EP2941327A1 - Method for the production of core sand and/or molding sand for casting purposes - Google Patents

Abstract

The invention relates to a method for producing core sand and/or molding sand for casting purposes. A granular mineral mold base material is mixed with at least one inorganic binder and additionally an inorganic expanding additive. According to the invention, water glass is used as the binder and expanded graphite is used as the expanding additive.

Description

 Process for producing a core and / or molding sand for

 foundry purposes

Description:

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

In a method of the structure described at the outset, as described, for example, in the applicant's EP 2 014 391 A2 or also US Pat. No. 4 505 750 A, bentonite is typically used as the 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 binding agent based on water glass, as described in DE 10 2004 042 535 A1. Generally, with the help of the inorganic binder, the individual grains of the granular mineral and refractory mold base material with each other

connected or glued. 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 additional intumescent additives added in the context of EP 2 014 391 A2 or also according to US Pat. No. 4 505 750 A now ensure that coring is facilitated. 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, DD 158 090 A1 deals 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 due to reactions of the core and / or molding sand with

let melt back. 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 consequently, ideally, the molding material trickles freely out of the casting.

Particularly in the case of inorganically bound cores with narrow contours in the millimeter range, as are observed, inter alia, in the case of cylinder heads with water jacket for the production of automotive engines, this results in core problems which are also intensified 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 binding 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 foundry technology for hardening molds and cores is basically known, as exemplified by DE 10 2004 042 535 A1, but not in combination with an additional intumescent 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. For example, the above expansion can be so

it can be determined 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 expandable graphites and the measurement of the expansion properties can be found inter alia in EP 1 489 1 36 A1. Thereafter, the expansion properties of expanded graphite can be determined, for example, 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. 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 also SOx and / or ΝΟχ.

In the context of the invention, in particular weakly expanding expandable graphites are used, which, on the one hand, significantly improve coring, and on the other hand have practically no adverse effects on the surface of the casting which forms after casting.

In order to demonstrate this fact quantitatively in particular, pouring tests were carried out with water glass bonded test specimens and additions of expanded graphite with different expansion rates.

For this purpose, a granular mineral molding base material (quartz sand) was used for the casting test, and with two different expansion rates as binder waterglass with 1, 6 wt .-% and 0.3 wt .-% expandable graphite, each based on the mold base material:

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

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 (see Fig. 4, sample number 2) Remains of expanded graphite macroscopically on the casting surface could recognize.

Compared to the sample number 1 or sample number 1 in Fig. 4, which had been treated with low 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 Abgießversuch no or hardly one - restrictions were 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 principle, it is also possible to work with an expansion rate of more than 350 cm ³ / g. Here, however, is expected with a reduced surface quality, as already described in Abgießversuch.

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 gives the

Volume increase of expandable graphite (in cm ³ ) relative to its mass (in g).

In the production of the expanded graphite according to the invention, sulfur or nitrogen compounds are generally incorporated into the individual layers of the graphite. It is therefore SOx or ΝΟχ expandable graphite. These typically have a starting temperature for expansion that is more than 1 80 ° 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 a Blähgraph it is typically such a used, the particle size is more than 20 μιτι. In particular, particles or grains in a diameter range from 20 μιτι to 150 μιτι used and preferably those with a grain size between 150 μιτι and 300 μιτι.

The described grain size of the expanded graphite up to a maximum of 300 μιτι takes into account inter alia 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 μιτι. In general, its grain size ranges from 100 μιτι to 300 μιτι. 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 humidity of the expanded graphite is in

Range of maximum 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 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 the 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. In essence, this can 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 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

Conclusions on the surface of the casting in principle not or practically can not occur.

Of particular importance beyond that high bending strengths are observed, which are located well above 100 N / cm ² , thus far exceeding the example given in EP 2 014 391 A2 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 for determining the bending strength given above can be produced, whereas according to the method according to EP 2 014 391 A2, such bending bars can not be produced at all. In any case, the strength over the teaching according to EP 2 014 391 A2 is significantly increased.

This fact is supported in particular by the comparative test values for 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 described, for example, in EP 2 014 391 A2 and uses bentonite as the inorganic binder. 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 inventive method from 1, 6 wt .-% water glass (based on the quartz sand or the granular mineral mold base material) and the same proportion expanded 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 test specimens, the following flexural 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 molded articles (measurements 1 to 3 refer to three test specimens of the same grammage) and production, which have been studied for statistical purposes):

Table 1 . Measurement series for testing the bentonite and water glass bonded specimens

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 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 here on the one hand additional material costs incurred for the molding boxes and on the other hand after each use a cleaning or reprocessing of the molding boxes is necessary, which also creates additional costs.

For comparison, water glass bonded moldings or molding materials without stabilizing mold frame (molding box) can be prepared so that they can be handled freely and inexpensively and so can be used as bentonite bonded moldings in terms of their casting technology application for a wider range of applications. Here are 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.

The core and / or molding sand produced by the process according to the invention can also be advantageously used 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 molds can be realized with the particularly filigree structures with

have 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 photograph shown in Fig. 1, a casting is shown in the left photo, which has been prepared without expandable graphite using a core and / or molding sand and water glass as a binder. The right-hand photograph in FIG. 1 shows the relevant workpiece with added expandable graphite in an amount of 0.1% by weight, based on the core or molding sand produced (likewise with water glass as binder, in both cases equal grammages for water glass and the core or molding sand as well as the same core or molding sand was used.

It is clear from the photographs in FIG. 1 that the casting surface is markedly improved even by the addition of expanded graphite, even in the stated amount, as the right-hand photograph in FIG. 1 shows. In contrast, when dispensing with expandable graphite, significant defects in the casting surface are to be expected, as the left-hand photograph in FIG. 1 makes clear. Theoretical considerations

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. 2 shows how predominantly hatched grains of sand 1

together with black represented inorganic binder or water glass 2 fill a water jacket core of a corresponding core shape. In FIG. 3A, by way of example, two grains or grains of sand 1 of the basic molding material are coupled to one another by a bridge of the inorganic binder or water glass 2 shown in black. FIG. 3B shows how, when the starting temperature is exceeded, a crack occurs in the bridge between the sand grains 1 formed by inorganic binders or the water glass 2. For this purpose, essentially the expanding graphite expanding above the starting temperature is responsible. Finally, FIG. 3C shows the breakage of the binder bridge 3 produced by the binder or water glass 2 in this way.

Claims

claims:

1 . Process for the production of a core and / or molding sand for foundry purposes, according to which a granular mineral molding base material is mixed with at least one inorganic binder and additionally an inorganic blowing additive, waterglass is used as the binding agent and expandable graphite is used as blowing additive.

2. The method according to claim 1, characterized in that the expandable graphite has an expansion rate of at most 350 cm ³ / g, in particular one of 10 to 100 cm ³ / g.

3. The method according to claim 1 or 2, characterized in that the starting temperature for expansion of the expanded graphite is settled at more than 180 ° C, in particular in the range between about 180 ° C and 220 ° C.

4. The method according to any one of claims 1 to 3, characterized in that the expandable graphite in a particle size of more than 20 μιτι, in particular in the range of 20 to 150 μιτι and preferably between about 150 μιτι and 300 μιτι is added.

5. The method according to any one of claims 1 to 4, characterized in that is used as expandable graphite SOx or ΝΟχ-expandable graphite.

6. The method according to any one of claims 1 to 5, characterized in that the expandable graphite has a carbon content of 85 wt .-% to 99.5 wt .-%.

7. The method according to any one of claims 1 to 6, characterized in that the expandable graphite of the mixture of the molding material and the water glass in a proportion of up to about 1 wt .-%, preferably with a proportion of

up to about 0.5 wt .-% and particularly preferably in an amount up to about 0.1 wt .-%, each based on the molding material is added.

8. The method according to any one of claims 1 to 7, characterized in that the expandable graphite is added to the waterglass and then mixed with the mold base material or added as a separate additive to the mold base including water glass.

9. Use of a produced according to the method according to claims 1 to 8 core and / or molding sand for the production of molds for

Aluminum casting alloys, iron-carbon alloys, copper alloys and / or magnesium alloys.

10. Use according to claim 9, characterized in that the casting mold is used in the automotive industry.