Abstract:
An injection molding apparatus is provided in which the rate of material flow during the injection cycle is controlled. According to one preferred embodiment, a method is provided for use in an injection molding apparatus including a hot runner assembly comprising a manifold and at least first and second injection nozzles, the hot runner assembly to direct material injected into said manifold through said at least first and second injection nozzles through a corresponding at least first and second gates to one or more mold cavities. The method includes the steps of injecting material into the manifold, controlling, in the hot runner away from the first gate, a first rate at which material is injected through the first gate, and controlling, in the hot runner away from the second gate, a second rate at which material is injected through the second gate, independently from the first rate.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to injection of pressurized materials through a manifold, such as injection molding of plastic melt in a hot runner system. More specifically, this invention relates to an improved injection molding hot runner system in which the rate of melt flow is controlled through the gate during an injection molding cycle.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    U.S. Pat. No. 5,556,582 discloses a multi-gate single cavity system in which the rate of melt flow through the individual gates is controlled independently via a control system according to specific target process conditions. This system enables the weld line of the part (the section of the part in which the melt from one gate meets the melt from another gate) to be selectively located. It also enables the shape of the weld line to be altered to form a stronger bond.  
           [0003]    The &#39;582 patent discloses controlling the rate of melt flow with a tapered valve pin at the gate to the mold cavity. It also discloses placing a pressure transducer inside the mold cavity. Placing the pressure transducer inside the mold cavity can result in the pressure transducer sensing pressure spikes which can occur when the valve pin is closed. A pressure spike sensed by the transducer can cause an unintended response from the control system, and result in a less precise control of the melt flow than desired.  
           [0004]    The control system disclosed in the &#39;582 patent uses the variables of valve pin position and cavity pressure to determine what position the valve pin should be in. Thus, the algorithm performed by the control system in the &#39;582 patent utilizes two variables to control the rate of melt flow into the cavity.  
         SUMMARY OF THE INVENTION  
         [0005]    An injection molding apparatus is provided in which the rate of material flow during the injection cycle is controlled. According to one preferred embodiment, a method is provided for use in an injection molding apparatus including a hot runner assembly comprising a manifold and at least first and second injection nozzles, the hot runner assembly to direct material injected into said manifold through said at least first and second injection nozzles through a corresponding at least first and second gates to one or more mold cavities. The method includes the steps of injecting material into the manifold, controlling, in the hot runner away from the first gate, a first rate at which material is injected through the first gate, and controlling, in the hot runner away from the second gate, a second rate at which material is injected through the second gate, independently from the first rate.  
           [0006]    According to another embodiment, a method is provided for use in an injection molding apparatus including a hot runner to direct material injected into the hot runner and through a gate and into one or more mold cavities. The method includes the steps of injecting material into the hot runner assembly, sensing, in the hot runner, a sensed condition related to a rate at which material is injected through the gate, and controlling the rate based on said sensed condition 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a partially schematic cross-sectional view of an injection molding system according to one embodiment of the present invention;  
         [0008]    [0008]FIG. 2 is an enlarged fragmentary cross-sectional view of one side of the injection molding system of FIG. 1;  
         [0009]    [0009]FIG. 3 is an enlarged fragmentary cross-sectional view of an alternative embodiment of a system similar to FIG. 1, in which a plug is used for easy removal of the valve pin;  
         [0010]    [0010]FIG. 4 is an enlarged fragmentary cross-sectional view of an alternative embodiment of a system similar to FIG. 1, in which a threaded nozzle is used;  
         [0011]    [0011]FIG. 5 is a view similar to FIG. 4, showing an alternative embodiment in which a plug is used for easy removal of the valve pin;  
         [0012]    [0012]FIG. 6 shows a fragmentary cross-sectional view of a system similar to FIG. 1, showing an alternative embodiment in which a forward shut-off is used;  
         [0013]    [0013]FIG. 7 shows an enlarged fragmentary view of the embodiment of FIG. 6, showing the valve pin in the open and closed positions, respectively;  
         [0014]    [0014]FIG. 8 is a cross-sectional view of an alternative embodiment of the present invention similar to FIG. 6, in which a threaded nozzle is used with a plug for easy removal of the valve pin;  
         [0015]    [0015]FIG. 9 is an enlarged fragmentary view of the embodiment of FIG. 8, in which the valve pin is shown in the open and closed positions;  
         [0016]    [0016]FIG. 10 is an enlarged view of an alternative embodiment of the valve pin, shown in the closed position;  
         [0017]    [0017]FIG. 11 is a fragmentary cross sectional view of an alternative embodiment of an injection molding system having flow control that includes a valve pin that extends to the gate; and  
         [0018]    [0018]FIG. 12 is an enlarged fragmentary cross-sectional detail of the flow control area. 
     
    
     DETAILED DESCRIPTION  
       [0019]    FIGS.  1 - 2  show one embodiment of the injection molding system according to the present invention. The injection molding system  1  is a multi-gate single cavity system in which melt material  3  is injected into a cavity  5  from gates  7  and  9 . Melt material  3  is injected from an injection molding machine  11  through an extended inlet  13  and into a manifold  15 . Manifold  15  distributes the melt through channels  17  and  19 . Although a hot runner system is shown in which plastic melt is injected, the invention is applicable to other types of injection systems in which it is useful to control the rate at which a material (e.g., metallic or composite materials) is delivered to a cavity.  
         [0020]    Melt is distributed by the manifold through channels  17  and  19  and into bores  18  and  20  of nozzles  21  and  23 , respectively. Melt is injected out of nozzles  21  and  23  and into cavity  5  (where the part is formed) which is formed by mold plates  25  and  27 . Although a multi-gate single-cavity system is shown, the invention is not limited to this type of system, and is also applicable to, for example, multi-cavity systems, as discussed in greater detail below.  
         [0021]    The injection nozzles  21  and  23  are received in respective wells  28  and  29  formed in the mold plate  27 . The nozzles  21  and  23  are each seated in support rings  31  and  33 . The support rings serve to align the nozzles with the gates  7  and  9  and insulate the nozzles from the mold. The manifold  15  sits atop the rear end of the nozzles and maintains sealing contact with the nozzles via compression forces exerted on the assembly by clamps (not shown) of the injection molding machine. An O-ring  36  is provided to prevent melt leakage between the nozzles and the manifold. A dowel  73  centers the manifold on the mold plate  27 . Dowels  32  and  34  prevent the nozzle  23  and support ring  33 , respectively, from rotating with respect to the mold  27 .  
         [0022]    The nozzles also include a heater  35  (FIG. 2). Although an electric band heater is shown, other heaters may be used. Furthermore, heat pipes (for example those disclosed in U.S. Pat. No. 4,389,002) may be disposed in each nozzle and used alone or in conjunction with heater  35 . The heater is used to maintain the melt material at its processing temperature up to the gates  7  and  9 . The nozzles  21  and  23  also include an insert  37  and a tip  39 . The insert can be made of a material (for example beryllium copper) having high thermal conductivity in order to maintain the melt at its processing temperature up to the gate by imparting heat to the melt from the heater  35 . The tip  39  is used to form a seal with the mold plate  27  and is preferably a material (for example titanium alloy or stainless steel) having low thermal conductivity so as to reduce heat transfer from the nozzle to the mold.  
         [0023]    A valve pin  41  having a head  43  is used to control the rate of flow of the melt material to the respective gates  7  and  9 . The valve pin reciprocates through the manifold. A valve pin bushing  44  is provided to prevent melt from leaking along stem  102  of the valve pin. The valve pin bushing is held in place by a threadably mounted cap  46 . The valve pin is opened at the beginning of the injection cycle and closed at the end of the cycle. During the cycle, the valve pin can assume intermediate positions between the fully open and closed positions, in order to decrease or increase the rate of flow of the melt. The head includes a tapered portion  45  that forms a gap  81  with a surface  47  of the bore  19  of the manifold. Increasing or decreasing the size of the gap by displacing the valve pin correspondingly increases or decreases the flow of melt material to the gate. When the valve pin is closed the tapered portion  45  of the valve pin head contacts and seals with the surface  47  of the bore of the manifold.  
         [0024]    [0024]FIG. 2 shows the head of the valve pin in a Phantom dashed line in the closed position and a solid line in the fully opened position in which the melt is permitted to flow at a maximum rate. To reduce the flow of melt, the pin is retracted away from the gate by an actuator  49 , to thereby decrease the width of the gap  81  between the valve pin and the bore  19  of the manifold.  
         [0025]    The actuator  49  (for example, the type disclosed in application Ser. No. 08/874,962) is mounted in a clamp plate  51  which covers the injection molding system  1 . The actuator  49  is a hydraulic actuator, however, pneumatic or electronic actuators can be used. The actuator  49  includes a hydraulic circuit that includes a movable piston  53  in which the valve pin  41  is threadably mounted at  55 . Thus, as the piston  53  moves, the valve pin  41  moves with it. The actuator  49  includes hydraulic lines  57  and  59  which are controlled by servo valves  1  and  2 . Hydraulic line  57  is energized to move the valve pin  41  toward the gate to the open position, and hydraulic line  59  is energized to retract the valve pin away from the gate toward the close position. An actuator cap  61  limits longitudinal movement in the vertical direction of the piston  53 . O-rings  63  provide respective seals to prevent hydraulic fluid from leaking out of the actuator. The actuator body  65  is mounted to the manifold via screws  67 .  
         [0026]    A pressure transducer  69  is used to sense the pressure in the manifold bore  19  downstream of the valve pin head  43 . In operation, the conditions sensed by the pressure transducer  69  associated with each nozzle are fed back to a control system that includes controllers PID  1  and PID  2  and a CPU shown schematically in FIG. 1. The CPU executes a PID (proportional, integral, derivative) algorithm which compares the sensed pressure (at a given time) from the pressure transducer to a programmed target pressure (for the given time). The CPU instructs the PID controller to adjust the valve pin using the actuator  49  in order to mirror the target pressure for that given time. In this way a programmed target pressure profile for an injection cycle for a particular part for each gate  7  and  9  can be followed.  
         [0027]    Although in the disclosed embodiment the sensed condition is pressure, other sensed conditions can be used which relate to melt flow rate. For example, the position of the valve pin or the load on the valve pin could be the sensed condition. If so, a position sensor or load sensor, respectively, could be used to feed back the sensed condition to the PID controller. In the same manner as explained above, the CPU would use a PID algorithm to compare the sensed condition to a programmed target position profile or load profile for the particular gate to the mold cavity, and adjust the valve pin accordingly.  
         [0028]    Melt flow rate is directly related to the pressure sensed in bore  19 . Thus, using the controllers PID  1  and PID  2 , the rate at which the melt flows into the gates  7  and  9  can be adjusted during a given injection molding cycle, according to the desired pressure profile. The pressure (and rate of melt flow) is decreased by retracting the valve pin and decreasing the width of the gap  81  between the valve pin and the manifold bore, while the pressure (and rate of melt flow) is increased by displacing the valve pin toward the gate  9 , and increasing the width of the gap  81 . The PID controllers adjust the position of the actuator piston  51  by sending instructions to servo valves  1  and  2 .  
         [0029]    By controlling the pressure in a single cavity system (as shown in FIG. 1) it is possible to adjust the location and shape of the weld line formed when melt flow  75  from gate  7  meets melt flow  77  from gate  9  as disclosed in U.S. Pat. No. 5,556,582. However, the invention also is useful in a multi-cavity system. In a multi-cavity system the invention can be used to balance fill rates and packing profiles in the respective cavities. This is useful, for example, when molding a plurality of like parts in different cavities. In such a system, to achieve a uniformity in the parts, the fill rates and packing profiles of the cavities should be as close to identical as possible. Using the same programmed pressure profile for each nozzle, unpredictable fill rate variations from cavity to cavity are overcome, and consistently uniform parts are produced from each cavity.  
         [0030]    Another advantage of the present invention is seen in a multi-cavity system in which the nozzles are injecting into cavities which form different sized parts that require different fill rates and packing profiles. In this case, different pressure profiles can be programmed for each respective controller of each respective cavity. Still another advantage is when the size of the cavity is constantly changing, i.e., when making different size parts by changing a mold insert in which the part is formed. Rather than change the hardware (e.g., the nozzle) involved in order to change the fill rate and packing profile for the new part, a new program is chosen by the user corresponding to the new part to be formed.  
         [0031]    The embodiment of FIGS. 1 and 2 has the advantage of controlling the rate of melt flow away from the gate inside manifold  15  rather than at the gates  7  and  9 . Controlling the melt flow away from the gate enables the pressure transducer to be located away from the gate (in FIGS.  1 - 5 ). In this way, the pressure transducer does not have to be placed inside the mold cavity, and is not susceptible to pressure spikes which can occur when the pressure transducer is located in the mold cavity or near the gate. Pressure spikes in the mold cavity result from the valve pin being closed at the gate. This pressure spike could cause an unintended response from the control system, for example, an opening of the valve pin to reduce the pressure—when the valve pin should be closed.  
         [0032]    Avoidance of the effects of a pressure spike resulting from closing the gate to the mold makes the control system behave more accurately and predictably. Controlling flow away from the gate enables accurate control using only a single sensed condition (e.g., pressure) as a variable. The &#39;582 patent disclosed the use of two sensed conditions (valve position and pressure) to compensate for an unintended response from the pressure spike. Sensing two conditions resulted in a more complex control algorithm (which used two variables) and more complicated hardware (pressure and position sensors).  
         [0033]    Another advantage of controlling the melt flow away from the gate is the use of a larger valve pin head  43  than would be used if the valve pin closed at the gate. A larger valve pin head can be used because it is disposed in the manifold in which the melt flow bore  19  can be made larger to accommodate the larger valve pin head. It is generally undesirable to accommodate a large size valve pin head in the gate area within the end of the nozzle  23 , tip  39  and insert  37 . This is because the increased size of the nozzle, tip and insert in the gate area could interfere with the construction of the mold, for example, the placement of water lines within the mold which are preferably located close to the gate. Thus, a larger valve pin head can be accommodated away from the gate.  
         [0034]    The use of a larger valve pin head enables the use of a larger surface  45  on the valve pin head and a larger surface  47  on the bore to form the control gap  81 . The more “control” surface ( 45  and  47 ) and the longer the “control” gap ( 81 )—the more precise control of the melt flow rate and pressure can be obtained because the rate of change of melt flow per movement of the valve pin is less. In FIGS.  1 - 3  the size of the gap and the rate of melt flow is adjusted by adjusting the width of the gap, however, adjusting the size of the gap and the rate of material flow can also be accomplished by changing the length of the gap, i.e., the longer the gap the more flow is restricted. Thus, changing the size of the gap and controlling the rate of material flow can be accomplished by changing the length or width of the gap.  
         [0035]    The valve pin head includes a middle section  83  and a forward cone shaped section  95  which tapers from the middle section to a point  85 . This shape assists in facilitating uniform melt flow when the melt flows past the control gap  81 . The shape of the valve pin also helps eliminates dead spots in the melt flow downstream of the gap  81 .  
         [0036]    [0036]FIG. 3 shows another aspect in which a plug  87  is inserted in the manifold  15  and held in place by a cap  89 . A dowel  86  keeps the plug from rotating in the recess of the manifold that the plug is mounted. The plug enables easy removal of the valve pin  41  without disassembling the manifold, nozzles and mold. When the plug is removed from the manifold, the valve pin can be pulled out of the manifold where the plug was seated since the diameter of the recess in the manifold that the plug was in is greater than the diameter of the valve pin head at its widest point. Thus, the valve pin can be easily replaced without significant downtime.  
         [0037]    [0037]FIGS. 4 and 5 show additional alternative embodiments of the invention in which a threaded nozzle style is used instead of a support ring nozzle style. In the threaded nozzle style, the nozzle  23  is threaded directly into manifold  15  via threads  91 . Also, a coil heater  93  is used instead of the band heater shown in FIGS.  1 - 3 . The threaded nozzle style is advantageous in that it permits removal of the manifold and nozzles ( 21  and  23 ) as a unitary element. There is also less of a possibility of melt leakage where the nozzle is threaded on the manifold. The support ring style (FIGS.  1 - 3 ) is advantageous in that one does not need to wait for the manifold to cool in order to separate the manifold from the nozzles. FIG. 5 also shows the use of the plug  87  for convenient removal of valve pin  41 .  
         [0038]    FIGS.  6 - 10  show an alternative embodiment of the invention in which a “forward” shutoff is used rather than a retracted shutoff as shown in FIGS.  1 - 5 . In the embodiment of FIGS. 6 and 7, the forward cone-shaped tapered portion  95  of the valve pin head  43  is used to control the flow of melt with surface  97  of the inner bore  20  of nozzle  23 . An advantage of this arrangement is that the valve pin stem  102  does not restrict the flow of melt as in FIGS.  1 - 5 . As seen in FIGS.  1 - 5 , the clearance  100  between the stem  102  and the bore  19  of the manifold is not as great as the clearance  100  in FIGS. 6 and 7. The increased clearance  100  in FIGS.  6 - 7  results in a lesser pressure drop and less shear on the plastic.  
         [0039]    In FIGS. 6 and 7 the control gap  98  is formed by the front cone-shaped portion  95  and the surface  97  of the bore  20  of the rear end of the nozzle  23 . The pressure transducer  69  is located downstream of the control gap—thus, in FIGS. 6 and 7, the nozzle is machined to accommodate the pressure transducer as opposed to the pressure transducer being mounted in the manifold as in FIGS.  1 - 5 .  
         [0040]    [0040]FIG. 7 shows the valve pin in solid lines in the open position and Phantom dashed lines in the closed position. To restrict the melt flow and thereby reduce the melt pressure, the valve pin is moved forward from the open position towards surface  97  of the bore  20  of the nozzle which reduces the width of the control gap  98 . To increase the flow of melt the valve pin is retracted to increase the size of the gap  98 .  
         [0041]    The rear  45  of the valve pin head  43  remains tapered at an angle from the stem  102  of the valve pin  41 . Although the surface  45  performs no sealing function in this embodiment, it is still tapered from the stem to facilitate even melt flow and reduce dead spots.  
         [0042]    As in FIGS.  1 - 5 , pressure readings are fed back to the control system (CPU and PID controller), which can accordingly adjust the position of the valve pin  41  to follow a target pressure profile. The forward shut-off arrangement shown in FIGS. 6 and 7 also has the advantages of the embodiment shown in FIGS.  1 - 5  in that a large valve pin head  43  is used to create a long control gap  98  and a large control surface  97 . As stated above, a longer control gap and greater control surface provides more precise control of the pressure and melt flow rate.  
         [0043]    [0043]FIGS. 8 and 9 show a forward shutoff arrangement similar to FIGS. 6 and 7, but instead of shutting off at the rear of the nozzle  23 , the shut-off is located in the manifold at surface  101 . Thus, in the embodiment shown in FIGS. 8 and 9, a conventional threaded nozzle  23  may be used with a manifold  15 , since the manifold is machined to accommodate the pressure transducer  69  as in FIGS.  1 - 5 . A spacer  88  is provided to insulate the manifold from the mold. This embodiment also includes a plug  87  for easy removal of the valve pin head  43 .  
         [0044]    [0044]FIG. 10 shows an alternative embodiment of the invention in which a forward shutoff valve pin head is shown as used in FIGS.  6 - 9 . However, in this embodiment, the forward cone-shaped taper  95  on the valve pin includes a raised section  103  and a recessed section  104 . Ridge  105  shows where the raised portion begins and the recessed section ends. Thus, a gap  107  remains between the bore  20  of the nozzle through which the melt flows and the surface of the valve pin head when the valve pin is in the closed position. Thus, a much smaller surface  109  is used to seal and close the valve pin. The gap  107  has the advantage in that it assists opening of the valve pin which is subjected to a substantial force F from the melt when the injection machine begins an injection cycle. When injection begins melt will flow into gap  107  and provide a force component F 1  that assists the actuator in retracting and opening the valve pin. Thus, a smaller actuator, or the same actuator with less hydraulic pressure applied, can be used because it does not need to generate as much force in retracting the valve pin. Further, the stress forces on the head of the valve pin are reduced.  
         [0045]    Despite the fact that the gap  107  performs no sealing function, its width is small enough to act as a control gap when the valve pin is open and correspondingly adjust the melt flow pressure with precision as in the embodiments of FIGS.  1 - 9 .  
         [0046]    [0046]FIGS. 11 and 12 show an alternative hot-runner system having flow control in which the control of melt flow is still away from the gate as in previous embodiments. Use of the pressure transducer  69  and PID control system is the same as in previous embodiments. In this embodiment, however, the valve pin  41  extends past the area of flow control via extension  110  to the gate. The valve pin is shown in solid lines in the fully open position and in Phantom dashed lines in the closed position. In addition to the flow control advantages away from the gate described above, the extended valve pin has the advantage of shutting off flow at the gate with a tapered end  1   12  of the valve pin  41 .  
         [0047]    Extending the valve pin to close the gate has several advantages. First, it shortens injection cycle time. In previous embodiments thermal gating is used. In thermal gating, plastication does not begin until the part from the previous cycle is ejected from the cavity. This prevents material from exiting the gate when the part is being ejected. When using a valve pin, however, plastication can be performed simultaneously with the opening of the mold when the valve pin is closed, thus shortening cycle time by beginning plastication sooner. Using a valve pin can also result in a smoother gate surface on the part.  
         [0048]    The flow control area is shown enlarged in FIG. 12. In solid lines the valve pin is shown in the fully open position in which maximum melt flow is permitted. The valve pin includes a convex surface  114  that tapers from edge  128  of the stem  102  of the valve pin  41  to a throat area  116  of reduced diameter. From throat area  116 , the valve pin expands in diameter in section  118  to the extension  110  which extends in a uniform diameter to the tapered end of the valve pin.  
         [0049]    In the flow control area the manifold includes a first section defined by a surface  120  that tapers to a section of reduced diameter defined by surface  122 . From the section of reduced diameter the manifold channel then expands in diameter in a section defined by surface  124  to an outlet of the manifold  126  that communicates with the bore of the nozzle  20 . FIGS. 11 and 12 show the support ring style nozzle similar to FIGS.  1 - 3 . However, other types of nozzles may be used such as, for example, a threaded nozzle as shown in FIG. 8.  
         [0050]    As stated above, the valve pin is shown in the fully opened position in solid lines. In FIG. 12, flow control is achieved and melt flow reduced by moving the valve pin  41  forward toward the gate thereby reducing the width of the control gap  98 . Thus, surface  114  approaches surface  120  of the manifold to reduce the width of the control gap and reduce the rate of melt flow through the manifold to the gate.  
         [0051]    To prevent melt flow from the manifold bore  19 , and end the injection cycle, the valve pin is moved forward so that edge  128  of the valve pin, i.e., where the stem  102  meets the beginning of curved surface  114 , will move past point  130  which is the beginning of surface  122  that defines the section of reduced diameter of the manifold bore  19 . When edge  128  extends past point  130  of the manifold bore melt flow is prevented since the surface of the valve stem  102  seals with surface  122  of the manifold. The valve pin is shown in dashed lines where edge  128  is forward enough to form a seal with surface  122 . At this position, however, the valve pin is not yet closed at the gate. To close the gate the valve pin moves further forward, with the surface of the stem  102  moving further along, and continuing to seal with, surface  122  of the manifold until the end  112  of the valve pin closes with the gate.  
         [0052]    In this way, the valve pin does not need to be machined to close the gate and the flow bore  19  of the manifold simultaneously, since stem  102  forms a seal with surface  122  before the gate is closed. Further, because the valve pin is closed after the seal is formed in the manifold, the valve pin closure will not create any unwanted pressure spikes. Likewise, when the valve pin is opened at the gate, the end  112  of the valve pin will not interfere with melt flow, since once the valve pin is retracted enough to permit melt flow through gap  98 , the valve pin end  112  is a predetermined distance from the gate. The valve pin can, for example, travel 6 mm. from the fully open position to where a seal is first created between stem  102  and surface  122 , and another 6 mm. to close the gate. Thus, the valve pin would have 12 mm. of travel, 6 mm. for flow control, and 6 mm. with the flow prevented to close the gate. Of course, the invention is not limited to this range of travel for the valve pin, and other dimensions can be used.  
         [0053]    Having thus described certain embodiments of the present invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereof.