EP3551358B1 - Method and mold tool or core tool for producing molds or cores - Google Patents

Description

The present invention relates to a method for the production of molds or cores for foundry purposes using electricity by adapting the specific electrical resistance of the core box material to a mixture of a molding material and a water-containing inorganic binder, which in dissolved form forms an electrolyte and a sufficient electrical Has conductivity. The invention also relates to a mold or core tool for producing molds or cores.

From the WO 2003/013761 A1 and US 2004/0192806 A1 a generic method is known in which magnesium sulfate is used as the inorganic binder, which is dispersed and / or dissolved in water and then mixed with foundry sand. This mixture of a molding material, that is to say for example foundry sand and the binding agent containing water, is then introduced into the mold or core tool and cured there by heating. The use of an inorganic binding agent is intended to prevent environmentally harmful gases from escaping when the mixture cures. This application is partly based on the patent application DE 24 35 886 A1 from 1974 on the hardening of sand cores by means of "passing an electric current through".

In the mentioned publication WO 2003/013761 A1 it is stated that the energy required for curing is made available by means of electricity. The electricity is applied via two or more electrodes to "at least partially electrically conductive, mutually insulated parts of the separable mold or core tools". The aforementioned application does not take into account the differences between the electrically specific Resistance properties of the core tool and the electrical specific resistance properties of the sand-binder mixture. It uses "mutually insulated parts of the separable mold or core tools".

From the DE 37 35 751 A1 A gas-permeable molding tool for the production of casting and core molds from hardenable molding sand is known, the tool being made of heteroporous, open-pored material and the wall of the molding tool having a first fine-pored layer area of 0.2-2 mm thickness adjoining the molding sand, 75-95% of the theoretical material density and pore diameter <50 µm, to which a second, massive area in the form of a large-pored supporting skeleton of <80% of the theoretical material density and an average pore diameter <100 µm is materially adjacent.

From the DE 24 35 886 A1 a method is known for producing foundry molds or cores by introducing a mixture of aggregate and binder into a mold or core box and heating the mixture, the heating being effected by passing an electric current through the mixture.

From the EP 3 103 562 A1 a template is known which has a frame-shaped or box-shaped configuration, preferably slightly tapering downwards, with a circumferential wall and, in the case of a box-shaped configuration, also a base.

Forming or core tools for inorganic processes are mainly made of metal such as steel or aluminum.

The disadvantage of the above-mentioned application is that an insulation layer is required between the parts of the mold or core tool, which is intended to prevent the short circuit when the voltage is applied and thus to cause the current to flow through the sand-binder mixture.

Another disadvantage of the technology arises despite the use of an insulation layer. The electric current always seeks the path of least resistance to balance the electric potentials.

Metallic core tools have a resistance range of, for example, 2x10 ^-7 ohmmeters (steel) with sand-binder mixtures in the range of approx. 10 ¹ to 10 ² ohmmeters. Since the resistance at the core box is significantly lower than in the sand-binder mixture, the current flows to the contact area inside the core box and is then passed through the sand-binder mixture for a short distance. As a result, there is almost no current flowing through thicker parts of the sand core, which means that there is insufficient heating. This means that the mixture does not harden evenly.

If such a only partially cured core is removed from the mold or core tool, it can be damaged or lead to damage when it is used later in a casting tool.

Another disadvantage is based on the same approach that electricity always seeks the path of least resistance. With core boxes made of non-conductive material and two opposing electrodes, the method would therefore only work for geometries with the same sand core thickness. For example, this is the case with cylinders and cuboids. The method can therefore only be used for simple geometric shapes.

Another disadvantage can be observed when curing by means of heat transfer. Since sand-binder mixtures are generally rather poor heat conductors, when heat is transferred from heated core boxes, shell formation occurs on the outer edges of the sand core, as the shell hardens sooner than the inside of the sand core. For economic reasons, it is not always necessary to wait for complete hardening before removal, so that the sand cores can break more easily.

Another disadvantage arises from the effect of the above-mentioned shell formation. Since the inside of the sand core has not yet completely hardened due to the shell formation, this limits the maximum sand core thicknesses that can be produced with existing methods. The maximum thickness of the sand core depends on the duration of the heating and the weight of the sand core. If the heating is not sufficient, the outer shell of the sand core cannot fully support the weight despite complete hardening and can thus lead to breakage of the sand core.

The present invention therefore deals with the problem of specifying an improved or at least an alternative embodiment for a method of the generic type which in particular overcomes the disadvantages known from the prior art.

According to the invention, this problem is solved by the subjects of the independent claims. Advantageous embodiments are the subject of the dependent claims.

The present invention is based on the general idea when selecting the material of the separable mold or core tools specific electrical conductivity must be taken into account in such a way that it corresponds approximately to the electrical conductivity of a (sand-binder) mixture during the optimum working temperature. The electrical specific conductivity of the mold or core tool (cavity) is therefore determined by the sand-binder mixture used.

This can achieve the special effect that a current introduced into the material finds almost the same electrical conductivity everywhere in this and in the mixture and thus does not look for a significantly shorter, in particular shortening, path through the mixture, whereby a uniform flow through the mixture with electricity and thus also a uniform heating and thereby also a uniform hardening of the same can be achieved regardless of the respective individual shape or shape of the core.

In general, in the method according to the invention, an electrically conductive material is first permanently introduced into a housing of the mold or core tool and there takes the previously described mixture of a molding material, e.g. sand (foundry sand), and water-containing binder, which is in dissolved form Forms electrolyte and has sufficient electrical conductivity.

The present invention is further based on the general idea of specifying a mold or core tool for producing molds or cores, for example casting cores, from a mixture of a molding material and a binder containing water, which in dissolved form forms an electrolyte and has sufficient electrical power Has conductivity, wherein the mold or core tool according to the invention has an electrically non-conductive housing consisting of at least two parts. The form or The core tool also has at least two electrodes, one electrode in each case being arranged in a part of the housing. Electrical energy is later introduced into the material via the two parallel electrodes and into the mixture via this, whereby the mixture is heated and thereby hardened.

The process requires direct contact between the conductive material and the electrodes of the core box. An insulation layer between the core box parts can thus be dispensed with.

The mixture is introduced for each cycle of sand core production, with the electrically conductive material being introduced once per production of the mold or core tool. The material thus forms the negative contour of the sand core or mold to be produced later in it. After the mixture is embedded in the material, electrical energy and above that heat is then supplied to the material via the electrodes arranged in / on the housing of the mold or core tool, which leads to hardening of the mixture.

As with existing patent applications, the housing only represents a container for holding the conductive material and does not have to be electrically conductive, since otherwise the current is only passed through the housing and not through the material or the mixture. The housing can be made of plastic and offers the advantage that it is comparatively light and therefore easy to handle. Alternatively, insulating ceramics or another electrically non-conductive material can also be used.

Parts of the housing can be connected to one another via one or more parting planes, the electrodes preferably being parallel can be arranged to one another or even embedded in a part of the housing.

In a further advantageous embodiment, a device for controlling / regulating the electrical voltage is provided on the electrodes. By means of such a device, the voltage applied to the electrodes can be regulated, for example increased, so that short cycle times for the curing process can be achieved. Short cycle times, in turn, enable the molds or cores to be manufactured in a comparatively cost-effective manner. The power / voltage can be regulated by means of an inverter / power controller or by connecting different voltages. Alternatively, the method can also be operated with a constant applied voltage.

As in the DE 24 35 886 A1 executed, the electrical energy can be supplied to the material and sand-binder mixture (mixture) in the form of alternating current or direct current. Alternating current is available everywhere and can be regulated in almost any way.

In addition, ventilation slots (nozzles) are to be provided in the material, in the electrodes and in the housing to allow the gases or water vapor to escape. As with existing processes, the gases or water vapor produced during the hardening process can be removed from the sand core (core) and the material, the electrodes and the housing via bores using core marks (nozzles). Alternatively, the material can also be porous and thus allow the gases or water vapor to escape.

Furthermore, holes for non-conductive ejector bolts are provided in the material, which are used to remove the (sand) cores. These allow the sand cores to be removed after the mixture has hardened and the moving apart of the housing parts. The ejector bolts should be made of non-conductive material to avoid a short circuit. The ejector bolts required are attached to the base plate of the tool in the ejector holes provided for this purpose.

Alternatively, conductive ejector bolts can also be used, provided that the design ensures that they do not come into contact with a conductive material while the current is switched on.

The solution according to the invention, according to which the specific electrical conductivity of the material at least approximately corresponds to the specific electrical conductivity of the mixture at working temperature, a uniform and in particular uniform passage of current or voltage through both the material and through the mixture can be achieved, whereby the latter evenly heated and can therefore be cured particularly evenly and thus of high quality.

Several steps are necessary to optimally select electrically conductive materials for this process. Each binder has an optimal working temperature which ensures the best possible curing. In the case of the binders tested, this was around 150-180 ° C and depends on the manufacturer's information and any binder additives used. Compared to methods known from the prior art, in which it was always feared that the mixture would have a locally different degree of hardening due to different internal electrical resistances, e.g. caused by different sand core thicknesses, the method according to the invention can be used for the first time to achieve a uniform, This means that uniform and also process-reliable curing of the mixture can be achieved, whereby molds or casting cores of can be produced in a particularly high quality regardless of their geometric structure. In addition, the method according to the invention prevents the risk of shell formation on a core surface or a mold surface, which would be the case, for example, with curing by means of external heat (eg oil heating).

With the mold or core tool according to the invention, a process-reliable production of molds or cores is thus possible for the first time by adapting the electrical specific conductivity of the mold / core box material to the sand-binder mixture. This allows the uniform transmission of electrical energy and thus uniform heating and thus uniform curing. This was previously not possible due to the disadvantages mentioned above.

By adapting the electrical resistance of the material to the sand-binder mixture, larger and more complex sand cores can also be produced economically using one electrode per core part, as there are no significant differences in resistance at any point due to sand core thicknesses due to different contours.

In addition, by adapting the specific electrical resistance, depending on the sand core thickness, you can also work in accordance with the guidelines for low voltage of up to 1000 V. This means that the process is not only more secure for employees, but is also more cost-effective. In principle, however, higher voltages as in existing patents are also possible. The rule is that the thicker the sand core, the higher the voltages should be used.

The direct heating of the sand core as well as the material without detours via external heating devices such as oil heating or steam increases the efficiency of the process and thanks to the uniform heat supply over the entire surface of the core, short heating phases and thus short cycle times result.

Another advantage results from the fact that no external heating devices are required. This not only increases the efficiency of the process, as described above, but also reduces the acquisition and maintenance costs for any external heating devices. In addition, this makes it possible to provide systems with a smaller space requirement, so that more systems can tend to be accommodated in the same area.

Another advantage arises for the core tool. Existing systems that require thermal energy for curing require that the heat from the heating source is supplied as close as possible to the sand core in the core box. This is partly solved by complicated heating bores within the base plate or the core box. These work steps can be omitted completely, as the heat is generated directly where it is needed: in the sand core and core box.

Another advantage results from the use of materials such as silicon carbide ceramic, which is a very hard material compared to existing core tool materials such as steel or aluminum (Mohs strength 9.5) and thus the life of the core box is extended due to less wear.

In general, a method according to the invention for the production of molds or cores for foundry purposes works by adapting the specific Electrical resistance of the material of the tool insert to the specific electrical resistance of a mixture of at least one molding material, in particular foundry sand, and at least one water-containing inorganic, thermosetting binder, which has sufficient electrical conductivity of at least 5 · 10 ^-3 S / m.

• wird in ein elektrisch nicht leitendes Gehäuse mindestens ein Werkzeugeinsatz aus einem elektrisch leitfähigen Material zur Aufnahme der Mischung eingebracht, wobei die elektrische Leitfähigkeit des Materials bei Betriebstemperatur zwischen 150 und 180 °C zumindest näherungsweise der spezifischen elektrischen Leitfähigkeit der Mischung bei einer Temperatur zwischen ca. 100°C bis 130°C entspricht,
• wird dem Werkzeugeinsatz über in/an dem Gehäuse parallel angeordnete und bei Bedarf vollflächigen Elektroden elektrische Energie und darüber Wärme zugeführt, die zum Aushärten der Mischung führt,
• besteht das Gehäuse aus mindestens zwei Gehäuseteilen, welche zum Beginn und Abschluss des Taktvorgangs der Form- oder Kernherstellung zusammen- bzw. auseinandergefahren werden und zusammengefahren eine direkte Kontaktfläche ohne isolierende Zwischenschicht bilden,
• sind benötigte Bohrungen für Ausstoßbolzen im Werkzeug, mindestens einer Elektrode sowie mindestens eines Teiles des Gehäuses zur Entnahme der Sandkerne vorhanden,
• sind zum Entweichen von Wasserdampf oder Gasen sowohl das Werkzeug als auch die Elektroden sowie mindestens ein Teil des Gehäuses porös ausgeführt und/oder Entlüftungsschlitze vorhanden und
• werden der oder die Formen oder Kerne nach dem Aushärten der Mischung und dem Auseinanderfahren der Gehäuseteile mittels Ausstoßbolzen aus dem Werkzeug gedrückt und entnommen.

There

• at least one tool insert made of an electrically conductive material for receiving the mixture is introduced into an electrically non-conductive housing, the electrical conductivity of the material at an operating temperature between 150 and 180 ° C at least approximately the specific electrical conductivity of the mixture at a temperature between approx ° C to 130 ° C,
• electrical energy is supplied to the tool insert via electrodes arranged in parallel in / on the housing and, if required, full-surface electrodes and, above this, heat, which leads to the hardening of the mixture,
• the housing consists of at least two housing parts, which are moved together or apart at the beginning and the end of the cycle process of mold or core production and when moved together form a direct contact surface without an insulating intermediate layer,
• Are the required holes for ejector bolts in the tool, at least one electrode and at least part of the housing for removing the sand cores available,
• Both the tool and the electrodes as well as at least part of the housing are designed to be porous and / or ventilation slots are provided for the escape of water vapor or gases
• the mold or molds or cores are pressed out of the tool and removed by means of ejector bolts after the mixture has hardened and the housing parts have moved apart.

Further important features and advantages of the invention emerge from the subclaims, from the drawings and from the associated description of the figures on the basis of the drawings.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention, which is defined by the claims.

Preferred exemplary embodiments of the invention are shown in the drawings and are explained in more detail in the following description, with the same reference symbols referring to the same or similar or functionally identical components.

Fig. 1

eine Schnittdarstellung durch ein erfindungsgemäßes Form- oder Kernwerkzeug,

Fig. 2

ein Phasendiagramm mit qualitativer Darstellung einer eingebrachten elektrischen Leistung und eines zugehörigen Widerstandes in einem Kern oder einer Form,

Fig. 3

eine Darstellung der Erwärmung mittels bestehenden elektrischen Verfahren ohne Anpassung des spezifischen Widerstandes des (Kernkasten-) Materials an das Sand-Bindergemisch (Mischung),

Fig. 4

eine Darstellung einer möglichen Kernkastenausführung,

Fig. 5

eine Befestigung des Materials mit isolierendem Gehäuse und Grundplatte,

Fig. 6

eine Darstellung von Entlüftungs- und Ausstoßbohrungen mit einer Ansicht von oben (Fig. 6 a.)), einer Ansicht von vorne (Fig. 6 b.)) und einer Seitenansicht (Fig. 6 c.)).

They show, each schematically,

Fig. 1

a sectional view through a mold or core tool according to the invention,

Fig. 2

a phase diagram with a qualitative representation of an electrical power input and an associated resistance in a core or a shape,

Fig. 3

a representation of the heating by means of existing electrical processes without adapting the specific resistance of the (core box) material to the sand-binder mixture (mixture),

Fig. 4

a representation of a possible core box design,

Fig. 5

an attachment of the material with an insulating housing and base plate,

Fig. 6

a representation of vent and discharge bores with a view from above ( Fig. 6 a.)), a view from the front ( Fig. 6 b.)) and a side view ( Fig. 6 c.)).

According to the Fig. 1 has a mold or core tool 1 according to the invention for producing molds 2 or cores 2 'for foundry purposes, a housing 3 which is electrically insulated towards the machine and which consists of two parts 4, 5 which are connected to one another via a parting plane 6. The housing 3 is fastened on a base plate 12. The housing 3 is made of plastic, insulating ceramic or some other non-conductive material and accommodates an electrically conductive material 7. The material 7 forms a mold for receiving a mixture 9 from which the core 2 ′ or the mold 2 is formed after the hardening. The material 7 can for example be a ceramic material. According to the invention, the specific electrical conductivity of the mixture 9 and the specific electrical conductivity of the material 7 are at least approximately the same, for example do not differ more than in phase 2 of Fig. 2 so that essentially the same specific electrical conductivity and the same specific electrical resistance prevail in the material 7 and the mixture 9. The molding or core tool 1 according to the invention also has at least two electrodes 10 which are arranged parallel to one another. A device 8 for regulating or controlling the voltage supplied to the electrodes 10 is provided.

According to the invention, the specific electrical conductivity of the material 7 of the core 2 ′ or of the form 2 now approximately corresponds to the specific one electrical conductivity of mixture 9 in phase 2 of Fig. 2 , whereby a comparatively uniform passage of electrical energy through the mixture 9 is possible.

With the molding or core tool 1 according to the invention, a mold 2 or a core 2 'or a casting core 2' can be produced at the highest level of quality, since the electrical conductivity of the mold 2 or the core is at least almost the same 2 'used mixture 9 and the material 7 a uniform passage of electrical current through the material 7 and the mixture 9 and thus a uniform heating and curing of the mixture 9 can take place regardless of the respective geometric dimensions of the mold 2 or the core 2 '.

The mold 2 or the core 2 'is produced as follows: First, after the material selection mentioned, the electrically conductive material 7 is introduced into the housing 3 of the mold or core tool 1 during the initial construction and forms a negative mold for the later mold 2 or The mixture 9 which will later form the core 2 'is then supplied to the material 7 via the electrodes 10 with electrical energy and thus heat, which leads to the mixture 9 hardening. The mixture 9 is hardened in particular by evaporation of water from the mixture 9, the mixture 9 containing an inorganic binder, water and foundry sand.

The inorganic binder used in the mixture 9 (sand-binder mixture) can be water-soluble, but contains at least water and is in any case electrically conductive. With the method according to the invention and with the molding or core tool 1 according to the invention, a casting core or core 2 ′ that is particularly uniformly heated and therefore also particularly uniformly cured and thus homogeneous can be created and this regardless of the respective geometrical dimensions of the core 2 'or the shape 2, since due to the preferably identical electrical conductivity of the mixture 9 for the core 2' and the material 7, the electric current does not seek shorter paths, as was previously the case with the Prior art mold or core tools was the case. This has so far led to the fact that, due to the electrical paths caused by the geometric dimensions of the core 2 'or the form 2, these may not yet have hardened uniformly and thus have areas with complete hardening and only partial or no hardening, as a result of which the quality of the molds or cores produced so far with the previous molding or core tools has often not been satisfactory.

The device 8 can in particular increase or decrease the voltage, whereby a cycle time for the production of the mold 2 or the core 2 ′ can be controlled.

The base plate of the tool 12 accommodates the housing 3 or the parts 4, 5 and the material 7, and insulating screws 13 and angles 14 ensure fastening. Insulating screws 13 can also be replaced by quick-release systems to enable easier and faster removal. The material "floats" on the electrode 10 and the electrode 10 is held in place by alignment bolts 15.

Table 1 is attached below for further understanding. Table 1 shows several series of measurements with different sand-binder mixtures 9. The finding is that the specific electrical conductivity depends on the desired sand-binder mixture 9 and can be influenced by varying the additives and / or by changing the percentage. Ever The stronger the electrically conductive part in the sand-binder mixture 9, the lower the specific electrical resistance in the sand-binder mixture 9. Table 1: Series of measurements of sand-binder mixtures. Series of measurements specific heat sand Area of test specimen cm2 Height of specimen cm2 Lowest measured resistance (optimal point) Ohm Water glass 2% 0.835J / g * K 6.1 2 1080 Water glass 3% 0.835J / g * K 6.1 2 1130 Water glass 3% and graphite 0.5% 0.835J / g * K 6.1 2 588 Water glass 3% graphite 1% measurement series 1 0.835J / g * K 6.1 2 529 Water glass 3% graphite 1% measurement series 2 0.835J / g * K 6.1 2 498 Water glass 4% measurement series 1 0.835J / g * K 6.1 2 523 Water glass 4% measurement series 2 0.835J / g * K 6.1 2 584 Water glass 10% and graphite 5.0% 0.835J / g * K 6.1 2 12.78 Innotek Binder from ASK 0.835J / g * K 6.1 2 781 Cordis Binder from Hüttenes Albertus 0.835J / g * K 6.1 2 683 Foundry tie (undisclosed) 0.835J / g * K 9.6 3.5 499

The procedure described below is therefore to be used to determine the specific electrical properties of the desired sand-binder mixture 9. However, this method can also be used if the (sand-binder) mixture 9 has not yet been defined. In this case, an attempt can be made to influence the specific electrical properties of the sand-binder mixture 9 in a targeted manner, e.g. by varying the additives, in order to improve the efficiency of the process.

Several steps are necessary to optimally select electrically conductive materials for this process. Each binder has an optimal

Working temperature which ensures the best possible curing. In the case of the binders tested, this was around 150-180 ° C and depends on the manufacturer's information and any binder additives used. First, the specific resistance curve of the desired inorganic sand-binder mixture 9 must be determined as a function of the temperature. Table 1 shows examples of selected resistance-temperature values for sand-binder mixtures based on inorganic binders and binder variations. Various water glass components and graphite additives were also examined. The curves were determined as follows:

First, a reference test specimen must be made. The test specimen consists of two opposing metallic electrodes and an insulating tube between the electrodes. The geometry (area and distance of the electrodes) of the body within the insulating tube must be determined. The cavity is filled with a green, not hardened sand-binder mixture 9. The sand-binder mixture 9 must correspond to the mixture 9 to be used later during production. The mixture 9 must be compressed according to real application conditions. Measuring devices for determining voltage, current and temperature are connected to the electrodes. A constant voltage is applied to the electrodes via a power supply. The calculated resistance results from the applied voltage divided by the measured current.

mit

Rho:

spezifischer elektrischer Widerstand der Mischung

R:

Widerstand vor Anstieg des elektrischen Widerstandes der Probe

A:

Elektrodenfläche der Mischung

I:

Dicke der Probe

The temperature-dependent specific resistance is calculated as follows: $$\mathrm{Rho}=\mathrm{R.}*\mathrm{A.}/\mathrm{I.}$$

With

Rho:

specific electrical resistance of the mixture

R:

Resistance before the electrical resistance of the sample increases

A:

Electrode area of the mixture

I:

Thickness of the sample

This results in a temperature-dependent resistance curve for each sand-binder mixture 9.

All measured resistance curves have the following characteristic form as in Figure 2 .

In Fig. 2 the typical course of the electrical resistance and the electrical power introduced of a conductively heated mixture 9 of any inorganic sand / binder mixture is shown. After switching on the voltage, the resistance drops significantly within a very short time (phase 1: capacitive load). Then phase 2 of the slowly falling electrical resistance begins in the curve (increase in charge carriers). During this time, the power absorbed by the sample increases continuously until charge carriers evaporate due to the temperature reached. The resistance now increases very quickly (phase 3).

For the selection of the specific electrical resistance (Rho) of the ceramic material for a later form, the point in time before the increase in the electrical resistance of the sample in phase 3 is optimal, since the greatest power can be introduced here (shortly before the end of phase 2). This is in Fig. 2 labeled 11.

Furthermore, specific electrical resistances, which result from the calculation of the values within phase 2, are also conceivable.

The specific electrical resistance of the tested mixtures 9 changes during the heating process. At below 100 ° C it is approx. 85

Ohmmeter and drops below 25 ohmmeter at over 130 ° C if heated further. With further heating, the specific resistance increases by leaps and bounds. Then, however, the energy required to expel the water from the binder, which leads to hardening, is also present in the sand-binder mixture 9.

For the optimal selection of electrically conductive materials for this method, after determining the temperature-resistance curve of the sand-binder mixture 9, it is possible to determine the material 7 based on the required specific resistance.

Based on the specific resistance of the sand-binder mixture 9, a material composition must be determined by means of test series, which has a suitable electrical specific resistance at a certain temperature. This specific temperature is based on the optimum temperature which the binder needs to best cure. In our tests, tested binders required temperatures of approx. 150 ° C to approx. 180 ° C in order to cure. The area around the optimal resistance was determined by means of a temperature resistance curve (see above) around 25 ohmmeters. Consequently, the tested binder mixture 9 requires a material 7 with a specific resistance of approx. 25 ohm meters at 150-180 ° C.

In principle, the specific resistance of the material 7 should be the same as the optimum specific resistance for the sand-binder mixture 9. If the specific resistance of the material 7 is higher than that of the sand-binder mixture 9 during implementation, this tends to lead to heating from the center of the core 2 in the direction of the core box material 7, since this is where the current finds the path of the lower resistance.

Should the specific resistance of the material 7 be lower during the implementation than in the sand-binder mixture 9, then there is a tendency for the core box material 7 to be heated in the direction of the center of the sand core.

Likewise, the course of the temperature-resistance curve of the material 7 should run similarly to the temperature-resistance curve of the sand-binder mixture 9. The smaller the deviation of the two curves, the more effective the method.

The test series to determine the material can be carried out as follows:
A starting material such as Example silicon carbide, is produced in the form of a small test plate. This material sample is then clamped in a device between two electrodes so that these electrodes are in direct contact with the sample plate. The temperature-resistance curve for this sample material is then determined. If the deviation between the specific resistance of the sample material and the optimal specific resistance of the sand-binder mixture 9 is too great, the material composition must be revised. In tests carried out, silicon carbide compositions with a variation in the proportion of graphite in the ceramic mixture have proven to be positive. But in principle there are also other material compositions or material additives that add to the electrical affect specific resistance, possible. The graphite content is bound in the ceramic and therefore has no influence on further casting processes. These tests have to be repeated until a suitable material composition has been found which has the desired specific resistance.

Furthermore, the selected material 7 must also meet the other physical properties for the environment of foundries. For example, breaking strength, surface roughness, thermal expansion and thermal conductivity are mentioned here.

For example, when the required operating temperature of approx. 180 ° C is reached, the ceramic selected for further tests has a specific resistance of approx. 30 ohmmeters for the above-mentioned sand-binder mixture 9. The maximum short-term load on the material 7 must then be determined at which no permanent damage to the material 7 occurs. This maximum short-term load subsequently plays an important role for the electrical control. This is determined with load tests and can lead to chipping on the material 7 if the maximum short-term load is exceeded.

In a further advantageous embodiment of the solution according to the invention, the material 7 mentioned above and below can be replaced by other materials as defined in the independent claims, provided that these are electrically conductive and the adjustment of the electrical resistivity corresponds to the selected mixture 9 and also the other requirements for the foundry are met.

The repeated term "adaptation" describes the aforementioned steps for selecting a suitable material 7 to the specific electrical properties of sand-binder mixtures 9. After the selection (adaptation) of the suitable material 7 according to the method described above has been successful and to the sand -Binder mixture 9 has been adapted, the structure of the core box can be produced for the application of the method. The most critical work step is the production of the material 7. In the case of the silicon carbide ceramic mentioned by way of example, the ceramic is produced in several production steps using common ceramic production processes. The fine machining after sintering in particular requires the greatest care because of the very hard material (Mohs hardness of approx. 9.5). The more precise the fine machining, the lower the later tolerance deviations for sand cores 2 produced with the method.

As soon as the finishing of the material 7 has been successfully completed, it can be fastened in the core box. The material 7 requires a direct contact area with the respective electrode on the opposite side of the contouring surface. In experiments it has been recommended to grind the contact surface flat in order to enable very good contact between the electrode 10 and the material 7. This leads to the desired effect of keeping the contact resistance low.

As in Figure 4 shown, the electrode 10 should be laid floating on the back of the material part. This is necessary because the material of the electrodes 10 normally has a higher thermal expansion than the core box material. For this purpose, two pins can be attached to the rear of the material, which hold the electrodes 10 in position during the production process.

Due to the parallel arrangement of the electrodes 10, a comparatively uniform transmission of electrical energy through the material 7 and the mixture 9 can be achieved, which in turn results in advantages with regard to uniform heating and uniform curing. One possible embodiment also provides for the electrodes 10 to be introduced into the material 7. In this case, no pins would be needed for alignment. The electrodes 10 and the material 7 are then received by means of a recess in an insulating material.

The multi-layer planes can be fastened by means of anchoring in the base plate 12 of the tool. Brackets 14 with screw connections 15 can be used for fastening, as in Figure 5 shown as an example. In order to enable a quick exchange of individual materials, quick locking systems can be used instead of screws.

The fastening screws 15 should be made of non-conductive material in order to avoid a current flow to the housing 3. In addition, ventilation slots 17 (nozzles) are to be provided in the material 7, in the electrodes 10 and in the housing 3, in order to allow the gases or water vapor to escape. As in existing processes, gases or water vapor produced during curing can be removed from the sand core 2 ″ (core) and the material 7, the electrodes 10 and the housing 3 via bores 17 using core marks (nozzles). Alternatively, the material can also be porous and thus allow the gases or water vapor to escape.

The electrodes 10 require a power supply which is connected to the external switchgear cabinet and thus enables an electrical control 8.

The electrical control 8 must be adapted to the core box and the process. The electrical control 8 takes on the task of supplying the core box with sufficient power by means of power supply and electrodes 10. For new systems, the electrical control 8 (device 8) must be planned accordingly. When converting existing systems to the new process, existing switchgear may be converted and adapted. It is important that the energy is supplied to the material 7 via electrodes 10. Alternating current or direct current is conceivable.

The control of the power supply must take into account the maximum short-term load of the selected material 7 as well as the resistance-temperature curve of the material 7 and the sand-binder mixture 9.

The electrical control 8 is to be selected so that the highest possible power input takes place by means of high voltage, but the maximum short-term load limit is never exceeded in order to prevent damage to the material 7 and thus ensure an economical process. The power input and the associated heat development in the sand-binder mixture 9 is dependent on the specific resistance and the applied voltage. Therefore, the power input and the temperature can also be controlled by regulating the voltage. In addition, the core box should have temperature sensors to prevent it from heating up beyond the prescribed working range of the binder, as too high a temperature would otherwise negatively affect the binding force.

The electrical control 8 also regulates the different process steps of the core shooter. Particularly when moving the core box parts together, care must be taken to ensure that they are brought together in one adjusted speed happens in order to avoid a shock effect in the core box material and thus a possible permanent damage.

In the case of core tools with several sand cores 2, either one pair of electrodes per sand core 2 "or one pair of electrodes that cover all sand cores 2 of the complete core box can be used Curing time, however, the temperature in the sand core 2 "can never rise above the point at which the binders lose their binding force.

Other devices for external heating of core boxes can be omitted. Other devices such as pressure ventilation can continue to be used.

The regular production process is divided into three processes. The first process describes the commissioning of the system after a short or long downtime.

A feature during this process is that the material 7 has not yet reached the planned operating temperature. The core box is heated in the same way as in the typical production process. The parts 4, 5 are brought together from their starting position and form a contact surface. Then the sand-binder mixture 9 can be shot into the core box. In the next step, the energy is supplied by means of electricity thanks to the electrical control 8. Due to the increased specific resistances of the material 7, the warming-up process takes a little longer than the regular production cycle times. During the warm-up process, the core box slowly warms up and, as the temperature rises, it falls Specific resistance of the material 7. The more the resistance falls, the faster the material 7 continues to heat up according to the principle of resistance heating. Since the heat input in the first sand cores 2 does not take place under optimal conditions, increased rejects can occur during this process.

As soon as the desired operating temperature for the binder is reached on the core box, the actual production process begins. The process parameters can be described as follows. The material 7 of the core box has the operating temperature and thus the optimal specific resistance of the sand-binder mixture 9. The core box parts 4, 5 have moved apart and the sand core cavity is empty. In the first step, the core box parts 4, 5 are closed and then the sand-binder mixture 9 is shot into the core box. The specific resistance is dependent on the temperature of the sand-binder mixture 9. The mixture 9 can be at room temperature or can already be preheated. As soon as the sand-binder mixture 9 has been shot into the core box, the direct contact surface with the sand-binder mixture 9 of the core box material cools down somewhat. This increases the resistance of the core box material 7 for a short time, with the specific resistance of the sand-binder mixture 9 falling at the same time thanks to the heat absorption. Since, as described above, the temperature-resistance curves of the material 7 and the sand-binder mixture 9 are similar, the deviation in the specific resistance remains limited. The electrical control 8 activates the current flow and this leads to a current flow through the material 7 as well as through the sand core 2 ″. As the temperature increases, the resistance of the sand-binder mixture 9 and in the material 7 decreases until the optimum resistance is almost reached. At this moment the power entry is optimal.

The sand-binder mixture 9 has now heated up from the initial temperature to approx. 100 to 130 ° C. within a few seconds, depending on its size. As soon as the free charge carriers are reduced as a result of the evaporation of the water content in the sand-binder mixture 9, the specific resistance of the sand-binder mixture 9 suddenly begins to increase. At this moment, the current flow within the sand core 2 is reduced. In order to achieve the desired optimum operating temperature for the sand-binder mixture 9, the remaining thermal energy must now be transferred via the core box material 7, as is the case with existing methods.

In tests carried out, the silicon carbide material is continuously further heated by means of a current flow in order to compensate for the heat loss of the material 7 to the sand core 2 ″.

The particular advantage of the method therefore lies in the heating of the sand-binder mixture 9 from the temperature at the point of injection up to approx. 130 ° C through the principle of resistance heating by means of current flow within the sand core 2. The further advantage is the efficient heating of the material 7 and thus the supply of heat in the phase from 130 ° C. to the desired operating temperature of the sand-binder mixture 9.

A sand-binder mixture 9 with an operating temperature of approx. 170 ° C. and an injection temperature of approx. 20 ° C. is used as an example. In total, approx. 150 ° C are required for heating. Using the method, 2/3 (approx. 100 ° C.) of the required thermal energy can therefore be generated very quickly by means of resistance heating within the sand core 2 and approx. 1/3 by means of heat transfer from the material 7 to the sand core 2 ″.

After reaching the operating temperature or after hardening, the sand core 2 ″ can be removed as with existing core shooting processes. Ejection bolts 16 required for ejecting the sand core from the cavity are fastened in the ejection bores 16 ′ provided for this purpose and enable the sand cores 2 to be detached from the material 7.

The third process describes the cool down phase before a break or shutdown. In this phase, the core box can simply cool down in the extended state and is then available again at any time for the first process step.

Compared to methods known from the prior art, in which it was always feared that the mixture 9 had a locally different degree of hardening due to different internal electrical resistances, for example caused by different sand core thicknesses, the method according to the invention can be used for the first time with a uniform , that is to say uniform and also reliable curing of the mixture 9 can be achieved, whereby molds 2 or casting cores 2 'of particularly high quality can be produced regardless of their geometric structure. In addition, the method according to the invention prevents the risk of shell formation on a core surface or a mold surface, which would be the case, for example, with curing by means of external heat (e.g. oil heating).

With the mold or core tool 1 according to the invention, a process-reliable production of molds 2 or cores 2 ′ is possible for the first time by adapting the electrical specific conductivity of the mold core box material 7 to the sand-binder mixture 9. This allows uniform passage of electrical energy and thus uniform heating and thus uniform curing. So far this has not been possible.

Claims (10)

1. A method for producing moulds (2) or cores (2') for foundry purposes by means of adapting the specific electrical resistance of the material of the tool insert to the specific electrical resistance of a mixture (9) of at least one moulding material, in particular, foundry sand, and at least one water-containing inorganic binder that can be cured by means of heat and that has an electrical conductivity of at least 5 · 10^-3 S/m,

   - at least one tool insert made of an electrically conductive material (7) for holding the mixture (9) being introduced into an electrically non-conductive housing (3), the electrical conductivity of the material (7) at an operating temperature between 150 and 180 °C at least approximately corresponding to the specific electrical conductivity of the mixture (9) at a temperature between approximately 100 °C and 130 °C,

   - electrical energy and thus heat being supplied to the tool insert (7) via electrodes (10) that are arranged in parallel in/on the housing (3), which leads to the curing of the mixture (9),

   - the housing (3) being made of at least two housing parts (4, 5), which are moved together or apart from each other at the beginning and upon completion of the cycle process of the mould or core production and forming a direct contact surface,

   - holes (16') for ejection pins (16) in the tool, at least one electrode (10), as well as at least at least a part (4, 5) of the housing (3) being provided for removing the sand core,

   - both the tool and the electrodes as well as at least a part of the housing (4, 5) being porous and/or ventilation slits (17) being provided, and

   - the mould(s) or the core(s) (2, 2') being pressed out of the tool and removed by means of ejection pins (16) after the curing of the mixture (9) and once the housing parts (4, 5) have been moved apart from each other,

   characterised in that a material (7) for tool inserts is used, which has the following characteristics:

   - it is a sintered solid body which

   - has a Mohs hardness of more than 4,

   - the specific electrical resistance of the material (7) being between 0.5 ohmmeters and 200 ohmmeters at an operating temperature of 150 °C to 180 °C, and

   - the heat conductivity being at least 0.56 W/(m*K).
2. The method according to claim 1,
   characterised in that
   the electrical energy is supplied in the form of alternating current or direct current to the tool insert (7) and the electrical voltage is controlled by means of a device (8) for open-loop/closed-loop control, under consideration of the specific temperature-resistance curve of the sand-binder mixture, the temperature of the tool insert (7) as well as the maximum short-term stress load of the tool insert material.
3. The method according to claims 1 to 2,
   characterised in that
   a sintered ceramic material primarily consisting of silicon carbide or silicon nitride is used as material (7).
4. The method according to claims 1 to 3,
   characterised in that,
   for the method for producing moulds (2) or cores (2'), at least one tool insert with at least one cavity is used for the mould (2) to be produced or the core (2') to be produced.
5. The method according to claims 1 to 4,
   characterised in that
   the ejection pins (16) for ejecting the sand cores are made of non-conductive material or are used on a technical constructive level in such a way that conductive ejection pins (16) do not come into contact during the production process of the moulds (2) or cores (2') with the electrically live components of the core box.
6. The method according to claim 1,
   characterised in that
   by adding additives, such as graphite or table salt, the electrical conductivity of the mixture (9) is influenced in such a way that a lower specific resistance is achieved.
7. A mould or core tool (1) for producing moulds (2) or cores (2') for foundry purposes, with a housing (3) consisting of at least two parts (4, 5), wherein

   - at least one tool insert made of an electrically conductive material (7) for holding a mixture (9) is introduced into an electrically non-conductive housing (3), wherein the material (7) consists of a sintered material primarily formed from silicon carbide or silicon nitride,

   - at least two electrodes (10) arranged in parallel are provided, wherein in each case at least one electrode (10) is arranged in at least a part (4, 5) of the housing (3),

   - both the mould or core tool (1) and the electrodes (10) as well as at least one part of the housing (4, 5) are porous and/or contain ventilation slits (17) for the escape of water vapour or gases.
8. The mould or core tool according to claim 7,
   characterised in that
   at least part (4, 5) of the housing (3) is made of plastic, electric insulation or insulating ceramic.
9. The mould or core tool according to one of claims 7 or 8,
   characterised in that
   the at least two parts (4, 5) of the housing (3) are connected to each other via at least one separation plane (6), the electrodes (10) being arranged parallel to one another and between the material (7) and the insulation layer.
10. The mould or core tool according to one of claims 7 to 9,
    characterised in that
    in at least one tool insert, at least one sand-core cavity is provided, which can be attached to a rapid-clamping system in the housing (3) and therefore, makes the quick replacement of the tool insert possible inside the core box.

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