Abstract:
A piston and valve stem assembly is held in either a gate open or gate closed position by a valve gate piston retention device until such time as sufficient air pressure is built up behind the piston, upon which the valve gate piston retention device is signaled to release the piston. The piston, being pre-charged, is thereby able to overcome both static and dynamic friction thus allowing it to move freely and immediately, and in the case of multiple pistons, simultaneously or sequentially, and without hesitation. Both simultaneous or sequential retention and release of the piston via mechanical or electromagnetic means can be controlled to actuate a plurality of pistons forward or back in an expeditious manner to overcome pneumatic losses and frictional forces in an effort to achieve more precise timing of the valve stem and optimize overall cycle time.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to, but is not limited to, injection molding systems, and more specifically the invention relates to a device for controlling the precise timing of the movement of a pneumatically actuated valve gate piston system, by retaining a piston in place, through either a mechanical or electromagnetic device, until a predetermined event or timing sequence is realized. Prior to actuation, air pressure, sufficient to overcome all frictions, losses and resisting forces, is introduced, after or upon which a signal causes release of the retention mechanism retaining the piston thus initiating piston motion. 
       BACKGROUND 
       [0002]    The nature of single or multiple pneumatically actuated valve gate piston and valve stem assemblies in injection molding is such that they are all subject to variability in the timing of their motion. For those skilled in the art, it is known that rapid and simultaneous movement of the pistons, and ultimately the valve stems, is desirable as valve stem position during the injection molding process affects molded part filling characteristics as well as the overall cycle time of each molding sequence. 
         [0003]    Timing variation of pistons and valve stems is due primarily to a combination of three factors, namely: (i) variability in the static and kinetic friction forces on both the piston, via the piston seal rings, and on the valve stem, due to the viscosity of the resin, as well as the fit and clearances to the manifold bushing (ii) the pressurized resin in the mold exerting a force against the valve stem which increases from zero, in the retracted position, and ramps up to a maximum just before the valve stem closes off the gate orifice, and (iii) the compressible nature of the air and the variability of the volume of the air lines, hoses, channels and chambers being used to actuate the piston. 
         [0004]    Pneumatic actuation is problematic in that the compressibility of air creates latency and inconsistency in piston movement, as the pressure wave propagation and air pressure losses in the system create variability in the timing of piston motion. This causes problems when it is a requirement that the valve gates must actuate at precise times, either simultaneously or sequentially. 
         [0005]    Typically, the pistons in a pneumatically actuated system are controlled by a single master solenoid, which pressurizes each side of all the pistons at once, including all air lines, hoses and chambers between the master solenoid and each piston. The nature of compressible flow and the variability of the force required to actuate the pistons causes them to move at different times. Because the pressure builds from zero gauge pressure, the force on the piston also ramps up proportionally as air rushes in. Each piston in a system begins to move when the force due to air pressure overcomes the forces resisting piston motion. Since there is variability in both forces, the pistons move at different times. 
         [0006]    The piston pressure force is variable due to differences in the flow characteristics and the effects of the motion of other pistons increasing the pressurized volume, and the force resisting the piston motion is variable due to different tolerances of the components, variations in frictional forces, and the like. 
         [0007]    U.S. Pat. Application Publication No. 2003/0143298, describes an injection molding nozzle utilizing pressurized air to open and close a valve stem and relies merely upon the pressure differential between the inlet and outlet sides of the piston to be satisfactorily dissimilar to enable movement of said piston in one direction, while not accounting for mechanical and pneumatic losses and effects as described above. Again, this arrangement subjects the pistons to inherently variable timing in movement. 
         [0008]    U.S. Pat. Application Publication No. 2004/0234645 describes back to back arrays of valve stems used in stack molding, each array being moved as one, either simultaneously or independently, utilizing one plate to carry each array for each mold face. This is accomplished by mechanically linking a plurality of valve stems to a plate which is actuated by a hydraulic, pneumatic or electromechanical driving mechanism. While this invention allows for simultaneous movement of each separate array of valve stems, it precludes individual control of each valve stem of an array for sequential movement and also is subject to timing and motion variability due to the frictional effects of the components and additional mechanical linkages. 
         [0009]    Additionally, in one embodiment, each valve stem carrier plate is in turn driven by the same pneumatics as with traditional piston arrangements, and so is subject to the same vulnerabilities as described above, which, in turn, affect an entire array of valve stems. Finally, should one, or more, valve stems of an array be unable to move forward, or back, due to unforeseen circumstances, such as frozen resin due to a failed nozzle heater, or foreign matter contaminating the melt channel, they would undoubtedly cause the entire array to move in an undesirable manner and speed, or in a worst case scenario, the momentum of the moving carrier plate could cause breakage of the lagging valve stem or stems. 
         [0010]    U.S. Pat. No. 7,210,922 further describes a valve gate assembly which is driven forward, either hydraulically, pneumatically or mechanically, by an actuation plate yet returned to its original position via springs. Again, while this en masse valve stem actuation ensures simultaneous movement in one direction, it precludes sequential control of any one valve stem, especially with any specific timing specification. Additionally, while the spring force is initially designed to overcome the frictional effects of the components and resin, the spring&#39;s consistent performance over time in a predominantly high temperature environment is uncertain, and may not offer exact, simultaneous valve stem retraction as desired. 
         [0011]    For the foregoing reasons, the present invention is directed to overcoming one or more of the problems or disadvantages set forth above, and for providing a mechanism which will retain a valve gate piston and valve stem assembly in a gate open or gate closed position until a predetermined pneumatic pressure is accumulated on a side of said piston, sufficient to overcome any and all opposing frictional forces, at which time the mechanism will release the piston for swift and unhindered travel, the timing of which movement may be simultaneous or sequential with neighboring valve stems. 
       SUMMARY 
       [0012]    The present invention is directed to a retention mechanism which retains a piston, and attached valve stem, in place until such time as compressed air can build up sufficient pressure on one side of the piston, thereby pre-charging the piston with enough energy to overcome friction so that upon release, the piston and valve stem will travel rapidly and uniformly to the next full stop position, thereby minimizing overall molding cycle time. The piston speed is increased due to the higher force present on the piston face before its release resulting in less variable motion initiation. The piston may be retained in either the valve stem retracted, gate open position or the valve stem forward, gate closed position for pre-charging. Additionally, when teamed with a plurality of pistons, the option exists to either release all the pistons in some preferred sequence or simultaneously with a decreased variability in their timing. 
         [0013]    In one aspect of the present invention, the piston and valve stem assembly is retained by at least one reciprocating iron core of a solenoid. 
         [0014]    In another aspect of the present invention, the piston and valve stem assembly is retained by an electromagnet. 
         [0015]    In yet another aspect of the present invention, the piston and valve stem assembly is retained by at least one retracting pin which engages with at least one mating slot in the piston. 
         [0016]    In a further aspect of the present invention, the piston and valve stem assembly is retained by at least one retracting roller. 
         [0017]    In one aspect of the present invention, the piston and valve stem assembly is retained by at least one retracting lever. 
         [0018]    In another aspect of the present invention, the piston and valve stem assembly is retained by at least one pawl which engages with at least one mating slot in the piston. 
         [0019]    In another aspect of the present invention, the piston and valve stem assembly is retained by at least one piezoelectric device. 
         [0020]    In yet another aspect of the present invention, the retention mechanism is activated by a pneumatic actuator. 
         [0021]    In a further aspect of the present invention, the retention mechanism is activated by an actuator which is a solenoid. 
         [0022]    In one aspect of the present invention, the retention mechanism is activated by an actuator which is a motor. 
         [0023]    In another aspect of the present invention, the actuator is signaled by a controller. 
         [0024]    In a further aspect of the present invention, the actuator is signaled by a timer. 
         [0025]    In another aspect of the present invention, the actuator is signaled by a transducer. 
         [0026]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a section view assembly of an entire valve gate nozzle stack showing one embodiment of the present invention, namely a solenoid. 
           [0028]      FIG. 2  is a section view detail showing a solenoid type retention mechanism retaining the piston in a gate closed position. 
           [0029]      FIG. 3  is a section view detail showing a solenoid type retention mechanism retaining the piston in a gate open position, controlled by a transducer. 
           [0030]      FIG. 4  is a section view detail showing an electromagnetic type retention mechanism retaining the piston in a gate open position. 
           [0031]      FIG. 5  is a section view detail showing a plurality of retracting pins retaining the piston in a gate closed position. 
           [0032]      FIG. 6A  is a section view detail showing a plurality of retracting rollers retaining the piston in a gate open position with  FIG. 6B  exchanging rollers for a sliding contact. 
           [0033]      FIG. 7  is a section view detail showing a plurality of retracting levers retaining the piston in a gate open position. 
           [0034]      FIG. 8  is a section view detail showing a plurality of pawls retaining the piston in a gate closed position. 
           [0035]      FIG. 9  is a section view detail showing a piezoelectric device in two possible configurations. 
           [0036]      FIG. 10  is a schematic illustrating the complete piston retention and release apparatus showing several possible embodiments. 
           [0037]      FIGS. 11A &amp; 11B  are section view details of the nozzle tip area showing the valve stem in a gate open and gate closed position respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Referring to the drawings, and initially to  FIG. 1 , a representation of a typical valve gate injection molding nozzle components in a hot runner system  101  is shown, including one embodiment of the present invention; a valve gate piston retention device  100 . While  FIG. 1  includes one particular retention mechanism  108 , specifically an iron core  117 , and an actuator  110 , in this case, a solenoid  116 , the components of the nozzle stack, and in particular, the pneumatically actuated items at the top of the figure, remain common to all embodiments of the present invention. The following description is not intended to describe each and every component of a hot runner system  101 , but rather to depict the parts necessary to understand and practice the present invention. 
         [0039]    Again, referring to  FIG. 1  in more detail, the hot runner system  101  is used to transfer molten resin from an injection molding machine (not shown) through to a gate orifice  136  to create a molded article  138  between a core plate  182  and a cavity plate  180 . The resin flow  134  is diverted from a manifold melt channel  135  within a manifold  166 , through to a nozzle housing melt channel  167  within a nozzle housing  168  via a manifold bushing melt channel  165  within a manifold bushing  164 , wherein the manifold melt channel  135  is in fluid communication with the manifold bushing melt channel  165  and the nozzle housing melt channel  167  is in fluid communication with the manifold bushing melt channel  165 . Additionally, a nozzle tip melt channel  133  is in fluid communication with the nozzle housing melt channel  167  as a nozzle tip  132  is operatively, and threadably, attached to the nozzle housing  168 . 
         [0040]    To maintain optimum temperature of the resin flow  134  throughout the hot runner system  101 , the manifold  166  is heated by a manifold heater  172  and the nozzle housing  168  and the nozzle tip  132  are heated by a nozzle heater  170  installed thereon. Both the manifold  166  and the nozzle housing  168  are housed within, but insulated from, a manifold plate  174 , by a plurality of air gaps  178  and minimal contact between low thermal conductivity components. 
         [0041]    In addition to diverting the resin flow  134 , the manifold bushing  164  also secures a backup pad  162  which, in turn, supports a cylinder  106  inside of which travels a piston  102 , though the primary function of the manifold bushing  164  is to guide a valve stem  104 . The valve stem  104  is removably attached to the piston  102  which, in operation, is caused to reciprocate within the cylinder  106  via air flow entering from either a piston forward air circuit  144  or a piston retract air circuit  146 , both of which are plumbed in a backing plate  176 . 
         [0042]    Referring now to  FIG. 2 , which is a detail of the pneumatic components of the hot runner system  101  shown in  FIG. 1 , the valve gate piston retention device  100  ensures that the piston  102  is held against, or in close proximity to, the manifold bushing  164  by way of a retention mechanism  108 , which, in this embodiment is an iron core  117 . The solenoid  116  is the device, in this case, which acts as an actuator  110  to cause the iron core  117  to cycle back and forth through a seal  107  in the cylinder  106  to either retain or release the piston  102  by sliding over the injection side  140  of the piston  102 . In this position, compressed air  114 , flowing from a piston retract air circuit  146 , is allowed to build up under the clamp side  142  of the piston  102 , until such time as the iron core  117  is retracted, and the piston  102  and valve stem  104  will travel to the gate open position  126 . 
         [0043]    Turning now to  FIG. 3 , the piston  102  is shown in the retracted or gate open position  126 . The embodiment of this invention varies slightly from that shown in  FIG. 2 , in that the retention mechanism  108 ; the iron core  117 , and the actuator  110 ; in this case, the solenoid  116 , is retaining the piston  102  while compressed air  114 , entering from a piston forward air circuit  144 , builds up on the injection side  140  of the piston  102 . A transducer  122 , shown in  FIG. 3 , is located such that it may sense the pressure behind the piston  102  and will feed that information back to a controller  120 , as shown in  FIG. 9 . Additionally, referring to  FIG. 3 , the transducer  122  may also be located in the resin flow  134  of the manifold  166 , manifold bushing  164 , nozzle housing  168 , nozzle tip  132 , cavity plate  180 , or mold cavity  184  in order to sense plastic pressure for feedback purposes. 
         [0044]      FIG. 4  presents yet another embodiment of the present invention, that being a retention mechanism  108  that is an electromagnet  112 . Though this particular rendering shows the electromagnet  112  attached to the top of the cylinder  106 , it may be noted that, in the case of other dissimilar configurations of the cylinder  106 , the electromagnet  112  will be located proximate to, but not attached directly to, the injection side  140  or the clamp side  142  of the piston  102 . The piston  102  may be retained in either the retracted or gate open position  126  or the forward or gate closed position  128 , or both positions, by such an electromagnet  112 , allowing the compressed air  114  to build up, until such time as power to the electromagnet  112  is removed, thereby rapidly releasing the piston  102  and allowing the compressed air  114  to drive the piston  102 . Additionally, if an electromagnet  112  were placed at both positions, an attractive force field could be applied to each respective electromagnet  112 , to hasten travel of the piston  102 . 
         [0045]    Referring now to  FIG. 5 , a further embodiment is illustrated in the form of a retracting pin  155  which engages a groove  152  in the piston  102 . The retracting pin  155  will withdraw to a position where the piston  102  is free to translate until such time as the retracting pin is energized by an actuator  110  to insert itself through the seal  107  in the cylinder  106  back into the groove  152  in the piston  102 . The retracting pin  155  may engage and retain the piston  102  at both the gate open position  126  and the gate closed position  128 , depending on the application, while allowing compressed air  114  to pressurize the area behind the piston  102 . 
         [0046]      FIG. 6A  depicts yet another embodiment of the present invention whereby the valve gate piston retention device  100  is in the form of a roller  154 . Similar to the retracting pin  155  as illustrated in  FIG. 5 , the roller  154  extends and retracts into the cylinder  106 , through the seal  107 , such that roller  154  makes contact with the clamp side  142  of the piston  102  thus preventing the piston  102  from moving while compressed air  114  builds up behind it on the injection side  140  of the piston  102 . The action of the roller  154  is controlled by an actuator  110  which causes it to extend or retract dependent upon the desired position of the piston  102 .  FIG. 6B  shows a slight variation of this embodiment wherein the roller is replaced with a sliding contact  153 . 
         [0047]    Yet another embodiment is illustrated in  FIG. 7  whereby the piston  102  and valve stem  104  are held in place by a retracting lever  156  which, when driven by the actuator  110 , pivots in and out of the path of the piston  102 , again, retaining it in place to allow compressed air  114  to pre-charge the piston  102  before release. The key aspect of this embodiment as compared to the previous roller  154  design is the pivoting action of the retracting lever  156  versus the linear travel of the roller  154 . 
         [0048]    Referring to  FIG. 8 , a plurality of pawls  150  are used to secure the piston  102  while allowing compressed air  114  to build up behind the injection side  140  of the piston  102 . The plurality of pawls  150  pivots to engage a plurality of fingers  158  into a slot  160  in the piston  102 . Once the actuator  110  is signaled to release the piston  102 , the plurality of pawls  150  opens sufficiently for the plurality of fingers  158  to disengage the piston  102 . 
         [0049]      FIG. 9  shows two different placements of a piezoelectric device  161 , one attached to the outer diameter of the cylinder  106  and one installed through the wall of the cylinder  106 . In the former position, the cylinder  106  is sufficiently compliant such that the force from the piezoelectric device  161  which is energized is enough to temporarily deform the inner diameter of the cylinder  106  to cause it to retain the piston  102  when it is proximate to said piezoelectric device  161 . Alternatively, when the piezoelectric device  161  is installed through the wall of the cylinder  106 , it is permitted to grip the piston  102  itself when energized. 
         [0050]      FIG. 10  illustrates the process required to fulfill the sequence of events which enable pre-charging of the piston  102  pneumatically. Central to the figure is the valve stem  104  which is operatively attached to the piston  102 . The piston  102  is retained in place by a retention mechanism  108 , three variations of which are shown, namely; an electromagnet  112 , a roller  154 , and an iron core  117 . 
         [0051]    The retention mechanism  108  is activated to either retain or release the piston  102  by a separate component which is the actuator  110 . The actuator  110  may be a solenoid  116 , a pneumatic device  111  or a motor  113 . The actuator  110  is provided a signal  118  to move or energize the retention mechanism  108 , the signal  118  originating from a timer  124  or a controller  120 , with the intention of releasing a plurality of pistons  102  simultaneously or sequentially. The retention mechanism  108  may be positioned such that it retains the piston  102  in either the gate open position  126  or the gate closed position  128  or both. 
         [0052]    By referring to  FIGS. 11A and 11B , the relationship between the valve stem  104  and its interaction with a gate orifice  136  may be elucidated. In  FIG. 11A , the valve stem  104  is shown in a retracted or a gate open position  126 , while  FIG. 11B  shows the valve stem  104  in a forward or a gate closed position  128 . In the gate open position  126 , a valve stem tip  130  is located sufficiently far away from the gate orifice  136  such that resin flow  134  is allowed to migrate through the nozzle tip  132 , through the gate orifice  136  and into the mold cavity  184  created by the cavity plate  180  and the core plate  182 , and forms the molded article  138 . In the gate closed position  128 , the valve stem tip  130  is in a fully forward position and so precludes the resin flow  134  from entering the gate orifice  136  and hence the mold cavity  184 . 
         [0053]    Description of the embodiments of the present inventions provides examples of the present invention, and these examples do not limit the scope of the present invention. It is to be expressly understood that the scope of the present invention is limited by the claims. The concepts described above may be adapted for specific conditions and/or functions, and may be further extended to a variety of other applications that are within the scope of the present invention. 
         [0054]    Having thus described the embodiments of the present invention, it will be apparent that modifications and enhancements are possible without departing from the concepts as described. Therefore, what is to be protected by way of letters patent are limited by the scope of the following claims: