Abstract:
An injection screw and barrel for small injection molding shots employs separate plunger and flight sections so that the plunger section may be arbitrarily reduced in diameter without adversely affecting the threaded portion of the flight section. Melt passes from the flight section to a metering area through a hollow bore in the plunger section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   BACKGROUND OF THE INVENTION 
   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. 
   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 fall enter the barrel at a feed zone and they 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. 
   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. 
   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. 
   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 
   The present invention provides an injector using an injection screw having a plunger portion of reduced diameter. This reduced diameter increases screw movement for a given shot size allowing more precise injection control. The plunger is thread-less, so molten plastic reaches the metering zone through a hollow channel in the plunger rather than around the threads as in a conventional injection molding system. By separating the metering function of the injection screw (in the plunger portion of the screw) from the shearing, mixing and plastic advancing functions of the injection screw (in the threaded portion of the screw), greater flexibility may be had in designing injectors for small shots. 
   Specifically, the present invention provides an injection screw for injection molding having: (1) a flight section supporting outwardly extending threads where the threads have an outer diameter and, (2) a plunger section continuing from an end of the flight section and having a central bore opening at a nozzle end of the injection screw. The plunger section has an outer diameter smaller than the diameter of the threads of the flight section and at least one passage communicates from the flight section to the bore of the plunger section. 
   It is thus one object of the invention to provide an extremely small diameter metering zone so as to improve the accuracy of the injection process. The problem of manufacturing small diameter threaded injection screws is avoided by eliminating threads on the plunger and using a hollow channel for plastic transport. 
   The injection screw may include a plug between the flight section and the plunger section, the plug having an outer diameter substantially equal to the inner diameter of the barrel and the passage may communicate through the plug between the flight section and the plunger section. 
   It is thus another object of the invention to provide a simple method of directing molten plastic from the flight section of the injection screw into the bore of the plunger. The plug may ride in the larger diameter portion of the bore, generally a cylinder sized to equal the outer diameter of the threads, providing a simple seal between the flight section and the plunger section. 
   The plunger section of the injection screw may include a check valve blocking the flow of melt backwards toward the flight section. 
   Thus it is another object of the invention to provide an anti-backup valve suitable for use with the present screw design. 
   The check valve may be a ball contained in the bore opposite a seat formed by the bore. 
   Thus it is another object of the invention to provide an extremely simple check valve structure. 
   The plunger section may have a substantially cylindrical outer wall of constant diameter. 
   It is therefore another object of the invention to provide a simple plunger shape that is easy to machine and seal. 
   The passage between the plunger section and the flight section may be a hole leading from a root of a thread at an end of the flight section near the plunger section. 
   Thus it is another object of the invention to provide a simple method of establishing a passage between the flight section and the plunger section that does not affect the clearance between the threads in the flight section and the barrel. 
   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 
       FIG. 1  is a perspective, cutaway view of a prior art injector showing the metering zone as defined by the outer diameter of the threads of the injection screw. 
       FIG. 2  is a side-elevation, partial cross-sectional view of a barrel of the present invention holding a two-part injection screw having a plunger and flight section separated by a plug, the injection screw not in cross-section; and 
       FIG. 3  is an enlarged view of  FIG. 2  with the injection screw also in cross-section showing the internal passageways in the injection screw from the flight section to the plunger section via a bore within the plunger section. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a prior art injector  10  includes a nozzle  12  such as may abut a mold (not shown) during the injection process. The nozzle  12  screws into a generally cylindrical barrel  14  having an inner cylindrical chamber  16 . A diameter  18  of the chamber  16  approximates the outer diameter of the threads  20  of an injection screw  22  positioned behind the nozzle  12 . 
   During operation, the injection screw  22  is actuated for rotary motion  24  to fill a metering zone  28  with molten plastic prior to an injection. As the plastic fills the metering zone  28 , the injection screw is actuated for longitudinal motion  26  to retract from the metering zone  28  to be displaced by melt. 
   When a sufficient shot is collected in the metering zone, the injection screw is again actuated for longitudinal motion, this time to extend into the metering zone  28  forcing the melt from the nozzle  12 . The amount of longitudinal motion  26  necessary for injection of the shot held in the metering zone  28  will be inversely proportional to the diameter  18  of the chamber  16 . Accordingly, for good control of the injection process, the diameter  18  of the chamber  16  should be reduced so as to provide an amount of longitudinal motion  26  comparable with that provided in larger shots. 
   Referring now to  FIG. 2 , the injector  30  of the present invention also provides a nozzle  12  attached to the front end of a barrel  32 . The barrel  32 , however, has a stepped chamber  31  defining a metering zone  34  near the nozzle  12 , and a mixing zone  36  behind the metering zone  34  with respect to the nozzle  12 , each of different diameters. The diameter  38  of the metering zone  34  is generally larger than the diameter  40  of the opening in the nozzle  12 , but smaller than the diameter  42  of the mixing zone  36 . 
   An injection screw  44  for the barrel  32  provides two sections corresponding generally to the mixing zone  36  and metering zone  34 , respectively. Specifically, the injection screw  44  provides a flight section  46  leading to a plunger section  50  closest to the nozzle  12 . 
   The flight section  46  of the injection screw  44  includes double helix threads  54  having an outer diameter (measured at the crest of the threads  54 ) substantially equal to the inner diameter  42  of the mixing zone  36 . A root diameter (measured at the root of the threads  54 ) is, in a preferred embodiment, substantially equal to the outer diameter of the plunger section  50  as will now be described. 
   Generally, the plunger section  50  has a cylindrical outer surface substantially equal to the inner diameter  38  of the metering zone  34  to fit like a piston smoothly within the cylindrical chamber of the metering zone. The longitudinal position of the plunger section  50  within the metering zone  34  defines the melt zone  52  in which melt will accumulate prior to injection. 
   In the preferred embodiment, the plug section  48  separates the plunger section  50  from the flight section  46 . The plug section  48  has a cylindrical outer surface substantially equal to the inner diameter  42  of the mixing zone  36  to seal against the cylindrical chamber of the mixing zone  36  preventing plastic flow around its outer edges. 
   The end of the injection screw  44  removed from the nozzle  12  may include a square drive coupling  45  of a type well known in the art. The drive coupling allows the screw to be attached to the mechanism providing for its rotation, and translation of the injection screw  44  according to methods well known in the art. 
   Referring now to  FIGS. 2 and 3  during operation of the injector  30 , melt  60  (shown in  FIG. 3 ) will be sheared, blended and advanced by the threads  54  of the flight section  46  toward the plunger section  50 , but will not be able to pass around the outer edges of the plug section  48  because of its tight clearance with the inner surface of the chamber of the mixing zone  36 . Nevertheless, plastic material in the mixing zone  36  will be able to pass into holes  58  (only one shown in  FIG. 2 ) leading from the thread roots  59 , at the opposite sides of the injection screw  44  near the plug section  48 . 
   The holes  58  conduct the melt  60  from the thread roots  59  of the flight section  46  to a bore  62  extending axially through the center of the plunger section  50 . The bore  62  leads to the melt zone  52  where the melt  60  can collect as the injection screw  44  is withdrawn. 
   The end of the bore  62  at the end of the plunger section  50 , prior to exiting the plunger section  50 , is enlarged to provide a valve body  64  having a valve seat  66  formed by a radially extending wall between the valve body and the bore  62 . A check valve ball  68  is held in the valve body  64  by a transverse pin  70 . The check valve ball  68  may move forward under the flow of melt  60  to hit the transverse pin and to be retained within the valve body while providing sufficient clearance to allow the melt  60  to pass out of the bore  62 . When the injection screw  44  is advanced, however, the check valve ball  68  is moved backward against the seat  66  sealing against the seat  66  to prevent backward flow of the melt  60  toward the flight section  46 . In this way, the advancing plunger section  50  can act like a piston to force melt  60  from the nozzle  12  into a mold. 
   By reducing the diameter  38  of the metering zone  34 , greater longitudinal motion  26  accompanies the injection of the shot. For a given linear precision of the actuator (not shown) driving the injection screw  44  in longitudinal motion  26 , a greater volume precision in the amount of melt  60  injected is obtained. The strength of the injection screw  44  is not adversely affected because the plunger section  50  need not be significantly reduced in strength from the flight section  46  which has a substantially equal root diameter. Generally there will be little or no torsion on the plunger section  50  which has no threads, so a further reduction in the diameter  38  of the plunger section  50  may easily be obtained. 
   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.