EP1832357B1 - Mould or blank, casting moulding material mix and method for its manufacture - Google Patents

Abstract

The mold uses quartz moulding sand and an aluminium oxide binder. The quartz sand has a particle size range of 0.05 to 5 mm and the aluminium oxide has a particle size of 1 to 200 mu m. The binder is arranged as a coating layer over the surface of the sand and contains an aqueous glass phase which draws together along the contacting faces of the quartz particles in knuckle manner and has a micro-porous structure in the boundary phases. Foundry moulding mixture is used to produce mould or moulding blank with additions of broken or ground emulsion-free aluminium oxide and an aqueous glass based binder. Independent claim describes method for manufacturing mould by mixing binder-oxide mixture with moulding sand and casting it under pressure in moulding box and drying liquid binder to produce micro-porous binder bridges between individual quartz sand particles.

Description

The invention relates to molds or foundries for foundry purposes according to the preamble of patent claim 1, a foundry molding material mixture according to the preamble of claim 4 and a method for producing a mold or a molding according to the preamble of claim. 7

Foundry-molding mixtures are known in many forms. Basically, a distinction must be made between clay-bound foundry sand, sand mixtures with inorganic or organic binders, and binderless molding sands with physical bonding. The requirements of the foundry-molding mixtures are very diverse and include properties such as flowability of the molding material mixture, solidification behavior, achievable ultimate strength, separability or mold release.

In the WO 2006/024540 describes a molding material mixture for the production of casting molds for metal processing, in which a free-flowing, refractory molding base material and a water glass-based binder is used. To the binder may be added a particulate metal oxide selected from a group consisting of silica, alumina, titania or zinc oxide, most preferably synthetic amorphous silica. Molded materials of spherical shape and / or organic additives are used to improve the surface quality of the casting. The strength of the molded article in a moist environment should be improved by intensively combining the surface of the particulate metal oxide with a strongly alkaline water glass phase in the binder. Since the binder has a deteriorated flowability, it should be achieved by addition of platelet-shaped lubricants that even complex shapes can be produced.

The particle size of the metal oxide is less than 200 micrometers in the known molding material mixture, wherein the proportion of metal oxide, based on the amount of binder is preferably between 4 and 40%. When using quartz sand as a molding material is preferably less than 5% binder contained in the known molding material mixture.

Object of the present invention

The invention has for its object to provide a molding material mixture having an improved flowability, wherein the solidification behavior and the mold release of the mold or molded article is improved. The castings produced with the molding or the molding material mixture according to the invention should allow an improved quality of the surface of the cast part.

Furthermore, the molding material mixture should have good disintegration properties after the intended use and the spent molding sand can be easily processed with low emission.

This object is achieved by a mold or a molding according to claim 1, a foundry molding material mixture according to claim 4 or a process for producing a mold or a molding according to claim 7.

Surprisingly, it has been found that aluminum oxide is suitable as a supplement to a foundry-molding material mixture if, after mixing and drying, it covers the sand or quartz particles of the molding material as an opaque layer. For this purpose, the alumina in a certain amount based on the binder and in a certain particle size based on the average diameter of the sand or quartz grains to use.

During processing, it was surprising that alumina significantly improves the flowability and the solidification behavior of the molding material mixture. This will be explained in more detail by means of subsequent comparative experiments. Furthermore, it was surprising that the molding / core or the mold had particularly good disintegration properties after their intended use. For example, it has been observed that the core contacted with water immediately disintegrates and can be fully processed as a homogeneous suspension.

Particular attention is paid to a molding material mixture after molding in foundries produced molds or moldings for foundry purposes. It was observed that the quality of the castings, in particular their surfaces, could be substantially improved with the addition according to the invention. On the basis of detailed investigations, the inventors assume that the improvement of the surfaces is caused, on the one hand, by a better impression behavior and, on the other hand, by a better mold release due to low adhesions of foundry sand etc. on the casting surface.

The inventors have then specifically investigated the conditions during casting in the contact area between molding sand and metal surface. It has been found that the wettability of the molding surface with the liquid metal plays a role on the one hand in the described processes, on the other hand, however opposite effect was observed during demolding or reprocessing of the molding sand. Thus, for a faithful reproduction, good wetting conditions are important; on the other hand, this sometimes leads to problems during demoulding, since with the removal of the casting, parts of the mold or of the molded article are also entrained in the form of fine sand particles. In particular, in a poor wetting behavior of the molding sand already in the state of mixing with the partially liquid binder adverse effects that led to poor reusability of the spent molding material mixture (segregation, inhomogeneities, etc.).

It was therefore surprising that with the addition of aluminum oxide under the mentioned problematic boundary conditions, both the flowability, the solidification behavior and the releasability of the molding together with a substantial improvement of the casting surface could be achieved. This occurred against the background of a complex physical, chemical and thermodynamic interdependence of the substances involved and process steps.

In the following the invention will be explained in more detail with reference to several embodiments.

In the preparation of the molding material mixture, an aluminum oxide with 99.9% purity was first added directly to a molding material mixture as an oxide and homogeneously distributed. It was found that homogenous distributions in common molding sands with mean particle sizes between 75 and 250 micrometers could only be obtained by repeated and lengthy mixing processes. In order to be able to effectively use above all established systems, which are designed for the use of liquid binders in the form of resins, alcohols, oils or inorganic suspensions, the oxide was first added to the binder, homogeneously dispersed and then added to the molding material via established methods. It has been found that aluminum oxides having a particle size of 1 to 200 micrometers can be dispersed in a liquid binder for the same short time and then introduced, with a consistently good homogeneity being achieved. In spite of the relatively high density (about 4 g / cm ³ ) of the Al ₂ O ₃ particles, the Al ₂ O ₃ binder dispersions showed no tendency to separate for several days. When using Al ₂ O ₃ particles with a particle size of over 200 microns unstable dispersions were obtained. For Al ₂ O ₃ particles with less than 1 micron mean grain size, the viscosity of the dispersion increased significantly, which made subsequent distribution of the dispersion in the molding sand more difficult and longer reaction times necessary. The Al ₂ O ₃ concentration was between 10% and 85% (by weight, all subsequent concentrations as well).

With the use of quartz sand as foundry sand homogeneous mixtures with liquid binders could always be produced in a mean particle size range of 0.05 to 5 mm. At grain sizes above 5 mm, the liquid binder was able to drain easily through the grain interspaces and allowed settling movements in the quartz sand, which led to inhomogeneities in the mixture. For grain sizes below 0.05 mm, both the amount of binder and the stirring and stirring time during mixing had to be increased significantly in order to overcome an abruptly increasing cohesive force between the grains of sand and to obtain a homogeneous mixture.

With molding material mixtures with quartz sand as molding sand with a mean particle size range of 0.05 mm to 5 mm and a liquid binder dispersion with Al ₂ O ₃ particles with 1 to 200 microns average particle size, a very good, consistent homogeneity could be achieved. The very good homogeneity was characterized by a complete distribution of the binder dispersion on the quartz grains, the quartz grains being covered by binder dispersion and being spaced apart by the Al ₂ O ₃ particles, while connected, free grain interspaces ensured the gas permeability necessary for the drying.

flowability

Flowability relates to the flow behavior of the molding material mixture while it is being filled into the mold. It is influenced by the cohesion of the molding material mixture components with each other and the adhesion of the molding material to the wall of the mold. Particularly in the field of dry molding mixtures, in which the ratio of foundry sand to binders and aggregates can be in the range of 3 to 1 - 2, the properties of the aggregates are clearly evident.

In order to determine the influence of the alumina aggregate, different mixtures of foundry sand and aluminum oxide were homogenized in a stirred mixer.
The mean grain size of the molding sand was 0.32 mm; the size of the alumina particles was 1.5-2.5 microns; also in the following experiments. Subsequently, the mixture was compacted in a cylindrical, vertically extended form. The upright mold was then pulled vertically upward with constant force, while a stationary die fixed the compacted mixture in place so that the mold was pulled upwards from the mix. The time t1 was determined, which was needed to completely remove the cylinder. Further, the time t2 at which the mixture broke up the cylindrical shape by its own weight and crumbled into a cone was determined. Finally, the inclination angle alpha of the conical flanks of the resulting cone was determined. Table 1: Tests for flowability I) each 1.5 kg of dry molding sand With 0% oxide With 1% oxide With 5% oxide With 10% oxide With 40% oxide t1 = 4 s t1 = 3.9 s t1 = 4.1 s t1 = 4 s t1 = 3.9 s t2 = 3.5 s t2 = 3.5 s t2 = 3.4 s t2 = 3.3 s t2 = 3.1 s Alpha = 115 Alpha = 118 Alpha = 117 Alpha = 121 Alpha = 141

Dry sand trials showed breakage of the mold during withdrawal of the cylinder. The cylinder then accelerated upward without resistance and triggered the timing t1. The alumina causes at a high content of the total mixture earlier breaking of the molding and a flatter angle of the cone flanks. Table 2: Flowability tests II) each 1.3 kg wet molding sand With 0% oxide With 1% oxide With 5% oxide With 10% oxide With 40% oxide t1 = 7 s t1 = 6.9 s t1 = 7 s t1 = 6.8 s t1 = 6.7 s t2 = 6.1 s t2 = 6,2s t2 = 6s t2 = 5.8S t2 = 5,6s Alpha = 91 Alpha = 94 Alpha = 96 Alpha = 95 Alpha = 101

Due to the moisture, the cohesion among the particles of the molding material mixture is greater and it comes only later to break up of the molding. The influence of the higher proportion of aluminum oxide is slightly lower. The fluidity of the sand is good for all mixtures. Table 3: Flowability tests III) 1.4 kg of wet molding sand + water glass binder (10%) in each case With 0% oxide With 1% oxide With 5% oxide With 10% oxide With 40% oxide t1 = 8.1 s t1 = 8 s t1 = 7.9 s t1 = 7.6 s t1 = 7.2 s t2 = 7 s t2 = 7 s t2 = 6.9 s t2 = 6.5 s t2 = 6.3 s Alpha = 93 Alpha = 97 Alpha = 96 Alpha = 98 Alpha = 107

The additionally added water glass binder enhances the cohesive forces between the particles of the molding material mixture. The breaking up of the molding occurred at comparable height of exposed form. This means that the significantly lower value for t1 and t2 can be explained by a higher take-off speed and a reduced adhesion to the mold wall with an aluminum oxide content of 40%.

The increase in pull-off speed with increasing alumina content and the flatter angles of the cone flanks indicate reduced interaction with the mold wall and better flowability. This was further investigated in the tests for solidification and separability.

Hardening behavior and separability

The solidification behavior describes the ability of a molding mixture to fill a mold while arranging its particles in the closest possible way. Separability or releasability relates to the interactions between the molding material mixture and the mold. If too strong adhesion forces occur, parts of the molding can adhere to the mold during demoulding and break out of the molding.

For verification, a foundry sand binder mixture with a binder content of 2.5% and a varied alumina content of 10% and 40% and 80% in the binder (percent by weight based on the binder) was injected via a core shooter into a mold until sufficient green strength pre-dried and removed. After examination of the green compacts for defects, these microwaves were completely dried ready-to-use moldings and finally examined.

In the form of a bar-shaped test specimens whose one side is smooth and the other side has profiles and undercuts with increasing fineness. Each 10 molds were made. The relative density was calculated after complete drying considering the different density of the alumina and the sand. Table 4: Tests for solidification behavior and separability with 10 moldings before and after final drying 0% oxide in the binder 10% oxide in the binder 40% oxide in the binder 80% oxide in the binder 1 mm profiles Yes; every 10th Yes; every 10th Yes; every 10th Yes; every 10th 0.5 mm profiles partly: 6 Yes; every 10th partly: 9 partly: 9 0.1 mm profiles partly: 2 partly: 3 partly: 3 partly: 4 Micrometer profiles No No No No dry density default equal equal equal Further errors (dry) Yes: 2 in 0.1 mm profile No No No

The mixtures with aluminum oxide additive show a consistently good hardening behavior. All test specimens have the same packing density.

The profile fidelity of the moldings of the molding material mixture with aluminum oxide additive is clearly superior to the mixture without additive in the area of submillimeter-sized profiles. This proves the better flow properties of a molding material mixture with aluminum oxide addition already indicated in the tests for flowability.

The blend without additive shows the occurrence of defects in the fine profiling during drying, while this is not the case with the mixtures according to the invention. In the drying behavior of the mixture according to the invention shows a better resistance of the molding material mixture against thermal effects.

To test the improved resistance to thermal effects, molding mixtures according to the invention based on quartz sand were prepared according to Table 3 and dried under different conditions and tested for their profile fidelity. The improved resistance to thermal effects was confirmed. The inventors assume that the free spaces in the molding material mixture, due to the Al ₂ O ₃ particles which space the quartz sand grains apart, allow the unhindered transfer of the solvent into the gas phase during drying. However, a dried in a drying oven at 60 ° C within 48 hours molding had too high final weight and showed in the microscopic examination bulky, coated with a glassy layer throughout binder bridges and had clearly set during drying. An explosively dried molded article exposed to a temperature of 80 ° C. directly after firing with 5000 watt microwave power showed on microscopic examination a foam-like solidified water glass phase around the Al ₂ O ₃ particles and had expanded beyond the hollow shape. From the observations of extremely slow and explosive drying, the inventors recognized that drying at medium speed proceeds well over rough evaporation sites. Especially on freshly generated fracture surfaces and / or breaklines drying apparently takes place preferably on the Al ₂ O ₃ particles, wherein the resulting gas is passed through spaces between the particles in the free inter-granular spaces and is led out through the free inter-granular spaces from the molding. Therefore, according to the invention, this process is directed so that the Al ₂ O ₃ particles are held as a porous, closed area, packed layer over the binder on the single quartz sand grain. In the area of the grain gussets porous binder bridges are formed, which connect the quartz sand grains together.

Examinations of the ready-to-use, dried moldings of the mixtures according to Table 4 with the aid of an optical microscope showed that the individual sand grains are completely enveloped by a covering layer of aluminum oxide particles and in their grain morphology by mountains or valleys of approximately half the grain size of the aluminum oxide particles were marked. At particularly high concentrations of oxide in the binder, thicker layers of oxide particles could be observed.

A test series with varied oxide content in the binder showed that with oxide contents from 10%, oxide particle coverage could be observed, while at 80% to 90%, the increasing concentration in the total mixture resulted in more and more particle-coated sand grains. Preference was given to working with contents of 40% to 60%, particularly preferably 50% of oxide.

As a result, the experiments showed that the addition of alumina to a foundry sand is associated with a surprising improvement in its flow properties and increased resistance of the molding mixture to thermal effects.

Use as foundry-molding material mixture

The resulting moldings were now tested as fine-profiled cores in a casting process with liquid aluminum. Aluminum was used because here in terms of Availability the greatest doubt existed. Aluminum and aluminum oxide have long been used as composites in combination. Therefore, it is expected that load-bearing adhesive bridges can be formed between the oxide particles and the liquid metal, which can lead to a casting surface contaminated with oxide particles.

However, the castings produced showed a significantly smoother surface compared to the standard castings after demoulding. The number of average adherent grains per square centimeter dropped from 47 to 49 to 0.4 to 0.5. In addition, the adhesion force of the individual grains to the metal surface was extremely low, so that cleaning could be carried out by means of compressed air or ultrasound instead of the usual sandblasting. This opens up the possibility of the final cleaning of the castings with methods such. B. ultrasonic baths or compressed air, which are significantly cheaper and faster compared to conventional sandblasting. In addition, the fine profiles were formed exactly in the manner specified in the molding.

The aforementioned effect can be used in particular in machine core production in conjunction with complicated castings. Thus, for example, oil-water channels with undercuts in the casting of automotive internal combustion engines can now be produced with a particularly smooth surface. A post-treatment e.g. by blasting the castings is no longer necessary.

There was an additional effect of coring the castings: while the castings produced in the usual way had to be shaken and turned for about 40 seconds in a frequency-regulated vibration eliminator in order to achieve complete gutting, the cored oxide castings already had coring Completed after 10 seconds. A microscopic examination of the pitted sand showed microporous binder bridges in the area of the grain gussets, which can be more easily loosened or broken at low-frequency vibration. The quadruple accelerated gutting was repeatable for each test specimen.

A review of the improved decorability with variation of binder composition and Al ₂ O ₃ particle size initially showed that the improved decorability always occurred in common with the previously described binder bridges. If the ratio according to the invention of the mean grain size of the foundry sand to the average grain size of the Al ₂ O ₃ particles was exceeded or undercut, then the Entkernbarkeit deteriorated, and the binder bridges showed a much more compact or significantly more porous broken structure. The inventors assume that binder bridges between the quartz grains, characterized by a framework of Al ₂ O ₃ particles, binders as adhesion-promoting phase and pores along the Al ₂ O ₃ particle interstices, represent optimal predetermined breaking points in the molding according to the invention Provide vibration after casting the improved decorability.

By addition of water, the barking out time for binders based on water glass was additionally lowered. This can be explained by an additional weakening of the adhesion-promoting phase of the binder bridges by dissolving with water. When the Al ₂ O ₃ particle size was reduced to less than 100 μm by grinding of coarser Al ₂ O ₃ particles, the coring time was found to be 20% lower compared to industrial products of the same particle size distribution. The improved barking out time for freshly ground Al ₂ O ₃ particles is attributed to a reduced adhesive force of the binder on the fresh fracture surfaces of the particles and a more easily detachable framework from the irregularly comminuted particles:

With a maximum Al ₂ O ₃ particle size of 2.5 μm, a gradual lowering of the coring out time was found in the case of aqueous ginning with binders based on waterglass. The core contacted with water disintegrated immediately and completely and could be further processed as a homogeneous suspension. Microscopic examination of the binder bridge structure showed that the bridging bridges between the Al ₂ O ₃ particles had pores of 0.1 μm to a maximum of 2.5 μm. The inventors assume that these micropores have such a high capillarity that the added water is absorbed and distributed in the binder bridges in a greatly accelerated manner. whereby the binder dissolved completely and the stability of the binder bridge is abruptly lowered.

Finally, the molding material mixture according to the invention using aluminum oxides of lower purity was tested with the same set grain sizes as described above. It was found that with a purity of the type AL90.0 and less increased adhesion of foundry sand occurred. This is therefore considered to be the lower limit for the purity of the alumina.

Hereinafter, oxides which have been adjusted in their morphology by grinding, crushing, crushing, blasting, impact milling, vibratory milling, etc., during production are referred to as milled oxides. It was found that even with ground aluminum oxides with a purity of 90%, the advantages according to the invention could be achieved.

In order to justify the various parameter range limits of the milled aluminum oxide defined above, various milled oxides were investigated. At grain sizes <1 micron, lumping occurred when mixed with reclaimed core sand. With grain sizes over 200 microns, it was found that complete coverage of the alumina-coated grain was not reliable.

Furthermore, it has been found that the finely ground aluminum oxides, especially in the limits of 100-200 microns, preferably immediately after the grinding must be added to the binder and used, otherwise there is a risk of dissolution or aluminate formation in the case of prolonged storage. The aluminate formation takes place by direct transition of the aluminum from the oxidic surface into the solution in the form of a negatively charged complex. The aluminum is kept in solution in the complex, spreads by diffusion, and tends to agglomerate and flocculate with longer residence time of the solution. This is especially at elevated temperatures as they can occur anywhere in foundry. The agglomeration and flocculation causes inhomogeneously altered Flow properties of the binder and makes use of the binder mixture impossible.

In summary, the foundry molding material mixture according to the invention consists of molding sand, binder, aggregates and aluminum oxide as an emulsion-free and thus emission-free additive. It results in improved flowability and resistance to thermal effects of the molding material mixture, a significantly reduced number of adhering grains on the finished casting, a reduced adhesive force of the adhering grains on the casting and a significantly reduced coring time.

The inventive method for producing a foundry molding mixture provides that the alumina with a purity of> 90% and a particle size of 1-200 microns is added directly to the binder and processed. The proportion may be between 10 and 85%, based on the amount of binder.

When using the molding material mixture according to the invention form between the sand grains microporous binder bridges, which allow faster and easier coring and final cleaning of the casting.

Claims (13)

1. A mould or moulded blank for casting purposes, comprising moulding sand, binding agents and additives, quartz sand being used as a moulding sand and aluminium oxide being used in the binding agent, characterised in that the quartz sand is used in a grain size range of from 0.05 to 5 mm and aluminium oxide with a grain size of from 1 to 200 micrometres being used as an additive, the aluminium oxide having fresh fracture planes or breaking edges and the binding agent provided with aluminium oxide being disposed on the surface of the quartz sand as a covering layer, Al₂O₃ particles being kept as a porous, closed, surface-covering, packed layer over the binder on the individual quartz sand grain, and a water glass phase contained in the binding agent being contracted on the contact surfaces of the quartz grains in a gusset-type manner, and having a microporous structure in the boundary phases in the form of porous binder bridges, binding agents and quartz sand being connected to one another such as to form the microporous binder bridges over the fracture planes or breaking edges of the aluminium oxide particles in the structure of the moulded blank.
2. The mould or moulded blank according to Claim 1, characterised in that a broken and/or ground aluminium oxide is used, the grain size of which is in the range of from 1 to 100 µm.
3. The mould or moulded blank according to any of the preceding claims, characterised in that the pore size of the microporous structure is in the range of from 0.1 to 2.5 µm.
4. A casting moulding material mixture for the production of a mould or of a moulded blank according to any of the preceding claims, comprising moulding sand, binding agents and additives, characterised in that the additives comprise a freshly broken or ground, emulsion-free aluminium oxide in a quantity > 10% in relation to the binding agent portion, that the aluminium oxide with a grain size of between 1 and 200 micrometres is contained in the binding agent in a quantity of 10-65%, that a binding agent with a water glass base is contained in the moulding material mixture with a binding agent content of 1-10%, and that quartz sand with a grain size range of 0.05 to 5 mm is used as a moulding sand.
5. The casting moulding material mixture according to any of the preceding claims, characterised in that the aluminium oxide additive is an alpha aluminium oxide.
6. The casting moulding material mixture according to any of the preceding claims, characterised in that the aluminium oxide is a pure aluminium oxide with a purity grade of greater than 90%.
7. A method for producing a mould or a moulded blank using a casting moulding material comprising moulding sand, binding agents and additives, characterised in that quartz sand with a grain size range of from 0.05 to 5 mm is used as a moulding sand, that as an additive freshly ground aluminium oxide with a grain size of from 1-200 micrometres is added in a quantity of 10-85% by weight of the binding agent to the latter and is mixed homogeneously with the binding agent, that the binding agent/oxide mixture is mixed with the moulding sand and injected into a moulding box under pressure and hardened, the binding agent/oxide to moulding sand mix ratio being kept at a ratio of 1-10 to 90, and the drying of the liquid binder taking place such that microporous binder bridges are produced between the individual quartz grains.
8. The method according to the preceding claim, characterised in that the aluminium oxide with an initial grain size > 200 µm is ground or broken into a grain size < 100 µm, and the ground product, keeping the fracture surfaces and breaking edges produced, is initially mixed with the binder at pH > 10 and is then mixed with the moulding sand within 1 to 10 seconds.
9. The method according to any of the preceding claims, characterised in that the ground aluminium oxide is mixed to a liquid binding agent, wherein the proportion of the binding agent/oxide mixture contributing to the overall mixture is 1.5-4 percent by weight.
10. A method for producing a cast part using a mould or a moulded blank according to any of the preceding claims, characterised in that in order to remove the core of the cast part, low-frequency vibration is applied to it for maximum 10 seconds.
11. The method according to the preceding claim, characterised in that the finished cast parts are finally freed of adhering grains of sand by subjecting to ultrasound.
12. The method according to any of Claims 9 to 10, characterised in that the mould or the moulded blank is broken up into its component parts by adding water.
13. The method according to any of Claims 9 to 11, characterised in that the breaking-up into the primary grain size takes place in a moist environment, wherein the handling time from starting to add moisture until total break-up is less than 1 second.