Abstract:
A drive device which is intended for displacing two linearly movable components of a plastics injection-molding machine at least partly at successive times, in particular for displacing the injection unit for bringing the injection nozzle into contact with a mold and for displacing the injection mechanism for injecting polymer into the mold. With a known drive device, arranged downstream of an electric rotating motor are two clutches, by which rotational movements are transmitted on the basis of frictional engagement, and with which wear accordingly occurs. Each clutch is followed by a threaded drive, by which the rotational movement of the output element of a clutch is transformed into the linear movement of the machine component to be displaced. To make the known drive device for a plastics injection-molding machine less susceptible to wear and less costly in a first way the threaded drive is arranged between the electric motor and the two clutches and, for displacing the one movable component, the linearly movable drive output element of the threaded drive is able to be moved beyond the displacement distance necessary for displacing the other movable component. In a second way the electric motor is an electric linear motor with a linearly movable drive output element, arranged downstream of which in the force chains are the two clutches, and for displacing the one movable component, the drive output element is movable beyond the displacement distance necessary for displacing the other movable component.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a drive device which is intended for displacing two linearly movable components of a plastics injection-molding machine at least partly at successive times, in particular for displacing the injection unit for bringing the injection nozzle into contact with a mold and for displacing the injection mechanism for injecting polymer into the mold. 
     Such a drive device is known from U.S. Pat. No. 4,540,359. Used there is an electric rotating motor, to move an injection unit including the injection nozzle up to an injection mold and away from the mold again, in order in this way to execute the so-called nozzle forward movement, and to move an injection mechanism including a plasticizing screw axially, for injecting polymer into the injection mold. The two movements take place completely separately at successive times. The electric motor is fastened laterally to the housing of the injection unit, its axis running parallel to the direction of the nozzle forward movement and the injection movement. The rotating electric motor can be used firstly for driving, by means of a belt gear mechanism, a shaft mounted rotatably in the housing of the injection unit, which passes through a gear wheel, which is either freely rotatable with respect to the shaft or can be coupled to the shaft by means of an electromagnetically operable clutch. The gear wheel meshes with a further gear wheel, which is fastened in a rotationally secure manner on a threaded spindle, which together with a linearly guided spindle nut forms a threaded drive, by means of which the rotating movement of the electric motor and the following elements of the gear mechanism is transformed into a linear movement of the injection mechanism. The intermediate shaft can be coupled by means of a second electromagnetically operable clutch to a threaded spindle which is in line with the intermediate shaft, is held in an axially fixed manner on the housing of the injection unit and meshes with a spindle nut arranged in a fixed manner on the machine frame. 
     To bring the injection unit up to the injection mold, the latter clutch is operated, while the first clutch is inoperative. Consequently, the threaded spindle arranged in line with the intermediate shaft is rotated and, as a result, the injection unit is brought up to the injection mold. When this has taken place, a blocking device is operated, which device prevents rotation of the second threaded spindle in relation to the housing of the injection unit. The second clutch is deactivated. 
     Then, the first clutch is operated and, as a result, a force chain from the electric motor to the first threaded spindle, and consequently to the injection mechanism, is closed. The latter moves axially forward, whereby previously plasticized polymer is injected into the injection mold. 
     A disadvantage of the known drive device is that the clutches, the input element and output element of which are rotatable in relation to each other, are subjected to relatively high wear. Altogether, the known drive device is quite complex, and consequently also very expensive. 
     SUMMARY OF THE INVENTION 
     The invention is accordingly based on the object of further developing a drive device of the introductory-mentioned features in such a way that it is less susceptible to wear and is of a relatively simple and low-cost construction. 
     The objective aimed for is achieved for a drive device of the introductory-mentioned features that is with a rotating electric motor, according to the invention wherein the mechanical gear mechanism is arranged between the electric motor and the two clutches, and, for displacing the one movable component, the linearly movable output element of the mechanical gear mechanism can be moved beyond the displacement distance necessary for displacing the other movable component. In the case of a drive device according to the invention, there is consequently no need for clutches in which the input element and output element rotate in relation to each other, and as a result can undergo wear. Moreover, only one threaded drive or one rack-and-pinion drive is necessary to allow both components to move. A threaded drive or a rack-and-pinion drive is generally quite expensive. 
     The objective aimed for is achieved in a second way in that the electric motor is an electric linear motor with a linearly movable output element, arranged downstream of which in the force chain are the two clutches and which, for displacing the one movable component, can be moved beyond the displacement distance necessary for displacing the other movable component. 
     Advantageous refinements of a drive device according to the invention are set forth herein. 
     The displacement distance by which the output element of the electric linear motor or the threaded or rack-and-pinion drive arranged downstream of the rotating electric motor can be moved can be readily made large enough that, the two movable components of the plastics injection-molding machine can be moved completely separately at successive times. 
     It is preferred for there to be in the force chain between the output element and a displaceable component a hydraulic gear mechanism with a first piston-cylinder unit, which has a first hydraulic cylinder, bounding a first cylinder chamber, and with a second piston-cylinder unit, which is located closer to the displaceable component in the force chain and has a second hydraulic piston, bounding a second cylinder chamber, the first cylinder chamber and the second cylinder chamber being fluidically connectable to each other. Such a hydraulic gear mechanism provides great flexibility with regard to the arrangement of the electric motor, since the distance between the two piston-cylinder units can be readily bridged by hydraulic lines. What is more, the effective area on the second hydraulic piston can be made slightly larger than on the first hydraulic piston, so that a force transmission is obtained, and the loading of the elements of the force chain that are located upstream of the hydraulic gear mechanism, for example the loading of a threaded drive, can be kept at a low level. The piston-cylinder units are, in particular, double-acting, so that the movable components can be displaced by the electric motor in opposite directions. It is conceivable in principle to form the blocking device for one movable component as a mechanical, positive locking mechanism. If, however, there is a hydraulic gear mechanism in a force chain, it is possible, in a particularly simple way for the blocking device to be a shut-off valve, by which the second cylinder chamber can be shut off with respect to the first cylinder chamber. 
     In the case of a design a second threaded drive and a second electric motor are used together with the hydraulic gear mechanism, the first threaded drive and the first electric motor. One of the two threaded drive elements which are in engagement with each other can be driven in a rotating manner by said second electric motor. One of the two threaded drive elements is arranged between the hydraulic gear mechanism and the displaceable component. This allows a force to be exerted on the movable component on the one hand via the gear drive and on the other hand via the hydraulic gear mechanism, the force exerted via the hydraulic gear mechanism not imparting any loading to the thread or any engagement elements of the threaded drive there may be. With the additional threaded drive it is possible to make the actual-value profile of the force and/or speed follow very exactly the desired-value profile, according to which the displaceable component is to be moved, since the mechanical force transmission is more rigid than that by means of the hydraulic gear mechanism. 
     The clutches are preferably constructed in a particularly simple manner according to features of the invention. 
     Two exemplary embodiments of a drive device according to the invention, by which the injection unit (nozzle forward movement) and the injection mechanism (injection movement) of a plastics injection-molding machine can be respectively displaced and which have a rotating electric motor and a threaded drive arranged downstream of the latter, are represented in the drawings. The invention is then explained in more detail on the basis of these drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the first exemplary embodiment, in which the screw of an injection mechanism can be displaced by a single electric motor by means of a threaded drive and a hydraulic gear mechanism arranged downstream of the latter, 
         FIG. 2  schematically shows a complete movement sequence of the threaded drive during the working cycle of a plastics injection-molding machine and 
         FIG. 3  shows the second exemplary embodiment, in which the screw of an injection mechanism can be displaced by a first electric motor by means of a threaded drive and a hydraulic gear mechanism arranged downstream of the latter can be displaced at the same time by a second electric motor by means of a second threaded drive. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to  FIG. 1 , a plastics injection-molding machine, which is not represented in any more detail in its entirety, has an injection unit  10  with a housing  11 , on which a plasticizing cylinder  12  is arranged. Mounted on the housing  11  in a rotatable and axially displaceable manner is an injection mechanism  13 , which comprises a screw  14  which is located substantially inside the plasticizing cylinder  12 . A conical end of the plasticizing cylinder  12 , facing an injection mold not represented, is designed as an injection nozzle  15 . Outside the plasticizing cylinder  12 , the screw  14  is adjoined by a splined shaft  16 , which is fixedly connected to said screw, is provided with axially running splines and slots and at the free end of which a disk  17  is fastened. This disk is part of a ball bearing  18 . 
     The splined shaft  16  is surrounded by a gear wheel  22 , which is rotatably mounted in an axially fixed manner on the housing  11  and engages with splines and slots on its inside diameter in the splines and slots of the splined shaft  16 . The gear wheel  22  is coupled by means of a toothed belt  23  to a pinion  25  seated on the motor shaft of an electric motor  24 . The injection mechanism comprising the screw and splined shaft  16  can consequently be driven in a rotating manner by the electric motor  24 , which is mounted on the housing  11 . This drive serves for plasticizing the polymer granules and conveying the plasticized composition into the space located inside the plasticizing cylinder  12  between the end of the screw and the nozzle  15 . 
     Fastened to the housing  11  of the injection unit  10  are the cylinder  28  of a piston-cylinder unit  30  and the cylinder  29  of a piston-cylinder unit  31 . The piston-cylinder unit  30  is in line, with its axis, with the axis of the screw  14  and of the splined shaft  16  and has a differential piston  32  with a piston rod  33 , which has at its free end a disk  34  with a collar protruding axially beyond the disk  17  of the splined shaft  16 . The disks  17  and  34  are parts of the rolling-contact bearing  18 , which ensures the rotatability of the splined shaft  16  in relation to the piston rod  33  and which can transmit axial forces in both directions between the piston rod  30  and the splined shaft  16 . The differential piston  32  divides the interior of the cylinder  28  into an annular cylinder chamber  35  on the side toward the piston rod and a circular-cylindrical cylinder chamber  36  on the side away from the piston rod. 
     The piston-cylinder unit  31  has a synchronizing piston  37 , which is provided on both sides with piston rods  38  of the same thickness and consequently divides the interior of the cylinder  29  into two annular cylinder chambers  39  and  40  of the same cross section. The two piston rods  38  are fastened to the machine frame in a way not represented in any more detail. The piston  37  consequently remains at rest with respect to the machine frame  46 . 
     The drive source for the linear movement of the injection unit for bringing the nozzle  15  into contact with the mold and for moving the nozzle away from the mold, and also for the axial movement of the injection mechanism  13 , is a second rotating electric motor  45 , which is fastened to the machine frame  46  underneath the injection unit in a way not represented in any more detail such that its axis runs parallel to the axis of the injection mechanism, and consequently parallel to the direction of the linear movements of the injection unit and of the injection mechanism. Seated in a rotationally secure manner on the motor shaft  47  is a pinion  48 , which is coupled by means of a toothed belt  49  to a spindle nut  50  which is mounted in an axially fixed manner on the machine frame  46  and provided with external toothing. The spindle nut  50  is provided on the inside with a ball rolling thread. Passing through it is a threaded spindle  51 , which is provided with an external thread formed as a ball rolling thread. Balls  52  engage in the internal thread of the spindle nut  50  and in the external thread of the threaded spindle  51 , so that these two parts are connected to each other by means of a screwed joint. The threaded spindle  51  is linearly guided in a way not represented in any more detail, consequently it cannot rotate, so that when there is rotation of the spindle nut  50  it moves linearly in one direction or in the opposite direction, depending on the direction of rotation. The electric motor  45  can rotate in two directions. At each end, the threaded spindle  51  bears a disk  53  and  54 , respectively, which represents the input element of an electromagnetically operable clutch  55  and  56 , respectively. 
     Arranged upstream of each input clutch disk  53 ,  54 , in line with the threaded spindle  51 , is a piston-cylinder unit  60  and  80 , respectively. The piston-cylinder unit  60  has in a cylinder housing  61  a differential piston  62  with a piston rod  63 , which protrudes from the cylinder housing  61  in the direction of the threaded spindle  51 . At its outer end, the piston rod  63  bears a disk  64 , which represents the output element of the clutch  55  and receives in it an electric coil  65 , and together with the disk  53  on the threaded spindle  51  forms the electromagnetically operable clutch  55 . The differential piston  62  divides the interior of the cylinder housing  61  into an annular cylinder chamber  66  on the side toward the piston rod and a circular-cylindrical piston chamber  67  on the side away from the piston rod. The cylinder chamber  66  is fluidically connected by means of a hydraulic line  68  permanently to the cylinder chamber  35  of the piston-cylinder unit  30 . From the cylinder chamber  67 , a hydraulic line  69  leads to the cylinder chamber  36  of the piston-cylinder unit  30 . In this line  69  there is an electromagnetically operable 2/2-way seat valve  75 , the rest position of which is the passage position and which can be brought by the electromagnet  86  into a shut-off position, in which it shuts off the cylinder chamber  36  with respect to the cylinder chamber  67  in a leak-free manner. According to  FIG. 1 , the effective area  70  of the hydraulic piston  62  adjacent to the cylinder chamber  67  is of precisely the same size as the effective area  71  of the hydraulic piston  32  adjacent to the cylinder chamber  36 . Similarly, the cross sections of the piston rods  33  and  63  are of the same size, so that the mutually opposite effective areas of the hydraulic pistons  62  and  32 , and consequently also the cross sections of the cylinder chambers  35  and  66 , are also of the same size. Accordingly, no force is transmitted between the two piston-cylinder units  60  and  30 . However, the hydraulic piston  62  can readily be made smaller than the hydraulic piston  32 , so that the force exerted by means of the piston rod  33  on the injection mechanism  13  becomes higher than the force introduced via the piston rod  63 . The cross sections of the piston rods  63  and  33  in relation to each other then also have the same ratio that exists between the cross sections of the cylinder chambers  67  and  36 . When there is force transmission between the two piston-cylinder units  30  and  60 , a high injection pressure can be applied, without the threaded drive  50 ,  51  and the toothed belt  49  being subjected to excessive loading. A smaller electric motor  45  can also be used than in the case of a solution without force transmission. 
     The piston-cylinder unit  80  upstream of the other end face of the threaded spindle  51  has a synchronizing piston  82 , which is located in a cylinder housing  81  and has a piston rod  83  respectively on both sides. The two piston rods have the same cross section and leave the cylinder housing  81  at opposite end faces. The piston rod  83  directed toward the threaded spindle  51  bears at its end a disk  84 , which receives in it a coil  85  and which, as an output element, together with the clutch disk  54  on the threaded spindle  51  forms the second clutch  56 . The synchronizing piston  82  divides the interior of the cylinder housing  81  into two cross-sectionally coinciding cylinder chambers  86  and  87 . The cylinder chamber  86  is fluidically connected by means of a hydraulic line  88  permanently to the cylinder chamber  40  of the piston-cylinder unit  31 . From the cylinder chamber  87  of the piston-cylinder unit  80 , a line  89  leads to the cylinder chamber  39  of the piston-cylinder unit  31 . In this line  89  there is a 2/2-way seat valve  95 , which at rest assumes its passage position and can be brought by the electromagnet  96  into a shut-off position, in which the cylinder chamber  39  is shut off with respect to the cylinder chamber  87 . 
     According to  FIG. 1 , the cross sections of the cylinder chambers  86  and  87  of the piston-cylinder unit  80  are greater than the cross sections of the cylinder chambers  39  and  40  of the piston-cylinder unit  31 . This means that between the two piston-cylinder units there is a step-down of the force and a step-up of the displacement distance. Consequently, a relatively small displacement distance of the hydraulic piston  82 , and therefore of the threaded spindle  51 , is sufficient to bring the injection nozzle from its rest position into contact with the mold and back again. 
     In  FIG. 1 , the injection unit  10  is intended to be shown in a state in which the injection nozzle  15  is at a distance from the injection mold. After plasticizing an appropriate amount of polymer material, the screw  14  and the entire injection mechanism as well as the hydraulic piston  32  with the piston rod  33  are in a withdrawn position. In the sub figure 2   a , the corresponding positions of the threaded spindle  51  and of the input clutch disks  53 ,  54  and the output clutch disks  64  and  84  of the two clutches  55  and  56  are schematically shown. These coincide with the positions from FIG.  1 . 
     For injecting polymer into the mold, the nozzle  15  then first has to be brought up to the mold. For this purpose, the electric motor  45  is driven in a rotating direction, [lacuna] that the threaded spindle  51  is moved to the left, in the view according to  FIGS. 1 and 2 , toward the clutch disk  64  of the clutch  55 . The clutch  56  is operated, so that the two clutch disks  54  and  84  of the clutch  56  adhere to each other, the hydraulic piston  82  follows the piston-cylinder unit  80 , that is the threaded spindle  51 . Pressure medium is forced out of the cylinder chamber  87  via the line  89  and the open shut-off valve  95  into the cylinder chamber  39  of the piston-cylinder unit  31 . As a result, the entire injection unit  10  moves to the left. After the displacement distance of the threaded spindle  51  necessary for bringing the nozzle  15  into contact with the mold, the clutch disk  53  has, as shown in  FIG. 2   b , reached the clutch disk  64  of the other clutch  55 . Then the shut-off valve  95  is first brought into its shut-off position and then the clutch  56  is deactivated. No pressure medium can flow out of the cylinder chamber  39  of the piston-cylinder unit  31 , so that the injection unit remains with the injection nozzle  15  against the mold. The electric motor  45  continues to rotate in the same direction, so that the threaded spindle  51  moves further to the left and as a consequence displaces the piston rod  63  and the hydraulic piston  62  to the left by means of the clutch disks  53  and  64 . As a result, pressure medium is forced out of the cylinder chamber  67  of the piston-cylinder unit  60  via the line  69  and the open directional control valve  75  into the cylinder chamber  36  of the piston-cylinder unit  30 . The hydraulic piston  32  displaces the injection mechanism  13  to the left, so that polymer material is injected into the mold. The positions of the threaded spindle  51  and of the various clutch disks at the end of the injecting operation are shown in the sub figure 2   c . While the threaded spindle  51  pushes the clutch disk  64  ahead of it by means of the clutch disk  53 , the clutch  55  can also be activated. It is then possible for the component of the plastics injection-molding machine that is to be moved also to be slowed down by the drive source. 
     At the end of the injecting movement, the valve  75  is brought into its shut-off position, so that the injection mechanism  13  remains in its forwardmost position. The electric motor  45  is driven in the reverse rotating direction and, as a result, the threaded spindle  51  is moved to the right. The clutch  55  is inoperative. When the threaded spindle  51  has traveled the displacement distance covered for injection in the reverse direction, it hits with the clutch disk  54  against the clutch disk  84  of the clutch  56 . Shortly before, the shut-off valve  95  is brought into its passage position again. The hydraulic piston  82  can then be displaced to the right, pressure medium being forced out of the cylinder chamber  86  into the cylinder chamber  40  and out of the cylinder chamber  39  into the cylinder chamber  87 . The injection unit  10  is moved away from the injection mold. The end position of the threaded spindle  51  and of the various clutch disks is shown in the sub figure 2   d . Then the shut-off valve  95  is brought into its shut-off position again, so that the injection unit  10  is blocked in its position. The clutch  56  is released. The electric motor  45  is driven in the first rotating direction again, so that the threaded spindle  51  is moved to the left up against the clutch disk  64  of the clutch  55 , as shown in the sub figure 2   e.    
     The polymer material is then to be plasticized and for this purpose a certain back pressure is to be maintained in front of the screw  14 . For plasticizing, the electric motor  24  is switched on, rotating the injection mechanism  13  together with the screw  14  in such a direction that polymer material is conveyed in front of the screw. A certain pressure builds up there, with the tendency to displace the injection mechanism  13  together with the hydraulic piston  32  backward in the sense of reducing the cylinder chamber  36  of the piston-cylinder unit  30 . With the shut-off valve  75  in the passage position, the speed, controlled by the electric motor  45 , with which the threaded spindle  51  is moved to the right and thereby allows pressure medium to be forced out of the cylinder chamber  36  of the piston-cylinder unit  30  via the valve  75  and the line  69  into the cylinder chamber  67  of the piston-cylinder unit  60 , can be used to set a specific back pressure or to proceed through a specific back pressure profile. A specific state of the threaded spindle  51  and of the clutch disks of the clutches  55  and  56  during the plasticizing operation is shown in the sub figure 2   f . At the end of the plasticizing operation, the clutch disk  64  has again reached the position shown in the sub figure 2   a . The threaded spindle  51  is moved further to the right up against the clutch disk  84  of the clutch  56 . 
     In the case of another exemplary embodiment (not shown) with a rotating electric motor  45 , the threaded spindle  51 , the threaded nut  50  and the toothed belt  49  can be replaced by a toothed rack, which meshes directly with the pinion  48 . 
     Furthermore, in the case of an exemplary embodiment with an electric linear motor, the axis of the latter is put in line with the piston rods  63  and  83  and provided at the two ends with the input element of the clutch  55  and the clutch  56 , respectively. 
     The exemplary embodiment according to  FIG. 3  differs from that according to  FIG. 1  substantially in that the electric motor  24  is used not only for rotating the screw  14  but also together with the electric motor  45  for displacing the screw axially, to inject polymer into the mold. For this purpose, there is a second threaded drive with a threaded spindle  101  and with a threaded nut  102  as drive elements. The threaded spindle  101  is inserted between the splined shaft  16  and the axial rolling-contact bearing  18 . The spindle nut  102  is supported axially on the housing  11  by means of an axial bearing  103 , counter to the direction of movement of the screw  14 , during injection. The spindle nut and threaded spindle have ball rolling threads and engage in each other by means of balls. The spindle nut is freely rotatable on the threaded spindle and can be blocked by a brake  104  against rotation in relation to the housing  11 . 
     For plasticizing, the electric motor  24  rotates in one direction and for injection it rotates in the opposite direction. In order that the screw does not co-rotate during injection, a freewheel  105  is arranged between the latter and the splined shaft  16 . 
     For plasticizing polymer, the electric motor  24  is driven in such a way that it rotates the splined shaft in a direction in which the freewheel  105  transmits the rotation to the screw  14 . The injection mechanism  13  together with the screw  14  is rotated in such a direction that polymer material is conveyed in front of the screw. A certain pressure builds up there, with the tendency to displace the injection mechanism  13  together with the hydraulic piston  32  backward in the sense of reducing the cylinder chamber  36  of the piston-cylinder unit  30 . With the shut-off valve  75  in the passage position, the speed of the threaded spindle  51 , controlled by the electric motor  45 , can be used as in the case of the exemplary embodiment according to  FIG. 1  to set a specific back pressure or to proceed through a specific back pressure profile. The spindle nut  102  can rotate freely during the plasticizing, with axial support provided by the axial bearing  103 , and does not hinder the backward movement of the threaded spindle  101  and consequently of the injection mechanism  13 . 
     Once sufficient polymer material has been plasticized, before polymer is injected into the mold, the nozzle  15  is first brought up to the mold. This takes place as in the case of the exemplary embodiment according to  FIG. 1  by the electric motor  45  rotating in a first rotating direction. The entire injection unit  10  moves to the left. After the displacement distance of the threaded spindle  51  necessary for bringing the nozzle  15  into contact with the mold, the clutch disk  53  has reached the clutch disk  64  of the other clutch  55 . Then the shut-off valve  95  is first brought into its shut-off position and then the clutch  56  is deactivated. No pressure medium can flow out of the cylinder chamber  39  of the piston-cylinder unit  31 , so that the injection unit remains with the injection nozzle  15  against the mold. The electric motor  45  continues to rotate in the same direction, so that the threaded spindle  51  moves further to the left and as a consequence displaces the piston rod  63  and the hydraulic piston  62  to the left by means of the clutch disks  53  and  64 . As a result, pressure medium is forced out of the cylinder chamber  67  of the piston-cylinder unit  60  via the line  69  and the open directional control valve  75  into the cylinder chamber  36  of the piston-cylinder unit  30 . The hydraulic piston  32  exerts a forward force on the injection mechanism  13 . 
     For injecting polymer, not only the electric motor  45  but also the electric motor  24  is driven. To be precise, the electric motor  24  rotates in a direction [lacuna] that the freewheel  105  does not transmit the rotation of the splined shaft  16  to the screw  14 . The brake  104  blocks the spindle nut  102  against rotation, so that the threaded spindle moves to the left at a speed determined by the rotational speed of the electric motor  24 . In this case, only part of the force necessary for the displacement has to be provided by the threaded drive  101 ,  102 . The greater part of the force is exerted by the hydraulic cylinder  30 , so that the threaded drive is not excessively loaded. On the other hand, because of the greater rigidity of the mechanical force transmission from the electric motor  24  via the threaded drive  101 ,  102  to the screw, the desired speed of the screw and the injection pressure can be maintained very exactly by changing the rotational speed or the torque of the electric motor  24 .