EP2941327B1 - Method for the production of core sand and or molding sand for casting purposes - Google Patents

Description

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

In a method of the structure described above, as for example in the EP 2 014 391 A2 the applicant or the US 4 505 750 A Bentonite is typically used as a binder. The additional added blowing additive may be perlite, vermiculite or expandable graphite. The bulking additive has a Blähzahl of at least 9, that is, the Blähadditiv in question multiplies its volume at a certain temperature accordingly. This temperature is typically at 300 ° C. As a result, harmful emissions in particular are avoided and the casting quality is improved.

Inorganic binders such as bentonite according to EP 2 014 391 A2 are equipped with the fundamental advantage over organic binders that significantly less pollutants are released during casting. In addition to bentonite as an inorganic binder for molds and cores it is also possible in principle to use molding material mixtures for the production of casting molds for metal processing, which use a glass-water-based binder, as described in US Pat DE 10 2004 042 535 A1 is described.

By the JP 58968446 A a core or foundry sand for foundry purposes has become known, which in addition to a molding material such as sand in addition on vermiculite, mica or other thermally expanding particles. In addition, water glass and phenolic resins and other binders are addressed. This should be improved in the end, the disintegration of the mold.

The US 3,848,655 A deals with the production of a steel bar. This is produced in a casting mold, for the realization of which a sand mixture is used. From the sand mixture, the mold is made using a binder. As possible binders various resins such as phenolic resins or water glass, cement and clay and mixtures are listed as examples. In addition, exothermic substances are used as an additive, which may be carbonaceous materials. Possible carbonaceous materials include coal dust or expanded graphite.

Generally, with the help of the inorganic binder, the individual grains of the granular mineral and refractory molding base material are bonded or glued together. The molding base material is typically sand or quartz sand. The physical curing of the binder, for example, from the water glass is done regularly by heating by moisture is removed by drying. Drying can be done in a hot core box, by hot air blowing in the core box concerned, or by microwave heating or in a conventional oven.

After curing, the grains of the molding base material are connected to each other via binder bridges produced with the aid of the binder. The under the EP 2 014 391 A2 or according to the US 4 505 750 A Added added Blähadditive now ensure that the coring easier becomes. Because the swelling additive ensures that, for example, the core can be separated from the casting.

Due to the special characteristics of the inorganic binder and especially of waterglass binders, difficulties may often arise at this point, so that the separation of the core or core sand and / or foundry sand from the respective casting is only partially or incompletely successful. For example, the DD 158 090 A1 with a method for controlling the strength of inorganic molding materials based on alkali metal silicate solutions. Here, the special characteristic of water glass is described as a binder and also the unsatisfactory decay properties are presented in this context.

Because this must be as accurate and accurate as possible. For example, surface defects can occur during the detachment of particles or grains. In addition, single or multi-phase inclusions are observed on the casting surface, which can be attributed to reactions of the core and / or molding sand with the melt. Such inclusions can sometimes be recognized macroscopically, ie with the naked eye, and regularly influence the mechanical strengths of a casting. This can result in extreme cases committee.

In the production of castings, not only is a perfect casting surface in focus, but the entire manufacturing process places particularly high demands on coring. That is, it is important that the molds and cores are properly separated from the casting after the casting is made. In order to support this process, mechanical energy is often supplied by shaking or vibrating, which in addition to the blowing additive ensures that the binder bridges between the individual grains are destroyed and therefore the mold base material ideally free-flowing out of the casting.

In particular, in the case of inorganic bonded cores with narrow contours in the millimeter range, as observed, inter alia, with water jacket cylinder heads for the production of automotive engines, there are Entkernprobleme, which are also reinforced by the relatively low casting temperatures of the aluminum alloys generally used. That is, in the formed thin channels often Kernsandreste get stuck or the channels are even closed. In addition, inorganic binders based on bentonite are generally associated with the disadvantage that the casting molds produced therefrom have relatively low strengths. However, high strengths are particularly important for the production of such complicated thin-walled cores (molds) and their safe handling.

The reason for the low strengths of bentonite-bonded shapes and cores compared to, for example, water-glass bonded shapes and cores can be attributed essentially to the fact that the bentonite-bonded casting molds have a slightly different bonding mechanism and still contain residual water from the binder. - This is where the invention starts.

The invention is based on the technical problem of developing a method of the type described above so that a perfect and rapid disintegration of the mold is associated with a perfect surface of the casting.

To solve this technical problem, a generic method in the context of the invention is characterized in that are used as a binder water glass and as blowing additive expandable graphite.

In the context of the invention, water glass is first used as the binder. Water glass is known to be solidified from a melt glassy water-soluble sodium and potassium silicates or their aqueous solutions. Depending on whether predominantly sodium or potassium silicates are contained, one speaks of soda water glass or potassium water glass. Such waterglasses are characterized by a high rate of setting and low emissions. The use of water glass in the foundry technology for hardening molds and cores is known in principle, as the DE 10 2004 042 535 A1 exemplified, but not in combination with an additional blowing additive in the form of expandable graphite.

In fact, expanded graphites are special graphites which typically expand by about 50 to 600 volume percent upon heating to temperatures above 150 ° C. The aforesaid expansion can be determined, for example, so that the expandable graphite 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 volume increase. Usually, a certain amount of expandable graphite (in g) is used in this process, so that not only the increase in volume can be specified, but also an expansion rate, ie the volume increase (in cm ³ ) per gram of expanded graphite used.

Details of the production of such expanded graphite and the measurement of the expansion properties can be, inter alia, the EP 1 489 136 A1 remove. Thereafter, the expansion properties of expanded graphite, for example determined by means of thermomechanical analysis (TMA).

With the aid of this thermochemical analysis dimensional changes of expanded graphite or individual graphite particles are measured as a function of temperature and time. For this purpose, the respective sample of expandable graphite is applied to a sample carrier and the dimensional changes of the sample are measured and recorded with the aid of a measuring probe as a function of the heating temperature and the heating time. Typically, the powdery sample of expanded graphite can be introduced into a corundum crucible, which is covered with a steel crucible. The steel crucible ensures the smooth transfer of the dimensional changes of the sample to the probe, which is in mechanical contact with the top of the steel crucible, as the sample expands. In addition, the probe is subjected to an adjustable load.

Further details of this thermochemical analysis (TMA) and also the calculation of the expansion of the substance in% or in cm ³ are in the previously mentioned EP 1 489 136 A1 described in detail. As a consequence of this, the expanded graphite can be characterized by its rate of expansion, that is to say the volume increase (in cm ³ ) relative to the mass (in g), inter alia.

The expandability of expanded graphite can be attributed to the fact that between the lattice planes of the graphite impurities are embedded, which cause the expansion of the lattice plane spaces when energized. These foreign constituents may be metallic groups, halogens, OH groups, acid residues or SOx and / or NOx.

In the context of the invention, weakly expanding expandable graphites are used which, on the one hand, considerably improve coring, and on the other hand have practically no negative influence on the surface of the casting which forms after casting.

In particular, in order to demonstrate this fact quantitatively, pouring tests with waterglass-bound test specimens and additions of expandable graphites with different expansion rates were undertaken in this connection.

1. 1. Expansionsrate < 120 cm³/g (Blähgraphit) Probe Nummer 1
2. 2. Expansionsrate > 350 cm³/g (Blähgraphit) Probe Nummer 2

For this purpose, a granular mineral molding base material (quartz sand) was used for the casting test, as well as waterglass with 1.6% by weight and 0.3% by weight expandable graphite, in each case based on the molding base material, with two different expansion rates:

1. 1. Expansion rate <120 cm ³ / g (expanded graphite) Sample number 1
2. 2. Expansion rate> 350 cm ³ / g (expanded graphite) Sample number 2

In the further course of the experiment, the moldings were introduced as inner cores in a common mold and poured. After cooling, a simple coring was visible in both, that is, a simple trickling out of the quartz sand from the casting was visible. When sawing both cast bodies, however, it was also apparent that the expanded graphite with the high expansion rate (> 350 cm ³ / g) had influenced the formation of the casting surface of the test specimen so strongly (cf. Fig. 4 , Sample No. 2) that, as a result, residues of the expandable graphite macroscopically on the casting surface could be detected.

Compared to sample number 1 or sample number 1 in Fig. 4 , who had been treated with weak expanding expandable graphite, these strong influences on the surface were not or only barely visible, so that with regard to the surface quality in the casting test little or no restrictions could be found.

From the pouring experiments described above, therefore, an expanded graphite having an expansion rate of more than 10 cm ³ / g and in particular such having an expansion rate of 10 to 100 cm ³ / g, a maximum of 120 cm ³ / g, has proven to be particularly favorable. The lower limit of 10 cm ³ / g is explained by the fact that only at such an expansion rate of expandable graphite a coring is made possible, that is, the shape without adherence to the casting properly disintegrates.

In general, therefore, expansion rates of up to 350 cm ³ / g and especially those of up to 100 cm ³ / g are particularly preferred. The expansion rate indicates the increase in volume of expandable graphite (in cm ³ ) relative to its mass (in g).

In the production of the expandable graphite according to the invention generally sulfur or nitrogen compounds are incorporated into the individual layers of graphite. It is therefore SOx or NOx expandable graphite. These typically have a starting temperature for expansion that is greater than 180 ° C. In particular, a starting temperature of about 220 ° C is observed. That is, only above the specified temperatures (> 180 ° C), the previously stated volume increase is observed.

As expandable graphite is typically used one whose particle size is more than 20 microns. In particular, particles or grains are used in a diameter range from 20 .mu.m to 150 .mu.m and preferably those with a grain size between 150 .mu.m and 300 .mu.m.

The described grain size of the expanded graphite up to a maximum of 300 μm takes into account, among other things, the fact that usually granular mineral sand, in particular quartz sand, is used as the molding base material. This is usually in a mean grain size ≤ 0.5 mm before, that is with a grain diameter of typically less than 500 microns. In general, its grain size ranges between 100 μm to 300 μm. As a result, the grains of on the one hand the expanded graphite and on the other hand the mold base material are approximately the same size, which favors the mixing of the molding material with the expandable graphite and its uniform distribution within the produced core and / or molding sand.

Expanded graphite generally has a carbon content of 85% to 99.5% by weight. The maximum moisture of the expanded graphite is in the range of at most 1 wt .-%. The PH value may be between 3 and 8. The starting temperature is in the range between 180 ° C and 220 ° C.

In most cases, the expandable graphite is added to the mixture in a proportion of up to about 1% by weight and preferably up to about 0.5% by weight. The mixture is the mixture of the granular mineral molding base material and the at least one inorganic binder. According to the invention, the inorganic blowing additive in the form of expandable graphite is added to this mixture. Particularly preferred is a proportion of expandable graphite in the mixture in question of about 0.1 wt .-%. The percentages by weight in each case relate to the molding material used.

Nevertheless, the introduced and due to the adaptation of the respective grain diameter of on the one hand the mold base and on the other hand, the expandable graphite evenly distributed Blähgraphitbestandteile in the mixture to simplify from the starting temperature, the coring. Because of the introduced expanded graphite virtually all bonds between the individual grains of the molding material are repealed.

At the same time, the expanded graphite, with its relatively low expansion rate of typically not more than 100 cm ³ / g or not more than 350 cm ³ / g, ensures that the surface of the casting that is formed is not or is not negatively affected. This can essentially be attributed to the fact that, on the one hand, the weak expansion of the expandable graphite does not exert excessive pressure on the grains of the basic molding material with pressure built up from the inside, but rather causes the moderate expansion rate mainly to break up the binder bridges. On the other hand, the expanding expanded graphite is in a particularly fine distribution, so that inclusions on the surface of the casting in principle can not or practically do not occur.

Of particular importance beyond that high bending strengths are observed, which are located well above 100 N / cm ² , therefore, for example, in the EP 2 014 391 A2 far exceed specified dry compressive strengths in the range of about 40 N / cm ² . This is because, according to the method of the invention, generally and generally bending bars can be produced for determining the bending strength given above, whereas according to the method according to FIG EP 2 014 391 A2 Such bending bars are not displayed at all. In any case, the resistance to the teaching after EP 2 014 391 A2 significantly increased.

This fact is supported in particular by the comparative test values of the flexural strength of bentonite-bonded and water-glass bonded specimens, as can be seen from Table 1. The bentonite-bonded test specimen corresponds to the state of the art, as used in the EP 2 014 391 A2 is described and used as an inorganic binder bentonite. In contrast, the water-glass-bonded test specimen belongs to the process according to the invention, in which water glass (in conjunction with expandable graphite as intumescent additive) is used as binder.

In this case, 5% by weight bentonite (based on the quartz sand) + water + quartz sand was used as the bentonite-bonded molding material. In addition, the bentonite-bonded molding material has 0.3 wt .-% expandable graphite (based on the quartz sand) with an expansion rate <100 cm ³ / g. The entire production of the test specimens was carried out in the laborkorkergergang and according to the Prüfkörperherstellung VDG leaflet P 69.

For comparison, the water-glass-bonded molding material according to the method of the invention from 1.6 wt .-% water glass (based on the quartz sand or the granular mineral mold base material) and the same proportion of expandable graphite as before and the rest quartz sand. The test specimen was produced here in a wing mixer and the hardening of the water-glass bonded cores in a drying oven.

When testing the bentonite- and waterglass-bonded specimens, the following bending strengths were determined, which reflect the large and already known differences in flexural strength, in particular the virtually non-measurable flexural strengths of bentonite-bonded molding materials (Measurements 1 to 3 refer to three test pieces of the same grammage and production, which were examined for statistical purposes): Table 1. Series of tests for testing the bentonite and waterglass bonded specimens Measurement Bending strength of the bentonite-bonded test specimens Bending strength of the waterglass-bonded specimens 1 below 20 N / cm ² 350 N / cm ² 2 below 20 N / cm ² 360 N / cm ² 3 below 20 N / cm ² 350 N / cm ²

Thus, the application of a bentonite-bonded molding material is limited to forms that are usually used only for forming the outer contour of molds, which represents a disadvantage overall. Because, for example, inner cores respectively inner contours can be hereby practically not realized.

In addition, for a possible stabilization and for the formation of the necessary strengths, such bentonite-bonded molding materials must always be located in a molding frame, also called a molding box, which compensates for this strength disadvantage of the binder.

This is also to be regarded as a further disadvantage, since on the one hand additional material costs for the molding boxes arise and, on the other hand, a cleaning or reprocessing of the molding boxes is necessary after each use, which also causes additional costs.

For comparison, water glass bonded moldings or molding materials without stabilizing mold frame (molding box) can be produced so that they can be handled freely and inexpensively and so can be used as bentonite bonded moldings also in terms of their casting technology application for a wider range of applications. Here, in particular, the production of cores to form inner contours, for example, water jacket cores in the production of molds for water-cooled engines to name, which can not be imagined with bentonite-bonded cores and handled. Here are the main benefits.

Also, the core and / or foundry sand produced by the process according to the invention can be used advantageously for the production of casting molds for iron-carbon alloys, aluminum alloys, copper alloys such as brass, bronze, etc. but also for magnesium alloys and castings produced therefrom. The casting molds in question are typically used in the automotive industry. In fact, this makes it possible to realize casting molds which have particularly delicate structures with thin contours and in particular core contours in the range of only a few millimeters. Such narrow contours and in particular channels for cooling water in the production of cylinder heads can be realized particularly advantageously with the aid of casting molds which have been produced on the basis of the inventively producing core and / or molding sand. Here are also the main advantages of the doctrine of invention.

Comparative Example 1

In the in Fig. 1 Photograph shown is a casting shown in the left photo, which without expandable graphite recourse to a core and / or molding sand and water glass has been prepared as a binder. The right photograph in the Fig. 1 shows the workpiece in question with added expanded graphite in an amount of 0.1 wt .-% based on the produced core or molding sand (also with water glass as a binder, in both cases the same grammages for water glass and the core or foundry sand and also the same core or molding sand was used.

Based on the photographs in the Fig. 1 It becomes clear that the addition of expanded graphite significantly improves the surface of the casting, even in the stated quantity, as the right - hand photograph in Fig. 1 shows. On the other hand, in the absence of expanded graphite, significant defects in the casting surface are to be expected, such as the left photograph in the Fig. 1 makes it clear.

Theoretical considerations

The Fig. 2 and 3A to 3C show the basic process in the production of a mold using the core and / or molding sand according to the invention. In the Fig. 2 one recognizes how predominantly hatched grains of sand 1, together with black inorganic binder or water glass 2, fill a water jacket core of a corresponding core shape. In the Fig. 3A By way of example, two grains or grains of sand 1 of the basic molding material are coupled together by a bridge made of the inorganic binder or water glass 2 shown in black. The Fig. 3B shows how when crossing the starting temperature, a crack in the formed by inorganic binder or the water glass 2 bridge between the sand grains 1 is formed. For this purpose, essentially the expanding graphite expanding above the starting temperature is responsible. The Fig. 3C finally shows the breakage of the binder bridge 3 produced by the binder or water glass 2 in this way.

Claims (10)

1. A method for producing a core and/or moulding sand for foundry purposes, according to which a granular mineral base moulding material is mixed with at least one inorganic binding agent and additionally an inorganic aerated additive, characterized in that sodium silicate is used as binding agent and expandable graphite is used as aerated additive, wherein the expandable graphite has a rate of expansion of at most 350 cm³/g.
2. The method according to Claim 1, characterized in that the expandable graphite has a rate of expansion of 10 to 100 cm³/g.
3. The method according to Claim 1 or 2, characterized in that the start temperature for the expansion of the expandable graphite is settled at more than 180°C, particularly lies in the range between approx. 180°C and 220°C.
4. The method according to one of Claims 1 to 3, characterized in that the expandable graphite is added at a particle size of more than 20 µm, particularly in the range from 20 to 150 µm and preferably between approx. 150 µm and 300 µm.
5. The method according to one of Claims 1 to 4, characterized in that SOx or NOx expandable graphite is used as expandable graphite.
6. The method according to one of Claims 1 to 5, characterized in that the expandable graphite has a carbon content of 85% by weight to 99.5% by weight.
7. The method according to one of Claims 1 to 6, characterized in that the expandable graphite is added to the mixture made up of the base moulding material and the sodium silicate with a proportion of up to approx. 1% by weight, preferably with a proportion of up to approx. 0.5% by weight and particularly preferably with a proportion of up to approx. 0.1% by weight, in each case with respect to the base moulding material.
8. The method according to one of Claims 1 to 7, characterized in that the expandable graphite is added to the sodium silicate and then mixed with the base moulding material or added as separate additive to the base moulding material including sodium silicate.
9. The use of a core and/or moulding sand produced according to the method according to Claims 1 to 8 for producing casting moulds for cast aluminium alloys , iron-carbon alloys, copper alloys and/or magnesium alloys.
10. The use according to Claim 9, characterized in that the casting mould is used in the automotive industry.

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