Abstract:
The invention relates to the production of molds or cores ( 2 ) for foundry purposes, wherein a mixture ( 3 ) of foundry sand and binder are produced and introduced into a mold or core tool ( 8 ), e.g. shot in a core shooter. A known binder or magnesium sulfate with and/or without at least one or additionally several crystallization waters is dispersed or dissolved in water and used as binder, which is then mixed with the foundry sand and introduced or shot into the mold tool or the core box ( 8 ). For hardening purposes, the water and a fraction of the crystallization water are vaporized by heating and driven out by a gaseous medium, all of which can be carried out very rapidly. After pouring, said core or mold consisting of foundry sand can be very rapidly removed from the tool with water and flushed due to the fact that the magnesium sulfate preserves its capability of dissolving.

Description:
BACKGROUND  
         [0001]    The invention relates to a process for producing molds or cores for foundry purposes from a mixture of foundry sand and binder, in which the foundry sand and binder are mixed and are introduced into a mold or core die, and the binder is then set, imparting the required strength to the mold or core.  
           [0002]    Such a procedure for the production of cores or molds for foundry purposes is known. The majority of cases which are currently in use employ organic binders, which result in good setting but generate gases during the casting operation as a result of their combustion, which can lead to the formation of voids in the cast workpiece which is formed. Furthermore, in particular cores which are not sufficiently dimensionally stable at elevated temperature when a foundry sand mixture of this type is used expand. Also, cleaning of the dies for the foundry molds or cores is a complex operation on account of the relatively high likelihood of the organic binders sticking.  
           [0003]    It is to be considered particularly unfavorable that in particular cores in which the sand mixture contains an organic binder can only be removed from the finished casting with considerable difficulty and with a high mechanical or thermal outlay.  
           [0004]    Therefore, it has also already been proposed, during production of molds or cores for foundry purposes, to admix inorganic binders, specifically water-glass, to the sand. Although this makes it possible to substantially avoid the evolution of environmentally harmful gases, in this case too demolding and removal in particular of the cores from the finished casting is a difficult and complex operation.  
         SUMMARY  
         [0005]    Therefore, the invention is based on the object of providing a process of the type described in the introduction, and also a device, which make it possible to produce foundry molds and/or cores which are intended to be highly dimensionally stable and strong even during the casting operation yet can nevertheless be removed from the finished casting in a simple way.  
           [0006]    To achieve this object, the process as defined in the introduction is characterized in that magnesium sulfate is dissolved and/or dispersed in water and mixed as binder with the foundry sand and is then shot or introduced into the mold or core die, and in that the water is then heated inside the mold or core die and is at least partially evaporated and expelled from the mold or core die.  
           [0007]    Tests have shown that a process of this type makes it possible to produce a stable core or a stable mold, with the melting point being increased greatly by the binder selected and the expulsion of water and if appropriate at least some water of crystallization from hydrated magnesium sulfate, so that a foundry mold or core of this type is able to resist and withstand even the high temperatures of the casting material without harmful gases being released. In this context, the invention exploits the fact that the expulsion of water of crystallization leads to a chemical change in the materials properties of the special binder, specifically of the magnesium sulfate.  
           [0008]    It has been found that foundry molds or cores of this type, after the cast workpiece has been cooled, can be flushed out using water, since the special binder then again seeks to take up water of crystallization so that it is chemically transformed back into a soluble substance which can be dissolved using a flushing or cleaning water of this type and can thereby be removed very easily even from a complicated casting without there being any need for mechanical vibrations or similar outlay. It may even be sufficient for the finished casting simply to be immersed in a water bath. It has been found that immersion of this nature for just half a minute can be sufficient to dissolve and flush out even a complicated core. Moreover, thereafter the foundry sand is available for reuse in virtually unchanged form and does not need any complex cleaning and treatment, since there is no need for any organic residues to be removed.  
           [0009]    The advantages which can be achieved by the invention therefore relate firstly to the foundry mold or cores and the properties thereof during the casting operation, in which harmful gases are not released, and secondly, at a later stage, to the cleaning of the finished casting, which is greatly simplified.  
           [0010]    It is expedient if the mixture of foundry sand and a dispersion and/or solution of magnesium sulfate in water is heated inside the mold or core die by means of microwave and/or infrared radiators. This represents a particularly simple form of heating for expelling the water and if appropriate at least some of the water of crystallization. In this context, microwaves can be used in a very targeted fashion and penetrate even into the very middle of relatively large cores.  
           [0011]    A particularly advantageous procedure can consist in the mixture of foundry sand and a dispersion and/or solution of magnesium sulfate in water being heated inside the mold or core die by the application of an electric voltage to the at least partially electrically conductive parts, which are insulated from one another, of the separable mold or core dies. Electrical energy is available virtually wherever molds or cores are produced, and consequently the heating for expulsion of the water from the mold or the core can be carried out in a correspondingly simple way.  
           [0012]    The electrically conductive core/mold, which consists of a mixture of foundry sand and a dispersion and/or solution of magnesium sulfate in water, may, in a simple and expedient way, be used as an electrical resistor of a resistance heating means and can be heated by means of an electric voltage applied to it and the current which flows as a result. This means that the heat is formed directly where the water is to be expelled.  
           [0013]    The electric voltage can be applied to electrodes which make contact with the core/mold, and the at least partially electrically conductive parts, which are insulated from one another, of the separable mold or core dies can be used for this purpose. The internal cavities of these dies, which receive the mold or core to be formed, therefore make contact, as electrodes, with the mold or core and provide the corresponding heating, since the mold or core is electrically conductive on account of the wet conditions or moisture and the remaining constituents.  
           [0014]    It is particularly expedient if an AC voltage is applied as the electric voltage. In this case, a pulsed, in particular square-wave voltage can be applied as the electric voltage. A suitable AC voltage can make use of the reactive properties of the sand mixture in the core or mold to heat the latter. Particularly good results can in this case be achieved with the pulsed and in particular square-wave voltages. As a result, it is possible in particular to control the introduction of power by varying the pulse width of the electric voltage. The voltage can therefore be selected to be controllable and in particular to be greater than 1000 V or greater than 1500 V, in order to achieve correspondingly rapid and powerful heating.  
           [0015]    Thorough drying within a short time can be achieved if an AC voltage with a frequency of over 1000 Hz, for example of 3000 Hz or more, is selected. Since the entire core box or the core or mold die is used as electrode surface, the energy can be transmitted very quickly and effectively, and therefore the corresponding core or mold can be dried within an extremely short time.  
           [0016]    It may be expedient if the water which has been evaporated as a result of heating is expelled from the die by means of a gaseous medium, such as nitrogen and/or carbon dioxide and/or air, it being possible for this gaseous medium, which is used to expel the evaporated water, to be transported through the die and therefore through the foundry mold or core which has been formed, by means of pressure or by suction and pressure reduction. In particular air is available in virtually unlimited quantities and can be used without problems to expel water vapor from the die.  
           [0017]    In this case, it is advantageous if the water vapor which is produced by the heating in the die is expelled using hot gas. This makes it possible to prevent the water vapor which is to be expelled from potentially condensing again prematurely, or means that slightly less heating of this water vapor may even be sufficient to allow it then to be expelled from the die as far as possible.  
           [0018]    An expedient configuration of the process may consist in the fact that magnesium sulfate without water of crystallization or with at least one mole of water of crystallization mixed with magnesium sulfate with more than one mole of water of crystallization, if appropriate with up to seven mol of water of crystallization, is dissolved and/or dispersed in water and mixed as binder with the foundry sand, and that the water and some of the water of crystallization are evaporated by heating and then expelled.  
           [0019]    This surprisingly makes it possible to reduce the quantity of water which is to be expelled. This is because the magnesium sulfate which does not contain any water of crystallization or contains only a small amount of water of crystallization can, during heating, take up such water of crystallization from the magnesium sulfate which contains more (polyhydrate) water of crystallization, so that corresponding crystals are formed inside the foundry mold or the core, leading to corresponding strengthening without the water of crystallization of the entire mixture having to be completely expelled.  
           [0020]    It is known that magnesium sulfate which does not contain any water of crystallization or contains only a small amount of, in particular just one mole of, water of crystallization, when combined with magnesium sulfate containing more than one mole of water of crystallization, when they are reacted with one another with heating, causes the corresponding crystals to become interlaced, which in the application according to the invention contributes to the formation of an extremely strong core or a correspondingly strong mold. An alternative or additional option for reducing the quantity of water or water vapor to be expelled during the process according to the invention may consist in a highly or more highly concentrated solution of magnesium sulfate with or without at least one mole of water of crystallization being mixed with a hydrocolloid, and this mixture being used as binder. The addition of hydrocolloid may make it possible to achieve higher salt concentrations in what is in relative terms a small quantity of dispersion and/or dissolution water, so that a correspondingly reduced amount of water has to be expelled.  
           [0021]    A further configuration of the process may consist in more magnesium sulfate being mixed with the quantity of dissolution water which is predetermined for a defined quantity of foundry sand than is required to produce a saturated solution, and in some of the magnesium sulfate being dispersed in the solution and mixed with the foundry sand as a dispersion. This makes it possible for the maximum possible amount of magnesium sulfate to be introduced as binder into the foundry sand and for the quantity of dissolution water required to be kept as low as possible, so that subsequently it is also the case that a correspondingly small amount of water vapor has to be expelled. At the same time, the advantages during subsequent removal of foundry sand residues on the casting with the aid of a simple water rinse or immersion in water are retained.  
           [0022]    The foundry sand can be mixed with the dispersed or dissolved binder in a weight ratio of from 97:3 to approximately 80:20.  
           [0023]    It is expedient if approximately 100 parts by weight of foundry sand are mixed with approximately 3 parts by weight to approximately 20 parts by weight of dispersed or dissolved binder, i.e. dissolved magnesium sulfate and/or without water of crystallization. An optimized procedure may involve mixing approximately 5 to 10 parts by weight of binder in dispersed or dissolved form with approximately 100 parts by weight of sand. Tests have shown that this leads to strong cores or foundry molds which are able to successfully withstand the casting operation and in which as little water as possible has to be expelled from the die.  
           [0024]    The invention also relates to a device for producing foundry molds or cores, having at least one heating device for setting purposes, wherein the device for producing foundry molds may be a molding machine and the device for producing cores may be a core-shooting machine. This device may be characterized in that at least one microwave generator is installed as heating device on the molding machine or on the core-shooting machine, and in that at least one microwave antenna which is or can be coupled to the microwave generator via a waveguide, is arranged in the region of the mold die for the foundry mold or for the core or cores. A feed opening of a gas purge hood which is known per se can in this case be used for expulsion of gases and/or of heated water vapor.  
           [0025]    At this point, it should be noted that the mold die for the foundry mold or for the core may also be a multi-cavity die in which, by way of example, a plurality of cores are molded and/or heated simultaneously.  
           [0026]    The device according to the invention may therefore advantageously be formed substantially by a known molding machine or core-shooting machine, which has been additionally equipped with a heating device, specifically with a microwave generator and a microwave antenna. Moreover, it is advantageous if the inlet openings for the foundry sand mixed with binder can be used to expel gases or heated water vapor, so that overall an inexpensive device is available. Even existing core-shooting machines or molding machines may if appropriate be retrofitted in order to allow the advantageous invention and in particular the process according to the invention to be employed therewith.  
           [0027]    It is expedient if, as a result of the device being set to the gas purge operation for the expulsion of gases or of water vapor, the microwave generator can simultaneously be coupled to the antenna via the waveguide. This makes it possible to simplify actuation of the device, since in practice only one setting movement is required in order to couple the microwave generator to the antenna and to trigger the heating operation.  
           [0028]    The setting movement to set the device to the gas purge operation may in this case automatically couple the microwave generator to the antenna. For this purpose, it is merely necessary for the corresponding coupling to be configured in such a way that closing the gas purge hood or the like simultaneously produces the corresponding coupling of the microwave generator to the antenna.  
           [0029]    According to an arrangement which is particularly simple in design terms, it is possible to provide that the path of the waveguide can be interrupted and has a coupling at the location where it is interrupted, and that the antenna-side part of the waveguide is optionally arranged on or connected to the gas purge hood or in the die. This coupling can therefore be closed or interrupted, when corresponding movements are carried out, in order to move a gas purge hood into the position of use or to move it back out of this position.  
           [0030]    A further configuration of the invention for intensifying and accelerating the heating operation may consist in the fact that the microwave generator can be coupled or is connected, via a branched waveguide or via two waveguides, to an antenna arranged in the gas purge hood and to an antenna arranged in the mold die.  
           [0031]    It has already been mentioned above that any water vapor can be expelled during the setting operation. This is expedient in particular if the foundry mold or the core is made from a mixture of foundry sand and a binder which is a dispersed or dissolved magnesium sulfate. In this case, the device can be set to the gas purge operation for expulsion of the water vapor formed during the heating operations, as has already been mentioned above.  
           [0032]    The overall advantageous result is that the molding machine or core-shooting machine and the actual mold dies can remain virtually unchanged, since the existing ventilation systems can also be employed in the device according to the invention and can be used for the expulsion of the heated and evaporated dissolution water in accordance with the invention. It is merely necessary to additionally install an antenna for the microwave, for example on the gas feed hood. In this case, of course, the dies are to be made from materials which are suitable for microwaves. This device having a heating device designed as a microwave generator and an antenna may, however, also be used for the production of molds or cores in which a different binder than the abovementioned dispersed or dissolved magnesium sulfate is used and a heating operation is required to set the binder.  
           [0033]    Another possibility which is worthy of protection provides a device for producing foundry molds or cores, having at least one heating device for setting purposes, wherein the device for producing foundry molds is a molding machine and the device for producing cores is a core-shooting machine, into which machine a mold or core die can be or is inserted. In this device, it is possible for the heating device provided to be an electrical resistance heating means, in which the electrically conductive core or the mold forms the electrical resistor, and the mold or core dies, which are composed of a plurality of parts in order to allow a mold or core to be removed, may be at least partially electrically conductive and may be insulated from one another at the locations where they are in contact with one another, and the parts of the dies may in each case have at least one electrical terminal for application of an electric voltage for the resistance heating device.  
           [0034]    In this way, the molds or cores, which initially contain dissolution water and/or water of crystallization and are electrically conductive on account of the further constituents which they contain, can be electrically heated in what is in design terms a very simple way in order for water to be expelled. The moist core or the moist mold constitutes an impedance, resulting in electrical conductivity. The voltage applied thereto can therefore be used for drying purposes.  
           [0035]    The resistance heating device may have a voltage source with a frequency converter for increasing the frequency and/or a pulse former for forming a pulsed voltage. Good results during heating can be achieved with a pulsed voltage.  
           [0036]    The resistance heating device may have a voltage source and a transformer for increasing the voltage, which are connected, via supply conductors, to the terminals on the parts of the mold or core die. This makes it possible to increase efficiency.  
           [0037]    At least one part of the mold or core die may have a plurality of electrical terminals, and switches for alternately or optionally applying a voltage to these terminals may be provided between the terminals and the voltage source, so that alternately one switch is closed and the others are open. This means that any polarities which occur at an electrode can always be reduced again and/or altered. The heating of a mold or core can be carried out correspondingly uniformly, and even very differing contours of molds or cores of this type can be taken into account by changing the electrical terminals which are active at any given time.  
           [0038]    In the case of a mold or core die comprising more than two parts, each part may have an electrical terminal and electrical supply conductors, and in each case two parts, cyclically, of a die of this type can always be connected to the current source. Multipart dies of this type are often required in particular for complicated cores. Nevertheless, with the abovementioned configuration it is possible to use the applied voltage in each case to form a resistance heating means so that the core is thoroughly heated.  
           [0039]    The overall result is a process and a device which allow mechanical production of molds or cores in standard core-shooting machines, and wherein the mold sand can set within about half a minute. In this context, the process makes use of the extremely different melting points of magnesium sulfate in its hydrated form, on the one hand, and in its anhydrous state, on the other hand. Specifically, magnesium sulfate in the form of its heptahydrate has a melting point of approximately 75° Celsius and in its anhydrous form has a melting point of 1124° Celsius. Therefore, by targeted removal of the chemically bonded water of crystallization, it is possible to achieve virtually instantaneous setting of the mold sand. Considerable interlacing of partially hydrated and fully hydrated magnesium sulfate is in this case a favorable property which can also be utilized in order to obtain a very high strength even with small quantities of magnesium sulfate, for example 1% based on the mold sand.  
           [0040]    The important ventilation for removal of the water of crystallization, which was originally chemically bonded, after heating may if appropriate be effected through specially arranged inlet and outlet nozzles, in which case a superatmospheric pressure of 1 to 6 bar of a dry gas, preferably of heated air, is expedient. The heating may expediently be effected using microwaves, since the quartz sand which is normally used is “transparent” to microwave radiation, so that such radiation can penetrate all the way through even relatively large molds or cores. Moreover, only the magnesium sulfate which contains water of crystallization is heated. As soon as the water of crystallization has escaped, this magnesium sulfate, which is now anhydrous, is also “transparent” and no longer presents any obstacle to the further penetration of the microwaves.  
           [0041]    However, the heating may expediently also be effected by resistance heating, as has been explained above.  
           [0042]    Therefore, the targeted removal of even the chemically bonded water of crystallization—at least in part—from the magnesium sulfate is essential. This leads to very rapid setting, which is advantageous for economically viable production. Moreover, sufficient strength is achieved with a relatively low concentration of magnesium sulfate. The mold parts or cores produced in this way are dimensionally stable up to at least 1124° Celsius and can be dissolved out of the metal casting using a small amount of water.  
           [0043]    If conventional binders are used, accelerated setting is likewise possible on account of the targeted heating by means of microwave or electrically conductive heating. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    Exemplary embodiments of a device for producing molds or cores for foundry purposes in accordance with the invention are described in more detail below with reference to the drawing, in which, in some cases in diagrammatic form:  
         [0045]    [0045]FIG. 1 shows a schematic illustration of a device for producing foundry molds or cores, having a microwave generator and corresponding antenna, in the form of a core-shooting machine,  
         [0046]    [0046]FIG. 2 shows, on an enlarged scale and in even more schematic form, a longitudinal section through part of the shooting unit after sand has been shot into a mold die formed as a core box and before this core box is moved to meet a purge hood located above it and is pressed onto the shooting head from below, or vice versa, with the microwave antenna for heating the core and expelling the dissolution water being arranged in this purge hood; the connection between the microwave generator and this emitting antenna is still open and can be closed automatically when the parts are moved together or lifted and pressed together,  
         [0047]    [0047]FIG. 3 shows an illustration corresponding to FIG. 2, with the emitting antenna arranged in the lower region of the core box designed as a mold die,  
         [0048]    [0048]FIG. 4 shows an embodiment which has been modified with respect to FIGS. 2 and 3 in a similar form of illustration; in this case, an antenna which is or can be coupled to the microwave generator in the position of use for heating the shot core is arranged in both the purge hood and the core box,  
         [0049]    [0049]FIG. 5 shows an illustration corresponding to FIGS.  2  to  4  of a modified embodiment, in which infrared radiators for heating the shot core are arranged in the core box,  
         [0050]    [0050]FIG. 6 shows an illustration corresponding to FIGS.  2  to  5  of a modified embodiment, in which the heating device provided is an electrical resistance heating means, in which the mold for the electrically conductive core is likewise electrically conductive and its parts are insulated at the locations where they are in contact, and an electrical terminal for a resistance heating device is provided at each part of the mold or core dies,  
         [0051]    [0051]FIG. 7 shows an arrangement and device corresponding to FIG. 6, in which a plurality of electrical terminals, which can be connected up optionally and alternately in switches, in order to avoid polarization at one of the terminals, are provided on one of the parts of the die for the core,  
         [0052]    [0052]FIG. 8 shows a further modified device, in which the core die comprises three electrically conductive parts which are insulated from one another and each of which has an electrical terminal, it being possible for in each case two of the three parts alternately to be connected to the voltage source via switches, and  
         [0053]    [0053]FIG. 9 shows an embodiment and arrangement corresponding to FIG. 6, in which, however, the sequence of the voltage transformer and the pulse former located behind the voltage source is reversed compared to the arrangement shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]    A device which is denoted overall by 1 and is illustrated diagrammatically and partially cut away in FIG. 1 is used to produce cores, but could also be employed to produce foundry molds. In the exemplary embodiment, the device is a core-shooting machine.  
         [0055]    The cores  2  which are to be produced with it (FIGS.  2  to  9 )—or by analogy foundry molds—are molded from a mixture  3  of foundry sand and binder, which is a magnesium sulfate which is dissolved in water and preferably includes at least one mole of water of crystallization, or alternatively is some other form of binder, this mixture  3  of sand and binder being introduced into a sand feed funnel  4  in a known way and as a result being introduced into the shot head  5  of a shooting unit denoted overall by 6. FIG. 1 also illustrates the air boiler  7 , which is essential to the shooting operation, in partially cut-away form.  
         [0056]    The core box  8  which is illustrated in each of FIGS.  2  to  9  and is assembled from a core box upper part  8   a  and a core box lower part  8   b  in the position of use, but could also be a correspondingly differently configured mold die if foundry molds are to be produced, belongs to this device  1  in the form of a core-shooting machine. FIG. 8 shows an embodiment in which the core box upper part  8   a  is in turn subdivided in order to allow the removal of a correspondingly complicated core after it has set.  
         [0057]    With this device, it is possible to produce molds or cores for foundry purposes from the mixture  3  of foundry sand and binder, with the foundry sand and binder initially being mixed and then being introduced into the mold or core die—in the exemplary embodiment the core box  8 —with the aid of the shooting unit  6 . This has already occurred in FIGS.  2  to  9 , and the binder can then set and impart the required strength to the mold or core  2 . The binder in this case is magnesium sulfate preferably with at least one mole of water of crystallization dissolved and/or dispersed in water, and this binder is then mixed with the foundry sand to form the mixture  3 . This mixture is then introduced or shot into the mold or core die  8 .  
         [0058]    Then, inside this die, the core box  8 , the dispersing and/or dissolution water and at least some of the water of crystallization is evaporated by heating and expelled from the mold or core die, i.e. from the core box  8 , by means of a gaseous medium.  
         [0059]    To carry out this process, at least one heating device, which is to be described in more detail below and can be used to heat and expel the dissolution water and/or the water of crystallization, is provided on the molding or core-shooting machine  1 .  
         [0060]    In the exemplary embodiments shown in FIGS.  1  to  4 , a microwave generator  9  is installed on the core-shooting machine  1  as heating device, and at least one microwave antenna  10 , which can be coupled, and in the position of use is coupled, to the microwave generator  9  via a waveguide  11 , is arranged in the region of the mold die, i.e. of the core box  8 , at different locations depending on the particular exemplary embodiment. In the exemplary embodiment, the corresponding coupling  12  is still open, since although a core  2  has already been shot, the core box  8  has not yet been brought to meet a gas purge hood  13  and the heating and setting by means of microwave have not yet been carried out.  
         [0061]    In all the exemplary embodiments, it is possible to see a feed opening  14 , which can be used, for example, to introduce hot air in order to expel the heated water or water vapor which is formed as a result of the heating with the aid of the heating device, i.e. the microwave  9  in the position of use.  
         [0062]    In FIGS.  1  to  4 , the connection between microwave generator  9  and antenna  10  is still open. Setting the device  1  to the gas purge operation for expulsion of the water vapor, i.e. the relative lifting motion of the core box  1  with respect to the gas purge hood  13  and with respect to the shooting head  5  or vice versa simultaneously allows the microwave generator  9  to be coupled to the antenna  10  via the waveguide  11  by virtue of the coupling  12  being closed during the abovementioned relative movement.  
         [0063]    hen, the heating with the aid of the microwave energy and, at the same time or slightly afterward, the expulsion of the water vapor which is formed can take place.  
         [0064]    The movement of setting to the gas purge operation can automatically couple the microwave generator  9  to the antenna  10 , so that the entire operation can be carried out quickly.  
         [0065]    The path of the waveguide  11  can therefore be interrupted, and the abovementioned coupling  12  is provided at the location where it is interrupted, it being possible for the antenna-side part of the waveguide  11  optionally to be arranged and connected at the gas purge hood  13 , as shown in FIG. 2, or in the mold die or core box  8 , as shown in FIG. 3, or even at both locations, as shown in FIG. 4. FIG. 4 shows that the microwave generator  9  can be coupled and is connected, via two waveguides  11 , to an antenna  10  arranged in the gas purge hood  13  and an antenna  10  arranged in the mold die or core box  8 , so that the foundry mold or the core  2  can be heated correspondingly quickly and powerfully and the time required to expel the dissolution water and/or water of crystallization can be shortened.  
         [0066]    [0066]FIG. 5 shows a modified embodiment, in which infrared radiators  15  are provided as the heating device at or in the mold die, in this case in the core box  8 ; the infrared radiators  15  may be provided as an alternative to heating by means of microwave or even in addition to heating by means of microwave, for example if an antenna  10  as shown in FIG. 2 were additionally to be provided in the gas purge hood.  
         [0067]    FIGS.  6  to  9  in turn show modified embodiments in which the heating device provided is an electrical resistance heating means, in which the electrically conductive core  2  forms the electrical resistance. The core die  8 , which for removal of a core  2  once again is composed of two parts (FIGS. 6, 7 and  9 ) or three parts (FIG. 8), is in this case at least partially, or expediently completely, electrically conductive, by virtue of it consisting, for example of aluminum or cast iron or steel. At the locations where they are in contact with one another, the parts  8   a  and  8   b  are insulated from one another, and this insulation  16  is diagrammatically depicted in FIGS.  6  to  9 .  
         [0068]    It can also be seen that the parts  8   a  and  8   b  of the dies or of the core box  8  each have an electrical terminal  17  for application of an electric voltage for the resistance heating device.  
         [0069]    The core box upper part  8   a  and core box lower part  8   b , i.e. the parts of the core die  8 , therefore belong to the resistance heating means, wherein the core  2  forms the actual resistor.  
         [0070]    In the usual way, this resistance heating device has a voltage source  19 , which in the present case leads through a three-phase network  20  to a frequency converter for increasing the frequency and/or a pulse former  21  for forming a pulse voltage.  
         [0071]    Moreover, this resistance heating device has a transformer  22  for increasing the voltage, from which supply conductors  23  lead to the terminals  17  on the parts  8   a  and  8   b  of the core die  8 . When the voltage is switched on, the moist core  2  inside the die  8  acts as a corresponding resistor or as an impedance, so that current flows in order to dry the core. The level of the voltage can be selected according to the thickness of the core  2 . Very intensive and effective drying is achieved since the parts  8   a  and  8   b  act as electrodes which bear against and make contact with the core and to which the electric voltage is applied, these “electrodes”  8   a  and  8   b  being isolated from one another by the insulation  16  in order to avoid a short circuit.  
         [0072]    The electric voltage may expediently be a sinusoidal or pulsed, in particular square-wave voltage, with an AC voltage of high frequency of over 1000 Hz, for example of 3000 Hz or even above, being particularly effective. It is also possible for the voltage to be controlled and to be selected to be greater than 1000 V. By changing the pulse width of the electric voltage, it is possible to control or regulate the introduction of power and to match it to the shape and size of a core  2 , and in the case of a mold being produced in a mold die, to the mold.  
         [0073]    Whereas in the exemplary embodiment shown in FIG. 6 two parts  8   a  and  8   b  form the core box  8  and each have one electrical terminal  17 , the exemplary embodiment shown in FIG. 7 reveals four electrical terminals  17  of this type on the core box lower part  8   b , these terminals being connected in parallel, and switches  24  for alternately or optionally applying a voltage to the various electrical terminals  17  being provided between these terminals  17  and the voltage source  19 , in which case alternately one switch  24  is closed and the others are open. This makes it possible to avoid polarization at a connection location on the core box lower part  8   b  and to heat the core  2  as uniformly as possible.  
         [0074]    [0074]FIG. 8 shows an embodiment in which the core die  8  is composed of more than two parts, the core box upper part  8   a  for its part being subdivided into two parts, which parts are electrically isolated from one another by an insulation  25 . This makes it possible to produce correspondingly complicated cores  2 .  
         [0075]    [0075]FIG. 8 illustrates that each of these three parts has an electrical terminal  17  and an electrical supply conductor  23 , which is initially composed of two parallel sections  23   a  and  23   b  in which switches  26  are arranged. These parallel-connected sections  23   b  enable in each case two parts  8   a  or  8   b , cyclically, of a multiply divided die  8  of this type to be connected to the voltage source  19  by the switches  26  being opened and closed cyclically. Therefore, in each case only two parts of the core box  8  are energized, cyclically, in order for the core  2  which is present therein to be used as a resistor and heated.  
         [0076]    In the embodiment shown in FIG. 9, which substantially corresponds to that shown in FIG. 6, it can be seen that the order in which the pulse former  21  and the transformer  22  are arranged may also be switched, so that the voltage transformer  22  is provided first, followed by the pulse former  21 , in series.  
         [0077]    In the embodiments shown in FIGS.  6  to  9 , as in the exemplary embodiments illustrated in FIGS.  2  to  5 , there is a gas purge hood  13  having a feed opening  14 , by means of which, by way of example, hot air can be introduced in order to expel the heated water or water vapor which is formed as a result of the heating with the aid of the electric voltage in the position of use.  
         [0078]    The gas purge hood  13  can be moved in the same way as in the exemplary embodiments described above in order for the gas purge operation to be carried out.  
         [0079]    A gaseous medium, for example nitrogen and/or carbon dioxide and/or air, preferably hot air or hot gas, can be supplied via the feed opening  14  in order to expel the evaporated water. Therefore, the expulsion of the evaporated water can best be effected by means of superatmospheric pressure.  
         [0080]    As has already been mentioned above, the mixture  3  contains, as binder, magnesium sulfate, without and/or with one mole or if appropriate more than one mole of water of crystallization, dissolved in water. By way of example, it is possible to use magnesium sulfate without any water of crystallization, with one mole of water of crystallization and magnesium sulfate with more than one mole of water of crystallization and/or also mixed with a hydrocolloid, as binder. In this context, it is particularly expedient if only magnesium sulfate or magnesium sulfate with hydrocolloid are used, since magnesium sulfate with water of crystallization can be successfully dissolved and/or dispersed in water and mixed as binder with foundry sand, but also can subsequently be successfully dissolved out of a cast workpiece again with the aid of water.  
         [0081]    According to one example of an expedient mixture of foundry sand and dispersed or dissolved binder, it is possible for approximately 100 parts by weight of foundry sand to be mixed with approximately 3 parts by weight to approximately 20 parts by weight of dissolved binder, in particular comprising magnesium sulfate in dissolved form.  
         [0082]    In this case, it is possible for approximately 100 parts by weight of sand to be mixed preferably with approximately 5 to 10 parts by weight of binder in dispersed or dissolved form. A correspondingly small amount of water has to be expelled from the core box  8  by heating and using a gas, which means that the process can be carried out correspondingly quickly.  
         [0083]    To produce molds or cores  2  for foundry purposes, a mixture  3  of foundry sand and binder is produced and introduced, for example shot in a core-shooting machine, into a mold or core die  8 . A known binder or magnesium sulfate without any water of crystallization and/or with at least one mole or alternatively more than one mole of water of crystallization dissolved or dispersed in water is used as binder and mixed with the foundry sand and introduced or shot into the mold die or the core box  8 . For setting purposes, the water and some of the water of crystallization is then evaporated by heating and expelled by means of a gaseous medium, an operation which can be carried out very quickly. After the casting operation, a core of this type or a mold of this type consisting of foundry sand can very quickly be dissolved out of the workpiece and flushed out by means of water, since the magnesium sulfate retains its ability to be dissolved.