Abstract:
An injection molding machine comprises: 
       a body ( 12 ) provided with a rectilinear cavity having a first end which is open and a second end ( 20   a ) opposite to the first one in which a nozzle ( 21 ) for injection into the mould ( 16 ) is disposed;    a floating punch ( 15 ) that is slidable within the cavity in the hollow body ( 12 ) between said first and second ends to define an injection chamber ( 20 ) with said second end ( 20   a ), said punch being provided with a passage ( 15   c ) to supply a way for access to the chamber ( 20 );    feeding means ( 13 ) adapted to be received in the first open end of the hollow body, to feed the injection chamber ( 20 ) with the material through the passage ( 15   c ) in the floating punch ( 15 );    thrust means ( 14 ) adapted to be received in the first open end of the hollow body ( 12 ) as an alternative to the feeding means ( 13 ), to make the floating punch ( 15 ) slide towards the injection nozzle ( 21 ), thereby causing injection into the mould of the material previously fed to the chamber ( 20 ).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an injection molding machine, in particular for rubber or plastic material.  
         [0003]     2. State of the Prior Art  
         [0004]     In the field of presses for injection molding, the injector unit for injecting the material into the mould has technical problems of great importance.  
         [0005]     The traditional machines of the known art (those having a reciprocal screw or a screw with a piston, for example) comprise a plasticizing screw assembly adapted to feed the injection chamber with the material for the mould. These machines also comprise a non-return valve necessary to prevent return of the material from the injection chamber to the plasticizing screw assembly when the rubber injection takes place. Movements enabling closure of the valve occur due to generation of the injection pressure within the chamber, and are of an uncertain duration. This fact involves an unsatisfactory precision in metering the injection amount because the whole material of a batch is not fully injected into the mould, but part of it flows back to the plasticizing screw assembly during the closure movement of the valve.  
         [0006]     In order to solve the problem concerning metering of the material, attempts have been made to use valves of the hydraulic type; this solution however, appears to be not very efficient, weakly reliable and subjected to quick wear. More recently molding machines have been devised that are provided with an injector unit operating in the absence of nonreturn valves and introducing the material from the same end of the chamber designed for subsequent delivery to the mould. These machines ensure a satisfactory and precise metering of the material to be injected into the mould. However, the work behavior of the injector unit is of the LIFO (Last In First Out) type, i.e. the last material portions introduced into the injector are also the first ones to come out therefrom and move to the mould. This situation is typically negative because part of the rubber remain within the injection chamber for longer periods than the rest of the material; this phenomenon can cause partial burns and lack of homogeneity in the material within the injection chamber (and therefore in the mould), which will bring about a decay in the quality of the final product.  
         [0007]     It is a general aim of the present invention to obviate the above mentioned drawbacks by providing an injection molding machine enabling a very precise metering of the material to be injected, a continuous change of the rubber (or the plastic material) within the injector and, consequently, a satisfactory homogeneity of the material injected into the mould.  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the above aim an injection molding machine has been devised, in accordance with the invention, which comprises: 
        a body provided with a rectilinear cavity having a first end which is open and a second end opposite to the first one, in which a nozzle for injection into the mould is disposed;     a floating punch that is slidable within the cavity in the hollow body between said first and second ends to define an injection chamber with said second end, said punch being provided with a passage to supply a way for access to the chamber;     feeding means adapted to be received in the first open end of the hollow body, to feed the injection chamber with the material through the passage in the floating punch;     thrust means adapted to be received in the first open end of the hollow body as an alternative to the feeding means, to make the floating punch slide towards the injection nozzle, thereby causing injection into the mould of the material previously fed to the chamber.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     For better explaining the innovative principles of the present invention and the advantages it offers as compared with the known art, a possible embodiment applying these principles will be described hereinafter, by way of non-limiting example, with the aid of the accompanying drawings. In the drawings:  
         [0014]      FIG. 1  is a view of the injector unit of an injection molding machine in accordance with the present invention;  
         [0015]      FIG. 2  is a sectional view of part of the injector unit in a first step of the injection cycle;  
         [0016]      FIG. 3  is a sectional view of part of the injector unit in a second step of the injection cycle;  
         [0017]      FIG. 4  is a sectional view of part of the injector unit in a third step of the injection cycle;  
         [0018]      FIG. 5  is a sectional view of part of the injector unit in a fourth and last step of the injection cycle. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     With reference to the drawings, shown in  FIG. 1  is an injector unit  11  of an injection molding machine, to inject rubber for example, in accordance with a preferred embodiment of the invention. This injector unit  11  comprises an injection body  12  within which the injection chamber is located, as widely described in the following. The body  12  is fastened to a support plate  40  and can be vertically moved along the four toothed bars  30  by driving the four cogwheels  32  (only two of which are shown in  FIG. 1 ) in rotation, said cogwheels engaging with the toothed bars  30 . The support element  44 , to which a counter-plate  17  and the respective horizontal movement system  18  (shown in  FIG. 2 ) are fastened, can move in a vertical direction along the toothed bars  30  in the same manner as the support plate  40 . In this case translation is carried out by setting the four cogwheels  31  (only two of which are shown in  FIG. 1 ) in rotation, said cogwheels too being in engagement with the toothed bars  30 .  
         [0020]     Shown in  FIG. 1  is also a plasticizing screw assembly  13  inserted in plate  40  and at the inside of the injection body  12 . This plasticizing screw assembly  13  is fastened to a translation carriage  43 . The carriage  43  is mounted on two pins  42  so that it can translate in a horizontal direction. This translation can take place when the plate  40  is at a lowered position and the plasticizing screw assembly  13  is no longer inserted in the injection body  12 . When the carriage  43  has translated to the right, to the end-of-stroke position, the pusher cylinder  41 , also fastened to carriage  43 , is over the injection body  12 . Projecting from the pusher cylinder  41  at the lower part thereof is an injection piston  14  (not shown in  FIG. 1 ) that is designed to replace the plasticizing screw assembly  13  within the injection body  12 , as widely described in the following.  
         [0021]     Also shown still in  FIG. 1  is part of the mould  16  and a block  19 . This block  19  is provided with a cavity  19   a  designed to receive the injection body  12  when the latter is in its lowered position for injection of rubber into the mould.  
         [0022]     As shown in FIGS.  2  to  5 , the body  12  is provided with an almost cylindrical vertical cavity within which a floating punch  15  can freely slide in a vertical direction. The floating punch  15  carries out a sealing action and defines an injection chamber  20  together with body  12 ; this chamber  20  has a variable volume depending on the position of the floating punch  15  relative to body  12 . The floating punch  15  is provided with a hole  15   c  that is coaxial with the cavity of body  12  so as to form an access way to the injection chamber  20 . Advantageously, in accordance with known techniques, the hole  15   c  is of truncated conical form and the major base thereof faces downwards. It is to be noted that the lower end  20   a  of the cavity of body  12  is of conical shape and the front portion  15   b  of the floating punch  15  is such shaped as to match the shape of said conical bottom. This front portion  15   b  tapers preferably with an inclination of 20° or more. Advantageously, the ratio between the longitudinal extension of the floating punch  15  and the punch diameter is included between 0.3 and 5.  
         [0023]     Positioned at the lower end  20   a  of the cavity of body  12  is an injection nozzle  21 . The cavity of body  12  is provided, at the upper part thereof, with an open end adapted to alternately receive the plasticizing screw assembly  13  or the thrust piston  14 .  
         [0024]     The plasticizing screw assembly  13  comprises a plasticizing screw  13   a  contained in a chamber formed in an outer casing  13   b . This screw assembly  13  also comprises a head portion  13   c  such shaped as to match the shape of the rear face  15   a  of the floating punch  15 . Formed in the middle of the head  13   c  is a feeding nozzle  13   d  adapted to engage the hole  15   c  of the floating punch  15 . Advantageously, the outer diameter of the plasticizing screw assembly  13  is smaller than the diameter of the cylindrical cavity of body  12  by at least 0.05 mm. It is to be noted that the plasticizing screw assembly  13  is fed from top and pushes the material towards the feeding nozzle  13   d  due to an appropriate rotatory motion of the screw  13   a.    
         [0025]     The injection piston  14  is of cylindrical shape and is adapted to be received into the cavity of body  12 , in the same manner as the plasticizing screw assembly  13 . This piston  14  is made up of a rod  14   a  and a thrust head  14   b  which is also such shaped as to match the shape of the rear face  15   a  of the floating punch  15 .  
         [0026]     Shown in  FIG. 2  is the configuration taken by the injector unit  11  at the beginning of the step of feeding the injection chamber  20 . Actually, this chamber  20  has a null volume in this stage of the molding cycle, since the floating piston  15  is disposed at the bottom of the cavity of body  12 . The plasticizing screw assembly  13  is inserted in the cavity and rests on the rear face  15   a  of the floating punch  15  by its head portion  13   c . Note that the screw assembly  13  is fixed in the vertical direction and can only carry out a horizontal translation; therefore, the configuration in  FIG. 2  is obtained by bringing the body  12 , fastened to plate  40 , to the raised position. The support  44  too, to which the counterplate  17  and the means  18  for the horizontal movement of same are fastened, is in a raised position. The counterplate  17  is such disposed as to close the injection nozzle  21 . Starting from this configuration, the step of feeding the injection chamber  20  can begin. The plasticizing screw  13   a  is driven in rotation to push the rubber material at the inside of the screw assembly  13  into hole  15   c  of the floating punch  15 . Simultaneously the cogwheels  31  and  32  are driven in a synchronous manner. This operation aims at causing translation of body  12  downwards while maintaining the injection nozzle  21  closed by means of the counterplate  17 . During translation, the floating punch  15  remains close to the plasticizing screw-assembly  13 , so that the injection chamber  20  is formed at the inside of body  12 . This chamber  20  receives the rubber material fed through hole  15   c . This rubber material is maintained to the correct compacting level because the descent movement of body  12  is regulated (electronically, for example) in such a manner as to oppose some resistance to entry of the rubber into the injection chamber  20 .  
         [0027]     Once the desired amount of rubber has been introduced into the injection chamber  20 , feeding from the screw assembly  13  is interrupted, lowering of body  12  stops and the configuration of the injector body as shown in  FIG. 3  is reached. Note that body  12  can still move downwards keeping the injection nozzle  21  closed by means of the counterplate  17 . In this manner the floating punch  15  can move upwards within the cavity of body  12  to release the pressure generated within the injection chamber  20 . This allows the injection nozzle  21  to be opened without rubber sprinkles taking place before the true injection begins. When the inner pressure of chamber  20  has been released, nozzle  21  is opened through operation of the movement means  18  to impose a translation movement to the left to the counterplate  17 .  
         [0028]     Once the counterplate  17  has been removed from the closed position, the body  12  can further move downwards, still by effect of a suitable rotation of the cogwheels  32  in engagement with bars  30 . This downward translation goes on until the configuration shown in  FIG. 4  is reached in which the body  12  is at the inside of the cavity  19   a  of block  19 . In particular, the injection nozzle  21  is coupled with the injection channel  16   a  in mould  16 . The plasticizing screw assembly  13  maintaining its position in height always fixed, is free from its engagement with the cavity of body  12  and is ready to begin its translation movement to the right to reach its rest position.  
         [0029]     In FIGS.  2  to  4  the injection piston  14  is always shown in its rest position. Once the situation seen in  FIG. 4  has been reached, this piston begins translating to the right, being rigidly connected with the plasticizing screw assembly  13 . This dual translation can be obtained, as previously described, through sliding of the translation carriage  43  in a horizontal direction on the two pins  42 . Once the injection piston  14  has reached a position coaxial with the injection chamber  20 , said piston  14  is moved downwards by the pusher cylinder  41  so that it enters the cavity of body  12  and pushes the floating punch  15  towards the injection nozzle  21 . In this manner rubber injection into mould  16  takes place, said mould having been suitably discharged and closed again to receive the new metered amount of rubber to be vulcanized.  
         [0030]     After the above description, it is apparent that the previously listed purposes of the present invention have been reached. In particular, an injector unit has been made that allows a very precise metering of the amount of material to be injected; in fact, the particular structure with a floating punch does not include any type of non-return valve, thereby avoiding occurrence of the technical problems connected with metering that were present in the machines of the known art, due to the valve arrangement. In addition, the described injection press enables an injection with a logic of the FIFO (First In First Out) type; in fact, the injection chamber is fed from top through the floating punch, while injection takes place through a nozzle placed to the lower end of the chamber. This situation allows a high degree of homogeneity of the material to be maintained within the injection chamber and, therefore, also within the mould. It is obvious that a material with an improved homogeneity allows a final product of better quality to be obtained.  
         [0031]     Obviously, the above description of an embodiment applying the innovative principles of the present invention is only given by way of non-limiting example and therefore must not be considered as a limitation of the scope of the patent rights herein claimed.