Abstract:
An injection assembly for injection molding machines for plastics material comprises a fixed frame ( 10, 21 ), a plastication cylinder ( 2 ) integral with the fixed frame, a plastication screw ( 5 ) acting inside the plastication cylinder, making a forward translational movement during the stage of injection of the plastics material into a cavity of a mold and a rotatory movement around its own axis and a rotatory translational movement of rotation around its own axis and retraction during the plastics material plastication stage, the plastication screw ( 5 ) being mounted integral in translation with a movable plate ( 23 ) mounted slidably on said fixed frame, there being provided drive means for translation of the movable plate ( 23 ), and drive means for rotation of the plastication screw ( 5 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an injection assembly for injection moulding machines for plastics material. 
     2. Description of Related Art 
     According to the prior art, in the injection moulding procedure, injection moulding machines or devices are used wherein resins, consisting of plastics material, are heated to be brought to a molten state in a heating cylinder and are injected at high pressure into a cavity of a mould so as to fill same. 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 is extracted and can be sent to storage or subsequent processing stages. 
     The injection moulding device comprises a moulding assembly and an injection assembly. The first assembly is generally formed of two moulds halves mounted respectively on a fixed plate and a movable plate, so that the mould can be opened and closed by causing the movable plate to advance and retract. 
     FIG. 1 shows schematically an injection assembly according to the prior art, denoted as a whole with reference numeral  100 . The injection assembly  100  comprises a heating cylinder  2  to bring the resins fed through a hopper  3  to a molten state, and an injection nozzle  4  to inject the molten material into the mould cavity. Inside the heating cylinder  2  is a plastication screw  5  which makes a rotatory movement around its own axis for plastication of the resins, a translational forward movement for injection of the molten material and a translational rearward movement which allows the resins to be introduced from the hopper  3 . 
     The plastication screw  5  is driven by means of a system of electric motors. For the translational movement of the plastication screw, a screw-nut and screw system is provided wherein a screw  6 , preferably a ball screw, engages in a screw nut  7 . The screw nut  7  is driven by a motor  8  by means of a reduction unit  9  fixed to the frame  10  of the machine. 
     Rotatory movement of the screw  6  around its own axis is prevented by an anti-rotation device. Said anti-rotation device comprises a rod  11  fixed to the screw  6  and a stop block  12  fixed to the frame  10  of the machine. In this manner the rod  11 , abutting against the stop block  12 , prevents rotation of the screw  6 , allowing only translation. Accordingly, operation of the electric motor  8  in one direction or in the opposite direction allows forward or backward movement of the screw  6 , which draws with it the plastication screw  5 . 
     As far as the movement of rotation of the plastication screw  5  is concerned, this is obtained by means of a splined shaft  13  connected by means of a joint  20  to the plastication screw  5 . The splined shaft  13  engages in a bush  14  set in rotation by an electric motor  15  by means of a reduction unit  16  fixed to the frame  10  of the machine. The splined shaft  13  is uncoupled from the screw  6  by means of a ball bearing assembly  17 , so that the rotatory movement of the plastication screw  5  can be independent of the translational movement imposed by the screw  6 . 
     The electric motors  8  and  15  are provided respectively with encoder type sensors  18  and  19  to carry out the speed adjustments required in the various phases of the work cycle. 
     The above described injection assembly, according to the prior art, has a drawback due to the fact that the stage of translation of the plastication screw takes place under very high axial loads. Therefore the driving mechanism based only on the ball screw  6  works in very demanding conditions if it is applied to large-sized moulding machines. The problems concern in particular the state of stress of the ball screw  6  determined by the high and variable loads which lead to wear on the screw and screw nut system and compromise the strength of the entire construction. 
     Said problem can be overcome only in part by replacing the ball screw with a planetary roller screw. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to eliminate said drawbacks by providing an injection assembly for injection moulding machines for plastics material suitable to be applied to large-sized moulding machines so as to allow high performances and high production capacity. 
     Another object of the present invention is to provide such an injection assembly for plastics material that is practical, economical, versatile and easy to make. 
     These objects are achieved in accordance with the invention with the characteristics listed in appended independent claim  1 . 
     Advantageous embodiments are apparent from the dependent claims. 
     The injection assembly for large-sized presses according to the invention is formed by a fixed base and a mobile structure translating on linear guides provided on the base. The plastication screw and the drive systems of the screw and of the movable structure are connected to the movable structure, whereas the plastication cylinder and the hopper are integral with the fixed part. 
     In a preferred embodiment of the invention the plastication assembly provides for use of four ball screws rigidly connected to the base, with the relative lead nuts positioned on the movable structure and free to rotate 
     The movement of pure translation for the injection stage is obtained by simultaneously setting in rotation the lead screws which pull the entire moveable structure and thus also the plastication screw. 
     To obtain synchronous, concordant movement of the lead nuts, a crown gear that engages simultaneously with four gear wheels directly connected to said lead nuts is used. 
     The injection assembly according to the invention has various advantages with respect to systems of the prior art. 
     The injection assembly according to the invention in fact has an extremely simple actuating mechanism, based principally on the use of a plurality of ball screws which allow the high axial load during the injection phase to be shared out. This arrangement offers high guarantees on the strength of the structure, the precision of movement of the plastication screw and the reproducibility of the operations of the work cycle. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Further characteristics of the invention will be made clearer by the detailed description that follows, referring to purely exemplary and therefore non-limiting embodiments thereof, illustrated in the appended drawings, in which: 
     FIG. 1 is a schematic side elevational view of an injection assembly according to the prior art; 
     FIG. 2 is a schematic, axonometric view of an injection assembly according to the invention, with some parts removed; the movable plate is illustrated with a thin line and broken off; 
     FIG. 3 is a schematic view from one end of the injection assembly according to FIG. 2, in which a first embodiment of its drive is illustrated; 
     FIG. 4 is a broken off sectional view along the plane of section  4 — 4  in FIG. 3; the movable plate is denoted with a dashed line; 
     FIG. 5 is a view similar to that of FIG. 3, illustrating a second embodiment of the drive of the injection assembly according to the invention; 
     FIG. 6 is a partially sectional, broken off view taken along the planes of section  6 — 6  of FIG. 5; the position of the mobile plate is illustrated with a dashed line; 
     FIG. 7 is a similar view to FIG. 6, illustrating a third embodiment of the drive of the injection assembly according to the invention; 
     FIG. 8 is a view from the right in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The injection assembly according to the invention is described with the aid of FIGS. 2-8. 
     Like or corresponding elements will be designated herein with the same reference numerals and will not be described in detail. 
     FIG. 2 shows schematically an injection assembly according to the invention as a whole, designated with reference numeral  1 . 
     The injection assembly  1  comprises a frame having an L-shaped longitudinal section, consisting of a base  10  and a fixed plate  21  which protrudes upward at right angles to the base  10 . Two guides  22  in the form of tracks parallel to each other are mounted on the base  10 . 
     A movable plate  23  is mounted slidably on the guides  22  so as to remain at right angles to the base  10  and parallel to the fixed plate  21 . The movable plate  23  has two grooves (not shown) which engage in the respective guide tracks  22 . 
     Rigidly fixed to the fixed plate  21  of the base  10  are four drive screws  30  disposed near the four corners of the fixed plate  21 . The drive screws  30  engage in respective lead screws or screw nuts  40  rotatably carried on the movable plate  23  near the four corners thereof. The drive screws  30  are preferably ball screws. 
     Again on the movable plate  23 , a crown gear  50  which has an outer toothing that meshes simultaneously with the outer toothing of the four lead screws is rotatably carried. In this manner the lead screws  40  are driven simultaneously by means of rotation around its own axis of the crown gear  50  which meshes with all the lead screws  40 , thanks to the toothed profile formed directly by tooling of the lead screws. The drive of the nuts  40  causes screwing and unscrewing thereof on the respective drive screws  30  which remain fixed and therefore translation of the movable plate  23  on the linear guides  22  occurs. 
     The arrangement of the drive screws  30  in the corners of the two vertical plates  21  and  23  allows the plastication cylinder  2  and the hopper  3  to be housed on the fixed plate  21  and easy access to the drive system of the plastication screw  5  which can be provided on the movable plate  23  and which will be described below. 
     The injection assembly  1 , thus structured, allows various possibilities of motorization and can thus be easily adapted and reconfigured. 
     Three possibilities for driving the injection assembly  1  according to he invention will be described below. 
     As shown in FIGS. 3 and 4, in this first embodiment of the drive system of the injection assembly  1  according to the invention, two independent motorizations are used; that is to say, a first electric motor M 1  to drive the plastication screw  5  and a second electric motor M 2  to drive the crown gear  50 . 
     The first electric motor M 1  can be carried on the movable plate  23  and connected in direct drive with the plastication screw  5 . Alternatively, a reduction unit can be interposed between the shaft of the motor M 1  and the plastication screw  5 . 
     The second electric motor M 2  can be connected, by means of a pinion, to the crown wheel  50  or alternatively a reduction unit can be interposed between the two. The motor M 2  is mounted peripherally on the movable plate  23  and, as shown in FIG. 4, has a pinion  52  which meshes with the crown gear  50  to set it in rotation. 
     Operation of this first embodiment of the invention will now be described. 
     In the injection phase only the motor M 2  which sets the lead screws  40  in rotation through the crown wheel  50  is activated; this rotation of the lead screws  40  around the respective drive screws  30  causes translation of the movable plate  23  integral with the plastication screw  5 . 
     The injection speed is controlled by governing the speed of rotation of the second motor M 2  by means of a suitable control which is not shown. 
     In the plastication stage the plastication screw  5  must perform a movement of rotation around its own axis. In this stage the plastication screw  5  must be able to simultaneously rotate and translate backward, as the plastic material is accumulated at the end of the plastication screw  5 . As it moves retracts the plastication screw  5  must ensure a constant but adjustable pressure on the plastics material so as to compact the plastics material. 
     The plastication stage begins with rotation (for example in a counter-clockwise direction with reference to FIG. 3) of the plastication screw  5  by means of the first motor M 1 . Control of the pressure on the plastic material is achieved by acting, by means of the motor M 2 , on the speed of retraction of the movable plate  23 . 
     Basically a closed loop circuit able to control the motor M 2  is formed. The motor M 2  initially maintains the lead screws  40  of the ball screws  30  blocked. In this manner there is an accumulation of plastic material in the injection chamber inside the cylinder  2  (the movable plate  23  and the plastication screw  5  do not retract in this stage). 
     The lead screws  40  remain still until the established pressure value is reached. Beyond this moment the system must provide a controlled rotation of the lead screws  40 , imparted by the second motor M 2 , so as to cause the moveable plate  23  integral with the plastication screw to retract, maintaining the required pressure value on the plastic material constant. 
     The second motor M 2  thus allows performance of the injection translation, the plastication screw return translation (shrinkage) and control of the pressure in the injection chamber during plastication. 
     It should be noted that the transmission ratio between the crown gear  50  and the pinion  52  of the motor M 2  can be exploited favourably at the time it is chosen and dimensioned. This configuration of the drive system does not impose any limits on the types of motors and possible other transmissions that can be used. 
     With reference to FIGS. 5 and 6, a second embodiment of the injection assembly  1  is illustrated, in which a main power motor Mp, dimensioned to perform both the injection stage and the plastication stage is used. The main motor Mp is flanked by an auxiliary motor Maux able to control the return of the plastication screw  5  during the plastication stage. 
     As shown in FIG. 6, the main motor Mp has a drive shaft  70  connected to the crown wheel  50  and connected through a joint  63  to the plastication screw  5 . A first one-way clutch device  60  is interposed between the shaft  70  and the crown gear  50  and a second one-way rotation clutch or device  61  is interposed between the shaft  70  and the joint  63 . Each device  60  and  61  allows relative rotation between the drive shaft  70  and the crown gear  50  and between the drive shaft  70  and the joint  63  in only one direction and not in the opposite direction. In this case the first device  60  allows an opposite rotation with respect to the direction of rotation of the second device  61 . 
     The auxiliary motor Maux has a pinion  71  which meshes on the outer toothing  72  of one of the four lead screws  40 . 
     Operation of this second embodiment of the drive system of the injection assembly according to the invention will now be described. 
     During the injection stage the main motor Mp causes its drive shaft  70  to rotate for example in a clockwise direction (with reference to FIG.  5 ). In this case the first device  60  is blocked and draws the crown gear  50  into rotation. The crown gear  50  in turn simultaneously draws into rotation the four lead screws  40  which, screwing into the respective drive screws  30 , cause the movable plate  23  and the plastication screw  5  to advance. In this manner a movement of pure translation of the plastication screw is obtained in that the second device  61  is free to rotate and sets the joint  63  free from the drive shaft  70  of the main motor Mp. 
     During the plastication stage the direction of rotation of the main motor Mp is reversed with respect to the injection stage. For example the drive shaft  70  is made to rotate in a counter-clockwise direction (with reference to FIG.  5 ). In this manner a rigid connection is made between the drive shaft  70  and the plastication screw  5  by means of the second device  61  and the joint  63 . In fact the second one-way clutch  61  is blocked and sets in rotation the joint  63  integral with the plastication screw. 
     The crown gear  50 , on the other hand, remains disconnected from the drive shaft  70 , since the first device  60  is free to rotate. 
     To allow the return of the plastication screw  5 , the auxiliary motor Maux is activated. The pinion  71  of the auxiliary motor Maux sets in rotation a lead screw  40  which in turn sets in rotation the crown gear  50 . The crown gear  50  thus also simultaneously sets in rotation the other three lead screws  40 . Consequently, the three lead screws  40  unscrew on their respective drive screws  30  causing retraction of the movable plate  23  integrally with the plastication screw  5 . 
     The return of the plastication screw  5  is controlled by the auxiliary motor Maux which intervenes in accordance with the pressure value measured on the plastic material during plastication. 
     If the auxiliary motor Maux is given a direction of rotation opposite to the direction of rotation that ensures the return of the plastication screw  5 , the movable plate is made to advance integrally with the plastication screw  5 . Thus the auxiliary motor Maux can also be employed, together with its main motor Mp, also to contribute to providing power during the injection stage. 
     In FIGS. 7 and 8 a third embodiment of the drive system of the injection assembly  1  is shown, in which a main motor Mp′ and an auxiliary motor Maux are connected to a combinatorial epicyclic reduction unit  80 . 
     The drive shaft  70 ′ of the main motor Mp′ is integral with a gear wheel  81  that acts as the first end wheel of the epicyclic reduction unit  80 . The second end wheel of the epicyclic reduction unit  80 , on the other hand, is formed by the crown gear  50  which meshes with the four lead screws  40 . One of the four lead screws  40  is driven by the auxiliary motor Maux, by means of the pinion  71 . 
     Gear wheels  86  forming part of a gear train  82  are interposed between the first end wheel  81  and the second end wheel  50  of the epicyclic reduction unit  80 . The gear train  82  is directly connected to the plastication screw  5  by means of an axially disposed joint  84 . Around the joint  84  of the gear train  82  is disposed a device  61 ′ with one-way clutch  61  that engages in a seat of the movable plate  23  allowing rotation of the gear train  82  in only one direction. 
     In the injection stage the two motors Mp′ and Maux, for example, are driven in the same direction of rotation. The main motor Mp′ sets in rotation the end wheel  81  which tends to make the gears  86  of the gear train  82  rotate. In this case the limited rotation mechanism  61 ′ is blocked and prevents rotation of the gear train  82  and thus of the plastication screw  5 . 
     The gear wheels  86  of the gear train  82 , being set in rotation by the first end wheel  81 , set in rotation the crown gear  50  which in turn simultaneously sets in rotation the four lead screws  40  which screw into the drive screws  30  allowing forward movement of the plastication screw  5 . 
     The auxiliary motor Maux, by means of its pinion  71 , sets in rotation one of the four lead screws  40  which in turn acts on rotation of the crown wheel  50 . Thus the crown wheel  50 , which in turn drives the lead screws  40 , is driven thanks to the intervention of the two motors Mp′ and Maux which suitably share the loads. 
     The plastication stage, on the other hand, takes place through the combination of the movements of the two motors Mp′ and Maux which will produce both rotation of the screw  5  and controlled return thereof. 
     That is to say, the main motor Mp′ is made to rotate in the opposite direction to the direction of rotation of the plastication stage and the auxiliary motor Maux is made to rotate in the opposite direction to the direction of the main motor Mp′. Consequently, the first end wheel  81  tends to set in rotation the gears wheels  86  of the gear train  82  and the one-way clutch  61 ′ of the gear train is free to rotate, thus the gear train sets in rotation the plastication screw  5 . 
     The auxiliary motor Maux acts on one of the four lead screws  40  in the direction of unscrewing of the lead screw of the drive screw  30 . Thus this lead screw sets in rotation the crown gear  50  which in turn drives the other three lead screws allowing retraction of the plastication screw  5 . 
     The epicyclic reduction unit is subject to Willis&#39; rule, which expresses the relationship between speed of rotation of the end wheels  81  and  50  and that of the frame of the gear train  82 . 
     If 
     ω 1 is the speed of rotation of the first end wheel  81   
     ω 2 is the speed of rotation of the second end wheel  50   
     ω 3 is the speed of rotation of the gear train  82 , and 
     τ 0  is the ordinary transmission ratio of the gearing 
     the following formula is obtained:          τ   0     =         ω      1     -     ω                 3           ω                 2     -     ω                 3                                
     Numerous variations and modifications 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 from the scope of the invention as set forth in the appended claims.