Process for casting nonferrous metals including light metals and casting mold

Abstract

The invention relates to a molding mixture for producing casting molds for metalworking, a process for producing casting molds, casting molds obtained by the process and also their use. To produce the casting molds, a refractory mold raw material and a binder based on water glass are used. A proportion of a particulate metal oxide selected from the group consisting of silicon dioxide, aluminum oxide, titanium oxide and zinc oxide is added to the binder, particular preference being given to using synthetic amorphous silicon dioxide. The molding mixture contains a phosphate as essential constituent. The use of phosphate can improve the mechanical strength of casting molds at high thermal load.

Claims

1. A process for casting nonferrous metals including light metals, which comprises the steps: producing a molding mixture by bringing together at least: a refractory mold raw material; a binder based on water glass; a particulate metal oxide comprising amorphous silicon dioxide; and 0.05 to 0.5% by weight, based on the refractory mold raw material, of a phosphorus-containing compound, wherein the phosphorus-containing compound is selected from the group consisting of sodium metaphosphate, sodium polyphosphate and mixtures thereof; and mixing molding the molding mixture; and curing the molded molding mixture by heating the molded molding mixture to give a cured casting mold or a cured casting core; and casting nonferrous metals including light metals in the cured casting mold or casting core, wherein the phosphorous-containing compound induces three-dimensional stability of the cured casting mold or the cured casting core during the casting process resulting in a reduced deformation under thermal load as measured by a BCIRA hot distortion test with an increase of at least 10% of the time elapsed until hot distortion starts; and wherein the combined use of amorphous silicon dioxide and the phosphorous-containing compound provides the cured casting mold or cured casting core with a hot strength 10 seconds after removal from a molding tool that is enhanced by at least 20% and a storage strength after 3 hours in a controlled-atmosphere cabinet at 25 degrees C. and 75% relative humidity that is enhanced by at least 20%, relative to a cured casting mold or cured casting core obtained from a molding mixture produced without both the amorphous silicon dioxide and the phosphorous-containing compound.

2. The process as claimed in claim 1 wherein the light metal is aluminum.

3. The process as claimed in claim 1, wherein the phosphorus-containing compound is added in the form of a solid to the moulding mixture.

4. The process as claimed in claim 1, wherein the phosphorus-containing compound is added in a dissolved form to the moulding mixture.

5. The process as claimed in claim 1, wherein the particulate metal oxide has a particle size of less than 300 m.

6. The process as claimed in claim 1, wherein the amorphous silicon dioxide comprises synthetic amorphous silicon dioxide.

7. The process as claimed in claim 1, wherein the phosphorus-containing compound comprises sodium polyphosphate.

8. The process as claimed in claim 1, wherein the molding mixture is molded and cured in a core shooting machine by heating the molded molding mixture.

9. The process as claimed in claim 1, wherein the proportion of the phosphorus-containing compound added to the molding mixture is in an amount of 0.05 to 0.3% by weight, based on the refractory mold raw material.

10. The process as claimed in claim 1, characterized in that the phosphorus-containing compound has a phosphorus content of from 0.5 to 90% by weight, calculated as P.sub.2O.sub.5.

11. The process as claimed in claim 1, characterized in that the amorphous silicon dioxide is precipitated silica and/or pyrogenic silica.

12. The process as claimed in claim 1, characterized in that the water glass has an SiO.sub.2/M.sub.2O ratio in the range from 1.6 to 3.5, where M represents sodium ions and potassium ions.

13. The process as claimed in claim 1, characterized in that the water glass has a solids content of SiO.sub.2 and M.sub.2O in the range from 30 to 60% by weight.

14. The process as claimed in claim 1, characterized in that the binder is present in a proportion of less than 20% by weight in the molding mixture.

15. The process as claimed in claim 1, characterized in that the particulate metal oxide is present in a proportion of from 2 to 60% by weight, based on the binder.

16. The process as claimed in claim 1, characterized in that the molding mixture is produced by the process of providing the refractory mold raw material; admixing the refractory mold raw material with solid constituents which comprise at least the particulate metal oxide and the phosphorus-containing compound, mixing the components to form a dry mix; and adding liquid components to the dry mix, the liquid components comprising at least the water glass.

17. The process as claimed in claim 1, wherein the heating of the molding mixture is cured by the action of microwaves.

18. The process as claimed in claim 1, wherein the proportion of the phosphorous-containing compound added to the molding mixture is in an amount of 0.05 to 0.45% by weight, based on the refractory mold raw material.

19. The process as claimed in claim 1, characterized in that the molding mixture is heated to a temperature in the range from 100 to 300 C. for curing.

20. The process as claimed in claim 1, wherein heated air having a temperature of 100 to 180 C. is blown into the molded molding mixture for curing.

Description

(1) The invention is illustrated below with the aid of examples and with reference to the accompanying figures. In the figures:

(2) FIG. 1 shows a schematic construction of a BCIRA Hot Distortion Apparatus (G. C. Fountaine, K. B. Horton, Hot Distortion of Cold-Box Sands, Giesserei-Praxis, No. 6, pp. 85-93, 1992)

(3) FIG. 2: shows a diagram of the BCIRA Hot Distortion Test of a phosphate-containing test specimen and of a test specimen without a phosphate fraction (Morgan, A. D., Fasham E. W., The BCIRA Hot Distortion Tester for Quality Control in Production of Chemically Bonded Sands, AFS Transactions, vol. 83, pp. 73-80 (1975);

(4) FIG. 3A: shows a schematic reproduction of a section of a casting, the casting mold having been produced in one case without phosphates; and

(5) FIG. 3B: shows a schematic reproduction of a section of a casting, in one case with addition of phosphates.

EXAMPLE 1

(6) Influence of synthetic amorphous silicon dioxide and phosphorous components on the strength of shaped bodies using silica sand as mold raw material.

(7) 1. Production and Testing of the Molding Mixture

(8) To test the molding mixture, Georg-Fischer test bars were produced. Georg-Fischer test bars are cuboidal test bars having the dimensions 150 mm22.36 mm22.36 mm.

(9) The composition of the molding mixture is indicated in Table 1. To produce the Georg-Fischer test bars, the following procedure was employed:

(10) The components indicated in Table 1 were mixed in a laboratory blade mixer (from Vogel & Schemmann AG, Hagen, Germany). For this purpose, the silica sand was firstly placed in the mixer and the water glass was added while stirring. A sodium water glass having proportions of potassium was used as water glass. The SiO.sub.2:M.sub.2O ratio, where M is the sum of sodium and potassium, is therefore indicated in the following tables. After the mixture had been stirred for one minute, the amorphous silicon dioxide if used and/or the phosphorus component was added while continuing to stir. The mixture was subsequently stirred for a further one minute;

(11) The molding mixtures were transferred to the stock hopper of an H 2.5 hot box core shooting machine from RperwerkGieereimaschinen GmbH, Viersen, Germany, whose molding tool had been heated to 200 C.;

(12) The molding mixtures were introduced into the molding tool by means of compressed air (5 bar) and remained in the molding tool for a further 35 seconds;

(13) To accelerate curing of the mixtures, hot air (2 bar, 120 C. at the inlet into the tool) was passed through the molding tool for the last 20 seconds;

(14) The molding tool was opened and the test bars were taken out.

(15) To determine the flexural strengths, the test bars were placed in a Georg-Fischer strength testing apparatus equipped with a 3-point bending rig (DISA Industrie AG, Schaffhausen, CH) and the force which led to fracture of the test bars was measured.

(16) The flexural strengths were measured according to the following scheme: 10 seconds after removal from the molding tool (hot strengths) 1 hour after removal from the molding tool (cold strengths) storage of the cooled cores for 3 hours in a controlled-atmosphere cabinet at 25 C. and 75% relative atmospheric humidity.

(17) TABLE-US-00002 TABLE 1 Composition of the molding mixtures Silica Amorphous sand Alkali metal silicon H32 water glass dioxide Phosphate 1.1 100 pbw 2.0 .sup.a) Comparison, not according to the invention 1.2 100 pbw 2.0 .sup.a) 0.5 .sup.b) Comparison, not according to the invention 1.3 100 pbw 2.0 .sup.a) 0.3 .sup.c) Comparison, not according to the invention 1.4 100 pbw 2.0 .sup.a) 0.5 .sup.b) 0.3 .sup.c) According to the invention 1.5 100 pbw 2.0 .sup.a) 0.5 .sup.b) 0.1 .sup.c) According to the invention 1.6 100 pbw 2.0 .sup.a) 0.5 .sup.b) 0.5 .sup.c) According to the invention 1.7 100 pbw 2.0 .sup.a) 0.3 .sup.c) Comparison, not according to the invention 1.8 100 pbw 2.0 .sup.a) 0.5 .sup.b) 0.3 .sup.c) According to the invention .sup.a) Alkali metal water glass having an SiO.sub.2:M.sub.2O ratio of about 2.3 .sup.b) Elkem Microsilica 971 (pyrogenic silica; produced in an electric arc furnace) .sup.c) Sodium hexametaphosphate (Fluka), added as solid .sup.d) Metakorin TWP 15 (polyphosphate solution from Metakorin Wasser-Chemie GmbH)

(18) TABLE-US-00003 TABLE 2 Flexural strengths After storage Hot Cold in a controlled- strengths strengths atmosphere cabinet [N/cm.sup.2] [N/cm.sup.2] [N/cm.sup.2] 1.1 70 420 20 Comparison, not according to the invention 1.2 170 500 400 Comparison, not according to the invention 1.3 60 410 20 Comparison, not according to the invention 1.4 160 490 390 According to the invention 1.5 170 500 400 According to the invention 1.6 150 460 350 According to the invention 1.7 80 430 30 Comparison, not according to the invention 1.8 160 450 380 According to the invention
2. Result
Influence of the Amount of Amorphous Silicon Dioxide and Phosphate Added

(19) All of the molding mixtures were prepared with a constant amount of molding material and of water glass. Examples 1.3 and 1.7 show that it is not possible to produce storable cores through the addition of phosphate alone. In Examples 1.2, 1.4, 1.5, 1.6 and 1.8 molding mixtures were prepared using amorphous silicon oxide. The hot strengths and strengths after storage in a controlled-atmosphere cabinet are much higher than for the other examples. Examples 1.4, 1.5 and 1.8 show that the hot strengths and cold strengths and also the strengths after storage in a controlled-atmosphere cabinet of molding materials comprising amorphous silicon dioxide as a constituent are not adversely affected by the addition of a phosphate-containing component. This means that the test bars produced using the molding mixture of the invention substantially retain their strengths even after prolonged storage. Example 1.6 suggests that, above a certain level of phosphate in the molding mixture, an adverse effect on the strengths is likely.

EXAMPLE 2

(20) 1. Measurement of Deformation

(21) The deformation under thermal load was determined by the BCIRA Hot Distortion Test (Morgan, A. D., Fasham E. W., The BCIRA Hot Distortion Tester for Quality Control in Production of Chemically Bonded Sands, AFS Transactions, vol. 83, pp. 73-80 (1975)).

(22) In the BCIRA Hot Distortion Test, which is shown in FIG. 1, a sample body of chemically bonded sand with dimensions of 256114 mm is clamped in as a cantilever and is heated on the flat side from below (G. C. Fountaine, K. B. Horton, Hot Distortion of Cold-Box Sands, Giesserei-Praxis, No. 6, pp. 85-93, 1992). As a result of this one-sided heating, the sample body bends upward toward the cold side as a result of the thermal expansion of the hot side. This movement on the part of the sample body is identified in the graph as the maximum expansion. To the extent that the sample body undergoes heating overall, the binder begins to disintegrate and to undergo transition to the thermoplastic state. On account of the thermoplastic properties of the various binder systems, the load through the load arm presses the sample body back downward again. This downward movement along the ordinate in the 0 line to the point of fracture is referred to as hot distortion. The time which has lapsed between the beginning of the maximum expansion on the graph, and the point of fracture, is identified as the time to fracture and represents a further parameter. The movement that occurs in this experimental system can in fact be observed in molds and cores.

(23) The molding mixtures were prepared in accordance with the method shown in Example 1, with the difference that the dimensions of the test bars were 25 mm6 mm114 mm.

(24) TABLE-US-00004 TABLE 3 Composition of the molding mixtures Silica Amorphous sand Alkali metal silicon H32 water glass dioxide Phosphate 2.1 100 pbw 2.0 .sup.a) 0.5 .sup.b) Comparison, not according to the invention 2.2 100 pbw 2.0 .sup.a) 0.5 .sup.b) 0.3 .sup.c) Comparison, not according to the invention .sup.a) Alkali metal water glass having an SiO.sub.2:M.sub.2O ratio of about 2.3 .sup.b) Elkem Microsilica 971 (pyrogenic silica; produced in an electric arc furnace) .sup.c) Sodium hexametaphosphate (Fluka), added as solid
2. Results

(25) The measurements for the deformation under thermal load are shown in FIG. 2. Without addition of phosphate (molding mixture 2.1) the test specimen is deformed after just a short period of thermal load. Test specimens produced using molding mixture 2.2, in contrast, exhibit a significantly improved thermal stability. Through the addition of phosphate it is possible to extend the time until hot distortion takes place and hence the time to fracture.

EXAMPLE 3

(26) Production of Casting Molds Using Phosphate-Free and Phosphate-Containing Shaped Bodies

(27) In order to investigate the improved thermal stability of shaped bodies that was shown in Example 2, cores were produced using the molding mixtures 2.1 and 2.2. These cores were tested for their thermal stability in a casting operation (aluminum alloy, approx. 735 C.). Here it was found that a circular segment of the shaped body was correctly reproduced in the corresponding casting mold (FIG. 3b) only in the case of molding mixture 2.2. Without the addition of the phosphate component, elliptical deformations were observed on the casting mold, shown schematically in FIG. 3a.

(28) From this it is evident that through the use of the molding mixture of the invention it is possible to lower the deformation tendency of shaped bodies during the casting operation and hence to improve the casting quality of corresponding casting molds.