Abstract:
A screw for injection molding provides a coaxial piston that allows the effective cross-sectional area of the screw to be varied during the injection cycle permitting small shot metering with relatively large diameter injection molding screws.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     — 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     — 
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates generally to injection molding, and more particularly, to an injection screw and barrel providing improved control of small shots of plastic during injection molding.  
         [0004]     The injection molding process employs an injector that forces a volume of thermoplastic material (a “shot”) under pressure into a mold cavity. A common injector design provides an outer barrel holding an injection screw. Pellets of thermoplastic resin from a hopper enter the barrel at a feed zone and are received by threads (“flights”) on the injection screw. The injection screw rotates within the barrel to shear, blend, and advances the molten plastic toward the front of the barrel near a nozzle that communicates with the mold cavity.  
         [0005]     As molten plastic is advanced toward the front of the barrel, the injection screw retracts, allowing molten plastic to fill a metering zone just behind the nozzle. At the time of the injection, the injection screw is moved like a piston to push the plastic from the metering zone into the nozzle and ultimately into the mold.  
         [0006]     In order to obtain consistent and high quality molded parts, the movement of the screw within the barrel must be accurately controlled. This is difficult for small shot sizes where very little screw movement occurs. For this reason for small shots of plastic, it is desirable to reduce the diameter of the bore of the injector barrel and the diameter of the injection screw so as to provide the largest possible amount of screw travel for the small shot volume.  
         [0007]     Small injection screws are difficult to manufacture, and there are practical limits on injection screw diameter resulting from the need for thread depth and sufficient root diameter to withstand the torque and compression placed on the injection screw.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention provides an injector screw having a coaxial piston that may be moved independently of the screw. The piston allows an effective variation in the cross section area of the screw allowing the injection shot to be controlled by (1) movement of the piston, (2) reduced movement of the screw alone, or (3) a combination of screw and piston motion. As a result, larger screw diameters may be fabricated and used with actuators having long strokes and high accuracy while still metering small amounts of plastic.  
         [0009]     Specifically, the present invention provides a screw for fitting within an injector barrel where the screw has outer threads to advance molten plastic toward the front of the barrel with rotation of the screw about an axis. The screw includes a central bore along the axis that receives a piston within the central bore to move with respect to the screw.  
         [0010]     Thus, it is one object of at least one embodiment of the invention to provide independent control of the effective cross-sectional area of the screw allowing large screw diameters to be used for metering small shots of plastic.  
         [0011]     The bore and piston may be cylindrical and the piston may be rotatably received within the central bore of the screw.  
         [0012]     Thus it is another object of at least one embodiment of the invention to provide simple construction of the screw and of the piston actuating mechanism.  
         [0013]     The piston may have a greater axial length than the screw.  
         [0014]     Thus it is another object of at least one embodiment of the invention to allow access to the piston through the rear of the screw typically outside the barrel.  
         [0015]     The screw may be used in an injection-molding machine having an injector barrel with a bore extending along an axis and terminating at a nozzle to abut a mold held by the injection-molding machine. A first actuator may be provided for rotating the screw about the axis, and the second actuator may provide relative movement between the piston and the screw.  
         [0016]     It is thus another object of at least one embodiment of the invention to provide an injection-molding machine suitable for use with the screw of the present invention.  
         [0017]     The molding machine may include a controller communicating with the actuators to rotate the screw to fill a front of the barrel with molten plastic while retracting the piston with respect to the screw away from the nozzle. The piston may be then be advanced by the controller with respect to the screw to eject molten plastic from the nozzle.  
         [0018]     Thus it is another object of at least one embodiment of the invention to provide a simple method of providing a range of small shot sizes by changing the diameter of the piston.  
         [0019]     Alternatively, the screw may be rotated to fill the front of the barrel with molten plastic and then advanced toward the nozzle while retracting the piston with respect to the screw to eject molten plastic from the nozzle.  
         [0020]     Thus it is another object of at least one embodiment of the invention to provide an extremely simple method of adapting current injection molding machines to small shot sizes, for example, by using the normal screw actuation mechanisms with a stationary piston that may subtract from the effective area of the screw.  
         [0021]     The piston may retract so that a nozzle end of the piston is within the screw.  
         [0022]     Thus it is an object of at least one embodiment of the invention to provide a screw that may be used with existing barrels having relatively small distances between the end of the screw and the barrel nozzle.  
         [0023]     Alternatively the piston may be extended from the screw so that when it is retracted, a nozzle end of the piston is not pulled within the screw.  
         [0024]     It is thus another object of at least one embodiment of the invention to provide a system in which molten thermoplastic is pulled within the interior of the screw.  
         [0025]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a simplified diagram of an injection molding having a barrel holding the small shot injector screw of the present invention;  
         [0027]      FIG. 2  is a fragmentary cross-sectional view of the barrel and screw of  FIG. 1  showing a mode of retraction of a coaxial piston to collect a shot volume; and  
         [0028]      FIG. 3  is a figure similar to that of  FIG. 2  showing an alternative extension mode of the coaxial piston as may define a shot volume. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Referring now to  FIG. 1 , an injection-molding machine  10  suitable for use with the present invention provides an injector barrel  12  extending along a longitudinal axis  14  having a nozzle  16  defining a front of the injector barrel  12 . As is generally understood in the art, one or more heater bands  26  may be placed about the barrel  12  and a rear of the barrel may support a vertically extending hopper  28  providing a source of thermoplastic pellets.  
         [0030]     The nozzle  16  of the injector barrels  12  may abut a mold portion  18  to inject a shot of plastic into a mold formed by the mold portions  18 . During the injection process, the mold portions  18  are held clamped together between a stationary platen  20  and a movable platen  22 , the latter sliding axially along tie bars  24 .  
         [0031]     Referring also to  FIG. 2 , the barrel  12  may include a central cylindrical bore  30  that may receive an injector screw  32 . The injector screw  32  extends generally along axis  14  and has screw threads  34  extending helically around the outside of the screw  32 .  
         [0032]     A rear end of the screw  32  (removed from the nozzle  16 ) may project beyond the barrel  12  to be mechanically attached to a screw rotation actuator  35  and a screw translation actuator  36  of types well known in the art. The screw rotation actuator  35 , for example, a hydraulic motor, allows controllable rotation of the screw  32  about axis  14  and screw translation actuator  36 , for example, a hydraulic cylinder, allows controllable translation of the screw  32  along axis  14 . Together, these motions allow melting of the pellets by the rotating screw  32  while allowing the screw  32  to be retracted as molten thermoplastic is accumulated toward the front the of barrel  12 . The controlled forward translation of the screw  32  then allows the molten thermoplastic to be ejected from the barrel  12  during the injection cycle.  
         [0033]     Referring still to  FIGS. 1 and 2  in the present invention, the screw  32  includes a coaxial bore  38  extending along axis  14  and passing the entire length through the body of the screw  32 . A cylindrical piston rod  40  may slide smoothly within the bore  38  with the outer circumference sealing against the inner surface of the bore  38  or seals (not shown) placed therein. The cylindrical piston rod  40  has a length along axis  14  greater than the length of the screw  32  so that the cylindrical piston rod  40  extends from the rear end of the screw  32  to be connected mechanically to a piston translation actuator  44  allowing motion of the piston rod  40  within the screw  32  independent of movement of translation movement of the screw  32 .  
         [0034]     Each of the actuators  35 ,  36  and  44  may provide connections to a controller  50  which may coordinate (1) the sequence of movements of the screw  32  in rotation and translation, and (2) the movement of the piston rod  40  in translation, both as a function of various stages of the injection molding process.  
         [0035]     Referring again to  FIG. 2  in a first stage of operation, screw  32  may be rotated per conventional injection molding techniques to collect plastic pellets from the hopper  28  and to elasticize them by grinding and moving them through the heated barrel  12 . Through this rotation, molten plastic  52  is moved to the nozzle end of the barrel  12 . As the plastic  52  accumulates at the nozzle end of the barrel  12 , the piston rod  40  may be retracted allowing plastic  52  to pass around the edges of the screw  32  and into a cavity formed by the retracting piston rod  40 .  
         [0036]     When piston rod  40  is retracted sufficiently to accumulate the necessary volume for the desired injection shot, rotation of the screw  32  may stop and screw  32  may be held in place while piston rod  40  is advanced forward to eject the plastic  52  from the cavity formed by the bore  38  of the injector screw  32 . This plastic  52  moves forward and out of nozzle  16 .  
         [0037]     In one variation of this embodiment, screw actuator  36  may be omitted as the screw  32  need only rotate and need not translate to cause the ejection of plastic  52 . In a second variation, the injection state is accompanied by motion of both of the screw  32  and piston rod  40 .  
         [0038]     Referring now to  FIG. 3  in an alternative embodiment, piston rod  40  may be initially extended out of the front of the screw  32  by an amount sufficient so that its volume in extension is equal to (or no less than) the desired shot of plastic  52 . During the plasticization step, molten plastic  52  pushes the extended portion of piston rod  40  into the screw  32  not necessarily causing the piston rod  40  to retract into the screw  32  so that molten plastic need be drawn into the screw  32 . The piston rod  40  is then re-extended to eject the necessary shot of plastic from the barrel  12 .  
         [0039]     Referring again to  FIG. 2 , in yet another mode of operation of the invention, once sufficient plastic  52  has accumulated toward the front the of the barrel  12 , the screw  32  may be advanced with the piston rod  40  held stationary (or retracted) with respect to the barrel  12 . This combined motion of the piston rod  40  with respect to the screw  32  decreases the effective cross-sectional area of the screw  32  causing a smaller amount of plastic  52  to be injected through nozzle  16  than would be obtained with a solid screw  32 . Note that at least one version of this mode of operations does not require absolute movement of the piston rod  40 , and thus does not require piston translation actuator  44 .  
         [0040]     It will be understood that the present invention also contemplates possible simultaneous relative movement of piston rod  40  forward and screw  32  backward to provide a range of possible metering solutions.  
         [0041]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.