Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to nozzles for hot runner systems of plastic injection molds, and in particular, to such nozzles equipped with dynamic seals. 
         [0002]    Multi-cavity molds use manifolds to transfer molten plastic from a central feeding point, connected to the sprue bar of the injection machine, to a plurality of injection points leading to the molding cavities. A common problem of hot runner systems is the difficulty to control the thermal expansion of the manifold while maintaining effective sealing between nozzles and manifold. Typical challenges may be: excessive thermal expansion of the components, which may cause failure due to pressures, component fatigue, and wear; insufficient thermal expansion, which may cause uneven or minimal contact between mold components, resulting in plastic leaks; or, in the style of nozzles directly screwed into the manifold, excessive lateral expansion of the manifold, causing lateral deflection of the nozzles. 
         [0003]    Previous designs depend on the axial thermal expansion of components to create the seal required to prevent plastic leakage. Thus, if the injection process is started before the system reaches full operating temperature, or if a nozzle heater burns out, the system is likely to leak, as the “cold gaps” are not fully closed. A solution is desired, allowing lateral thermal expansion of the manifold while eliminating lateral deflection of the nozzles, and providing improved axial sealing between the nozzles and the manifold without relying solely on the thermal expansion of the components to achieve such sealing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    An injection molding nozzle system includes a mold plate defining a first and a second pocket. A nozzle is provided in the second pocket, and defines a nozzle orifice extending in an axial direction to inject molten material into a mold from a downstream end of the nozzle orifice. A manifold is disposed in the first pocket, and defines a manifold orifice to supply the material to the nozzle orifice. A downstream end of the manifold orifice is in fluid communication with the upstream end of the nozzle orifice. A centering support ring is provided in the second pocket, to maintain the nozzle stationary with respect to the second pocket. A bushing extends between the first and second pockets, fixedly attached to the manifold and laterally movable with respect to the pockets and to the nozzle. 
         [0005]    At ambient temperature, the centers of the nozzle orifice and the manifold orifice are offset from one another in the lateral direction, a preload is defined between abutting surfaces of the nozzle and the bushing, an additional preload is defined between abutting surfaces of the manifold and the nozzle, and a gap is defined between the support ring and a shoulder at a downstream end of the second pocket. In operation, the manifold and the nozzle reach respective operating temperatures and thermally expand, such that the manifold moves laterally within the first pocket, and the nozzle remains substantially stationary within the second pocket. The centers of the nozzle orifice and the manifold orifice are thus aligned in the lateral direction, and the gap is closed by thermal expansion of the various components. 
         [0006]    The gap may be closed to a preload of approximately 0.001″ to approximately 0.003″. 
         [0007]    An additional gap may be defined between the support ring and the bushing at the ambient temperature. 
         [0008]    The system may further include dowels disposed between the nozzle and the manifold to prevent rotational movement of the nozzle with respect to the manifold. 
         [0009]    The bushing may include a flange abutting a downstream end of the manifold, and a collar defining an inner shoulder portion slidable within a notch of the nozzle. 
         [0010]    The system may further include a puck disposed between the manifold and the bushing. 
         [0011]    For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an exemplary vertical sectional view of an injection molding nozzle system typical of the prior art. 
           [0013]      FIG. 2  is an exemplary vertical sectional view of an injection molding nozzle system in accordance with a preferred embodiment of the invention. 
           [0014]      FIG. 3  is an exemplary vertical sectional view of the injection molding nozzle system of  FIG. 2 , taken along line A-A of  FIG. 2 . 
           [0015]      FIG. 4  is an exemplary horizontal sectional view taken along line B-B of  FIG. 3 . 
           [0016]      FIG. 5  is an exemplary exploded perspective view of the injection molding system of  FIG. 2 . 
           [0017]      FIG. 6  is an enlarged view of  FIG. 2 . 
           [0018]      FIG. 7  is an enlarged view of  FIG. 3 , showing the injection molding nozzle at operating temperature, where axes X-X and Y-Y are in alignment. 
           [0019]      FIG. 7   a  is an enlarged view of  FIG. 3 , showing the injection molding nozzle in cold condition, where axes X-X and Y-Y are out of alignment. 
           [0020]      FIG. 8  is an exemplary vertical sectional view of an injection molding nozzle system in accordance with an alternative embodiment of the invention. 
           [0021]      FIG. 9  is an exemplary vertical sectional view of the injection molding nozzle system of  FIG. 8 , taken along line A 1 -A 1  of  FIG. 8 . 
           [0022]      FIG. 10  is an exemplary horizontal sectional view along line B 1 -B 1  of  FIG. 9 . 
           [0023]      FIG. 11  is an exemplary exploded perspective view of the injection molding system of  FIG. 8 . 
           [0024]      FIG. 12  is an enlarged view of  FIG. 8 . 
           [0025]      FIG. 13  is an enlarged view of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    With reference to  FIG. 1 , which shows a typical prior art system, a nozzle  2 , manifold  3 , and pressure pad  4  create a compression packet  5  between mold plates  6  and back plate  7 . Such systems are designed to have a cold gap between the nozzle  2  and the manifold  3  of typically 0.000″ to 0.003″ (inches), or as high as 0.012″ in some cases. The term “cold gap,” as used herein, refers to the distance between the nozzle  2  and the manifold  3  at about room temperature. The typical nozzle/manifold assembly has an axial thermal expansion of 0.007″-0.017″ when heated to an operating temperature, usually around 400° F.-600° F. Consequently, substantial pressure forces are exerted on the manifold assembly steels and on to the mold plates and the fasteners securing them. 
         [0027]    A detailed description of the preferred embodiment of the invention follows, with reference to the sectional views of  FIGS. 6 and 7 . A manifold  10 , located in a pocket  12  of a mold plate  14  and backed by a pressure pad  11  secured to a back plate  15  (or to manifold  10 , securing means not shown), has an outlet hole  16  directing the flow of molten plastic to a nozzle  18  extending through a pocket  20  of mold plate  14 . The system is described herein with reference to one such nozzle; however, it should be understood that a plurality of nozzles may be in use with the manifold and mold plates, as necessitated by the scope of the application. A portion  22  of manifold  10  adjacent to nozzle  18  has an annular groove  22   a  concentric with outlet hole  16 , the inner surface of this annular groove being a cylindrical centering surface  24 , while the outer surface of the annular groove has a threaded portion  26 . 
         [0028]    The system further comprises a bushing  28  and a centering support ring  30 . Bushing  28  has a protrusion  32  threadably connected to manifold  10  via thread  26  and centered about axis X-X of outlet hole  16  by the cylindrical surface  24 . The cylindrical surface  24  not only centers the bushing  28 , but also serves to protect thread  26  from seizing laterally, which might otherwise occur due to lateral movement of the manifold during opertion. Bushing  28  further has a flange  34  and a collar  36  having an inner shoulder portion  38 . Outer surface of collar  36  is shaped to allow easy torquing (as necessary for installation of bushing  28  into manifold  10  via thread  26 ), having a pair or series of opposing flat surfaces, such as, for example, a hexagonal or an octagonal shape. 
         [0029]    Pocket  20  of mold plate  14 , having axis Y-Y as shown in  FIG. 6 , has a lead-in portion  40 , a centering cylindrical portion  42 , and a clearance portion  44 . The centering cylindrical portion  42  terminates at its bottom (in  FIG. 6 ) end at a shoulder  42   a.  Lead-in portion  40  may be of conical shape, or a combination of conical and cylindrical surfaces such as that shown in  FIG. 6 , where the cylindrical lead-in portion is of larger diameter than the centering cylindrical portion  42 , in order to allow easy insertion of support ring  30  into the pocket. Cylindrical portion  42  thus centers support ring  30  about axis Y-Y of pocket  20 . An inner cylindrical portion  46  of support ring  30 , concentric with cylindrical portion  42 , further centers nozzle  18  along axis Y-Y. 
         [0030]    As can be further seen in  FIG. 6 , bushing  28  and portion  22  of manifold  10  form a packet  48  centered about axis X-X of outlet hole  16 , while nozzle  18  and support ring  30  form a packet  50  centered about axis Y-Y of pocket  20  of mold plate  14 . The system is designed with a preset lateral misalignment of axes X-X and Y-Y in cold (i.e. room temperature) condition (shown exaggerated for clarity in  FIG. 7   a , which is seen from the direction perpendicular to  FIG. 6 ), the two axes aligning as the system reaches operating temperature. 
         [0031]    The nozzle system of  FIG. 6  has a preload at interfaces  52  (between manifold  10  and nozzle  18 ) and  54  (between shoulder of nozzle  18  and inner shoulder of bushing  28 ), and a cold gap at interfaces  56  (between bushing  28  and support ring  30 ) and  58  (between support ring  30  and shoulder  42   a ). The size of the cold gap varies with the application, being dependent on the thermal growth of the manifold thickness, such that the gap is reduced to zero at operating temperature. As an example, the system may have a preload of 0.001″ to 0.003″ when fully heated. While the forces resulting from the high preloads of conventional systems are transferred to mold plates and their fasteners, putting them under considerable strain, the present design transfers forces (of much smaller preloads, as explained above) from manifold  10 , to shoulder of nozzle  18 , to inner shoulder  38  of bushing  28 , and back onto the manifold through flange  34  of bushing  28 , thus not subjecting the mold plates to unnecessary forces, consequently extending the life of the mold. 
         [0032]    Furthermore, as the nozzle assembly is installed by threading bushing  28  into the manifold and torquing until flange  34  is in firm contact with the manifold at interface  64  according to proper torquing methods, the preload pressure at interfaces  52  and  54  creates a mechanical seal, preventing plastic leaks even if the injection process were to start prematurely, before reaching full operating temperature. Even if plastic were to leak at interface  52 , the mechanical seals at interfaces  54  and  64  would prevent it from reaching the mold plate pocket, which is a common problem of conventional systems, where plastic leaks may fill up the mold plate pockets, damaging nozzle heaters and causing extensive downtime, resulting in costly repairs. 
         [0033]    The nozzle system disclosed herein provides a pre-set, fully controlled pre-loaded system. Prior art systems start anywhere from zero to 0.003″ gap at ambient temperature, while at operating temperatures the system could load to 0.017″ and higher, putting considerable compression strain on the mold steels. The system disclosed herein, on the other hand, allows the pre-loads to be set mechanically at ambient temperature, and maintains those pre-loads at operating temperatures. The loads caused by thermal expansion of mold components are now prevented from being transferred to the mold steels. 
         [0034]    In more detail, as bushing  28  is hand-torqued into the annular groove  22   a  of manifold  10 , the inner shoulder portion  38  (of collar  36  of bushing  28 ) compresses nozzle  18  against manifold  10 . At this stage, there is a pre-set gap between flange  34  of bushing  28  and manifold  10  at interface  64 . As the desired amount of torque is further applied with a torque wrench, the inner shoulder portion  38  distorts, causing flange  34  to deflect and close the gap. Thus, the portion of bushing  28  projecting outwardly from the face of the manifold (i.e. collar  36 , inner shoulder portion  38 , and flange  34 ) in effect becomes a high-tension spring, holding constant tension on the nozzle against the manifold, thus applying the appropriate preloads mechanically (i.e. without the need to bring the system to operating temperature). The size of the gap at interface  64  is tightly toleranced, chosen to achieve the adequate amount of mechanical preload at interfaces  52  and  54  in cold condition. It should be noted that protrusion  32  of bushing  28  is not in contact with the manifold at the top of the groove  22   a;  there is a small gap between these elements (somewhat visible in  FIG. 6 ). Thus, the flange  34  abutting the manifold  10  at  64 , and not the protrusion  32  abutting the manifold, limits how far the bushing  28  can be threaded into the annular groove  22   a.    
         [0035]    The mechanical preloads of the system described herein allow cold start-ups without danger of leaks. A drawback of conventional systems is that they rely on axial thermal expansion of components to achieve sealing against plastic leaks. If the injection process is started before reaching full operating temperature, the system is likely to leak as the cold gaps are not fully closed. The system described herein eliminates this problem by having mechanical preloads pre-set in cold condition, as explained above. 
         [0036]    In conventional systems of multi-cavity applications, if there is a problem with one of the cavities, it is common practice to shut off its nozzle heater, resulting in a cold nozzle, i.e. a different preload on one nozzle compared to the other nozzles, which may cause leaks and flashing. Similarly, if a nozzle heater burns out without the mold operator noticing, there is danger of plastic leaks. However, the mechanically set and held preloads of the system described herein prevent flashing even if a nozzle changes temperature for any reason. 
         [0037]    A further advantage is extended life. As mentioned previously, a typical nozzle/manifold assembly can have a thermal axial expansion as high as 0.017″ when heated to operating temperature. This results in high compressive loads on the steels of the mold plates and manifold, the manifold being susceptible to hobbing between nozzles and pressure pads, sometimes leading to indentations so large they are visible with the naked eye. Such wear shows that prior art systems have a greater chance of leakage as time progresses. The current system, with a zero gap or a preload of 0.001″ to 0.003″ when fully heated, will have a considerably longer life than conventional systems. 
         [0038]    Further features and benefits of the system described herein include the preloaded seal at interface  64  via flange  34 , the preload force being calculated such that it retains nozzle  18  in place and seals it, but all along allowing the manifold to expand laterally so that axes X-X and Y-Y become aligned at operating temperature. The dimension of the cylindrical centering surface  24  compared to the inner diameter of protrusion  32  is such that when there is lateral movement of the manifold, it doesn&#39;t subject thread  26  to extreme forces, preventing lateral seizing of the threads. The combination of all dimensional tolerances and cold gaps, according to some embodiments, allows the system to have growth of only 0.001″ to 0.003″ subjected on the manifold and mold plates, thus eliminating excessive hobbing on manifold. As mentioned above, cold start-up leakage and heater failure leakage are prevented. 
         [0039]    A further advantage of the present design is that it allows shipment of pre-assembled hot runner systems to customers, and easy on-site installation into molds. Although not shown in the figures, the nozzles can be provided with pre-installed and pre-wired heaters, and with nozzle tips suited to the specific application. 
         [0040]    A further feature of this system allows for easy nozzle tip change without removal of the nozzle from the hot runner system. As shown in  FIG. 6 , a pair of dowels  60 , fixedly attached to manifold  10  in the embodiment shown, project into clearance pockets  62  of nozzle  18 . Pockets  62  are shaped to allow the relative motion caused by thermal expansion of the manifold relative to the nozzles as explained above (in the direction perpendicular to the page in  FIG. 6 , left and right in  FIGS. 7 and 7   a ), but preventing nozzle  18  from rotating about axis Y-Y as the nozzle tip (not shown) is threadably disengaged or re-engaged into front end  68  of the nozzle, to deliver the flow of molten plastic into the molding cavity. Any suitable designs of nozzle tips may be used with the nozzle system described above, such as those disclosed in applicant&#39;s U.S. Pat. Nos. 7,207,795; 7,329,117; and 7,413,431, all of which are incorporated by reference herein. Although not shown in the figures, it should be understood that dowels  60  may alternatively be fixedly secured to nozzle  18 , and pockets  62  may be accordingly provided in portion  22  of manifold  10 . 
         [0041]    A brief description of an alternative embodiment of the invention follows with reference to  FIG. 12 . 
         [0042]    This embodiment additionally includes a puck  128  centered into a manifold  110  by a cylindrical surface  124  (concentric with a manifold outlet hole  116  having an axis X 1 -X 1 ) and secured to manifold  110  by way of a thread  126 . Puck  128  extends beyond the face of manifold  110 , its protrusion having an outer thread  127 . A collar bushing  136 , having an inner shoulder portion  138 , is threadably secured to puck  128  via thread  127  until collar bushing  136  is torqued firmly against manifold  110  at interface  164  according to proper torquing methods. A nozzle  118  and a centering support ring  130  (similar to those shown in the first embodiment) complete the nozzle assembly. Support ring  130  is centered along axis Y 1 -Y 1  of mold plate pocket  120  by a cylindrical centering portion  142  of the mold plate pocket. Nozzle  118  is in turn centered along axis Y 1 -Y 1  via an inner cylindrical surface  146  of support ring  130 . This system has a preload at interfaces  152  (between puck  128  and nozzle  118 ) and  154  (between shoulder of nozzle  118  and inner shoulder portion  138  of collar bushing  136 ), and a cold gap at interfaces  156  (between puck  128  and centering support ring  130 ) and  158  (between centering support ring  130  and the bottom of the mold plate pocket). 
         [0043]    As can be further seen in  FIG. 12 , puck  128  and collar bushing  136 , together with portion  122  of manifold  110  adjacent the nozzle, form a packet  148  centered about axis X 1 -X 1  of outlet hole  116 , while nozzle  118  and support ring  130  form a packet  150 , centered about axis Y 1 -Y 1  of pocket  120  of mold plate  114 . As the system is brought to operating temperature, axes X 1 -X 1  and Y 1 -Y 1  align, while the thermal expansion of the components achieves axial preload, effectively sealing against plastic leaks (puck  128  being outfitted with a seal ring  170  behind the thread, as an added measure). With the exception of the differences presented above, this system is similar to the previous embodiment, and will therefore not be described in more detail. 
         [0044]    It will be appreciated that prior art designs suffer from a loss of alignment: the nozzle is in contact with the cooled mold plate at one end, and threadably secured to the expanding manifold at the opposite end, such that lateral thermal expansion of the manifold causes the nozzle axis to deflect at mold heat-up. A further challenge caused by such deflection is that nozzles threaded into the manifold must have a minimum length in order to allow axial deflection without snapping, minimum lengths usually being around 5.5″-6.0″. The present design, allowing relative motion between packets  48  and  50  (or  148  and  150 ) as explained above, prevents the lateral thermal expansion of the manifold from deflecting the nozzle axis, thus posing no limitations on the shortness of nozzles allowed, therefore resulting in more compact stack sizes.

Technology Category: 7