Abstract:
This invention teaches a hot runner injection molding apparatus for co-injecting at least two different materials into a mold cavity. A pin valve gated hot runner nozzle includes separate melt channels for each material and a melt chamber for accurately metering one of the materials. The melt chamber is in communication with an injection piston. The controlled movement of the valve pin and of the injection piston insures that the desired amount of a first and at least a second material is injected into the mold cavity. Depending of the composition and the processing window of the materials, the co-injection hot runner nozzle is in communication with either a single or multiple manifolds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of provisional application No. 60/430,358, filed Dec. 3, 2002, which is incorporated by reference in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a co-injection molding apparatus, and in particular to a metering device for a hot runner nozzle, which injects a predetermined quantity of melt into a mold cavity. The present invention is applicable to molding applications in which two or more materials are injected simultaneously and/or sequentially into a single mold cavity. 
     BACKGROUND OF THE INVENTION 
     In a co-injection molding apparatus, two or more molten materials are injected into the same mold cavity, either simultaneously or in sequence using a single or a plurality of injection manifolds. A typical co-injection molding apparatus comprises first and second injection manifolds that receive pressurized melt streams from respective molten material sources. Each manifold distributes a melt stream of molten material to a plurality of nozzles. The two melt streams are forced through separate channels in the nozzle and into a plurality of mold cavities. The two melt streams may enter the mold cavities simultaneously or, alternatively, the two melt streams may enter in sequence. A combination in which the melt streams first enter the mold cavities in sequence and then simultaneously may also be used. Once both materials have been deposited in the mold cavities, the melt is cooled in the mold cavities and the molded parts are released so that another cycle can begin. 
     Co-injection is used for example to produce food packaging products having a predetermined and very accurate amount of an inner material, such as for example oxygen barriers or having a percentage of recycled, or post-consumer material or having a percentage of a different colored material. 
     In general, the amount of the inner material that enters the mold cavity after injecting the first outer material must be very precise in order to produce a quality molded part. In the case of a multi cavity molding system, the quantity of the inner material must also be the same in each molded material. This inner material can be a barrier material. 
     It is desirable to use as much recycled material in a molded part as possible without exceeding a maximum allowable amount. As such, the amount must be measured precisely. 
     In order to ensure that the molded product has a consistent appearance, the amount of colored material that enters the mold cavity must be precisely measured. 
     In a co-injection molding apparatus, the volume of the inner or core material, such as a barrier, recycled or colored material transferred in each shot is very important. Several devices have been developed to control the volume of melt that is injected into a multi-material mold cavity, however, these devices tend to be inaccurate, difficult to operate, complex and costly to manufacture. 
     U.S. Pat. No. 5,223,275 to Gellert discloses a co-injection molding apparatus having two manifolds. Two separate channels are provided in a plurality of nozzles to receive material from the respective manifolds. The volumes of the first and second materials flowing into a mold cavity are controlled by the machine nozzle and therefore are not precise. 
     U.S. Pat. No. 5,112,212 to Akselrud et al. discloses a shooting pot, which is used as a metering device, for use in a co-injection molding apparatus. The shooting pots are remotely located with respect to the hot runner nozzle and are used to control the timing and the volume of one of the two molten materials injected into the cavity. The shooting pot includes a piston that is axially movable within a cylinder to force molten material from the cylinder into a nozzle, which leads to a mold cavity. The cylinder includes an inlet that delivers melt from a melt source to a reservoir, which is located in a lower end of the piston. The piston is rotatable to move the reservoir out of communication with the inlet to seal it off so that when the piston is lowered, a known volume of melt is forced into the mold cavity. 
     Other shooting pot arrangements for use in co-injection are shown in U.S. Pat. Nos. 5,143,733 and 5,200,207 and European Patent Application No. EP 0 624 449. 
     A disadvantage of these manifold shooting pots is that they are remotely located from the nozzle and the mold cavity and this makes the whole apparatus more space consuming. Also these shooting pots located in the manifold or adjacent the manifold include separate mechanisms located in the manifold that open and close the access of the metered molten material to the shooting pot and these mechanisms are space consuming, difficult to manufacture and hard to synchronize in a multi-cavity mold. By using these known co-injection molding devices, the measured volume of inner melt injected from the shooting pots may vary from one molding cycle to the next and from one cavity to another. This occurs because there is a large volume of melt that is located between the shooting pot and the mold cavity, i.e., the melt in the nozzle, the melt in the manifold channel and the melt in the shooting pot. This large volume of quasi metered melt introduces several process variables. Minor deviations in temperature or pressure, for example, may result in significant variations of the known volume. The sizable distance between the shooting pot and the mold cavity further causes the melt to have a long residence time outside of the nozzle between the injection of one article to the next. This results in molded parts that are not of the highest quality because the temperature of the melt coming from the shooting pot may be either under heated or over heated. 
     It is therefore an object of the present invention to provide a metering device for a nozzle of a co-injection molding apparatus, which obviates or mitigates at least one of the above disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention generally provides at least one manifold for delivering at least two different materials to at least one mold cavity through a single or a plurality of hot runner nozzles. Each hot runner nozzle includes a metering device that is used to deliver a predetermined and accurate amount of a molten material into each mold cavity. 
     According to one aspect of the present invention there is provided an injection molding apparatus comprising: 
     a first manifold having a first manifold channel for receiving a first melt stream of moldable material under pressure, the first manifold channel having a first outlet for delivering the first melt stream to a nozzle channel of a nozzle; 
     a second manifold having a second manifold channel for receiving a second melt stream of moldable material under pressure, the second manifold channel having a second outlet for delivering the second melt stream to a second nozzle channel of a nozzle; 
     a mold cavity receiving the first melt stream and the second melt stream from the nozzle, the first nozzle channel and second nozzle channel communicating with the mold cavity through a mold gate; 
     a gating mechanism for selectively enabling communication between the first nozzle channel, the second nozzle channel and the mold gate; 
     an injection piston extending through a channel located between the first outlet of the manifold and the first nozzle channel of the nozzle, the injection piston being slidable through the channel and having an outer wall for abutting an inner wall of the nozzle channel, the injection piston being movable from a retracted position to an extended position to force melt towards the mold cavity; 
     wherein movement of the injection piston towards the extended position forces melt located in a melt chamber of the nozzle channel to flow into the mold cavity. 
     According to another aspect of the present invention there is provided a method of forming a molded product from at least two different materials comprising: 
     injecting a first material into a mold cavity, the mold cavity being in communication with a hot runner nozzle to receive the first material under pressure therefrom, a machine injection unit providing the first material under pressure to the hot runner nozzle through a manifold; and 
     injecting a second material into the mold cavity, the mold cavity being in communication with a melt chamber that is located at least partially in the hot runner nozzle, an injection piston forcing the second material from the melt chamber into the mold cavity. 
     According to yet another aspect of the present invention there is provided an injection molding apparatus comprising: 
     a hot runner injection nozzle having a first melt channel and a second melt channel; 
     a valve gating mechanism to control the flow of a first molten material and a second molten material through a mold gate; 
     a melt chamber located in the hot runner injection nozzle; and 
     an injection piston in communication with the hot runner injection nozzle to force a metered amount of molten material into the mold cavity. 
     The present invention provides an advantage for multi-material molding in that a metered quantity of a melt is delivered accurately and consistently to a single or a plurality of mold cavities via a melt chamber located in a hot runner nozzle. The metered amount of melt is delivered by actuating an injection piston located in fluid communication with the nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which: 
         FIG. 1  is a side sectional view of a co-injection molding apparatus according to the present invention; 
         FIG. 2  is a side sectional view of a portion of the co-injection molding apparatus of  FIG. 1  including a mold cavity, the co-injection molding apparatus in a closed position; 
         FIG. 3  is a side sectional view of a portion of the co-injection apparatus of  FIG. 1  in a first molding position; 
         FIG. 4  is a side sectional view of a portion of the co-injection apparatus of  FIG. 1  in a second molding position; 
         FIG. 5  is a side sectional view of a portion of the co-injection apparatus of  FIG. 1  in the closed position of  FIG. 2  with a completed part in the mold cavity; and 
         FIG. 6  is a side sectional view of an embodiment of the present invention showing a single manifold to guide at least two molten materials towards a single nozzle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , portions of a co-injection molding apparatus are generally shown at  10 . The co-injection molding apparatus  10  includes a first manifold  12  having a first manifold melt channel  14  for receiving a first melt stream of moldable material under pressure from a first manifold bushing  16 . The first manifold bushing  16  is in communication with a first machine nozzle (not shown). Bores  18  extend through the first manifold  12  at distal ends of the first manifold melt channel  14 . The bores  18  are in communication with the first melt channel  14  and extend generally perpendicular thereto. 
     The injection molding apparatus  10  further comprises a second manifold  22  having a second manifold melt channel  24  for receiving a second melt stream of moldable material under pressure through an inlet  26  from a second manifold bushing (not shown). Bores  28  extend through the second manifold  22  at distal ends of the second manifold  22  and extend generally perpendicular thereto. The bores  28  of the second manifold  22  are aligned with bores  18  of the first manifold channel  12 . The second melt stream exits the second manifold  22  through outlets  29 . 
     Spacers  40  are located between a lower surface of the first manifold  12  and an upper surface of the second manifold  22 . Each spacer  40  includes a bore  42  that extends therethrough. The bore  42  is aligned with the bores  18  and  28  of the first and second manifolds  12  and  22 , respectively. Each spacer  40  further includes a flange  44  that projects outwardly from a lower surface  46  thereof. The flange  44  is seated in a recess  48  provided in the upper surface of the second manifold  22  to locate the spacer  40  relative to the second manifold  22 . 
     Hot runner nozzles  30  are coupled to a lower surface of the second manifold  22 . Each nozzle  30  includes a nozzle body  32  having a mold gate  34  located adjacent a tip thereof. The mold gates  34  are openable to allow delivery of melt to respective mold cavities  60  (shown in  FIG. 2 ). 
     Each nozzle  30  further includes an inner nozzle divider  36  having a nozzle flange  38 . The nozzle flange  38  is supported by a shoulder  50 , which is formed in an upper end of the nozzle body  32 . The inner nozzle divider  36  includes a first nozzle channel gate  56 , which is spaced from the tip of the nozzle  30 . The inner nozzle divider  36  separates a first nozzle channel  52  from a second nozzle channel  54 . The first nozzle channel  52  is aligned with bores  18 ,  42  and  28  of the first manifold  12 , the spacer  40  and the second manifold  22 , respectively. The first nozzle channel  52  receives the first melt stream from the first manifold  12 . The second nozzle channel  54  is in communication with the annular outlet  29  of the second manifold  22  and receives the second melt stream therefrom. The second nozzle channel  54  generally surrounds the first nozzle channel  52 . 
     Any number of nozzles  30  can be used to feed either a single or a plurality of mold cavities  60  (shown in  FIG. 2 ). The mold cavities  60  may be of the same size and shape or they may differ. Manifold heaters (not shown) and nozzle heaters  62  maintain the melt stream at a desired temperature and cooling channels (not shown) facilitate cooling of the mold cavities  60 . 
     A valve pin  58  extends through the bores  18 ,  42  and  28  of the first manifold  12 , the spacer  40  and the second manifold  22 , respectively, and the first nozzle channel  52 . The valve pin  58  is generally a gating mechanism that is pneumatically driven by a valve pin head  64 , which is slidable within a cylinder  66 . The valve pin  58  has three positions: open, partially open and closed. 
     Referring to  FIG. 2 , the valve pin  58  is shown in the closed position. In this position, the valve pin  58  engages the mold gate  34  to block melt from flowing from the nozzle  30  into the mold cavity  60 . The valve pin head  64  further communicates with a stroke limiting device  79 . The stroke limiting device has two positions that control the location of the valve pin  58  with respect to the first nozzle channel gate  56 . 
     Referring to  FIG. 3 , the valve pin  58  is shown in the partially open position. In this position, the valve pin  58  extends through the first nozzle channel gate  56  to block the first melt stream from the first nozzle channel  52  to the mold cavity  60 . Because the valve pin  58  is not in contact with the mold gate  34 , the second melt stream is able to enter the mold cavity  60 . 
     Referring to  FIG. 4 , the valve pin  58  is in the open position. In this position, both the first melt stream and the second melt stream are able to flow from the nozzle  30  into the mold cavity  60 . 
     The valve pin  58  is not limited to being driven pneumatically, it may be also driven hydraulically or by any other suitable means, including electrical and electromagnetic motors. In addition, it will be appreciated that the valve pin  58  may be replaced with another suitable gating system. 
     Referring to  FIGS. 2–5 , a hot runner metering device for the first or inner melt material is disclosed in more detail in the form of a hot runner injection piston  70  and a melt chamber  78 . The injection piston  70  is slidable through an injection manifold channel  80 , which communicates with the bore  42  of the spacer  40 , the bore  28  of the second manifold  22  and the first nozzle channel  52 . The injection piston  70  is pneumatically driven by a piston head  74  that is slidable in a second cylinder  73 . The injection piston  70  is not limited to being driven pneumatically, it may be also driven hydraulically or by any other suitable means, including electrical and electromagnetic motors. 
     The injection piston  70  includes a central bore  72  which allows the valve pin  58  to slide through the injection piston  70  into the open, partially open and closed positions that have been previously described. The injection piston  70  includes an outer surface  76 , which selectively blocks the communication between the first manifold channel  14  and the injection channel  80 . The clearance  81  between the channel  14  and the channel  80  is defined by the position of the injection piston  70 . The metered melt chamber  78  has a constant volume which is defined by the amount of melt located in the channels  52 ,  28  and  42  when the injection piston  70  closes the communication between channel  80  and channel  14 . 
     The injection piston  70  is movable from a retracted position, which is shown in  FIGS. 2 and 3 , to an extended position, which is shown in  FIGS. 4 and 5 . In the retracted position, melt flows from the first manifold channel  14  into the injection channel  80  via clearance  81 . In the extended position, communication between the first manifold channel  14  and the injection channel  80  is blocked and the melt chamber  78  (shown in  FIG. 4  and  FIG. 5 ) is formed. 
     When the injection piston  70  is in the retracted position and the valve pin  58  is in either the closed or partially open positions, the melt chamber  78  (shown in  FIGS. 4 and 5 ) is opened and accessible to be filled with the first or inner melt in the injection channel  80  between a forward end  75  of the piston body  74  and the first nozzle channel gate  56 . The volume of melt in the melt chamber is known. Because the stroke of the injection piston  70  from the retracted position to the extended position is known and constant among each nozzle  30 , the volume of melt injected into the mold cavity  60  from the injection channel  80  is also known and is constant from one nozzle to another and from one injection cycle to the next. The close proximity of the known volume of melt to be injected and the mold cavity  60  reduces the variability experienced by prior art devices 
     In operation, the first inner or core pressurized melt stream flows through the first manifold bushing  16  to the first manifold channel  14  of the first manifold  12  and into the first nozzle channel  52  which is closed by valve pin  58  to form the melt chamber  78  of a known size. The second outer or skin pressurized melt stream flows through the second manifold bushing (not shown) to the second manifold channel  24  of the second manifold  22  and into the second nozzle channel  54 . Referring to  FIG. 2 , the co-injection molding cycle may begin according to an embodiment of this invention with both the mold gate  34  and the first nozzle channel gate  56  in the closed position and the injection piston  70  in the retracted position. 
     Referring to  FIG. 3 , the valve pin  58  is retracted from the closed position of  FIG. 2  into the partially open position to allow the second melt stream to flow from the second nozzle channel  54  into the mold cavity  60 . The position of the valve pin  58  is blocked by the stroke limiter  79 , which is in the forward position. According to an embodiment of the current invention, following the injection of a certain volume of the second melt into the mold cavity  60 , the melt chamber  78  is filled with the first material and then the injection piston  70  is moved into an intermediate position to block the manifold melt channel  14  while keeping the valve pin  58  in the closed position of the first nozzle channel gate  56 . The movement of the injection piston  70  to block the communication between the manifold melt channel  14  and the bore  42  creates the metered volume of the inner or core second material which is located mostly in the first nozzle channel  52  and above it to form the melt chamber  78 . The volume of the melt chamber  78  is always the same from one shot to the next and from one nozzle to the others. During the next injection step, the valve pin  58  is moved into the open position of  FIG. 4 . As the valve pin  58  moves toward the open position, the injection piston  70  is further extended so that melt flows simultaneously from both the first and second nozzle channels  52 ,  54  into the mold cavity  60 . As shown, the first melt stream generally flows inside the second melt stream so that the barrier, colored or post-consumer layer is generally centrally disposed within the molded product. The volume of the core or inner melt that can be a barrier, colored or post-consumer layer is pre-determined so that the quantity of these substances that is injected into each product is controlled. 
     Referring to  FIG. 5 , the valve pin  58  is returned to the closed position once the predetermined volume of first melt has been injected into the mold cavity  60 . The mold cavity  60  is then cooled and the molded product is released from the mold cavity  60 . From the position of  FIG. 5 , the injection piston  70  is returned to the retracted position of  FIG. 2  and the injection molding cycle is repeated. According to another embodiment of the current invention, the second or the inner or the core material can be injected from the melt chamber  78  in a different manner for certain co-injection applications where there is a need for a faster injection cycle or for a simpler controller of the movements of the injection piston  70  and the valve pin  58 . In this case, shown in  FIG. 3 , after the second or the skin material is injected in the mold cavity  60  and the first or core material is injected in the first nozzle channel  52 , the valve pin  58  is moved to the fully retracted position to open the first nozzle channel gate  56  and, simultaneously the injection piston  70  is moved gradually to a fully extended position to inject the first material from the melt chamber  78  into the mold cavity  60 . 
     As will be appreciated, the injection molding apparatus  10  described herein ensures that the volume of melt injected from the first nozzle channel  52  and the melt chamber  78  into the mold cavity  60  is equal for each mold cavity  60  and is constant for every cycle. 
     Because a manifold typically supports more than one nozzle, it will be appreciated by a person skilled in the art that the movement of the individual pistons of each nozzle may be staggered so that the pressure from the machine nozzle can remain constant. 
     In a further embodiment, the mold cavities  60  are of different sizes. In order to properly fill each mold cavity  60 , the melt chamber  78  of each nozzle  30  must be sized to accommodate the correct volume of melt. The nozzles  30  associated with each mold cavity  60  are identical, however, each injection piston  70  must be sized accordingly. 
     Referring to  FIG. 6 , another embodiment of a co-injection molding apparatus  10   a  is shown, in which like reference numerals have been used to denote like parts. The co-injection molding apparatus  10   a  includes a manifold  12   a  having a first manifold melt channel  14   a  and a second manifold channel  24   a  extending therethrough. The first manifold melt channel  14   a  receives a first melt stream of moldable material under pressure from a first manifold bushing  16   a , which is in communication with a first machine nozzle (not shown). Bores  18   a  extend through the manifold  12   a  at distal ends of the first manifold melt channel  14   a . The bores  18   a  are in communication with the first melt channel  14   a  for receiving the first melt stream therefrom. 
     The second manifold melt channel  24   a  receives a second melt stream of moldable material under pressure from a second manifold bushing (not shown). The second melt stream exits the manifold  12   a  through outlets  29   a.    
     Hot runner nozzles  30   a  are coupled to a lower surface of the manifold  12   a . Each nozzle  30   a  includes a nozzle body  32   a  having a mold gate  34   a  located adjacent a tip thereof. The mold gates  34   a  are openable to allow delivery of melt to respective mold cavities. The nozzles  30   a  and the remaining components are similar to those of  FIG. 1  and therefore will not be described further here. 
     The co-injection molding apparatus  10   a  of  FIG. 6  operates in a similar manner to the co-injection molding apparatus of  FIG. 1 , with the exception that the first and second manifolds have been replaced with a single manifold  12   a.    
     Although a preferred embodiment of the present invention has been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.