METHOD AND DEVICE FOR THE PRODUCTION OF MOLDS OR MACHINES FOR FOUNDRY PURPOSES
DESCRIPTION OF THE INVENTION The invention relates to a method for the production of molds or cores for casting purposes from a mixture of foundry sand and binder, being that the foundry sand and the binder are introduced mixed and in a tool of mold or male, and then the binder hardens and gives the mold or core the required strength. Proceeding in this way in the production of cores or molds for foundry purposes is known. In the vast majority of cases currently in vogue, organic binders are used that result in a good hardening, but during the smelting process generate gases due to combustion, gases that can lead to a formation of porosities (plugs) in the molten piece of work that is produced. In addition, the dies in particular with a mixture of foundry sand of this type do not have sufficient shape stability at higher temperatures. Because organic binders have a relatively high tendency to adhesion, cleaning tools for casting molds or cores is also expensive. REF: 153297 2
It is considered particularly unfavorable that, in particular, the males whose sand mixture contains an organic binder can only be separated from the finished casting part with great difficulty and a high mechanical and thermal cost. For this reason, it has already been proposed to use inorganic binders, specifically potassium silicate, to mix them with the sand in the production of molds or cores for foundry purposes. It is true that it is possible to avoid the generation of ecologically harmful gases to a large extent, but it is also difficult and costly to remove the mold or the separation, in particular of the cores, of the finished workpiece. For this reason there is the task of creating a method of the type under consideration, and also a device, with which it is possible to produce molds and / or casting cores, which must also have great stability of shape and strength during the process of casting, and which however can be separated in a simple manner from the finished cast workpiece. To solve this problem, the method defined at the beginning is characterized in that it disperses and / or dissolves magnesium sulfate in water, and mixes as a binder with the foundry sand, and after that
introduces or triggers into the mold or male tool, and because then the water is heated and evaporated at least partially within the mold or male tool, and is ejected from the mold or male tool. Tests have shown that by means of such a process it is possible to produce a stable male or a stable mold, the melting point being considerably increased by the selected binder and the expulsion of water and optionally at least a part of water of crystallization of the hydrated magnesium sulfate, so that a casting mold of this type or a male of this type can also withstand and withstand the high temperatures of the casting material, without this damaging gases being released. In this aspect the invention takes advantage of that by means of the expulsion of the water of crystallization a chemical change takes place of the material properties of the special binder, specifically magnesium sulphate. It has been found that this type of casting molds or cores can be separated by flotation with water after cooling of the molten workpiece, since the special binder then exhibits again the tendency to absorb water of crystallization, and thereby chemically transformed again in a soluble substance, which it is possible to detach with a washing water or
cleaning and thus can be very easily separated even from a complicated casting piece, without requiring mechanical vibrations or such expenses. It may even be sufficient to simply submerge the finished casting in a water bath. It has been shown that a dive of this kind for only a half minute can be enough to dissolve and float even a complicated male. In addition, after this, the foundry sand is again available virtually without any change, and does not require expensive cleaning and conditioning, since it is not necessary to eliminate organic waste. In other words, the advantages obtained by the invention refer on the one hand to the casting mold or the cores and their properties during the casting process, during which no harmful gases are released, and on the other hand after cleaning of the finished casting, which is considerably simplified. It is desirable that the mixture of foundry sand and a dispersion and / or magnesium sulfate solution in water be heated inside the mold tool or male by means of microwaves and / or infrared radiation. This constitutes a particularly simple way of heating to expel the water and optionally at least a part of the water.
of crystallization. In this aspect it is possible to apply the microwaves in a very specific way, and penetrate even the deepest of larger males. A particularly advantageous procedure may be that the mixture of foundry sand and a dispersion and / or solution of magnesium sulfate in water is heated inside the mold or core tool by applying an electrical voltage to the insulated parts. one with respect to another of the separable mold and male tools, which are at least partially electrically conductive. Electric power is available almost anywhere where molds or cores are produced, so that the heating for ejection of the water from the mold or the core can be carried out in a correspondingly simple manner. The male / electrically conductive mold constituted by a mixture of foundry sand and a dispersion and / or solution of magnesium sulfate in water can be used simply and conveniently as an electrical resistance of a resistance heater, and can be heated through of an electrical voltage that is applied to it and the current that flows through it. In this way the heat is produced directly where the water must be expelled. The electrical voltage can be applied to electrodes that are in contact with the male / mold, and for this 6
purpose it is possible to use the parts of the separable mold or male tools, isolated from one another, which are at least partially electrically conductive. Their internal cavities that will lodge the mold to be formed or the core to be formed are, therefore, in contact with the mold or the core as electrodes, and they are responsible for the corresponding heating, since the mold or the core is electrically drivers under wetting or moisture and the remaining components. It is particularly favorable to apply an alternating voltage as an electrical voltage. In this aspect it is possible to apply an electrical voltage in the form of pulses, in particular a rectangular voltage. Suitable alternating voltage can take advantage of the reactive properties of the sand mixture in the core or the mold for heating. It is possible to obtain particularly good results with pulse-shaped, and in particular rectangular, voltages. In particular, it is possible by this to control or regulate the power input by modifying the pulse width of the electrical voltage. That is to say that the voltage can be selected in a regulatable manner, and in particular higher than 1000 V or higher than 1500 V, in order to obtain a correspondingly fast and intense heating. It is possible to achieve a good thorough drying in a 7
short time if an alternating voltage with a frequency higher than 1000 Hz is selected, for example, 3000 Hz or more. Since the entire core box, or the core or mold tool is used as an electrode surface, it is possible to transmit the energy very quickly and effectively and, therefore, to dry the corresponding core in the shortest time or the corresponding mold. It may be convenient that the water evaporated by heating is expelled from the tool by means of a gaseous medium such as nitrogen and / or carbon dioxide and / or air, the gaseous medium serving for the ejection being transported through the tool, and thereby through the casting mold formed or through the core, with pressure or by suction and reduced pressure. First of all, the air is practically available in unlimited form, and can be used to expel water vapor from the tool without problems. In this aspect it is convenient if the water vapor that is produced in the tool due to the heating is expelled with hot gas. By this it is possible to prevent the water vapor to be expelled from possibly returning to condense too prematurely or, respectively, a smaller heating of this water vapor may be sufficient to be able to subsequently expel a large part of the water vapor.
tool A convenient design of the method may be that the magnesium sulfate without or with a minimum of a water of crystallization is dispersed and / or dissolved in water mixed with magnesium sulfate with several waters of crystallization, optionally with up to seven waters of crystallization, and it is mixed as a binder with the foundry sand and the water and a part of the water of crystallization are evaporated by heating and then expelled. Surprisingly it is by this possible to decrease the amount of water to be expelled. And is that the magnesium sulfate that does not have or only very little water of crystallization can receive this water of crystallization of magnesium sulfate that contains more water of crystallization, so that they are produced inside the casting mold or the male the corresponding formations of crystals and the corresponding solidifications, without it being necessary to completely expel the crystallization water from the entire mixture. It is known that magnesium sulphate with or without a little, in particular with only one water of crystallization produces with those having several waters of crystallization an interlacing of the respective crystals during the reaction and mutual heating, which cooperate in the use of conformance with the invention for 9
form an extremely resistant male or an extremely resistant mold. A different or additional possibility of decreasing the quantity of water or water vapor to be expelled during the method according to the invention may consist in mixing a solution of high or higher concentration of magnesium sulphate without or with a minimum of a water of crystallization with a hydrocolloid, and use this mixture as a binder. The addition of hydrocolloid may have the effect of obtaining higher salt concentrations in a relatively small amount of dispersion water and / or solution, so that it is necessary to expel correspondingly less water. Another embodiment of the method can be that with the amount of solution water pre-set for a given amount of foundry sand, more magnesium sulfate is mixed than is required for a saturated solution, and that part of the magnesium sulfate is dispersed in the solution and mix with the foundry sand as a dispersion. By this it is possible to introduce as much magnesium sulphate as possible as binder in the foundry sand, and to maintain the necessary amount of solution water as low as possible, so that it will then also only be necessary to expel. correspondingly little water vapor. Simultaneously 10
they preserve the advantages during the subsequent elimination of the remains of foundry sand from the casting, with the aid of a simple washing with water or a dip in water. The foundry sand can be blended with the dispersed or dissolved binder in a weight ratio of 97: 3 to about 80:20. It is convenient if approximately 100 parts by weight of foundry sand are mixed with about 3 parts by weight to about 20 parts by weight of dispersed or dissolved binder, ie dissolved magnesium sulfate and / or without water of crystallization. An optimized procedure can consist in that about 5 to 10 parts by weight of binder are mixed in dispersed or dissolved form with approximately 100 parts by weight of sand. Tests have shown that this leads to resistant casting cores or molds, which can withstand the melting process well and in which it is necessary to expel from the tool a minimum of water possible. The invention also relates to a device for producing casting molds or cores with at least one heating device for hardening, the device for the production of the casting molds being a molding machine, and the device for producing the same.
production of the cores a core molding machine by injection with compressed air. This device can be characterized in that the molding machine or in the core molding machine by compressed air injection is installed as a heating device at least one microwave generator, and in that in the region of the mold tool for the mold of or for the core (s) at least one microwave antenna is provided, which can be coupled or is coupled with the microwave generator through a hollow conductor. For the expulsion of gases and / or hot water steam, a feed opening of a gas flushing hood can serve in this respect. At this point we would like to mention that the mold tool for the casting mold or for the core can also be a multiple tool, in which, for example, several cores are molded and / or heated simultaneously. That is to say that the device according to the invention can conveniently be constituted to a large extent by a machine of molding or machine for molding a cores by injection by means of compressed air, now known, which is equipped with a heating device, specifically with a generator. microwave and a microwave antenna. It is also convenient that for the expulsion of gases or steam from hot water it is possible 12
use the inlet openings of the mixed foundry sand with the binder, so that in total there is an economical device available. Optionally it is also possible to retroactively modify the core molding machines by injection with compressed air or the existing molding machines, in order to be able to use with them the advantageous invention, and in particular the method according to the invention. It is convenient if by adjusting the device for the gas sweeping process to expel gases or water vapor the microwave generator can be coupled simultaneously with the antenna through the hollow conductor. By this it is possible to simplify the operation of the device, since in this way only an adjustment movement is required to couple the microwave generator with the antenna and trigger the heating process. The adjustment movement to the gas sweeping process can for this purpose automatically effect the coupling of the microwave generator with the antenna. For this purpose it is only necessary to carry out the corresponding coupling so that the closure of the gas flushing hood or the like simultaneously establishes the corresponding coupling of the microwave generator with the antenna. A construction grouping particularly 13
simple can provide that the extension of the hollow conductor is separable and comprises a coupling at the point of separation, and that, optionally, the part of the hollow conductor on the side of the antenna is arranged on or connected to the gas flushing hood or the tool. This means that this coupling can be closed or detached by carrying out the corresponding movements to put a gas flushing hood in the position of use or also to remove it from it again. Another refinement of the invention for increasing and accelerating the heating process may be that the microwave generator can be coupled or is connected through a bifurcated hollow conductor or through two hollow conductors with an antenna arranged in the sweeping hood of gas and one arranged in the mold tool. In the foregoing it was already mentioned that during hardening it is possible, optionally, to expel water vapor. This is convenient, in particular, if the casting mold or core is produced from a mixture of foundry sand and a binder which is a dispersed or dissolved magnesium sulfate. In this case, the device can be adjusted to the gas sweeping process to expel water vapor that is produced when heating or heating, as already mentioned above.
14
In total, it is conveniently achieved that the molding machine or the core molding machine by compressed air injection and the mold tools themselves can practically be preserved without modifications, since the existing ventilations can also be used with the device according to the invention, and can be used for the expulsion according to the invention of hot and evaporated solution water. It is only necessary to additionally install an antenna for the microwaves, for example, in the gas supply hood. Naturally, in this aspect it is necessary to manufacture the tools of suitable materials for microwaves. But this device with a heating device configured as a microwave generator and antenna can also be used in the manufacture of molds or cores in which a different binder is used than the dispersed or dissolved magnesium sulfate mentioned, and in which Hardening requires a heating process. Another possibility that has meaning worthy of protection proposes a device to produce molds of cast iron or cores with a minimum of a heating device for hardening, being that the device for the manufacture of casting molds is a molding machine and the device for the manufacture of cores is a machine 15
male molding machine by injection by compressed air, machine in which it is possible to place or is placed a tool mold or male. As the heating device, an electric resistance heater can be provided in which the male or electrically conductive mold 'constitutes the electrical resistance, and the mold or male tool composed of several parts for the extraction of a mold or a core can be at least in part electrically conductive and insulated at their contact points, and the parts of the tool may in each case comprise an electrical connection for applying an electrical voltage to the resistance heating device. In this way it is possible to heat with a very simple construction form the molds or cores that first contain solution water and / or water of crystallization, and that because of this are electrically conductive by virtue of the other components they contain, to expel the Water. The wet male or the wet mold constitutes an impedance, so that the electrical conduction capacity results. That is to say that the tension applied to them can be used to dry. The resistance heating device may comprise a power source with a frequency rectifier for increasing the frequency and / or a pulse former to form a pulse-shaped voltage. With a 16
tension that has the form of impulse it is possible to obtain good results with the heating. The resistance heating device may comprise a power source and a transformer for increasing the voltage, which are connected through the intake lines with the connections in the parts of the mold or male tool. Through this it is possible to increase the effectiveness. At least a part of the mold or core tool can comprise several electrical connections, and between these connections and the power source can be provided switches to selectively change or apply a voltage to the electrical connections, so that alternately a switch It is closed and the others are open. By means of this it is possible to suppress or always modify again the polarities that eventually occur in an electrode. The heating of a mold or of a core can take place in a correspondingly uniform manner, it being also possible to take into account the different contours of these molds or cores by changing the actual electrical connections in each case. In the case of a mold or male tool consisting of more than two parts it is possible for each part to have an electrical connection and power lines, and
it is possible that two parts of a tool of this type are always connected to the power source in a cyclic way. First of all, for complicated males, these multi-part tools are often required. However, with the aforementioned configuration it is possible to form a resistance heater by applying the applied voltage and to heat the core thoroughly. In total there results a method and a device with which a mechanized production of molds or cores in conventional male injection molding machines by means of compressed air is possible, and the sand of the mold can harden within about half a minute. In this respect, the method takes advantage of the extremely different melting points, on the one hand magnesium sulphate in its hydrated form and on the other hand in its state free of water of crystallization. As a heptahydrate magnesium sulfate has a melting point of about 75 ° Celsius, and in its water-free form of crystallization a melting point of 1124 ° Celsius. By the specific removal of the chemically bound crystallization water, it is therefore possible to obtain an almost sudden hardening of the mold sand. In this aspect, a favorable entanglement of magnesium sulphate partially hydrated and completely hydrated is advantageous, which can be used for already with quantities of magnesium sulfate.
very small magnesium sulfate, for example, 1% in relation to the sand of the mold, get a very high resistance. The important gassing to remove the crystallization water originally chemically bound after the heating can optionally take place by inlet and outlet nozzles that are arranged in a special way, being that an overpressure of 1 to 6 bar of a dry gas is convenient. , preferably of air of the heated environment. The heating can take place, conveniently, with microwaves, since the quartz sand that is usually used is "transparent" for the microwave radiation, so that it can completely penetrate into larger molds or cores. In addition, only magnesium sulfate containing water of crystallization is heated. As soon as the water of crystallization escapes, this magnesium sulphate, which is then without water of crystallization, is "transparent" and no longer constitutes an obstacle to the additional penetration of microwaves. But the heating can also be carried out conveniently by heating by resistance, as explained in the foregoing. That is to say, what is important is the specific elimination also of the water of crystallization chemically bound - at least in part - of magnesium sulphate. With 19
this produces a very fast hardening, which is convenient for an economical production. In addition, sufficient strength is obtained with a comparatively low concentration of magnesium sulfate. The mold parts or the males produced in this way have a stable shape up to at least 1124 ° Celsius, and can be detached from the metal casting with little water. If conventional binders are used, accelerated hardening by virtue of specific heating by microwave or electrical conductive heating is also possible. In the following, the embodiments of a device for the production according to the invention of molds or cores for casting purposes are described in more detail on the basis of the drawing. Shows in a partially schematic representation: Fig. 1 in graphic representation a device for the production of casting molds or cores with a microwave generator and the corresponding antenna in the form of a core molding machine by means of compressed air injection, Fig. 2 a amplified scale and still more schematized, a longitudinal cut through a part of the firing unit after firing the arena in a 20
mold tool configured as a box of males, and before coupling this box of males with a hood of sweep that is on it and to tighten it below to the firing head or vice versa, being that the microwave antenna for heating the male and to expel the solution water is arranged in this sweeping hood, and the connection between the microwave generator and this emitting antenna is still open, and can be closed automatically during the coupling or respectively lifting or tightening, Fig. 3 a representation analogous to FIG. 2, wherein the emitting antenna is arranged in the lower region of the core box configured as a mold tool, FIG. 4 an embodiment modified in relation to FIGS. 2 and 3 in representation analogous, in which both the sweep bell and the male box have an antenna to heat the shot that in the position of use can be of coupling or respectively is coupled with the microwave generator, Fig. 5 a representation analogous to Fig. 2 to 4 of a modified embodiment in which infrared radiator cores are arranged in the box to heat the shot male, Fig. 6 a representation analogous to Fig. 2 to 5 21
of a modified embodiment, in which an electric resistance heating is provided as a heating device in which the mold for the electrically conductive male is also electrically conductive, and its parts are insulated at their points of contact and in each part of the mold or male tools an electrical connection for a resistance heating device is provided, Fig. 7 a grouping and device analogous to Fig. 6, in which several of the parts of the tool for the core are provided electrical connections that can be connected at choice and alternately through switches, to avoid a polarization in one of the connections, Fig. 8 a newly modified device, in which the tool of the male is constituted by three electrically conductive and isolated parts of another, of which each one has an electrical connection, being that by means of a switch in each case two of the three parts can alternatively be connected to the power source, and FIG. 9 a form of embodiment and grouping analogous to FIG. 6, in which, however, the order of succession of the pulse former is reversed. and the voltage transformer located behind the power source, in relation to the arrangement of Fig. 6.
22
A device that is designated in its entirety with 1 and that in Fig. 1 is represented in schematic and partially cut-out form is used for the manufacture of cores, but it could also be used for the production of foundry molds. In the exemplary embodiment, it is a core molding machine by injection with compressed air. The cores 2 (Fig. 2 to 9) that must be produced with this device - or similarly, casting molds - are molded from a mixture 3 of foundry sand and binder, which is a magnesium sulfate, preferably with a minimum of one water of crystallization dissolved in water or else any other binder, being that this mixture 3 of sand and binder is introduced in a known manner into a sand feeding hopper 4 and by this is filled in the head 5 of firing of a firing unit which is designated as a whole by 6. In FIG. 1, the air compressor 7 essential for the firing process is shown partially cut out. To this device 1 in the form of a core molding machine by injection by compressed air belongs the core box 8 which is shown respectively in FIGS. 2 to 9, which consists of the assembly of an upper part 8a of the core box. and a lower part 8b of the core box, for the production of molds 23
of casting but also of a mold tool configured in a correspondingly different manner. In Fig. 8 an embodiment is shown in which the upper part 8a of the core box is divided in its part, in order to allow the extraction of a correspondingly complex male after its hardening. With this device it is possible to produce molds or cores for casting purposes from the mixture 3 of foundry sand and binder, where first the foundry sand and the binder are mixed, and then they are introduced in the mold tool or male - in the example of embodiment, the male box 8 - with the aid of the trigger unit 6. In Figures 2 to 9 this has already taken place, and the binder can harden and provide the mold or core 2 with the required strength. As the binder, magnesium sulfate is used, preferably with a minimum of one water of crystallization, dispersed and / or dissolved in water, which is mixed with the foundry sand, to form the mixture 3. That is, it is introduced or triggered afterwards. of this in the tool 8 of mold or male. After this, the core box 8, the dispersion water and / or solution, and at least a part of the water of crystallization, and being expelled from the mold or male tool, are evaporated by heating inside this tool. say, of the box 8 of males, by means of a medium 24
gaseous In order to carry out this method, at least one heating device, which will still be described in more detail, is provided in the molding machine or core molding machine by compressed air injection, with which it is possible to heat and expel the solution and water. / or crystallization. In the exemplary embodiments according to the embodiments according to FIGS. 1 to 4, a microwave generator 9 is installed in the injection molding machine 1 by means of compressed air, and in the region of the molding tool a heating generator 9 is installed as a heating device. , that is to say, of the male box 8, there is arranged - in a different place according to the example of embodiment - at least one microwave antenna 10, which can be coupled with the microwave generator 9 through a hollow conductor 11 , and that in the position of use is coupled. In the embodiment, the corresponding coupling 12 is still open, since although it is true that a male has already been fired, the core box 8 is still before its connection movement with the gas scavenging bell 13 and before of heating and hardening by microwave. In all the embodiments, a feed opening 14 is recognized, with which hot air can be introduced, for example, to eject the water.
hot and respectively water vapor, which in the use position is produced by heating with the aid of the heating device, ie the microwave 9. In Figures 1 to 4 the connection between the microwave generator 9 is still open and the antenna 10. By adjusting the device 1 to the gas sweeping process to eject the water vapor, i.e., by the relative raising movement of the core box 1 against the gas sweeping bell 13 and against the 5 or vice versa it is possible to simultaneously couple the microwave generator 9 with the antenna 10 through the hollow conductor 11, by closing the coupling 12 with the mentioned relative movement. After this, the heating can be carried out with the help of the microwave energy, and simultaneously, or starting a little later, the expulsion of the water vapor that is generated. The adjustment movement to the gas sweeping process can automatically effect the coupling of the microwave generator 9 with the antenna 10, so that the whole process can be carried out quickly. That is, the extension of the hollow conductor 11 is separable, and at the point of separation the aforementioned coupling 12 is provided, wherein the part of the hollow conductor 11 that is on the side of the antenna can be localized and connected at choice, according to 26
the Fig. 2, to the gas scavenging bell 13 or, according to Fig. 3, in the mold tool or box 8 of cores, or, according to Fig. 4, even in both places. Fig. 4 shows that the microwave generator 9 can be coupled through the hollow conductor 11 and joined with an antenna 10 arranged in the gas-sweeping hood 13 and one which is disposed in the mold tool or box 8 of males, so that the mold or respectively the casting male 2 is heated in a correspondingly rapid and intense manner, and the time required to expel the solution and / or crystallization water can be shortened. Fig. 5 shows a modified embodiment, in which as heating device against or inside the mold tool, infrared radiators are provided in the core box 8, which can be provided as an alternative to heating by means of microwave, or optionally even in addition to this, if, for example, another antenna 10 according to FIG. 2 was additionally provided in the gas flushing hood. In FIGS. 6 to 9, modified embodiments are again shown in FIGS. that an electric resistance heater is provided as the heating device, in which the electrically conductive core 2 constitutes the electrical resistance. The tool 8 of 27
a male that to remove a male 2 again consists of two (Fig. 6, 7 and 9) or three (Fig. 8) parts is at least partially or conveniently fully electrically conductive, being constituted, for example, of aluminum or cast iron or steel At their points of contact, the parts 8a and 8b are isolated from one another, and this insulation 16 is shown in schematic form in Figs. 6 to 9. It is further recognized that the parts 8a and 8b of the tools or respectively the box 8 of cores have in each case an electrical connection 17 for applying an electrical voltage to the resistance heating device. That is, the upper part 8a of the core box and the lower part 8b of the core box, ie the parts of the core tool 8 belong to the resistance heater, in which the core 2 constitutes the resistance itself. . Conventionally, this resistance heating device has a power source 19, which in the present case through a three-phase current network 20 leads to a frequency transformer for increasing the frequency and / or to a pulse generator 21 for form a tension that has the form of impulse. This resistance heater device further comprises a transformer 22 for increasing the
voltage, from which lead lines 23 to the connections 17 in the parts 8a and 8b of the tool 8 of male. If the voltage is connected, the wet male 2 inside the tool 8 acts as a respective resistance or as an impedance, so that current flows to dry the core. The intensity of the tension can be selected according to the thickness of the core 2. This results in a very intensive and effective drying, since the parts 8a and 8b act as electrodes that abut and are in contact with the core to which they are attached. the electric voltage is applied, since these "electrodes" 8a and 8b are separated from one another by the insulation 16 to avoid a short circuit. The electrical voltage can conveniently be a sine-shaped or pulse-shaped voltage, in particular a rectangular voltage, wherein a high-frequency alternating current of more than 1000 Hz, for example of 3000 Hz or even more, is particularly effective. It is also possible to regulate the attention, and select it higher than 1000 V. By varying the pulse width of the electrical voltage it is possible to control or regulate the power supplied and adapt it to the shape and size of a male 2 - and in the case of the production of a mold in a mold tool, to the mold. While in the embodiment according to Fig. 6 the male box 8 is formed by two parts 8a and 29
8b, and in each case have an electrical connection 17, the exemplary embodiment according to Fig. 7 shows in the lower part 8b of the core box four of these connections 17 which are connected in parallel, where between these connections 17 and the power source 19 are provided with switches 24 for the alternate application or at the choice of a voltage to the different electrical connections 17, alternately a switch 24 being closed, and the others open. By this it is possible to avoid a polarization at a contact point of the lower part 8b of the core box and to obtain the most uniform heating possible of the core 2. FIG. 8 shows an embodiment in which the tool 8 of FIG. The male is made up of more than two parts, the part 8a of the core box being divided into two parts, parts of which are electrically separated from one another by means of an insulation 25. This makes it possible to produce males 2 correspondingly complicated. Fig. 8 shows that each of these three parts has an electrical connection 17 and an electrical connection line 23, which first consists of two parallel branches 23a and 23b in which switches 26 are located. These branches 23b connected in parallel they allow to connect cyclically to the power source always two parts 8a or 8b 30
of a tool 8 of this type several times subdivided, when opening and closing in a cyclical way the switches 26. In other words, only two parts of the core box 8 are cyclically always present with current in order to use as resistance and to heat the male 2 that is inside her. In the embodiment according to FIG. 9, which substantially corresponds to that according to FIG. 6, it is shown that the sequence of the arrangement of the pulse former 21 and of the transformer 22 can also be reversed so that The voltage transformer 22 is then supplied first and then the pulse former 21. In the embodiments according to FIG. 6 to FIG. 9, as in the embodiment examples according to FIGS. 2 to 5, a gas scavenging bell 13 is provided with a feed opening 14 with which it can be introduced, for example, hot air to expel the hot water or steam, which is generated by heating with the help of the electric voltage in the position of use. For the gas flushing process, the gas flushing hood 13 is approximated analogously to the previously described embodiments. To eject the evaporated water it is possible to feed medium through the feed opening 14.
gaseous, for example, nitrogen and / or carbon dioxide and / or air, preferably hot air or hot gas. In this way the expulsion of the evaporated water can be carried out better by means of overpressure. The mixture 3 contains, as already mentioned, as a binder magnesium sulphate dissolved in water without and / or with one or optionally also several waters of crystallization. It is possible to use as a binder, for example, magnesium sulphate without, with a water of crystallization and magnesium sulphate with several crystallization waters combined and / or also mixed with a hydrocolloid. In this aspect it is particularly favorable if magnesium sulphate or magnesium sulfate is exclusively used with hydrocolloid, because the magnesium sulphate with water of crystallization is dispersed and / or dissolves well in water and, mixed as a binder with foundry sand later It can also be detached well from a work piece melted with the help of water. An example of a convenient mixture of melt sand and dispersed or dissolved binder may provide for mixing about 100 parts by weight of foundry sand with about 3 parts by weight to about 20 parts by weight of dissolved binder which primarily consists of sulphate of magnesium in dissolved form. It is possible to mix approximately 100 parts by weight of sand preferably with about 5 to 10 32
parts by weight of binder in dispersed or dissolved form. It is only necessary to expel a correspondingly small amount of water from the core box 8 by heating with a gas, and the process can be carried out correspondingly quickly. For the production of molds or cores 2 for casting purposes, a mixture 3 of melting sand and binder is produced and introduced into a mold or male tool 8, for example, it is fired into a core molding machine by injection compressed air. A known binder or magnesium sulfate is used as binder without and / or with at least one or more water (s) of crystallization, dispersed or dissolved in water, and mixed with the foundry sand and introduced or fired inside. in the mold tool or respectively the core box 8. For hardening, the water and a part of the water of crystallization are then evaporated by heating and expelled by means of a gaseous medium, which can be carried out very quickly. After casting, it is possible to detach and detach very quickly from the workpiece, by flotation by water, a male of this type or a mold of this type made up of foundry sand, since magnesium sulphate retains its ability to enter in solution.
33
It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.