MOULD MATERIAL MIXTURE CONTAINING RESOLS AND AMORPHOUS SILICON DIOXIDE, MOULDS AND CORES PRODUCED THEREFROM AND METHOD FOR THE PRODUCTION THEREOF

Abstract

The invention relates to mold material mixtures for producing molds and cores for metal casting, consisting of at least one refractory material, a binder based on resols and amorphous silicon dioxide. The invention also relates to a multicomponent system and methods for producing molds and cores using the mold material mixtures as well as molds and cores for metal casting produced according to this method.

Claims

1. A mold material mixture for producing molds or cores comprising at least: a) a refractory basic mold material, b) alkaline resols and water as a binder or as a binder component and c) amorphous SiO.sub.2 as additive.

2. The mold material mixture according to claim 1, wherein the mold material mixture is obtainable by bringing together a multicomponent system comprising at least the following components (A), (B) and (F) existing spatially separate from one another: (A) amorphous SiO.sub.2 in powder form, no water and no resol, (B) a binder component (B) comprising the alkaline resol, water and no amorphous SiO.sub.2 and (F) a free-flowing refractory component (F) comprising a refractory basic mold material and no resol.

3. A multicomponent system for producing molds or cores comprising at least the following components (A), (B) and (F) existing spatially separate from one another: (A) amorphous SiO.sub.2 in powder form, no water and no resol, (B) a binder component (B) comprising the alkaline resol, water and no amorphous SiO.sub.2 and (F) a free-flowing refractory component (F) comprising a refractory basic mold material and no resol.

4. The mold material mixture according to claim 1, wherein the refractory basic mold material comprises quartz sand, zirconia sand, chrome sand; olivine, vermiculite, bauxite, fireclay, glass beads, granular glass, aluminum silicate microspheres and mixtures thereof and preferably consists of more than 50 wt.-% quartz sand based on the refractory basic mold material.

5. The mold material mixture system according to claim 1, wherein more than 80 wt.-%, preferably more than 90 wt.-%, and particularly preferably more than 95 wt.-%, of the mold material mixture is refractory basic mold material.

6. The mold material mixture according to claim 1, wherein the refractory basic mold material has mean particle diameters of 100 m to 600 m, preferably between 120 m and 550 m, determined by sieve analysis.

7. The mold material mixture according to claim 1, wherein the amorphous silicon dioxide has a surface area determined by BET of between 1 and 200 m.sup.2/g, preferably greater than or equal to 1 m.sup.2/g and less than or equal to 30 m.sup.2/g, particularly preferably of less than or equal to 15 m.sup.2/g.

8. The mold material mixture according to claim 1, wherein the amorphous silicon dioxide is selected from the group consisting of: precipitated silica, pyrogenic silicon dioxide produced by flame hydrolysis or in an electric arc, amorphous silicon dioxide produced by thermal decomposition of ZrSiO.sub.4, silicon dioxide produced by oxidation of metallic silicon with an oxygen-containing gas, quartz glass powder with spherical particles produced from crystalline quartz by melting and rapid recooling, and mixtures thereof and preferably contains or consists of amorphous silicon dioxide produced by thermal decomposition of ZrSiO.sub.4.

9. The mold material mixture according to claim 1, wherein the mold material mixture contains the amorphous silicon dioxide in quantities of 0.1 to 2 wt.-%, preferably 0.1 to 1.5 wt.-%, in each case based on the basic mold material.

10. The mold material mixture according to claim 1, wherein the amorphous silicon dioxide has a water content of less than 5 wt.-% and particularly preferably less than 1 wt.-%.

11. The mold material mixture according to claim 1, wherein the amorphous silicon dioxide is particulate amorphous silicon dioxide and preferably has a mean particle diameter, determined by dynamic light scattering, between 0.05 m and 10 m, in particular between 0.1 m and 5 m and particularly preferably between 0.1 m and 2 m.

12. The mold material mixture according to claim 1, wherein the mold material mixture contains the resols in a quantity of 1 to 10 wt.-%, preferably 1 to 5 wt.-% and particularly preferably of 1 to 4 wt.-%, in each case based on the weight of the basic mold material.

13. The mold material mixture according to claim 1, wherein the resols are CO.sub.2-curable.

14. The mold material mixture according to claim 1, wherein the mold material mixture contains oxyanions, in particular 1 to 4 wt.-%, in each case based on the resol binder comprising resol, water and base, preferably as part of component (B).

15. The mold material mixture according to claim 13, wherein the oxyanions contain boron and/or aluminum, preferably both, and the Al:B atomic ratio is 0.05:1 to 1:1, preferably 0.1:1.

16. The mold material mixture according to claim 1, wherein the mold material mixture contains 10 to 40 wt.-% bases, preferably alkali hydroxides, advantageously as a constituent of component (B), in particular between 10 to 35 wt.-% bases, particularly preferably 12 to 25 wt.-%.

17. The mold material mixture according to claim 1, wherein the mold material mixture contains water as a constituent of component (B) or as the binder component, or exclusively as a constituent of component (B), in particular 25 to 50 wt.-% based on the weight of component (B).

18. The mold material mixture according to claim 1, wherein the resols are added in the form of an aqueous alkaline solution, preferably with a solids fraction of 30 to 75 wt.-%, and also independently thereof, have a pH above 12.

19. The mold material mixture according to claim 1, wherein a curing agent is added to the mold material mixture, in particular at least one alkaline-hydrolyzable ester, preferably as a constituent of component (B) or as an additional component.

20. A method for producing molds or cores comprising: providing the mold material mixture by bringing together and mixing the substances or components according to any one of claims 1 or 3, introducing the mold material mixture into a mold, and curing the mold material mixture by introducing carbon dioxide into the mold.

21. A method for producing molds or cores comprising: providing the mold material mixture by bringing together and mixing the substances or components according to any one of claim 1 or 3 i) comprising at least one ester liquid at room temperature, introducing the mold material mixture into a mold, and curing the mold mixture by introducing ii) gaseous esters into the mold, wherein either features i) and ii) are present jointly or only i) or ii) is present.

22. The method according to claim 20 or 21, wherein the mold material mixture, for curing, is exposed to a temperature of 5 to 60 C., preferably of 5 to 25 C.

23. (canceled)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0024] As the refractory basic mold material (also simply called basic mold material in the following) usual and known materials for the production of casting and mixtures thereof may be used. Suitable materials are, for example, quartz, zirconia or chrome sands; olivine, vermiculite, bauxite, fireclay and so-called synthetic basic mold materials, thus basic mold materials brought into spherical or approximately spherical (for example, ellipsoid) form by industrial methods. Examples are synthetic, spherical, ceramic sandsso-called Cerabeads but also Spherichrome, SpherOX, and microspheres such as those that can be isolated as components from fly ash, among others.

[0025] Particularly preferred are basic mold materials containing more than 50 wt.-% quartz sand based on the refractory basic mold material. Refractory basic mold materials are defined as substances with a high melting point (melting temperature). The melting point of the refractory basic mold material is advantageously above 600 C., preferably above 900 C., particularly preferably above 1200 C. and especially preferably above 1500 C.

[0026] The refractory basic mold material preferably makes up more than 80 wt.-%, especially more than 90 wt.-%, particularly preferably more than 95 wt.-% of the mold material mixture.

[0027] The mean diameter of the refractory basic mold materials is generally between 100 m and 600 m, preferably between 120 m and 550 m and particularly preferably between 150 m and 500 m. The particle size can be determined, e.g., by sieving according to DIN ISO 3310. Particularly preferred are particle shapes with good ra-tios of the largest dimension to the smallest dimension (at right angles to one another and for all directions in space) of 1:1 to 1:5 or 1:1 to 1:3, i.e., those that, for example, are not fibrous.

[0028] The refractory basic mold material preferably has a free-flowing state, especially to allow processing of the mold material mixture according to the invention in conventional core shooting machines.

[0029] As additional components, the mold material mixture according to the invention contains resols in a quantity of 1 to 10 wt.-%, preferably of 1 to 5 wt.-% and particularly preferably of 1 to 4 wt.-%, in each case based on the weight of the basic mold material.

[0030] Resols in the sense of the present invention are aromatics bonded over methylene groups (CH.sub.2) and/or over ether bridges (in particularCH.sub.2OCH.sub.2) each having at least oneOH group (hydroxyaromatic). Suitable hydroxyaromatics are phenols, substituted phenols, such as cresols or nonylphenol, 1,2-dihydroxybenzene (pyro-catechol), 1,2-dihydroxybenzene (resorcinol) or 1,4-dihydroxybenzene (hydroqui-none) or phenolic compounds such as bisphenol A, optionallyinsofar as non-phenols are also usedpreferably in a mixture with phenol.

[0031] The resols can be obtained, e.g., by condensation of one or more hydroxyaromatics with one or more aldehydes, in particular in the presence of a basic catalyst, such as ammonium hydroxide or an alkali metal hydroxide. Preferably alkali metal hydroxide catalysts are used.

[0032] Suitable aldehydes are formaldehyde, paraformaldehyde, butyraldehyde, glyoxal and mixtures thereof. Particularly preferred is formaldehyde or mixtures containing primarily (based on the molar quantity) formaldehyde.

[0033] The molar ratio of aldehyde (optionally as formaldehyde) to phenol in the resol resin can vary within the range of 1:1 to 3:1, but is preferably in the range of 1.6:1 to 2.5:1.

[0034] The production of resols is disclosed, e.g., in EP 0323096 B2 and EP 1228128 B1.

[0035] Preferred resols are those in which adjacent hydroxyaromatics are bonded at ortho and/or para positions (relative to the hydroxy group of the incorporated phenol/aromatics) over the methylene bridges and/or the ether bridges, i.e., most of the attach-ments are in para and/or ortho position.

[0036] Additional resol-based binders are described, for example, in U.S. Pat. No. 4,426,467, U.S. Pat. No. 4,474,904 and U.S. Pat. No. 4,468,359. In all three patents the resols are cured with esters, wherein in the first two the curing is performed by addition of a liquid curing agent, e.g., a lactone (U.S. Pat. No. 4,426,467) or of triacetin (U.S. Pat. No. 4,474,904), whereas in U.S. Pat. No. 4,468,359 the mold material mixture is gassed with a volatile ester, e.g., methyl formate, for curing the binder. These binders may also be combined amorphous SiO.sub.2, but the strength-increasing effect is not so pronounced as in the CO.sub.2-curable resols. Resols that cure with liquid esters do not necessarily require the addition of oxyanion. However, oxyanion are preferably used for curing with CO.sub.2.

[0037] The resols are used in the form of an aqueous alkaline solution and/or slurry, e.g., with a solids fraction of 30 to 75 wt.-%, in particular the pH is above 12 or even above 13. The viscosity of the aqueous alkaline solution at 25 C. is, e.g., 100 to 800 mPas, in particular 300 to 700 mPas. The viscosity is determined using a Brookfield rotational viscometer (spindle 21 and 50 rpm).

[0038] In the scope of this invention, oxygen-containing anions are called oxyanion. Suitable boron-containing oxyanion are in particular borates and/or aluminum-containing oxyanion e.g., aluminates. The boron-containing oxyanion can be used alone or in combination with aluminum-containing oxyanion. The latter is preferred.

[0039] The addition of the oxyanion during binder synthesis can take place directly in the form of their salts. The salts preferably contain alkali or alkaline earth metals as the cation, wherein in particular sodium and potassium salts are preferred. However, it is also possible to produce the oxyanion in situ. For example, aluminates form during the dissolution of aluminum compounds such as aluminum hydroxide. A solution of a boron compound such as boric acid in sodium hydroxide is a suitable solution of a boron-containing oxyanion. The alkali can be the solution of a base in water, and is likewise used for mixing with the resol.

[0040] The molar ratio of the oxyanion (expressed as B, Al etc.) to hydroxyaromatic group is preferably between 0.1:1 and 1:1 and when exclusively a boron-containing oxyanion is used, is particularly preferably between 0.3:1 and 0.6:1. In the case of a combination of a boron-containing and an aluminum-containing oxyanion, the Al:B atomic ratio is preferably varied within the range of 0.05:1 to 1:1. The particularly preferred range is between 0.1:1 and 0.8:1.

[0041] As the base (generally a constituent of the resol, e.g., from the production of the resol) alkali hydroxides such as sodium hydroxide and potassium hydroxide are preferably used. The molar ratio of hydroxide ions to hydroxyaromatic groups (such as phenol) in the binder system is preferably 0.5:1 to 3:1.

[0042] In addition to the previously mentioned components, the binder system contains water, preferably in a quantity of 25 to 50 wt.-% based on the weight of the composition. The water serves to dissolve the base and possibly the oxyanion.

[0043] In addition, the binder may contain up to 25 wt.-% additives such as alcohols, gly-cols, surfactants and silanes.

[0044] The binder is produced in that the resols are mixed with the base, the water and the oxyanion. It is possible first to mix the resol resin with an aqueous solution of the base and then to also mix in the oxyanion(s), e.g., as a solid or in the form of an aqueous solution. It is also possible first to mix the oxyanion with at least part of the base and at least part of the water, and to mix this mixture with the resol resin. Then optionally the remainder of the base, and optionally the remainder of the water as well as the conventional additives are mixed in.

[0045] Furthermore, the mold material mixtures according to the invention contain a portion of an amorphous SiO.sub.2. In particular, this is particulate amorphous SiO.sub.2. Synthetically produced particulate amorphous silicon dioxide is particularly preferred.

[0046] The amorphous SiO.sub.2 can in particular involve the following types: [0047] a) amorphous SiO.sub.2 obtained by precipitation from an alkali silicate solution, [0048] b) amorphous SiO.sub.2 obtained by flame hydrolysis of SiCl.sub.4, [0049] c) amorphous SiO.sub.2 obtained by reduction of quartz sand with coke or anthracite to silicon monoxide followed by oxidation to SiO.sub.2, [0050] d) amorphous SiO.sub.2 obtained from the process of thermal decomposition of ZrSiO.sub.4 to form ZrO.sub.2 and SiO.sub.2, [0051] e) amorphous SiO.sub.2 obtained by oxidation of metallic Si with an oxygen-containing gas, and/or [0052] f) amorphous SiO.sub.2 obtained by melting crystalline quartz with subsequent rapid cooling. [0053] c) includes both processes in which the amorphous SiO.sub.2 is deliberately produced as the main product and those in which it is obtained as a byproduct, such as in the production of silicon or ferrosilicon.

[0054] The amorphous SiO.sub.2 used may be either synthetically produced or naturally occurring silicas. The latter are known, for example, from DE 102007045649, but are not preferred, since as a rule these contain appreciable crystalline fractions and are therefore classified as carcinogenic. The term synthetic is applied to non-naturally occurring amorphous SiO.sub.2, i.e., its manufacture comprises a deliberately performed chemical reaction, such as that induced by a human being, e.g., the production of silica sols by ion exchange processes from alkali silicate solutions, precipitation from alkali silicate solutions, flame hydrolysis of silicon tetrachloride, and the reduction of quartz sand with coke in an electric arc furnace in the manufacturing of ferrosilicon and silicon. The amorphous SiO.sub.2 produced according to the last two methods mentioned is also called pyrogenic SiO.sub.2.

[0055] Occasionally synthetic amorphous silicon dioxide is defined exclusively as precipitated silica (CAS No. 112926-00-8) and SiO.sub.2 produced by flame hydrolysis (Pyrogenic Silica, Fumed Silica, CAS No. 112945-52-5), while the product formed during the manufacturing of ferrosilicon or silicon is merely called amorphous silicon dioxide (Silica Fume, Microsilica, CAS No. 69012-64-12). For the purposes of the present invention, the product formed during the production of ferrosilicon or silicon will also be defined as synthetic amorphous SiO.sub.2.

[0056] Preferably used are precipitated silicas and pyrogenic silicas, i.e., silicon dioxide produced by flame hydrolysis or in an electric arc. Particular preference is given to the use of amorphous silicon dioxide produced by thermal decomposition of ZrSiO.sub.4 (described in DE 102012020509) as well as SiO.sub.2 produced by oxidation of metallic Si with an oxygen-containing gas (described in DE 102012020510). Also preferred is powdered quartz glass (mostly amorphous silicon dioxide) produced from crystalline quartz by melting and rapid recooling, so that the particles are present in spherical rather than splintery form (described in DE 102012020511).

[0057] The mean primary particle size of the particulate amorphous silicon dioxide can be between 0.05 m and 10 m, in particular between 0.1 m and 5 m, particularly preferably between 0.1 m and 2 m. The primary particle size can, e.g., be determined by dynamic light scattering (e.g., Horiba LA 950) as well as by scanning elec-tron microscopy (SEM photography with, e.g., Nova NanoSEM 230 from the FEI company). Furthermore, the use of SEM photography makes it possible to see details of the primary particle shape down to the order of magnitude of 0.01 m. For the SEM measurements the silicon dioxide samples were dispersed in distilled water and then placed on an aluminum holder layered with copper tape before the water was evaporated.

[0058] In addition, the specific surface of the particulate amorphous silicon dioxide was determined using gas adsorption measurements (BET method) according to DIN 66131. The specific surface of the particulate amorphous SiO.sub.2 is between 1 and 200 m.sup.2/g, in particular between 1 and 50 m.sup.2/g, particularly preferably less than 17 m.sup.2/g or even less than 15 m.sup.2/g. Optionally the products may also be mixed, e.g., to sys-tematically obtain mixtures with certain particle size distributions.

[0059] The particulate amorphous SiO.sub.2 can contain different amounts of byproducts. For example, the following may be mentioned in this regard: [0060] carbon in the case of reduction of quartz sand with coke or anthracite, [0061] iron oxide and/or Si in the case of production of silicon or ferrosilicon, and/or [0062] ZrO.sub.2 in the case of thermal degradation of ZrSiO.sub.4 to form ZrO.sub.2 and SiO.sub.2.

[0063] Additional byproducts may be, e.g., Al.sub.2O.sub.3, P.sub.2O.sub.5, HfO.sub.2, TiO.sub.2, CaO, Na.sub.2O and K.sub.2O.

[0064] It is preferred that the amorphous SiO.sub.2 used for the invention have a SiO.sub.2 content of at least 70 wt.-%, preferably at least 80% and especially preferably at least 90 wt.-%.

[0065] The quantity of amorphous SiO.sub.2 added to the mold material mixture according to the invention is usually between 0.05 wt.-% and 3 wt.-%, preferably between 0.1 wt.-% and 2.5 wt.-% and particularly preferably between 0.1 wt.-% and 2 wt.-%, in each case based on the basic mold material.

[0066] The addition of the amorphous SiO.sub.2 to the basic mold material can take place in the form of an aqueous paste, as a slurry in water or as a dry powder. The latter is preferred.

[0067] The amorphous SiO.sub.2 is preferably present in particulate form. The particle size of the particulate amorphous silicon dioxide is advantageously less than 300 m, preferably less than 200 m and especially preferably less than 100 m and has, e.g., a mean primary particle size between 0.05 m and 10 m. The sieve residue of the particulate amorphous SiO.sub.2 in the case of passage through a sieve with a mesh size of 125 m (120 mesh) advantageously amounts to no more than 10 wt.-%, particularly preferably no more than 5 wt.-% and most particularly preferably no more than 2 wt.-%. Independently of this, the sieve residue on a sieve with a mesh size of 63 m amounts to less than 10 wt.-%, advantageously less than 8 wt.-%. The sieve residue is determined by the machine sieving method described in DIN 66165 (Part 2), in addition a chain ring is used as a sieving aid.

[0068] The particulate amorphous silicon dioxide advantageously used according to the present invention has a water content of less than 15 wt.-%, in particular less than 5 wt.-% and particularly preferably of less than 1 wt.-%.

[0069] The particulate amorphous SiO.sub.2 is advantageously used in the form of a powder (including dusts).

[0070] The order of addition of resol resins, binder and amorphous SiO.sub.2 to the basic mold material is not of decisive importance. It can take place either before or after or together with the binder. Preferably, however, first the amorphous SiO.sub.2 is added and then the binder. In any case, however, the binder must not be already cured before the amorphous SiO.sub.2 is added to the basic mold material.

[0071] The mold material mixture can additionally if desired contain other additives such as iron oxide(s), ground wood fibers or mineral additives.

[0072] The invention will be explained in further details based on the examples that follow, without being limited to these.

Examples

1. Preparation of the Mold Material Mixtures

[0073] 1.1 without Addition of Amorphous SiO.sub.2

[0074] Quartz sand was filled into the bowl of a Hobart mixer (Model HSM 10). Then under agitation the binder was added and mixed intensively with the sand for 1 min. The sand used, the type of binder and the respective quantities added are shown in Tab. 1. The quantities are given in parts by weight (PBW).

1.2. With Addition of Amorphous SiO.SUB.2

[0075] The method as given under 1.1 was followed, with the difference that first particulate amorphous SiO.sub.2 was mixed in for 1 min and then the binder addition took place. The types of amorphous SiO.sub.2 used and the quantities added are presented in Tab. 1.

TABLE-US-00001 TABLE 1 Quartz sand Amorphous H 32.sup.a) Binder .sup.b) SiO.sub.2 [PBW] [PBW] [PBW] 1.1 100 2.5 not according to invention 1.2 100 2.5 0.5.sup.c) according to invention 1.3 100 2.5 1.sup.c) according to invention 1.4 100 2.5 0.5.sup.d) according to invention 1.5 100 2.5 1.sup.d) according to invention .sup.a)Quarzwerke Frechen GmbH .sup.b) Novanol 240 (ASK Chemicals GmbH) .sup.c)Microsilica 971 U (Elkem AS; manufacturing: Production of silicon/ferrosilicon .sup.d)Microsilica POS B-W 90 LD (Possehl Erzkontor GmbH, manufacturing process: production of ZrO.sub.2 and SiO.sub.2 from ZrSiO.sub.4

2. Preparation of the Test Pieces

[0076] Part of a mold material mixture produced according to 1.1 and 1.2 was transferred to the storage chamber of a H 1 core shooting machine from Rper Gieereimaschi-nen GmbH, Viersen. The remainder of the mold material mixture was stored in a carefully closed vessel to protect it from drying before use for refilling the core shooting machine.

[0077] From the storage chamber of the core shooting machine, the mold material mixtures were delivered using compressed air (4 bar) into a molding tool provided with 2 engraved parts for producing rectangular box-shaped test pieces with dimensions of 15022.3622.36 mm (so-called Georg Fischer bar). For curing, 1 litre of CO.sub.2 was passed through the molding tool for 30 seconds.

[0078] Then the test pieces were removed from the tool and their strengths determined after preset times. The storage of the test pieces for strength determination was conducted in the laboratory at 23 C. and 50% rel. humidity or in a climate chamber from the Rubarth company at 23 C. and 98% rel. humidity.

3. Heating the Test Pieces

[0079] In each case 2 of the test pieces stored at 23 C./50% rel. humidity per mold material mixture, at 10 min. after their production. were held in a circulating air oven for 30 min at 150 C. After removal from the oven and cooling to room temperature (1 h) the strengths were determined.

4. Coating the Test Pieces

4.1. Water Coating (Refractory Mold Material Coating)

[0080] In each case 4 test pieces per mold material mixture stored at 23 C./50% rel. humidity, 10 min. after their production, were dipped for 3 sec. in the water coating, Miratec DC 3 (commercial product of ASK Chemicals GmbH) and placed on a rack in the laboratory to dry. After holding for 30 min., the strengths of 2 test pieces were determined. The other two test pieces were held in a circulating air oven at 150 C. for 30 min. for complete drying of the coating. After removal from the oven and cooling to room temperature (1 hr.) the strengths were determined.

4.2. Alcohol Coating (Refractory Mold Coating)

[0081] In each case 4 test pieces per mold material mixture, stored at 23 C./50% rel. humidity, 10 min. after their manufacturing were dipped for 3 sec. in the alcohol coating Velvacoat GH 701 (commercial product of ASK Chemicals GmbH) and placed on a rack to dry at 23 C./50% rel. humidity. The strengths of 2 test pieces each were determined after holding for 30 min. and 24 hr.

5. Strength Testing

[0082] The bending strengths were determined as a measure of the strengths of the test pieces. For this purpose the test pieces were placed in a Georg Fischer strength testing device, equipped with a 3-point bending device, and the force (in N/cm.sup.2) required for breaking the test pieces was measured.

[0083] The results are shown in Tab. 2.

TABLE-US-00002 TABLE 2 not according according to according to according to according to to invention invention invention invention invention 1.1 1.2 1.3 1.4 1.5 no coating, room temp. 30 sec. 70/70 80/80 80/90 80/90 90/90 0.5 h. 120/120 130/130 140/140 110/120 160/170 1 h. 110/120 130/150 140/150 160/180 170/180 2 h. 130/130 140/160 160/170 170/180 190/200 24 h. 160/170 170/180 180/200 190/200 220/240 24 h./98% rel. humidity 100/110 110/120 140/140 110/130 150/150 no coating, 30 min. 150 190/200 200/220 220/240 220/220 280/290 water coating 30 min. wet 100/100 120/120 130/140 120/130 150/160 30 min. wet/30 min. 150 150/150 190/200 210/220 200/210 250/270 alcohol coating 30 min. air drying 100/110 100/120 130/130 120/120 140/140 24 hr. air drying 150/160 160/170 170/190 180/190 190/200

Results:

[0084] It is apparent from Tab. 2 that the addition of amorphous SiO.sub.2 to the mold material mixtures has advantageous effects on the bending strengths of the test pieces. The effect of the amorphous SiO.sub.2 obtained by thermal decomposition of ZrSiO.sub.4 to ZrO.sub.2 and SiO.sub.2 (Ex. 1.4 and 1.5) is greater in these tests than that of the SiO.sub.2 obtained from the production of silicon/ferrosilicon (Ex. 1.2 and 1.3).