Abstract:
A hot runner nozzle system ( 500 ) comprising a nozzle tip ( 100 ) adjacent to a nozzle ( 405 ) in series, and a retainer ( 400 ) adjoining both the nozzle tip ( 100 ) and the nozzle ( 405 ) in parallel, the retainer ( 400 ) having a first retainer thread ( 435 ), for connection to a nozzle thread ( 420 ) to form a first seal ( 450 ) between the nozzle end ( 440 ) of the retainer ( 400 ) and a nozzle shoulder ( 430 ), a second retainer thread ( 460 ), for connection to a tip thread ( 455 ) to form a second seal ( 470 ) between the inlet end ( 475 ) of the nozzle tip ( 100 ) and the gate end ( 425 ) of the nozzle ( 405 ), a seal ring ( 115 ) for creating a gate seal ( 496 ) with a gate insert ( 120 ), and a plurality of flats ( 410 ) thereon to facilitate torquing of the retainer ( 400 ) to the nozzle ( 405 ), the torque value applied to the retainer ( 400 ) being about two to four times of that applied to the nozzle tip ( 100 ) to facilitate removal of the nozzle tip ( 100 ) independent of the retainer ( 400 ).

Description:
CROSS REFERENCES 
   None. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to the field of injection molding equipment and, more particularly, to a hot runner nozzle system whereby a nozzle tip and a retainer are threadably secured to a nozzle. 
   BACKGROUND OF THE INVENTION 
   A hot runner is utilized to transfer molten material, typically plastic resin, from an injection molding machine to a mold. A hot runner generally includes a manifold plate, a manifold housed in the manifold plate, and a backing plate that encloses the manifold in the manifold plate. The manifold, typically heated via a plurality of tubular heaters embedded therein, routes molten resin from a sprue bushing, which mates with an injection unit on an injection molding machine, to a plurality of nozzles which inject the molten resin into cavities in the mold. The manifold divides the flow of the molten resin into a network of a plurality of melt channels as it flows from the sprue bushing to the nozzles, all the while maintaining a near constant temperature of the resin throughout. 
   The state of the art includes various nozzles and nozzle tips for a hot runner which is typically of either a valve gate style or a hot tip style. In the valve gate style, a valve stem reciprocates within the nozzle, nozzle tip and a gate orifice acting as a valve to selectively preclude or allow the flow of resin through the nozzle tip and into a mold cavity. In the hot tip style, a small volume of resin at the end of the nozzle tip, in the gate orifice, solidifies during each molding cycle thus precluding the flow of resin into the mold cavity. The present invention describes the hot tip style nozzle. 
   It is important to note that the nozzle tip is subject to many influences which help determine its size and makeup. The nozzle tip must be able to withstand loads from injection pressures that may reach 40,000 psi (275 MPa) or more, endure corrosion and chemical attack, and resist abrasion and wear from resins filled with glass or other particulate materials. Paramount to the nozzle tip is its ability to provide the correct amount of heat to the gate orifice to allow sufficient flow of resin to the mold cavity yet promote solidification of the resin once the mold cavity is filled. To enable this feature, a heater is installed to encircle the nozzle in an area proximate to the nozzle tip, and the nozzle tip is typically constructed of a highly thermally conductive alloy, usually a copper alloy, which, by nature, tends to be relatively low in hardness. All these factors contribute to the nozzle tip eventually wearing out or failing thus necessitating its replacement, generally more frequently than most other components usually replaced during regular, periodic maintenance of the hot runner. For this reason, it is desirable to be able to service the nozzle and the nozzle tip in a quick and efficient manner without necessarily disassembling the entire hot runner or even removing and re-wiring the heater. 
   A common, and simple, nozzle housing and nozzle tip configuration involves a nozzle tip, having a male thread, being installed into a nozzle housing which has a female thread. The nozzle housing, typically made of a high-hardness tool steel, extends over the nozzle tip, beyond the threaded connection, to include, at its end, a thin, raised band of material; a seal ring, configured to fit diametrically inside a similarly sized bore in a gate insert within a mold, with some clearance at room temperature, such that at operating temperature, its radial, thermal expansion creates a gate seal therebetween to preclude molten resin from leaking between the seal ring and the gate insert. 
   When the mold, and consequently, the gate insert, is removed from the hot runner during maintenance or product changeover, the seal ring of the nozzle housing is disengaged from the bore of the gate insert. Though there is a nominal clearance between the two surfaces at room temperature, if disassembly is performed before the nozzle housing has cooled sufficiently from its operating temperature to reduce its radial, thermally-expanded diameter, or if the two surfaces are slightly misaligned, the result will be abrasion of the two mating surfaces. Any slight scratches or abrasion of the seal ring on the nozzle housing may potentially provide a path for pressurized, molten resin to leak by, during operation, resulting in catastrophic damage to the hot runner. Over time, this abrasion will require replacement of the entire nozzle housing to prevent, or repair from, resin leakage, thus necessitating significant down time of the hot runner for its maintenance as the entire hot runner must be disassembled in order to remove the nozzle housing from between the manifold plate and the manifold. 
   The thin section of the seal ring of the nozzle housing is also its weakest point, and is subjected to the same high injection pressures as the nozzle tip. The trend of the injection molding industry to reduce the cost of a molded part by reducing the amount of resin required to fill it, necessitates a thinner molded part wall thickness thus requiring higher injection pressures. To utilize stronger materials to make the seal ring of the nozzle housing more robust, is cost prohibitive as the seal ring and the entire length of the nozzle housing and are integral. 
   To address these needs and concerns, a two piece tip assembly is commonly utilized, as is illustrated in U.S. Pat. No. 6,609,902 B1 to Blais et al, for example. A removable tip insert is secured against a nozzle by a tip retainer which is typically threadably connected to the nozzle, whereby a flange of the tip insert is trapped by a mating shoulder of the tip retainer. The tip retainer also has the added feature of having the seal ring included at its gate end. The relatively inexpensive tip insert can be removed and replaced by unscrewing the tip retainer, installing a new tip insert, and re-attaching the tip retainer. Such a tip arrangement is cost effective in that the tip retainer is not discarded. 
   However, this two piece design is not without its limitations. In order to create sufficient seal force, the flange of the tip insert is subjected a high torque load by the retainer, creating a stress concentration at the corner of the flange and the tip insert. When subjected to resin at operating temperature and pressure, the tip insert is prone to cracking and failing at the base of the flange. Additionally, the cumulative design of the flange and retainer assembly imposes restrictive size limitations on the diameters of the components thereby limiting the injection pressures and loads they may withstand. 
   For the foregoing reasons, the present invention is directed to overcoming one or more of the problems or disadvantages set forth above, and for providing a hot runner nozzle system with replaceable componentry capable of withstanding high injection pressures. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is to provide a hot runner nozzle system which comprises a nozzle tip which abuts a nozzle in series, while a retainer adjoins both the nozzle tip and the nozzle in parallel. 
   In accordance with the above aspects of the invention, there is provided a retainer, having a first retainer thread, which connects to a nozzle, having a nozzle thread, creating a first seal where the nozzle end of the retainer contacts the nozzle shoulder. The retainer has a plurality of flats thereon to facilitate torquing on to the nozzle thread. To prevent resin leakage beyond the gate insert of the mold, the retainer is configured to have a seal ring at its gate end such that, when heated, radial, thermal expansion of the outer diameter of the seal ring will make forced contact with the gate insert, thus precluding the passage of pressurized molten resin during operation. A second seal is created whereby the nozzle tip, having a tip thread which engages a second retainer thread during installation, is torqued into the retainer, forming the second seal with the nozzle housing to preclude resin leakage from the melt channel. 
   In another embodiment of the present invention, the wall thickness of the retainer is reduced, owing to the implementation of a higher strength material, while the wall thickness of the nozzle tip is increased to accommodate high pressure applications. 
   In yet another embodiment of the present invention, the wall thickness of the retainer is reduced, owing to the implementation of a higher strength material, while the wall thickness of the nozzle tip remains unchanged, providing for a thicker layer of insulative resin to occupy a gap therebetween. 
   These aspects are merely illustrative of the various aspects associated with the present invention and should not be deemed as limiting in any manner. These and other objects, aspects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the referenced drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the drawings which illustrate the best known mode of carrying out the invention and wherein the same reference numerals indicate the same or similar parts throughout the several views. 
       FIG. 1A  is a partial cross-sectional view of a typical nozzle tip threadably installed in a one piece nozzle housing as is known in the prior art. 
       FIG. 1B  is a partial cross-sectional view of a finite element analysis of the nozzle tip and one piece nozzle housing shown in  FIG. 1A  showing stresses under load. 
       FIG. 2  is a sectional view of a tip insert and tip retainer assembly known in the prior art. 
       FIG. 3  is a partial cross-sectional view of a finite element analysis of the tip insert of  FIG. 2  showing stresses under load. 
       FIG. 4A  is a view of the exterior of the present invention showing the flats used to torque the retainer onto the nozzle. 
       FIG. 4B  is an isometric view of the exterior of the present invention showing a flat used to torque the retainer onto the nozzle. 
       FIG. 5  is a cross-sectional view of the present invention illustrating the interaction of the nozzle, the nozzle tip and the retainer. 
       FIG. 6  is a partial cross-sectional view of the present invention illustrating the interaction of the nozzle, the nozzle tip, and the retainer with the gate insert. 
       FIG. 7  is a chart illustrating the comparative strength and hardness values of choice materials for the retainer versus the nozzle. 
       FIG. 8  is a partial cross-sectional view of the present invention illustrating an embodiment where the thicknesses of both the retainer and the nozzle tip may be optimized for high pressure applications. 
       FIG. 9  is a partial cross-sectional view of the present invention illustrating an embodiment where the gap between the nozzle tip and the retainer is optimized for thermal considerations. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. For example, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. 
   The prior art of  FIG. 1A  shows a nozzle tip  100  threadably engaged with a nozzle housing  105  for the purpose of understanding the function and interaction of each component. The nozzle housing  105  has a tip end  110  which is integral with the nozzle housing  105  in its entirety and therefore both aspects are made of one material suitable for the temperatures and pressures for which it is intended. Situated at the tip end  110  is a seal ring  115 , being a precisely sized, raised band of material, the outer diameter of which thermally expands to contact a gate insert  120 , on its inner diameter to seal and prevent the flow of resin thereby. The intimate contact between the seal ring  115  and the gate insert  120  results in abrasion and wear of the seal ring  115  which, over time, would allow resin to leak by, necessitating removal of the nozzle housing  105  as a whole for replacement of the seal ring  115 , an undertaking requiring complete disassembly of a hot runner system (not shown). 
   The finite element analysis of  FIG. 1B  illustrates the forces generated by the loads endured under operating conditions during the injection molding process on the nozzle tip  100  and the nozzle housing  105 . When molten resin is injected under pressure into a mold cavity (not shown), it also surrounds the nozzle tip  100  and enters a chamber  135  where it acts to insulate the heat conducted by the nozzle tip  100  to the tip end  110  of the nozzle housing  105  as well as enhancing the sealing mechanism. The pressurized resin exerts an outward force on the tip end  110  and since the tip end  110  is unsupported behind the seal ring  115 , a first high stress concentration  140  occurs there. In an effort to combat the first high stress concentration  140  and to prevent failure of the nozzle housing  105  in this area, a sufficiently strong material is desirable, but because of the integral design of the nozzle housing  105  and tip end  110 , the overall cost could be prohibitive. Therefore, the tip end  110  of the nozzle housing  105  must be made sufficiently thick to withstand such loads, thereby decreasing the insulative thickness of the chamber  135 , resulting in undesired conduction of heat away from the nozzle tip  100  and nozzle housing  105 . 
   A two piece design  200  comprising a nozzle housing  105 , a tip insert  210  and a tip retainer  215 , as shown in the prior art of  FIG. 2  from U.S. Pat. No. 6,609,902 B1 to Blais et al, illustrates how a flange  220  of the tip insert  210  is trapped between the tip retainer  215  and the nozzle housing  105  when the tip retainer  215  is threadably attached to the nozzle housing  105 . The tip insert  210  is subjected to a compressive load on the flange  220  as the tip retainer  215  is torqued onto the nozzle housing  105  to ensure sufficient seal off pressure is created between the tip insert  210  and the nozzle housing  205  at an interface  225 . In doing so, a second high stress concentration  230  is created at the corner of the flange  220 , as illustrated in  FIG. 3 , resulting in potential failure of the tip insert  210  over time or under high operating temperatures and pressures. 
   Referring now to an embodiment of the present invention shown in  FIGS. 4A and 4B , to facilitate attachment of a retainer  400  to a nozzle  405 , the retainer  400  is configured to have a plurality of flats  410  thereon, which may be engaged with an open ended wrench. Similarly, the nozzle tip  100  is configured to have a plurality of serrations  415  thereon to facilitate torquing of the nozzle tip  100  into the retainer  400  with a mating socket (not shown). 
   The embodiment of the present invention shown in the section view of  FIG. 5  illustrates the interaction between the nozzle  405 , the nozzle tip  100 , and the retainer  400 . The nozzle  405  is configured to have a nozzle thread  420 , located at its gate end  425 , and a nozzle shoulder  430  proximate to the nozzle thread  420 . The retainer  400  is configured to have a first retainer thread  435  located at its nozzle end  440 , which, when the retainer  400  is torqued, via the plurality of flats  410  located at about a midsection  412 , threadably engages the retainer  400  to the nozzle  405 , forming a first threaded connection  445 . Additionally, an interference fit is formed when the retainer  400  abuts the nozzle shoulder  430  creating a first seal  450 . 
   Referring still to  FIG. 5 , when the nozzle tip  100  is threadably installed into the retainer  400 , a tip thread  455  engages a second retainer thread  460  thus forming a second threaded connection  465 . Similar to the first seal  450 , a second seal  470  is created when an inlet end  475  of the nozzle tip  100  is compressed sufficiently against the gate end  425  of the nozzle  405 . It is this second seal  470  which initially prevents molten resin from a melt channel  480  from leaking therebetween. It is preferable that the minimum seal pressure at both the first seal  450  and the second seal  470  be at least about 20% greater than the injection pressure in the melt channel  480 . 
   To match industry standard, it is preferred that both the first threaded connection  445  and the second threaded connection  465  each be right hand, where both the tip thread  455  and the nozzle thread  420  each be male, and both the first retainer thread  435  and the second retainer thread  460  each be female. It is recommended that the torque value used to attach the retainer  400  to the nozzle  405  be about two to four times greater than the torque value used to install the nozzle tip  100  into the retainer  400 . This hierarchy of torque values will allow ease of maintenance of the hot runner nozzle system  500  by ensuring that the nozzle tip  100  can be removed from the retainer  400  without causing the retainer  400  to loosen from the nozzle  405 . Conversely, the retainer  400  and nozzle tip  100  may be removed together as one from the nozzle  405  and the nozzle tip  100  may be subsequently removed from the retainer  400  utilizing the plurality of flats  410  on the retainer  400  for stability while the nozzle tip  100  is unscrewed. Additionally, the present invention allows for a heater  497  to be installed on, or removed from, the nozzle  405  without prior removal of the nozzle tip  100  or the retainer  400  and consequently, the nozzle tip  100  may also be installed in the retainer  400  without requiring removal of the heater  497 . 
   An inner diameter  485  of the retainer  400  is sized such that a tip shoulder  490  may engage it to ensure proper alignment of the nozzle tip  100  while it is being torqued into the retainer  400 . Located distally from the nozzle end  440  of the retainer  400  is a seal ring  115 , whose function will be better understood upon viewing  FIG. 6 , where the present invention is shown installed in a gate insert  120 . The seal ring  115  is sized to match the inner diameter of the gate insert  120 , with some clearance in cold condition that is taken up once the nozzle  405  thermally expands during operation. Molten resin, under pressure, also travels from the melt channel  480  to a gate bubble area  495  which further acts against the inner diameter  485  to force the seal ring  115  of the retainer  400  against the gate insert  120  thereby forming a gate seal  496 . It is this gate seal  496  which acts to prevent molten resin from leaking from the gate bubble area  495  to the exterior of the nozzle  405 . 
   Referring to the chart of  FIG. 7 , while typically a nozzle housing  105  is made of AISI H-13 tool steel hardened to 42-44 Rockwell C (Rc), it is the intention of the present invention to provide flexibility in the choice of material for the retainer  400  in an effort to increase the overall longevity of the hot runner nozzle system  500 . Accordingly, to increase the fatigue life of the retainer  400  from 15% to 125% respectively, it may be made from materials with higher endurance limits than that of AISI H-13 hardened to 42-44 Rc, such as AISI H-13 hardened to 49-52 Rc, Ph 13-8, Custom-465 (a steel alloy and registered trademark of the Carpenter Technology Corporation), AISI-4340, Aermet-100 (a martensitic alloy steel and registered trademark of the Carpenter Technology Corporation) or Vascomax C-300 (a specialty steel and registered trademark of the Allegany Technologies Company). These material selections allow for a seal ring  115  that is more robust and has greater wear resistance than that typical of a nozzle housing  105  and with the flexibility of replacing only the retainer  400  when necessary versus the nozzle housing  105  in its entirety when the seal ring  115  becomes worn and unusable. It may be realized that within the scope of the present invention, the retainer  400  being a separate piece and made from the more robust materials as listed previous, it is now permissible to make the nozzle  405  of the present invention from a lower grade or hardness material, such as AISI H-13 hardened to 42-44 Rc, as it is not integrated with the seal ring  115 , compared to the nozzle housing  105  of  FIG. 1A , thereby reducing its cost. 
   Turning now to  FIG. 8 , since the material of the retainer  400  may be made from different material than the nozzle  405 , it is another embodiment of the present invention to reduce the thickness ‘Y’ of the retainer  400 , thereby increasing its inner diameter  485 , while still maintaining the necessary mechanical properties required for extended longevity. Consequently, the thickness ‘X’ of the nozzle tip  100  may also be increased to afford a thicker wall section to be able to withstand higher injection pressures in the melt channel  480 . 
   Referring to  FIG. 9  now, yet another embodiment of the present invention illustrates, similar to  FIG. 8 , how, for the same reasons described previous, the thickness ‘Y’ of the retainer  400  is reduced while that of the nozzle tip  100  is not affected. The resulting increase of the inner diameter  485  of the retainer  400  allows for a respective increase of a gap ‘Z’  498  to exist between the nozzle tip  100  and the retainer  400 , thereby permitting a thicker insulative layer of resin to occupy said gap ‘Z’  498  to retard the unwanted transfer of heat from nozzle tip  100  to the retainer  400 . 
   Other objects, features and advantages of the present invention will be apparent to those skilled in the art. While preferred embodiments of the present invention have been illustrated and described, this has been by way of illustration and the invention should not be limited except as required by the scope of the appended claims and their equivalents.