Claims (12)

1. A casting core containing sand, organic binder and hydrophobic oxidic aerogel granules.
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 either of claims 1 or 2, characterized in that said oxidic aerogel granules includes SiO₂, TiO₂ and/or ZrO₂.
4. The core according to any of claims 1 to 3, characterized in that said aerogel granules have a grain size distribution on the order of that of the sand.
5. The core according to any of claims 1 to 4, 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.
6. The core according to any of claims 1 to 5, characterized in that said aerogel granules have a grain size within a range of ≤ 0.5 mm.
7. The core according to any of claims 1 to 6, characterized in that the proportion of the aerogel granules is within a range of from 3 to 15%, preferably 8 to 12%, by volume.
8. The core according to any of claims 1 to 7, characterized in that the proportion of the aerogel granules in the core is within a range of from 0.05 to 0.24%, preferably 0.13 to 0.19%, by weight.
9. The core according to any of claims 1 to 8, characterized in that said binder is 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.
10. A process for producing a casting core according to any of claims 1 to 9, 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.
11. Use of a casting core according to any of claims 1 to 9 in metal casting, especially in non-ferrous metal, light metal or iron casting.
12. The use according to claim 11, characterized in that said core is removed by thermal treatment at an elevated temperature, especially a temperature of ≥ 300 °C, or by a fluid wetting the core, especially water.