Abstract:
Device for moving in the longitudinal direction a shaft with screw rotationally driven by a motor, comprising at least one cylinder coaxial with said shaft and provided with at least one rotating piston rigidly connected to the shaft and able to produce a displacement in either direction of the shaft from a first retracted end-of-travel position into a second advanced end-of-travel position, and vice versa.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Italian Patent Application No. M12006A 000321, filed 22 Feb. 2006, which is hereby incorporated by reference. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not Applicable. 
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0004]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    The present invention relates to a device for moving in the longitudinal direction a screw shaft for mixing machines and a mixing machine equipped with said device. 
         [0007]    2. Description of Related Art 
         [0008]    It is known, for example in the technical sector relating to machines for the injection-moulding of plastics that there exists the need to allow displacement of the mixing screw in both directions and pushing of the material; in greater detail the screw must: 
         [0009]    during a first step, be retracted with respect to the material inlet mouth by means of a screw-like (rotary translational) movement which pushes forwards the granules of material which melt gradually until the required plasticity necessary for achieving correct extrusion is obtained; 
         [0010]    during a second step, be able to advance towards the outlet mouth in order to inject into the mould the melted material which, once solidified, will assume the shape of the final plastic article. 
         [0011]    Devices which are able to allow the axial movement, in both directions, of the mixer screw are also known; these known devices may be of the following type: 
         [0012]    with a motor for actuating the screw, fixed to the housing of the mixing machine and therefore axially stationary; in this case it is envisaged providing a cylinder arranged between the motor and the shaft of the screw so that the latter is able to be displaced; 
         [0013]    with a motor fixed to a flange which is axially movable on respective guides and to which flange the shaft of the screw is also fastened. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    Although fulfilling their function, these solutions nevertheless have the drawback consisting in the fact that it is required to provide an axial bearing, or equivalent mechanical thrust-bearing device, in order to prevent the screw from transmitting axial thrusts onto the driving shaft owing to the forces produced by the thrust acting on the material. 
         [0015]    Since the latter are very high, the dimensions of the bearing and the machine structure must necessarily be increased in order to safeguard the driving shaft with an obvious increase in the amount of machining, the weight as well as the production, storage and despatch costs. 
         [0016]    The technical problem which is posed, therefore, is that of providing a device for actuating shafts of screws for moving material inside mixing chambers, such as, for example, presses for the injection-moulding of plastics, which is able to absorb the axial thrusts imparted by the said shaft without the need for auxiliary means such as axial bearings or the like. 
         [0017]    In connection with this problem it is also required that this device should have small dimensions, be easy and inexpensive to produce and assemble and be able to be installed also on ready-existing machines without the need for complicated adaptation. 
         [0018]    These results are obtained according to the present invention by a device for moving in the longitudinal direction a screw shaft which is rotationally driven by a motor, comprising at least one cylinder coaxial with said shaft and provided with at least one rotating piston rigidly connected to the shaft and able to produce a displacement in either direction of the shaft from a first retracted end-of-travel position into a second advanced end-of-travel position, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]    Further details may be obtained from the following description of a non-limiting example of embodiment of the subject of the present invention, provided with reference to the accompanying drawings in which: 
           [0020]      FIG. 1  is a schematic cross-sectional view, along an axial vertical plane, of a machine with the device according to the present invention during the advancing movement of the screw; 
           [0021]      FIG. 2  is a cross-section along the plane indicated by II-II in  FIG. 1 ; 
           [0022]      FIG. 3  is a cross-sectional view, similar to that of  FIG. 1 , with the device during retraction of the screw; 
           [0023]      FIG. 4  is a cross-sectional view, along an axial vertical plane, of a second embodiment of the device according to the present invention during the advancing movement; 
           [0024]      FIG. 5  is a cross-section, similar to that of  FIG. 4 , with the device during retraction; 
           [0025]      FIG. 6  is a plan view of a further embodiment of the device according to the invention during retraction; and 
           [0026]      FIG. 7  is a schematic cross-sectional view, along a vertical plane, of the machine according to  FIG. 6  during the advancing movement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    As shown in  FIG. 1  and with reference to longitudinal and transverse axes of orientation, X-X and Y-Y respectively, conventionally assumed for the sake of convenience of the description, but not limited thereto, the device  100  according to the present invention is applied to a machine  1  for injecting a polymer  2   a , in the form of granules, radially fed to a chamber  3 , the outlet of which (not shown) forms the mouth for feeding the liquid polymer  2   b  ( FIG. 3 ) to a downstream machine such as, for example, an extrusion head. 
         [0028]    Below, a front part corresponding to the extended part of the screw and a rear part opposite to the former will also be assumed. 
         [0029]    In detail, the device  100  comprises a chamber  110  which is axially closed by a front flange  112  for fastening the device to the machine  1 , and by a rear blind cover  113 . 
         [0030]    The front flange  112  has a coaxial opening, with associated seals  112   a,  with associated seals  112   b,  able to receive the shaft  120  forming the shank of the screw  121  for mixing the polymer  2   a,   2   b.    
         [0031]    The side wall  10   a  of the chamber  110  is also provided with two seats  115  extending in the longitudinal direction and able to receive two pins  145 —or equivalent anti-rotation device—radially projecting from the housing  141  of the motor  140  for actuating the shaft  120 , which motor is coaxially arranged inside the said chamber  110 . 
         [0032]    The annular extension of the seats  115  of the cylinder is slightly greater than the outer diameter of the pins  145 —or equivalent anti-rotation device—so that the latter are able to move inside the respective seat in the longitudinal direction, but not rotate about the longitudinal axis X-X. 
         [0033]    In this way the motor  140  may therefore be displaced in the longitudinal direction, but not rotate about the corresponding axis X-X. 
         [0034]    The chamber  110  has internally two pistons, i.e. rear piston  131  and front piston  132 , which are provided with respective annular seals  131   a,    132   a,  are mounted on the shaft  120  on opposite sides of the motor  140  in the axial direction and are both able to rotate about the longitudinal axis X-X. 
         [0035]    In this way, the chamber  110  forms a double-acting cylinder which houses internally, in an axially movable manner, a rod  120  forming the shaft of the screw  121  for mixing the polymer. 
         [0036]    The side surface of the cylinder  100  is also provided with two nozzles, i.e. rear nozzle  144   a  and front nozzle  144   b,  for supplying/discharging the actuating fluid. 
         [0037]    With the configuration shown, the operating principle of the device is as follows: 
         [0038]    during start-up and mixing and with the motor operating, the cylinder  100  is supplied via the nozzle  144   a,  with the nozzle  144   b  being set simultaneously for discharging; 
         [0039]    the fluid acts on the front rotating piston  132 , causing the backward movement of the motor  140  and therefore the shaft  120  attached to it as far as the end-of-travel point defined by contact of the rear piston  131  against the rear flange  113 ; 
         [0040]    the movement of the screw  121  which rotates and at the same time retracts pushes forwards the plastic granules  2   a  which, along the travel path, melt owing to the heat produced by heating elements (not shown); 
         [0041]    supplying and discharging of the nozzles  144   a  and  144   b  are reversed; 
         [0042]    as a result the fluid pushes the rotating piston  132  forwards as far as the respective front stop  112 , restoring the initial position; 
         [0043]    from here a new cycle commences. 
         [0044]      FIGS. 4 and 5  show a second embodiment of the device according to the present invention which in this configuration has the double-acting cylinder  1100  mounted externally on the rear cover  1113  of the chamber  110  containing the motor  140 . 
         [0045]    In greater detail, the cylinder  1100  is mounted coaxially with and at the rear of said chamber  110  and has a single piston  1131  actuated in both senses in the longitudinal direction X-X by means of the fluid supplied via the nozzles  1144   a,    1144   b  which are alternately set to assume the supplying or discharging condition. 
         [0046]    As shown, in this case the shaft  1120  is coaxially extended towards the rear by a section  1120   a  until it is connected to the rotating piston  1131  via an opening  1113   a  with seals  1113   b  in the rear cover  1113 . 
         [0047]    The operating principle of the device is entirely similar to that already described, although with this embodiment it is easier to replace, if necessary, the actuating cylinder  1100  should it be required to use a different sized actuating device for displacement of the screw when the same mixing machine is used for different applications. 
         [0048]      FIGS. 6 and 7  show a further embodiment of a machine with a device according to the present invention; in this configuration it is envisaged that the cylinder  2100  is rigidly attached to a fixed support  2101  and that the motor  2140  for rotationally driving the shaft  2120  of the screw  121  is situated outside the said cylinder and fixed to a case  2114  which, in this case, is movable in an axial direction on guide rails  2113  from a retracted position substantially in contact with the front flange  2112  of the fixed cylinder  2100  into an advanced end-of-travel position ( FIG. 7 ). 
         [0049]    As shown, the case  2114  has internally a gear  2122  rigidly connected to the shaft  2120  and provided with radial teeth for engagement with corresponding gears  2141  actuated by the motor  2140 . 
         [0050]    In this case also, alternate supplying/discharging of the chamber  2110  of the cylinder via the respective rear nozzle  2144   a  and front nozzle  2144   b  causes the movement of the shaft  2120  from the retracted position into the advanced position and vice versa. 
         [0051]    This solution is very flexible in that it is possible to couple several motors to the connecting member which therefore acts as a reduction unit, the number of motors being able to vary depending on the power, torque and speed required. 
         [0052]    An equal number of motors moreover keeps the system more balanced from the dynamic point of view. 
         [0053]    It is therefore clear how, with the device according to the invention, it is possible to achieve movement in the axial direction of the shaft for actuating screws of machines for injecting polymers and similar materials, with substantial simplification of the overall structure, with an increase in the working life and reliability and without the need for incorporating special axial bearings or equivalent mechanical thrust-bearing devices which are replaced by the hydraulic thrust of the same cylinders which move the injection screw. 
         [0054]    With the device it is also possible to design the shaft  120 ,  1120  as a single body or as several coaxial parts fastened together.