Abstract:
A method and apparatus for the installation of a mixer in a hot runner manifold is disclosed. An adapter plate is affixed to the manifold and directs the fluid flow through a mixer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This a Continuation-In-Part of co-pending application Ser. No. 09/605,763 filed Jun. 28, 2000 which is a Continuation-In-Part of co-pending application Ser. No. 09/435,965 filed Nov. 8, 1999. Both of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to injection molding plastics. More specifically, this invention relates to an adapter apparatus and method for the insertion of a mixer in the melt stream of an injection molding machine.  
           [0004]    2. Summary of the Prior Art  
           [0005]    The large number of variables in the injection molding process creates serious challenges to creating a uniform and high quality part. These variables are significantly compounded within multi-cavity molds. Here we have the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot.  
           [0006]    Shear induced flow imbalances occur in all multi-cavity molds that use the industry standard multiple cavity “naturally balanced” runner system whereby the shear and thermal history within each mold is thought to be kept equal regardless of which hot-runner path is taken by the molten material as it flows to the mold cavities. These flow imbalances have been found to be significant and may be the largest contributor to product variation in multi-cavity molds.  
           [0007]    Despite the geometrical balance, in what has traditionally been referred to as “naturally balanced” runner systems, it has been found that these runner systems can induce a significant variation in the melt conditions delivered to the various cavities within a multi-cavity mold. These variations can include melt temperature, pressure, and material properties. Within a multi-cavity mold, this will result in variations in the size, shape and mechanical properties of the product. Though the effect is most recognized in molds with eight or more cavities, it can create cavity to cavity variations in molds with as few as two cavities.  
           [0008]    The flow imbalance in a mold with a geometrically balanced runner is created as a result of shear and thermal variations developed across the melt as it flows through the runner. The melt in the outer region (perimeter) of the runner&#39;s cross-section experiences different shear and temperature conditions than the melt in the center region. As flow is laminar during injection molding, the position of these variations across the melt stream is maintained along the length of the runner branch. When the runner branch is split, the center to perimeter variation becomes a side to side variation after the split. This side to side variation will result in variations in melt conditions from one side to the other of the part molded from the runner branch.  
           [0009]    If the runner branches were to split even further, as in a mold with 4 or more cavities, there will exist a different melt in each of the runner branches. This will result in variations in the product created in each mold cavity. It is important to note that as consecutive turns and/or splits of the melt channel occur, the difference in melt temperature and shear history is further amplified. This cumulative effect is clearly recognized in large multi-cavity molds where the runner branches split and turn many times.  
           [0010]    In an attempt to reduce this variation, the prior art has been primarily directed at various mixing devices that are located within the runner nozzle which is typically just prior the mold cavity. Examples of this can be found in U.S. Pat. No. 4,965,028 to Manus et al. and U.S. Pat. No. 5,405,258 to Babin.  
           [0011]    Mixers at various locations within the injection molding machine are also well known. Examples of mixers in the hot runner manifold include U.S. Pat. No. 5,683,731 to Deardurff et al., European Patent 0293756, U.S. Pat. No. 5,688,462 to Salamon et al. and U.S. Pat. No. 4,848,920 to Heathe et al. (all incorporated herein by reference). An example of mixers installed within the injection unit can be found in U.S. Pat. No. 3,156,013 to Elphee (incorporated herein by reference).  
           [0012]    Within the prior art, at least as much as known, there is no retrofit apparatus or method for installation of a mixer in an already existing injection molding machine, specifically in the hot runner manifold. Attempts at alleviating runner imbalance has been directed at correcting the problem within the injection nozzle or further upstream in the machine nozzle or sprue bar.  
           [0013]    There exists a need for an adapter apparatus and method that allows for the retrofit of injection molding machines for the placement of a mixer in the melt stream. Preferably, the mixer should be installed just upstream of where the melt channel splits or divides.  
         SUMMARY OF THE INVENTION  
         [0014]    The primary objective of the present invention is to provide an apparatus and method for retrofitting an existing hot runner for the installation of a mixer in the melt stream.  
           [0015]    Another object of the present invention is to provide a method and apparatus that reduces flow imbalances that are inherent in an injection molding machine.  
           [0016]    Yet another object of the present invention is to provide a melt mixer apparatus and method that exhibits substantially reduced pressure drop.  
           [0017]    Still yet another object of the present invention is to provide an adapter apparatus and method that easily and/or removably attaches to a hot runner manifold which alters the melt channel to allow for the insertion of a mixer in the melt channel.  
           [0018]    The foregoing objects are achieved in one preferred embodiment by providing an adapter plate specifically adapted to interface and communicate with the melt channel of a hot runner manifold. The adapter plate alters the path of the melt channel so that space for a mixer is achieved. Within the adapter plate a means for insertion of a mixer is provided which helps alleviate or substantially reduce flow imbalances within the flowing melt. The mixer is preferably installed in close proximity to where the melt channel branches into a plurality of melt channels. To effect the change in melt channel configuration, various plugs and holes in the hot runner manifold are used in conjunction with the adapter plate and the mixer.  
           [0019]    Further objections and advantages of the present invention will appear hereinbelow.  
       
    
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a top plan view of a hot runner manifold with two adapter plates installed thereon;  
         [0021]    [0021]FIG. 2 is a side cross-sectional view of the hot runner manifold;  
         [0022]    [0022]FIG. 3 is an enlarged cross-sectional view of the hot runner manifold and the adapter plate of a preferred embodiment of the present invention;  
         [0023]    [0023]FIG. 3 a  is an enlarged cross-sectional view of a hot runner before modifications are made in accordance with a preferred embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is a side cross-sectional view of a representative mixer used in conjunction with a preferred embodiment of the present invention;  
         [0025]    [0025]FIG. 4A is a top view of the mixer shown in FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring first to FIG. 1, a preferred embodiment  10  of the present invention is generally shown. A hot runner manifold  12  preferably constructed of a large flat plate comprises a plurality of primary melt channels  16  running therein for communication of a hot and pressurized melt stream to a plurality of injection nozzles (not shown) for the formation of a molded article. A heater  42  in thermal communication with manifold  12  maintains the flowing melt at an elevated temperature.  
         [0027]    Primary melt channels  16  branch out into a plurality of secondary melt channels  28 . An adapter plate  14  is rigidly affixed, preferably (but not by limitation) by at least one fastener  44 , to a face of the manifold  12 , adjacent the point where the primary melt channel  16  branches out into secondary melt channels  28 . It should be noted, that while the description of the present invention is directed at a specific location in the hot runner manifold, one of ordinary skill could easily place the invention at any point along the melt flow path. All such locations are fully contemplated by the present invention.  
         [0028]    Referring now to FIG. 2 and FIG. 3, a cross-sectional side view of the preferred embodiment  10  of the present invention is shown. Adapter plate  14  comprises a third melt channel  20  which interfaces with first melt channel  18  which is located in manifold  12 . Third melt channel  20  goes through a substantially 180° turn by completion of two 90° turns within adapter plate  14  such that the melt stream is communicated to a mixer  22 . The mixer  22  is located in adapter plate  14  by means of a mixer bore  24  and a complementary mixer bore  24   a  located in manifold  12 . In the preferred embodiment, a predetermined amount of axial sealing force is applied to the mixer  22  when the adapter plate  14  is secured to the manifold  12 .  
         [0029]    A pocket  41  is provided in the backing plate  40  which surrounds the adapter plate  14 . This allows the backing plate  40  to be attached to manifold  12  in a similar fashion before the adapter plate  12  was installed.  
         [0030]    For ease of manufacture, a plug  27  is inserted and rigidly affixed into adapter plate  14  by a threaded plug  26 . Plug  27  is provided with a smooth front edge which forms part of the wall of third melt channel  20  to help alleviate hang-up or stagnation points that may occur in the flowing melt. A hole  34  in manifold  12  aligns with a hole  32  in adapter plate  14  when the adapter plate is installed on manifold  12 . A locator  30  inserted into hole  32  and  34  ensures precise alignment of the various features of the adapter plate  14  to the manifold  12 . In the preferred embodiment, the locator  30  is a dowel pin, spring pin or any other suitable locating device known in the art.  
         [0031]    An elongated plug  38  is inserted in manifold  12  and rigidly affixed therein by threaded insert  36 . Similar to plug  27 , the end of plug  38  is smooth and forms part of the melt channel where primary melt channel  16  communicates with first melt channel  18 . A fifth plug  46  is inserted perpendicular to plug  38 , and fills the melt channel that was present before the hot runner manifold was modified for installation of the mixer and adapter plate. Plug  46  helps direct the melt flow to the plurality of secondary melt channels  28 .  
         [0032]    Mixer assembly  22  is trapped inside bores  24  and  24   a  between adapter plate  14  and manifold  12 . The mixer assembly  22  is comprised of a flow inlet  60  and a flow exit  62 . Flow inlet  60  communicates with third melt channel  20  for the transmission of the flowing melt through the mixer  22  to the flow exit  62 . Flow exit  62  communicates with a plurality of secondary melt channels  28 .  
         [0033]    Referring now to FIG. 3 a  (where like features have like numerals), a typical hot runner manifold  12  is shown before modifications are performed for installation of the mixer assembly. By comparison of FIG. 3 to FIG. 3 a , it can be seen that very little modifications to the hot runner manifold  12  are required to install the mixer assembly  22 . Phantom lines are provided to show where first melt channel  18  would be located as well as where mixer bore  24   a  would be located. A shorter plug  38 ′ is shown, which will need to be elongated as shown by plug  38  in FIG. 3. to effect the change in the direction of primary melt channel  16 .  
         [0034]    Referring now to FIGS. 4 and 4A, in this preferred embodiment (and not by limitation), mixer assembly  22  is comprised of two pieces. A mixer bushing  50  with at least one spiral groove  54  formed therein running from the inlet  60  to the outlet  62  for communication of the fluid through the mixer assembly  22 . An elongated torpedo  52  is inserted into the mixer bushing  50  and is maintained in a preferably coaxial position by at least one land  56  formed between the spiral groove  54 . At the flow inlet  60 , the torpedo  52  is comprised of an annular disk  58  which abuts against one end of the mixer bushing  50 . A plurality of spokes  64  extend from the center of the torpedo  52  to annular disk  58 , thereby creating space for the flowing melt as it enters the mixer assembly  22 . As the spiral groove  54  and lands  56  travel along the direction of the melt flow, a gap  57  which increases in the direction of the melt flow, is formed between the elongated torpedo  52  and the mixer bushing  50 . The cross-sectional area of the spiral groove  54  also decreases in the direction of the melt flow.  
         [0035]    As the melt travels through mixer  22 , more and more of the melt gradually spills out of the spiral groove  54  and over lands  56  such that the melt flow transitions from all helical to all annular flow. This mixing action has been shown to substantially eliminate flow imbalances that occur inside a melt stream.  
         [0036]    It should be noted that while the foregoing description provided only a single description for a mixer, one skilled in the art could easily envision alternative mixer arrangements, and as such, all such mixer embodiments are fully contemplated within the scope of the present invention.  
         [0037]    It is to be understood that the invention is not limited to the illustrations described herein, which are deemed to illustrate the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. The invention is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.