Abstract:
In a moulding machine for plastics comprising an injection assembly ( 1 ) mounted slidably on a frame of the machine and a moulding assembly ( 30 ) comprising a mould half ( 8 ) mounted on a fixed plate ( 6 ) integral with the machine frame, a movement apparatus ( 200 ) for the injection assembly comprises a chain transmission ( 209 ) connected to the injection assembly ( 1 ), a drive unit ( 211 ) that drives the chain transmission ( 209 ) to move the injection assembly ( 1 ) toward the moulding assembly ( 30 ) and return means ( 220 ) connected to the injection assembly ( 1 ) to allow the injection assembly ( 1 ) to be moved away from the moulding assembly ( 30 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention refers to a driving or movement system for moving the injection assembly up to the mould closing assembly on injection moulding machines for plastics. 
     2. Description of Prior Art 
     According to the prior art, in the injection moulding procedure, use is made of injection moulding machines or apparatus wherein resins consisting of plastic materials are heated in order to be brought to a molten state in a heating barrel and are injected at high pressure into a cavity of a mould so as to fill it. The resin in the molten state is cooled inside the mould so as to harden and form the moulded product. At this point the mould is opened and the moulded product, which can be taken to storage or to subsequent processing stages, is ejected. 
     The injection moulding apparatus comprises an injection assembly and a moulding assembly. The injection assembly comprises a barrel externally heated by means of electric resistances and containing a plasticizing screw. 
     The plasticizing screw has two functions. In the first function the plasticizing screw, by rotating, conveys and plasticizes the material in the front part of the barrel. In the second function the plasticizing screw, by translating axially, transfers the material, by now in the molten state, from the barrel to the inside of the mould. 
     In particular the mould is divided into two mould halves mounted respectively on a fixed plate and on a movable plate. In this manner the mould can be opened and closed by moving the movable plate back and forth. 
     The moulding assembly comprises a double toggle mechanism that provides the movable plate with the movements necessary for opening/closing of the mould as quickly as possible. Furthermore the double toggle mechanism develops the mould closing force to be able to resist the pressure of the material during injection. 
     The two injection and moulding assemblies are disposed one in front of the other on a machine base. In particular, the injection assembly is not permanently constrained to said base. In fact it can slide on shoes or columns to translate back and forth to allow the injection nozzle to be moved away from or toward the mould. 
     By means of a driving apparatus the injection-plasticization assembly is given the motion of translation and a contact pressure between the injection nozzle and the mould is generated. Said contact pressure between nozzle and mould is necessary during injection of the plastic material to prevent the plastic material from escaping in the area of contact between the nozzle and the mould. 
     FIG. 1 shows schematically and partially a moulding machine according to the prior art comprising an injection and plasticization assembly  1  and a moulding assembly  30 , shown partially. The machine has a movement apparatus for the injection assembly, according to the prior art, designated as a whole by reference numeral  100 . 
     The moulding assembly  30  comprises a movable mould half, not shown, and a fixed mould half  8  mounted on a fixed plate  6  and in turn mounted integrally on a base  5  of the machine. 
     The injection assembly  1  comprises a hopper  2  into which solid plastic material is fed and a plasticizing barrel  3  inside which a plasticizing screw (not shown in FIG. 1) operates. The plasticizing barrel  3  ends in an injection nozzle  7  for injection of molten material into the mould. 
     The plasticizing assembly  1  is mounted slidably on profiled guides  4  provided on the base  5  of the moulding apparatus. In this manner the plasticizing assembly  1  can translate toward the moulding assembly  30 . That is to say, the injection assembly  1  moves toward the fixed plate  6  until the injection nozzle  7  comes into contact with the fixed mould half  8 . 
     Movement of the injection assembly  1  is achieved by means of the driving apparatus  100 . The driving apparatus  100  has a transmission  109  with metal cables or ropes  109 ,  110  respectively connected to the front and rear part of the injection assembly  1 . 
     The cables  109  and  110  extend respectively on pulleys  113  and  114  which are mounted on the fixed plate  6  of the mould and on the machine base  5 , respectively. The cables  109  and  110  are driven by a common winding drum  112 , in turn powered by an electric motor  111 . 
     The phase of bringing the injection assembly  1  toward the moulding assembly  30  is obtained by winding the cables  109  on the drum  112  while the cables  110  are unwound. Vice versa for withdrawal of the injection assembly  1  from the moulding assembly  30  the direction of rotation of the electric motor  111  and thus of the drum  112  which causes winding of the cables  110  and unwinding of the cables  109  is inverted. Clearly the cables  109  and  110  can be parts of the same cable wound for a few turns on the winding drum  112 . 
     The cables  109 , in particular, are connected to the structure of the injection assembly  1  by means of small pistons  116  that act on a series of disc springs  115  provided in special cylindrical seats  131  of the injection assembly structure. The disc springs  115  when compressed make it possible to achieve the contact force necessary to hold the injection nozzle  7  against the fixed mould half  8  during injection of the plastic material. In fact, during this injection phase, the high pressure on the plastic material would lead to retraction of the injection assembly  1 . 
     The movement apparatus  100  according to the prior art has various drawbacks due to slipping of the cables  109  and  110  under load and the problems relating to winding of the cables  109  and  110  on the pulleys  113  and  114  or on the small-diameter drums  112 . 
     In fact, because of the high forces that have to be developed to maintain the contact between the injection nozzle and the mould, large-diameter cables must be provided, whereas because of problems of size the diameter of the idler pulleys and the winding drum cannot be increased too much. 
     In order to solve the problems of slipping between the cables and the pulleys, an alternative prior art solution provides for the cables  109  and  110  to be replaced with respective chains, maintaining the same structure of the movement system. 
     Both solutions, that providing for use of cables and that providing for use of chains, require adjustment of the movement system, so as not to create a backlash between the cable or chain and the pulley during reversal. 
     Moreover, both cables and chains must be long enough to be able to connect the front part and the rear part of the injection assembly, resulting in greater difficulty in driving or moving them. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to eliminate the drawbacks of the prior art, providing an apparatus for moving the injection assembly that is able to ensure a high level of performance, allowing a high movement speed and a tight seal between the injection nozzle and the mould. 
     Another object of the present invention is to provide such a driving apparatus of the injection assembly that is reliable and able to reduce problems of jamming and adjustment of its mechanical components to a minimum. 
     Yet another object of the present invention is to provide such a driving apparatus for the injection assembly that is practical, economical, versatile and simple to make. 
     The movement apparatus for moving the injection assembly of a moulding machine, according to the invention, has an injection assembly slidably mounted on a machine base and driven or moved by means of a drive or movement unit that drives a metal chain transmission, in which one end of the chains is connected to the injection assembly. The other end of the chains is connected by means of spring means to a fixed frame of the machine. 
     The chains, driven by the drive unit, generate the forward movement of the injection assembly toward the moulding assembly, whilst the spring means keep the pull of the chains constant. The backward movement of the injection assembly away from the moulding assembly is generated by return means. Said return means are preferably extension spring means which connect the injection assembly to the frame of the machine. 
     However, the return means can also be another chain transmission driven by the same drive unit as the first chain transmission or by another drive unit. 
     The advantages of the movement apparatus of the injection assembly according to the invention are obvious. In fact, said movement apparatus allows the use of metal chains which ensure a greater reliability with respect to the metal cables used in the apparatus of the prior art. 
     Furthermore, the return means make it possible to avoid using long chains connected to the rear part of the injection assembly to allow its return. 
     Moreover, the spring means connected to one end of the chain make it possible always to keep the chain under tension, thus avoiding the creation of backlash between the chain and its driving sprocket wheels and therefore also the problems of jamming and adjustment of the transmission sprocket wheels of the chain. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non-limiting embodiment thereof, illustrated in the appended drawings, in which: 
     FIG. 1 is side elevational view, partially broken off and partially in section, illustrating an injection moulding machine with a driving apparatus for the injection assembly according to the prior art; 
     FIG. 2 is a side elevational view, like that in FIG. 1, partially broken off and partially in section, illustrating an injection moulding machine with a movement apparatus for the injection assembly according to the invention, in which the injection assembly is shown in a retracted position; 
     FIG. 3 is a top plan view from above of the moulding machine in FIG. 2, shown broken off; 
     FIG. 4 is a sectional view along sectional plan IV—IV in FIG. 2; 
     FIG. 5 is a view, as in FIG. 2, illustrating the injection assembly in the forward position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinunder, elements like or corresponding to those illustrated in FIG. 1, relating to the moulding machine of the prior art, are designated with the same reference numerals and are not described. 
     In FIG. 2 an injection moulding machine, substantially similar to the machine described with reference to FIG. 1, is shown schematically and broken off. 
     The moulding machine of FIG. 2 has a movement or driving apparatus for the injection assembly, according to the invention, designated as a whole with reference numeral  200 . 
     Some elements making up the movement apparatus  200  are shown singly in FIG. 2, however they can be provided in pairs on the two sides of the machine. Therefore, even if reference will be made hereinunder to a single element, it must be considered that an identical element can be disposed symmetrically on the other side of the machine. 
     The injection assembly  1  is slidably mounted on a profiled guide  204  provided on a support  240  stationary mounted on the base  5  of the machine. 
     A bracket  219  which ends in a pivot or pin  241  slidably mounted in a slot  242  formed in the support  240  is fixed to the injection assembly  1 . A spring  220  has one end engaged with the pin  241  and the other end engaged with a peg  243  fixed to the rear end of the support  240 . The spring  220  is preferably a helical extension spring. 
     When the injection assembly  1  is in its retracted position (FIG.  2 ), that is to say, when the injection nozzle  7  is not engaged in the aperture of the fixed mould half  8 , the spring  220  is not urged and the pin  241  is near to its rearward end of stroke inside the slot  242 . 
     A drive unit  211  rotates drive shaft  245  (FIG. 4) arranged transversally beneath the base  5 . The drive unit  211  preferably has a worm screw geared motor. The output shaft  245  of the drive unit  211  is connected to two pinions  212  (FIG.  4 ). Each pinion  212  drives a chain  209  which extends on a first sprocket  213  rotatably mounted on the fixed plate  6  of the moulding assembly and on a second sprocket  214  rotatably mounted in the front side part of the machine base  5 . 
     One end of the chain  209  is connected by means of a tie rod  216  to the injection assembly  1  and the other end of the chain  209  is connected by means of a spring  221  to the machine base  5 . 
     As also shown in FIG. 3, each tie rod  216  has one end fixed to an end link of the chain  209  and the other end housed in a seat  231  of the injection assembly  1 . A spring  215  which is interposed between the seat  231  and an end part  250  of the tie-rod  216  is positioned in the seat  231 . The spring  215  is a disc compression spring or Belleville spring and provides contact pressure between the nozzle  7  and the mould  8  during the injection phase. 
     In each chain  209 , the chain end opposite to the chain end engaged with the respective tie rod  216  is connected to an end of the spring  221 . The spring  221  is a helical extension spring which serves to recover the chain  209  and always keep it under tension, avoiding disengagement thereof from the pinion  212 . 
     As shown in FIG. 2, when the injection assembly  1  is in the retracted position, the spring  221  is extended and thus loaded so that it exerts a pulling force on the chain  209  which remains under traction. 
     Operation of the drive assembly  200  according to the invention is described below. 
     During the plasticizing stage, the injection assembly  1  is in its retracted position, shown in FIG.  2 . When the plasticizing stage is completed, the driving apparatus  200  comes into operation to move the injection assembly  1  toward the moulding assembly  30 . 
     For this purpose the electric motor  211  which rotates the pinions  212  in an anticlockwise direction (in FIG. 2) is actuated. The pinions  212  drive the respective chains  209  which, by means of tie rods  216 , operate traction of the injection assembly  1 . Consequently, the injection assembly  1  slides on the guides  204  to move toward the moulding assembly  30 . At the same time the extension springs  221  recall the respective chains  209 , keeping them under tension. 
     Since the injection assembly  1  moves forward, the bracket  219  moves forward with it, pulling the end of the spring  220  fixed to the pin  241  which moves forward in the slot  242  of the support  240 . Consequently the spring  220  is loaded and extends. 
     FIG. 5 shows the injection assembly  1  in its advanced position, that is to say when the injection nozzle  7  is engaged against the half-mould  8 . 
     In this situation the disc-shaped compression springs  215  are compressed and ensure the contact pressure between the nozzle  7  and the mould half  8 , with such a force as to contrast the pulling action of the spring  220  and the reaction on the injection assembly exerted during the injection phase. The traction springs  221  are released and ensure tension of the chains  209 . The pin  241  of the bracket  219  is at its forward end of stroke inside the slot  242  and consequently the extension spring  220  is loaded. The extension spring  220  which is loaded exerts a traction force on the bracket  219  which tends to cause the injection assembly  1  to retract. However, in this situation, thanks to the action of the motor brake of the geared motor  211 , the traction force exerted by the spring  220  does not cause the injection group  1  to retract. 
     On completion of the injection phase, the injection assembly  1  must be moved away from the moulding assembly  30 . For this purpose the geared motor  211  is actuated, so as to release the motor brake. Consequently the extension spring  220 , which is loaded, tends to cause the injection assembly  1 , which slides on the guides  204 , to retract. At the same time the springs  221  extend and are thus loaded until the injection assembly  1  again reaches its retracted position, shown in FIG.  2 . 
     During retraction of the injection assembly  1 , the geared motor  211 , instead of releasing its motor brake, can be driven in the opposite direction with respect to the direction of the advancement phase. In this manner the pinions are caused to rotate in a clockwise direction (FIG.  5 ). 
     Numerous variations and changes of detail within the reach of a person skilled in the art can be made to the present embodiment of the invention, without thereby departing form the scope of the invention set forth in the appended claims.