Abstract:
The invention provides a replaceable bushing in the base of a nozzle of a hot runner assembly and provides access through a manifold and a rear plate for removal and replacement of the bushing without having to disassemble the hot runner assembly. According to one embodiment of the invention, an actuator access port is provided for removing the valve pin actuator and a bushing access port is provided for accessing the bushing upon removal of the valve pin actuator, both of which may be removed from a second or rear plate of the injection molding assembly.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to injection molding apparatus. More particularly, this invention relates to hot runner arrangements utilizing a valve pin to control melt flow. Still more particularly, this invention relates to a guide bushing for guiding reciprocal movement of the valve pin and replacement of the bushing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Valve gated injection molding apparatus is well known. Such apparatus utilizes a valve pin slidably received in a valve gate to control flow of melt when molding a part, to ensure a positive shut-off of the gate. While this technology has been utilized for decades in view of its positive shut-off attributes, it suffers from leakage problems upstream of the gate.  
         [0003]     The valve stem moves slidably within a tight fitting bushing secured to a rear face of a nozzle housing. The nozzle housing provides a melt passage from a manifold to a nozzle tip mounted to the nozzle housing. The nozzle has a cap which tip registers with the gate. The valve pin extends through the bushing, through the melt passage and into a melt opening in the gate. The bushing supports the valve pin for its slidable movement and furthermore provides a seal to prevent melt from flowing out of the nozzle housing along the valve pin.  
         [0004]     As one would expect, over a period of time, as the valve pin rubs against the bushing, wear will occur. Eventually the wear will create a significant gap between the valve pin and the bushing enabling melt to leak out between the two components, which may affect movement of the valve pin.  
         [0005]     None of the present valve gated systems utilizing a valve pin bushing are maintenance friendly. Accordingly, replacement of the bushing generally requires removal and complete disassembly of the mold. Generally, the bushings are retained within the nozzle housing and accessible only upon removal of the nozzle housing which in turn requires removal of the portions of the mold in which the nozzle housing is mounted. At a minimum, a rear plate of the mold must be removed which in effect amounts to disassembly of the mold whether or not this is done in situ.  
         [0006]     It is an object of the present invention to facilitate bushing removal for valve gate maintenance to enable replacement of such wear components without disassembly of the hot runner.  
       SUMMARY OF THE INVENTION  
       [0007]     In very general terms, the invention provides a replaceable bushing in the base of a nozzle of a hot runner assembly and provides access through a manifold and a rear plate for removal and replacement of the bushing without having to disassemble the hot runner assembly. According to one embodiment of the invention, an actuator access port is provided for removing the valve pin actuator and a bushing access port is provided for accessing the bushing upon removal of the valve pin actuator, both of which may be removed from a second or rear plate of the injection molding assembly.  
         [0008]     More particularly, a valve gated injection molding assembly is provided which has first and second plates in face to face juxtaposition with the front face of the second plate facing the first plate and defining a manifold cavity therebetween for receiving a melt manifold. A nozzle extends through the first plate and has a base for contacting the manifold and a tip opposite the base. The nozzle has a passage extending therethrough between the base and the tip along which melt can flow from the manifold to the tip. A valve pin is slidably mounted in the passage. The valve pin has a tip end for extending through the nozzle tip to open and close a melt passage through a gate. The valve pin further has an actuator end opposite the tip end. A valve pin guide bushing extends from the base of the nozzle for receiving and guiding the valve pin for movement along the passage. The valve pin guide bushing removably engages the base of the nozzle for removal in a direction away from the base toward the second plate. A valve pin actuator is mounted to the second plate and connected to the valve pin activator end for moving the valve pin along the passage for the opening and closing of the melt outlet. The second plate has an actuator access port for removal of the valve pin actuator from a rear face of the second plate opposite the front face. The second plate further has a bushing access port for removal of the bushing therethrough absent the actuator.  
         [0009]     The valve pin bushing may extend through the manifold. Space may be provided between the valve pin bushing and the manifold to allow thermal expansion of the manifold without such thermal expansion exerting pressure on the valve pin bushing.  
         [0010]     The actuator may be a piston slidably mounted in a bore in the second plate and slidable along the bore in response to fluid pressure.  
         [0011]     A heat insulator (also referred to as a “manifold insulator”) may extend between the second plate and the manifold, with the valve pin extending through the insulator.  
         [0012]     A retainer may extend about the valve pin between the valve pin and the insulator. The retainer may abut against the valve pin bushing to retain the valve pin bushing against movement away from the nozzle. The retainer may be secured to at least one of the insulator and the manifold. The retainer is removable through the bushing access port.  
         [0013]     A gap may be provided between the valve pin and the retainer to allow for movement of the retainer radial to the valve pin without applying pressure to the valve pin.  
         [0014]     The retainer may threadedly engage the insulator. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0015]     Preferred embodiments of the present invention are described below with reference to the accompanying illustration in which:  
         [0016]      FIG. 1  is a sectional view along a valve pin axis of a valve gated injection molding assembly according to the present invention.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]     An injection molding assembly is generally indicated by reference  100 . It includes a first plate  112  in face to face juxtaposition with a front face  120  of a second plate  113 . A manifold cavity  122  is defined between the first plate  112  and  113 . The manifold cavity  122  receives a manifold  114  having a melt passage  108  along which melt flows.  
         [0018]     A nozzle comprising a nozzle housing  107  and a nozzle tip  111  extends through the first plate  112 . The nozzle housing has a base  124  which contacts the manifold  114 . The nozzle has a passage  126  extending therethrough between the base  124  and the nozzle tip  111 . The passage  126  fluidly communicates with the melt passage  108  to provide a path for melt from the melt passage  108  to the nozzle tip  111 . The nozzle tip  111  has a melt outlet  121  which registers with a corresponding outlet or “gate”  123  in a gate insert  116  which forms part of a mold  115 . The valve tip  111  is retained in place in the nozzle housing  107  by a valve cap  110  which threadedly engages the nozzle housing  107  and presses against a base flange  109  of the nozzle tip.  
         [0019]     A valve pin  104  is slidably mounted in the passage. The valve pin  104  has a tip end  130  for interfacing with the outlet  123  in the gate insert to open and close the gate  123 . The valve pin  104  has an actuator end  132  opposite the tip end  130 .  
         [0020]     A valve pin guide bushing  106  extends from the base  124  of the nozzle housing  107  for receiving and guiding the valve pin  104  for movement along the passage  126 . The valve pin guide bushing  106  removably engages the base  124  of the nozzle housing  107  and is removable in a direction away from the base  124  toward the second plate  113  (i.e. to the right as illustrated).  
         [0021]     The valve pin guide bushing  106  would generally be a tight sliding fit in the nozzle housing  107  and would typically have a melt passage  134  registering with the melt passage  108  and fully communicating with the passage  126  to provide a conduit for melt from the manifold  114  into the passage  126 .  
         [0022]     While this is “typical”, it is conceivable that the valve pin guide bushing  106  would be shorter than illustrated and wouldn&#39;t require a passage therethrough to act as a melt conduit.  
         [0023]     A valve pin actuator having a piston  101  is mounted to the second plate  113  and connected to the valve pin connector  132 . The piston  101  is mounted within a bore  146  in an insert  100  is secured to the valve pin  104  at the valve pin connector  132 . The valve pin actuator is responsive to fluid pressure between the bore  146  and the piston  101  to cause movement of the valve pin  104  along the passage  126  for opening and closing the melt outlet  112 .  
         [0024]     The second plate  113  has an actuator access port  140  through which the valve pin actuator may be removed from a rear face  142  of the second plate  113  which is opposite the front face  120 . In the arrangement illustrated this would require first removing the insert  100 . The second plate  113  further has a bushing access port  144  between the actuator access port and the bushing  106 . The access port is dimensioned to have a breadth or diameter greater than that of the bushing  106 . Accordingly upon removal of the actuator  100 ,  101 , which would typically be accomplished by also withdrawing the valve pin  104 , the bushing  106  may be withdrawn through the bushing access port through the second plate. Accordingly, bushing removal may be effected without separating the first and second plates  112  and  113  respectively, removal of the manifold  114  or disassembly of the nozzle assembly  107 ,  111 .  
         [0025]     In order to avoid stress being applied to the valve pin  104  as a result of differential thermal expansion of the various components of the injection molding assembly  100  and in particular the manifold  114  relative to the first plate  112  and  113 , a space  150  is provided between the manifold  114  and the valve pin bushing  106 .  
         [0026]     In order to provide adequate clamping force between the manifold  114  and the base  124  of the nozzle housing  107 , preferably the manifold  114  is clamped between the first plate  112  and  113 . It is however desirable to avoid loss of heat from the manifold  114  as this would in turn cause thickening of melt flowing along the melt passage  108 . Accordingly, a heat insulator  103  may be provided which extends between the second plate  113  and the manifold  114 . In addition, the nozzle housing  107  generally extends beyond an inside face  160  of the first plate  107  enabling the nozzle housing  107  to contact the manifold  114  without the manifold  114  abutting against the first plate  112 . Accordingly, the manifold  114  is clamped between the first plate  112  and  113  by virtue of being pressed between the base  124  of the nozzle housing  107  and the insulator  103 .  
         [0027]     The valve pin  104  extends through the insulator  103  and a retainer  102 , such as the tubular element illustrated may be provided through the insulator  103  to press against the valve pin bushing  106  to prevent its movement toward the second plate in response to melt pressure within the passage  126 . The retainer  102  is illustrated as threadedly engaging the insulator  103 . Other arrangements may be possible, for example the retainer  102  may simply threadedly engage the second plate  113  without requiring securement to the insulator  103 . Other securing arrangements may also be possible such as using screw fasteners to secure the retainer  102  to the insulator  103 .  
         [0028]     As with the manifold  114 , a gap  162  may be provided between the valve pin  104  and the retainer  102  to allow for movement of the retainer radial to the valve pin  104  without applying pressure to the valve pin  104 .  
         [0029]     Although a piston and cylinder type of valve pin actuator  101 ,  100  is illustrated, other arrangements may be possible such as lever, screw or solenoid type arrangements. The invention resides in accessing the valve pin bushing through the second plate  113  upon removal of the valve pin actuator  101 ,  100  and not the specific valve pin actuator assembly selected.  
         [0030]     Variations to the above preferred embodiments may be apparent to persons skilled in such structures without departing from the spirit and scope of the present invention which is defined below in the accompanying claims.