Abstract:
An injection molding apparatus having a manifold and several manifold melt channels communicating with several hot runner nozzles includes a melt redistribution element. The melt redistribution element is placed at specific locations along the melt channels to balance the uneven shear stress profile accumulated during the flow of a melt along the manifold channels. The melt redistribution element has an unobstructed central melt bore having at its inlet a narrowing tapered channel portion. The melt redistribution element also includes a helical melt pathway portion that surrounds the central melt bore. The incoming melt is first subjected to a pressure increase by the tapered portion that causes the melt to flow at a higher velocity through the central melt bore. The outer portion of the melt is forced to flow along the helical path and thus it changes direction multiple times and partially mixes with the melt flowing through the central melt bore. Accordingly, at the outlet of the melt redistribution element the shear stress profile is more evenly distributed than at the inlet of the redistribution element.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a melt redistribution element and method to improve the homogeneity of melt material in an injection molding hot runner apparatus. 
     BACKGROUND OF THE INVENTION 
     In a conventional injection molding apparatus, melt material is delivered from an injection molding machine and flows through a hot runner manifold generally having a plurality of circular cross-section manifold channels. The manifold has an inlet and a plurality of outlets communicating with a plurality of hot runner nozzles and is heated to maintain the melt at a consistent and flowable temperature. Several variables affect the quality of the molded parts produced using hot runner manifolds. One such variable is the shear stress-induced flow imbalance that can be observed or measured along the melt channels and at the outlets of the manifold. This flow imbalance is unavoidable and is characterized by a variable, non-symmetrical, cross-sectional distribution (or profile) of the temperature, viscosity and velocity of the melt along each melt channel of the manifold. Therefore, the temperature, viscosity and velocity cross-sectional distribution or profile of the melt leaving the manifold varies at the entrance of each nozzle. This explains why in many applications the molded parts differ from one cavity to the other and from one batch to the other in terms of weight, density, size, appearance, etc. 
     As the melt material flows in generally circular channels, the material in the center of the channel has a higher velocity than the material along the sides of the channel. Since material along the sides of the channel moves more slowly than material in the center, it is exposed to the heat from the manifold for a longer period of time than the faster, more centrally-disposed material causing a temperature imbalance between material in the middle of a channel and material along the sides of a channel. At the same time, melt material against the sides of a channel are further heated and stressed (i.e., sheared) by the friction generated as the melt moves against the side channels. Higher temperatures and shear stress create changes in viscosity of the material. 
       FIG. 1  shows, in cross-section, a conventional two-level hot runner manifold  112  of a multi-channel injection molding apparatus. Melt material enters the manifold along a channel  102 . The melt is maintained at a moldable temperature by manifold heaters  128 . The melt then splits and enters identical and opposite branches  103  and flows around a first approximately 90-degree turn  104 . The melt then splits again and enters identical and opposite branches  105  and  106 , curves around a second approximately 90-degree turn  107  in each branch and exits the manifold through outlets  108  and  109 , respectively. The outlets  108  and  109  are in fluid communication with two hot runner nozzles (not shown) to deliver the melt to either a single or two mold cavity system (not shown). 
     The shear stress created along the walls of channel  102  is schematically shown in  FIG. 1A  in a cross-sectional shear profile along line A-A. When channel  102  splits into branches or melt channels  103 , the shear stress from melt channel  102  is greater along side  103   a  than on side  103   b  of melt channels  103 . As melt material flows through branch  103 , shear stress is naturally created at a lesser extent along a side  103   b . However, any shear stress formed by friction along side  103   a  is added to the shear history from channel  102  along side  103   a , forming a side-to-side, or asymmetrical, shear stress profile imbalance within channel  103 . Further, shear stress profile imbalance occurs as melt moves around turns  104  and further flows along melt channel  103 . Where channel  103  splits into branches or melt channels  105  and  106 , the shear stress and thus the temperature and velocity profile along and across these melt channels and towards outlets  108  and  109  becomes even more unevenly distributed and unevenly balanced. The variations of shear stress profile from side  105   a  to side  105   b  are shown schematically in a cross-sectional shear stress profile along line B-B, shown in  FIG. 1B . Shear stress profile variations from side  106   a  to side  106   b  are generally shown in a cross-sectional shear profile along line C-C, shown in  FIG. 1C . Thus, at manifold outlets  108  and  109  each of the cross-sectional shear stress profiles of  FIGS. 1B and 1C  indicate distinct side-to-side variations and thus uneven shear stress, temperature and viscosity cross-sectional distribution with respect to a central axis  115  of the manifold melt channels. 
     Further, comparison of cross-sectional shear profiles of  FIGS. 1B and 1C  indicates that the amount of shear stress between manifold channels  105  and  106  differs greatly. Since shear stress profiles are also an indication of temperature, velocity and viscosity profiles, the melt that leaves branch  105  through outlet  108  has a much higher temperature on the outer and intermediate portion of the melt than the melt material that leaves channel  106  through outlet  109 . Thus, a temperature and pressure suitable for molding a product from the melt in branch  105  may be different from a temperature and pressure suitable for molding a product from the melt material in branch  106 . Since it is very difficult to adjust or locally correct the temperature and pressure differences to a particular channel in a multi-channel injection molding apparatus, variations in shear stress profiles lead to inconsistent molded products from one mold cavity to another and from one batch of molded products to the next. 
     Further, side-to-side (or uneven or non-symmetrical) shear stress and temperature cross-sectional profiles may cause different flow characteristics from one side to the other of a single molded product, causing poor quality parts to be produced. 
     Attempts have been made to either reduce, eliminate, redistribute or rotate the non-symmetrical profile of the temperature, viscosity or velocity of the melt flowing inside a manifold towards several nozzles in order to provide at the manifold outlets more homogeneous, identical or similar profiles that would improve the processing conditions. However, these attempts generally require splitting the melt stream via a mechanical obstruction, which may lead to flow lines, particularly with materials that are sensitive to the development of flow lines. 
     Reference is made in this regard to European Patent Publication No. EP 0293756, U.S. Pat. No. 5,421,715 and U.S. Pat. No. 6,572,361 that show so-called manifold melt mixers, U.S. Pat. No. 5,683,731 that shows one so-called manifold melt redistributor and U.S. Pat. No. 6,077,470 that shows a so-called melt flipper, or melt rotating device. Further, reference is also made to U.S. Patent Application Publication No. 2004/0130062 that shows yet another melt mixing device and method. Each of these references is incorporated by reference herein in its entirety, respectively. 
     There is a need to provide a melt redistribution device and method that will provide a melt flow through a hot runner system with an improved temperature, viscosity, pressure and shear stress cross-sectional profile at various stages of the melt flow through the system. 
     SUMMARY OF THE INVENTION 
     This invention discloses an injection molding apparatus and an injection molding method that provides a molten material having more symmetrical shear stress, temperature, viscosity and velocity cross-sectional profiles at each of a plurality of outlets of an injection manifold. 
     According to one embodiment of the invention, a melt redistribution element is placed at specific locations inside an injection manifold along the melt channels. The melt redistribution element is provided with an unobstructed melt bore having at its inlet an inlet tapered section that increases the melt&#39;s pressure and generates a pressure increase, or what is known in the injection molding trade as a pressure drop. The melt redistribution element further includes a helical melt pathway that surrounds the melt bore. The incoming melt, flowing along the manifold melt channel and having accumulated an uneven shear stress profile, is first subjected to the pressure increase that causes the melt to flow at a higher velocity through the central melt bore of the melt redistribution element. Next, a central portion of the melt stream continues to flow along the melt bore and an outer portion of the melt stream flows along the helical melt pathway. Initially, the outer portion has a more non-symmetrical shear stress profile than the central portion. The helical melt pathway changes the direction of flow of the outer portion of the melt stream to reorient the shear stress and to mix some of the outer portion of the melt stream with some of the adjacent central portion of the melt stream. At the outlet of the melt redistribution element, the shear stress cross-sectional profile of the melt is more even, or symmetrical, than at the inlet. 
     In another embodiment of the invention, the melt redistribution element has several helical melt pathways, which may provide an additional reorientation and mix of the outer portion of the melt stream, further improving the shear stress cross-sectional profile at the outlet of the melt redistribution element. 
     By directing the melt directly through a melt redistribution element without using a blocking, or splitting, mechanical device inside the melt bore, the melt redistribution element according to this invention works as a non-invasive device reducing the occurrence of flow lines, for example. 
     One aspect of the present invention is directed towards having a more consistent shear stress cross-sectional profile of the melt material exiting each of a plurality of manifold channel outlets within a multi-channel injection molding apparatus. 
     Another aspect of the invention is to have a melt material with a more uniform shear stress cross-sectional profile when exiting a particular outlet of a manifold channel of an injection molding apparatus. In each case, an unobstructed melt redistribution element is used that does not require the mechanical separation of an incoming melt stream. 
     According to an embodiment of the present invention, there is provided an injection molding apparatus including a manifold having a manifold channel with an inlet for receiving a melt stream of moldable material under pressure and a plurality of outlets, a melt redistribution element having an inlet and an outlet communicating with the manifold channel, and a plurality of nozzles each having a nozzle channel for receiving the melt stream from the outlet of the manifold channel. 
     The melt redistribution element has an unobstructed melt bore that is further provided with a melt bore surface having a helical melt pathway. The incoming melt stream is first subjected to a pressure increase, alternatively referred to as a pressure drop, and then flows through the melt bore of the melt redistribution element and through the helical melt pathway. 
     According to another embodiment of the invention, the melt redistributing method includes providing a melt redistribution element that: a) provides a non-invasive local pressure increase of the melt by reducing the diameter of the manifold melt channel at the inlet and b) provides a non-invasive melt splitting and redirection of the melt stream by allowing a central portion of the pressurized melt to continue to follow a straight pathway and forcing an outer portion of the melt stream to follow a helical melt pathway. A certain degree of mixing between the central portion of the melt stream and the outer portion of the melt stream may occur while the melt flows through the melt redistribution element. The degree of mixing depends of various factors and variables such as the level, or the amount, of the pressure increase, the length of the melt redistribution element&#39;s melt bore and the helical melt pathway, the pitch or the density of the helical pathway and the viscosity and the temperature of the melt. More mixing and better homogenizing is achieved with some molten materials when the melt redistribution element has more than one helical melt pathway. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which like reference numerals indicate similar structure. 
         FIG. 1  is a side view partly in section of a conventional multi-channel manifold of an injection molding apparatus. 
         FIG. 1A  is a cross-sectional shear profile taken along a line A-A in  FIG. 1 ;  FIG. 1B  is a cross-sectional shear profile taken along a line B-B in  FIG. 1 ; and  FIG. 1C  is a cross-sectional shear profile taken along a line C-C in  FIG. 1 . 
         FIG. 2  is a side sectional view of an embodiment of an injection molding apparatus according to the present invention. 
         FIG. 3  is an enlarged view of a portion of  FIG. 2 .  FIG. 3A  is an alternative enlarged view of the portion of  FIG. 2  illustrated in  FIG. 3 .  FIG. 3B  is another alternative enlarged view of a portion of  FIG. 2  illustrated in  FIG. 3 .  FIG. 3C  is another alternative enlarged view of a portion of  FIG. 2  illustrated in  FIG. 3 . 
         FIG. 4  is an enlarged view of a portion of  FIG. 2 . 
         FIG. 5  is an enlarged view of a portion of another embodiment of an injection molding apparatus according to the present invention. 
         FIG. 6  is a side sectional view of another embodiment of an injection molding apparatus according to the present invention.  FIG. 6A  is a side sectional view of a portion of another embodiment of injection molding apparatus according to the present invention. 
         FIG. 7  is a side sectional view of another embodiment of an injection molding apparatus according to the present invention. 
         FIG. 8  is a side sectional view of another embodiment of an injection molding apparatus according to the present invention. 
         FIG. 9  is a side sectional view of a portion of another embodiment of an injection molding apparatus according to the present invention. 
         FIG. 10  is a side sectional view of another embodiment of an injection molding apparatus according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 2 , an injection molding apparatus according to an embodiment of the invention is generally shown at  210 . Injection molding apparatus  210  includes a two level manifold  212  having manifold channels  214 . As shown, manifold channels  214  are in communication with an inlet melt channel  202  that splits into at least two melt branches  203  downstream of a manifold inlet  218 . Melt from channel  202  enters a melt redistribution element  252  within each branch  203 . Melt redistribution element  252  will be discussed in further detail below, with respect to melt redistribution element  352  of  FIG. 3 . Each branch  203  turns a corner  204  and splits again into branches  205  and  206 . Manifold  212  is spaced between a mold block back plate  222  and a cavity mold plate  224 . Manifold  212  is located relative to the cavity mold plate  224  by a locating ring  226 . A sprue bushing  216  is coupled to the manifold inlet  218 . The sprue bushing  216  receives melt from a machine nozzle (not shown) and delivers the melt through manifold inlet  218  to channel  202  of manifold  212 . The melt travels through branches  203 ,  205  and  206 . Before exiting manifold  212  through outlets  208  and  209  of branches  205  and  206 , respectively, melt travels again through melt redistribution elements  253 . Melt redistribution element  253  also will be discussed in further detail below with respect to melt redistribution element  453  of  FIG. 4 . Manifold  212  is heated by a manifold heater  228 . 
     Nozzles  230  are received in openings  232  in a mold plate  221 . Nozzles  230  are heated by heaters  236 . Each nozzle  230  includes a nozzle channel  234  for receiving melt from a respective outlet of manifold channel  214  and delivering the melt through a mold gate  238  to a respective mold cavity  240 . Mold cavities  240  are provided between cavity mold plate  224  and a mold core  225 . Cooling channels  242  are provided to cool mold cavities  240 . 
     According to an embodiment of the invention, manifold  212  is a two-level manifold for a multi-cavity injection molding apparatus. However, an injection molding apparatus may have a manifold with any number of channels leading to any number of cavities wherein a shear stress profile of a melt flow within a channel is imbalanced. 
     Melt redistribution element  252  according to an embodiment of the current invention is illustrated in more detail in  FIG. 3  as melt redistribution element  352 . Melt redistribution element  352  is located, in one embodiment of the invention, in a manifold  312  having a melt channel  303  therein. Melt redistribution element  352  is coaxial with manifold melt channel  303  along central axis  315 . Melt redistribution element  352  includes a body portion  354  and a central melt bore  363 . Melt bore  363  has an inlet  362  and an outlet  364 . Melt bore  363  includes a first bore section  355 , an inlet tapered section  357 , a second bore section  366 , an outlet tapered section  361  and a third bore section  356 . The melt redistribution element  352  also includes a helical melt pathway  358  formed into body portion  354  along a surface  360  of second bore section  366 . Helical melt pathway  358  also includes a depth  359  measured from surface  360  of second bore section  366 . The angle of the helical melt pathway, the pitch or the density of the helical turns, and distance  359  may be varied depending upon the application for which melt redistribution element  352  is being used. The melt travels to inlet  362  through manifold melt channel  303  having accumulated a nonsymmetrical or uneven shear stress cross-sectional profile, depending on the location of element  352 . From inlet  362 , the melt moves through first bore section  355  having a first diameter  368 . First diameter  368  is generally the same as a diameter of manifold melt channel  303 . First bore section  355  is generally aligned flush with channel  303 . Melt then travels through inlet tapered section  357 . Inlet tapered section  357  has a gradually reducing diameter. 
     As the melt flows into inlet tapered section  357 , the pressure of the melt increases and thus the melt applies an increased pressure against second bore section  366 . This higher pressure forces the melt to flow at a higher velocity through melt redistribution element  352 . While a central portion of the melt flows unobstructed through second bore section  366 , an outer portion of the melt, having an uneven cross-sectional shear stress profile, is forced to follow helical melt pathway  358  that reorients the outer shear stress profile and makes it more evenly distributed. Also, the outer melt portion may be partially mixed with the central portion of the melt stream flowing unobstructed through second bore section  366  of melt redistribution element  352 . Depending on the application, the injection molding processing conditions, and the type of the melt, the melt redistribution element  352  may have a single or several helical melt pathways  358  of similar or different geometries. Unlike other melt mixers or flippers known in the art that have a mechanical obstruction or bullet therein requiring the melt to split and flow around the obstruction, redistribution element  352  allows the melt to flow unobstructed, which provides an additional advantage in color change applications. Unobstructed flow prevents the accumulation of melt and the formation of so-called “dead spots” where melt is trapped and does not flow. 
     Melt material then enters second bore section  366 , which has a second diameter  370  which is smaller than first diameter  368 . At the end of helical melt pathway  358 , melt material enters outlet tapered section  361 , which has a gradually increasing diameter. Melt then flows into third bore section  356  which has a diameter  371  about equal to diameter  368  of first bore section  355  and to a diameter of manifold melt branch  303 . Third bore section  356  is also generally flush with branch  303  at outlet  364 . 
       FIG. 3A  illustrates an alternative melt redistribution element  352   a . Melt redistribution element  352   a  is similar to melt redistribution element  352  except that it does not feature outlet tapered section  361 . Instead, helical melt pathway  358   a  and second bore section  366   a  end abruptly at outlet  364   a .  FIG. 3B  illustrates yet another alternative melt redistribution element  352   b . Melt redistribution element  352   b  is also similar to melt redistribution element  352   a  in that it does not feature outlet tapered section  361 . Also, the diameter  370   b  of second bore section  366   b  gradually increases in the downstream direction such that the diameter of second bore section  366   b  at outlet  364   b  is substantially the same as the diameter of melt channel  303  at outlet  364   b . Since depth  359   b  of helical melt pathway  358   b  is constant over the length of second bore section  366   b , the turns of helical melt pathway  358   b  become gradually larger in the downstream direction.  FIG. 3C  illustrates another alternative melt redistribution element  352   c . Melt redistribution element  352   c  is similar to melt distribution element  352   b , in that it does not have outlet tapered section  361  and diameter  370   c  of second bore section  366   c  gradually increases in the downstream direction as described with reference to element  352   b . However, depth  359   c  of helical melt pathway  358   c  is not constant along the length of second bore section  366   c . Instead, depth  359   c  of helical melt pathway  358   c  becomes gradually smaller in the downstream direction such that the outer diameter of helical melt pathway  358   c  at outlet  364   c  is substantially the same as and aligned with the diameter of melt channel  303  at outlet  364   c.    
     Melt redistribution element  352 , or any of the melt redistribution elements disclosed herein, may be made from two or more blocks, which when placed together form either a cylindrical-shaped or a square-shaped insert or plug. For example, a melt redistribution element  352  made from two blocks may have a portion of melt bore  363  machined into each block. The two blocks are then positioned adjacent one another to obtain the overall shape of melt redistribution element  352 . The blocks may be brazed, welded, bonded, or otherwise fused together or may be mechanically held together, such as by clamping, etc. Alternatively, two or more blocks may be received within a recess or bore made in a manifold or a manifold plug, as discussed in more detail below, such that the positioning of the blocks and thermal expansion due to heating of the manifold could be used to hold the two the blocks together. In another embodiment, melt redistribution element  352  may be cast, such that melt bore  363  is formed within a block. In another embodiment, one skilled in the art can appreciate a complex boring process may be used to form melt bore  363  of melt redistribution element  352  within a single block. In yet another embodiment, melt bore  363  of melt redistribution element  352  may be formed along with, and as part of, a portion of manifold channel, such as manifold channel  214  of  FIG. 2 . 
     According to another embodiment of the invention, melt redistribution elements  453  are shown in  FIG. 4  disposed between the two-level manifold  412 , similar to manifold  212  of  FIG. 2 , and nozzles  430 . Both redistribution elements  453  are the same and communicate with branches  405  and  406  of manifold  412 , respectively. One skilled in the art will appreciate that other melt redistribution elements  453  may be provided in manifold  412  having a plurality of manifold outlets. Melt redistribution elements  453  operate identically to melt redistribution element  352  discussed in detail above. Also, the internal structure of melt redistribution elements  453  are similar to that discussed above with respect to melt redistribution element  352 . Similarly, melt redistribution elements  453 , or any of the melt redistribution elements disclosed herein, may have an internal structure similar to those described above with respect to any of melt redistribution elements  352   a ,  352   b  and  352   c  of  FIGS. 3A-3C . 
     The external structure of redistribution elements  453 , however, are particularly adapted for easy installation at outlets  408  and  409  of manifold  412 . Melt redistribution elements  453  can be press-fit, shrink-fit, brazed or threaded into manifold  412 . Melt redistribution elements  453  according to this embodiment can be also used to retrofit existing manifolds as they are very easy to install, align, clean and eventually replace after a lengthy service. 
     Melt redistribution elements  453  can be added downstream of redistribution elements  352 . Also, “annular flow,” where melt flows around an inserted device, for example a torpedo, positioned within a channel, is prevented. Because the melt does not split, flow lines are not created in the melt stream using melt redistribution elements  453 . 
     The melt redistribution elements disclosed herein, such as elements  453 , may be made of various materials, such as carbides and stainless steel that provide wear resistance in case glass-filled and other abrasive melt materials are used. In other applications, the redistribution elements, such as melt redistribution elements  453 , are made of high, thermally-conductive materials, such as copper and copper alloys. In particular, melt redistribution elements  453 , if located in the proximity of the manifold heater  428  as illustrated in  FIG. 4 , will improve the heat transfer to a nozzle head. 
     A portion of another embodiment of the invention is shown in an injection molding apparatus  510  illustrated in  FIG. 5 . Although only a portion of the apparatus  510  is shown, it will be appreciated that the apparatus  510  is generally similar to apparatus  210  of  FIG. 2 . In injection molding apparatus  510 , a manifold plug  572  is received in a bore  574 , which is provided in a manifold  512 . Plug  572  allows a melt redistribution element  552  to be inserted more easily within a melt channel  514  of single piece manifold  512 . Plug  572  includes a melt channel  576  that extends therethrough along a flow axis  578 . Melt redistribution element  552  is received in a bore  580  of the plug  572 . Melt redistribution element  552  is similar or equivalent in function and structure to those described above for melt redistribution elements  352 ,  352   a ,  352   b  or  352   c , shown in FIGS.  3  and  3 A- 3 C. 
     In another embodiment of the invention, melt redistribution element  552   a  is formed as a portion of manifold plug  572   a  and is illustrated in  FIG. 5A . In this arrangement, helical melt pathway  558  is cut into, or formed as an integral part of, the manifold plug  572   a . The melt redistribution element  552   a  is oriented with respect to the melt channel  514  using a dowel  511  or other fastener, such as a bolt. 
       FIG. 6  illustrates another embodiment of an injection molding apparatus  610 , having a one-level manifold  612 . Manifold  612  includes a manifold channel  614  with at least two branches  603   a  and  603   b , yet still imbalances occur such that a melt redistribution element  652  is useful. Alternatively, manifold  612  may be a two-level manifold similar to that described above with respect to manifold  212  of  FIG. 2 . 
     In the embodiment of  FIG. 6 , melt redistribution element  652  is placed within the manifold prior to assembling the manifold in its final form. Manifold  612  is a split manifold having a first manifold plate  644  and a second manifold plate  646 . Recesses  648 ,  650  are provided in first manifold plate  644  and second manifold plate  646 , respectively, for receiving melt redistribution element  652 . Manifold plates  644 ,  646  are welded, fused, brazed, bonded or otherwise fused together, for example as described in U.S. Pat. No. 4,648,546, which is herein incorporated by reference in its entirety. Alternatively, plates  644 ,  646  may be connected or positioned together by another manner apparent to one skilled in the art. For example, they may be mechanically clamped or positioned with close tolerances within injection molding apparatus  610  so as to have the effect of being fused. Although only one melt redistribution element  652  is shown in branch  603   a , it will be appreciated by a person skilled in the art that a melt redistribution element may be provided in each of branches  603   a  and  603   b  of the manifold channel  614 . In  FIG. 6A , melt redistribution element  652   a  is located in the manifold melt channel  603   a  with no need to form recess in the manifold body. Because of its fit entirely within melt channel  603   a , melt redistribution element  652   a  does not include first bore section  355  or third bore section  356 , as illustrated in melt redistribution element  352  of  FIG. 3 . A locating dowel  611  or other fastener, is used to retain the redistribution element in a fixed position. 
     Although melt redistribution elements are shown in  FIGS. 2 ,  4  and  6  in distinct locations, one skilled in the art can appreciate that they may appear in a variety of positions along a manifold melt channel. For example, in a manifold  712  of an injection molding apparatus  710 , as illustrated in  FIG. 7 , melt redistribution element  752  is instead positioned within branch  703  of a melt channel  714  downstream of turn  704 . In this position, melt redistribution element  752  will redistribute any shear imbalance that may have generated in a melt stream from such a turn. Likewise, as shown in an injection molding apparatus  810  of  FIG. 8 , melt redistribution elements  853  may be positioned anywhere within branches  805  and  806 , such as upstream of turns  807 . 
       FIG. 9  shows yet another injection molding apparatus  910  wherein only melt redistribution elements  952  are present within a manifold  912 . Likewise,  FIG. 10  shows an injection molding apparatus  1010  wherein only melt redistribution elements  1053  are present within a manifold  1012 . Since the different melt redistribution elements, such as melt redistribution element  952  of  FIG. 9  and melt redistribution element  1053  of  FIG. 10 , serve different purposes within a two-level manifold, one skilled in the art can appreciate that only one may be useful in a particular manifold design. Melt redistribution elements  752 ,  853 ,  952  and  1053  are not illustrated in detail because any of the melt redistribution elements disclosed herein are suitable for use in the positions as illustrated in  FIGS. 7-10 . 
     The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.