Abstract:
Methods for enhanced purging of an injection molding shooting pot assembly are provided. Old melt is purged from a shooting pot having an injection plunger slidably received in an injection cylinder. The plunger is moved by a powered piston, which moves the injection plunger to a purging position. The plunger is then arrested in the purging position. Sufficient new melt is injected through an inlet positioned such that the new melt sweeps substantially an entire volume of the injection cylinder ahead of the injection plunger in flowing between the inlet and a single outlet remote from the inlet.

Description:
FIELD OF THE INVENTION 
     This invention relates to pre-plasticizing injection molding machines using shooting pots as intermediate reservoirs of melted plastic resin. More particularly this invention relates to apparatus and methods for improving colour changing of resins to reduce changeover time and resin wastage. 
     BACKGROUND OF THE INVENTION 
     A conventional two stage or “pre-plasticizing” injection molding machine comprises, in general, a plasticizing component having a screw in a heated barrel and an injection component having an injection plunger. Polymer resin plasticized in the plasticizing component is transferred to the injection component&#39;s chamber, or “shooting pot”, by way of a feed channel and is injected into a mold cavity through a nozzle by the injection plunger. Conventionally, a distributor or ball check valve in the feed channel prevents resin in the shooting pot from flowing backwards to the plasticizing component when the injector plunger injects the resin. 
     When the resin is changed to a different colour or different resin type, the previously processed resin must be purged from the extruder and the shooting pot before processing using the new colour or resin can begin. Typically, the new resin is used to purge the old resin. 
     U.S. Pat. No. 4,290,701 to Schad shows a typical pre-plasticizing injection unit for a molding machine. The shooting pot design shown in the &#39;701 patent is a “last-in-first-out” style because the same port to an injection cylinder is used to both fill the cylinder and empty it. Although the injection plunger has a profile matching both the cylinder bore and an internal surface of a shooting pot head covering an end of the cylinder, and the injection plunger can be “bottomed out” when advanced until it contacts the cylinder head, some resin remains behind when purging. Because the “remaining” resin may remain in the shooting pot longer than most of the through-feeding resin, it can degrade and mix into the melt stream causing imperfections in the molded articles. Multiple feedings of the new resin to and from the shooting pot are typically required to clean out the prior resin. This can be both time consuming and wasteful of expensive resin material. For example, if a large component such as a garbage bin is being molded, changeover may require 50 to 100 cycles and takes a significant amount of time to complete. 
     U.S. Pat. No. 2,950,501 to Harkenrider is an early example of a “first-in-first-out” shooting pot design. By placing an entry port to the shooting pot at the maximum volume location of the plunger, the first resin entering the cylinder will move to the cylinder head end of the injection cylinder adjacent an exit port so that when the plunger is advanced, the “first-in” resin is injected into the mold, or purged as the case may be. Similarly, the “last-in” resin will be the last to be expelled. The disadvantage of this design is that variable shooting pot volumes, requiring a different shooting pot piston starting position other than maximum volume cannot be easily accommodated. The piston must be fully retracted each time the shooting pot is to be loaded prior to a molding cycle. Partial fillings cannot be made without risking air entering the cylinder and mixing with the resin. 
     U.S. Pat. No. 5,380,186 to Hettinga et al. shows a shooting pot cylinder having a hollow piston through which the resin is fed to the shooting pot. This “first-in-first-out” style allows for various volume fillings of the cylinder, but means that the construction of the injection unit becomes very long as the extruder and shooting pot are mounted on the same center line. 
     U.S. Pat. No. 5,814,358 to Bock shows another arrangement of a “first-in-first-out” shooting pot and extruder. In this arrangement, the shooting pot plunger is mounted inside a moveable shooting pot cylinder which is itself mounted inside an outer heated cylinder. The complication of this construction adds more places for the resin to “hangup” and degrade and increases cost. 
     U.S. Pat. No. 5,858,420 to Szajak et al. shows a “first-in-last-out” arrangement having separate channels used for the transfer of resin and the injection of resin. Transferred resin is brought into the shooting pot head behind resin retained from previous shots by the use of a plurality of channels communicating with the periphery of a conical tip of the shooting pot plunger head. The melt is therefore distributed more uniformly into the shooting pot and during injection, resin retained from the last cycle is injected first through the separate injection channel. Although this arrangement gives improved purging, some stagnation nevertheless exists requiring numerous cycles to properly effect a resin type or colour changeover. 
     SUMMARY OF THE INVENTION 
     A method is provided for purging a shooting pot having an injection plunger slidably received in an injection cylinder. The method comprises the steps of: 
     i) moving the injection plunger to a purging position; 
     ii) arresting the injection plunger in the purging position; 
     iii) injecting melt into the injection cylinder through a melt inlet; and, 
     iv) discharging melt through an outlet remote from the inlet. 
     Preferably, the inlet and outlet are positioned to cause injected melt to sweep substantially an entire volume of the injection cylinder ahead of the injection plunger as injected melt flows between the inlet and the outlet. 
     In one embodiment, in the purging position, a tip of the injection plunger is positioned adjacent a corresponding shooting pot head covering an end of the injection cylinder. The melt inlet is adjacent an outer periphery of the tip and the melt outlet is generally co-axial with the injection cylinder. 
     In an alternate embodiment, the injection cylinder is provided with at least one outlet ordinarily covered by the piston and in moving the injection plunger into the purging position, the injection plunger is overstroked to uncover the outlet. 
     The outlet in the alternate embodiment may be a drool port which leads from a channel extending circumferentially about the injection cylinder and in the purging position the injection plunger at least partially uncovers the channel. 
     The position of the injection plunger in the purging position may be incrementally variable to vary a degree to which the circumferential outlet is uncovered to optimize purging. 
     A shooting pot is provided for an injection molding machine. The shooting pot has an injection cylinder with a shooting pot head covering an end thereof. An injection plunger is slidably received in the bore in a substantially fluid sealed manner. A melt passage extends through the shooting pot head into a first end of the bore. A drool port is located distal the melt passage for discharging melt seeping between the injection plunger and the injection cylinder. An actuator moves the injection plunger reciprocally along the bore over a first distance corresponding to an injection stroke. The injection plunger covers the drool port over the injection stroke. An overstroker is provided to move the injection plunger along the bore to a purging position in which the drool port is at least partially uncovered. The shooting pot has an arrestor for arresting the injection plunger in the purging position. 
     The actuator may also act as the overstroker and a channel may extend circumferentially about the bore at the drool port. The shooting pot may also include a distributor valve for selectively providing fluid communication between the melt passage and either of an extruder feed line and a nozzle. 
     An alternate embodiment shooting pot is provided which has an injection cylinder with a bore, a shooting pot head over an end of the bore and an injection plunger slidably received in the bore in a substantially fluid sealed manner. At least one inlet extends through the shooting pot head and at least one outlet also extends through the shooting pot head. The shooting pot includes a reconfigurable flow controller moveable between load, eject and purge configurations. In the load configuration, the flow controller allows melt flow into the inlet and prevents melt flow at the outlet. In the eject configuration, the flow controller prevents melt flow at the inlet and allows melt flow out of the outlet. In the purging configuration, the flow controller allows melt flow into the bore through the inlet and out of the bore through the outlet. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described below with reference to the accompanying drawings in which: 
     FIG. 1 is an isometric view of a shooting pot according to the present invention; 
     FIG. 2 is a section on line  2 — 2  of FIG. 1; 
     FIG. 3A is a partially cut away enlarged isometric view of the area encircled by circle  3  in FIG. 2; 
     FIG. 3B is a partially cut away plan view of the area encircled by circle  3  in FIG. 2; 
     FIG. 4 is a view corresponding to FIG. 3B of an alternate embodiment of the present invention; 
     FIG. 5 is a cut away isometric view of an alternate embodiment of a shooting pot head according to the present invention. 
     FIG. 6 is a section corresponding to FIG. 5 illustrating a flow controller in an eject position; and, 
     FIG. 7 is a view similar to FIG. 6, but illustrating the flow controller in a load position. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A shooting pot according to the present invention is generally indicated by reference  10  in FIGS. 1 and 2. The shooting pot  10  includes an injection cylinder  12  having a bore  14 . A shooting pot head  16  extends over and covers an end of the bore  14 . The shooting pot head  16  contains an extruder feed passage  18  and a nozzle passage  20 . The extruder feed passage  18  fluidly communicates, through a connecting passage  24  in a connecting member  26 , with a head  27  of a plasticizing extruder  28  (The connecting member  26  and plasticizing extruder  28  do not appear in FIG.  1 ). 
     A distributor valve  30  is located in the shooting pot head  16  for selectively providing fluid communication between a melt passage  32  extending through the shooting pot head  16  and either of the extruder feed passage  18  and the nozzle passage  20 . The distributor valve  30  closes the nozzle passage during filling of the bore  14  and closes the extruder feed passage  18  during ejection of melt from the bore  14 . 
     An injection plunger  34  is slidably received in the bore  14  and reciprocally moveable along the bore  14  in the direction of arrows  36  by an actuator  38  which, as illustrated, may be a hydraulically operable piston. The actuator  38  is connected to the shooting pot  10  by an injection housing  40 . 
     During use for injection molding, the injection plunger  34  is reciprocally moveable over an injection stroke in which a tip  42  of the injection plunger moves from adjacent the shooting pot head  16  to the position indicated by dashed lines  44 . 
     Although the injection plunger is substantially sealed relative to the bore  14 , some melt seepage is typically permitted between the injection plunger  34  and the bore  14  for lubrication. The melt seeping past the plunger  34  is captured in a channel  46  extending about the bore  14  and discharged through a drool port  48  extending through the injection cylinder  12 . The channel  46  and drool port  48  are ordinarily covered by the injection plunger  34  over the injection stroke. 
     The injection plunger  34  is movable into an overstroked position illustrated in solid lines in FIG. 2 wherein the tip  42  of the injection plunger  34  uncovers the channel  46  and thereby also uncovers the drool port  48 . Overstroking may be achieved by an “overstroker” which may be the hydraulically actuatable piston used for the actuator  38  or possibly a separate apparatus. 
     For purging, the injection plunger  34  is moved into a purging position corresponding to the overstroked position described above. The injection plunger  34  is “arrested” in the purge position by any suitable arrestor such as hydraulic blocking of the piston forming the actuator  38 . The distributor valve  30  is moved to a configuration to admit melt from the plasticizing extruder  28  into the bore  14 . The incoming melt flows through the bore  14  into the channel  46  to exit through the drool port  48 . This provides enhanced purging as melt is caused to flow through what is usually a stagnant area at the injection plunger tip  42  adjacent the wall of the bore  14 . 
     FIGS. 3A and 3B illustrate the relationship between the injection plunger tip  14  and the channel  46  in more detail. In the purging position a gap  50  is defined between the tip  42  of the injection plunger  34  and the channel  46 . The gap  50  acts as a flow restriction which will locally cause shear heating of the melt as it enters the channel  46 . This local reheating of the resin causes it to flow more easily and flush out of the system. The breadth of the gap  50  may be varied to suit the characteristics of the particular resin being purged to create more or less shear as required. A large gap causes less shear and less heating whereas a narrower gap increase the amount of both shear and heating. 
     In order to vary the gap the injection plunger  34  should be incrementally moveable in the overstroked position. This may be achieved either using the actuator  38  or other overstroker if a separate one is provided. 
     FIG. 4 illustrates an alternate embodiment in which a plurality of drool ports  48  are provided to direct purged resin to a collector channel  52  extending about the cylinder  34  and having an outlet  54  at a lower part thereof. The FIG. 4 embodiment provides further passages for purged material to better flush out the purged material than would be expected with the single, bottom, gravity influenced drool port  48  in the FIGS. 3A and 3B embodiment. 
     FIGS. 5,  6  and  7  illustrate an alternate embodiment of a shooting pot head generally indicated by reference  116  which is adapted for purging according to the method disclosed herein. The shooting pot head  116  has a plurality of inlet passages  132  (two are shown) terminating in inlets  134  and a single outlet  136  extending into an outlet passage  138  extending therethrough. Typically four inlets  134  would be provided but other numbers are also feasible. The inlet passages  132  may all branch off of a common inlet passage  140  across from an extruder feed passage  118 . The extruder feed passage fluidly communicates with a connecting passage  124  of a connecting member  126  for connecting the shooting pot head  116  with a plasticizing extruder (not shown). 
     The outlet passage  138  is directly across from a nozzle passage  120  extending through a nozzle  122 . Bolts  142  are provided to mount the shooting pot head  116  onto a shooting pot cylinder  112  in FIGS. 6 and 7. 
     A reconfigurable flow controller  130  having a body  131  is provided to control melt flow as required. The flow controller  130  has a number of bores  144 ,  146 ,  148  and  150  extending therethrough. The flow controller  130  is axially movable in the direction of arrows  152  between load, eject and purge configurations. 
     The eject configuration is illustrated in FIGS. 5 and 6. In this configuration the bore  144  registers with the nozzle passage  120  and the outlet passage  138  to allow melt to flow out of the outlet  136 . In the eject configuration, the flow controller body  131  blocks melt flow between the extruder feed passage  118  and the common inlet passage  140  thereby preventing melt flow through the outlets  134 . The eject configuration would correspond to an injection stroke. 
     The load configuration is illustrated in FIG.  7 . In the load configuration the bore  146  is slid into registry with the extruder feed passage  118  and common inlet passage  140  to allow melt to flow from the plasticizing extruder through the inlets  134  and into the bore. In the load configuration the flow controller body  131  blocks melt flow between the outlet passage  138  and the nozzle passage  120 . The load configuration corresponds to the filling of the shooting pot. 
     For purging, bores  148  and  150  are provided which simultaneously allow melt flow through the inlets  134  and out of the outlet  136 . In the purging configuration, the bore  148  registers with the extruder feed passage  118  and the common inlet passage  140  allowing melt flow therebetween. In the purging configuration the bore  150  registers with the outlet passage  138  and the nozzle passage  120  allowing melt flow therebetween. 
     For purging, an injection plunger  160  having a tip  162  is moved into the position illustrated in FIG. 6 in which the injection plunger tip  162  is adjacent the shooting pot head  116 . The injection plunger is arrested in this position. Melt is fed through the inlets  134  from where it flows across the portion of the injection cylinder bore defined between the injection plunger tip  162  and the shooting pot head  116 , to flow out through the outlet  138 . Preferably, the position of the injection plunger is incrementally variable in the purging position to vary the breadth of a gap  164  defined between the injection plunger tip  162  and the shooting pot head  116 . The breadth of the gap  164  will affect the temperature of the melt by affecting the degree of shearing. The breadth of the gap  164  will affect the temperature of the melt by affecting the degree of shearing. The breadth of the gap  164  may be selected to suit the particular characteristics of the resin being purged. 
     It will be appreciated that the above embodiments for a shooting pot design enable enhanced purging as compared to previous designs in which an injection plunger is used for moving melt for purging. According to the method of the present invention, the injection plunger is kept stationary during purging and melt is fed in a path extending either radially inwardly or radially outwardly between an inlet and an outlet of an injection cylinder to substantially sweep the entire volume of the injection cylinder bore during purging. This is much more effective than moving melt with the injection plunger during purging as it causes melt to flow through areas which tend to be stagnant in injection plunger induced melt flow. 
     The above description is intended in an illustrative rather than a restrictive sense. Variations to the exact embodiments described above 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.