Abstract:
A hot runner system including a shoot pot system for transferring melt from a single shooting pot to multiple nozzles. Melt is fed from a source of melt into the cavity through the multiple nozzles, and a valve isolates melt in the cavity from melt in the source. A plunger within the cavity is driven forward to inject melt in the cavity into a mold cavity at high pressure without significantly increasing the pressure of melt in the source. The plunger optionally functions as both the plunger and the valve by opening and closing communication between the cavity and the manifold as it is rotated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of prior U.S. patent application Ser. No. 11/931,106, filed on Oct. 31, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     APPENDIX 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to injection molding systems and relates in particular to the injection of metered amounts of melt and to the injection of melt at high pressure. 
     2. Related Art 
     In many applications it is desirable to mold plastic parts with the least amount of plastic necessary to perform the desired function of the finished part without premature failure. Therefore, as resins are made increasingly stronger, part wall thickness can correspondingly be made thinner and more molded parts can be made with the same amount of melt. In addition, since thinner parts are quicker to cool, set and eject, parts with thinner walls can be made at a faster cycle time than parts with thicker walls, which increases maximum machinery output rates. 
     Thinner parts generally require higher injection pressures than thicker parts of similar size and shape. Therefore, machinery injection units capable of creating increasingly higher injection force are required to fill mold cavities for increasingly thin-walled parts. Prior designs attempting to meet this need have utilized high pressure injection units coupled with hot runner manifold systems capable of withstanding high pressures. These high pressure injection units and manifold systems are often more expensive and more difficult to maintain because higher quality materials capable of withstanding high pressures must be used. These systems also suffer from the fact that a significant amount of pressure is lost as the melt passes through the manifold and the nozzle, which makes achieving desired pressures within the mold cavity more challenging still. 
     In many applications it is also desirable to reliably produce molded parts with statistically consistent part characteristics. In many instances customers require stringent and repeatable molding processes to be verified with sensors, instrumentation and/or fixed and documented molding parameters. One area of particular concern is part weight, which is perceived as an indication of complete part filling and consistency of part quality and/or uniformity. 
     In many prior designs, this is accomplished by precision design and manufacturing of hydraulically balanced melt channel layouts, carefully thermally balanced heat distribution of the manifold and nozzles, use of valve gated cavity filling orifices in the manifold, and valve pin position sensors to confirm the opening and closing of each cavity position during the injection cycle. 
     SUMMARY OF THE INVENTION 
     The present invention provides an injection apparatus capable of injecting melt into a mold cavity at high pressure while utilizing a low pressure injection unit and manifold. The apparatus according to the present invention is also capable of precisely metering the amount of melt injected into a mold cavity during each injection cycle. 
     The apparatus according to the present invention has a cavity contained within and defined by a housing. This cavity receives melt at low pressure from a source of melt. The source of melt can include conventional equipment used for low pressure injection molding such as, for example, a low pressure injection unit and a low strength manifold. When the cavity is appropriately filled with melt, a selectively closable valve intermediate to the source of melt and the cavity closes, thereby isolating melt in the cavity from melt in the source. A plunger within the cavity is then driven forward increasing the pressure of the melt within the cavity and injecting melt in the mold cavity at high pressure. The valve prevents any substantial backflow of melt into the source of melt during the injection and also prevents any substantial increase in the pressure of melt within the source. As backflow into the source of melt is prevented and the cavity is proximate to the injection outlet of the nozzle, the amount of melt injected into the mold cavity can be precisely metered by monitoring the distance the plunger is pressed forward. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a sectional view of an injection molding apparatus constructed in accordance with an embodiment of the present invention. 
         FIG. 2  is a sectional view an injection molding apparatus having a reduced vertical profile and constructed in accordance with an embodiment of the present invention. 
         FIG. 3  is a sectional view of an injection molding apparatus having an alternative valve mechanism and constructed in accordance with an embodiment of the present invention. 
         FIG. 4  is a sectional view of an injection molding apparatus having an alternative valve and valve stem construction and constructed in accordance with an embodiment of the present invention. 
         FIG. 5  is a close-up sectional view of a thermal shut-off injection molding apparatus constructed in accordance with an embodiment of the present invention. 
         FIG. 6  is a sectional view of an injection molding apparatus having an alternative valve mechanism and constructed in accordance with an embodiment of the present invention. 
         FIG. 7  is a perspective view of a shooting pot assembly used to supply molten material to a plurality of nozzles. 
         FIG. 8  is a cross-sectional view of the shooting pot assembly used to supply molten material to a plurality of nozzles. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     As illustrated in  FIG. 1 , a hot runner system for injection molding is provided having a shooting pot assembly  10  contained within a nozzle  12 . According to this embodiment, melt is injected at low pressure into a manifold channel  14  of a manifold  16 . Manifold channel  14  is provided with melt by way of a hot runner system leading from a source or supply means, such as an extruder. Melt passes from manifold channel  14  into a bushing cavity  18 . In some embodiments, melt passes through a bushing channel  20  prior to entering bushing cavity  18 . Melt passes into bushing cavity  18  at an inlet  22 . A valve  24  separates manifold channel  14  from bushing channel  20 . Valve  24  opens to fill bushing cavity  18  with melt and closes during or prior to injection of melt into a mold cavity  26  to isolate melt in bushing cavity  18  from melt in manifold channel  14 . 
     According to the embodiment depicted in  FIG. 1 , a valve stem  28  is provided within bushing cavity  18  to open and close an injection outlet  30  defined by the bushing  38 . Valve stem  28  is driven to open and close injection outlet  30  by a suitable valve stem actuator  32 , such as, by way of example, a pneumatic drive or electric motor. 
     A plunger  34  is also provided in bushing cavity  18 . Preferably, plunger  34  is dimensioned so as to form a seal between an outer surface of plunger  34  and side walls defining the bushing cavity  18 . Plunger  34  is driven by a plunger actuator  36  capable of providing sufficient force to create a desired pressure within bushing cavity  18 , such as, by way of example, a hydraulic drive or electric motor. In the depicted embodiment, valve stem  28  passes through the center of plunger  34  and is actuated independently from plunger  34 . 
     At the time of injection of melt into mold cavity  26 , valve  24  is closed and valve stem  28  is pulled away from injection outlet  30  to allow melt in bushing cavity  18  to pass through injection outlet  30 . With injection outlet  30  open, plunger  34  is actuated to move forward to inject melt into mold cavity  26  at high pressure. The closed valve  24  facilitates a high pressure differential between melt in bushing cavity  18  and melt in manifold channel  14  during injection of melt into mold cavity  26 . Thus, higher pressure is achieved within the bushing  38  while lower pressure is maintained within manifold channel  14  and structures upstream of manifold  16 . 
     As high pressure is isolated to nozzle  12 , a low performance injection unit that handles and discharges melt at low pressure can be used in conjunction with the present invention to produce pieces requiring injection of melt at high pressure, such as parts having thin walls. Additionally, a low strength manifold  16 , such as one made with low grade steel or through free form fabrication, can be used in the production of such molded pieces requiring high pressure injection. According to an embodiment of the present invention shown in  FIG. 6 , a heated manifold  16  having a flexible melt distribution system is employed. 
     Furthermore, in applications in which precise control over the quantity of melt injected into each mold cavity  26  is desirable, the present invention can be employed to ensure a metered amount of melt is positively and repeatedly injected into each individual mold cavity  26 . In a hot runner system employing multiple nozzles  12 , this feature facilitates precise balance between each nozzle  12 . Synchronized filling and consistent part weight can be adjusted and controlled through plunger  34  start and stop positions. These start and stop positions can be confirmed with sensors for greater precision and reliability. 
     Another embodiment of the present invention is depicted in  FIG. 2 . This embodiment is similar to the embodiment depicted in  FIG. 1 , except the height of the system is reduced by situating plunger actuator  36  side-by-side with nozzle  12  rather than on top of nozzle  12 . According to this embodiment, plunger  34  and plunger actuator  36  are attached to a plate  40  that transmits power from plunger actuator  36  to plunger  34 . 
     In an embodiment depicted in  FIG. 3 , plunger  35  serves as both valve  24  and plunger  34  shown in  FIG. 2  respectively, to isolate melt in bushing cavity  18  from melt in manifold channel  14 . According to this embodiment, melt at low pressure passes from manifold channel  14  to bushing channel  20  without passing through a valve  24  as shown previously in  FIGS. 1 and 2 . A portion of one side of plunger  35  has a recess  42  that forms a channel between bushing  38  and plunger  34 . When filling bushing cavity  18  with melt, plunger  35  is rotated to align recess  42  with inlet  22  and pulled back. Thus, while filling bushing cavity  18 , melt flows from manifold channel  14 , into bushing channel  20 , then through inlet  22  into the channel formed by recess  42  between plunger  34  and bushing  38 , thereby filling bushing cavity  18 . 
     Prior to pressing plunger  35  forward to inject melt into mold cavity  26 , plunger  35  is rotated such that recess  42  is not aligned with inlet  22 , substantially preventing melt in bushing cavity  18  from flowing back into bushing channel  20  and manifold channel  14 . After it is rotated, plunger  35  is actuated to move forward to inject melt into mold cavity  26  at high pressure through nozzle  12 . In this manner the interaction between plunger  35  and bushing  38  serves as valve  24  to prevent pressurization of melt in manifold channel  14  and structures upstream therefrom during injection of melt into mold cavity  26 . 
     In the depicted embodiment means for rotation of plunger is provided in the form of a rack or gear  44 . Rack  44  is preferably motivated by an actuator (not shown) such as, for example, a hydraulic piston or electric motor, and interacts with teeth  46  formed on plunger  35  to cause plunger  35  to rotate as rack  44  is actuated to move up and down. Various other known means could be employed to rotate plunger  35  such as, by way of example, an arm and link system as disclosed in U.S. Pat. No. 5,112,212, the entire specification of which is incorporated herein by reference. 
     In the embodiment depicted in  FIG. 4 , valve stem  28 , having a smaller cross-sectional area, is fixed to the end of plunger  35 , having a larger cross-sectional area, and valve stem  28  and plunger  35  move together as a single unit. According to this embodiment, plunger  35  contains a plunger channel  48  passing substantially through the center of plunger  35 . Plunger channel  48  has ingress  50  at an opening located on the surface of plunger  35 , and egress  52  that opens into bushing cavity  18 . In the depicted embodiment, egress  52  is two openings at the junction of plunger  35  and valve stem  28 ; however, the placement and number of such openings is a design choice, and more or less openings could be used for egress  52 . 
     According to this embodiment, when filling bushing cavity  18  with melt, melt flows from manifold channel  14  to bushing channel  20  without passing through valve  24 . Valve stem  28 /plunger  35  combination is pulled back and rotated to align ingress  50  with inlet  22  such that melt passes from manifold channel  14 , then through bushing channel  20  into plunger channel  48  and then empties into bushing cavity  18 . 
     When injecting melt into mold cavity  26 , valve stem  28 /plunger  35  combination is rotated such that ingress  50  is not aligned with inlet  22  so as to prevent backwash into, and pressurization of, manifold channel  14 . Valve stem  28 /plunger  35  combination is pressed forward such that melt in bushing cavity  18  is injected into mold cavity  26  at high pressure. When valve stem  28 /plunger  35  combination reaches its most advanced position, valve stem  28  will close injection outlet  30 . Preferably, after an appropriate cooling period and with injection outlet  30  closed, the injection molded piece is expelled from mold cavity  26  and the cycle begins again. 
     As depicted in  FIG. 4 , immediately surrounding nozzle housing is a heater  54  that heats bushing  38  to maintain melt within bushing cavity  18  at a desired temperature. It is preferable to heat melt while limiting the amount of heat transferred to manifold plate  56  and mold  58 . Accordingly, air space  60  is provided as an insulator between much of bushing  38  and manifold plate  56 . Flanges  62  are also provided within bushing cavity  18  to increase contact area between the hot bushing  38  and melt, while decreasing the area in which the hot bushing  38  is in direct contact with mold  58 . The pockets  64  formed between flanges  62  and mold  58  may be filled with a thermoset material, or simply allowed to fill with melt. 
     In an alternate embodiment depicted in  FIG. 5  thermal shut-off is employed as a means of preventing melt from drooling out of injection outlet  30  after the part is ejected from mold cavity  26  and prior to a new injection of melt into mold cavity  26 . This embodiment can be employed with plunger  37  configurations similar to those depicted in FIGS.  1 - 4 ; however, no valve stem  28  is required to close injection outlet  30 . According to this embodiment, heater  54  is provided surrounding nozzle  12  near injection outlet  30 . Heater  54  maintains melt in the nozzle at an appropriate pre-injection temperature. Prior to reaching injection outlet  30 , melt passes through a tip insert  66 . 
     Similar to the prior embodiments, melt is injected into bushing channel  20  at low pressure and isolated from manifold channel  14  by rotating plunger  37  which is then pressed forward to inject melt into mold cavity  26  at high pressure. At the end of an injection cycle, when mold cavity  26  is appropriately filled with melt, melt within injection outlet  30  is cooled and solidifies. This solidified melt serves as a plug to prevent molten melt from passing through injection outlet  30  while the injection molded piece is expelled from mold cavity  26 . On the next cycle, when melt is injected into mold cavity  26 , pressure in bushing channel  20  pushes the solidified melt through injection outlet  30  into mold cavity  26  where it melts and mixes with the fresh stream of molten melt. 
     Turning now to  FIGS. 7 &amp; 8 , the embodiments of the shooting pot assembly  10  previously described may be modified such that the plunger  34  feeds two or more nozzles  12 . In the embodiment shown in  FIGS. 7 &amp; 8 , the plunger  34  is actuated to move forward to inject melt through two or more nozzles  12  and then into the mold cavity  26  at high pressure. 
     As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.