Patent Publication Number: US-2019193144-A1

Title: Method and device for producing mould material moulds for the casting of metals

Description:
RELATED APPLICATIONS 
     This application is a U.S. National Stage filing of PCT/IB2016/053537, filed Jun. 15, 2016, which claims priority to German Patent Application No. 10 2015 109 805.9, filed Jun. 18, 2015, and German Patent Application No. 10 2015 109 640.4, filed Jun. 17, 2015. The disclosures of each of these documents is hereby incorporated, by reference, in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This disclosure relates to a method for producing a molding-material mold having a predetermined or pre-determinable minimum strength for the casting of metals. In the process, a molding material is inserted into a molding box and the molding material is compacted in the molding box. According to the invention, the compaction is carried out in two steps: a first step in which the molding box is moved across a first distance to stop against a press head, and a second step in which the molding material is moved across a second distance to an end position and is further compacted thereby. A length of the second distance is dependent on a condition of the molding material. 
     Molding substances or molding materials which are used in the production of molding-material molds for the casting of metals have a higher structural strength on the force application side after compaction than on the side facing away from force application. The differences in structural strength are proportional to the forces acting in the respective areas. This effect is based on the fact that the forces from the force application side are transferred under the angle of repose of the molding material to be compacted. That is, the forces are proportionally supported on lateral limitations due to friction, for example, on a molding box wall or upright or vertical pattern contours. 
     This uneven effect of the forces results in a variable compaction of the molding material in the molding box. This, in turn, results in an inconsistent strength of the molding-material mold. Since the 1960s, various pre-compaction methods have been developed for reducing or compensating these differences in molding-material compaction. Thus, the differences in compaction between the molding-material mold side, to which the force is applied, and the opposite side have been reduced. As a result, the average structural strength of the molding-material mold is improved. 
     However, the improvement in quality of the molding-material molds achieved by post-compaction, i.e. the improved compaction of the material caused thereby in areas remote from the site of force application, has not resulted in a constant structural strength or quality of the molding-material molds which is desirable for permanently producing high-quality products with little waste. 
     DE 44 25 334 C2 (DIS, Dansk Industrie), cf. column 1, lines 38 to 45, wishes to form the surface of the mold sufficiently hard. Allegedly, it is pressure, not force that is mentioned therein which nevertheless also means force with an identical surface area. However, this prior art suggestion does not operate with a molding box and a pattern plate (on which the pattern stands and which is moved with respect to the molding box) arranged therein, but with the compaction of sand poured into a molding chamber, cf. column 2, lines 29 to 39. Each one of the press plates 3, 8 thereof has a pattern 4, 5 at its surface, which plates are moved laterally towards each other so that the filled-in molding sand is compacted in the molding chamber and the desired molded part (cast molded part) is formed. 
     DE 602 17 205 T2 (Sintokogio) is concerned with a similar problem, considers the properties of the molding sand, and has realized that different molding sands have different properties, and that thus compensation controls are required for achieving the uniform height of the sand mold, which is described in paragraph [06] thereof, and what is more, under optimum conditions. The best possible definition is found in the sequence of paragraphs [042] to [045]. However, a pressure sensor is mentioned in paragraph [69] only, which has the function of differentiating between the first phase of compaction (primary compaction, as mentioned in paragraph [042]) and the second phase of compaction (secondary compaction, as mentioned in paragraph [043]) which is reflected in claim  5  thereof. The pressure sensor causes switching from primary compaction to secondary compaction. 
     There is a need for a method for producing molding-material molds for the casting of metals which performs a more uniform compaction, thereby improving the structural strength of the molding-material mold. Thus, a consistently high quality of the metal casting products is achieved in a more effective manner than so far and the number of defective molding-material molds (and cast parts) is reduced. There is further a need for a system for producing molding-material molds with a consistent quality of the molds in the long run. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention(s) to produce improved molds for the casting of metals and to prevent the complexity of the production from increasing. “Improved” may be defined such that a mold consisting of the molding material permanently has a surface of uniform hardness, even in the event of a change or variation in the quality of at least one of a plurality of properties of the molding material. 
     This object is achieved by the disclosed method as well as by the disclosed device which are incorporated in their entirety herein. 
     The two-stage method which is claimed as well includes a first step and a second step, wherein, for the second step, there is a second distance which is adjusted or changed in accordance with the third feature. A change is required when a property of the sand is changed. If the sand is not changed, no control intervention is intended. This is the so-called “steady state” of the controlled system given here. A measurement of a preceding compaction process has a controlling effect on the next compaction process (or one of the subsequent compaction processes). 
     As a working method, the claimed inventions operate in a completely different manner as compared to the state of the art. A different reference variable is provided, namely the force determined for the quality and hardness of the mold of a preceding molding, that adjusts the distance for the subsequent moldings which is given as a second distance prior to lifting the pattern plate. 
     The second distance is adjusted, or as a person skilled in the art would put it “has been adjusted” when the molding box abuts the press head in an upper position. In that case there is virtually no further upward movement for the pressing device operating from below. Then, the movement is carried out and the pattern plate is moved further upwards by the pressing device, the molding box itself cannot be moved any further, but the pattern plate is moved across a second distance in order to have a target, namely the lower edge of the molding box. 
     The second distance is adjusted as a function of a force. This force is representative of the hardness of the mold at its surface and is used as a reference variable by the invention, from which the distance required in the second molding results. Inherently, it is a strength or hardness control of the sand surface by a predetermination of the pattern stroke as a higher-level control. According to the invention, this is not accomplished in the same process, but in a time delayed manner after the end of the previous molding. DE 602 17 205 T2 shows a box requiring position control. DE 44 25 334 C2 does not disclose a box, so the criterion of the position to be reached relative to the box does not apply here. 
     This is very clear from for example, two equal distances S2 of approx. 5 mm with two different compaction curves (force-distance characteristics of the two different sands) yield a highly variable force. Thus, they also yield a highly variable strength, e.g. the right-hand representation of  FIG. 8 a   . The invention will perform readjustment here, which difference control is exemplarily shown in the right-hand representation of  FIG. 8 . The distance is readjusted such that the force is achieved as best as possible at the end of the distance. 
     The object thus has a double aspect, a force has to be achieved and this force has to be achieved at a point in time when the pattern plate reaches the lower edge of the molding box. For this purpose, this is to be repeated accurately. 
     If a person skilled in the art interprets the problem differently and starts from different sands having different distance-force characteristics, the need for a change in distance arises with respect to the invention so that the force (hardness or strength) is appropriate at the end of compaction. If a person skilled in the art would work differently and control only a force, the distance would not be correct if he started from changes in properties of the molding material (molding sand) not sufficiently compacted. According to the invention, both objects are achieved, wherein the invention uses the, or one of the preceding measurements of the force at the end of the corresponding compaction as a basis. 
     In terms of results, it may be said that the force can be too high when the lower edge of the molding box is reached. In that case it is useless and even a prolonged application of this force using a timer according to DE 602 17 205 T2 would not improve anything. However, if the force is too low because the sand could be compacted too easily, the possible strength of the sand surface has not yet been exhausted at the end of the stroke (assumed to be firmly adjusted). 
     The invention can thus be viewed from various perspectives and is hence not anticipated by the prior art if the prior art also orients the end of its process towards aligning the pattern plate with the lower edge or lower side of the molding box. This is absolutely necessary for the production of a reasonable or useful mold, but the way how this necessity is achieved is different according to the invention, in fact completely different from the way suggested in DE 602 17 205 T2. 
     Designs of the claimed basic inventions are defined in the dependent claims. They can be combined with each other in a technically useful manner. The description, especially in connection with the drawings, additionally characterizes and explains the invention. 
     The granular pourable molding material (also called molding sand or molding substance; referred to as “molding material” hereinafter in a simplifying manner), from which the molding-material molds for the casting of metals are produced, is usually a bentonite-bonded molding material, briefly also referred to as “sand”. The molding material is used again and again in a cycle, wherein the starting material gradually mixes, for example, with core sand or greensand (sand in mint condition), of which insert core(s) for the cast parts to be produced may be formed, and fines contents, such as ground grains of sand, in a non-deterministic manner. This mixture of pure molding material, greensand or used core sand and fines changes the property of the molding material. 
     The molding material may also have a varying grain size distribution which has a direct effect on how much force has to be applied for compacting the molding material to a target value. In practice, the molding-material properties of a material batch are determined by a standard test. In this test, a standard container is filled with material which is compacted by use of a stamp with a predetermined force. The penetration depth of the stamp into the standard container is measured when the predetermined force is reached. 
     The measured value, i.e. the distance by which the stamp penetrates into the standard container, is the value which is set as a constant value for the post-compaction of the molding-material molds produced from the material of the one material batch in prior art systems. 
     However, it is not taken into account that the force curve across the distance is not a constant, but changes depending on the specific properties of the molding material of every single molding-material mold.  FIG. 1  shows the result of a plurality of measurements of the molding material of one material batch. It is clearly discernible that, with a given force of 3,000 N with which the stamp is pressed into the standard container, the penetration depth of the stamp into the standard container varies between approx. 12 mm and more than 50 mm. 
     One invention relates to the method for producing a molding-material mold for the casting of metals with a pre-determinable minimum strength of the mold. 
     In the method, the molding material (granular molding material) is filled into a molding box and the material is compacted within the molding box in a molding system. Compaction is performed in two steps. 
     In a first step, the molding box including the filled-in molding material is moved across a first distance by a pressing device to stop against a press head. The press head is usually arranged above the molding box so that the molding box is pressed against the press head from below. 
     The press head may comprise a stamp or multi-stamp protruding from the press head towards the molding box. The stamp or multi-stamp is retained in a predetermined position during the first step and penetrate(s) into the molding material previously filled into the molding box while in this position during the first step. Less preferably, the stamp or multi-stamp can be actively pressed towards the molding box during the first step. 
     In a second step, the pattern plate including the pattern (in the case of a stationary molding box) is moved across a second distance to an end position by the pressing device for hardening or compacting (creating) of the molding-material mold. The second distance is varied as a function of the condition of the molding material, in particular, is automatically adjusted for each molding box preferably as a function of the composition of the molding material. The new adjustment of the current process of compaction preferably results from the measurement of the force of the previous compaction process. 
     In other words, an individual second distance depending on the composition of the molding material is determined for each (upcoming) molding-material mold, and this second distance is individually adjusted in the molding system for each next molding-material mold. This also includes the case in which the second distance does not have to be adjusted between two or more molding-material molds since the molding material for the two or more successive molds has an identical, or at least substantially identical composition. However, the method is also capable of causing the adjustment of the distance (of the stroke of the stamp to be traveled) when a variation in at least one property of the molding material occurs. This is the object of a control which intervenes only when readjustment is required, i.e. a system deviation is measured. 
     Thus, the control achieves both, the adjustment of the force at the end of the compaction process, which force determines the quality of the mold (its surface hardness), and the stroke required for reaching the lower edge of the molding box at least substantially with a maximum tolerance of ±5% of the height of the molding box (as the best possible comparative measure). 
     The end of the hardening or compaction process is not required to define a point in time. It can span a time range ranging from the time of the end of the compaction, when the lower edge of the molding box is reached, to at most the duration of the molding cycle or sampling cycle of the control (T). In this range, no or no noticeable change in mold hardness occurs; thus, a measurement of the force of compaction can be carried out immediately when the force is being still applied (end of compaction) or a little later, can be determined using a different measuring device which detects the surface hardness of the mold, but is to be performed before the next pressing (compaction) starts. If the control is given more time, i.e. if more molded molding boxes are accepted between measurement of the mold hardness and readjustment of the distance s, the control is still functional, but has an internal inserted run-time (a dead time in the sense of control). In the example, the strength of the first compacted mold is measured, but is used only for the fourth mold to be compacted as a control variable. There are two molded molding boxes between the measurement and the change in distance for the upcoming compaction (box  4  is measured, boxes  3  and  2  are located in between, box  1  is currently being compacted, for which purpose the measurement of box  4  and its molding-material mold is used). Then, box  3  is measured and has an effect on box  0 , etc. 
     In the replacement circuit of this control, such a dead time in a first approximation has the effect of a PT 1  element, i.e. a delay which does not allow such a direct control as compared to an effect of the measured value used immediately after molding (effect of box  1  on box  0 ) and the effect of the control error on the compaction process following the measurement, however, also this kind of control is still functional. 
     During the second step, the stamp or multi-stamp of the press head can be retained or moved as described with respect to the first step. After completion of the second step, i.e. after completion of a pressing stroke of the pressing device, the stamp or multi-stamp can be pressed into the molding material with increased pressure. 
     The fact that a first distance and a second distance are mentioned here means that the molding box is initially moved across the first distance, is stopped at the end of the first distance, and the pattern plate is then moved for the second distance. 
     It may be preferred that the molding box passes the first distance and the second distance in a continuous movement. The adjustment of the second distance may be carried out before the movement the pattern plate or the second movement of the pressing device starts, and/or while the molding box is moving across the first distance. 
     The compaction of the molding material by the pressing device and/or the press head can determined or measured, in particular, by means of at least one sensor or a pressure sensor. The sensor can be arranged in or close to an area of the molding-material mold which is critical in terms of structural strength. For example, the sensor may be a part of the molding box, especially may be integrated into an internal wall of the molding box so that it directly detects the pressure transferred to the molding material in this position or in this area. When larger cast parts or cast parts having a complicated geometry are used, a plurality of sensors may be provided in corresponding critical positions. 
     Alternatively, the sensor or a sensor may be an optical sensor measuring the compaction of the molding material in an area within the molding material; for example, a laser sensor, the penetration depth of which into the material is adjustable. 
     Finally, a sensor or the sensor may be a sonic pulse sensor, such as e.g. sonar, which detects a degree of material compaction in the molding-material molded part or in an area of the molding-material molded part by means of sonic waves. 
     The force applied to the molding-material molded part and measured by the pressure sensor is converted into a signal which is fed to a controller in a wired or wireless manner. The controller may be a central controller of the molding system, preferably it is a local controller by which the received signals can be processed faster than in the standard controllers of the molding systems which are, in part, already 30 years old. 
     The controller may comprise a storage medium in which a nominal value or limit values of a nominal range for the measured applied force is/are stored. A program stored in the controller, e.g. a computer, may include an algorithm by means of which the value received from the sensor can be compared to the nominal value or the limit values in the storage medium, and a possible deviation of the measured actual value from the predetermined nominal value or nominal range can be determined. 
     What has been said in the previous section applies—mutatis mutandis—also in case the sensor is the optical sensor or sonic pulse sensor. Also in these cases, a nominal value or a nominal value range having clearly defined limit values can be stored in the controller or the storage medium as reference values for the current actual-value measurement. 
     If a deviation between the currently measured actual value and the stored nominal or limit value is determined, a correction value can be calculated from this deviation by an algorithm. This correction value can then be converted into a signal, and the signal can be transmitted to an actuator of the molding system which changes a length of the second distance, i.e. lengthens or shortens the second distance. 
     The second distance can be adjusted by a predetermined length of distance, for example, 0.5 mm, 1 mm, 1.5 mm, or any other length of distance irrespective of the size of the deviation. In other words, the control signal predetermines only a direction of the adjustment movement of the actuator and possibly the number of the steps required for changing the length of distance, but no absolute value of the change in length of distance. 
     Alternatively, the controller can determine the change in length of distance as a function of the calculated correction value, i.e. in this case, the control signal predetermines a direction of the adjustment movement of the actuator and a measure of the adjustment movement of the actuator. 
     The predetermined nominal value or the predetermined nominal value range can be a value which is input into the controller by a user or a corrected value which is determined by the controller during creation or production for the molding-material mold produced immediately prior to the current measurement. 
     The latter means that a nominal value is input into the controller at the start of the production of the molding-material molds, i.e. prior to the start of the compaction of the first molding-material mold of a production. This nominal value is then compared to the actual measured value of the first molding-material mold and is possibly corrected in the controller. The measured actual value or the corrected value calculated by the controller then serves as a predetermined nominal value etc. for the actual value of the second molding-material mold of the same production. In the n th  molding-material mold of the current production, the measured actual value or the correction value calculated by the control for the n−1 th  molding-material mold serves as a nominal value to which the actual-value measurement of the n th  molding-material mold is compared. 
     Finally, the correction value can also be determined from information on the molding material for the molding-material mold to be currently created. For receiving this information, for example, the molding material can be scanned when it is filled into the molding box so that a minimum, medium and maximum grain size of the molding material as well as the volumetric content thereof in the molding material can be determined. From this information, supplemented by further information, such as temperature, moisture, etc. of the molding material, a force can then be calculated which is required for producing a molding-material mold having a predetermined strength. 
     Values, such as temperature and moisture, can also be incorporated into the calculation when the applied force is detected by a sensor, as described above. 
     A further invention relates to a molding system for molds capable of casting made of a granular molding material (molding substance), for example, bentonite-bonded molding sand, for the casting of metals. 
     The molding system comprises a linearly movable pressing device for exerting pressure on the molding-material mold or casting mold being created which includes a molding box for receiving the mold and a filling frame for receiving an upper portion of the molding material for the mold. The molding system further comprises a press head including at least one mold stamp which may comprise a drive that is decoupled from the drive of the pressing device. The press head is arranged in a direction of closing of the pressing device ahead of (in most cases below) the molding box and is not moved by the pressing device. In other words, the pressing device moves the (filled) molding box, the filling frame and the pattern-plate carrier with the pattern resting thereon towards the press head during closing (start of compaction). When the filling frame reaches the frame of the press head, the molding box remains stationary. Then, the pattern plate including the pattern is moved relative to the molding box until it reaches the lower edge thereof. This is a necessary requirement, which has to be fulfilled at least substantially. The molding-material mold is supposed to line up with the lower edge of the molding box. 
     The press head comprises at least one, preferably a plurality of mold stamps distributed over the inner surface of the molding box, wherein the at least one mold stamp is fixed in a position relative to the molding box, or the mold stamp is pressed actively into the molding material of the mold to be compacted by a drive while the molding box is moved against the press head, and/or after the molding box has stopped against the press head. Preferably, the press head comprises more than one mold stamp, wherein the plurality of stamps may form a multi-stamp. 
     The molding system further comprises an actuator coupled to the pressing device or the molding box, which actuator comprises a linear drive that is decoupled from a drive of the pressing device. The possible effective directions of the linear drive of the actuator and the drive of the pressing device may be identical. The fact that the linear drive of the actuator is decoupled from the drive of the pressing device means, in particular, that the actuator can be linearly moved relative to the pressing device in and opposite to the possible direction of movement of the pressing device. 
     A controller adjusts a distance (s 1 ) between the pressing device and the molding box via the actuator when the molding box abuts the press head or a frame of the press head. This “when” is not to be understood in the sense of a temporal correlation in the claimed molding system. It is the possibility at which this adjustment is to be present when the compaction is performed subsequently thereto. The change can also be carried out prior to the abutment against the press head or a frame of the press head; it may also be adjusted already during the first stroke, i.e. it has an entire time range, however, also a structural end, at which it should be adjusted at the latest in order to still have an effect. 
     The adjustment travel of the actuator can be between 20 mm and 100 mm, preferably the adjustment travel is between 30 mm and 90 mm, particularly preferably between 40 mm and 80 mm. The adjustment travel is determined by the height of the molding box or the size of the filling material mold or the cast part to be produced by the mold. The adjustment travel may also be greater or less than the preferred adjustment travel. 
     Due to the adjustment of the actuator, a total distance is changed which the pressing device travels from an initial position, in which the molding box does not stop against the press head, to an end position, where the pressing process for the production of the casting mold is completed, during production of the casting mold. By means of the actuator, the total distance of the pressing device or the total stroke of a pressure cylinder of the pressing device can be lengthened or shortened. 
     Furthermore, the system comprises at least one force sensor which measures a force applied to the molding material or exerted on the molding material by the pressing device and/or the press head. Instead of or in addition to the force sensor, an optical sensor or a sonic pulse sensor may be used for measuring the compaction of the molding material in an area below the surface. To this end, it is referred to the explanations with respect to the method. 
     Finally, the system comprises a controller, wherein the controller is connected to at least the sensor and the actuator in terms of signaling. The controller automatically adjusts a distance between the pressing device and the molding box or a lower side of the molding box on the basis of the signal from the sensor. This adjustment can be started prior to the start of the movement of the pressing device and has to be completed just before the molding box stops against the press head at the latest. 
     The controller may be a central controller of the molding system, however, preferably it is a separate controller having extremely short control times. 
     The molding system may comprise further features which can be taken—mutatis mutandis—from the description of the method. Basically, it holds true that all features of the method can be interpreted to apply also to the system and, vice versa, all features of the system may apply to the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by means of examples and not in a way that transfers or incorporates limitations from the Figures into the patent claims. Same reference numerals in the Figures indicate same or similar elements. 
         FIG. 1  shows, in a graphic representation, results of a plurality of samples of one material batch which have been compacted using a predetermined force. 
         FIG. 2  shows, in a graphic representation, the achieved compaction as a function of a firmly adjusted stroke. 
         FIG. 3  shows, in a graphic representation, a relationship between structural strength and the applied pressing force. 
         FIG. 4  shows an enlarged cutout of a graphic representation of a target area for structural strength. 
         FIG. 5  shows a cutout of a molding system  1  including force sensors  30 ,  30 ′. 
         FIG. 6  shows a cutout of another molding system V in which the force is measured in a different way at the end of the preceding (e.g. immediately preceding) compaction process. 
         FIG. 7  shows a control unit  102  or  102 ′ with the sampling cycle T. A force (of compaction) is determined at the end of the previous compaction process. The control unit  102  then changes the distance from s 0  to s 1  (or from s 1  to s 2 ) for the subsequent compaction process until the pattern plate  46  reaches the lower edge of the molding box  40 . Thus, the stroke of the second portion of the pressing process is changed (indirectly) as well. This is due to the (determined, i.e. measured or calculated from other values, e.g. pressure) differential stability between nominal value and actual value which is fed to the control unit  102  or  102 ′. This is the system deviation of the differentiator  99 . 
         FIG. 8  shows a control process in which the control unit  102  in the controller  100  reduces the distance s for the next molding, in this case because the force F (applied by the lifting cylinder as a press) was too high at the end of the previous molding. 
         FIG. 8 a    shows an end of the molding including the traveled distance s with a force F 50 . The initial position is shown on the left and the end position, when the lower edge  40   a  of the molding box  40  is reached, is shown on the right. The (changed) force-distance characteristic of a changed molding material results in a different force F with an identical distance s. 
         FIG. 8 b    shows the start of the molding including the not yet traveled distance s without force. The initial position is shown on the left and the same position is shown (enlarged) on the right. The (precise) force-distance characteristic of the molding material  41  to be compacted is still unknown. 
         FIG. 9  shows isolated force-distance characteristics of two molding materials A and B or one molding material  41 , the immanent property of which has changed during use. Apart from compactability, the fines content and the grain-size distribution have a considerable effect on the force-distance characteristic of one or two molding materials to be compared. An identical distance s is illustrated for both sands A, B. However, a difference of almost a factor of 2 of achieved force (or strength) can be seen on the ordinate what a difference. If an increase in distance is achieved for sand B, a mold hardness may result therefrom which is equal to that achieved for sand A. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows the already mentioned standard container which can be filled with a sample of the molding material. The molding material is provided to be compacted to form a molding-material mold for the casting of metals in a molding system. After being filled into the standard container, the molding material can be compacted by use of a stamp. The stamp is connected to, for example, a hydraulic cylinder which presses the stamp into the standard container with an adjustable force. When the stamp is pressed into the standard container with the predetermined force at the maximum, the penetration depth of the stamp into the standard container can be measured. 
     This measurable value is representative of the compaction behavior of the material in the standard container and is considered to be representative of the compaction behavior of an entire batch in the prior art. The value of the measurement is used for adjusting a distance or stroke for the post-compaction in a pressing device for producing a molding-material mold. According to the prior art, an entire batch of the molding material is then processed in a molding system using this post-compaction adjustment. 
     The graphic representation next to the standard container exemplarily shows the result of pressing processes of a plurality of material samples of a single batch of corresponding molding material with an identical force. In the table, the force acting on the stamp is plotted against the penetration depth of the stamp into the standard container. 
     The measuring results show that the molding material of a batch is not nearly homogeneous, but that the penetration depth of the stamp S into the standard container B is between approx. 12 mm and approx. 50 mm when the samples are compacted with an identical force. 
       FIG. 2  also shows a graphic representation, as already conveyed with  FIG. 1 . It is graphically shown by means of arrows that, when a firmly adjusted stroke (distance) is used, the granular molding material is compacted with a force of a minimum of approx. 1700 N and a maximum of 2400 N depending on the composition or property of the molding material. In other words, the molding-material molds produced from these materials with an identical pressure have highly variable strengths which is unfavorable for a smooth production and may result, for example, in increased waste. 
       FIG. 3  shows in a further graphic representation that, in the molding-material molds, the achievable strength of the mold is directly (substantially linearly) dependent on the application of force to the molding-material mold at the end of the compaction process, or the force with which the molding material is compacted at the end of the compaction process. 
     For showing the relationship between structural strength and the force applied to the molding material at the end of the stroke or distance illustrated in the graph, numerous samples of a molding material were compacted with three different final forces in each case. Thus, it could be shown that a relationship between structural strength and the force applied to the molding material can be described by a linear function with sufficient precision. 
       FIG. 4  shows an enlarged cutout of a graphic representation including a target area for an intended structural strength of a molding-material mold. The target area is delimited by a lower limit value F min  and an upper limit value F max . In other words, a force at the end of the stroke has to be reached in such a manner that a curve in a diagram showing the strength across a distance (of  FIG. 2 ) is within the target area at the end of the stroke. This area can be a hysteresis, or may only have a value as a “switching value” which is to be achieved or exceeded at least a little bit. 
     In the diagram cutout of  FIG. 4  a curve I can be seen which rises to a maximum value I max  and then falls again. When a predetermined force is applied, the highest strength is achieved at the point I max . The maximum value I max  is clearly not within the target area. For compacting the same material to such an extent that the force reaches the target area, only the control variable “force” can be changed. 
     This requires that the application of force must be increased. The result of the increase in force is curve II, the maximum value II max  of which is now within the target area. 
     In addition, it can be taken from  FIG. 4  that an increase in force or application of force can be achieved via a change of the stroke or distance which is predetermined by the not shown actuator of the molding system. The distance (the stroke travels) is reduced as compared to the previously adjusted stroke. 
     The stamp (the pressing device) thus travels a different distance, even though the end of this changed distance is still the lower edge of the molding box. However, the force at the end of the changed distance is a different one, namely a force corresponding to the target value which is representative of the mold hardness at the surface (in most cases at the surface of the pattern). 
       FIG. 5  shows an exemplary structure of a pressing device  1  of a molding system in which a molding material  41  can be compacted to form a molding-material mold. 
     A pressing device  1  of a molding system for the production of molding-material molds or casting molds for the casting of metals is shown in a vertical longitudinal section. The pressing device comprises a lifting cylinder  2  which can be moved with an adjustable pressure in the direction of the arrow. For lowering the lifting cylinder  2 , the cylinder can simply be switched powerless whereby it preferably returns to an initial position by its own weight only. The lifting cylinder  2  may be a cylinder which can be charged with oil or air. Instead of the lifting cylinder  2 , a linear drive can be used, for example, a gear rack which can be moved linearly by a gear drive. 
     The lifting cylinder  2  is connected to the lower side of a molding box  40  via a connector  3 . The molding box  40  comprises a filling frame  42 . In the following, the molding box  40  and the filling frame  42  will be subsumed under the term “elevated molding box”  40 . The molding box  40  is filled with a molding material  41 . A press head  10  including a multi-stamp  11  is arranged above the molding box  40 . A stop  13  protrudes from the press head  10  towards the molding box  40 , which stop delimits a movement of the molding box  40  in the direction of the arrow. 
     The connector  3  comprises an actuator  20  including a drive  21  and a support cylinder  22  into which the actuator  20  can be retracted at least in part when the lifting cylinder  2  moves to its end position. The drive  21  is decoupled from the drive of the lifting cylinder  2 . A distance between the upper side of the lifting cylinder  2  (or the pattern carrier  46 ) and the lower side or lower edge  40   a  of the molding box  40  (the pattern plate with the pattern standing on top of it for predetermining the cavity of the molding-material mold) can be increased or decreased by means of the actuator  20 . 
     In the embodiment, the adjusted distance is s 1 . The distance can be a minimum of zero. A maximum value is determined by the structural design of the actuator  20 . 
     In this embodiment, two sensors  30  are arranged within the molding box  40  which measure a force acting from the lifting cylinder  2  and/or the press head  10  on the molding material  41 . The force measured by the sensors  30  is transmitted to a controller  100 . The controller  100  comprises a storage medium  101  which stores a predetermined strength value for the molding-material mold to be produced, or limit values within which a desired strength value lies (control variable or target variable). 
     A microprocessor  102 , as a controller (also referred to as control unit  102 ), functions as a “control” (a functionally adapted technical program or a plurality of such modules as a control unit) by means of which a strength value measured by the sensor  30  can be compared to the strength value held in the memory  101  or specified separately. 
     When a deviation (as a system deviation) is determined here, a correction value can be calculated which is output to the actuator  20  in the form of a signal. The signal causes activation of the drive  21  which can move the actuator  20  from its position to another position. From the point of view of control, position s 1  is changed to a second position (or a second distance) s 2 . 
     An operating cycle of the pressing device  1  of the molding system may be as follows . . .
         The molding box  40  being filled with molding material  41  is inserted into the pressing device  1 .   The support cylinders  22  of the actuator  20  are extended to the calculated position and arrested in the extended position.   The lifting cylinder  2 , together with the molding box  40 , moves towards the press head  10  until the molding box  40  abuts the stop  13 , thereby pressing the multiple stamps  11  into the molding material  41 .   The lifting cylinder  2  moves further upwards and overcomes the distance s 1  adjusted by the actuator  20 .   The multiple stamps  11  are pressed further into the molding material  41  thereby.   The multiple stamps  11  re-squeeze at a defined pressure.   The lifting cylinder  2  and the multiple stamps  11  return to their respective initial positions.   The molding box  40  is removed.       

       FIG. 6  shows an exemplary structure of a pressing device  1 ′ of a comparable molding system in which a molding material  41  is compacted to form a molding-material mold, but in which the controller and its measured values operate in a different manner. 
     The force is not measured at the pattern here, but is calculated (determined) via the pressure P of the lower press stamp resulting from the control  90  thereof. The determination is carried out via a proportional factor (force per surface area is pressure). The adjustment of the changed distance s 2  after measuring the force F 2  (in the preceding compaction process) is carried out as the distance of the pattern plate to the lower edge  40   a  of the molding box  40 . It may be an Δs to the previous adjustment. Thus, s 2 =s 1 +Δs, wherein Δs may also be negative. 
     At the end of the compaction of this process, the pattern plate is also at the height of the lower edge of the molding box. However, the force has a different value at this time due to the readjustment of the distance to s 2 , in which the stamp traveled a changed stroke. 
     The determined force F 2  is transmitted to a controller  100 ′. This controller  100 ′ comprises a memory  101 ′ which stores a predetermined strength value for the molding-material mold to be produced, or limit values within which a desired strength value lies (control variable or target variable). A microprocessor or an ASIC  102 ′, as a controller (also referred to as control unit  102 ′), functions as a “control” (also here, a functionally adapted technical program or a plurality of such modules as a control unit) by means of which the determined strength value can be compared to the strength value held in the memory  101 ′ or specified separately in order to obtain a system deviation from which the control unit calculates a change in control value Δs. 
     In this case, the control unit reduces the distance for next molding process by Δs, because the force was too high. In another case, the distance is increased by Δs if the determined force was too low (and thus also the intended strength was too low). 
     At the end of the preceding or pre-preceding compaction process, the force is determined which presses the pattern into the molding sand at the end of the stroke s i . The work cycle of compaction is T. For each T, there is a strength value in the form of the (measured or determined) force at the end of a molding process (as a compaction process). This value is subtracted from nominal value w in order to obtain a system deviation at the differentiator  99 , cf.  FIG. 7 . The new s 2  or (generally) s i , with i=1 to n, is adjusted by the control unit  102  or  102 ′, which may be a proportional controller, using the system deviation Δw. 
     The control unit changes the distance s 0 , s 1 , s 2  until the pattern plate  46  reaches the lower edge  40   a  of the molding box  40 . Thus, the stroke of the second phase of a twin-press compaction is changed (indirectly) as well. This is due to the differential stability between nominal value and actual value which is fed to the control unit  102 . 
       FIGS. 8 a  and 8 b    are per se self-explanatory with respect to sequence. They show the beginning of the second compaction, i.e. the approach of the pattern carrier  46  towards the lower edge of the molding box  40  and, in  FIG. 8 a   , the end thereof, at which this lower edge is reached, wherein the resulting force is apparent from the diagram at F s0 . Here, another molding material would have reached only the force (and strength) shown as achieved by the underlying curve. 
       FIG. 9  illustrates the force-distance characteristic of two “sands” (molding materials). Two curves with non-identical force-distance characteristics illustrate the highly variable force (structural strength) obtained with an identical distance s. The identical distance is indicated by arrows of equal length which results in a clearly variable force (shown on the ordinate on the left) of approx. 1.5 kN and approx. 2.8 kN (sand A). 
     If the force is to remain constant despite a change in force-distance characteristic, even if gradual, at the end of a respective compaction process, the distance can be adapted. This is precisely the route the solution takes when using the force control and change in distance in the second compaction process (the second stroke), and since the end of the second stroke is compulsory, namely the lower edge  40   a  of the molding box  40 , the stroke to be traveled has to be changed by the above described Δs. 
     Thus, the result can be scientifically explained, wherein an interposition of a distance changed in a controlled manner achieves both, the constant force describing the structural strength as being constant, and a target point of the upward movement which is procedurally defined for the continued use of the mold half  41  (then compacted). 
     
       
         
           
               
             
               
                   
               
               
                 Legend of Figures 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 FIG. 1 (Figur 1): 
                   
               
               
                 Kraft [N] 
                 =Force [N] 
               
               
                 Weg [mm] 
                 =Distance [mm] 
               
               
                 FIG. 2 (Figur 2): 
                   
               
               
                 Hub fest 
                 =Constant stroke 
               
               
                 Kraft 
                 =Force 
               
               
                 Verdichtungsweg 
                 =Compaction distance 
               
               
                 FIG. 3 (Figur 3): 
                   
               
               
                 Formfestigkeit [N/cm 2 ] 
                 =Structural strength [N/cm 2 ] 
               
               
                 Kraft bei Hubende 
                 =Force at end of stroke 
               
               
                 FIG. 7 (Figur 7): 
                   
               
               
                 Sollwert w FESTIGEKEIT 
                 =Nominal value w STRENGTH 
               
               
                 Regler 102′ oder 102 
                 =Control unit 102′ or 102 
               
               
                 S 0  Nachstellen, als S 1 (T) 
                 =readjust s 0  to s 1 (T) 
               
               
                 Verdichtungsvorgang 
                 =Compaction process 
               
               
                 Störgrö    e z (t)-veränderte 
                 =Disturbance variable z (t)-changed 
               
               
                 Eigenschaften des Formstoffs 
                 properties of the molding material 
               
               
                 Kraft F s0   
                 =Force F s0   
               
               
                 Festigkeit 
                 =Strength 
               
               
                 Kraft F s0   
                 =Force F s0   
               
               
                 Kraft-oder Druck-Sensor 
                 =Force or pressure sensor 
               
               
                 FIG 8 (Figur 8): 
                   
               
               
                 Weg s 0 , s 1   
                 =Distance s 0 , s 1   
               
               
                 Kraft F s0  (entspricht der 
                 =Force F s0  (corresponds to strength) 
               
               
                 Festigkeit) 
                   
               
               
                 F lst   
                 =F Actual   
               
               
                 Differenz 
                 =Difference 
               
               
                 F soll   
                 =F Nominal   
               
               
                 Weg s 
                 =Distance s 
               
               
                   
               
            
           
         
       
     
     Der Regler  100  reduziert in diesem Fall den Weg für die nächste Abformung, weil die Kraft in der vorigen Abformung zu hoch gewesen ist. 
     =In this case, the control unit  100  reduces the distance for the next molding since the force was too high in the preceding molding. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 FIG. 8a (Figur 8a): 
                   
               
               
                   
                 Weg s 0   
                 =Distance s 0   
               
               
                   
                 Kraft F s0   
                 =Force F s0   
               
               
                   
                 Weg s 
                 =Distance s 
               
               
                   
                 FIG. 8b (Figur 8b): 
                   
               
               
                   
                 Weg s 0   
                 =Distance s 0   
               
               
                   
                 S 0 ′ = S 0  − d 46   
                 S 0 ′ = S 0  − d 46   
               
               
                   
                 d 46  = Konst. 
                 d 46  = constant 
               
               
                   
                 Kraft F s0   
                 =Force F s0   
               
               
                   
                 Weg s 
                 =Distance s 
               
               
                   
                 FIG. 9 (Figur 9): 
                   
               
               
                   
                 Kraft [kN] 
                 =Force [kN] 
               
               
                   
                 Erreichte Kraft F A   
                 =Force F A  achieved 
               
               
                   
                 bei Sand A 
                 with sand A 
               
               
                   
                 Erreichte Kraft F B   
                 =Force F B  achieved 
               
               
                   
                 bei Sand B 
                 with sand B 
               
               
                   
                 Grundfestigkeit bei Beginn 
                 =Basic strength at the beginning 
               
               
                   
                 des zweiten Hubs 
                 of the second stroke 
               
               
                   
                 Weg [mm] 
                 =Distance [mm] 
               
               
                   
                 Weg s2 (zweiter Hub) 
                 =Distance s2 (second stroke) 
               
               
                   
                 bei Sand A 
                 for sand A 
               
               
                   
                 Weg s2 
                 =Distance s2 
               
               
                   
                 bei Sand A 
                 for sand A 
               
               
                   
                 Grundfestigkeit aus der ersten 
                 =Basic strength from the first 
               
               
                   
                 Phase (dem ersten Hub) der 
                 phase (the first stroke) of 
               
               
                   
                 Konturanpassung 
                 adaptation of contour