Abstract:
An injection molding system having an improved valve gate arrangement that provides molded articles with improved vestige and or surface quality.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to injection molding systems. More specifically, the present invention relates to a valve gating system found in injection molding systems.  
           [0003]    2. Summary of the Prior Art  
           [0004]    Injection molding nozzles are well known and are used to inject materials, such as plastic, into the cavity of a mold. For example, such nozzles receive molten material, such as plastic, from an injection molding machine and direct the same into a mold cavity through a passage called a gate. When an injection operation is complete, and prior to opening the mold cavity to eject the molded part, the transfer of molten material through the gate must be stopped. Generally, two methods exist for stopping the transfer of molten material through the gate, namely: thermal, or open, gating; and valve gating.  
           [0005]    In thermal gating, the gate is an open aperture through which molten material passes during an injection operation. The gate is rapidly cooled at the end of the injection portion of the cycle, when the injection pressure is removed, to “freeze” the injected material into a plug. This plug remains in the gate to prevent drool of molten material from the gate when the mold is open for the ejection of the molded part. In the next injection portion of the cycle, the cooling applied to the gate is removed and hot molten material from the injection molding machine pushes the plug into the mold cavity, where it melts and mixes with the newly provided molten material.  
           [0006]    In valve gating, the opening and closing of the gate is independent of injection pressure and/or cooling and is achieved mechanically with a valve stem. This stem can be moved between an open position, wherein flow of molten materials through the gate is permitted, and a closed position wherein the gate is closed by entry of the valve stem into the gate which establishes a seal, preventing molten materials from passing through the gate. Valve gating is well known and examples of such systems are shown in U.S. Pat. Nos. 2,878,515; 3,023,458; and 3,530,539, each being incorporated herein by reference.  
           [0007]    Generally, for situations that require improved aesthetics, valve gating is preferable to thermal gating because it can reduce the undesired gate vestige which results on the finished molded part. However, there are problems with valve gating systems.  
           [0008]    Specifically, the valve stem and gate each have a complementary sealing portion which are brought into contact to seal the gate. Typically there is a 0.001″-0.002″ diametrical clearance between the valve stem and the gate sealing portion. As the valve stem is moved into alignment with the sealing portion of the gate to effect sealing, a slight misalignment of the stem with the gate will cause the stem to strike the gate sealing portion. Over time, this will cause the gate area to wear and become misshapen. Now that the gate sealing area is worn, the stem no longer stops the flow of molten material and a small amount of molten material will migrate between the stem and the worn gate sealing area. This leakage adversely impacts the vestige quality because as the mold is opened, the now solidified material between the gate and the valve stem will cause a tear or blemish to form along the vestige of the part, and in extreme cases, the tearing can propagate to the surface of the molded article or preform.  
           [0009]    Following the injection cycle, typically the mold halves will open and the molded article in a somewhat solidified state will be removed from the area of the stem/gate area. Due to the entrapped molten material between the worn gate area and the stem, the molded article will not break away cleanly when the mold is opened, but rather will tear away from the gate area, which results in a blemished vestige on the molded article.  
           [0010]    Referring to FIGS. 1 and 2 this phenomenon can be clearly seen. As well known in the art, a nozzle assembly  10  is comprised of an elongated nozzle bushing  12  with a nozzle tip  16  affixed co-axially therein. Optionally, an insulator  14  is affixed to a proximal end of the nozzle tip  16  thereby thermally insulating the heated nozzle assembly  10  from the cooled cavity plate  34 . A movable valve member  18  extends co-axially in the nozzle assembly  10  and is selectably positioned in or out of a passageway/gate area  22 . A melt channel  20  surrounds the valve member  18  and runs the length of the nozzle assembly  10  to communicate a flowable material to a mold cavity  28 . When the valve member  18  is placed in a fully closed position (as shown in FIG. 1), a sealing portion  25  in the cavity plate  34  sealingly surrounds the valve member  18  to shut off the flow of material to the mold cavity  28 . As shown in FIG. 1, a face portion  21  of valve member  18  defines the entire top of the vestige  26  of the molded article. A chamfer  36  is typically provided along the face of the valve member  18  to help guide the valve member into the gate area and reduce wear of the valve member and cavity plate  34 .  
           [0011]    Due to the close fit of the valve member  18  to the sealing portion  25 , any misalignment that exists between their respective interfaces will cause the valve member  18  to strike the surface of the sealing portion  25  which will ultimately lead to a deterioration of the seal portion  25  and/or the valve member  18 .  
           [0012]    At the end of the injection cycle, the valve member  18  is moved into its closed position as previously described and the mold cavity is held in a closed position with a core  30  for a predetermined cycle time to allow the molten material to cool and solidify, thereby forming the molded article. Once the molded article has been allowed to cool to a sufficient level, the core  30  with the molded article thereon is moved in the direction as denoted by arrow A, and the vestige  26  is pulled away from the face portion  21  of the valve member  18 . If enough wear exists between the valve member  18  and the sealing portion  25 , a small amount of molten material will migrate therein, and as the mold core  30  and the molded article  27  are moved to an open position, a peeled edge  38  will form on the vestige  26  of the molded article  27 .  
           [0013]    Also, as the valve member  18  is in the flow of molten material when the gate is open, it can become quite hot. When the gate is closed by the valve member  18 , the hot tip of the valve member  18  can be difficult to cool as the mold cavity  28  is cooled and this can result in a need for increased cycle times to permit the necessary cooling, and/or can result in undesirable characteristics in the molded article  27 . Specifically, as the material in the mold cavity  28  adjacent the valve member  18  is cooled less efficiently due to the hot tip, parts molded from thermally sensitive materials such as PET can suffer from an enlarged area of crystallinity  40  or other undesired characteristics. In addition, since the entire top surface of the vestige  26  is in contact with the face portion  21  of the hot valve member  18 , the molten material adjacent the face portion  21  remains somewhat molten and stringing and an uneven edge forms when the mold is opened.  
           [0014]    Therefore there is a need for an improved injection-molding machine with a valve gate system that reduces or obviates some or all of the drawbacks of the prior art.  
         SUMMARY OF THE INVENTION  
         [0015]    The primary objective of the present invention is to provide an improved injection molding system with a valve gating system that reduces or obviates the drawbacks of the prior art.  
           [0016]    Another object of the present invention is to provide an insert that interfaces with a valve member in an injection molding system that reduces or eliminates the formation of peeled edges along a vestige of a molded article.  
           [0017]    Yet another object of the present invention is to provide a gate insert in the mold plate adjacent the valve member that may be easily replaced.  
           [0018]    The foregoing objects are achieved by providing a mold cavity with a vestige cross-sectional area that is larger than the cross-sectional area of the valve member so that the periphery of the vestige is cooled quicker than the interior portion of the vestige. In another preferred embodiment, a replaceable insert is provided to help guide the valve member into a sealing position with the gate. Replacement of this insert can easily be performed whenever the wear of the insert reaches a predetermined and unacceptable level.  
           [0019]    Further objections and advantages of the present invention will appear hereinbelow.  
       
    
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a simplified cross-sectional view of an injection molding nozzle in accordance with the prior art;  
         [0021]    [0021]FIG. 2 is a partial cross-sectional view of a molded article in accordance with the prior art;  
         [0022]    [0022]FIGS. 3 and 3 a  are simplified cross-sectional views of exemplicative embodiments in accordance with the present invention;  
         [0023]    [0023]FIGS. 4 a  and  4   b  are partial cross-sectional views of alternate exemplicative embodiments in accordance with the present invention;  
         [0024]    [0024]FIG. 5 is a simplified partial cross-sectional view of a molded article.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring now to FIG. 3, an injection molding nozzle assembly  10  in accordance with the present invention is located in a cavity plate  34  for the communication of a flowable material to a mold cavity  28  for the formation of a molded article therein. Cavity plate  34  is provided with a plurality of cooling passageways  32  therein for the communication of a cooling fluid for the removal of heat from the cavity plate to cool and solidify the flowable material in the mold cavity  28 .  
         [0026]    The nozzle assembly  10 , as well known in the art, is comprised of an elongated nozzle bushing  12  with a nozzle tip  16  affixed to a proximal end of the nozzle bushing. In a preferred embodiment, the nozzle tip  16  is threaded to the nozzle bushing  12 , but any such suitable means could be used. Typically, a heater  17  is wrapped around the nozzle assembly  10  to maintain the flowable material in a viscous state. In the preferred embodiment an optional insulator  14  is located between the nozzle tip  16  and the cooled cavity plate  34  to reduce the transfer of heat from the hot nozzle tip  16  to the cooled cavity plate  34 . Located co-axially in the nozzle assembly  10  is a movable valve member  18  that extends adjacent a vestige  26  of the mold cavity  28 . In a preferred embodiment, the valve member  18  is a slender elongated cylindrical piece that is moved up and down to an open and closed position respectively. When the valve member  18  is in the open position as shown by phantom line  50 , the flowable material in melt channel  20  is allowed to enter the mold cavity  28 . When placed in the closed position, as shown in FIG. 1, the valve member  18  is in sealing communication with a sealing portion  25  thereby stopping the flow of material to the mold cavity  28 .  
         [0027]    In one preferred embodiment, an insert  42  with a passageway  41  formed therein is placed in a cavity  44  located in the cavity plate  34  in alignment with the valve member  18 . In this arrangement, the sealing portion  25  is located in this replaceable insert  42  to allow for easy maintenance when leakage around the valve member  18  starts to occur.  
         [0028]    The insert  42  may optionally provide a first chamfer  46  to help guide the valve member  18  when it first enters the passageway  41  and a second chamfer  48  to help guide the valve member further into the sealing portion  25 . These chamfers act to reduce wear on both the valve member  18  and the insert  42  and prolong the useable life of both components.  
         [0029]    Vestige  26 , as shown in the figures, has a cross-sectional area larger than the face portion  21  of the valve member  18 . As such, a portion  23  of the vestige  26  is in thermal communication with the insert  42 . Given that the insert  42  is placed in the cooled cavity plate  34 , the insert  42  will cool portion  23  faster than the portion in contact with face portion  21  of the hot valve member  18 . This differential cooling action will allow portion  23  to solidify before the area adjacent face portion  21 . When core  30  is retracted to remove the molded article from the mold cavity  28 , this now solidified portion  23  will tend to breakaway more cleanly than the prior art. In addition, due to the location of the sealing portion  25  being internal and displaced from the outer surface of the finished vestige  26 , any tearing that may occur when the mold is opened is reduced or eliminated because the tear is not on the surface of the vestige/preform as in the prior art.  
         [0030]    As shown in FIG. 5, this clean break will result in a more uniform and flat vestige  26  than previously seen. In addition, a reduced area of crystallinity  40  will form inside the molded article  27  due to the improved cooling of the vestige  26 .  
         [0031]    Referring to FIG. 3A, an alternative embodiment in accordance with the present invention is shown which is identical to the embodiment in FIG. 3 except for the removal of the insert  42 . As shown in FIG. 3A, the sealing portion  25  is now located in the cavity plate  34 . Cooling of portion  23  will still occur quicker than in the remainder of the vestige  26 , which will allow for a substantially clean break when mold core  30  is retracted.  
         [0032]    [0032]FIGS. 4 a  and  4   b  (where like features have like numerals) show alternative embodiments of the insert  42  and the valve member  18 . As shown in FIG. 4 a , the valve member  18  has a chamfer  54  near the vestige  26 . A reduced diameter section of the valve member is in sealing communication with the sealing portion  25  when in the closed position. At least one elongated recess  56  is formed in the surface of the valve member  18  which allows the flowable material to be forced up along the valve member  18  as the valve member is brought to the closed position. Chamfers  46  and  48  help guide the valve member  18  as it enters the passageway  41  and seats in the sealing portion  25 . FIG. 4 b  shows the valve member  18  as one continuous cylinder down to the chamfer at the very bottom. The passageway  41  in the insert  42  is also a uniform diameter for most of its length, except for the lead in chamfer  46 . In this embodiment, the sealing portion  25  can be longer and provide a longer lasting seal. The recess  56  allows the flowable material to flow up out of passageway  41  as the valve member  18  is brought to a closed position.  
         [0033]    It is to be understood that the invention is not limited to the illustrations described herein, which are deemed to illustrate the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. The invention is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.