DE102020209100B4 - Process for producing sand cores suitable for foundry purposes - Google Patents

Abstract

Method for producing sand cores which can be used for foundry purposes, in which, in a core shooting system, a molding tool (2), the inner contour of which corresponds to the outer contour of at least one sand core to be produced, which consists of an electrically conductive ceramic material, is filled with a mixture (1) of molding sand and an electrically conductive binder and, after filling, an electrical voltage is applied to the molding tool (2) so that the molding tool (2) heats up to a predeterminable first temperature; wherein after reaching the predeterminable first temperature, the electrical current flow to the molding tool (2) is switched off and an electrical current flows through the mixture (1) formed from molding sand and binding agent via electrodes (5.3) which are arranged electrically insulated from the molding tool (2) and are connected to the same or a separate electrical voltage source, so that the mixture (1) is heated to a second predeterminable temperature and after reaching the second predeterminable temperature, at which the sand core(s) formed with the mixture has/have achieved sufficient strength, the electrical current flow through the sand core(s) formed is terminated and the sand core(s) is/are removed from the molding tool (2) with one or more ejection bolts (6) which are guided through the molding tool (2).

Description

The invention relates to a method for producing sand cores that can be used for foundry purposes, whereby the sand cores are to be produced as efficiently and harmlessly as possible using a core shooting machine.

Overall, 90% of casting production today is carried out using sand cores based on quartz sand, which enable the generation of cavities in the castings. As a result, both quartz sand as a molding material and the manufacturing process used are very important in casting production. The most common process for core production from sand today is so-called core shooting. The core box is filled by suddenly releasing a large volume of air. The molding material mixture (core sand) is made flowable by fluidization. This creates a two-phase flow through which the core sand is transported and thus forms the core contour. Due to the very short time period of 0.3 s to 0.8 s, the entire process is referred to as core shooting. The so-called cold box process has become established on the market for the core shooting process. It is the most commonly used process for producing sand cores. The sand mixture, which consists mainly of quartz sand, is hardened at room temperature after the core contour has been formed by means of chemical reactions of amine gases. However, the organic binding agents required for the cold box process are harmful to the environment and toxic. The replacement of this process with an ecologically harmless process for producing sand cores is still an open task. The prerequisite for successful substitution is that the new process is faster and more cost-effective.

The hot box process, which has also been used to date, uses thermally curable binders, but is significantly slower than the cold box process in terms of cycle time. The required temperature increase is achieved by heating the core tools with external heating elements (gas, oil, liquid heaters, electric heating cartridges).

According to WO 2003/013761 A1 Magnesium sulfate is used as a binding agent for the core sand, dispersed in water. The sand injected into the mold is heated and thereby hardened. It is suggested that electricity be passed through it to heat the mold.

Likewise, in DE 24 35 886 A1 and EP 3 103 562 A1 the passage of electrical current through the sand mixture of the mold. Since the molds are made of metallic materials whose electrical resistance is significantly lower than that of the molding sand, the passage of the current is not possible or only possible with considerable technical effort to isolate the molds from the molding sand. Therefore, the current flows through the metallic molds and heats them up or does not reach sufficient heating due to the low electrical resistance.
Local heating of the core on the outside leads to uneven and delayed curing. Large-volume or complex-shaped cores are cured to an insufficient quality and do not fulfill their function in the casting process.
A significant improvement should be achieved with the EN 10 2017 217 098 B3 described process. A ceramic mold material is used in a housing which has an electrical resistance adapted to the electrical resistance of an electrically conductive binder that is mixed into the molding sand. This enables a more uniform flow of current through the sand core. Since the usual inorganic binder is water glass, this only achieves technically usable electrical conductivity at higher temperatures > 130°C. To increase the electrical conductivity, it is therefore proposed to add carbon to the binder. However, this can have a negative impact on the quality of the cast products. The addition of carbon should therefore be avoided as far as possible. This requirement significantly limits the range of applications for the proposed process. Either the problem of uneven hardening remains or sand cores are produced that cannot be used for all foundry processes.

In addition, the mold material and binder must always be coordinated with each other, so that if the binder is changed, a different mold material usually also has to be selected, which limits flexibility in production.

It is therefore an object of the invention to provide possibilities for the production of sand cores that can be used in foundry technology, with which production is simpler and more flexible.

According to the invention, this object is achieved by a method that uses the features of claim 1.

The invention addresses the above-mentioned problems of adapting the electrical resistances of the mold and molding sand as well as the process-appropriate composition of the molding sand.

According to the invention, in a core shooting system, a molding tool whose inner contour corresponds to the outer contour of at least one sand core to be produced, which consists of an electrically conductive ceramic material, is filled with a mixture of molding sand and an electrically conductive binder.

After filling, an electrical voltage is applied to the mold so that the mold heats up to a predeterminable first temperature. The predefined first temperature should be high enough to correspond to at least 80% of the temperature at which a respective sand core in the mold has hardened with sufficient strength and should correspond to a second predeterminable temperature. The second predefined temperature is essentially determined by the respective binding agent, possibly in conjunction with the heat capacity of the respective sand core.

After reaching the predeterminable first temperature, the electrical current flow to the molding tool is switched off and an electrical current flows through the mixture formed from molding sand and binding agent via electrodes arranged electrically insulated from the molding tool and connected to the or a separate electrical voltage source, so that the mixture is heated to a second predeterminable temperature and after reaching the second predeterminable temperature, at which the sand core(s) formed with the mixture have achieved sufficient strength, the electrical current flow through the sand core(s) formed is terminated.

The preset first temperature should be at least 150 °C and the preset second temperature should be in the range 160 °C to 300 °C.

It can work with an electrical voltage of 40 V with a power in the range 200 W to 2000 W, whereby the most suitable power depends on the shape and mass of the respective sand core.

The sand core(s) can preferably be removed from the mold using one or more ejection pins guided through the mold.

A composite material used to form the mold can be sintered together in different compositions so that macroscopic composite components can be produced as sand cores. This makes it possible to produce electrically insulating, electrically conductive and defined electrical resistances within a material system as a monolithic ceramic component. The combination of these properties makes it possible to combine a heating function, an electrode function and an insulation function in the ceramic mold. This makes it possible to dispense with adapting the molding sand with the binder to the electrical resistance of the mold. The mold can be heated very quickly to a high first temperature because it is thermal shock resistant. This heating also allows the binder, in particular water glass, to be quickly brought into a technically usable conductivity range with additives if necessary, and hardens in a short time without the need for carbon additives.

In addition to water glass, phosphate, borate or magnesium sulfate-based salts can also be used as binding agents.

When designing the heating process, it is advantageous that the electrically conductive electrodes for supplying the electrical energy can be constructed in such a way that the control of the core shooter in the start-up phase supplies the mold with electrical current until the first preset temperature is reached, and when the required first preset temperature is reached, the electrical current flows exclusively via the mixture of molding sand and binding agent. The composite structure of the mold also makes it possible to electrically isolate the mixture of molding sand and binding agent from the ceramic heater without limiting the heat conduction between the heater and the molding sand. This makes it possible to control the mold separately.

The core mold box used can be designed according to the respective requirements for curing the respective sand core and the control of the respective core shooting system and the molding tool.

The electrical voltage source(s) used to heat the mold and the mixture can advantageously be operated in a controlled manner. The measured temperature or, preferably, the currently measured electrical resistance can be used as the controlled variable, as it changes depending on the current temperature. Depending on the temperature or electrical resistance value, the electrical voltage and/or the respective electrical current can be regulated and when the first and second predeterminable temperatures are reached, the respective electrical current flow through the mold and the resulting hardened sand core can be switched off. An electronic control unit can be integrated into or connected to the electrical voltage source(s).

A molding tool can be used which is formed with Si ₃ N ₄ , SiALON or ALN and a silicide of a metal selected from Mo, W, Ta, Nb, Ti, Zr, Hf, V and Cr. A molding tool is preferred which is formed with Si ₃ N ₄ and MoSi ₂ or with Si ₃ N ₄ , SiC and MoSi ₂ , wherein the proportion of MoSi ₂ or MoSi ₂ and SiC is at least 50% by mass, preferably at least 60% by mass.

Ejection bolts and compressed air options can be provided over open-pored areas. This allows gases released when the mixture is heated to be dissipated in order to avoid defects in the sand core. For this purpose, at least one open-pored ejection bolt can be inserted into the mold, which is guided through the wall of the mold. Ejection bolts can be used very efficiently in multiple functions. Since these ejection bolts should also be made of the electrically conductive ceramic material used for the mold, e.g. Si ₃ N ₄ -SiC-MoSi ₂ , surface areas of a respective ejection bolt can itself be used as a forming part of the mold. They have a similar thermal expansion coefficient and can be designed as an electrically insulating or electrically conductive variant depending on the specific requirements of the core shooting process.

The open porosity of at least one ejection bolt makes it possible to remove gases released during heating from the mixture using a connected device that generates a vacuum. During the ejection process, the sand core(s) can be removed from the mold using compressed air. If necessary, the ejection bolts can be moved inwards via the mechanism in the core box so that the respective sand core can be removed from the mold. The core box can be designed in such a way that, for example, a vacuum system and, alternately, a compressed air system can be integrated or connected.

The core shooter enables the two-stage heating process to be controlled using a new control system. The electrical resistance of the mold can be used as a sensor signal for the temperature reached by the mold. When an electrical resistance value corresponding to a previously known R-T curve is reached, the electrical current supply can be directed entirely to the electrodes touching the molding sand and the mold can no longer be included in the heating process. The current electrical resistance can also be calculated from the respective current values for the electrical current and the electrical voltage or taken into account accordingly.

With the start of the direct curing process in the sand core, the vacuum pump can be switched on as an example of a unit that generates a negative pressure to extract the gases that are produced, which can then be pumped out via the porous ejection bolt(s).

The course of the current-voltage curve can in turn be used to stop the curing process after complete curing when the second preset temperature is reached. The ejection process can then be initiated by opening the mold and applying compressed air.

To assist, the ejection bolts can be moved inwards to loosen the hardened sand core. An important advantage of the porous ejection bolts used in the dual function of gas conduction and mechanical movement is that if the porosity becomes clogged by the escaping gases, they can be easily replaced without having to replace the entire mold.

A mold can be used whose inner contour is formed with an electrically non-conductive ceramic material, and the electrically non-conductive ceramic material is surrounded by an electrically conductive ceramic material on the outside. Si ₃ N ₄ , SiALON or AIN can be used as electrically non-conductive ceramic material. The ceramic material used should be selected so that it is also contained in the rest of the mold. This facilitates joint sintering and only minimal mechanical stresses occur when the temperature changes.

This electrically non-conductive inner contour forms an electrical insulator to the rest of the mold, which is electrically conductive. This means that the electrodes used for the electrical current flow used to heat the mixture and harden the sand core can be arranged in the cavity of the mold and preferably embedded in the electrically non-conductive ceramic material. For this purpose, corresponding recesses can be formed there into which these electrodes can be inserted in a fitting manner. These electrodes can thus participate in the external shaping of the respective sand core. It is then only necessary to connect the electrical supply lines to these To electrically insulate electrodes, particularly from the mold.

If a mold is used that is made entirely of electrically conductive ceramic material, the electrodes through which electrical current is to flow through the mixture must be electrically insulated from the mold. This can be achieved, for example, by a locally defined coating of these electrodes in the direction of the mold and electrical insulation of the supply lines.

The use of the ceramic composite material variant offers additional advantages such as high abrasion resistance, high thermal shock resistance, electrical variability, high mechanical strength, very good thermal conductivity and low thermal expansion.

In particular, the use of the composite material based on the Si ₃ N ₄ -SiC-MoSi ₂ system for a molding tool enables the production of an electrically multifunctional component from practically one material. This significantly improves the reliability and variability of the molding tool.

• - Schnellere Taktzeiten durch erhöhte Aufheizgeschwindigkeit
• - Vermeidung von gesundheitsschädlichen Gasen während des Herstellungsprozesses
• - Verbesserte Qualität der Sandkerne
• - Verlängerte Lebensdauer der Formwerkzeuge
• - Verminderung von Wartungszyklen für die Kernformkästen
• - Fehlerresistentes Ausstoßkonzept für Sandkern

The high mechanical strength, high thermal conductivity and high thermal shock resistance enable a novel approach to the production of sand cores for foundry production.

• - Faster cycle times due to increased heating speed
• - Avoidance of harmful gases during the manufacturing process
• - Improved quality of sand cores
• - Extended service life of the molds
• - Reduction of maintenance cycles for the core mold boxes
• - Error-resistant ejection concept for sand core

The invention can be used for any mixture of molding sand and binding agent. Only the predeterminable first and second temperatures and, if necessary, the electrical parameters for the two separate heating processes must be adjusted.

The invention will be further explained below by way of example.

• 1 in schematischer Form ein Beispiel eines Formwerkzeugs, wie es bei der Erfindung einsetzbar ist.

It shows:

• 1 in schematic form an example of a molding tool as can be used in the invention.

A mixture 1 of core sand and electrically conductive binding agent is contained in a molding tool 2. The molding tool 2 consists of a core which is formed with electrically conductive ceramic material, on the inner and outer surface of which a layer of electrically non-conductive ceramic material 3 is formed.

In addition, an electrode 5.1 is formed on an end face of the mold 2, which is in contact with the electrically conductive material of the mold 2. There is a further electrode 5.3 on the mold, which is in electrically conductive contact with the mixture 2. Between the electrodes 5.1 and 5.3 there is an electrically insulating region 5.2, which consists of electrically non-conductive ceramic material.

Furthermore, there is an injection opening 5.4 through which the mixture 1 can be introduced into the mold 2.
A further opening is also formed on the mold 2, at which an open-porous ejection pin 6 is arranged and is also at least partially guided therein.

The electrodes 5.1 and 5.3 are connected to an electrical voltage source (not shown) which is controlled as explained in the general part of the description.

1. 1. zur Herstellung eines keramischen Formwerkzeugs in konventioneller keramische Technologie erfolgen folgende Schritte:
   • ◯ Pulveraufbereitung für ein Pulver A mit einer Mischung mit Si₃N₄ (d₅₀ 0,6 µm) 40 Masse-%, SiC (d₅₀ 0,8 µm) 30 Masse-%, MoSi₂ (d₅₀ 1,0 µm) 30 Masse-% sowie Sinteradditiven Y₂O₃ und Al₂O₃ mit einem Gesamtanteil von 10 Masse-% zu den restlichen Bestandteilen zur Herstellung des Heizwiderstands in der Form
   • ◯ Pulveraufbereitung Für ein Pulver B mit einer Mischung mit Si₃N₄ (d₅₀ 0,6 µm) 30 Masse-%, SiC (d₅₀ 0,8 µm) 30 Masse-%, MoSi₂ (d₅₀ 1,0 µm) 40 Masse-% sowie Sinteradditiven Y₂O₃ und Al₂O₃ mit einem Gesamtanteil von 10 Masse-% zu den restlichen Bestandteilen zur Herstellung der Kontaktelemente in der Form
   • ◯ Pulveraufbereitung für ein Pulver C mit einer Mischung mit Si₃N₄ (d₅₀ 2 µm) 60 Masse-%, SiC (d₅₀ 1,5 µm) 30 Masse-%, MoSi₂ (d₅₀ 1,0 µm) 10 Masse-% sowie Sinteradditiven Y₂O₃ und Al₂O₃ mit einem Gesamtanteil von 10 Masse-% zu den restlichen Bestandteilen zur Herstellung der porösen Ausstoßbolzen in der Form
   • ◯ Herstellung von jeweiligen Pressgranulaten aus den Pulvern A, B und C durch Zugabe von üblichen organischen Bindern Suspensionsherstellung und Sprühgranulierung
   • ◯ Formgebung durch uniaxiales Trockenpressen → isostatisches Nachverdichten → Grünbearbeitung
   • ◯ Zusammensetzen der Kontaktkomponenten und der Heizwiderstandkomponenten zum Formwerkzeug und der porösen Ausstoßbolzen mit einem dichten Mantel aus dem Widerstandswerkstoff
   • ◯ Ausbrennen des organischen Binders unter Stickstoffatmosphäre bei 800 °C und 1 h Haltezeit an den Formteilen, Formwerkzeug (Kontakte + Widerstand + poröse Verbindung) sowie poröse Ausstoßbolzen
   • ◯ Gasdrucksintern des Formwerkzeugs unter Stickstoffatmosphäre bei 1700 °C und einer Haltezeit von 1 h
   • ◯ Finischbearbeitung am Formwerkzeug und den porösen Austauschbolzen zur exakten Einpassung
   • ◯ Oxidation des Formwerkzeugs bei 1000 °C unter Luftatmosphäre über 1 h zur Schaffung einer elektrisch isolierenden Oxidhaut mit einer Stärke zwischen 1 µm - 2 µm, anschließendem Anschleifen der Kontakteflächen zur Kontaktierung und Entfernung der Oxidhaut
   • ◯ Montage des Gesamtformwerkzeugs aus monolithischem Heizwiderstand/Kontaktteil und den porösen Ausstoßbolzen
2. 2. Für die Herstellung eines Kernkastens zur Herstellung von Sandkernen in einer Kernschießmaschine erfolgen folgende Schritte:
   • ◯ Aufbau eines Kernkastenrahmens aus Aluminium zur Aufnahme des Formwerkzeugs
   • ◯ Einsatz eines keramischen Dämmmaterials zur thermischen Isolation von Formwerkzeug und Kernkastenhülle
   • ◯ Verlegung elektrischer Leitungen zur Stromzuführung an das Formwerkzeug innerhalb des Formkastens
   • ◯ Verlegung von Rohrleitungen zum Anschluss an die Ausstoßbolzen, hochtemperaturstabile Kunststoffanschlüsse für die keramischen Ausstoßbolzen
   • ◯ Verlegung von Leitungen für die Temperatursensorik zur Ermittlung der vorgebbaren ersten und zweiten Temperatur
   • ◯ Schaffung elektrischer Anschlüsse am Kernkasten zum Anschluss an die Steuerung der Kernschießmaschine
3. 3. Für die Verbindung der Steuerung der Kernschießmaschine mit dem Kernkasten/Formwerkzeug wird wie folgt vorgegengen:
   • - Nutzung einer kommerziellen Steuer- und Regeleinheit zur Regelung des keramischen Formwerkzeugs
   • - Anschlüsse für Druckluft (1 bar - 5 bar) und Vakuum (10^-3 mbar) und Integration eines Druckluftkompressors und einer Vakuumpumpe in der Kernschießmaschine, Ansteuerung über die Steuer- und Regeleinheit
   • - Messwerterfassung der Temperatur am Formwerkzeug durch die Steuer- und Regeleinheit
   • - Kopplung der Kernschießmechanismen der Maschine mit der Steuer- und Regeleinheit
   • - Programmierung der Steuer- und Regeleinheit entsprechend folgender Vorgaben:
     • ◯ Einschießen des Kernsands in das Formwerkzeug
     • ◯ Starten des Aufheizvorgangs des Formwerkzeugs durch Freigabe des maximalen elektrischen Stromflusses an den Elektroden des Formwerkzeugs
     • ◯ Zuschaltung des Vakuumdrucks auf die porösen Ausstoßbolzen zur Absaugung entstehender Gase und des Wasserdampfes durch Aushärtung des Sandkerns
     • ◯ Registrierung des Erreichens der vorgebbaren ersten Temperatur (z.B. 150°C) am Formwerkzeug
     • ◯ Absenken des elektrischen Stromflusses auf 30% des Maximums und Halten der vorgebbaren ersten Temperatur T1 innerhalb von 10 s
     • ◯ Zuschalten des elektrischen Stromflusses auf den erwärmten Sand und gleichzeitiges Abschalten des elektrischen Stromflusses auf das Formwerkzeug
     • ◯ Ermittlung der Temperatur über die Sensorik des Formwerkzeugs und Erhöhung der elektrischen Spannung im Formsand, Regelgröße ist die Temperatursteigerung entsprechend einer vorgegebenen Rate in der Steuerungseinheit
     • ◯ Bei Erreichen der vorgebbaren zweiten Temperatur und einem Absinken des Vakuumdrucks auf 10^-3 mbar (Indikator für ein Ende der Aushärtung) wird der Stromfluss und damit der Aufheizvorgang nach 5 s gestoppt
     • ◯ Nach Ende des Heizvorgangs wird der Druckluftkompressor in der Kernschießmaschine gestartet und Druckluft zu den porösen Ausstoßbolzen geleitet. In der Startphase wird ein geringer Druck < 1 bar als Gasströmung durch die poröse Komponente geleitet. Diese dient der Lockerung des Kerns im Formwerkzeug
     • ◯ Nach 5 s wird ein Druckluftstoß mit einem Druck > 2 bar auf die porösen Ausstoßbolzen geleitet. Dieser führt dazu, dass die Bolzen in das Formwerkzeug gedrückt werden und den Sandkern anheben. Damit kann der Kern dem Werkzeug entnommen werden
     • ◯ Der Kernaushärtungsvorgang einschließlich Ausstoßvorgang kann in Abhängigkeit von der Kerngröße innerhalb von 30 s - 200 s abgeschlossen sein.

A device (not shown) for generating a negative pressure is connected to the ejection bolt 6.

1. 1. To produce a ceramic mold using conventional ceramic technology, the following steps are carried out:
   • ◯ Powder preparation for a powder A with a mixture of Si ₃ N ₄ (d ₅₀ 0.6 µm) 40 mass%, SiC (d ₅₀ 0.8 µm) 30 mass%, MoSi ₂ (d ₅₀ 1.0 µm) 30 mass% and sintering additives Y ₂ O ₃ and Al ₂ O ₃ with a total proportion of 10 mass% to the remaining components for the production of the heating resistor in the form
   • ◯ Powder preparation For a powder B with a mixture of Si ₃ N ₄ (d ₅₀ 0.6 µm) 30 mass-%, SiC (d ₅₀ 0.8 µm) 30 mass-%, MoSi ₂ (d ₅₀ 1.0 µm) 40 mass-% and sintering additives Y ₂ O ₃ and Al ₂ O ₃ with a total proportion of 10 mass-% to the remaining components for the production of the contact elements in the form
   • ◯ Powder preparation for a powder C with a mixture of Si ₃ N ₄ (d ₅₀ 2 µm) 60 mass-%, SiC (d ₅₀ 1.5 µm) 30 mass-%, MoSi ₂ (d ₅₀ 1.0 µm) 10 Mass-% and sintering additives Y ₂ O ₃ and Al ₂ O ₃ with a total proportion of 10 mass-% to the remaining components for the production of the porous ejection pins in the form
   • ◯ Production of the respective pressed granules from powders A, B and C by adding conventional organic binders, suspension production and spray granulation
   • ◯ Shaping by uniaxial dry pressing → isostatic compaction → green processing
   • ◯ Assembling the contact components and the heating resistance components to form the mold and the porous ejection pins with a tight jacket made of the resistance material
   • ◯ Burning out the organic binder under a nitrogen atmosphere at 800 °C and 1 h holding time on the mold parts, mold tool (contacts + resistance + porous connection) and porous ejection pins
   • ◯ Gas pressure sintering of the mold under nitrogen atmosphere at 1700 °C and a holding time of 1 h
   • ◯ Finishing of the mold and the porous replacement bolts for exact fitting
   • ◯ Oxidation of the mold at 1000 °C under air atmosphere for 1 h to create an electrically insulating oxide skin with a thickness between 1 µm - 2 µm, subsequent grinding of the contact surfaces to make contact and remove the oxide skin
   • ◯ Assembly of the entire mold consisting of monolithic heating resistor/contact part and the porous ejection bolts
2. 2. To produce a core box for producing sand cores in a core shooting machine, the following steps are carried out:
   • ◯ Construction of a core box frame made of aluminium to accommodate the mould tool
   • ◯ Use of a ceramic insulation material for thermal insulation of the mold and core box shell
   • ◯ Laying of electrical cables for supplying power to the mold inside the mold box
   • ◯ Laying of pipes for connection to the ejection bolts, high temperature stable plastic connections for the ceramic ejection bolts
   • ◯ Laying of cables for the temperature sensors to determine the preset first and second temperatures
   • ◯ Creation of electrical connections on the core box for connection to the control system of the core shooting machine
3. 3. To connect the control of the core shooter to the core box/moulding tool, proceed as follows:
   • - Use of a commercial control and regulation unit to control the ceramic mold
   • - Connections for compressed air (1 bar - 5 bar) and vacuum (10 ^-3 mbar) and integration of a compressed air compressor and a vacuum pump in the core shooter, controlled via the control and regulation unit
   • - Measurement of the temperature on the mold by the control and regulation unit
   • - Coupling of the core shooting mechanisms of the machine with the control and regulation unit
   • - Programming of the control and regulation unit according to the following specifications:
     • ◯ Injection of the core sand into the mould
     • ◯ Start the heating process of the mold by releasing the maximum electrical current flow to the electrodes of the mold
     • ◯ Switching on the vacuum pressure on the porous ejection bolts to extract gases and water vapor generated by hardening the sand core
     • ◯ Registration of the reaching of the preset first temperature (e.g. 150°C) on the mold
     • ◯ Reducing the electrical current flow to 30% of the maximum and maintaining the preset first temperature T1 within 10 s
     • ◯ Switching on the electric current flow to the heated sand and simultaneously switching off the electric current flow to the mold
     • ◯ Determination of the temperature via the sensor system of the mold and increase of the electrical voltage in the molding sand, the controlled variable is the temperature increase according to a predetermined rate in the control unit
     • ◯ When the preset second temperature is reached and the vacuum pressure drops to 10 ^-3 mbar (indicator for the end of the curing) the current flow and thus the heating process is stopped after 5 s
     • ◯ After the heating process has ended, the compressed air compressor in the core shooter is started and compressed air is fed to the porous ejection bolts. In the start-up phase, a low pressure of < 1 bar is passed through the porous component as a gas flow. This serves to loosen the core in the mold
     • ◯ After 5 s, a compressed air blast with a pressure > 2 bar is directed onto the porous ejection bolts. This causes the bolts to be pressed into the mold and lift the sand core. This allows the core to be removed from the tool
     • ◯ The core curing process including ejection process can be completed within 30 s - 200 s depending on the core size.

Claims (8)

Method for producing sand cores that can be used for foundry purposes, in which in a core shooting system a mold (2), the inner contour of which corresponds to the outer contour of at least one sand core to be produced, which consists of an electrically conductive ceramic material, is filled with a mixture (1) of molding sand and an electrically conductive binding agent and after filling an electrical voltage is applied to the mold (2) so that the mold (2) heats up to a predeterminable first temperature; wherein after reaching the predeterminable first temperature, the electric current flow to the molding tool (2) is switched off and an electric current flows through the mixture (1) formed from molding sand and binding agent via electrodes (5.3) arranged electrically insulated from the molding tool (2) and connected to the same or a separate electrical voltage source, so that the mixture (1) is heated to a second predeterminable temperature and after reaching the second predeterminable temperature, at which the sand core(s) formed with the mixture has/have achieved sufficient strength, the electric current flow through the sand core(s) formed is terminated and the sand core(s) is/are removed from the molding tool (2) with one or more ejection bolts (6) guided through the molding tool (2). Procedure according to Claim 1 , characterized in that the electrical voltage source(s) are operated in a controlled manner, the respectively measured temperature or preferably the currently measured electrical resistance being used as the controlled variable. Method according to one of the preceding claims, characterized in that a molding tool (2) is used which is formed with Si ₃ N ₄ , SiALON or ALN and a silicide of a metal selected from Mo, W, Ta, Nb, Ti, Zr, Hf, V and Cr. Method according to one of the preceding claims, characterized in that a molding tool (2) is used which is formed with Si ₃ N ₄ and MoSi ₂ , or with Si ₃ N ₄ , SiC and MoSi ₂ , wherein the proportion of MoSi ₂ or MoSi ₂ and SiC is at least 50 mass%, preferably at least 60 mass%. Method according to one of the preceding claims, characterized in that gas released during the heating of the mixture is removed from the mold (2) by at least one open-pore ejection bolt (6) by means of a device generating a negative pressure. Method according to one of the preceding claims, characterized in that at least one ejection pin (6) is used which is made of the same electrically conductive material as the molding tool (2). Method according to one of the preceding claims, characterized in that a molding tool (2) is used, the inner contour of which is formed with an electrically non-conductive ceramic material (3), and the electrically non-conductive ceramic material is enclosed by electrically conductive ceramic material. Method according to the preceding claim, characterized in that Si ₃ N ₄ , SiALON or AIN is used as the electrically non-conductive ceramic material.