Abstract:
A drive device has a drive element movable axially by means of an electric motor and a hydraulic unit. In the drive device it is important to execute a rapid adjusting movement and then to exert high forces. This causes high reaction forces on the lifting spindle. High reaction forces are avoided if a force intensifier with two pistons movable in relation to one another and differing from one another in the size of their active surfaces along with an intermediate part which, together with the pistons, encloses a pressure space filled with a pressure fluid is used, if the smaller active surface is connected mechanically to the drive element, for the adjusting movement, the hydraulic unit can be moved as a whole, and if, for exerting a high force by means of the larger active surface, the intermediate part can be blocked against displacement in relation to a fixed stand.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The invention relates to a drive device which is provided, in particular, for driving a movable part of the closing unit or of the injection unit of a plastic injection molding machine. 
   Within the closing unit of a plastic injection molding machine, the drive device moves the movable mold clamping plate of the machine. Such a drive device must fulfill two important different requirements. On the one hand, it is to move the mold clamping plate as quickly as possible in order to close and to open the mold, so that the cycle time for producing a molding can be kept short. On the other hand, it is to be capable of keeping the mold clamping plate and consequently the entire mold shut with great force against the high injection pressure. On the one hand, therefore, adjusting movements are to be executed at high speed and, on the other hand, high forces are to be exerted without any appreciable movement. Requirements of this kind may arise not only with regard to the closing unit, but also in respect of the injection unit of a plastic injection molding machine. For example, when plastic is being injected into the mold, the plasticizing worm is moved at relatively high speed in the direction of the mold, until the mold is filled completely with plastic. When the plastic melt contained in the mold is subsequently exposed to what may be referred to as dwell pressure, the drive has to apply a high force without any appreciable movement of the plasticizing worm. 
   U.S. Pat. No. 4,030,299 discloses a purely hydraulic drive for the movable mold clamping plate of a plastic injection molding machine, said drive also containing a hydraulic force intensifier. The latter has a movable piston with a small active surface, a further movable piston with a large active surface and a cylinder which, together with the pistons, encloses a pressure space filled with a pressure fluid. The cylinder is arranged at a fixed location on the stand of the injection molding machine. The drive includes, moreover, hydraulic cylinders which move the movable mold clamping plate in order to close and open the mold. When the mold is in the open state, the volume of the pressure space of the hydraulic force intensifier is minimal. If, then, the movable mold clamping plate is moved by the hydraulic cylinders with the effect of a closing of the mold, the large piston of the hydraulic force intensifier is taken up, the volume of the pressure space of the hydraulic force intensifier increasing and pressure medium flowing out of a reservoir into the pressure space via a suction follow-up valve. Thereafter, the small piston of the hydraulic force intensifier is moved into the pressure space, thereby generating a high pressure which brings about a high closing force via the large active surface of the large piston. The small piston is moved hydraulically due to the supply of pressure fluid. Thus, in the drive device according to U.S. Pat. No. 4,030,299, various hydraulic drive components are present for the adjusting movement of the movable mold clamping plate and for exerting a high force. During the adjusting movements of the mold clamping plate, a large amount of pressure fluid flows back and forth between the pressure space and the reservoir, thus necessitating correspondingly large valves and fluid ducts. 
   A drive device is known from DE 41 11 594 A1. In this drive device, a hydraulic cylinder having a large active surface is firmly connected to the movable mold clamping plate. The unit consisting of the movable mold clamping plate and of the hydraulic cylinder can be moved by an electric motor, via a mechanism comprising a lifting spindle and a spindle nut, in order to close the mold quickly and open it quickly. The high closing force is applied by the action of pressure upon the hydraulic cylinder movable together with the mold clamping plate. In this case, the entire reaction force is diverted to the machine stand via the spindle and the spindle nut. The spindle must therefore have a highly robust design, is overdimensioned for the adjusting movement and is correspondingly costly. 
   SUMMARY OF THE INVENTION 
   The object on which the invention is based is to develop further a drive device such that, at low outlay, on the one hand, a rapid adjusting movement is possible and, on the other hand, a high force effect can also be achieved. 
   The said aim is achieved In such a drive device, therefore, a hydraulic force intensifier is used, in the pressure space of which a defined volume of a pressure fluid is enclosed, at least during the adjusting movement and during the subsequent exertion of a high force, apart from changes in volume due to a pressure change. Other hydraulic components are not necessary, in principle, for the drive device according to the invention. According to the invention, the small first hydraulic piston of the hydraulic force intensifier is connected mechanically to the drive element axially movable by means of the drive motor designed particularly as an electric motor. Furthermore, according to the invention, for the adjusting movement of an element to be driven, the hydraulic unit is movable as a whole, with the result that the speed of the large second hydraulic piston coupled mechanically to the element to be driven is equal to the high speed of the drive element moved axially by the electric motor. So that a high force can be exerted, the intermediate part of the hydraulic force intensifier is blocked against displacement in relation to a fixed stand, so that, as a result of a further movement of the first hydraulic piston over a relatively short distance, a high pressure can be built up in the pressure space of the force intensifier, which generates a high force at the large active surface of the second hydraulic piston. In this case, only a fraction of the force, which corresponds to the active surface of the first hydraulic piston, has to be supported via the drive element. Particularly with regard to what may be referred to as dual-plate machines, the drive motor and the drive element are preferably arranged with respect to the movable part in such a way that, when the high force is being built up and exerted, the drive element is subjected to tensile stress. Although one drive element is mentioned here, this, of course, also embraces the situation where the drive device has a plurality of drive elements interacting in parallel. 
   So that the hydraulic unit can be moved as a whole, three of its components are coupled to one another in such a way that the intermediate part and the second hydraulic piston follow the drive element. 
   Particularly with regard to large plastic injection molding machines with a high force for keeping the mold shut, a relatively high tear-open force, which lies in the range of between 5% and 10% of the keeping-shut force, is also necessary for opening the mold. Since, conversely to what occurs during keeping shut, in a dual-plate machine, the drive element is therefore subjected to compressive load during the tearing-open operation, it may be the case that the dimensioning of the drive element must be governed rather by the necessary tearing-open force than by the necessary keeping-shut force, in order to prevent a bending of the drive element. What is achieved here is that, even when a cost-effective drive element designed for tensile load during keeping shut is used, there is no risk of bending during the tearing-open operation. Hence, both for keeping shut and for tearing open the mold of a plastic injection molding machine, a hydraulic force intensifier is provided for each drive element, so that the drive element or drive elements is or are subjected to only low load. In this case, the piston portion, having the small diameter, of a third hydraulic piston designed as a stepped piston forms the small piston of the hydraulic force intensifier active during the tearing-open operation. During the adjusting movement, the third hydraulic piston is pressed with the large piston portion, with high force in the direction of the adjusting movement, against a stop which is fixed in relation to the housing on which or in which the first hydraulic piston and the third hydraulic piston are located. The force is higher than the force which is generated at the first hydraulic piston by a pressurizing pressure acting upon the first pressure space and the active surface of the first hydraulic piston, the pressurizing pressure being selected such that it is above the pressure necessary for accelerating and moving the second hydraulic cylinder and the intermediate part in the first pressure space. The third hydraulic piston then acts in the same way as a fixed stop for the first hydraulic piston. The high pressurizing pressure in the first pressure space causes the fluid cushion in the first pressure space to seem like a rigid connection between the first hydraulic piston and the housing. On the other hand, the third pressure space adjacent to the large piston portion of the third hydraulic piston can be relieved of pressure, so that the third hydraulic piston can be displaced in order to tear open the mold. 
   A third hydraulic piston may, in principle, be arranged laterally in relation to the drive element and/or to the first hydraulic piston. Preferably, however, according a feature of the invention, it is in alignment with and upstream of the first hydraulic piston. 
   If, according to another feature of the invention the first active surface of the third hydraulic piston is larger than the active surface of the first hydraulic piston, the force excess on the third hydraulic piston is obtained even when the third pressure space and the first pressure space are acted upon by the same pressure. This pressure may be set by means of a single hydraulic apparatus. Preferably, according to still another feature of the invention, the first pressure space and the third pressure space can be connected to the same pressure source, for example to a hydraulic pump, so as to be acted upon by the same pressure. According to yet another feature of the invention, a switchable valve is expediently used for the change between pressure action upon and pressure relief of the third pressure space. 
   According to another feature of the invention, the fourth hydraulic piston is adjacent to a pressure space with a boundary wall fixed in relation to the machine stand. The fourth hydraulic piston can therefore be firmly connected to or at least be supportable by the second hydraulic piston or to the machine stand or the mold clamping plate fixed in relation to the stand, the corresponding pressure space then being located between the fourth hydraulic piston and the mold clamping plate or the second hydraulic cylinder. The latter version is preferred, since the fluidic connection from the pressure space at the small piston portion of the third hydraulic piston to the pressure space at the fourth hydraulic piston can then be short, particularly when the pressure space at the second piston portion is likewise located in the second hydraulic piston. 
   According to still another feature of the invention, the fourth hydraulic piston may also be active between the second hydraulic piston and the intermediate part, in which case said fourth hydraulic piston can be fixed with respect to the intermediate part or to the second hydraulic piston, that is to say is also movable, or is fixed, that is to say permanently immovable, and, in each case with the other part of the two parts, intermediate part and hydraulic piston, delimits a pressure space which is connected to a pressure space at the second piston portion of the third hydraulic piston. 
   In particular, according to another feature of the invention the fourth hydraulic piston is located in or on the intermediate part or in or on the second hydraulic piston and is connected to a fifth hydraulic piston in each case in the other part via a piston rod. The fifth hydraulic piston serves, for example, in order, after the closing of the mold, when the second hydraulic piston or the movable mold clamping plate connected to the latter bears against the fixed mold clamping plate, to displace the intermediate part also somewhat toward the second hydraulic piston into a locking position, in so far as said intermediate part can be blocked only at discrete points. 
   In a design according to yet another feature of the invention, a once-only setting of the intermediate part and of the second hydraulic piston in relation to one another when a new molding die is used is possible with the aid of the fifth hydraulic piston. After setting, the fifth hydraulic piston can remain blocked with respect to the part on which it is located, because, for the fourth hydraulic piston, the sixth hydraulic piston implements on the other part a stop which, during the build-up of the high keeping-shut force and the associated slight movement of the second hydraulic piston, can yield, but, for tearing open the mold, acts as a fixed stop by being acted upon by pressure. According to another feature of the invention, the sixth hydraulic piston is preferably arranged in alignment with the fourth hydraulic piston. 
   To make the fluidic connections between the individual pressure spaces adjacent to the hydraulic pistons, it is beneficial if, according to yet another feature of the invention, the fourth hydraulic piston or the fourth and the fifth hydraulic piston are located on the same housing as the first and the third hydraulic piston. 
   It may be pointed out here, in quite general terms, that the reception spaces for the hydraulic pistons may be formed in each case directly in the intermediate part or in the second hydraulic piston or in plates connected thereto or else separate cylinders having the reception spaces may be provided, said cylinders being fastened to the intermediate part or to the second hydraulic piston. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Three exemplary embodiments of a drive device according to the invention, which are intended in each case for what may be referred to as a dual-plate plastic injection molding machine, are illustrated in the drawings. The invention, then, is explained in more detail with reference to these drawings in which: 
       FIG. 1  shows the first exemplary embodiment, in which a fourth hydraulic piston is guided in a cylindrical receptacle of the second hydraulic piston and, with the mold closed, can be supported on the fixed mold clamping plate for the purpose of tearing open said mold, 
       FIG. 2  shows the second exemplary embodiment, in which the fourth hydraulic piston is likewise guided in the second hydraulic piston, moreover is connected via a piston rod to a sixth hydraulic piston and serves with the latter for the releasable coupling of the second hydraulic piston and of the intermediate part, and 
       FIG. 3  shows the third exemplary embodiment which is constructed in a similar way to the second exemplary embodiment, but in which a sixth hydraulic piston guided in the second hydraulic piston forms a movable stop for the fourth hydraulic piston. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the exemplary embodiments shown, two spindle nuts are rotatably mounted axially at a fixed location in a stationary mold clamping plate  10 , diametrically opposite one another with respect to a center axis  11 , via self-aligning roller bearings in a way not illustrated in any more detail, each of said spindle nuts being capable of being driven in rotation by an electric motor  12  fastened to the mold clamping plate  10 . Each spindle nut is in engagement via balls with a threaded portion  13  of a rectilinearly movable lifting spindle  14  secured against rotation. A piston rod  15  of a first hydraulic piston  16 , which is a small piston of a hydraulic force intensifier  17 , extends from the threaded portion  13  of a lifting spindle  14 , parallel to the center axis  11 , sealed off, through a passage  18  into a cavity  19 , having three cylindrical portions differing from one another in their diameters, of a second hydraulic piston  20  which is the large piston of the hydraulic force intensifier  17 . Designated here as the second hydraulic piston is a machine part which is of essentially plate-shaped design and has a projecting piston portion  21  ( FIGS. 2 and 3 ), which penetrates into a corresponding piston receptacle  23  of an intermediate part  22  of the hydraulic force intensifier  17 , or, set back, a piston receptacle  23  ( FIG. 1 ), into which a projecting piston portion  21  of the intermediate part  22  penetrates. The piston portion  21  has an active surface  25  which is adjacent to a pressure space  24  and which is substantially larger, for example thirty times larger, than the active surface  31  of a hydraulic piston  16 . The movable mold clamping plate of the machine is fastened to the hydraulic piston  20  and is guided along spars in the direction of the center axis  11  in a way not illustrated in any more detail. 
   The passage  18  is followed, in the hydraulic piston  20 , first by a cavity portion  27 , the diameter of which is about three times as large as the diameter of the piston rod  15 . The following cavity portion  28  has a larger diameter and is shorter than the cavity portion  27 . Finally, the diameter of the last blind-hole-like cavity portion  29  is about as large as the diameter of the passage  18  and consequently of the piston rod  15 . The hydraulic piston  16  is located in the cavity portion  27  and on the piston-rod side is adjacent with an annular active surface  30  to an annular pressure space  31 . On the side facing away from the piston rod, the hydraulic piston  16  carries a short tappet  32 . 
   A third hydraulic piston  35  is designed as a stepped piston and is located with its piston portion  36  of large diameter in the cavity portion  28  and penetrates with a piston portion  37  of small diameter into the cavity portion  29  in a plunger-like manner. The free space upstream of the piston portion  37  is a pressure space which is filled with pressure medium and may be designated by  33 . Located on that side of the piston portion  36  which faces away from the hydraulic piston  16  is an annular pressure space  38  which can be acted upon by system pressure or can be relieved of pressure. The space  39  between the two hydraulic pistons  16  and  35  is permanently relieved of pressure. By the action of pressure upon the annular active surface  40 , which is larger than the annular active surface  30  on the hydraulic piston  16 , from the pressure space  38 , the hydraulic piston  35  can be pressed with the piston portion  36  against the step between the two cavity portions  27  and  28 , said step constituting an axial stop  41 . 
   The pressure spaces  31  and  38  can be supplied with a fluid pressure medium which is sucked in from a tank  46  by a hydraulic pump  45  and is conveyed into a pump line  47 , to which a pressure limiting valve  48  is connected in order to set a system pressure. A first valve  50  can be changed over from a middle working position into two lateral working positions. In the middle working position of the valve  50 , the pressure space  31  at the hydraulic piston  18  is fluidically connected to the pressure space  24  at the piston portion  21  of the hydraulic piston  20 . In a first lateral working position, the pressure space  31  is connected, throttled, to the pump line  47  and the pressure space  24  is relieved to the tank. In the other lateral working position, both pressure spaces  24  and  31  are relieved to the tank. A second switching valve  51  can assume two working positions, the pressure space  38  being connected, throttled, to the pump line  47  in one working position and being relieved to the tank in the other working position. 
   In all three exemplary embodiments shown, the intermediate part  22  of the hydraulic force intensifier  17  can be firmly connected to the hydraulic piston  20  via a releasable coupling device, so that, during the closing of the mold, that is to say during the adjusting movement, the hydraulic piston  20  and the intermediate part  22  can be moved as a unit. After the closing of the mold, the coupling between the hydraulic piston  20  and the intermediate part  22  is released and the intermediate part is retained in relation to the machine stand. In principle, the blocking device between the intermediate part and the machine stand or the mold clamping plate fixed in relation to the latter may be designed in such a way that, within a defined zone, blocking at infinite intervals is possible at any desired point. With regard to the exemplary embodiment according to  FIG. 1 , such a possibility may be assumed to be afforded. 
   In the exemplary embodiment according to  FIG. 1 , the hydraulic piston  20  possesses, distributed at uniform intervals about the center axis  11 , a plurality of blind-hole-like cylindrical receptacles  54  which are open toward the fixed mold clamping plate  10 . In each receptacle  54 , a plunger piston  55  is guided, as the fourth hydraulic piston of the exemplary embodiment according to  FIG. 1 , is held in the receptacle  54  by a spring  56 , projects from the receptacle and, with the mold closed, can be supported on the fixed mold clamping plate  10 . The pressure space  57  in the receptacle  54  upstream of the plunger piston  55  is connected to the pressure space  33  upstream of the piston portion  37  of the hydraulic piston  35 . 
   In the exemplary embodiments according to  FIGS. 2 and 3 , the intermediate part  22  can be interlocked, with the aid of toothed locking rods  59  fastened to the fixed mold clamping plate and extending in the direction of the center axis  11 , together with the fixed mold clamping plate  10  and consequently with the machine stand at discrete points spaced slightly apart from one another. The hydraulic piston  20  receives a fourth hydraulic piston  65  in each of a plurality of cylindrical cavities  64  distributed uniformly about the center axis  11 . The intermediate part  22  likewise receives a fifth hydraulic piston  67  in each case in each of a plurality of cylindrical cavities  66  which are in alignment with the cavities  64  in the hydraulic piston  20 . The two hydraulic pistons  66  and  67  lying in alignment with one another are firmly connected to one another via a piston rod  68 . The pressure space  69 , on the side facing away from the piston rod, at the hydraulic piston  65  is either connected, throttled, to the pump line  47  or relieved to the tank via a switching valve  70  having two working positions. The circular-cylindrical active surfaces  71 , on the side facing away from the piston rod, of the two hydraulic pistons  65  and  67  are of equal size. The pressure space  57 , on the piston-rod side, at the hydraulic piston  65 , said pressure space corresponding to the identically designated pressure space from  FIG. 1 , is connected permanently to the pressure space  33  at the piston portion  37  of the stepped piston  35 . 
   In the version according to  FIG. 2 , the hydraulic piston  65  can bear directly against the hydraulic piston  20  on the piston-rod side. By contrast, in the exemplary embodiment according to  FIG. 3 , the stop on the side of the piston rod  68  for the hydraulic piston  65  is formed by a sixth hydraulic piston  75  and can be switched to active and inactive. The hydraulic piston  75  has a larger outside diameter than the hydraulic pistons  65  and  67  and is located in a cavity  76  of larger diameter which is adjacent to the cavity  64  on the same side as the piston rod  68 . The piston rod  68  passes, sealed off, through a central orifice of the hydraulic piston  75 . On that side of the hydraulic piston  75  which faces away from the hydraulic piston  65 , the pressure space  77  is either connected, throttled, to the pump line  47  or relieved to the tank via a switching valve  78  having two working positions. When the pressure space  77  and consequently an annular active surface  79  at the hydraulic piston  75  are acted upon by system pressure, the hydraulic piston  75  bears against the step  80  between the two cavities  64  and  76  and acts in the same way as a fixed boundary of the cavity  64 . System pressure prevailing in the pressure space  69  cannot lift off the hydraulic piston  75  from the step, since the active surface  79  at the hydraulic piston  75  is larger than the active surface  71  at the hydraulic piston  65 . The hydraulic piston  65  bears against the hydraulic piston  75  via a collar having an increased diameter in relation to the diameter of the piston rod  68 . The annular pressure space between the two hydraulic pistons  65  and  75  corresponds to the pressure space  57  of the exemplary embodiment according to  FIG. 2  and is connected to the pressure space  33  at the piston portion  37  of the hydraulic piston  35 . During the relief of the pressure space  77 , the hydraulic piston  65  can lift off the hydraulic piston  75  from the step  80 , the volume of the pressure space  57  between the two hydraulic pistons increasing. When the hydraulic piston  75  approaches the step, the volume decreases. So that the change in volume can be compensated, the two pressure spaces  33  and  57  are connected to the tank in a second of two working positions of a switching valve  85 . In the first working position, the two pressure spaces are shut off, free of leakage. 
   In the exemplary embodiment according to  FIG. 3 , after a new die has been mounted, when the pressure spaces  69  and  77  are acted upon by system pressure with the aid of the hydraulic piston  67 , at the hydraulic piston  20  the intermediate part  22  is brought into a position lying as near as possible to the hydraulic piston  20 , such that, with the mold closed, said intermediate part assumes with respect to the locking rods  59  a position such that it can be locked. For this purpose, the pressure spaces on both sides of the hydraulic piston  67  can be connected to the pressure source and to a tank via a throttling control valve not illustrated in any more detail. The pressure which is established in one or the other pressure space of the hydraulic piston  67  during the setting operation is substantially lower than the system pressure. After this setting, the two pressure spaces are shut off, free of leakage. As long as the same die is used, in principle, no further setting is necessary. 
   Assuming an open mold, then, the mold is to be closed. The valve  50  assumes the working position in which the pressure space  24  is relieved to the tank and the pressure space  31  is connected to the pump line  47 . The pressure space  69  is also connected via the valve  70  and the pressure space  77  via the valve  78  to the pump line  47 . System pressure prevails in the pressure spaces. The pressure spaces  33  and  57  are relieved to the tank via the valve  85 . If, then, the lifting spindle is moved in the direction of the fixed mold clamping plate  10 , the hydraulic piston  20  is taken up via the hydraulic piston  16  and via the pressurized, that is to say compressed pressure-medium cushion located in the pressure space  31 , without the pressure medium in this case being compressed any further. There is a virtually rigid connection between the lifting spindle  14  and the hydraulic piston  20 . The intermediate part  22  is taken up, likewise in a virtually rigid manner, by the hydraulic piston  20  via the pressure-medium cushion in the pressure space  77 , the two hydraulic pistons  75  and  65 , the piston rod  68 , the hydraulic piston  67  and the pressure-medium cushion in one pressure space of the latter. 
   After the closing of the mold, the intermediate part  22  is locked with respect to the machine stand. The two valves  70  and  78  change over, so that the two pressure spaces  69  and  77  are relieved to the tank. The valve  50  is brought into the working position in which the two pressure spaces  24  and  31  are connected to one another. The pressure space  38  remains connected to the pump line  47  via the valve  51 , so that the hydraulic piston  35  is held reliably against the stop  41 . If, then, the hydraulic piston  16  is moved further on in the direction of the mold clamping plate  10 , a high keeping-shut pressure lying above the system pressure can build up in the pressure spaces  24  and  31  connected to one another and generates a very high keeping-shut force at the large active surface  25  of the hydraulic piston  20 . In the event of a slight movement of the hydraulic piston  20  which still takes place in this case, the hydraulic piston  75  upstream of the hydraulic piston  65  can readily be taken up, since the pressure space  77  is relieved of pressure. 
   After the injection of the plastic into the mold, after a possible dwell-pressure phase and after a solidification time, the lifting spindle  14  is moved in the direction away from the fixed mold clamping plate  10  and the pressure medium in the pressure spaces  24  and  31  is thereby expanded. After expanding, the hydraulic piston  16  bears with the tappet  32  against the hydraulic piston  36 . The valve  50  is brought into the working position in which the two pressure spaces  24  and  31  are relieved to the tank. The valve  51  changes over, so that the pressure space  38  is relieved to the tank. The valve  78  changes over, so that the pressure space  77  is acted upon by the system pressure. The valve  85  changes over in order to shut off the two pressure spaces  33  and  57  relative to the tank. As a result of the movement of the lifting spindle  14 , then, the hydraulic piston  35  is moved by the latter, via the hydraulic piston  16  and the tappet  32 , with the effect of a reduction in the pressure space  33 , so that pressure medium is displaced out of this pressure space into the pressure space  57 . A pressure builds up there, which generates at the hydraulic piston  75  a force which reduces the bearing force of said hydraulic piston against the step  80  of the hydraulic piston  20 . The reaction force is absorbed via the hydraulic pistons  65  and  67  by the intermediate part  22  and consequently by the machine stand. Finally, the bearing force of the hydraulic piston  75  is reduced to an extent such that the excess force generated by the system pressure prevailing in the pressure space  77  is sufficient to separate the mold halves from one another. The progress of this tearing open of the mold may in this case be controlled via the traveling speed of the lifting spindle  14 . The pressure space  69  is in this case reduced due to the relative movement between the hydraulic piston  20  and the hydraulic piston  65 . 
   After the tearing-open operation, the locking of the intermediate plate  22  is released. The valve  85  switches and connects the two pressure spaces  33  and  57  to the tank. Moreover, the two valves  51  and  70  change over, so that pressure medium can flow, throttled, to the pressure spaces  38  and  69  from the pump line  47 , as indicated in the respective circuit diagram by the nozzle. The hydraulic piston  20  and the intermediate part  22  move relative to one another and to the lifting spindle in such a way that they assume the positions, shown in  FIG. 3 , in relation to one another and to the lifting spindle. 
   The valve  50  is then brought into the working position in which it connects the pressure space  31  to the pump line  47 . The pressure space  31  is acted upon by system pressure, so that the hydraulic piston  20  and the intermediate part  22  continue to follow exactly the movement of the lifting spindle  14  in the opening direction into the opening position. 
   The exemplary embodiment according to  FIG. 2  lacks, as compared with the exemplary embodiment according to  FIG. 3 , the stop, to be made active and inactive, for the hydraulic piston  65 . During the closing of the mold, the valves  50 ,  51  and  70  assume the same positions as in the exemplary embodiment according to  FIG. 3  in the same movement phase. The hydraulic piston  65  bears against the hydraulic piston  20  on the piston-rod side, and the hydraulic piston  67  is blocked with respect to the intermediate part  22 . 
   When the mold is closed, the intermediate part  22  is positioned at the hydraulic piston  67 , by the supply of pressure medium into one pressure space and by the discharge of pressure medium out of the other pressure space, in such a way that said intermediate part can be interlocked with the locking rods  59 . During positioning, the pressure space  24  is still connected to the tank via the valve  50 , so that a change in volume can be compensated. After the locking of the intermediate part  22 , the hydraulic piston  67  is switched so as to be freely movable by means of the connection of the two pressure spaces adjacent to it to the tank. The valve  50  is brought into the position in which the pressure spaces  24  and  31  are connected to one another. As a result of a further movement of the lifting spindle  14  and consequently of the hydraulic piston  16  in the closing direction, a high pressure builds up in the pressure spaces  31  and  24  and generates a high keeping-shut force at the large active surface  25 . In the event of a slight movement of the hydraulic piston  20  which still takes place in this case, the hydraulic piston  65  can readily be taken up, since the hydraulic piston  67  is switched free. 
   After the injection of the plastic into the mold, after a possible dwell-pressure phase and after a solidification time, the lifting spindle  14  is moved in the direction away from the fixed mold clamping plate  10  and the pressure medium in the pressure spaces  24  and  31  is thereby expanded. After expansion, the hydraulic piston  16  bears with the tappet  32  against the hydraulic piston  35 . The valve  50  is brought into the working position in which the two pressure spaces  24  and  31  are relieved to the tank. The valve  51  changes over, so that the pressure space  38  is relieved to the tank. The hydraulic piston  67  is blocked hydraulically with respect to the still locked intermediate part  22 . As a result of the movement of the lifting spindle  14 , then, as in the exemplary embodiment according to  FIG. 3 , the hydraulic piston  35  is moved by the latter, via the hydraulic piston  16  and the tappet  32 , with the effect of a reduction of the pressure space  33 , so that pressure medium is displaced out of this pressure space into the pressure space  57 . A pressure builds up there, which acts directly upon the hydraulic piston  20  at a surface corresponding to the annular surface of the hydraulic piston  65  and which generates a tearing-open force. The reaction force is absorbed via the hydraulic pistons  65  and  67  by the intermediate part  22  and consequently by the machine stand. 
   After the tearing-open operation, the locking of the intermediate plate  22  is released. The two valves  51  and  70  change over, so that pressure medium can flow, throttled, to the pressure spaces  38  and  69  from the pump line  47 . The hydraulic piston  20  and the intermediate part  22  move relative to one another and to the lifting spindle in such a way that they assume the positions, shown in  FIG. 2 , in relation to one another and to the lifting spindle. 
   The valve  50  is then brought into the working position in which it connects the pressure space  31  to the pump line  47 . The pressure space  31  is acted upon by system pressure, so that the hydraulic piston  20  and the intermediate part  22  continue to follow exactly the movement of the lifting spindle  14  in the opening direction into the opening position. 
   In the exemplary embodiment according to  FIG. 1 , during the closing of the mold, the valves  50  and  51  assume the same positions as in the exemplary embodiment according to  FIG. 3  in the same movement phase. The pressure spaces  31  and  38  are therefore connected to the pump line  47  and are acted upon by system pressure. 
   When the mold is closed, the intermediate part  22  is blocked in relation to the machine stand. The valve  50  is thereafter brought into the position in which the pressure spaces  24  and  31  are connected to one another. As a result of the further movement of the lifting spindle  14  and consequently of the hydraulic piston  16  in the closing direction, a high pressure builds up in the pressure spaces  31  and  24  and generates a high keeping-shut force at the large active surface  25 . 
   After the injection of the plastic into the mold, after a possible dwell-pressure phase and after a solidification time, the lifting spindle  14  is moved in the direction away from the fixed mold clamping plate  10  and the pressure medium in the pressure spaces  24  and  31  is thereby expanded. After expansion, the hydraulic piston  16  bears with the tappet  32  against the hydraulic piston  35 . The valve  50  is brought into the working position in which the two pressure spaces  24  and  31  are relieved to the tank. The valve  51  changes over, so that the pressure space  38  is relieved to the tank. As a result of the movement of the lifting spindle  14 , then, as in the exemplary embodiments according to  FIGS. 2 and 3 , the hydraulic piston  35  is moved by the latter, via the hydraulic piston  16  and the tappet  32 , with the effect of a reduction of the pressure space  33 , so that pressure medium is displaced out of this pressure space into the pressure space  57 . A pressure builds up there, which acts directly upon the hydraulic piston  20  at a surface corresponding to the active surface of the hydraulic piston  55  supported on the stationary mold clamping plate  10  and which generates a tearing-open force. 
   After the tearing-open operation, the blocking of the intermediate plate  22  is released. The valves  50  and  51  change over, so that pressure medium can flow, throttled, to the pressure spaces  31  and  38  from the pump line  47 . The hydraulic piston  20  and the intermediate part  22  move in relation to the lifting spindle in such a way that they assume the positions in relation to the lifting spindle which are shown in  FIG. 1 . The hydraulic piston  20  and the intermediate part then exactly follow the movement of the lifting spindle  14  in the opening direction into the opening position.