Abstract:
A drive for a movable part, such as a mold closure platen, includes two hydraulic cylinder-and-piston units defining first and third pressure chambers in a first of the hydraulic units and defining fourth and fifth pressure chambers in a second of the hydraulic units, and wherein the piston of one of the hydraulic units is a stepped piston defining a second pressure chamber, thereby providing five pressure chambers. An arrangement of valves provides for selective coupling of hydraulic fluid among the pressure chambers whereby pressure medium is forced from one of the hydraulic units, during an advance of its piton, to the other of the hydraulic units.

Description:
The invention relates first of all to a drive device for a movable part in an advance stroke, a power stroke and a return stroke, especially a mold closure device for an injection molding machine. 
     FIELD AND BACKGROUND OF THE INVENTION 
     As a mold closure device for an injection molding machine, the drive device moves the movable die platen of the machine. Such a drive device has to satisfy two important requirements. First, it must move the mold platen as fast as possible to close and open the mold, so that the cycle time for the production of a molding can be kept short. Secondly, it must be able to hold the mold platen and hence the entire mold closed with great force against the high injection pressure. First, then, adjustment movements must be performed with high acceleration, high speed and powerful braking; secondly, high forces must be exerted without any substantial movement. Such requirements may arise not only with the closure unit but also with the ejectors or the injection unit of a plastics injection molding machine. For example, when plastic is injected into the mold, the plasticizing screw has to be moved toward the mold at relatively high speed until the mold is completely filled with plastic. If subsequently the plastic contained in the mold is exposed to what is referred to as holding pressure, the drive has to apply a high force without substantial movement of the plasticizing screw. 
     A mold closure device in which an attempt was made to satisfy the above-mentioned requirements is disclosed in DE 195 23 420. In this drive device, there is connected to the movable mold platen the piston part of a hydraulic cylinder-and-piston unit having a hydraulic piston, which borders with a first active surface on a first pressure chamber and with a second active surface, which is smaller direction thereto, on a second pressure chamber, and comprises a further hydraulic piston, which is fixedly connected to the first hydraulic piston, and possesses a third active surface, which borders on a third pressure chamber and is active in the same direction as the second active surface. Via a valve arrangement, the second pressure chamber can be connected to the first pressure chamber and can be relieved of pressure separately from the first pressure chamber. The further hydraulic piston is formed by a plunger-like projection on the side of the first hydraulic piston facing the second pressure chamber. 
     During the adjustment movement in the closing direction, the first pressure chamber and the second pressure chamber are connected to one another, so that only the quantity of pressure medium determined by the difference between the first active surface and the second active surface has to flow to the cylinder-and-piston unit and a high speed can be achieved. For the application of the locking force, the second and the third pressure chambers are relieved of pressure, so that the first active surface is fully available to apply the locking force. During the adjustment movement in the opening direction, in the conventional mold closure device, all three pressure chambers are connected to one another and to a source of pressure medium, so that the quantity of pressure medium to be fed to the cylinder-and-piston unit for a desired speed is determined by the cross-sectional surface area of a piston rod starting from the first hydraulic piston, emerging from the cylinder after traversing the first pressure chamber and fixed to the movable part. 
     In the conventional mold closure device, the cylinder-and-piston unit for closing and opening the mold is operated in open hydraulic circuits, the pressure chambers being connected to a tank at the end of the closure operation and at the end of the opening operation and the movable part evidently being braked only by friction. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention further to develop a drive device of the conventional type so that it can be operated at higher dynamics. 
     This object is achieved with a drive device according to the invention. In this drive device, a first, receiving hydraulic cylinder-and-piston unit has a first pressure chamber, which can be subjected to the action of pressure in the power stroke alone, and a third pressure chamber to be subjected to the action of pressure on the return stroke. A second, delivering cylinder-and-piston unit has a fourth pressure chamber which is connected to the third pressure chamber at least on the advance stroke and on the return stroke, and a fifth pressure chamber which can be connected to the first pressure chamber. By a drive motor, in particular an electric drive motor, the piston part and cylinder of the second cylinder-and-piston unit are movable relative to one another. A second pressure chamber of the drive device is located in one of the two cylinder-and-piston units. A valve arrangement is present via which, during the advance stroke, pressure medium can be forced from the second pressure chamber into the first pressure chamber and, during the power stroke, the second pressure chamber can be relieved of pressure separately from the first pressure chamber. In addition, the third pressure chamber and the fourth pressure chamber have cross sections such that pressure medium forced out of the third pressure chamber during the advance stroke can be received by the fourth pressure chamber. The fourth pressure chamber may be permanently connected to the third pressure chamber. However, the fourth pressure chamber may also be formed by two partial pressure chambers, of which, during a working step to tear open the mold, only one partial pressure chamber is connected to the third pressure chamber. In this manner, the active size of the fourth active surface is diminished. A stronger force transmission can be achieved. 
     The method according to the invention for operating a drive device according to the invention is one characterized in that, during the movement of the movable part, all five pressure chambers are subjected to the action of an elevated pressure greater than atmospheric pressure. Preferably, the pressure during the braking of the movable part remains above atmospheric pressure in each of the five pressure chambers, so that the braking force is particularly high and the braking travel correspondingly short. 
     Advantageous embodiments of a drive device according to the invention are listed below. 
     The second pressure chamber of a drive device according to the invention may be located on the second cylinder-and-piston unit. 
     However, particular preference is given to an embodiment in which the first hydraulic cylinder-and-piston unit comprises a first hydraulic piston, which borders with a first active surface on the first pressure chamber and with a second active surface, which is smaller than the first active surface and faces in the opposite direction thereto, on the second pressure chamber, and comprises a further hydraulic piston, which is fixedly connected to the first hydraulic piston, and possesses a third active surface, which borders on the third pressure chamber and is active in the same direction as the second active surface. The second hydraulic cylinder-and-piston unit comprises a second hydraulic piston, which is connected to a fourth active surface at the fourth pressure chamber, which is permanently connected to the third pressure chamber, and borders with a fifth active surface, which faces in the opposite direction to the fourth active surface, on the fifth pressure chamber. In addition, the dimensional relationship between the fifth active surface and the difference between the first and second active surfaces is equal to the dimensional relationship between the fourth active surface and the third active surface. 
     Particularly preferably, the further hydraulic piston is formed in a simple manner by a plunger-like projection on the side of the first hydraulic piston or second hydraulic piston facing the second pressure chamber. 
     Advantageously, in a drive device according to the invention, the first cylinder-and-piston unit is formed in the same way as that according to DE 195 23420 C1. 
     If either the third pressure chamber or the fourth pressure chamber comprises a first partial pressure chamber and a second partial pressure chamber, of which one partial pressure chamber is connected jointly with the other partial pressure chamber via a valve to the other pressure chamber and can be connected separately from the partial pressure chamber connected to the other pressure chamber to a low-pressure tank, a force transmission to tear open the mold of a plastic injection molding machine is possible. 
     Preferably, the fourth pressure chamber and the third pressure chamber can be connected via a valve to a low-pressure tank. This reliably prevents the formation of a vacuum in the third and in the fourth pressure chambers when a high pressure is built up in the first pressure chamber by movement of the second hydraulic piston, during which time the first hydraulic piston and the further hydraulic piston move only slightly, if at all. The risk of cavitation is then slight. The low-pressure tank is expediently a closed tank with no connection to atmosphere and with a volume compensation provided by a flexible wall or a gas cushion. In this manner, the hydraulic system of the drive device can be so formed that the pressure medium has no contact with atmosphere. If the pressure medium is oil, this slows the process of aging. If the pressure medium is water, this results in no oxygen being introduced from the atmosphere so that corrosion of the metal parts that come into contact with the water is minimized. 
     It is advantageous if the fifth active surface on the second hydraulic piston is smaller than the first active surface on the first hydraulic piston. The second hydraulic piston and the first hydraulic piston then form a hydraulic force transmitter, so that with a relatively low load on the drive motor and on a rotation/translation converter installed downstream thereof a high force can be exerted with the first hydraulic piston. The force transmission naturally corresponds to a reduction of travel. 
     The drive device can be operated at particularly high dynamics if it is operated with elevated prestress pressures in the pressure chambers. In order to permit such operation in an advantageous manner, the fifth pressure chamber on the second hydraulic piston is connected in a first position of a further valve to the first pressure chamber and in a second position of the further valve to the low-pressure tank. In a “prestress” working step before the opening of the mold, it is now possible, by moving the second hydraulic piston in the “mold opening” direction, for pressure medium to flow from the low-pressure tank into the fifth pressure chamber, so that after the further valve has been switched over and the fifth pressure chamber connected to the first pressure chamber, the level of the prestress pressures is largely maintained. The level falls only slightly, because the pressure medium present in the fifth pressure chamber is raised to the pressure level of the first pressure chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Four examples of embodiment of a drive device according to the invention, which are conceived as mold closure devices, are shown in the drawings. The invention will now be explained in detail with reference to those drawings, in which: 
     FIG. 1 shows the first example of embodiment in a position occurring during the closure of the mold; 
     FIG. 2 shows the first example of embodiment at the end of the locking of the mold, in other words at the end of the build-up of a high pressure in pressure chambers one and five to hold the mold closed; 
     FIG. 3 shows the first example of embodiment after the delivering of the mold, in other words after the reduction of the high pressure in pressure chambers one and five to hold the mold closed; 
     FIG. 4 shows the first example of embodiment after the build-up of prestress pressures in pressure chambers one to four; 
     FIG. 5 shows the first example of embodiment in a position occurring at the start of the opening of the mold; 
     FIG. 6 shows the second example of embodiment in a position occurring during the closure of the mold; 
     FIG. 7 shows the third example of embodiment in a position occurring during the closure of the mold; and 
     FIG. 8 shows the fourth example of embodiment in a position occurring during the closure of the mold. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to FIGS. 1 to  5 , the first example of embodiment of a mold closure device, shown there only in sketch form, possesses a first, receiving cylinder-and-piston unit  10 , which is mechanically coupled to the movable mold platen  11  of a two-platen plastics injection molding machine, a rotary electric motor  12  as the drive motor and a second, delivering cylinder-and-piston unit  13  disposed in the power chain between the electric motor  12  and the first cylinder-and-piston unit. The cylinder  14  of the first cylinder-and-piston unit  10  is fixedly connected to the frame of the machine and forms a first cavity  15  having a regular cylindrical cross section and a second cavity  16  having a regular cylindrical cross section, the diameter of which is less than the diameter of the cavity  15  and which is open toward the cavity  15 . 
     The cylinder-and-piston unit  10  comprises two hydraulic pistons  17  and  18  disposed in the same plane and combined to form a stepped piston. The first hydraulic piston  17  is located in the cavity  15  and divides the latter sealingly into a first pressure chamber  19  and a second pressure chamber  20 , which is located on the side of the hydraulic piston  17  facing the cavity  16 . On the other side, a piston rod  21  projects away from the hydraulic piston  17 , traverses the pressure chamber  19 , emerges outward from the cylinder  14  and is fixed to the mold platen  11  which is movably guided on the machine frame. The further hydraulic piston  18  projects in the manner of a plunger away from the side of the hydraulic piston  17  facing the cavity  16  and dips sealingly into the cavity  16 . The pressure chamber  19  is bordered by the hydraulic piston  17  with an annular active surface  22  (first active surface of the drive device), whose size is determined by the diameter of the cavity  15  and the diameter of the piston rod  21 . The pressure chamber  20  is bordered by the hydraulic piston  17  with an annular active surface  23  (second active surface of the drive device), whose size is determined by the diameter of the cavity  15  and the diameter of the plunger piston  18 . The diameter of the plunger piston  18  is greater than the diameter of the piston rod  21 , so that the active surface  22  is greater than the active surface  23 . 
     The plunger piston  18  borders, with a third active surface  24 , whose size is equal to the cross section of the plunger piston and which points in the same direction as the second active surface  23 , on a third pressure chamber  25 . The active surface  24 , in the embodiment of the hydraulic pistons  17  and  18  shown, is naturally greater than the difference between the first active surface  22  and the second active surface  23 . 
     The second, delivering cylinder-and-piston unit  13  is, in the example of embodiment shown in FIGS. 1 to  5 , a simple differential cylinder having a cylinder  29  and having a hydraulic piston  30  and having a piston rod  31 . The cylinder space of the differential cylinder is sealingly subdivided by the hydraulic piston  30  into a non-piston-rod side pressure chamber  32  (fourth pressure chamber of the drive device) of regular cylindrical cross section, on which borders the hydraulic piston  30  with a regular cylindrical active surface  33  (fourth active surface of the drive device) and a piston-rod side pressure chamber  34  (fifth pressure chamber of the drive device) of annular cylindrical cross section, on which borders the hydraulic piston  30  with an annular cylindrical active surface  35  (fifth active surface of the drive device). The sizes of the five active surfaces are so selected that the dimensional relationship between the fifth active surface  35  and the difference between the first active surface  22  and the second active surface  23  is equal to the dimensional relationship between the fourth active surface  33  and the third active surface  24 . 
     The piston rod  31  is connected to a threaded spindle  36  which is secured against twisting. The electric motor  12  is a hollow rotor having a hollow shaft  37 , which is provided with an internal thread with which the threaded spindle  36  is in engagement directly or via ball bearings. The hollow shaft is thus the spindle nut of a rotation/translation converter  38  comprising spindle nut and threaded spindle. 
     According to the figures, the two pressure chambers  25  and  32  are permanently connected to one another via a line  39 . Connected to this line  39  is a 2/2-way seat valve  40 , which in the fully open position that it adopts in FIGS. 2 and  3  connects the line  39  to a low-pressure tank  41  for a pressure medium. The pressure medium may be hydraulic oil or water. Irrespective thereof, the low-pressure tank is completely closed toward atmosphere. A compensation system for various quantities of pressure medium to be received is permitted by a flexurally slack or elastic wall  42 . Depending on the tension under which the wall is placed, the pressure in the tank is equal to atmospheric pressure or slightly higher. 
     The mold closure device shown comprises, in addition to the 2/2-way seat valve  40 , a further two directional seat valves  45  and  46  with three connections and two switching positions, which, as in the case moreover of the valve  40 , can preferably be brought by a solenoid against the force of a spring from one switching position into the other switching position and are restored again by the spring. Of the three connections of a first 3/2-way seat valve  45 , one connection is connected to the first pressure chamber  19 , one connection to the second pressure chamber  20  and one connection to the line  39 , the two pressure chambers  19  and  20  being open to one another in a first switching position of the valve  45 , which it adopts in the illustration shown in FIGS. 1,  4  and  5 . The connection connected to the line  39  is shut off from the other two connections. In the other switching position of the valve  45 , which is shown in FIGS. 2 and 3, the pressure chamber  20  is connected to the line  39 , while the connection to the pressure chamber  19  is shut off. 
     The further valve  46  connects the fifth pressure chamber  34 , in one switching position which is shown in FIGS. 1,  2 ,  3  and  5 , to the first pressure chamber  19  and in the other switching position, which is shown in FIG. 4, to the low-pressure tank  41 . The further connection which is not necessary in each case is shut off. 
     The two valves  40  and  45 , which are each switched at the same time, can also be combined to form a single 4/2-way valve, which valve, by comparison with the valve  45 , would have a fourth connection connected to the low-pressure tank  41 . 
     In FIG. 1, the mold is just being closed. The valve  40  is in the closed position, the valve  45  connects the two pressure chambers  19  and  20  to one another and the valve  46  the two pressure chambers  34  and  19 . The electric motor  12  is operated, during the closure of the mold, in a direction of rotation such that the threaded spindle  36 , the piston rod  31  and the hydraulic piston  30  move toward the right as seen in FIG.  1 . As a result, pressure medium is forced from the pressure chamber  34  via the valve  46  into the pressure chamber  19  of the cylinder-and-piston unit. The hydraulic piston  17  moves toward the right and, via the piston rod  21 , carries with it the mold platen  11 . For a particular travel of the hydraulic piston  17  in this case it is only necessary for a volume of pressure medium to flow out of the pressure chamber  34  to the pressure chamber  19  which is equal to the difference in surface area between the two active surfaces  22  and  23  multiplied by the travel of the hydraulic piston  17 . The remaining increase in volume of the pressure chamber  19  is filled by the pressure medium forced out of the pressure chamber  20 . As the difference between the two active surfaces  22  and  23  is much less than the active surface  35 , the hydraulic piston  17  covers, for a particular travel of the hydraulic piston  30 , a much greater travel. The pressure medium forced out of the pressure chamber  25  is entirely received by the pressure chamber  32 , as the dimensional relationship of the active surface  33  and hence of the cross section of the pressure chamber  32  to the active surface  24  and hence to the cross section of the pressure chamber  25  is equal to the dimensional relationship of the active surface  35  and hence of the cross section of the pressure chamber  34  to the difference between the active surfaces  22  and  23  and hence to the difference between the cross sections of the pressure chambers  19  and  20 . 
     As the pressure chambers  19 ,  20  and  34  are connected to one another, the same pressure prevails in them, apart from minor flow losses. In the pressure chambers  25  and  32  the pressure, considering a static state, is far higher than atmospheric pressure. Therefore the pressure in the pressure chambers  19 ,  20  and  34  is also elevated. This is described as a prestressed system. The effect of this prestress is that, in the event of a movement of the hydraulic piston  30  from the position of rest, not only does the pressure increase in the pressure chambers  19 ,  20  and  34  as a result of the diminution of the pressure chamber  34 , but also the pressure in the pressure chambers  25  and  32  falls because of the diminution of the pressure chamber  32 , as the hydraulic pistons  17  and  18  cannot initially follow the hydraulic piston  30  because of their own inertia and the inertia of the movable mold platen  11 . Because the force acting in the closing direction increases and the counterforce falls, a large force resultant is also obtained for the acceleration of the mold platen  11 . 
     Shortly before the mold has closed, the hydraulic piston  30  comes to a halt. Because of inertia, the hydraulic pistons  17  and  18  move onward, so that the pressure in the pressure chambers  25  and  32  rises sharply and that in the pressure chambers  19 ,  20  and  34  falls sharply. A large force results, whereby the mold platen  11  is braked. The prestress pressures are so great that the pressure does not fall below atmospheric pressure in any of the pressure chambers either during the acceleration or during the braking of the mold platen. Cavitation is thus prevented. A very great resultant force is also obtained in each case. If in fact atmospheric pressure were reached, a further pressure drop would in practice make no further contribution to increasing the force resultant. 
     When the mold is closed, the two valves  40  and  45  are switched over and then adopt the positions shown in FIG.  2 . The pressure chambers  20 ,  25  and  32  are then connected to the low-pressure tank  41 , so that a pressure equal or close to atmospheric pressure prevails therein. The valve  46  continues to connect the two pressure chambers  19  and  34  to one another. During the further movement of the hydraulic piston  30  toward the right, while the hydraulic pistons  17  and  18  and the mold platen  11  move only slightly, if at all, a high locking force now builds up in the pressure chambers  34  and  19 . This pressure generates, on the one hand, only a slight force at the relatively small active surface  35  of the hydraulic piston  30 , which force must be absorbed by the rotation/translation converter  38 , and on the other hand a high mold locking force at the active surface  22  of the hydraulic piston  17 , which active surface  22  is larger than the active surface  36  and is now fully effective because of the let-down of the pressure chambers  20  and  25 . The increase in size of the pressure chamber  32  is compensated by the inflow of pressure medium from the low-pressure tank  41 . 
     When the injection operation has ended, the high pressure in the pressure chambers  19  and  34  is initially let down to the pressure or close to the pressure in the low-pressure tank  41 , the hydraulic piston  30  being moved toward the left by operating the electric motor  12  in the corresponding direction of rotation. When this occurs, pressure medium is forced out of the pressure chamber  32  via the valve  40  into the low-pressure tank  41 . 
     After the reduction of pressure, all three valves  40 ,  45  and  46  are switched over, so that, as shown in FIG. 4, the pressure chamber  34  is connected to the low-pressure tank via the valve  46  and the two pressure chambers  19  and  20  to one another via the valve  45 . There now follows a phase in which the prestress pressures for the displacement of the movable mold platen  11  are built up. For this purpose, the hydraulic piston  30  is moved further toward the left and the pressure chamber  34  is thereby reduced in size and the pressure medium in the pressure chambers  25  and  32  compressed. The pressure in the pressure chamber  25  generates a force at the active surface  24  of the hydraulic piston  18  which acts on the structural unit comprising the hydraulic pistons  17  and  18  in a manner such as to push the piston rod  21  out and push the hydraulic piston  18  into the cavity  15  of the cylinder  14 , and thus, because the cross section of the hydraulic piston  18  is greater than the cross section of the piston rod  21 , in a manner such as to reduce the size of the free volume of the cavity  15 . Therefore, if compressibility is disregarded, the hydraulic pistons  17  and  18  do not move. The pressure in the pressure chambers  19  and  20  rises, rather, to a value such that the force acting on the hydraulic piston  17  is equal to the force acting on the hydraulic piston  18 . During the build-up of the prestress pressure, pressure medium flows out of the low-pressure tank  41  via the valve  46  into the enlarging pressure chamber  34 . 
     Connected to the pressure chamber  19  is a pressure sensor whereby the prestress pressure and the locking force are monitored. Instead of or in addition to the pressure sensor connected to the pressure chamber  19 , one pressure sensor could be provided to record the pressure in each of the pressure chambers  32  and  34 . 
     Once the desired prestress pressure has been reached, the valve  46  is switched over. This may take place slowly, so that the pressure compensation between the pressure chambers  19  and  20  on the one hand and the pressure chamber  34  on the other takes place gently. The hydraulic piston  30  is moved from the position shown in FIG. 4 further toward the left. The hydraulic pistons  17  and  18 , the piston rod  21  and the mold platen  11  follow this movement, because the pressure medium, which is forced out of the pressure chamber  19 , can flow, to the extent that it exceeds the capacity of the pressure chamber  20  to receive it, into the enlarging pressure chamber  34 . During the acceleration and braking of the mold platen  11 , high forces again take effect, as the pressure rises in the pressure chamber  25  and falls in the pressure chambers  19  and  20  during acceleration and rises in the pressure chambers  19  and  20  and falls in the pressure chamber  25  during braking. 
     In the phase of build-up of the prestress pressures, the positions of the valves  40 ,  45  and  46  for which are shown in FIG. 4, the hydraulic piston  30  is moved toward the left, so that even allowing for any leakages during the performance of the adjustment movements and during locking it could drift toward the left. This drift can be compensated by a reference movement when necessary or regularly during each working cycle, for which purpose the position of the hydraulic piston  30  is recorded directly by a travel indicator, not shown in detail, or indirectly by an angle indicator on the electric motor  12 . The reference movement can, for example, be performed before the build-up of the prestress pressures. For this purpose, the valve  46  is brought into the position shown in FIG.  4  and the valve  40  into the fully open position, after the pressure reduction following an injection operation has ended. The hydraulic piston can then be brought toward the right into the starting position for the prestress phase, pressure medium being forced out of the pressure chamber  34  and flowing via the valves  46  and  40  to the pressure chamber  32 . The reference movement can also be performed after the closure of the mold and before the build-up of the locking pressure, the two valves  40  and  46  again being brought into the above-mentioned positions. It is advantageous here if the direction of movement of the hydraulic piston  30  need not be changed between the closure and locking of the mold. 
     In the example of embodiment shown in FIG. 6, the individual components and pressure chambers, where they correspond to components and pressure chambers of the example of embodiment in accordance with FIGS. 1 to  5 , are provided with the same reference numerals. According to FIG. 6, the mold closure device which is merely sketched there possesses, exactly like the first example of embodiment, a first, receiving cylinder-and-piston unit  10 , which is mechanically coupled to the movable mold platen  11  of a two-platen plastics injection molding machine, a rotary electric motor as the drive motor and a second, delivering cylinder-and-piston unit  13  disposed in the power chain between the electric motor  12  and the first cylinder-and-piston unit. Once again, the cylinder  14  of the first cylinder-and-piston unit  10  is fixedly connected to the frame of the machine. In contrast to the example of embodiment shown in FIGS. 1 to  5 , the receiving cylinder-and-piston unit  10  is now a simple double-rod cylinder whose hydraulic piston  17  has on one side the piston rod  21  connected to the movable mold platen  11  and borders on that side with the (first) active surface  22  on the first pressure chamber  19 . On the other side of the hydraulic piston  17  is a pressure chamber, which corresponds to the third pressure chamber  25  of the example of embodiment shown in FIGS. 1 to  5  and which because of an additional piston rod  55  is exactly the same size in cross section as the pressure chamber  19  and borders on the hydraulic piston  17  with a (third) active surface  24 , which is exactly the same size as the active surface  22 . 
     Instead of the receiving cylinder-and-piston unit  10 , the second, delivering cylinder-and-piston unit  13  now comprises two hydraulic pistons disposed in the same axis, which are combined to form a stepped piston. One hydraulic piston  30  (second hydraulic piston of the drive device) divides the part of the cylinder  29  that is larger in diameter into an annular (fourth) pressure chamber  32  and a further pressure chamber. The latter corresponds to the second pressure chamber  20  of the first cylinder-and-piston unit of the first example of embodiment and is therefore provided with the same reference numeral. The hydraulic piston  30  borders with a (fourth) active surface  33  on the pressure chamber  32 , the active surface  33 , because of a piston rod  58  traversing the pressure chamber  32  and having the same diameter as the piston rod  55 , and because of its cylinder diameter, being exactly the same size as the active surface  24  of the hydraulic piston  17 . The pressure chamber  32  is permanently connected via the line  39  to the pressure chamber  25 . 
     As in the first example of embodiment, connected to this line  39  is a 2/2-way seat valve  40 , which in the fully open position connects the line  39  to a low-pressure tank  41  having a flexurally slack or elastic wall. 
     On the side remote from the piston rod  56 , there is attached to the hydraulic piston  30  a hydraulic piston  58  of lesser diameter, whose diameter is greater than that of the piston rod  56  and which dips sealingly, in the manner of a plunger, into the smaller-diameter part of the cylinder  29 . The pressure chamber  20 , like the pressure chamber  32 , is annular. The active surface  23  of the hydraulic piston  30  bordering on the pressure chamber  20  is likewise annular. Because the diameter of the plunger piston  56  is greater than the diameter of the piston rod  58 , the active surface  33  is larger than the active surface  23 . The plunger piston  58  borders, with a (fifth) active surface  35  whose size is equal to the cross section of the plunger piston and which points in the same direction as the second active surface  23 , on a (fifth) pressure chamber  34 , which is permanently connected to the pressure chamber  19 . 
     A piston rod  31  projects away from the plunger piston  58  and traverses the pressure chamber  34 , has the same diameter as the piston rod  56  and is connected to a threaded spindle  36 , which is secured against twisting and is in engagement directly or via ball bearings with the hollow shaft  37 , provided with an internal thread, of the electric motor  12 . The hollow shaft is thus the spindle nut of a rotation/translation converter  38  comprising spindle nut and threaded spindle. 
     In the second example of embodiment, the sizes of the five active surfaces  22 ,  23 ,  24 ,  33  and  35  are selected so that the dimensional relationship of the sum of the fifth active surface  35  and the second active surface  23  and the first active surface  22  is equal to the dimensional relationship between the fourth active surface  33  and the third active surface  24 . 
     The mold closure device shown in FIG. 6 comprises, like those shown in FIGS. 1 to  5 , in addition to the 2/2-way seat valve  40 , a further two directional switching valves  45  and  46  with three connections and two switching positions. As in the first example of embodiment, of the three connections of the valve  45 , one connection is connected to the first pressure chamber  19 , one connection to the second pressure chamber  20  and one connection to the line  39 , the two pressure chambers  19  and  20  being open to one another in a first switching position of the valve  45 , which it adopts in the illustration shown in FIG.  6 . The connection connected to the line  39  is shut off from the other two connections. In the other switching position of the valve  45 , which is shown in FIGS. 2 and 3, the pressure chamber  20  is connected to the line  39 , while the connection to the pressure chamber  19  is shut off. 
     The further valve  46  connects the fifth pressure chamber  34 , in one switching position, which is shown in FIG. 5, to the first pressure chamber  19  and in the other switching position to the low-pressure tank  41 . The further connection which is not necessary in each case is shut off. 
     As in the second example of embodiment, the two valves  40  and  45 , which are each switched at the same time, can also be combined to form a single 4/2-way seat valve, which valve, by comparison with the valve  45 , would have a fourth connection connected to the low-pressure tank  41 . 
     When the mold is closed, the valves  45  and  46  adopt the switching positions shown in FIG.  6 . The valve  45  connects the two pressure chambers  19  and  20  to one another and the valve  46  the two pressure chambers  19  and  34 . The valve  40  is in the closed position. The electric motor  12  is operated, during the closure of the mold, in a direction of rotation such that the threaded spindle  36 , the piston rod  31  and the hydraulic pistons  30  and  58  move toward the right as seen in FIG.  1 . As a result, pressure medium is forced from the pressure chambers  20  and  34  of the second cylinder-and-piston unit  13  into the pressure chamber  19  of the cylinder-and-piston unit  10 . The hydraulic piston  17  moves toward the right and, via the piston rod  21 , carries with it the mold platen  11 . Because the piston rods  21  and  31  have the same diameter, the travels of the hydraulic pistons  30  and  58  on the one hand and of the hydraulic piston  17  on the other are the same. The pressure medium forced out of the pressure chamber  25  is entirely received by the pressure chamber  32 , because these two pressure chambers are the same size. 
     Shortly before the mold has closed, the hydraulic pistons  30  and  58  come to a halt. Because of inertia, the hydraulic piston  17  moves onward, so that the pressure in the pressure chambers  25  and  32  rises sharply and the hydraulic piston  17  is braked. 
     When the mold is closed, the two valves  40  and  45  are switched over. The pressure chambers  20 ,  25  and  32  are then connected to the low-pressure tank  41 , so that a pressure equal or close to atmospheric pressure prevails therein. During the further movement of the hydraulic pistons  30  and  58  toward the right, while the hydraulic piston  17  and the mold platen  11  move only slightly, if at all, a high looking pressure now builds up in the pressure chambers  34  and  19 . This pressure generates, on the one hand, only a slight force at the relatively small active surface  35  of the hydraulic piston  30 , which force must be absorbed by the rotation/translation converter  38 , and on the other hand a high mold locking force at the active surface  22  of the hydraulic piston  17 , which active surface  22  is larger than the active surface  35 . The increase in size of the pressure chamber  32  is compensated by the inflow of pressure medium from the pressure chamber  20  and from the low-pressure tank  41 . 
     When the injection operation has ended, the high pressure in the pressure chambers  19  and  34  is initially reduced to the pressure or close to the pressure in the low-pressure tank  41 , the hydraulic pistons  30  and  58  being moved toward the left by operating the electric motor  12  in the corresponding direction of rotation. When this occurs, pressure medium is forced out of the pressure chamber  32  via the valve  40  into the low-pressure tank  41 . 
     After the reduction of pressure, the two valves  40  and  46  are switched over, so that the pressure chamber  34  is connected to the low-pressure tank  41  via the valve  46  and the pressure chambers  20 ,  25  and  32  are separated from the low-pressure tank. There now follows a phase in which the prestress pressures for the displacement of the movable mold platen  11  are built up. For this purpose, the hydraulic piston  30  is moved further toward the left and the pressure chamber  32  is thereby reduced in size and the pressure medium in the pressure chambers  20 ,  25  and  32  compressed. The pressure in the pressure chamber  25  generates a force at the active surface  24  of the hydraulic piston  17  which acts in a manner such as to push the piston rod  21  out of the cylinder  14  and reduce the size of the pressure chamber  19 . Since no pressure medium can escape from the pressure chamber  19 , the same pressure develops in the pressure chamber  19  as in the pressure chambers  20 ,  25  and  32 . During the build-up of the prestress pressure, pressure medium flows out of the low-pressure tank  41  via the valve  46  into the enlarging pressure chamber  34 . 
     Connected to the pressure chamber  19  is a pressure sensor  47  whereby the prestress pressure and the locking pressure are monitored. Instead of or in addition to the pressure sensor connected to the pressure chamber  19 , one pressure sensor could be provided to record the pressure in each of the pressure chambers  32  and  34 . 
     Once the desired prestress pressure has been reached, the valves  45  and  46  are switched over, so that the configuration of the valves is once again as shown in FIG.  6 . The switching may take place slowly, so that the pressure compensation between the pressure chambers  19  and  20  on the one hand and the pressure chamber  34  on the other takes place gently. The hydraulic piston  30 , and with it the hydraulic piston  58 , is moved further toward the left. The hydraulic piston  17 , the piston rod  21  and the mold platen  11  follow this movement, because the pressure medium, which is forced out of the pressure chamber  19 , can flow into the enlarging pressure chambers  20  and  34 . During the acceleration and braking of the mold platen  11 , high forces again take effect, as the pressure rises in the pressure chamber  25  and falls in the pressure chambers  19  and  20  during acceleration and rises in the pressure chambers  19  and  20  and falls in the pressure chamber  25  during braking. 
     In the second example of embodiment, the following are examples of what appear to be favorable values for the individual active surfaces: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 First active surface 22: 
                 100 units of area 
               
               
                   
                 Second active surface 23: 
                  90 units of area 
               
               
                   
                 Third active surface 24: 
                 100 units of area 
               
               
                   
                 Fourth active surface 33: 
                 100 units of area 
               
               
                   
                 Fifth active surface 35: 
                  10 units of area 
               
               
                   
                   
               
             
          
         
       
     
     The example of embodiment shown in FIG. 7 is identical to the example of embodiment shown in FIG. 6 to the extent that here again only the first pressure chamber  19  and the third pressure chamber  25  are located at the receiving  20 , the fourth pressure chamber  32  and the fifth pressure chamber  34  are located at the delivering cylinder-and-piston unit  13 . Apart from the pressure chambers, the individual components of the example of embodiment shown in FIG. 7, where they correspond to components of the examples of embodiment in accordance with FIGS. 1 to  6 , are provided with the same reference numerals. The receiving cylinder-and-piston unit  10  is, according to FIG. 7, a differential cylinder having a cylinder  14  which is fixedly connected to the frame of a machine and having a hydraulic piston  17  formed as a differential piston, which is connected via a piston rod  21  to the movable mold platen  11 . On the piston-rod side of the hydraulic piston  17  is the annular first pressure chamber  19  and on the non-piston-rod side is the third pressure chamber  24 , which is completely cylindrical and thus larger than the first pressure chamber by the cross section of the piston rod  21 . The hydraulic piston  17  borders on the first pressure chamber  19  with the first active surface  22  and on the third pressure chamber  25  with the third active surface  24 . 
     In contrast to the example of embodiment in accordance with FIG. 6, the second, delivering cylinder-and-piston unit  13  now has two differential cylinders  59  and  60  arranged side by side in parallel and having a common cylinder housing  61 , in which are two cylinder chambers whose cross sections differ in size. The hydraulic piston received by the larger cylinder space substantially corresponds to the hydraulic piston  30  and the hydraulic piston received by the smaller cylinder space substantially corresponds to the hydraulic piston  58  of the example of embodiment from FIG.  6 . Both hydraulic pistons  30  and  58  are differential pistons, from which piston rods  62  and  63  extend in the same direction, these being fixedly connected to one another and to the threaded spindle  36  that can be driven by the electric motor  12 . On the piston-rod side of the hydraulic piston  30 , and limited by the active surface  23  of the hydraulic piston  30 , is an annular pressure chamber, which is the second pressure chamber  20  of the drive device. The annular pressure chamber located on the piston-rod side of the hydraulic piston  58 , limited by the active surface  35  of the hydraulic piston  58 , is the fifth pressure chamber  34 , which according to FIG. 7 is permanently connected via a line  64  to the first pressure chamber  19 . 
     The two hydraulic pistons  30  and  58  therefore do not completely correspond to the identically referenced hydraulic pistons from FIG. 6 because in the example of embodiment according to FIG. 7 the two completely cylindrical pressure chambers on the piston-rod sides of the two hydraulic pistons  30  and  58 , which open to one another for fluid purposes, together form the fourth pressure chamber  32  of the cylinder-and-piston unit  13 , in other words are only partial chambers  32 ′ and  32 ″ of the fourth pressure chamber. This again, as with the examples of embodiment described above, is connected via a line  39  to the third pressure chamber  25  of the cylinder-and-piston unit  10 . The two piston surfaces  33 ′ and  33 ″ bordering on the two partial chambers together form the fourth active surface  33  of the cylinder-and-piston unit  13 . The cross sections of the cylinder chambers in the cylinder-and-piston units  10  and  13  and the cross sections of the piston rods  21 ,  62  and  63  are selected so that, as in the second example of embodiment, the dimensional relationship of the sum of the fifth active surface  35  and the second active surface  23  to the first active surface  22  is equal to the dimensional relationship between the fourth active surface  33  and the third active surface  24 . 
     In the example of embodiment according to FIG. 7, the two valves  40  and  45  of the examples of embodiment described above are combined to form a single directional valve  65  having two switching positions and four connections. One connection is connected to the low-pressure tank  41 , one connection to the pressure chamber  20 , one connection to the pressure chamber  25  and a final connection to the pressure chamber  19 . In a spring-induced position of rest, which is illustrated in FIG. 7, the valve  65 , via two of its connections, connects the two pressure chambers  19  and  20  to one another, while the other two connections are shut off. In its other switching position, the connection connected to the pressure chamber  19  is shut off, while, via the other three connections, the pressure chambers  20 ,  25  and  32  are connected to the low-pressure tank. 
     In the example of embodiment according to FIG. 7, there is no valve corresponding to the valve  46  of the examples of embodiment described above. 
     When the mold is closed, the valve  65  adopts the switching position shown in FIG.  7 . The electric motor  12  is operated, during the closure of the mold, in a direction of rotation such that the threaded spindle  36 , the piston rods  62  and  63  and the hydraulic pistons  30  and  58  move toward the right as seen in FIG.  7 . As a result, pressure medium is forced from the pressure chambers  20  and  34  of the second cylinder-and-piston unit  13  into the pressure chamber  19  of the cylinder-and-piston unit  10 . The hydraulic piston  17  moves toward the right and carries with it the mold platen  11  via the piston rod  21 . The pressure medium forced out of the pressure chamber  25  is entirely received by the pressure chamber  32 . 
     Shortly before the mold has closed, the hydraulic pistons  30  and  58  come to a halt. Because of inertia, the hydraulic piston  17  moves onward, so that the pressure in the pressure chambers  25  and  32  rises sharply and the hydraulic piston  17  is braked. 
     When the mold is closed, the valve  65  is switched over. The pressure chambers  20 ,  25  and  32  are then connected to the low-pressure tank  41 , so that a pressure equal or close to atmospheric pressure prevails therein. During the further movement of the hydraulic pistons  30  and  58  toward the right, while the hydraulic piston  17  and the mold platen  11  move only slightly, if at all, a high locking pressure now builds up in the pressure chambers  34  and  19 . This pressure generates, on the one hand, only a slight force at the relatively small active surface  35  of the hydraulic piston  58 , which force must be absorbed by the rotation/translation converter  38 , and on the other hand a high mold locking force at the active surface  22  of the hydraulic piston  17 , which active surface  22  is larger than the active surface  35 . The increase in size of the pressure chamber  32  is compensated by the inflow of pressure medium from the pressure chamber  20  and from the low-pressure tank  41 . 
     When the injection operation has ended, the high pressure in the pressure chambers  19  and  34  is initially reduced to the pressure or close to the pressure in the low-pressure tank  41 , the hydraulic pistons  30  and  58  being moved toward the left by operating the electric motor  12  in the corresponding direction of rotation. When this occurs, pressure medium is forced out of the pressure chamber  32  via the valve  40  into the low-pressure tank  41 . 
     After the pressure reduction, the valve  65  is brought back into the switching position shown in FIG.  7 . The hydraulic pistons  30  and  58  are moved further toward the left. The hydraulic piston  17 , the piston rod  21  and the mold platen  11  follow this movement, because pressure medium is forced out of the pressure chamber  32  of the cylinder-and-piston unit  13  into the pressure chamber  25  of the cylinder-and-piston unit  10 . The pressure medium which is forced out of the pressure chamber  19  of the cylinder-and-piston unit  10  flows into the enlarging pressure chambers  20  and  34  of the cylinder-and-piston unit  13 . 
     Instead of a single differential cylinder  60 , as a development of the example of embodiment according to FIG. 7, two or more differential cylinders  60  may be provided, being disposed as a plurality symmetrically to the axis of the threaded spindle, their axis then coinciding with the axis of the differential cylinder  59 . 
     The example of embodiment shown in FIG. 8 has the same first cylinder-and-piston unit  10  as the example of embodiment according to FIG.  7 . The receiving cylinder-and-piston unit  10  is a differential cylinder having a cylinder  14  which is fixedly connected to the frame of a machine and having a hydraulic piston  17  formed as a differential piston, which is connected via a piston rod  21  to the movable mold platen  11 . On the piston-rod side of the hydraulic piston  17  is the annular first pressure chamber  19  and on the non-piston-rod side is the third pressure chamber  24 , which is completely cylindrical and thus larger than the first pressure chamber by the cross section of the piston rod  21 . The hydraulic piston  17  borders on the first pressure chamber  19  with the first active surface  22  and on the third pressure chamber  25  with the third active surface  24 . 
     The second, delivering cylinder-and-piston unit  13  has a cylinder  69  in the axis of which is a cylinder rod  70 , so that the cylinder chamber is annular. An annular piston  71  is displaceable within the cylinder space, a busing-like piston rod  72  projecting away from one end of the annular piston  71 , passing through an annular aperture in one end face of the cylinder  69  to the outside, and being mechanically connected to the threaded spindle  36 . The internal diameter of the piston rod is greater than the diameter of the cylinder rod  70 , so that a hydraulic piston  73  at one end of the cylinder rod divides the interior of the piston rod  72  into two chambers. The chamber defined axially between the part of the annular piston  71  projecting inward beyond the piston rod  72  and the hydraulic piston  73 , and radially between the piston rod  72  and the cylinder rod  70 , corresponds to the respective pressure chamber  34  of the examples of embodiment described above, and is thus the fifth pressure chamber  34  of the drive device according to FIG.  8  and is permanently connected via the line  64  to the pressure chamber  19 . The part of the annular piston  71  projecting inward beyond the piston rod  72  accordingly comprises the fifth active surface  35 . The other chamber within the piston rod  72  is a partial chamber of the fourth pressure chamber  32 , specifically the partial chamber corresponding to the partial chamber  32 ″ from FIG.  7 . This partial chamber  32 ″ is permanently connected via a line  39  to the pressure chamber  25  of the first cylinder-and-piston unit  10 . The corresponding active surface inside on the piston rod  72  is designated  33 ″. 
     The chamber defined axially between one end surface of the cylinder  69  and the part of the annular piston  71  projecting outward beyond the piston rod  72 , and radially between the piston rod  72  and the cylinder  69 , corresponds to the respective pressure chamber  20  of the examples of embodiment described above, and is thus the second pressure chamber  20  of the drive device according to FIG.  8 . The part of the annular piston  71  projecting outward beyond the piston rod  72  accordingly comprises the second active surface  23 . 
     The chamber defined axially between the other end surface of the cylinder  69  and the annular piston  71  corresponds to the partial chamber  32 ′ of the example of embodiment according to FIG. 7, thus belonging to the fourth pressure chamber  32 . The border active surface of the annular piston  71  is the active surface  33 ′. The cross sections of the pressure chambers are again selected so that, as in the second and third examples of embodiment, the dimensional relationship of the sum of the fifth active surface  35  and the second active surface  23  to the first active surface  22  is equal to the dimensional relationship between the fourth active surface  33  and the third active surface  24 . 
     In the example of embodiment according to FIG. 8, a valve  45  with three connections and two switching positions is provided, this corresponding to the valve  45  of the examples of embodiment shown in FIGS. 1 to  6 , in other words connecting the pressure chamber  20  in one switching position, which is shown in FIG. 8, to the pressure chamber  19  and in the other switching position to the low-pressure tank  41 . 
     A further valve  75  is provided, having three connections and three switching positions, and performing inter alia the function of the valve  40  of the example of embodiment according to FIGS. 1 to  6 , connecting, in one lateral switching position, the two partial chambers  32 ′ and  32 ″ of the pressure chamber  32  to the low-pressure tank  41 . In the central position it connects only the partial chamber  32 ′ with the low-pressure tank  41 . In the other lateral switching position, the position shown, the valve  75  connects the partial chamber  32 ′ to the line  39  and hence the whole pressure chamber  32  to the pressure chamber  25 . 
     When the mold is closed, the valves  45  and  75  adopt the switching positions shown in FIG.  8 . The electric motor  12  is operated, during the closure of the mold, in a direction of rotation such that the threaded spindle  36 , the piston rod  72  and the hydraulic piston  71  move toward the right as seen in FIG.  8 . As a result, pressure medium is forced from the pressure chambers  20  and  34  of the second cylinder-and-piston unit  13  into the pressure chamber  19  of the cylinder-and-piston unit  10 . The hydraulic piston  17  moves toward the right and carries with it the mold platen  11  via the piston rod  21 . The pressure medium forced out of the pressure chamber  25  is entirely received by the pressure chamber  32 . 
     When the mold is closed, the valve  45  is switched over and the valve  76  is brought into the other lateral switching position. The pressure chambers  20 ,  25  and  32  are then connected to the low-pressure tank  41 , so that a pressure equal or close to atmospheric pressure prevails therein. During the further movement of the hydraulic piston  71  toward the right, while the hydraulic piston  17  and the mold platen  11  move only slightly, if at all, a high locking pressure now builds up in the pressure chambers  34  and  19 . This pressure generates, on the one hand, only a slight force at the relatively small active surface  35  of the hydraulic piston  71 , which force must be absorbed by the rotation/translation converter  38 , and on the other hand a high mold locking force at the active surface  22  of the hydraulic piston  17 , which active surface  22  is larger than the active surface  35 . The increase in size of the pressure chamber  32  is compensated by the inflow of pressure medium from the pressure chamber  20  and from the low-pressure tank  41  and into the pressure chamber  20 . 
     When the injection operation has ended, the high pressure in the pressure chambers  19  and  34  is initially reduced to the pressure or close to the pressure in the low-pressure tank  41 , the hydraulic piston  71  being moved toward the left by operating the electric motor  12  in the corresponding direction of rotation. When this occurs, pressure medium is forced out of the pressure chamber  32  via the valve  75  into the low-pressure tank  41 . 
     In a special working step for tearing open the mold, the valve  75  is brought into its central position, in which the pressure chamber  32 ′ is connected to the low-pressure tank. When the hydraulic piston  17  is now moved toward the left, a pressure builds up in the chambers  32 ″ and  25  which generates a force at the hydraulic piston  17  to tear open the mold. Because of the relatively small active surface  33 ″, at which the pressure generates a force on the piston rod  72 , the rotation/translation converter is not overloaded at this point. Pressure medium forced out of the chamber  32 ′ passes via the valve  75  into the low-pressure tank  41  and via the two valves  45  and  75  into the pressure chamber  20 . 
     After the mold is torn open, the two valves  45  and  75  are again brought into the switching positions shown in FIG.  8 . The hydraulic piston  71  is moved further toward the left. The hydraulic piston  17 , the piston rod  21  and the mold platen  11  follow this movement, because pressure medium is forced out of the pressure chamber  32  of the cylinder-and-piston unit  13  into the pressure chamber  25  of the cylinder-and-piston unit  10 . The pressure medium which is forced out of the pressure chamber  19  of the cylinder-and-piston unit  10  flows into the enlarging pressure chambers  20  and  34  of the cylinder-and-piston unit  13 .