Abstract:
A valve gate system for an injection molding machine, having a valve gate unit configured to be in contact with a manifold of an injection molding machine for delivering a molten plastic flow from a hot runner system to an injection chamber. The valve gate unit has a valve pin for controlling the flow of the molten plastic from a hot runner system to an injection chamber and an activating unit coupled with the valve gate unit. The activating unit is configured to be mounted external to a mold unit that houses the injection chamber. In addition, the activating unit has an element that extends through the mold unit to engage the valve pin, so as to control the molten plastic flow from a runner system to an injection chamber.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Patent Application No. 60/502,341, filed Sep. 12, 2003, the teachings of which are incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to injection molding machines, and in particular to valve gate systems for injection molding machines, and injection molding machines having molds using valve-gate systems for controlling the injection of molten plastic into the mold chamber.  
         [0003]     Valve-gate systems have the advantage of creating a clean, flush gate mark, when minimal vestige height is required on the molded part. Apart from a cosmetic viewpoint, larger orifices allowed by valve gates prevent drooling, reduce shear heat and molded-in stress, provide easier filing and reduce injection pressure. Valve-gates are typically part of a larger unit (commonly referred to as “valve-gate unit”) that is mounted behind the gate area, in firm contact with the hot runner&#39;s manifold. More issues regarding existing valve-gate unit designs are raised below.  
         [0004]     While existing valve gate systems create quality gates on molded parts, they also suffer from certain shortcomings, as described below. 
        The valve pin or stem of a valve gate unit is actuated typically by pneumatic or hydraulic systems, included in the body of the valve-gate unit, which contributes to increase valve pin length.     Pneumatic or hydraulic actuating systems included in heated valve-gate units are continuously subjected to high temperatures, and therefore likely to suffer from problems associated with thermal expansion.     Pneumatic or hydraulic actuating systems mounted behind the manifold require cooling. If no cooling is available, they generally will require regular maintenance checks (e.g., to inspect and/or change o-rings, etc.), which adds to the overall cost of the operation of the machine.     Presence of pneumatic or hydraulic systems in valve-gate units may limit the use of back-to-back gating for stack molds. In such cases, when using a single manifold, staggered placement of gates may be required, resulting in increased projected area. It is noted that back-to-back mounting can be achieved if using multiple manifolds, but, in such cases, equalizing flow in all runners (e.g., to avoid preferential flow) becomes an issue.     Many of the existing valve-gate systems have no form of adjustment of the valve pin length. An adjustment of some sort is typically necessary to bring the valve pin flush with surrounding molding surface. Existing systems that have this adjustment still require a fair amount of work, even with the mold in the injection press, resulting in increased downtime.        
 
         [0010]     There is therefore a need for an improved valve-gate unit that does not suffer from these issues.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a valve gate system and an injection molding machine having such a valve gate system, where the activating unit of the valve gate system is located in an unheated area of the injection molding machine and where an element of the activating unit extends through the injection molding machine to engage and activate the valve gate of the valve gate unit.  
         [0012]     In one embodiment, the present invention provides a valve gate system for an injection molding machine, having a valve gate unit configured to be in contact with a manifold of an injection molding machine for delivering a molten plastic flow from a hot runner system to an injection chamber. The valve gate unit has a valve pin for controlling the flow of the molten plastic from a hot runner system to an injection chamber and an activating unit coupled with the valve gate unit. The activating unit is configured to be mounted external to a mold unit that houses the injection chamber. In addition, the activating unit has an element that extends through the mold unit to engage the valve pin, so as to control the molten plastic flow from a runner system to an injection chamber.  
         [0013]     For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings. The drawings described below are merely exemplary drawings of various embodiments of the present invention which should not limit the scope of the disclosure and claims herein. One of ordinary skill would recognize many variations, alternatives, and modifications. These variations, alternatives, and modifications are intended to be included within the scope of the present invention, which is described herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an exemplary schematic diagram of one embodiment of a valve gate unit in accordance with the present invention shown as a part of a single-face multi-cavity mold.  
         [0015]      FIG. 2  is an exemplary schematic diagram showing an enlarged detail view of the valve-gate unit of  FIG. 1 .  
         [0016]      FIG. 3  is an exemplary schematic diagram of one embodiment of a valve gate unit and the activating unit in accordance with the present invention shown as a part of a single-face multi-cavity mold.  
         [0017]      FIG. 4  is an exemplary schematic diagram showing an enlarged detail view of the activating unit of  FIG. 3 .  
         [0018]      FIG. 5  is an exemplary schematic diagram showing another enlarged detail view of the activating unit of  FIG. 3 .  
         [0019]      FIGS. 6A and 6B  are exemplary schematic diagrams showing additional detail views of the activating unit of  FIG. 3 .  
         [0020]      FIG. 7A  is an exemplary schematic diagram of a curved activating slot of the valve gate unit of  FIG. 1 .  
         [0021]      FIG. 7B  is an alternate exemplary schematic diagram of an activating slot of the valve gate unit of  FIG. 1 .  
         [0022]      FIGS. 8-9  are exemplary schematic diagrams showing engagement positions on the slot of  FIG. 7A .  
         [0023]     FIGS.  10 A-B are exemplary schematic diagrams showing engagement positions of the activating rod on the slot of  FIG. 7A .  
         [0024]      FIGS. 11-13  are exemplary schematic diagrams of alternate embodiments of a valve gate unit in accordance with the present invention shown as a part of a single-face multi-cavity mold.  
         [0025]      FIG. 14  is an exemplary side view schematic diagram of an alternate embodiment of a valve gate unit and the activating unit having a multi-piece activation bar in accordance with the present invention shown as a part of a single-face multi-cavity mold.  
         [0026]      FIG. 15  is an exemplary schematic diagram showing engagement positions of the activating rod of  FIG. 14 .  FIG. 15  shows the mold of  FIG. 14  opened, for example, for the removal of valve gate unit(s).  
         [0027]      FIGS. 16-19  are exemplary detailed view schematic diagrams of the multi-piece activation bar of  FIG. 14 ; with activating inserts and connecting bars shown separated in a top view ( FIG. 16 ); front view ( FIG. 17 ) and assembled shown in top view ( FIG. 18 ) and front view ( FIG. 19 ).  
         [0028]      FIG. 20  is an exemplary detailed schematic diagram of a connection of the pieces of the multi-piece activation bar of  FIG. 14 .  
         [0029]      FIG. 21  is an exemplary cross sectional diagram through a stack mold using back-to-back gating.  
         [0030]      FIG. 22A  is an exemplary plan view diagram of a multi-cavity mold (seen from the parting line), shown with two activating units.  
         [0031]      FIG. 22B  is an alternate exemplary plan view diagram of a multi-cavity mold (seen from the parting line), shown with two activating units and using a multi-piece activating bar.  
         [0032]      FIG. 22C  is an alternate plan view diagram of  FIG. 22B , seen from an opposite end.  
         [0033]      FIG. 23  is an exemplary side view diagram of a stack mold, shown from the side where the activating units are mounted.  
         [0034]     FIGS.  24 A-C are exemplary schematic diagrams showing a first alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0035]     FIGS.  25 A-C show the embodiment of FIGS.  24 A-C with the valve gate closed.  
         [0036]     FIGS.  26 A-C are simplified views of the embodiment of FIGS.  24 A-C, shown with the valve gate open.  
         [0037]     FIGS.  27 A-C simplified views of simplified views of this embodiment, shown with the valve gate closed.  
         [0038]     FIGS.  28 A-C are exemplary schematic diagrams showing a second alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0039]     FIGS.  29 A-C show the embodiment of FIGS.  28 A-C with the valve gate closed.  
         [0040]     FIGS.  30 A-C are simplified views of the embodiment of FIGS.  28 A-C, with the valve gate open.  
         [0041]     FIGS.  31 A-C are simplified views of the embodiment of FIGS.  28 A-C, with the valve gate closed.  
         [0042]     FIGS.  32 A-C are exemplary schematic diagrams showing a third alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0043]     FIGS.  33 A-C show the embodiment of FIGS.  32 A-C with the valve gate closed.  
         [0044]     FIGS.  34 A-C are simplified views of the embodiment of FIGS.  32 A-C, with the valve gate open.  
         [0045]     FIGS.  35 A-C are simplified views of the embodiment of FIGS.  32 A-C, with the valve gate closed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]     The embodiments of the present invention, described herein, may be used for single cavity molds, as well as with multi-cavity (e.g., single-face and stack) molds.  
         [0047]     The embodiments of the present invention use a combination of pneumatic-mechanical actuating system for the movement of the valve pin. The pneumatic component (e.g., pneumatic cylinder) of the actuating system is brought outside the mold, leaving only mechanical components in the mold. The pneumatic cylinder runs cold, which helps protect its components from heat expansion and extend the life of its seals (e.g., o-rings). Also, maintenance checks and service are easier for cylinders located outside the mold, where they are easily accessible. The pneumatic actuating component being removed from the valve-gate unit, enables the back-to-back mounting of valve-gate units for stack molds.  
         [0048]     An embodiment of the valve-gate unit in accordance with the present invention is shown in  FIG. 1  as part of a single-face multi-cavity mold. Such a mold typically includes the following items: a bottom plate  1 , stripper plate  2 , stripper rings  3  (secured to stripper plate  2 ), cores  4  (secured to bottom plate  1 ), cavity blocks  5  secured to cavity plate  6 , gate inserts  7  (secured in cavity blocks  5 ), manifold plate  8 , housing manifold  9 , top plate  10 , and valve-gate units  11  (secured to cavity plate  6 ). It should be understood that additional components (not shown or described here) can be part of such a mold, and different mounting methods than the one described can be used, without departing from the scope of the present invention.  
         [0049]     In a manner typical to the injection process, at the beginning of each injection cycle the mold closes and molten plastic is injected, through the hot runner system (e.g., as shown including a manifold  9  and a nozzle unit  12 ), in the injection chambers  13  formed between the active faces of cores  4  and cavity blocks  5 . The active end of nozzle unit  12  shown in  FIG. 1  is housed in gate insert  7 , but can be housed directly in a pocket in cavity block  5  (e.g., gate insert  7  is optional). At the end of the injection cycle, the mold opens and the stripper plate  2  moves away from bottom plate  1  for a short distance, causing the stripper rings  3  to strip the molded parts  14  off cores  4 . The molded parts  14  fall through the opening between the mold halves, and the injection machine closes the mold for the beginning of a new cycle.  
         [0050]     The valve-gate system in accordance with the embodiments of the present invention includes two main units: the valve-gate unit  11  (as shown in  FIG. 1 ), secured to cavity plate  6  and in contact with manifold  9 , and the activating unit  31  (as shown in  FIG. 3 ), mounted on the side of the mold, and having elements that go through the mold, to valve-gate units  11 .  
         [0051]     Melt-flow channels through manifold  9  bring molten plastic to valve-gate units  11 . Valve-gate units  11  can have one flow channel connecting to nozzle unit  12 , or they can have two flow channels (e.g., as shown in  FIG. 3 ), diverging from a common entry point (e.g., matching exit channel of manifold  9 ), and converging at interface with nozzle unit  12 . Sealing between manifold  9  and valve-gate unit  11 , and between valve-gate unit  11  and nozzle unit  12 , is achieved by the thermal expansion of these components. In single-face molds, pressure pads  78  are mounted between manifold  9  and top plate  10 , in line with the gate (one pressure pad for each injection point—e.g., see  FIG. 1 ). Pressure pads  78  are used to counteract the injection pressure from the gate, and aid with sealing when components expand during mold cycles. In stack molds with back-to-back gating, pressure pads may not be needed as the injection pressures equalize on sides of manifold.  
         [0052]     An enlarged detail of the valve-gate unit  11  from  FIG. 1  is shown in  FIG. 2 . It includes a valve pin or stem  15  going through nozzle unit  12  and through a central hole in the body of valve-gate unit  11 . It has an enlarged cylindrical portion  16 , followed by a reduced cylindrical end  17 . A flanged sleeve  18  is mounted on this end, followed by a retainer  19 , these two components being locked in place with a retaining ring  20 . Although these items are employed and described in the present design, it should be understood that any system producing a similar result could be used on this end of valve pin  15 . Flanged sleeve  18  and retainer  19  move in a pocket  21  in the body of valve-gate unit  11 . Pocket  21  is round on one side, and open to the other side, towards the exterior of the body of valve-gate unit  11 . A yoke  22  is located in the open end of pocket  21 , pivoting around a pivot-pin  23  secured in the body of valve-gate unit  11 . The forked end  24  of yoke  22  is located in the space between flanged sleeve  18  and retainer  19  (mounted on reduced cylindrical end  17  of valve pin  15 ). Yoke  22  has a spherical end  25  on the opposite side, which can move in a rounded slot/activating profile  26  in an activating bar  27 . A cover cap  28 , bolted to body of valve-gate unit  11 , acts as guide for activating bar  27 . A thermal plate  29  prevents heat transfer from body of valve-gate unit  11 , which is heated, to activating bar  27  and cover cap  28 . A cover plate  30  is bolted at top of valve-gate unit  11 , to separate pocket  21  from manifold  9 .  
         [0053]     One activating bar  27  can be used to activate several valve-gate units  11  located along the same axial line. The activating bar  27  extends to one side of the mold, where it is connected to the activating unit  31 , as shown in  FIG. 3 . An enlarged detail of the activating unit  31  of  FIG. 3  is shown in  FIG. 4 . The activating unit  31  includes a base guide  32 , an adjusting nut  33 , an adjustable cylinder support  34  and a pneumatic cylinder  35 . Base guide  32  is a round piece, extended with a squared base  36  that is secured to the side of the mold with bolts  37  (as shown in  FIGS. 4, 5  and  6 B). On the opposite end, base guide  32  has an outer thread  38 , for engagement of adjusting nut  33 . Base guide  32  has a central cylindrical hole with one axial slot  39 . Adjustable cylinder support  34  is in the shape of a sleeve with a flanged end. A transversal key  40  is press-fit in an axial slot  41  on the outer surface (on the sleeve portion) of adjustable cylinder support  34 . Sleeve portion of adjustable cylinder support  34  is inserted in central hole of base guide  32 , with transversal key  40  sliding in axial slot  39 . Transversal key  40  prevents rotation of adjustable cylinder support  34  in reference with base guide  32 . Adjustable cylinder support  34  is loosely secured to adjusting nut  33  with shoulder bolts  42 . As shown in  FIG. 6A , outer surface of adjusting nut  33  has a notched portion  43  (for ease of handling), extending with a narrow cylindrical portion  44 , marked with a number of indentations  45 . One “origin” indentation  59  is marked on the outer surface of flange portion of adjustable cylinder support  34 . Indentations  45  are used for precise adjustment in reference with “origin” indentation  59 .  
         [0054]     Pneumatic cylinder  35  is secured onto the end face of adjustable cylinder support  34  with bolts  46 , as shown in  FIG. 4 . External end  48  of piston  47  of pneumatic cylinder  35  has an internal thread  49 . A connector  50 , in the shape of a square prism, has a threaded extension  51  at one end (for engagement in piston  47 ) and a central slot  52  at the other end (as shown in  FIG. 5 , which is a top view of system from  FIG. 4 ). End  53  of activating bar  27  is secured in central slot  52  with a low-head bolt  54 . Four (4) access holes  55  are located, at 90° intervals, on middle portion of base guide  32 , to provide access to low-head bolt  54 . Piston  47  of pneumatic cylinder  35  actuates connector  50 , which in turn directs activating bar  27  in a push-pull movement. Bar  27  has one rounded slot/activating profile  26  for each valve-gate unit  11  it activates. Activating slot  26  runs along a curve/spline  56  (as shown in  FIGS. 3, 7A ,  8 ,  9 ,  10 , and  14 ), and holds the spherical end  25  of yoke  22  previously described. Yoke  22  cannot move axially (in the direction of movement of activating bar  27 ), as it is held in body of valve-gate unit  11 , but can pivot around pivot pin  23 . The push-pull movement of activating bar  27  makes the rounded slot  26  guide the spherical end  25  of yoke  22  in an up-and-down movement, in a manner that will be described in more detail later. In other words, the up-and-down movement of the spherical end  25  causes yoke  22  to pivot around pivot-pin  23 , which makes the forked end  24  of yoke  22  move down-and-up respectively, bringing the valve pin  15  with it. Valve pin  15  opens and close once per injection cycle. The pneumatic cylinder  35  receives a signal from the injection machine, which correlates movement of valve pin  15  with mold cycles.  
         [0055]     The 3 positions on curve  56  (as shown in  FIG. 7A ), are next described, with correlation to  FIGS. 8, 9 ,  10 A, and  10 B. In the case described here, curve  56  is an arc (e.g., a portion of a circle). 
        Position “0” (zero), also shown in  FIGS. 8 and 10 A, corresponds to valve-gate being closed (when injection is stopped). Piston  47 , connector  50  and activating bar  27  are fully retracted ( FIG. 10A ), which corresponds to 0° rotation of yoke  22 . In this position, forked end  24  of yoke  22  is lowered, bringing valve pin flush with surrounding surface of injection chamber  13 .     Position “1”, also shown in  FIGS. 9 and 10 B, corresponds to valve-gate being fully opened (when injection is in progress). Piston  47 , connector  50  and activating bar  27  are extended at full stroke S ( FIG. 10B ), which corresponds to rotation “A” of yoke  22 . In this position, forked end  24  of yoke  22  is lifted at maximum, retracting valve pin  15  by amount “B” ( FIG. 9 ). Note: Spherical end  25  of yoke  22  moves repeatedly from “0” to “1” and back to “0” during mold cycles (once per mold cycle).     Position “2” is at the quadrant of curve  56  traveled by spherical end  25  of yoke  22 . Valve pin  15  can be adjusted to move towards injection chamber  13  (to bring it flush with surrounding surface, or to eliminate plastic leaks at gate, etc.) by moving spherical end  25  of yoke  22  anywhere between “0” and “2”. Quadrant “2” is the highest position the spherical end  25  can reach, and corresponds to the furthest out the valve pin  15  can go towards injection chamber  13 . If spherical end  25  of yoke  22  is at “2” and valve pin is below surrounding surface of injection chamber  13 , it cannot be adjusted any further and will need to be replaced with a longer pin.        
 
         [0059]     Stroke S is an in-built feature of pneumatic cylinder  35  used, and its value is thus typically a constant. Values “A” and “B” are a result of the combination of stroke S of pneumatic cylinder  35  used, geometry of curve  56 , and shape and size of yoke  22 . These values can be varied depending on desired result.  
         [0060]     Procedure to adjust activating unit:  
         [0061]     In order to adjust the activating unit, the following procedure may be followed: 
        1. Mold is stopped.     2. Shoulder bolts  42  are loosened slightly (but not removed) to allow a little clearance between adjustable cylinder support  34  and adjusting nut  33 .     3. Adjusting nut  33  is rotated while adjustable cylinder support  34  is slowly pulled away from (or moved inward into) base guide  32 , as shoulder bolts  42  bolted in adjusting nut  33  rotate in annular groove  57  of adjustable cylinder support  34 . This movement increases or reduces adjustment gap  58  between front of base guide  32  and flanged portion of adjustable cylinder support  34 .     4. Indentations  45  of adjusting nut  33  help mold operator control the adjustment precision in reference with the origin indentation  59  of adjustable cylinder support  34 .     5. When desired adjustment has been reached, shoulder bolts  42  are tightened, locking adjusting nut  33  and adjustable cylinder support  34  together. When these two items are locked together, they are also locked into position, in reference to base guide  32 . This is achieved by the combination of transversal key  40  and thread  38 . As transversal key  40  allows only axial movement of adjustable cylinder support  34  in reference to base guide  32 , when shoulder bolts  42  are tightened, they also force the threads of adjusting nut  33  against the opposing threads of base guide  32 , resulting in a solid, precise engagement of all the components of activating unit  31 .     6. Steps 2, 3, 4, and 5 are repeated for each activating unit  31  mounted on mold, depending on performance of valve pins  15 .     7. Once all activating units  31  have been adjusted, the mold can be started again.        
 
         [0069]     A more detailed explanation of the correlation between adjustment on activating unit  31  and location of spherical end  25  of yoke  22  on curve  56  follows, in reference with  FIGS. 7A, 7B ,  8 ,  9 ,  10 A and  10 B. Position “1” is at the bottom of rounded slot/activating profile  26 . Position “0” is located, along the length of the activating bar  27 , at a distance, from “1”, equal to the stroke S of pneumatic cylinder  35 . Position “2” is always at the quadrant of curve  56 . When adjustment gap  58  is altered (unit  31  is being adjusted), adjusting nut  33 , adjustable cylinder support  34 , and pneumatic cylinder  35  move relative to base guide  32 , bringing connector  50  and activating bar  27  with them. This means that adjustments modify location of position “0” relative to position “2” on curve  56 . Since distance, along length of activating bar  27 , between “0” and “1” is constant (equal to stroke S of pneumatic cylinder  35 ), position “1” also moves with every adjustment. Valve pin  15  will need to be replaced with a longer one when it requires adjustment beyond position “2”.  
         [0070]     A feature of curve  56  (of rounded slot/activating profile  26 ) that influences the closing speed of valve pin  15  is discussed below, with reference to  FIGS. 7A and 7B . When spherical end  25  of yoke  22  moves along curve  56  from “1” to “0”, its speed decreases as the angle of the curve reduces. This translates into the valve pin slowing down slightly as it reaches the gate, allowing for a smooth closing. For comparison purposes,  FIG. 7B  shows a straight activating slot  26  (straight from “1” to “0”), which would result in a constant closing speed of the valve pin  15 .  
         [0071]     For comparison purposes,  FIG. 10A  shows activating unit  31 , complete with activating bar  27 , in position “0”, while  FIG. 10B  (below it) shows same system in position “1”. Piston  47  is retracted in  FIG. 10A , bringing spherical end  25  of yoke  22  in position “0”, and extended in  FIG. 10B , bringing spherical end  25  in position “1”. Yoke  22  is shown at the left of the figures for clarity.  
         [0072]     One embodiment of this invention is directed to the use of a one-piece activating bar  27 , the distance between activating slots  26  being determined by the pitch of the mold. An alternate embodiment, however, uses a multi-piece activation bar ( FIGS. 14, 15 ), where the activating profile  26  is part of an activating insert  60 , made of high-wear material. The mold pitch influences the length of connecting bars  61  that connect activating inserts  60 . As shown in  FIGS. 16 through 20 , slotted activating inserts  60  and connecting bars  61  have a tongue-and-groove style joint  62 , locked with a transversal key  63  of square section. Transversal key  63  has a cylindrical extension  64  with a groove  65 . A washer  66  and a retaining ring  67  (pushed in groove  65 ) lock the transversal key  63  in place, which in turn locks the slotted activating inserts  60  in connecting bars  61  and in activating bar  27 . Transversal key  63  has a knurled cylindrical flange  68  at opposite end, which is used for handling.  
         [0073]     The multi-piece embodiment has several advantages in regards to servicing of the valve-gate unit. For a single-face mold (as shown in  FIGS. 11, 12  and  13 ) the procedure to service the valve-gate unit is as follows: 
        1. Mold is closed in the injection machine. Valve-gates must be closed (pistons  47  of pneumatic cylinders  35  are fully retracted).     2. Safety straps  69  are installed between top plate  10  and cavity plate  6  (shown with phantom lines). Mold is opened and bolts  70  are removed. Mold is closed again.     3. Safety straps  69  are then installed between cavity plate  6  and bottom plate  1 .     4. Mold is opened slowly, as shown in  FIG. 12 , bringing cavity plate  6 , manifold plate  8  (which is secured to cavity plate  6 ), and valve-gate units  11  (secured to cavity plate  6 ) with the core half, away from cavity side.     5. Manifold  9  stays with top plate  10 , as it is secured to top plate  10  with bolts  71 .     6. When mold is opened this way, valve-gate units  11  are exposed and can be removed, one at a time, for service, cleaning etc. To do that, bolts  72  (that secure valve-gate unit  11  to cavity plate  6 ) can be removed, as shown in  FIG. 13 . Retaining rings  66  (see  FIG. 20 ) are removed from grooves  65  of cylindrical extensions  64 , and transversal keys  63  are then removed. Slotted activating insert  60  can then be easily disengaged from connecting bars  61  (which will stay in the mold, attached to adjacent valve-gate units) and valve-gate unit  11  (together with its activating insert  60 ) can be lifted out of the mold, using threaded portion of holes for bolts  72  as jacking holes  73 . After changes, cleaning, service etc. valve-gate unit  11  can be returned to the mold and secured back in it, in reverse order. Another valve-gate unit  11  can then be removed in the same manner.        
 
         [0080]      FIG. 14  shows a side view of a single-face multi-cavity mold, in closed position, using a multi-piece activating bar  27 .  FIG. 15  shows same mold being opened in the manner described above, for removal of valve-gate units  11 .  FIGS. 16 through 19  are detailed views of multi-piece activating bar  27 , with activating inserts  60  and connecting bars  61  shown separated in top view ( FIG. 16 ) and front view ( FIG. 17 ), and assembled (complete with transversal keys  63 , washers  66 , and retaining rings  64 ), shown in top view ( FIG. 18 ) and front view ( FIG. 19 ).  
         [0081]      FIG. 21  shows a cross section through a stack mold using back-to-back gating. Valve-gate units  11  are shown, complete with activating bar  27  (one-piece option shown) and activating units  31  mounted to side of mold. Valve-gate units  11  are be heated, to hold desired temperature of molten plastic as it transits from manifold  9  to nozzle unit  12 . Different types of heating elements  74  can be used (coil heaters wrapped around body of valve-gate unit  11 , or bar-type heaters inserted in the body of the valve-gate unit  11  as shown in  FIG. 21 , etc.). Wires  75  extending from heaters  74  are directed through pockets in the mold, similar with wires  76  coming from nozzle unit  12 , and wires  77  coming from heaters of manifold  9 .  
         [0082]      FIG. 22A  is a plan view of a multi-cavity mold (seen from the parting line), shown with two activating units  31 . The two cavities at the bottom of the mold are shown with valve-gate open (one-piece activating bar  27  is extended at full stroke S of pneumatic cylinder  35 , as shown just below the plan view). The two cavities at the top of the mold are shown with valve-gate closed (activating bar  27  is retracted fully, as shown above the plan view). At the right of the page, valve gate units, complete with nozzle units, are shown open (bottom) and closed (top), corresponding to plan view.  
         [0083]      FIG. 22B  shows the same mold in plan view, but using a multi-piece activating bar  27 .  FIG. 22C  is a plan view of the same mold from  FIG. 22B , seen from opposite end—after top plate  10  and manifold  9  are removed. The valve-gate units  11  and multi-piece activating bars  27  are visible, and valve-gate units  11  can be removed, one by one, as previously described.  
         [0084]      FIG. 23  is a side view of a stack mold, shown from the side where the activating units  31  are mounted.  
         [0085]     FIGS.  24 A-C are exemplary schematic diagrams showing a first alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0086]     The embodiment of FIGS.  24 A-C uses a pair of activating bars  27 ′ working in parallel as a rigid unit. They can be of either one-piece or multi-piece design, and are connected in a rigid assembly by means known to those of skill in the art. An activating unit  31 ′, mounted externally on the injection mold, activates both bars  27 ′ simultaneously. A pair of bars  27 ′ may activate one or several valve gate units  11 ′ located along the same axial line. Cover caps  28 ′, secured to opposing sides of the body of valve gate unit  11 ′, act as guides for activating bars  27 ′. Each activating bar has a slot/activating profile  26 ′ for each valve gate unit  11 ′ activated. This embodiment shows a linear, sloped slot, but it should be understood that a rounded slot such as those described above may be used.  
         [0087]     The valve gate unit  11 ′ of this embodiment has a round pocket  21 ′, disposed centrally, opening to the side which comes in contact with manifold  9 ′. A cylindrical guide  80 , in threaded engagement  81  with a cylindrical cage  82 , is located in round pocket  21 ′. A valve stem  15 ′ has a cylindrical flange  83 , located centrally in cage  82 . Flange  83  is firmly held between base of cage  82  and bottom of threaded extension of guide  80 , with no freedom of axial motion. Guide  80 , flange  83  of valve stem  15 ′, and cage  82  form a sliding unit  84 , which can move axially in pocket  21 ′ to repeatedly close or open a valve gate opening into an injection chamber  13 ′ of the injection mold. Such motion of the sliding unit is achieved by a transversal pin  85 , fixedly engaged in guide  80 , and having symmetrical extensions on sides of guide  80 . Ends of transversal pin  85  pass through vertical slots  86  on sides of valve gate unit  11 ′, continuing on through activating profiles  26 ′, and being secured with some means such as retaining rings (as shown) against accidental sliding out of profiles  26 ′. With each extension of the piston  47 ′ of a pneumatic cylinder  35 ′ of activating unit  31 ′, the pair of bars  27 ′ extends, causing the activating profiles  26 ′ to force transversal pins  85  to retract sliding units  84 , so that valve stems  15 ′ open the valve gates. With each retraction of the piston  47 ′, the pair of bars  27 ′ retracts, causing the activating profiles  26 ′ to force transversal pins  85  to extend sliding units  84 , so that valve stems  15 ′ close the valve gates. Vertical slots  86  only allow extend/retract motions along axis of valve stem  15 ′, preventing any sideway motions as could be caused by slots  26 ′ of activating bars  27 ′, acting against transversal pin  85 . A cover plate  30 ′, secured at the top of the valve gate unit  11 ′, separates pocket  21 ′ from manifold  9 ′.  
         [0088]     FIGS.  25 A-C show the embodiment of FIGS.  24 A-C with the valve gate closed.  
         [0089]     FIGS.  26 A-C are simplified views of the embodiment of FIGS.  24 A-C, shown with the valve gate open.  
         [0090]     FIGS.  27 A-C simplified views of simplified views of this embodiment, shown with the valve gate closed.  
         [0091]     FIGS.  28 A-C are exemplary schematic diagrams showing a second alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0092]     The embodiment shown in FIGS.  28 A-C uses a pair of activating bars  27 ′ working in parallel as a rigid unit. They can be of either one-piece or multi-piece design, and are connected in a rigid assembly by means known to those skilled in the art. An activating unit  31 ′, mounted externally on the injection mold, activates both bars  27 ′ simultaneously. A pair of bars  27 ′ may activate one or several valve gate units  11 ′ located along the same axial line. Rollers  87  and support pads  88  guide the extend/retract motions of bars  27 ′, as activated by unit  31 ′. Activating bars  27 ′ transfer this motion, through pins  89 , to side arms  90 , which transfer it further, through transversal pin  85 ′, to a sliding unit  84 ′ (similar to the one described above). Vertical slots  86 ′ in opposite sides of valve gate unit  11 ′ allow extend/retract motions of pin  85 ′, as activated by bars  27 ′. Such motions of pin  85 ′ are transferred directly to valve stem  15 ′ through sliding unit  84 ′. When activating bars  27 ′ are extended, they cause side arms  90  to pull pin  85 ′ to the bottom end of slots  86 ′. Pin  85 ′ brings the whole sliding unit  84 ′ down, which causes the valve stem  15 ′ to close the valve gate as shown in  FIGS. 29A , B, and C. When activating bars  27 ′ are retracted, they cause side arms  90  to push pin  85 ′ to the top end of slots  86 ′, bringing the whole sliding unit  84 ′ up, and causing the valve stem  15 ′ to open the valve gate (as shown in  FIGS. 28A , B, and C).  
         [0093]     FIGS.  29 A-C show the embodiment of FIGS.  28 A-C with the valve gate closed.  
         [0094]     FIGS.  30 A-C are simplified views of the embodiment of FIGS.  28 A-C, with the valve gate open.  
         [0095]     FIGS.  31 A-C are simplified views of the embodiment of FIGS.  28 A-C, with the valve gate closed.  
         [0096]     FIGS.  32 A-C are exemplary schematic diagrams showing a third alternate embodiment of the valve gate unit in accordance with the present invention.  
         [0097]     The embodiment shown in FIGS.  32 A-C uses a pair of activating bars  27 ′ working in parallel as a rigid unit. They can be of either one-piece or multi-piece design, and are connected in a rigid assembly by means known to those of skill in the art. An activating unit  31 ′, mounted externally on the injection mold, activates both bars  27 ′ simultaneously. A pair of bars  27 ′ may activate one or several valve gate units  11 ′ located along the same axial line. Roller bearings  91  guide the extend/retract motions of bars  27 ′, as activated by unit  31 ′. Activating bars  27 ′ transfer this motion, through a toggle system  92 , to a transversal pin  85 ′, to a sliding unit  84 ′ (similar to the one described above) and to a valve stem  15 ′. The toggle system  92  has of two side arms,  93  and  94 , their connecting pins  95  and  96 , and the transversal pin  85 ′. Pins  95  are fixedly secured onto opposite sides of the valve gate unit  11 ′. Pins  96  connect side arms  93  and  94 , and are allowed motion in vertical slots  97  of activating bars  27 ′. Side arms  94  are further connected to ends of transversal pin  85 ′. Vertical slots  86 ′ on opposite sides of valve gate unit  11 ′ only allow pin  85 ′ an extend/retract motion along axis of valve stem  15 ′. Such motions of pin  85 ′ are transferred directly to valve stem  15 ′ through sliding unit  84 ′. When activating bars  27 ′ are extended, vertical slots  97  cause pins  96  to move simultaneously along horizontal direction of activating bars  27 ′ and vertically towards bottom of slots  97  (which are open at the top). Since side arms  93  can only pivot around pins  95  (when actuated by activating bars  27 ′), the resulting combined horizontal/vertical motion of pins  96  causes side arms  94  to pull transversal pin  85 ′ to bottom end of vertical slots  86 ′. Pin  85 ′ transfers this motion to the sliding unit  84 ′, causing the valve stem  15 ′ to close the valve gate, as shown in  FIGS. 33A , B, and C. When activating bars  27 ′ are retracted, vertical slots  97  cause pins  96  to move simultaneously along horizontal direction of activating bars  27 ′ and vertically towards top of slots  97 . Side arms  93  pivot around pins  95 , the resulting combined horizontal/vertical motion of pins  96  causes side arms  94  to push transversal pin  85 ′ to top end of vertical slots  86 ′. Pin  85 ′ transfers this motion to the sliding unit  84 ′, causing the valve stem  15 ′ to open the valve gate, as shown in  FIGS. 32A , B, and C.  
         [0098]     FIGS.  33 A-C show the embodiment of FIGS.  32 A-C with the valve gate closed.  
         [0099]     FIGS.  34 A-C are simplified views of the embodiment of FIGS.  32 A-C, with the valve gate open.  
         [0100]     FIGS.  35 A-C are simplified views of the embodiment of FIGS.  32 A-C, with the valve gate closed.  
         [0101]     It should be noted that in all three alternate embodiments described above, S 1  is the stroke of the activating bars  27 ′, along a direction perpendicular to that of valve stem  15 ′. Sliding unit  84  and valve stem  15 ′ have a stroke S 2 , along the centerline of the valve stem  15 ′. Both strokes are shown on  FIGS. 24A , B and C of the first alternate embodiment. For the other two embodiments, however, only stroke S 2  is shown, for clarity of the drawing. The two extreme positions of these embodiments (when valve is open and when valve is closed) were not shown on the same drawing to avoid unnecessarily cluttering the figures.  
         [0102]     As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, instead of the pneumatic cylinder, a hydraulic one may be used may, or alternately the motive force may be supplied by an electric motor drive. These other embodiments are intended to be included within the scope of the present invention, which is set forth in the following claims.