Abstract:
A valve gating arrangement for an injection molding system having an insulated manner and a heated torpedo across from a gate. The valve gating arrangement includes a valve member sidably received within a passage extending through the torpedo. The valve member terminates at one end with a pin reciprocally movable into and out of the gate. An actuator is connected to the valve member to effect the reciprocal movement.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to plastic injection molding systems. More specifically, this invention relates to insulated runner systems and in particular, to the commencement and cessation of melt flow at a gate associated with such runner systems. 
     BACKGROUND OF THE INVENTION 
     There are two basic types of hot runner molds, namely, insulated runner molds and true hot runner molds. In an insulated runner mold, an unheated runner extends between a machine nozzle and a mold cavity. In use, molten plastic (“melt”) flows from the nozzle through the runner into a space defined between respective mold faces of a mold cavity and a mold core to form a part. During use, a portion of the plastic solidifies adjacent the runner walls and acts as an insulator for a molten core of plastic, hence the term “insulated runner”. 
     Immediately adjacent the mold face of the mold cavity is an aperture referred to as a “gate” through which melt exits the insulated runner to form the part. It is of course necessary to interrupt flow through the gate for mold opening and stripping portions of an injection molding cycle. In insulated runner systems, melt is generally allowed to “freeze” in the gate prior to mold opening. Early systems would re-open the gate by pushing the frozen “slug” of plastic into the next part by incoming melt which would also melt the slug. 
     More recently developed systems use a nozzle heater, generally referred to by persons skilled in such devices as a “torpedo” to better control melt temperature adjacent the gate. The torpedo is situated in an area referred to as a “runner drop” at a gate end of the insulated runner. The use of a heated torpedo allows longer cycle times by extending the critical time in which freezing could occur which would require “restarting” of the molding process. 
     A drawback to insulated runner systems is the presence of a “vestige” on the part in the vicinity of the gate and “stringing” which result from melt in the gate adhering to the part during mold opening. In order to produce more acceptable gate vestiges, the gate should ideally be closeable other than by a slug of plastic after injection and before mold separation. Various arrangements have been devised to date. Each has its drawbacks. 
     U.S. Pat. No. 3,159,878 to Scott teaches an insulated runner mold that uses a moveable core plug in the mold to keep the gate closed until sufficient pressure has built up inside the runner to overcome the closing force of the plug thereby “blasting open” the gate to rapidly fill a thin wall container mold. 
     U.S. Pat. No. 5,037,598 to Akselrud teaches a moveable heated torpedo in an insulated runner drop that acts as a gate valve. However, to open the gate the torpedo is advanced toward the gate to melt the cold slug therein and thereafter the torpedo is retracted to allow melt to flow. When the mold is filled the gate remains open and freezes off, it is not closed by advancing the torpedo. 
     Both these approaches produce gate vestiges on the part the size of which are a function of gate diameter. The larger the diameter the larger the vestige. Large gate diameters promote good flow and rapid filling of the mold cavity when high viscosity, or filled resins are being processed. In contrast smaller gate diameters, which give smaller vestiges, make filling slower and more difficult for filled resins and for making thin walled parts. 
     It is an object of the present invention to provide a valve gated insulated runner to enable maximization of gate opening while substantially eliminating the gate vestige to produce a substantially blemish free part in any application. 
     SUMMARY OF THE INVENTION 
     A valve gating arrangement for an injection molding system having an insulated runner terminating in a gate through which melt is dischargeable into a space defined between a mold core and a mold cavity. The valve gating arrangement includes a fixed torpedo having a heated tip and a passage extending longitudinally therethrough. The heated tip extends into the insulated runner adjacent the gate. A valve member with a valve stem is slidably received in the passage. The valve member has a pin reaching from the valve stem into the gate. The valve member is reciprocally slidable between a closed configuration and an open configuration. In the closed configuration the pin extends into the gate to block melt flow. In the open configuration the pin is withdrawn from the gate to permit melt flow therethrough. An actuator is connected to the valve member for moving the valve member between the open and closed configurations. 
     The actuator may be a pneumatically operable piston. 
     A thermocouple may be provided in the heated tip to monitor melt temperature in the vicinity of the gate. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described below with reference to the accompanying drawings in which: 
     FIG. 1A is a sectional view through a prior art insulated runner mold having a fixed heated torpedo; 
     FIG. 1B is an enlargement of the area indicated by reference  1 B in FIG. 1A; 
     FIG. 2A is a sectional view through a prior art insulated runner mold having a movable heated torpedo; 
     FIG. 2B is an enlargement of the area indicated by reference  2 B in FIG. 1A; and, 
     FIG. 3 is a sectional view through a valve gated runner mold according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B are illustrative of a conventional insulated runner mold arrangement generally indicated by reference  10  in FIG.  1 A. An insulated runner  12  has an inlet  14  and is illustrated as supplying two gates  16 , each of which supplies melt for the molding of respective parts  18 . Although two gates  16  and parts  18  are illustrated, persons skilled in such arrangements will appreciate that other numbers are feasible. 
     A heated torpedo  20  extends into a runner drop  22  adjacent each gate  16 . The torpedo  20  has a a heater  24  adjacent a tip  26  which provides heat in an amount sufficient to permit melt flow. This arrangement, which is discussed in the background above, relies on melt solidification rather than valve gating to interrupt melt flow. 
     FIGS. 2A and 2B illustrate an arrangement in which a movable torpedo  30  is utilized for valve gating. Insofar as the structure illustrated in FIGS. 2A and 2B is similar to the structure illustrated in FIGS. 1A and 1B, similar reference numerals have been used and the above description applies. 
     The torpedo  30  has a tip  32  heated by a heater  34 . Electrical leads  36  extend from the heater  34  along an inside of the torpedo  34  and exit the torpedo  30  to the right as illustrated. The electrical leads  36  provide power to the heater  34 . 
     The torpedo  30  is slidably mounted in a bushing  38  to allow axial movement of the torpedo  34 . A double acting pneumatically actuatable piston  40  is connected to the torpedo  30  to cause axial movement of the torpedo  30  as indicated by the arrows at reference  42 . The movement of the torpedo  30  by the piston  40  causes the tip  32  of the torpedo  30  to move into and out of the gate  16  to respectively close and open the gate  16 . 
     Although the FIGS. 2A and 2B arrangement does provide a good vestige it does have certain drawbacks. Firstly, it has a relatively large diameter as it must act both as a heater and as a valve. Secondly, as the torpedo  30  is hot, it is prone to back leakage between it and the bushing  38 . Finally, the electrical leads  36  move with each back and forth cycle of the torpedo  30  thereby giving rise to frequent electrical failures. 
     A valve gating arrangement according to the present invention is generally indicated by reference  50  in FIG.  3 . The valve gating arrangement  50  includes a torpedo  52  having a heated tip  54  extending into an insulated runner  56  in a runner drop  58  adjacent a gate  60 . A passage  62  extends longitudinally through the torpedo  52 . 
     The torpedo is fixed in that it is securely mounted to a mold base  64  so as to be immovable relative to the gate  60 . Electrical leads  66  extend from the heated tip  54  providing electrical current to a tip heater  55  in the tip  54 . 
     A valve member  70  has a stem  72  slidably received in the passage  62 . The valve member  70  has a pin  74  which reaches from the valve stem  72  in the passage  62  toward the gate  60 . Although the pin  74  is illustrated as being of reduced diameter this may not always be the case as the diameter of the pin  74  will depend on the breadth of the gate  60 . 
     The valve member  70  is reciprocally slidable in the direction of arrows  76  between a closed configuration illustrated at the bottom of FIG.  3  and an open configuration illustrated at the top of FIG.  3 . In the closed configuration the pin  74  extends into the gate  60  to block the gate  60 . In the open configuration the pin  74  is withdrawn from the gate  60  to allow melt to flow through the gate  60 . 
     An actuator such as a pneumatically actuatable piston  80  is connected to the valve member  70  to move the valve member  70  between the closed and open configurations. The valve member  70  is illustrated as having an actuator end  82  of the valve stem  72  distal the pin  74  connected to the piston  80 . 
     In order to provide enhanced control of melt temperature, a thermocouple  84  may be provided in the heated tip  54  to monitor the temperature of the tip  54 . Thermocouple leads  86  extend from the thermocouple  84  out of the torpedo  52 . 
     As the torpedo  52  is fixed, the electrical leads  66  and the thermocouple leads  86  do not move back and forth as in the arrangement illustrated in FIGS. 2A and 2B above. Hence the reliability of the thermocouple  82  and the heater  55  are enhanced as lead failure is less likely. 
     As the torpedo  52  is fixed, it can be properly sealed to eliminate melt seepage to which the movable torpedo arrangement illustrated in FIGS. 2A and 2B is prone. 
     In the valve gating arrangement of the present invention, the valve member  70  is relatively slender and lightweight compared to the size and weight of a typical torpedo such as the torpedo  30  in FIGS. 2A and 2B thereby significantly reducing the reciprocating mass to be moved in and out of the gate  60 . 
     The above description is intended in an illustrative rather than a restrictive sense. Variations may be apparent to persons skilled in injection molding apparatus without departing from the spirit and scope of the invention as defined by the claims set out below.