EP2014391B1 - Method for producing a core and/or mould sand for casting purposes - Google Patents

Description

The invention relates to a method for producing a core and / or molding sand for foundry purposes, after which a granular mineral molding base material is mixed with an inorganic additive and an inorganic binder.

Core sands for foundry purposes are generally used to define cores in castings. In contrast, molding sand generally refers to a sand or a sand-like mold base material, which dictates the outer shape of the desired casting. Core sand and foundry sand fall largely under the generic term foundry sand or foundry mold base material. That is, in the present case, it is not necessarily sands, but generally granular foundry molds.

In the production of core and / or foundry sands for foundry purposes, a lustrous carbon generator or, in general, coal dust / hydrocarbon resin is usually added in addition to a binder such as bentonite, as described, for example, in US Pat DE 30 17 119 A1 is described. The main reasons for the use of coal dust in the foundry sand foundry are that an improved casting surface is achieved and sand buildup is largely avoided. In addition, the mold can be advantageously separated from the sand and casting errors are reduced. Finally, the dosage can be easily made and the costs are relatively low.

However, these advantages are offered at a disadvantage, which can be seen, for example, in the fact that the strength of the casting mold or casting mold suffers from the addition of dust and harmful emissions due to the organic constituents are observed in the foundry mold sand or foundry mold base, for example carbon monoxide or sulfur dioxide and benzene emissions. Furthermore, it can not be prevented that the usually recirculated and reprocessed molding base or molding sand is contaminated by organic condensation products, benzene and so on. In addition, a growing moisture content is observed, which can lead to casting defects.

A method of the design described above is the approach in the US 2,828,214 presented. Here, Example II discloses a composition for foundry cores and molds consisting of sand, binders and additives that are mixed. Suitable additives are those based on organic substances, for example cellulose-containing, are used. As a result, in particular emissions are still to be feared and ultimately casting errors as described can not be ruled out.

As a result, this also applies to teaching according to the US 4 505 750 A , For here a core and / or foundry sand for foundry purposes is described, which is composed, inter alia, of quartz sand, a smectite binder and here in particular concrete and in addition an organophilic and water-absorptive fiber material. Here, for example, cotton fibers are used.

The invention is based on the technical problem of further developing such a method so that in particular harmful emissions are avoided and the quality of the casting is improved.

To solve this technical problem, the invention proposes a method according to claim 1. This is a total as an additive inorganic Blähadditiv used with a Blähzahl of at least 9, so an inorganic blowing additive, which in any case has a higher Blähzahl than coal.

In the context of the invention, therefore, an inorganic additive is expressly used, specifically a special additive, namely a bulking additive. In the context of the invention, such bulking additives are characterized by having a free swelling index of at least 9, as defined in more detail in DIN 51741. The swelling index preferably even assumes values of more than 10, in particular more than 20. In general, the swell ratio is even around 100.

The Blähzahl expresses that the inorganic swelling additive in question at a certain (high) temperature (expansion temperature) multiplied its volume, for example, tenfold. From the increase in volume can then be deduced on the Blähzahl, wherein the multiplication of the volume as a factor in about the Blähzahl corresponds. This fact can essentially be attributed to the fact that the inorganic swelling additive, such as, for example, perlite, vermiculite or (expanded) graphite, has a relatively high moisture content in the form of intracrystalline water. Also, a chemical in the interior is conceivable, which provides an expansion of the Blähadditives when exposed to heat. In fact, it is now due to this heat effect to the volume increase described, which can largely be attributed to the fact that the previously bound in the respective structure of water or the chemical evaporates abruptly and provides the desired expansion. This effect is basically known, including on the DE 24 53 552 C3 referenced.

The indicated blowing rates are generally determined so that the inorganic swelling additive in question is optionally ground and then heated in a crucible. From a comparison of the volume before and after heating can then be deduced on the Blähzahl. In most cases, (blowing) temperatures of more than 300 ° C are used at this point.

It must of course be ensured that the inorganic blowing additive before addition to the molding sand and especially before the eientlichen casting has not reached the above-mentioned Blähtemperaturen of about 300 ° C or even more. For in the context of the invention, the respective bulking additives arrange advantageously in the range of binder bridges, which are built up by the inorganic binder between the individual granular grains of the molding material in order to represent the mold in the desired shape.

The inorganic swelling additive present in the area of these binder bridges now ensures that the blowing additives in question expand at the casting temperatures that are achieved, which are usually above the specified expansion temperatures of about 300 ° C. and more. As a result, the binder bridges are broken, so that the granular mineral molding material decomposes immediately after completion of the mold, because the binder does not fulfill its original binding function for the preparation of the binder bridges. That is, the bond between the individual grains of the molding base material or of the molding sand is physically or mechanically dissolved, by the inorganic blowing additive, which expands targeted and initiated by the casting process. In this case, the abovementioned decomposition process can be controlled or regulated via the temperature and / or selection or modification of the respective blowing additives.

Because the various inorganic blowing additives mentioned show a different temperature expansion behavior and consequently also different blow molding temperatures at which the expansion process starts or reaches its maximum. In addition, it is conceivable to work with mixtures as an inorganic blowing additive, for example to mix vermiculite with perlite in a certain ratio. As a result, the swelling behavior of the inorganic swelling additive can be precisely adjusted and controlled both with regard to the temperature behavior of the expansion as such and with regard to the achieved expansions.

In any case, according to the invention, organic additives are dispensed with throughout, be it for the preparation of the binder or as additive or bulking additive. As a result, the unavoidable in the prior art emissions of, for example, carbon monoxide or benzene, as well as contaminants based thereon reliably and already prevented from the beginning. In addition, the expanding nature of the expanding additive closes any pores remaining in the mold during casting, which greatly reduces the casting surface and its roughness. Because the liquid metal finds in the mold no pores, in which it can penetrate.

In addition, for example, expanding graphite as an inorganic blowing additive has complementary positive properties to the effect that any separating oils, condensates and any resulting benzene are bound. This can be attributed to the high porosity of graphite and its non-polar character. In addition, graphite can be loaded or combined with additional materials, which are stored in the compulsory spaces. Advantageously, sulfur can be used at this point. For example, one knows so-called Graphite bisulfate, which is prepared by the treatment of highly crystalline natural graphite with a mixture of sulfuric acid and with the addition of various oxidants.

In any case, it becomes clear that the inorganic bulking additive can not only take over or take over the function of blasting the binder bridges between the individual grains of the granular mineral molding base material, but also being able to bind individual potentially harmful emissions, such as oil, benzene or other mostly carbonated condensation products. In addition, the inorganic blowing additive can be specifically modified by additives or stored materials such as sulfur. The stored materials are automatically released during the onset of the blow molding process and can develop the desired effect.

For the blowing additive, the finished mixture of the molding base material, the blowing additive and the binder is compacted, taking into account a density increase of at least 20 g / dm ³ . As a result of this increase in density, the core and / or molding sand or the foundry sand or foundry molding base material can generally be prepared in such a way that casting defects are practically reduced to a minimum. Because of the increased compression of the mold, penetrations of the liquid casting material or metal into the casting mold can be reduced to a minimum. In any case, it is an inorganic additive, so that so far observed in the prior art harmful emissions virtually no longer occur.

By sodium ion activated bentonite has proved to be beneficial as a binder. In addition, an inorganic additive with the inorganic Blähadditiv be combined in the production of core and / or foundry sand for foundry purposes.

There are various options for further refinement in detail. Thus, the binder can be mixed with the inorganic additive or blowing additive before its addition to the granular mineral molding base itself. Consequently, the inorganic binder and the inorganic blowing additive or additive form an inorganic premix, which may also be present as a mixture particles or premix particles themselves (pellet).

Moreover, it has proven to be advantageous if the binder and the swelling additive or additive are processed by coextrusion to the mixing particles or the pellets. Of course, other methods of preparation are conceivable in order to process the binder and the swelling additive or additive to the mentioned mixture particles or pellets. As grain size for the mixing particles or premix particles, the invention recommends such a thickness of about 5 .mu.m to 500 .mu.m, in particular from 10 .mu.m to 200 .mu.m. The average grain diameter may be about 65 microns, wherein the mixture particles are subjected to a total grinding process to adjust the specified particle size spectrum.

This can be done in detail so that after the extrusion of the grinding process described is carried out and then separated according to the grain sizes, for example via cyclones or other discharge facilities.

With regard to the inorganic blowing additive or additive, the invention recommends to provide a separate screening / grinding process at this point and to work with a grain size in the range of 10 nm to 3000 nm. The average particle diameter should be about 1000 nm or 1 μm. In this way, a particularly intimate connection between the binder and the inorganic blowing additive can be achieved. Because the binder is usually in a grain size of originally about 10 microns to about 200 microns ago. The shell or binding envelope responsible for the use of, for example, bentonite for the bonding process between the individual grains of the molding base material has a layer thickness of about 3.5 μm. In this way, the maximum of 3 microns (3000 nm) particles or grains of Blähadditivs easily bring into the binding envelope in question (with greater thickness than the largest grain diameter) of the bentonite or the binder or store. As a result, the bulking additive is arranged exactly in the region of the bonding bridges already mentioned above and can unfold its effect of breaking the binding bridge in each case.

As a molding material, the invention usually resorts to a granular mineral sand, in particular quartz sand, which is usually present in a mean grain size smaller than 0.5 mm, wherein the grain usually moves in the range between 0.10 mm to 0.30 mm ,

Thus, the inorganic additive is incorporated directly into the shell or binding shell of the binder or the bentonite or can be incorporated, it is advisable to process both of the aforementioned ingredients (binder and blowing additive) together. This can be done by the already mentioned extrusion process, which directly ensures that the swelling additive penetrates into the binding envelope in question. Because this is ensured by the pressure prevailing in the compulsory molding press for the extrusion process. After also in this way the mixture particles of the binder and the blowing additive have been formed, the question Mixtured particles or pellets as already mentioned sieved, wherein then grain sizes are preferably observed in the range of 10 .mu.m to 200 .mu.m with a mean grain diameter in the range of about 65 microns.

The abovementioned mixture particles or premix particles of the binder with embedded swelling additive and / or a loose premix of the binder and the swelling additive is then mixed with the molding material for final mixing. In the finished mixture, the proportion of the molding base material is usually about 80 wt .-% or more, whereas the remaining 20 wt .-% filled in the maximum of the binder, the organic additive or the blowing additive and possibly one or more other additives become. These additives may be porous mineral materials or particles such as clinoptilolite, a catalyst such as manganese oxide, (silver treated) zeolite, or the like, which will crack any harmful hydrocarbon emissions. In addition, it has proven useful if, alternatively or additionally, an oxidizing agent is added in order to burn or split any absorbed organic components.

The aforementioned oxidizing agent can be added as an additive of calcium carbonate, for example, the binder in a weight proportion of about 10 wt .-%. In addition, the oxidizing agent can be realized according to the invention in such a way that the binder or the bentonite is activated with sodium oxalate or comparable additives. As bentonite is usually one used, which contains at least 85 wt .-% montmorillonite as the main component.

The finished mixture of the granular mineral molding material, the binder and the blowing additive and optionally the additives is then finally compacted. This can be done by means of a stamping device, which may be, for example, a molding press or a pile driver with a plurality of rams. Most of the time, however, compression methods or corresponding devices are used today, which provide for a corresponding compaction by shooting, by generating air pulses, by means of pressing. As a result of this compaction process, a core and / or molding sand or a finished mixture is observed whose density has a density increase in the range of more than 20 g / dm ³ , in particular more than 30 g / dm ³ .

As a result of this increase in density, the molding sand can be processed with high compression, and consequently the casting mold produced therefrom is also present in the increased density. As a result of this high compression of the mold, penetration of the liquid casting material (metal) into the casting mold is reduced to a minimum. As a result, particularly smooth casting surfaces are observed. In addition, by the addition of the catalyst and / or an oxidizing agent any harmful emissions are bound or split during casting. For example, bentonite with an addition of more than 9% by weight of carbonate, based on the bentonite or the binder, can be used as the oxidizing agent.

Overall, the inventive addition of the inorganic additive respectively Blähadditivs with a Blähzahl of at least 9 and the inorganic binder or their use for the production of a core and / or molding sand for foundry purposes ensures that the fluidity of the foundry molding sand is increased and its disintegration after making the Gusswerkstückes is accelerated and otherwise takes place almost residue-free.

The additional introduction of additives, such as zeolite as a catalyst or of oxidants optimizes the absorption of possibly still resulting organic emissions, their application anyway by the absence of organic binders and organic additives and also by resorting to only inorganic additives per se on a Minimum is reduced. Here are the main benefits.

Embodiment 1

Sodium-activated calcium bentonite as a binder is mixed with an inorganic bulking additive, which is expanded graphite or vermiculite. The individual compositions are given in the table below. On the one hand, a distinction is made between a loose premix of the additive with the inorganic binder and, on the other hand, between pellets which have been produced by prior combined extrusion of the additive with the blowing additive. The abovementioned pellets or the premix was dried and ground, so that on the output side a water content of about 10% by weight is observed and an average particle diameter of about 0.063 mm.

The pellets or premix particles are then sieved and ground to a mean grain size of about 65 μm. If a loose premix of the additive and the inorganic blowing additive has been investigated, an average grain size of about 65 μm has also been set. Subsequently, the foundry molding sand or the core and / or foundry sand for foundry purposes was prepared by filling the granular mineral molding base material, in the exemplary embodiment quartz sand, into a pug mill.

The quartz sand was mixed with 2.5 wt% (deionized) water for a period of one minute. Subsequently, the bentonite was added to about 7 wt .-%, based on the molding material, and the inorganic blowing additive to about 5 wt .-%, based on the binder or the bentonite. Alternatively, the pellets were filled from the binder including embedded swelling additive in the edge of the pan.

Following this, the finished mixtures were sieved, taking into account a 3 mm sieve, so that on the output side a foundry molding sand or a lump-free ready mix of the original grain distribution was available. Thereafter, the molding foundry sand in question was compacted, in the example in a cylinder of length 100 mm with a diameter of 50 mm at a pressure of 100 N / cm ² .

In this way, a compression or a percentage reduction in volume of foundry sand in the cylinder of about 40% ± 2% could be achieved. In fact, it has been found that a compaction of at least 30%, i. H. a volume reduction of at least 30% is required in order to represent the desired properties, in particular with regard to the bondability of the surface of the foundry molding sand and consequently of the casting mold. Any adjustments can be controlled by more or less water addition.

The following Table 1 shows in column 1 a granular molding material with an addition of only 7 wt .-% bentonite as a binder, based on the molding material. Column 2 shows the mold base in question with an addition of 7 wt .-% bentonite as a binder and 2 wt .-% carbon as an organic additive, each based on the molding material, according to the prior art.

In column 3, a granular molding base material with an addition of 7 wt .-% bentonite and 5 wt .-% expandable graphite is reproduced. Column 4 shows as well as column 3 the granular molding material with 7 wt .-% bentonite as a binder and 5 wt .-% vermiculite added as an inorganic blowing additive, each as loose mixture components and compacted according to the example described.

In column 5, the molding material plus 7 wt .-% bentonite and 5 wt .-% expandable graphite according to the example in column 3 is shown, however, in such a way that the bentonite was extruded together with the expandable graphite to the pellets or premix particles , Column 6 finally shows the granular molding material with an addition of 7 wt .-% bentonite and 5 wt .-% vermiculite as in the example of column 4, but again in the form of mixture components or pellets, which have been prepared by co-extrusion. All ready mixes have been compacted as described. <b> Table 1 </ b> 1 2 3 4 5 6 Moisture content in% by weight % 2.45 2.65 2.14 2.25 2.24 2.3 Compaction or degree of compaction % 40.1 40.2 40.5 40.8 40.9 41.2 "green" pressure resistance N / cm ² 15.6 15.0 15.7 15.2 12.8 12.8 Dry compressive strength at 150 ° C / 3 hrs. N / cm ² 23.3 32 28.4 31.2 26.8 24.4 Dry compressive strength at 350 ° C / 1.5 hrs. N / cm ² 22.5 16.6 0 0 24.3 21.9 Dry compressive strength at 550 ° C / 45 min. N / cm ² 26.8 14.5 0 0 17.0 13.8 Dry compressive strength at 750 ° C / 30 min. N / cm ² 3.45 2.4 0 0 2.8 2.1 Hot shear strength at 15 sec. N / cm ² 2.46 2.27 2.38 2.32 2.0 2.4 Hot shear strength at 30 sec. N / cm ² 2.72 1.72 2.73 2.70 2.1 2.00 Hot shear strength at 60 sec. N / cm ² 4.7 5.2 1.28 3.60 3.4 3.4 Hot compressive strength at 980 ° C / 12 min. psi 23.4 9.62 0 0 10.9 11.2

To measure the "green" compressive strength, three samples were made according to the respective example and compacted in the compacting apparatus with the cylinder to a diameter of 50 mm. The "green" compressive strength was then measured with a compressive strength tester. In order to determine the dry compressive strength, the individual samples were prepared as in the determination of the "green" compressive strength. However, before the samples were measured, they have been brought to different temperatures for certain times as indicated.

In order to determine the hot shear strength, a narrow area at the head of a specimen has been heated very strongly to temperatures of about 1000 ° C. After a certain time of 15, 30 or 60 seconds, the binding force of the foundry sand was measured. The hot compressive strength was measured with a Simpson & Gerosa high temperature compressive strength tester. For this purpose, the sample was introduced into the tester and heated to temperatures up to 980 ° C for a period of 12 min. Thereafter, the maximum compressive strength was measured.

Embodiment 2

In this case, a foundry molding sand was prepared by adding a mixture of bentonite, a macrocrystalline graphite, expandable graphite, and a porous additive component or a porous additive to the masterbatch as a premix, as described in Example 1. The compounding ingredients added to the mold base include about 85% by weight of bentonite, 8.5% by weight of the macrocrystalline graphite, 3% by weight of the expanded graphite and 3.5% by weight of natural zeolite (clinoptilolite). The abovementioned premix constituents as a whole are added to the molding material at about 8% by weight, based on the finished mixture, so that the Form base material in the finished mixture occupies a share of about 92 wt .-%.

This foundry molding sand mixture according to the invention is compared with a conventional mixture of a molding base with 7 wt .-% bentonite and 2 wt .-% coal dust, which is characterized in the following Table 2 as "prior art". Table 2 State of the art invention Compaction or degree of compaction % 40 40 density g / dm ³ 1505 1545 Dry compressive strength at 150 ° C / 3 hrs. N / cm2 27 35 Dry compressive strength at 350 ° C / 1.5 hrs. N / cm2 19 0 Dry pressure strength at 550 ° C / 45 min. N / cm2 16 0

It is clear from Tables 1 and 2 that (dry) compressive strengths of 0 N / cm ^{2 are} often already observed for the foundry molding sand according to the invention during a treatment at 300 ° C. and a duration of 1.5 hours. That is, the foundry molding sand in question according to the invention is predominantly disintegrated during the treatment. In the range below 350 ° C comparable compressive strengths as in conventional foundry molding sands observed so that molds can be produced with conventional properties.

Also, the binding effect has drastically decreased, which supports the previously given interpretation that the foundry molding sand according to the invention disintegrates reliably immediately after production of the casting material. In the process, the mold base material as a whole is not destroyed and can easily be reprocessed, with recycling rates or regeneration rates for the basic molding material of up to 98% being observed. That is, in a casting usually go only a maximum of 2 wt .-% of mold base lost.

A comparison of the two columns according to Table 2 shows that according to the invention, the density has been increased by about 40 g / dm ³ , that corresponds to a corresponding density increase. In fact, a density increase of about 35 g / dm ³ has already been achieved in a single compaction process. Despite this increase in density and the overall high density of the foundry molding sand according to the invention excellent disintegration is observed, even at 350 ° C, as the dry compressive strength of 0 N / cm ² makes clear.

The following two electron micrographs support this rating. One recognizes in the Fig. 1 a foundry molding sand according to the embodiment 2. In the Fig. 1 is the beginning of the breaking up of the respective binding bridge to recognize, while the Fig. 2 represents the broken bond bridge.

It is clear that the bond bridge is broken between the individual grains of the molding material represented. Actually Brick-shaped bridges from the binder with the help of the inorganic Blähadditivs and blasted off. As a result, the thickness of any Schamotthülle, which surrounds the single grain of sand, reduced. This facilitates cleaning during reuse and supports the desired decomposition process, so that after cooling of the foundry molding sand a good separation of the casting and molding material or mold by sieving is possible. This can be done immediately after production of the cast workpiece. - Of course, the described method can also be combined with other additives, such as conventional coal.

Claims (9)

1. A method for producing a core sand and/or a mould sand for foundry purposes, according to which

   - a granular, mineral base mould material is mixed with an inorganic swelling additive having a swelling index of at least 9 and an inorganic binding agent, and

   - the completed mixture of base mould material, swelling additive and binding agent is compacted until the density thereof increases to at least 20 g/dm³.
2. The method according to claim 1, characterised in that the binding agent is first mixed with the swelling additive, and this mould mixture is then added to the base mould material.
3. The method according to claim 1 or 2, characterised in that the binding agent and the swelling additive are extruded together to form mixture particles (pellets) for addition to the base mould material.
4. The method according to claim 3, characterised in that the mixture particles are sieved to yield a grain size from 5 µm to 500 µm, particularly from 10 µm to 200 µm.
5. The method according to any of claims 1 to 4, characterised in that the premixture consisting of the binding agent and the swelling additive is dried until the water content thereof if less than 20% by weight, particularly less than 10% by weight.
6. The method according to any of claims 1 to 5, characterised in that the premixture consisting of the binding agent and the swelling additive contains approximately 80% by weight or more binding agent and approximately 20% or less swelling additive, plus other additives as required.
7. The method according to claim 6, characterised in that, for example, a catalyst and/or an oxidising agent, such as bentonite containing more than 10% by weight carbonate, may be utilised as additional substances to the swelling additive and the binding agent.
8. The method according to any of claims 1 to 7, characterised in that the completed mixture consisting of the base mould material, the swelling additive and the inorganic binding agent as well as any other additive is compacted by means of densification processes such as firing, with pulsed air, ramming and the like.
9. The method according to any of claims 1 to 8, characterised in that the swelling additive is used with a grain size in a range from 10 nm to 3,000 nm.

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