EP2014392B1 - Moulding material mixture, moulded blank for moulding purposes and method for producing a moulded blank - Google Patents

Description

The invention relates to a molding material mixture for foundry purposes, consisting of molding sand, caustic soda, alkali-silicate-based binders and aggregates and a molding for foundry purposes, prepared using the molding material mixture. The invention also relates to a method for producing a molded article.

Formstoffmischungen of the type mentioned are for example from the DE 102004042535 A1 (AS LÜNGEN GmbH) known. In this case, the binder used is an alkali water glass in combination with a particulate metal oxide, for example silica, alumina, titania or zinc oxide, in order to improve the strength of casting molds both immediately after shaping and curing and after storage and increased air humidity. The particle size of the metal oxides is preferably less than 300 microns, according to the examples, the sieve residue on a sieve with a mesh size of 63 microns less than 10% by weight, preferably less than 8% by weight.

Another method for producing molding mixtures which is intended to achieve high strength in combination with a polyphosphate or borate binder is disclosed in U.S. Pat US 5,641,015 described. In column 4, line 39 of the US patent, the release of water is mentioned as a result of the drying of polyphosphate and borate-containing binders, which is to be absorbed by the addition of ultrafine silica. The ultrafine silica consists of porous primary particles produced by a precipitation process with grain sizes between 10 and 60 nm, which agglomerate to secondary particles with a particle size of several μm (column 3, lines 64-66 of the US patent).

An inorganic binder system for molding materials is in the EP 1095719B1 described. According to the EP 1095719B1 For a binder based on alkali metal silicate with addition of sodium hydroxide, the flowability can be improved by adding 8-10% by weight of silicone oil with respect to the binder. This improvement was accompanied by a higher moisture content of the core sand.

• Hier ist an erster Stelle die Fließfähigkeit zu nennen, die als wichtiger Parameter für die Eignung des Formstoffes beim Verschießen in einer Kernschießmaschine bekannt ist.

In addition to the known measures for improving the strength, in particular the flexural strength of molded articles, further influencing variables which determine the quality of a molding material mixture must be taken into account:

• Here, the flowability is first mentioned, which is known as an important parameter for the suitability of the molding material when shooting in a core shooting machine.

Another important parameter is the progress of curing and the reduction of sensitivity to humidity.

The main quality feature, however, is the surface quality of the casting achievable with the molding material mixture. Unfortunately, the known processes are not sufficiently stable under the conditions prevailing in mass production, so that again and again high reject rates and unacceptable additional costs result from reworking. As a yardstick for the assessment of the surface quality, the determination of the area fraction of sand deposits on the cast part has proven itself.

It is therefore an object of the present invention to provide a novel molding material mixture for foundry purposes and a molded article which can be produced by a simple drying process, in which the abovementioned criteria, ie good flowability, high flexural strength and a high curing speed are achieved, and at the same time the surface quality, measured by the area fraction at Sandanhaftungen is significantly improved.

This object is achieved by the features specified in the claims.

It has been found that the use of an aggregate of amorphous, spherically-formed silica provides the desired benefits when the finely divided silica grains are used in two narrow grain spectra at approximately equal volumes in the form of a suspension, a crucial measure being uniformity of the suspension to distribute in the molding material mixture and to achieve a specifically formed substructure by the subsequent drying.

The measures of distribution and drying are shown in the method claims, with even more measures as preferred method steps from the dependent claims can be removed. In particular, care must be taken that no agglomeration of the finely divided particles occurs during mixing, but rather that a uniform distribution of the particles takes place in the respective grain classification. Fluid mixers, and especially the wing mixers in continuous operation, have proved their worth.

In the formation of the substructure, the drying process has an outstanding influence on the formation of the roughness on the surfaces of the moldings. In particular, it is necessary to influence the distribution of the mountain and valley structure in such a way that a relief structure arises which has a height-depth difference ratio of at most 300 nm. As a drying process, both thermal drying and microwave drying into consideration, with very good storage capabilities, especially without microwave drying were achieved even under extreme storage conditions at humidities above 78% and storage temperatures of more than 33 ° C.

During drying, the binder layer present on the particles in the molding material mixture shrinks to form a substructure of mountains and valleys. By successive shrinking and residual shrinkage, a morphology of the substructure is formed, which is characterized by a height-depth difference of max. 300 nm, caused by cracking during the two-stage shrinking process. In the physical drying used in the first stage, for example by microwave, energy is introduced directly into the moist binder shell. The resulting hardening of the binder cover (surface) leads by the subsequent thermal drying to crack formation in the nano range (substructure).

In the following examples, the invention is described in comparison to other molding material mixtures and molded articles produced therefrom. For standardization, the same basic mixtures of Haltern foundry sand with a mean grain size of 0.32 mm were used. Grain size determination was carried out according to Brunhuber, 16th edition, page 400. The suspension according to the invention containing 25% by volume of NanoSiO ₂ and 25% by volume of MikroSiO ₂ and 50% by volume of water was used as the additive.

The fluidity is given as GF-flowability, the determination was carried out according to Brunhuber, 16th edition, page 352/353.

As test specimens standard test specimens measuring 22.5 × 22.5 × 180 mm were produced and subjected to the respective experimental conditions.

In summary, the improvements of the molding compound mixtures assembled according to the invention in terms of flowability and reduced wettability with respect to liquid aluminum result convincingly. Since liquid aluminum has strongly wetting properties in the casting process compared with silicon dioxide and in particular tends to completely wet SiO.sub.2 particles and penetrate the interspaces, it was highly surprising that with the mold set according to the invention only small sand-adhering surface areas of less than 10% were achieved.

In combination with an alkali water glass binder, which is uniformly distributed on the molding sand particles, a quartz sand-based molding material mixture could be produced, which far exceeds the properties of the known products in terms of flowability, flexural strength and curing, if used as aggregate the two particle size classifications mentioned in claim 1 become.

In the adjusted molding material mixture, the micrometer-sized, amorphous SiO ₂ spheres are intended to allow the individual molding sand grains to be spaced apart from each other and to allow them to slide off relative to one another in a facilitated manner. This "skating effect" was confirmed by flowability measurements, for example by the drastically sinking stirring resistance during the introduction of the suspension composed of two different grain classifications according to the invention in a wing mixer. The power consumption of the wing mixer decreased by more than 50%, while the effect without surcharge was below 10%, based on the current consumption before addition.

For the mixing process, the dosing order of the individual components and their mixing time must be taken into account. The dosing order is: 1. The quartz sand is mixed with caustic soda. 2. An alkali silicate binder is added. 3. The addition of suspension according to the invention with NanoSiO ₂ and MikroSiO ₂ plus water is added to the basic mixture.

The mixing time depends on the type of mixing unit used and should be determined experimentally. In this case, the minimum required duration for the mixture is the respectively desired state (homogenization / uniform distribution).

EXAMPLES

• In the experiments, Halterner foundry sand was used as a basic mixture. The experimental procedure is explained below by comparison with a traditional binder system:

a) Improvement of fluidity

1. 1. die Grundmischung ohne erfindungsgemäße Suspension, nachfolgend auch als Additiv C bezeichnet,
2. 2. die Grundmischung mit Suspension, welche sich zusammensetzt aus einer Suspension aus 25% NanoSiO₂ 25% MikroSiO₂ und 50% Wasser, und
3. 3. die Grundmischung mit der äquivalenten Wassermenge und Formsand aus der Suspension.

To illustrate the improved flowability, through the combined addition of NanoSiO ₂ (0.01-0.05 μm) and MicroSiO ₂ (1-5 μm), the following test results were compared.

1. 1. the base mixture without suspension according to the invention, hereinafter also referred to as additive C,
2. 2. the masterbatch with suspension, which is composed of a suspension of 25% NanoSiO ₂ 25% MikroSiO ₂ and 50% water, and
3. 3. the basic mixture with the equivalent amount of water and molding sand from the suspension.

The term "masterbatch" indicates a mixture of foundry sand, NaOH and alkali silicate binder in varying composition.
1. Basic mixture of a classic binder system
Halterner molding sand, determined according to Brunhuber p. 400 NaOH 0.20% Alkali silicate binder 1.80% GF flowability 73% additive: - GF flowability determined according to Brunhuber p. 352/353; F = [(h ₁ -h) / (h ₁ -h ₂ )] * 100%
2nd basic mix + suspension NaOH 0.20% Alkali silicate binder 1.80% GF flowability 87% Additive C * 1.00% (Additive C: Suspension of 25% NanoSiO ₂ , 25% MicroSiO _2, and 50% water, with the NanoSiO ₂ spheres having a mean diameter of 0.03 μm and the MicroSiO ₂ spheres having an average diameter of 3 μm.)
3. Basic mixture and equivalent amount of water and molding sand from the suspension NaOH 0.20% Alkali silicate binder 1.80% GF flowability 73% Water + molding sand 0.50%

FIG. 1 graphically reproduces the listed results. The comparison of the test results clearly shows that the suspension causes an improvement in flowability. In addition, it becomes clear that the addition of the equivalent amount of water from the suspension has no influence on the flowability.

For comparison with known processes, molding material mixtures as described in DE '535 of AS Luengen and in EP' 719 were prepared with the same base mixture and investigated as described above. The results are in Fig. 7 graphically reproduced, wherein the comparative examples according to Fig. 6 were selected. Mix of basic mix flowability Binding system according to EP'719 73% Molding material mixture according to DE '535 80% Basic mixture + additive C 87%

FIG. 7 illustrates that the flowability (after GF) of the core sand increases by the inventive addition of present in 2 grain classifications SiO ₂ balls. The microSiO ₂ spheres are kept at a distance by the NanoSiO ₂ spheres and allow the so-called "roller skate effect", ie a rolling of the sand grains through the MikroSiO ₂ spheres arranged between them

b) Increasing the bending strength:

1st basic mix NaOH 0.20% Alkali silicate binder 1.40% additive: - flexural strength Removal strength: 289 N / cm ² Core storage time 1 h: 284 N / cm ² Core storage time 3h: 281 N / cm ² Core storage time 24h: 287 N / cm ² 2. Base Mix + Additive C NaOH 0.20% Alkali silicate binder 1.40% Additive C * 1.00% (Additive C: Suspension of 25% NanoSiO ₂ , 25% Micro-SiO ₂ and 50% Water) flexural strength Removal strength: 475 N / cm ² Core storage time 1 h: 483 N / cm ² Core storage time 3h: 476 N / cm ² Core storage time 24h: 475 N / cm ²

The determined bending strengths are in Fig. 2 graphically illustrated. The comparison of the flexural strength of a core sand base mixture without additive C and the flexural strength of a core sand base mixture containing the additive C (suspension of 25% NanoSiO ₂ + 25% microSiO ₂ and 50% water) clearly shows that with a surcharge according to the invention a 3 increased flexural strength.

c) Increasing the cure rate:

1st basic mix NaOH 0.20% Alkali silicate binder 1.40% additive: - extraction strength extraction strength extraction strength 1st test bar (after 25 sec) 64 N / cm ² 65 N / cm ² 65 N / cm ² 2nd test bar (after 50 sec) 62 N / cm ² 65 N / cm ² 64 N / cm ² 3. Test bar (after 75 sec) 63 N / cm ² 64 N / cm ² 65 N / cm ² 2. Base Mix + Additive C NaOH 0.20% AWB-AL binder 1.40% Additive C * 1.00% (Additive C: suspension of 25% NanoSiO ₂ , 25 microSiO ₂ and 50% water) extraction strength extraction strength extraction strength 1st test bar (after 25 sec) 81 N / cm ² 84 N / cm ² 80 N / cm ² 2nd test bar (after 50 sec) 95 N / cm ² 92 N / cm ² 95 N / cm ² 3. Test bar (after 75 sec) 109 N / cm ² 102 N / cm ² 105 N / cm ²

The results of the experiments are in FIG. 3 shown graphically. Due to the present experimental set-up, it is possible that the three simultaneously produced test bars can only be tested individually and at intervals of about 25 seconds.

In determining the flexural strength of the masterbatch, this time difference also does not matter, i. the strength of all three test bars are approximately equal.

On the other hand, if the test bars provided with the additive C are examined, it is found that the bending strength increases steadily during the test procedure (from the first to the second test bar).

d) Reduction of the sensitivity to air humidity:

1st basic mix NaOH 0.20% Alkali silicate binder 2.40% Silicone oil: 0.10% masterbatch Core storage time [h] (storage in a humidity cabinet) Flexural strength with microwave drying Bending strength without microwave drying 0 289 N / cm ² 57 N / cm ² 1 240 N / cm ² 86 N / cm ² 3 200 N / cm ² 50 N / cm ² 24 25 N / cm ² 22 N / cm ² 2. Base Mix + Additive C NaOH 0.20% Alkali silicate binder 1.40% Additive C * 1.00% (Additive C: suspension of 25% NanoSiO ₂ , 25% MicroSiO ₂ and 50% water) Basic mixture with additive C Core storage time [h] (storage in a humidity cabinet) Bending strength with microwave drying Bending strength without microwave drying 0 475 N / cm ² 87 N / cm ² 1 409 N / cm ² 106 N / cm ² 3 303 N / cm ² 73 N / cm ² 24 85 N / cm ² 87 N / cm ²

The results of the experiments are in the FIGS. 4 and 5 graphically illustrated. To assess the shelf life of the cores even under extreme conditions (air humidity 78%, temperature 33 ° C), the cores were stored in a humidity cabinet.

In the FIGS. 4 and 5 the evaluation is shown, which shows that the additive C has a positive effect on the storage life.

Especially if the cores were not dried in the microwave ( Fig. 5 ) this effect comes to bear.

e) Surface comparison of several castings with regard to sand buildup

• Zur Bestimmung der Qualität von Gussstückoberflächen wurden wannenförmige Kerne mit den Abmessungen 150 mm x 80 mm verwendet werden. Dieser Kern wird aus dem zu prüfenden Formstoff in einem Laborflügelmischer der Firma Vogel und Schemann AG gemischt. Dazu wurde zunächst der Quarzsand vorgelegt und unter Rühren erstens NaOH und das Wasserglas als nächstes zugegeben. Nachdem die Mischung für 1 Minute gerührt worden war, wurde das amorphe Siliziumdioxid (erfindungsgemäße Beispiele) bzw. für die Vergleichsbeispiele Polyphosphatlösung (gemäß US 5,641,015 oder amorphes SiO₂ in Kugelform, gemäß DE '535) unter weiteren Rühren zugegeben. Die Mischung wurde anschließend noch eine weitere Minute gerührt.

Explanations to Fig. 6 :

• Trough-shaped cores measuring 150 mm x 80 mm were used to determine the quality of casting surfaces. This core is mixed from the test material to be tested in a laboratory wing mixer Vogel and Schemann AG. For this purpose, the quartz sand was initially introduced and, with stirring, firstly NaOH and the waterglass were added next. After the mixture was stirred for 1 minute, the amorphous silica (Examples of the present invention) and for the comparative examples, polyphosphate solution (according to US 5,641,015 or amorphous SiO ₂ in spherical form, according to DE '535) with further stirring. The mixture was then stirred for a further minute.

The molding material mixtures were transferred into the storage bunker of a hot box core shooter of the company Rölperwerk foundry machines, the mold was heated to 180 ° C. The molding material mixtures were introduced into the mold by means of compressed air (5 bar) and remained in the mold for a further 35 seconds. The mold was opened and the molding was removed. In order to achieve the maximum strength, the molding is post-dried in the microwave. Subsequently, the casting was poured off in open hand casting.

After cooling the casting, the molding was removed and the casting surface was evaluated for the type and amount of sand buildup.

casting parameters:
Dimensions casting: 150 x 80 x 40 mm
Weight casting: 900 g
Alloy used: AlSi 7 mg
Casting temperature: 740 ° C
Static casting height: 200 mm
Measured sand deposits in area percent related to the respective surface mixture Surface with sand deposits Basic mixture without surcharge 75% Basic mixture with polyphosphate and borate content 60%
(US '015) Base mixture with glass beads of 100 to 200 μm thickness according to Table 5 No. 3.7 of AS Lüngen DE 102004042535 25%
(DE'535) Inventive base mixture with spread grain spectrum <10% (invention)
according to example a) 2

FIG. 8 illustrates the molding used to make the casting used here. The percentages of the sand adhesions relate to the outer surface in the region of the bulged casting region R, which is formed as a continuously curved bulge R in the molding.

FIG. 6 gives the test results graphically. With the molding material mixture according to the invention, a significantly improved casting surface is achieved in comparison to the base mixture according to Ex. A) 1, according to US '015 (amorphous SiO ₂ spheres constructed from nanoparticles) and according to DE' 535 (amorphous, synthetic silicic acid in spherical form).

Claims (7)

1. A molding material mixture for foundry purposes containing mold sand, sodium hydroxide aqueous solution, an alkali-silicate binding agent and additives, characterised in that said mold sand particles having a grain size of 0.1 to 1mm; said molding material mixture comprising 0.1 to 10 percent by weight of a sodium hydroxide aqueous solution in relation to the weight of the sand, wherein said sodium hydroxide aqueous solution comprises a concentration of 20 to 40 percent by weight of sodium hydroxide; said molding material mixture comprising 0.1 to 5 percent by weight of an alkali-silicate binding agent with a solid fraction of 20 to 70 %; said molding material mixture comprising 0.1 to 3 percent by weight of an additive suspension with a solid matter percentage of 30 to 70 % of amorphous, spherical SiO₂, in two grain size classifications in the suspension with a first grain size classification A containing SiO₂ particles with a grain size ranging between 1 and 5 micrometers and with a second grain size classification B containing SiO₂ particles with a grain size ranging between 0.01 and 0.05 micrometers, and wherein, for the volume percentages of the two grain size ranged A, B, the following distribution rule applies: 0.8 to 1.0-1.2 to 1.
2. A molded part for foundry purposes, produced from a molding material mixture according to claim 1, wherein the surface of the individual mold sand grain in the molded part comprises a primary structure out of SiO₂ particles with a grain size ranging between 1 and 5 micrometers, the micrometer-sized amorphous SiO₂ spheres space the individual quartz sand particles from one another; and wherein the molded part comprises a substructure of SiO₂ particles with a grain size ranging between 0.01 and 0.05 micrometers which are distributed in a binding agent layer which is 0.5 to 2 micrometers thick and is uniformly distributed on mold sand grains, the nanometer-sized, amorphous SiO₂ spheres form adjoining peaks and valleys of up to 300 nanometers of height/depth.
3. A method of producing a molded part according to claim 2, characterised in that the molding sand is provided, mixed with the sodium hydroxide aqueous solution and laced with the binding agent based on alkali silicate, with the binding agent then being uniformly and homogeneously distributed over all the mold sand grains in the form of a binding agent envelope, that, into the binding agent envelope there is fed a mixture of SiO₂ particles with two grain size classifications and that the molding material mixture is dried to form a molded part, wherein the binding agent envelope shrinking during the drying process, forming a roughness structure with a maximum height differential of 300 nanometers.
4. The process according to claim 3, wherein 0.10 to 0.30% of sodium hydroxide aqueous solution is mixed with said mold sand, then 1 to 4% of binding agent on alkali silicate basis is added and the binding agent is uniformly and homogenously distributed over the mold sand grains in the form of a binding agent envelope with a thickness of 0.5 to 2 micrometers.
5. The process according to any of the preceding claims 3 or 4, wherein, during the drying process, the binding agent envelope shrinks by 50 to 70 percent by volume.
6. The process according to any of claims 3 to 5, wherein the drying process is a physical one, and wherein the binding agent envelope is pre-shrunk by 40 to 60 percent by volume and wherein the remaining shrinking process takes place thermally.
7. The process according to any of claims 3 to 6, wherein the drying process takes place in a microwave oven.

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