Abstract:
An injection molding apparatus is disclosed that includes a manifold having at least one manifold melt channel therethrough, a mold plate defining a gate for transmitting melt flow to a mold cavity, and at least one nozzle including a nozzle body and nozzle tip defining a nozzle melt channel in communication with the manifold melt channel. The nozzle tip includes a substantially conical forward portion having a terminal end that is aligned with and spatially offset a predetermined distance “d” from the gate, the nozzle melt channel having a discharge opening that is located rearward of the terminal end for discharging melt flow from the nozzle melt channel towards the gate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/619,685, filed Oct. 19, 2004, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an injection molding apparatus and a nozzle for an injection molding apparatus. 
     BACKGROUND OF THE INVENTION 
     A hot runner system is used to produce plastic parts from a mold; the hot runner delivers molten material to the mold cavity through a manifold, a nozzle, and a gate. A gate vestige mark is created on plastic parts from a hot runner system; this mark is created at the interface between the nozzle, gate and the mold cavity. In an application that requires a good esthetic appearance the size and shape of the gate vestige is important. There are many variables which can worsen the appearance of the gate vestige mark on a part, including stringing. Stringing occurs when the melt at the front of the nozzle tip and in the gate area is still relatively molten and therefore fluid or flowable when the part is ejected. The melt or molten plastic is drawn out in a line or string as the part is ejected. Stringing necessitates cleanup of the injection nozzle and tip causing downtime of the injection molding apparatus, which can be quite substantial for some applications. If the melt is sufficiently frozen at the front of the nozzle when the part is ejected, the plastic breaks or shears off, leaving behind a clean gate vestige mark on the finished plastic part. 
     Although the problems of stringing and gate vestige are common to many injection molding applications, these problems are a particular concern when using molding resins such as polypropylene, and when using faster cycle times. In such applications, the nozzle is generally operated at a higher temperature or does not allow for significant cooling times. 
     Thus, there is a need for an injection molding nozzle and tip which reduces or eliminates the problems associated with stringing and gate vestige. 
     SUMMARY OF THE INVENTION 
     Embodiments hereof are directed to an injection molding apparatus that includes a manifold having at least one manifold melt channel therethrough, a mold plate defining a gate for transmitting melt flow to a mold cavity, and at least one nozzle including a nozzle body and nozzle tip defining a nozzle melt channel in communication with the manifold melt channel. The nozzle tip includes a substantially conical forward portion having a terminal end that is aligned with and spatially offset a predetermined distance “d” from the gate, the nozzle melt channel having a discharge opening that is located rearward of the terminal end for discharging melt flow from the nozzle melt channel towards the gate. The nozzle further includes a thermally insulative tip retainer for securing the nozzle tip to the nozzle body. In an embodiment, the nozzle tip has a rounded terminal end. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 sectional view of an injection molding apparatus in which embodiments of the present invention may be used, showing the right nozzle in cross-section. 
         FIG. 2  is a cross-sectional view of an example embodiment of an injection molding nozzle. 
         FIG. 3  is an expanded cross-sectional view of an end of the injection molding nozzle of  FIG. 2 , showing a first example embodiment of an injection molding nozzle tip. 
         FIG. 4  is an expanded cross-sectional view of an end of the injection molding nozzle of  FIG. 3 , showing a second example embodiment of an injection molding nozzle tip. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to  FIG. 1  which shows an injection molding apparatus indicated generally by reference numeral  10 . The injection molding apparatus  10  is an example environment in which embodiments of the present invention may be used. The injection molding apparatus shown in  FIG. 1  is provided for the purpose of illustrating the use of embodiments of the present invention and is not intended to be limiting. The injection molding apparatus  10  includes a manifold  12  having a manifold melt channel  14  extending therethrough. As shown in  FIG. 1 , the manifold  12  is fixed to a back plate  15  of the injection molding apparatus  10 . However, a floating manifold may also be used to compensate for thermal expansion and contraction of adjacent components, if desired. An inlet  16  of manifold channel  14  receives a melt stream of moldable material from a machine nozzle (not shown). The melt stream flows through manifold channel  14  to outlets  18 . Manifold heaters (not shown) are provided in manifold  12  to maintain the melt stream at a desired temperature. 
     Nozzles  20  are located between the manifold  12  and respective mold cavities  30 . The nozzles  20  are received in wells or openings  32  provided in a cavity mold plate  34 . Although a single mold plate  34  is shown, multiple mold plates or a modular mold plate assembly may also be used. Each nozzle  20  includes a nozzle head  24  and a nozzle tip  26 . As shown in  FIG. 1 , the nozzle tip  26  is a one-piece tip; however, other types of nozzle tips may also be used such as a two-piece nozzle tip. An insulating ring or flange  36  surrounds the nozzle head  24  and abuts a seat or shoulder  38  of the mold plate  34 . The flange  36  is provided to force an inlet surface  25  of the nozzle head  24  against an outlet surface  28  of the manifold  12  when the injection molding apparatus  10  is heated to an operating temperature. This provides a seal between the nozzles  20  and the manifold  12 . 
     Each nozzle  20  includes a nozzle melt channel  22  for receiving the melt stream from the respective manifold outlet  18  and delivering the melt stream to the respective mold cavity  30  through respective mold gates  31 . The mold gates  31  are provided at the entrance to the mold cavities  30 , adjacent nozzle tips  26  of the nozzles  20 . Each nozzle  20  is further provided with a heater  40 , which helps to maintain the melt stream at a desired temperature as it passes through the nozzle  20 . The nozzles  20  may also have a thermocouple  114 . Cooling channels  42  are located adjacent the mold cavities  30  and transport a cooling fluid to cool and solidify the melt in the mold cavities  30 . 
     In operation, a melt stream of moldable material is injected from a machine nozzle and flows through manifold melt channel  14 , nozzle melt channel  22  and past mold gate  31  into mold cavity  30 . The melt in mold cavity  30  is then cooled creating a molded part that is subsequently ejected from the injection molding apparatus  10 . 
       FIG. 2  shows another example of a nozzle in which embodiments of the invention can be used. The nozzle of  FIG. 2  is substantially the same as the nozzle discussed above in respect of  FIG. 1 , except that instead of having a one-piece nozzle tip/seal, the nozzle shown in  FIG. 2  includes a nozzle tip  102  and tip retaining device  108  that together function as a two-piece nozzle tip/seal. Tip retainer  108  positions the nozzle tip  102  within a nozzle body  104  of the nozzle. In this embodiment, tip retainer  108  is threadingly engaged through threads  236  on an outer wall of tip retainer  108  with complementary threads  238  on an inner wall of nozzle body  104 . When engaged, a shoulder  242  of tip retainer  108  abuts a corresponding portion of nozzle tip  102  to secure it to nozzle body  104 . 
       FIG. 3  shows an enlarged portion of the nozzle of  FIG. 2  having a nozzle tip constructed according to a first example embodiment of the present invention. The nozzle body  104  and a nozzle tip  102  define a nozzle melt channel  106  for receiving a melt stream from a corresponding nozzle melt channel  22  and delivering the melt stream to the respective mold cavity  30  through the respective mold gates  31 . As shown in  FIG. 3 , the tip is generally conical at its downstream end and extends to close to the mold gate  31 . The nozzle tip  102  is removably attached to the nozzle body  104  by tip retainer  108 . The nozzle also includes a heating element  112  and a thermocouple  114 . Although the nozzle tip  102  shown has a central melt channel  106  for the purposes of this invention it may also be of the torpedo type, which is well known in the industry (not shown) and provides an annular melt channel. 
     The tip retainer  108  is provided between nozzle tip  102  and the inner wall of the opening  32 . The tip retainer  108  defines with tip  102  a tip retainer melt channel  109  in fluid communication with and downstream from the nozzle melt channel  106 . The tip retainer  108  may also act as a seal and prevent backflow of melt from traveling further into opening  32  from nozzle tip  102  by providing an annular sealing portion  110  that contacts the mold plate  34  at an inner surface of the opening  32  within a melt chamber  120 . Melt chamber  120  forms a portion of the opening  32  adjacent to mold gate  31 . In the cold condition, a gap exists between the end surface  122  of the tip retainer  108  and surface  124  of the mold plate  34 . This gap allows for heat expansion of the nozzle towards the gate  31  when the hot runner is brought up to operating temperature. In the heated condition, the end surface of the tip retainer  108  may abut a surface  124  of the mold plate  34 , or in some instances a portion of the gap may remain to provide insulation between the tip retainer  108  and the mold plate  34 . This is dependent on the requirements of the molding application. 
     The nozzle tip  102  may be formed from a tip material having a relatively high thermal conductivity to facilitate the conduction of heat from the heating element  112  to the melt in the tip retainer melt channel  109 . Some examples of suitable materials for the tip  106  are Be—Cu (Beryllium-Copper), Beryllium-free Copper such as, for example, Ampco 940™, TZM (Titanium/Zirconium carbide), Aluminum or Aluminum-based alloys, Inconel™, Molybdenum or suitable Molybdenum alloys, H13, mold steel or AerMet 100™. 
     The nozzle tip  102  has a terminal end  116  which is rounded to provide a spatial offset of a predetermined distance “d” between the terminal end  116  and an end surface  118  of the mold plate  34 . The mold gate  31  is provided in the end surface  118  adjacent the nozzle tip  102 . The distance “d” between the terminal end  116  of the nozzle tip  102  and the mold gate  31  and the curvature of the terminal end  116  may vary based on the melt material, gate size, cooling conditions and cycle time, among other considerations. In some embodiments, the terminal end  116  is rounded to provide an offset of up to 1 mm. 
     The offset of the nozzle tip  102  from the mold gate  31  reduces the thermal mass for heat conduction in the gate area and creates a temperature gradient between the terminal end  116  and the mold gate  31 . The temperature gradient allows some of the melt to cool and partially solidify within the opening  32  within or adjacent to the mold gate  31 . The partially solidified melt forms a small plug or skin over the mold gate  31  that allows the molded part to be removed by a shearing separation, thereby reducing or eliminating stringing or interference from the melt. The skin or plug which forms can be readily re-melted on a subsequent injection cycle without clogging the nozzle or burning or thermally degrading the melt inside of the nozzle. Further, the rounded terminal end  116  does not create a significant problem of aerodynamic dead spots in front of the nozzle. In some example embodiments, the use of a rounded tip permits a greater relative thermal mass to be placed closer to the set-off distance “d” than would be possible using a similar nozzle with a non-rounded pointed nozzle tip that had the same set-off distance “d”. 
       FIG. 4  shows nozzle having a nozzle tip  202  constructed according to a second embodiment of the present invention. The nozzle differs from the nozzle shown in  FIG. 3  in the construction of the nozzle tip, but is similar in other respects. The nozzle tip  202  has a tip body comprising a generally cylindrical body portion  204  and a generally conical portion  206 . A bead  208  constructed of a relatively high wear resistant material is welded or brazed to the end portion  210  of the conical portion  206  of the nozzle tip  202 . The nozzle tip  202  may be constructed by, for example, obtaining a nozzle tip with a cutoff end and welding, brazing or thermal bonding a bead of suitable wear resistant material thereto to form a rounded terminal end  212  of the nozzle tip  202 . 
     A nozzle tip undergoes significant wear over its useful life, which may adversely affect processing conditions and require the nozzle tip to be replaced. One adverse effect that may occur as the nozzle tip wears is an increase in the size of the gate vestige created by the tip, resulting in unpredictable gate vestige over time. The use of a wear resistant material for the bead  208  may increase the wear resistance of the nozzle tip and thereby increase its useful life. Further, as wear resistant materials are typically poor heat conductors, the bead material can be selected to provide a nozzle tip having a terminal end with a relatively high wear resistance and relatively low thermal conductance compared with the remainder of the tip. In some example embodiments, the body portion  204  is constructed from Beryllium-Copper (Be—Cu) and the wear resistant bead  208  is constructed from a wear resistant ceramic, steel, or carbide material, for example, to form a bimetallic nozzle tip. 
     As for the nozzle tip  202 , the terminal end  212  of the nozzle tip  202  is rounded such that it is spaced at a predetermined distance “d” from the end surface  118  of the mold plate  34 . The distance “d” between the terminal end  206  of the nozzle tip  202  and the mold gate  31  may vary based on the melt material, gate size, cooling conditions and cycle time among other considerations. In some embodiments, the terminal end  206  is rounded to provide an offset of up to 1 mm. 
     It will be appreciated by a person skilled in the art that embodiments of the present invention could be utilized in systems utilizing multiple injection molding nozzles with a single mold cavity. Thus, according to at least one example embodiment is a nozzle tip for an injection molding nozzle to be installed in an injection molding apparatus, comprising a tip member having a front end and a rear end, and defining a melt channel between the front and rear end. The front end has a rounded terminal end. The nozzle tip may, in some embodiments include a tip body having an end portion and being formed from a first material, and a bead formed from a second material and attached to the end portion of the tip body such that it provides a rounded terminal end and the second material may in some embodiments have a wear resistance greater than a wear resistance of the first material and/or in some embodiments the second material may have a thermal conductivity less than a thermal conductivity of the first material. In some example embodiments, the first material is a beryllium-copper alloy, and the second material is titanium or a titanium alloy. In some example embodiments, the bead is attached to the end portion end using welding, brazing or thermal bonding. In some example embodiments, the nozzle tip body includes a body portion and a conical portion, the end portion of the tip body being disposed on the conical portion. In some example embodiments, the rounded terminal end is offset a distance of 0.05 to 1 millimeters from a mold gate. 
     The 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.