Abstract:
A method and apparatus for rotating a cross-sectional asymmetrical condition of a laminar flowing material is provided in a hot runner system for supplying a laminar flowing material. The hot runner system has (i) an upstream melt passage, (ii) a pair of intermediary melt passages downstream from the upstream melt passage, and (iii) for at least one intermediary melt passage, an associated pair of downstream melt passages downstream from the at least one intermediary melt passage. The cross-sectional asymmetrical condition of a laminar flowing material is rotated by providing a bending path for orienting at least one path outlet relative to a path inlet to rotate the cross-sectional asymmetrical condition of the laminar flowing material such that the cross-sectional asymmetrical condition is substantially equally divided between the two downstream portions.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to an injection molding apparatus and, more specifically, to a multi-cavity injection molding apparatus that includes a balanced hot runner layout in a stack mold or manifold.  
         BACKGROUND OF THE INVENTION  
         [0002]    The use of stack molds and manifolds in a hot runner injection molding apparatus is well known. Furthermore, it is well known that in many applications it is important that a runner layout be provided such that each cavity receives the same flow of melt having the same temperature and the same composition. Balancing of the runner system results in overall higher quality molded parts because consistency is achieved from mold cavity to mold cavity in a multi-cavity application. Even in multi-runner, single cavity applications, the benefits of balancing are well known and important.  
           [0003]    A well-known technique for balancing a manifold or stack mold is to match runner diameters and lengths and to match the number of turns in the runners, so that the pressure drop through the manifold or stack mold to each cavity is the same. Occasionally, however, different flows are provided to different cavities, in spite of the runner layout having matched runner lengths and turns. This is at least partly due to a combination of shear heating of the melt flow combined with the layout of the runner system.  
           [0004]    When melt is forced under pressure through a bore, as is done in a hot runner system whether in a manifold or a stack mold, the melt experiences friction or shear in the area adjacent to the channel wall. This results in a localized elevation of the temperature of the melt. The result is a differential in temperature across the bore, with the center of the channel being cooler than the material closer to the bore. Many hot runner systems split the melt flow from a primary runner through two or more secondary runners. When this occurs, the heat distribution profile in the melt is divided as well. This occurs because the flow through the runners is laminar, and therefore the shear-heated material remains adjacent to the wall as the corner is turned. After the corner, the heated peripheral portion is no longer annular, but is instead generally crescent-shaped and remains on one side of the melt flow. The mass flow through each of the secondary runners is substantially equal; however, the heated peripheral portion in each secondary runner is asymmetrically distributed about the periphery. If, as is usually the case, each secondary runner is divided into a plurality of tertiary runners, the asymmetric heated peripheral portion may be unequally divided between these plurality of tertiary runners. As a result, the material flowing into one of the tertiary runners from a secondary runner may include a higher proportion of shear-heated material compared to the melt flowing into the other of the tertiary runners downstream from that secondary runner. This phenomenon can, in some applications, cause preferential flow to some drop locations, and can cause out-of-spec product from portions of a molding machine. Specifically, there will typically be preferential flow to the tertiary runner receiving a higher proportion of shear-heated material from its upstream secondary runner compared to the other of the tertiary runners fed by that secondary runner.  
           [0005]    This problem of asymmetric division of shear-heated material has been recognized in a cold runner context; however, it has not been as clearly recognized in a hot runner context. That is, melt in a hot runner is typically less viscous than in a cold runner. As a result, shear-induced heating has been thought of as less of a problem, as there is less resistance to shear. Instead, imbalance of flow in hot runner context has been attributed to other factors.  
           [0006]    Different devices have been developed to address the problem for both cold runner and hot runner applications. In cold runner injection, the mold component includes the runners as well as the mold cavities. The mold component is made up of two halves that mate together. All the runners and the mold cavities lie in the plane of the mating faces of the two halves. At the end of an injection cycle the mold component is parted and the molded parts and the solidified melt in the runners are ejected. Cold runner layouts are typically simple in nature since all the runners lie on a common plane.  
           [0007]    For cold runner applications, U.S. Pat. No. 4,123,496 to Gallizia et al. discloses an equalization device in a conduit carrying a melt flow, wherein different portions of the melt flow are reoriented to achieve a relatively uniform heat distribution in the melt flow.  
           [0008]    U.S. Pat. No. 6,077,470 to Beaumont discloses a similar device for achieving similar balancing results in cold runner applications. Beaumont discloses a device for achieving balanced melt flow in cold runner applications. The device is positioned upstream of the split that first produces an asymmetric flow. Beaumont&#39;s device applies to cold runners particularly because the device is dependent on the simple, planar nature of the cold runner mold. Beaumont&#39;s device, for example, would not be applicable in a situation in which one of the runners from a downstream split extended out of the plane of the parting line of the mold component.  
           [0009]    Hot runner stack mold systems typically include a plurality of mold components, which taken together, define a hot runner system and a plurality of mold cavities. Similarly, hot runner manifolds provide a hot runner system for providing melt to a plurality of mold cavities. In a stack mold, the primary runner or upstream runner of the runner system is typically provided by passages in a first, second and third mold component. In such stack molds, the second mold component is between and adjoins the first mold component and the third component. In the third mold component, the primary runner divides into two secondary runners. One of these secondary runners proceeds back into the second mold component where it divides into a pair of tertiary runners. The other of the secondary runners projects into the fourth mold component where it divides into two tertiary runners. The tertiary runners in the second and fourth mold components then provide melt to cavities in the third mold component. In operation, the second, third and fourth mold components are separable to eject the formed product from the mold cavities. Because of the differences in hot runner and cold runner systems, cold runner technologies are not typically applied to hot runner molding machines. For hot runner systems, other devices have been developed.  
           [0010]    European Patent Application No. 963,829 to Goldwin et al. discloses the use of cylindrical heaters positioned at different points in a hot runner manifold. The heaters are positioned around the runners themselves and heat the melt passing through the runners so that cooler portions of the melt flow are heated to a temperature similar to the shear-heated portions of the melt flow.  
           [0011]    U.S. Pat. No. 5,683,731 to Deardurff et al. discloses a device for use in hot runner manifolds having a double X layout. The device separates the hotter portion of an asymmetric shear-heated melt flow and redistributes it into each runner of the X, so that the runners receive melt having roughly equal temperatures.  
           [0012]    Also for hot runner systems, it is known to pass a melt flow through one or more static mixers positioned in the runners. This creates a relatively uniform heat distribution so that any downstream split in a runner system divides the heat content in the melt flow generally equally in the runners after the split. Many injection molders, however, perform color changes during production runs and cannot tolerate cross-contamination between successive colors. Static mixers have complex internal structures, and are therefore difficult and time consuming to clean, making them poorly suited to many injection molding applications, such as those where color changes are common and cross-contamination is not tolerated.  
           [0013]    Accordingly, there is a need for a hot runner system that offers improved balancing of the resin melt flows while facilitating efficient resin color changes between molding operations.  
         SUMMARY OF THE INVENTION  
         [0014]    An object of an aspect of the present invention is to provide an improved injection molding method.  
           [0015]    In accordance with this first aspect of the present invention, there is provided a method of controlling a cross-sectional asymmetric condition of a laminar flowing material. The method comprises (a) providing a hot runner system, the hot runner system having an upstream melt passage, a plurality of intermediary melt passages downstream from the upstream melt passage, and for at least one of the intermediary melt passage, an associated pair of downstream melt passages downstream from the intermediary melt passage; (b) providing the laminar flowing material to the hot runner system; (c) heating the laminar flowing material within the hot runner system; and, (d) for the at least one intermediary melt passage, orienting one of (i) the cross-sectional asymmetric condition of the laminar flowing material in the intermediary melt passage, and (ii) the associated plurality of downstream melt passages, such that the cross-sectional asymmetric condition is substantially equally divided between the associated two downstream melt passages.  
           [0016]    An object of a second aspect of the present invention is to provide an improved injection molding apparatus.  
           [0017]    In accordance with this second aspect of the present invention, there is provided an injection molding apparatus comprising: (a) a hot runner system for supplying a laminar flowing material, the hot runner system having (i) an upstream melt passage, (ii) a plurality of intermediary melt passages downstream from the upstream melt passage, and (iii) for at least one intermediary melt passage, an associated pair of downstream melt passages downstream from the intermediary melt passage; (b) for the upstream melt passage and the at least one intermediary melt passage, a flow path for orienting the cross-sectional asymmetric condition of the laminar flowing material in the at least one intermediary melt passage such that the cross-sectional asymmetric condition is substantially equally divided between the associated pair of downstream melt passages; and, (c) a plurality of hot runner nozzles in communication with and downstream from the downstream melt passages.  
           [0018]    An object of a third aspect of the present invention is to provide an improved injection molding apparatus.  
           [0019]    In accordance with this third aspect of the present invention, a flow rotator for rotating a cross-sectional asymmetrical condition of a laminar flowing material is provided in a hot runner system for supplying a laminar flowing material. The hot runner system has (i) an upstream melt passage, (ii) a pair of intermediary melt passages downstream from the upstream melt passage, and (iii) for at least one intermediary melt passage, an associated pair of downstream melt passages downstream from the at least one intermediary melt passage. The flow rotator comprises: (a) an inlet for receiving the laminar flowing material; (b) at least one outlet for discharging the laminar flowing material; and, (c) a bending path for orienting the at least one outlet relative to the inlet to rotate the cross-sectional asymmetrical condition of the laminar flowing material such that the cross-sectional asymmetrical condition is substantially equally divided between the two downstream portions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings, showing articles made according to a preferred embodiment of the present invention, in which:  
         [0021]    [0021]FIG. 1 a , in a side view, illustrates a portion of a stack mold in accordance with the prior art;  
         [0022]    [0022]FIG. 1 b , in a schematic side view, illustrates a hot runner manifold in accordance with the prior art;  
         [0023]    [0023]FIG. 2 is a view on A-A of FIG. 16;  
         [0024]    [0024]FIG. 3, in a sectional view, illustrates the melt flow in a primary runner of the hot runner manifold of FIG. 1 b  as it branches into a pair of secondary runners;  
         [0025]    [0025]FIG. 4, is a view on B-B of FIG. 16;  
         [0026]    [0026]FIG. 5, is a view on C-C of FIG. 16;  
         [0027]    [0027]FIG. 6, in a sectional view, illustrates the melt flow of FIG. 4 as the secondary runner branches into a pair of tertiary runners;  
         [0028]    [0028]FIG. 7, is a view on D-D on FIG. 1 b;    
         [0029]    [0029]FIG. 8, is a view on E-E of FIG. 1 b;    
         [0030]    [0030]FIG. 9, in a plan view, illustrates a flow-rotating plug in accordance with an embodiment of the invention;  
         [0031]    [0031]FIG. 10, in a perspective view, illustrates the flow-rotating plug of FIG. 8;  
         [0032]    [0032]FIG. 11 a , in a side view, illustrates a portion of a stack mold having the plug of FIG. 9 installed at the juncture of the primary runner with the pair of secondary runners;  
         [0033]    [0033]FIG. 11 b , in a schematic side view, illustrates a manifold having the plug of FIG. 9 installed at the juncture of the primary runner with the pair of secondary runners;  
         [0034]    [0034]FIG. 12 is a view on B′-B′ of FIG. 11 b;    
         [0035]    [0035]FIG. 13 is a view on C′-C′ of FIG. 11 b;    
         [0036]    [0036]FIG. 14 in a sectional view, illustrates the melt flow in the secondary runner of FIG. 12 as this melt flow is divided between two tertiary runners;  
         [0037]    [0037]FIG. 15 is a view on D′-D′ of FIG. 11 b;    
         [0038]    [0038]FIG. 16 is a view on E′-E′ of FIG. 11 b;    
         [0039]    [0039]FIG. 17, in a side sectional view, illustrates a portion of a stack mold injection molding apparatus having a second plug in accordance with a further aspect of the present invention installed at the juncture of a secondary runner with its downstream tertiary runners;  
         [0040]    [0040]FIG. 18, in a sectional view, illustrates the second plug of FIG. 17;  
         [0041]    [0041]FIG. 19, is a view on G-G of FIG. 17;  
         [0042]    [0042]FIG. 20, is a view on H-H of FIG. 17;  
         [0043]    [0043]FIG. 21, is a view on I-I of FIG. 18;  
         [0044]    [0044]FIG. 22, is a view on J-J of FIG. 18;  
         [0045]    [0045]FIG. 23, in a schematic view, illustrates a portion of a hot runner system in accordance with an aspect of the invention;  
         [0046]    [0046]FIG. 24, is a view on L-L of FIG. 23;  
         [0047]    [0047]FIG. 25, is a view on M-M of FIG. 23;  
         [0048]    [0048]FIG. 26, is a view on N-N of FIG. 23;  
         [0049]    [0049]FIG. 27, is a view on O-O of FIG. 23;  
         [0050]    [0050]FIG. 28, in a schematic view, illustrates a hot runner system incorporating an auxiliary plug in accordance with a further aspect of the present invention;  
         [0051]    [0051]FIG. 29, in a sectional view, illustrates the auxiliary plug of FIG. 28;  
         [0052]    [0052]FIG. 30, is a view on M′-M′- of FIG. 28;  
         [0053]    [0053]FIG. 31, is a view on N′-N′ of FIG. 28; and,  
         [0054]    [0054]FIG. 32, is a view on O′-O′ of FIG. 30. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0055]    Referring to FIG. 1 a , there is illustrated in a side view, a portion of a stack injection molding apparatus  10  in accordance with the prior art. The stack injection molding apparatus  10  includes a runner system  12  including a manifold  14 . The runner system  12  includes a primary runner  16  for receiving melt from a melt source (not shown). At a first branch  18 , the primary runner  16  branches into two secondary runners  20 . The secondary runners  20 , at respective second branches  22 , then branch into tertiary runners  24 . The tertiary runners  24  supply melt to associated nozzles  26 , which inject the melt into associated mold cavities (not shown).  
         [0056]    Referring to FIG. 1 b , there is illustrated in a side view, a manifold  14   b  of an injection molding apparatus  10   b  in accordance with the prior art. The manifold  14   b  includes a runner system  12   b . The runner system  12   b  includes a primary runner  16   b  for receiving melt from a melt source (not shown). At a first branch  18   b , the primary runner  16   b  branches into two secondary runners  20   b . The secondary runners  20   b , at respective second branches  22   b , then branch into tertiary runners  24   b . The tertiary runners  24   b  supply melt to associated nozzles (not shown), which inject the melt into associated mold cavities (not shown).  
         [0057]    In the description that follows, like reference numerals are used to refer to analogous elements of the runner systems  12   a  and  12   b  of the injection molding apparatus  10  and the manifold  14   b , respectively. The runner systems  12 , 12   b  differ, however, both include primary  16 , 16   b , secondary  20 , 20   b , and tertiary  24 ,  24   b  runners, through which the melt flow characteristics are similar. FIGS.  2 - 8  are described with respect to the manifold  14   b  only for simplicity.  
         [0058]    Referring to FIG. 2, there is illustrated a sectional view of primary runner  16   b  at A-A of FIG. 1 b . A heated peripheral portion  28  of the melt around the runner wall of the primary runner  16   b  is shown by shading. As can be seen from FIG. 2, the hotter melt adjoining the runner wall is substantially uniformly distributed about the runner wall.  
         [0059]    At the first branch  18   b , the heated peripheral portion  28  of the flow is divided into two, as shown in FIG. 3. Each of these halves of the heated periphery then flows into the secondary runners  20   b  of the manifold  14   b.    
         [0060]    Referring to FIGS. 4 and 5, there is illustrated in sectional views, the melt flow at sections B-B and C-C in the secondary runners  20   b  of FIG. 1 b . As shown in FIGS. 4 and 5, in the secondary runners  20   b , the heated periphery  28  is no longer substantially uniformly distributed about the runner wall. Instead, the portion of the runner wall that is closer to the side at which the primary runner  16   b  connects to the secondary runner  20   b  receives most of the heated periphery  28  from the primary runner  16   b . Consequently, this side of the secondary runner  20   b  will have more heated melt than the opposite side of the secondary there of. However, as both secondary runners  20   b  receive substantially the same proportions of heated and relatively unheated melt, there will be substantially equal mass flow through the two secondary runners  20   b . This will not, necessarily, be the case where the secondary runners  20   b  branch at the second branch  22   b  into the tertiary runners  24   b.    
         [0061]    Referring to FIG. 6, melt flow downstream from section B-B, at the second branch  22   b  where the secondary runner  20   b  branches into two tertiary runners  24   b , is illustrated. As can be seen from the sectional view, the heated peripheral portion  28  is more symmetrical than it was at the section B-B, due to heating of the relatively unheated side; however, there remains a substantial asymmetry in terms of the degree of heating on either side of the secondary runner  20   b . This substantial asymmetry is, at the second branch  22   b , unequally divided as shown in FIG. 6. Referring to FIG. 7 and to FIG. 8, the downstream consequences of this asymmetric division are illustrated in the sectional views D-D and E-E.  
         [0062]    Referring to FIG. 7, the sectional view D-D illustrates that this tertiary runner  24   b  receives a disproportionate share of the heated peripheral melt  28 , while the sectional view E-E shown in FIG. 8 indicates that the opposite tertiary runner  24   b  receives substantially less of the heated peripheral melt  28 . The heated melt moves faster as it is less viscous. As a result, the tertiary runner  24   b  including section D-D will receive more melt than the tertiary runner  24   b  including section E-E, resulting in a flow imbalance between the nozzles and cavities supplied by these respective tertiary runners  24   b . A similar situation arises with respect to the tertiary runners  24   b  fed by the opposite secondary runner  20   b.    
         [0063]    Referring to FIG. 9, there is illustrated in a sectional view, a flow-rotating plug  30  in accordance with an embodiment of the invention. The flow rotating plug  30  is installed at the juncture of the primary runner  16   b  and the pair of secondary runners  20   b  in the manifold  14   b . The flow-rotating plug  30  includes an inlet  32  and an inlet conduit  34  that branches into two outlet conduits  36 . Each of the outlet conduits  36  leads to a separate outlet  38 . The inlet conduit  34  follows an arcuate path. As a result, the intersection of the axis of the inlet conduit  34  with the axes of the two outlet conduits  36  is in a plane substantially perpendicular to the plane in which the primary runner  16   b  intersects the two secondary runners  20   b . The outlet conduits  36  then curve back toward the secondary runners  20   b , such that the outlets  38  release the melt into the secondary runners  20   b . However, due to the plane of the branch within the plug  30  being perpendicular to the plane of the first branch  18   b , the heated peripheral melt  28  will be rotated 90 degrees. Referring to FIG. 10, the plug  30  of FIG. 9 is shown in a perspective view with hidden details shown using dashed lines.  
         [0064]    Referring to FIG. 11 a , another embodiment of a stack injection molding apparatus  100  is generally shown. The stack injection molding apparatus  100  is similar to the stack injection molding apparatus  10  of FIG. 1 a , however, further includes the flow rotating plug  30  of FIG. 9. The plug  30  is installed at first branch  118  between primary runner  116  and secondary runners  120 .  
         [0065]    Referring to FIG. 11 b , a manifold  114   b  incorporating the flow-rotating plug  30  of FIG. 9 is shown at a first branch  118   b  between a primary runner  116   b  and a pair of secondary runners  120   b  in the runner system  112   b.    
         [0066]    The melt flow through the primary  116   b , secondary  120   b  and tertiary  124   b  runners will now be described with respect to FIGS.  12 - 18  it will be appreciated that the melt flow through the primary  11   b , secondary  120  and tertiary  124  runners of FIG. 11 a  is similar and therefore will not be described separately.  
         [0067]    The section of the melt taken at A′-A′ in FIG. 11 b  showing the melt in the primary runner  116   b  upstream from the first branch  118   b  and the plug  30  will be the same as the section A-A shown in FIG. 2. However, downstream from the first branch  118   b  and plug  30 , the sectional views differ. Referring to FIG. 12, there is illustrated in a sectional view, the melt in the secondary runner  120   b  at section B′-B′. The heated peripheral portion  28  of the secondary runner  120   b  is shown as shaded in. Comparing FIG. 12 to FIG. 3, it is apparent that the shaded portion has been rotated 90 degrees. Similarly, the sectional view at C′-C′ of FIG. 11 b  is shown in FIG. 13. Compared to the analogous sectional view C-C shown in FIG. 4, the sectional view C′-C′ shown in FIG. 13 is rotated 90 degrees. In FIG. 11 b , the shaded portions at sections B′-B′ and C′-C′ are both in the plane of the page, as opposed to being in a plane perpendicular to the page. Consequently, they will both be evenly divided at a second branch  122   b  where the secondary runners  120   b  divide into respective pairs of tertiary runners  124   b . Each of the tertiary runners  124   b  in turn supplies melt to an associated nozzle (not shown).  
         [0068]    Referring to FIG. 14, there is illustrated in a sectional view, melt downstream from the sectional view B′-B′ at the second branch  122   b  where the secondary runner  120   b  branches into two tertiary runners  124   b . As with the sectional view of FIG. 6, the melt is asymmetrically distributed about the periphery of the secondary runner  120   b . However, in FIG. 14, this asymmetrical distribution is symmetrically divided such that each of the tertiary runners  124   b  will receive substantially equal halves of the heated melt. Referring to FIGS. 15 and 16, there is illustrated in sections D′-D′ and E′-E′ respectively, the melt flow in the tertiary runners  124   b . As is apparent from FIGS. 15 and 16, the heated melt is substantially equally divided between these two tertiary runners  124   b . Accordingly, the amount of melt provided to the associated nozzles by these tertiary runners  124   b  will be substantially equal, and of substantially the same temperature.  
         [0069]    According to other embodiments of the invention, the flow is not rotated between the primary runner  116   b  and secondary runners  120   b , but is instead rotated between the secondary runners  120   b  and tertiary runners  124   b . However, this requires a different configuration of flow-rotating plug  30 .  
         [0070]    Referring to FIG. 17, there is illustrated in a sectional view a portion of a runner system  212  of a stack injection molding apparatus  200 . The stack injection molding apparatus  200  is similar to the stack injection molding apparatus  10  of FIG. 1 a , however, it incorporates a second plug  230  in accordance with a further embodiment of the invention. The plug  230  could alternatively be installed in the manifold  14   b ,  114   b  of FIGS. 16 and 11 b , respectively. This second plug  230  is also illustrated in the sectional view of FIG. 18. The section of the melt taken in a primary runner  216  upstream from a first branch  218  between the primary runner  216  and the secondary runners  220  at section G-G is shown in FIG. 19. As shown, this section is the same as the section A-A as shown in FIG. 2. In the runner system  212 , there is no plug between the primary runner  216  and the secondary runners  220 . Accordingly, the section H-H in the secondary runner  220  upstream from the second plug  230  as shown in FIG. 20 is the same as the section B-B shown in FIG. 4. Thus, as shown in FIGS. 7 and 8, a heated peripheral portion  228  will be unequally divided between tertiary runners  224  (shown in FIGS. 21 and 22) unless it is rotated. Accordingly, in FIG. 17, the second plug  230  is installed at the juncture between the secondary runner  220  and tertiary runners  224  to rotate the heated peripheral portion  228  such that it is equally divided between the two downstream tertiary runners  224 .  
         [0071]    Referring back to FIG. 18, the second plug  230  includes an inlet  232 , an inlet passage  234 , two outlet passages  236  in fluid communication with the inlet passage  234 , an internal branch  240  between the inlet passage  234  and the two outlet passages  236 , and two outlets  238  at the downstream ends of the two outlet passages  236 . The inlet passage  234  is curved, and the internal branch  240  is located, so as to substantially equally divide the peripheral heated portion  228  in the inlet passage  234  between the two outlet passages  236 . As a result, substantially equal amounts of flow are provided to the tertiary runners  224  via the outlet passages  236  and outlets  238  of the second plug  230 . Sections of the melt downstream from the second plug  230  at sections I-I and J-J are shown in FIGS. 21 and 22 respectively. As can be seen from FIGS. 21 and 22, the peripheral heated portion  228  of the melt is equally divided between the tertiary runners  224 .  
         [0072]    Some stack molds and manifolds may comprise more than three levels of runners. That is, in addition to primary, secondary and tertiary runners, they may include quaternary runners. In such runner systems, each of the tertiary runners branches into a quaternary runner, which quaternary runner in turn supplies melt to an associated nozzle. In such hot runner systems, depending on the asymmetry existing in the tertiary runner, it may be necessary to include secondary plugs downstream from a primary plug, which primary plug may be the plug of FIG. 9 or the plug of FIG. 17. That is, the primary plug rotates an initial asymmetry such that it is symmetrically divided between tertiary runners. However, the secondary plug must subsequently rotate that asymmetry in the tertiary runners to ensure that it is substantially equally divided between the quaternary runners. Unlike the above-described primary plugs, this secondary plug does not rotate the flow by 90 degrees, as the asymmetry must only be rotated 45 degrees in one direction or 135 degrees in the other direction, to be symmetrically divided between the two quaternary runners.  
         [0073]    Referring to FIG. 23, there is illustrated in a schematic view, a runner system  312  of a stack injection molding apparatus in accordance with an aspect of the present invention. Melt within a secondary runner  320  of the runner system  312  of FIG. 23 is illustrated in the sectional view of FIG. 24 along the line L-L. As is apparent from FIG. 24, the melt within the secondary runner  320  has been rotated such that it is aligned for a substantially equal division between the tertiary runners  324  downstream from the secondary runner  320 .  
         [0074]    Melt flow at a section M-M of a tertiary runner  324  downstream from the secondary runner  320  is shown in FIG. 25. From this drawing, it is apparent that the heated peripheral portion  328  at section M-M is not positioned for equal division between quaternary runners  325  downstream from this tertiary runner  324 . Melt flow within one of these quaternary runners  325  at section N-N is illustrated in FIG. 26, and melt flow in the other of these quaternary runners  325  at section O-O is illustrated in FIG. 27. As can be seen from these figures, the peripheral heated portion  328  shown in FIG. 25 is unequally divided between these quaternary runners  325 , as the melt shown in FIG. 26 includes more of this heated peripheral portion  328  than the melt shown in FIG. 27. As a result, there may be preferential melt flow to the quaternary runner  325  of FIG. 26.  
         [0075]    Referring to FIG. 28, there is illustrated in a schematic diagram a runner system  400  of a stack injection molding apparatus including an auxiliary plug  430  (shown in FIG. 29) in accordance with a further aspect of the present invention. Referring to FIG. 29, this auxiliary plug  430  is illustrated in a sectional view. The auxiliary plug  430  includes an auxiliary plug inlet  432 , an auxiliary plug branch  440 , and two auxiliary plug outlets  438 . However, instead of including a 90 degree bend, the auxiliary plug  430  includes two 45 degree bends. In operation, the auxiliary plug  430  is installed at the juncture of secondary runner  420  and tertiary runners  424 . The auxiliary plug branch  440 , which divides the melt flow from the secondary runner  420  into two auxiliary plug passages  442  including 45 degree bends. As a result, the peripheral heated portion  428  in the tertiary runners  424  is rotated by 45 degrees to be aligned for substantially equal division between the downstream quaternary runners  425 .  
         [0076]    Referring to FIG. 30, there is illustrated in a sectional view, the melt flow at section M′-M′ downstream from the auxiliary plug  430  (the melt flow at section L′-L′ upstream from the auxiliary plug  430  is unchanged from that of FIG. 24). As shown, the melt flow has been rotated by 45 degrees from the melt flow shown at the same location in FIG. 25. When this melt flow reaches the juncture of the tertiary runner  424  with the two quaternary runners  425 , it will be equally divided between the quaternary runners  425  due to its position. The melt flow in one of the quaternary runners  425  downstream from this tertiary runner  424  is shown at section N′-N′ in FIG. 31, and the melt flow in the other of the quaternary runners  425  downstream from this tertiary runner  424  at section O′-O′ is shown in FIG. 32. From a comparison of FIGS. 31 and 32, it is apparent that the melt flow has been symmetrically divided between the two quaternary runners  425 . From FIGS. 31 and 32, it is also apparent that the shear-heated peripheral portion  428  at both sections N′-N′ and O′-O′ are positioned to be symmetrically divided if the quaternary runners  425  subsequently branch into two downstream runners that run substantially parallel to the tertiary runner  424 . However, this is not required if the quaternary runners  425  supply melt directly to their associated nozzles, but this is only an incidental feature resulting from the way the shear-heated portion  428  of the melt in the tertiary runner  424  is divided between the two quaternary runners  425 .  
         [0077]    While the preferred embodiments are described above, it will be appreciated that the present invention is susceptible to modification and change without departing from the fair meeting of the accompanying claims. For example, instead of the auxiliary plug being positioned at the juncture of the secondary runner and tertiary runners, auxiliary plugs might be positioned at the juncture of the tertiary runners and the quaternary runners, or positioned solely within the tertiary runners. Further, the invention may be implemented without using plugs. Instead of plugs, the runners in the manifold of stack mold may be configured in the same way as the plugs to rotate the melt for equal division between downstream runners. The purpose, in every case, is simply to rotate the melt within the relevant runner such that it is symmetrically divided in the next runner, or, such that it is aligned in the next runner for subsequent symmetric division in the runners immediately downstream from the next runner. All such modifications or variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.