Abstract:
Disclosed herein are a method and apparatus for combining two or more streams of a polymeric material to form a plastic object. The method and apparatus are capable of ending an interior layer of the plastic object at a desired length to avoid the need to clean selected surfaces of components used to form the plastic object. The method and apparatus increase the velocity of the polymeric material used to form the plastic object in certain components used to form the plastic object. The increase in the velocity of the polymeric material facilitates the ending of the interior layer of the plastic object.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/472,550, filed May 21, 2003, and entitled Co-Injection Nozzle with Improved Interior Layer Termination. 

   BACKGROUND OF INVENTION 
   The present invention relates to the co-extrusion of two or more streams of plastic material and the like, for introduction into a molding apparatus or similar devices. More particularly, the invention is directed to structure that enables better control of such co-extrusion, thereby providing for greater flexibility in the use of a wide range of suitable materials, extruding temperatures, and other conditions affecting the extrusion process. 
   With specific reference to injection systems for co-injecting at least two materials, the present invention relates to an improved technique, apparatus and resulting article for combining different annular flow streams of material where an interior layer of the combined annular flow stream can be terminated in a more abrupt fashion. 
   One conventional method of creating a multilayer object by co-injection molding is to inject annular layers of flowing material through a nozzle assembly into a mold. The result is a multilayer object having annular layers of material. The resulting multilayer object has an inner layer, an outer layer, and at least one interior layer sandwiched between the inner and outer layers. Depending on the end use requirements for the molded multilayer object it is often desirable to create a structure containing three or more annular layers of material. For example, in the case of a Polyethylene Teraphalate (PET) preform for a blown bottle, it is desirable to create a structure that contains three or more annular layers of material. The inner and outer layers of the preform are PET and at least one interior layer is formed from a material chosen to enhance the overall performance of the resulting plastic object, or to reduce the cost of the resulting plastic object. For example, interior layers may include one or more layers of a barrier material (MXD6 Nylon or EVOH), oxygen scavenging material, recycled material, or other performance-enhancing or cost-reducing material. 
   One problem in the field of co-injection molding resides in the need to end an interior layer of the material flow in a quicker or more abrupt manner. When molding a multilayer object it is often desirable to encapsulate the trailing edge of an interior layer of material with the inner and outer layers of material. The type of material used for the interior layer is often different from the type of material used for the inner and outer layers and as such, requires a region in the combining element extending from a stream combination area to a gate of a mold cavity to be free or clean of the interior layer material before the start of the next controlled volume shot of material into the mold cavity. This region must be free of the interior layer material so the inner and outer layer materials combine into a single encapsulating structure, or skin, that encapsulates the interior layer material. If this region is not free of the interior layer material the next controlled volume shot of material into the mold becomes contaminated with the interior layer material that remains in this region. Conventional nozzles for co-injection molding sequentially add layers of material to form a multiple layer output stream. As such, intermediate surface layers of conventional nozzles require cleaning, which is burdensome due to the sequential build of material layers. 
   Moreover, it is often desirable to have the interior layer material remain in close proximity to a base or gate portion of the resultant molded object. In the case of a PET preform, where the interior layer material can be a barrier layer, it is important to have the barrier layer extend as close as possible to the gate portion of the preform. Extending the barrier layer as close as possible to the gate portion of the preform results in a significant benefit when the preform is blown into a bottle. That is, a substantial portion if not the entire sidewall of the resulting bottle has the interior barrier layer. Absent a barrier layer that extends the entire sidewall length, the barrier property of the bottle can be adversely affected. The sidewall extends from a neck portion to a base portion of the resulting bottle. However, it is not always necessary for the gate portion of the bottle to include an interior layer as compared to the sidewall of the bottle, for the gate portion of the blown bottle tends to have a thickness sufficient to provide an adequate barrier to protect the contents of the bottle. Thus controlling a distribution of the material forming the interior layer of a molded object is important to the value and performance of the resulting molded object. 
   One conventional method of accomplishing this goal of controlling a distribution of the material forming the interior layer of a molded plastic object is to stop injecting the interior layer material into the mold while continuing to inject into the mold the inner and outer layer material. That is, when the flow of the interior layer material is stopped, the inner and outer layer material surrounding the interior layer material continues to flow dragging with them, in a downstream direction, the material that exited the interior layer material outlet of a combining element (e.g., nozzle assembly). In this manner, a stretching occurs between the interior layer material that remains substantially stationary in the outlet of the combining means and the interior layer material that has already exited the interior outlet of the combining element. This thinning eventually leads to breaking of the interior layer material from the combining element. Consequently, the inner and outer layer material makes contact once the interior layer material breaks from the combining means. 
   The breakage results in the formation of two interior layer components. The first is a trailing edge of the interior layer material just injected into a cavity of a mold. The second is a leading edge of the interior layer material which remains in the combining means for the next shot of material into the cavity. A further consequence of the breaking of the trailing edge of the interior layer material in this manner is the cleaning of such material from the gate of the combining element. As a result, the combining element is ready for the next injection cycle. 
   In the molded object, the structure of the trailing edge can be observed by coloring the discrete layers or by delaminating one or more layers of material after molding. Often, the trailing edge of the interior layer material has at least two observable regions. The first starts at the nominal core thickness of the molded object and quickly thins to an immeasurable thickness. The second is more burdensome to detect and can typically be detected by the lack of bonding between the inner and outer layers of material. This second region has only a microscopic quantity of interior layer material but it is enough to prevent the bonding of the inner and outer layer materials. In general, the first region accounts for ⅓ of the total tail length and the second region accounts for the remaining ⅔ of the total tail length. 
     FIG. 1  illustrates a partial cross section of a prior art three-layer co-injection nozzle assembly  200 . Nozzle assembly  200  includes nozzle body  210 , first nozzle member  212 , second nozzle member  214 , nozzle tip  216 , and valve pin  218 . Nozzle assembly  200  includes a third nozzle member (not shown) adapted to receive two or more material flows from respective material sources. Valve pin  218  in conjunction with second nozzle member  214  form inner flow channel  238  for carrying inner material stream  230  from an entrance orifice (not shown) to inner material egress orifice  226 . First nozzle member  212  in combination with second nozzle member  214  form interior material flow channel  236 . The interior material flow channel  236  directs interior material stream  232  from an entrance orifice (not shown) to interior material egress orifice  224 . Nozzle body  210 , first nozzle member  212 , and nozzle tip  216  combine to form exterior material flow channel  240 . The exterior material flow channel  240  directs an outer material stream  228  from an entrance orifice (not shown) to outer material egress orifice  222 . 
   Nozzle tip  216 , first nozzle member  212 , second nozzle member  214 , and valve pin  218  together define a combination volume  220 . Combination volume  220  provides an area of combination where the inner material exiting orifice  226 , the interior material exiting orifice  224  and the outer material stream  228  exiting orifice  222  combine to form combined output stream  234 . 
   Outer material egress orifice  222 , interior material egress orifice  224 , and inner material egress orifice  226  are positioned in a common plane to provide combination volume  220  with three annular material streams. The three annular material streams flow substantially parallel to each other as each stream passes through each respective egress orifice  222 ,  224 , and,  226  into combination volume  220 . 
   Use of the conventional three layer co-injection nozzle  200 , with an inner layer material stream  230  and an outer layer material stream  228  (i.e., skin) consisting of PET having an intrinsic viscosity (IV) of about 0.84 and an interior (core) layer material stream  232  consisting of MXD6 Nylon with a relative viscosity (RV) of about 2.65, the trailing edge or tail of the interior layer material in a molded preform with a 4 mm wall thickness often has a length of between about 15 mm and about 20 mm. One skilled in the art will recognize that the tail length of the interior layer material is a function of the viscosities of the materials used (i.e., skin and core materials) as well as the wall thickness of the molded preform. That is, as the preform wall becomes thinner, the tail of the interior layer material becomes longer in an inversely proportional relationship. For example, a preform with a 2 mm wall thickness formed with the conventional three layer co-injection nozzle  200  and the same core and skin materials identified above would have a tail length in a range of between about 30 mm and about 40 mm. 
     FIG. 2  illustrates an exemplary prior art preform  250  produced with the conventional three-layer co-injection nozzle  200 . Preform  250  has an inner layer  256  and an outer layer  258  formed of PET having an IV of about 0.84 and an interior layer  252  formed of MXD6 nylon with a RV of about 2.65. The wall thickness of preform  250  is about 4 mm. As such, interior layer  252  has a tail  254  with a length of between about 15 mm and about 20 mm. 
   SUMMARY OF INVENTION 
   The present invention addresses the above-described limitations of the conventional nozzle assemblies for co-injecting two or more materials into a cavity to form a molded object. The present invention provides an approach to increase the control of a volume of material forming an interior (core) layer of the molded object. The controllability of the volume of interior layer material provided by the methods and assemblies disclosed herein extend the length of a tail section of the interior layer and increase the volume amount of material in the tail section of the interior layer to provide an interior layer that extends from a neck portion to a gate portion of a preform without having the tail of the interior layer extend into the gate portion. Furthermore, the controllability of the volume of interior layer material provided by the present invention benefits other configurations of molded objects, for example a molded object having a five layer construction. Other exemplary configurations include, but are not limited to molded objects formed by offsetting the leading edge of an interior layer material from a velocity gradient in a controlled volume shot. A significant result of this controllability are manufactured objects having improved barrier layer protection which, in turn, extends the shelf life of products contained in such manufactured objects. Thus, the present invention beneficially extends the shelf life of goods and reduces the scrap rate and cost of such goods caused by shelf life expiration. 
   In one embodiment of the present invention, a nozzle assembly is disclosed. The nozzle assembly includes a first inlet to receive a first polymeric material and a second inlet to receive a second polymeric material. A first channel of the nozzle assembly has an inner passage to receive a first portion of the first polymeric material from the first inlet and feeds a combination area with the first polymeric material. A second channel of the nozzle assembly has an inner passage to receive a second portion of the first polymeric material from the first inlet and feed the combination area with the first polymeric material. A third channel of the nozzle assembly has an inner passage to receive a portion of the second polymeric material from the second inlet and feed the combination area with the second polymeric material. The combination area simultaneously combines the polymeric materials from the first, second, and third channels to form an annular output stream having multiple annular layers. Additionally, the combination area is configured to terminate formation of an interior layer of the annular output stream after termination of the flow of the second polymeric material from a second material source using a minimum volume of material flowing from the first and second channel while avoiding flow instabilities. 
   In another embodiment of the present invention, a method performed in a system for co-extruding a first polymeric material stream and a second polymeric material stream for introduction into a mold cavity to form a plastic piece is disclosed. The method positions a flow of a first portion of the first polymeric material stream substantially parallel to a central longitudinal axis of a nozzle assembly of the system to direct the flow of the first portion of the first polymeric material stream into a combination area of the nozzle assembly substantially parallel to the central longitudinal axis. The method positions a flow of a second portion of the first polymeric material stream to direct the flow into the combination area of the nozzle assembly at an angle offset from the central longitudinal axis. The method also positions a flow of the second polymeric material stream to direct the flow of the second polymeric material stream into the combination area of the nozzle assembly at angle offset from the central longitudinal axis. Performance of the method in the system simultaneously combines the flow of the first portion of the first polymeric material stream, the flow front of the second portion of the first polymeric material stream, and the flow of the second polymeric material stream in the combination area. 
   In one embodiment of the present invention, method for co-injection is disclosed. The method includes a step of forming a number of flow streams from two or more streams of plastic material that flow into a nozzle. Performance of the method combines the flow streams in a combination area of the nozzle to form an output stream having a number of annular layers. The output stream includes exterior layers that substantially form the inner and outer portion of a resulting plastic part and at least one interior layer enveloped by the exterior layers. The interior layer has a tail portion with a length of between about 3 mm and about 12 mm. The exterior layers of the output stream in the combination area has a cross sectional area of between about 70 mm 2  and about 160 mm 2 . 
   In another embodiment of the present invention, a plastic object formed by the following steps is disclosed. The steps include receiving two or more polymeric materials at a nozzle and combining the two or more polymeric materials in the nozzle to form an output stream having a number of annular layers. The output stream includes exterior layers that substantially form the inner and outer portion of the plastic object and at least one interior layer that is enveloped by the exterior layers. The interior layer has an abrupt termination to form an end portion of the interior layer having a length of between about 3 mm and about 12 mm when a cylindrical wall portion of the plastic object has a wall thickness of about 4 mm. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following description and apparent from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles of the invention and, although not to scale show relative dimensions. 
       FIG. 1  is a partial cross sectional view of a prior art nozzle assembly configured to combine three separate material flows into one material flow for injection into a cavity. 
       FIG. 2  is a cross section view of an exemplary object formed in a cavity supplied with a combined material flow from the nozzle assembly illustrated in  FIG. 1 . 
       FIG. 3  is a schematic block diagram of a system configured for injecting a cavity with a combined material flow in accordance with the teachings of the present invention. 
       FIG. 4  is a cross section view of a nozzle assembly for forming a combined fluid flow from a plurality of materials in accordance with the teachings of the present invention. 
       FIG. 5  is a partial cross section of a molded plastic object having at least one undesirable feature. 
       FIG. 6  is an exemplary cross section of an annular output stream formed by a nozzle assembly in accordance with the teachings of the present invention. 
       FIG. 7  is an exemplary cross section view of a portion of the nozzle assembly illustrated in  FIG. 4  which illustrates an area of the nozzle configured to combine the plurality of material flows to form a combined material flow. 
       FIG. 8  is a more detailed cross section view of the orifices that feed a stream combination area of the nozzle assembly illustrated in  FIG. 4 . 
       FIG. 9  is a cross section view of a portion of the prior art nozzle illustrated in  FIG. 1  illustrating orifices that feed a combination cavity with material for combination into a combined fluid flow. 
       FIG. 10  graphically illustrates velocity profile differences of material flowing through the orifices entering the stream combination area of a nozzle assembly in accordance with the teaching of the present invention and material flowing through orifices entering the combination cavity of the prior art nozzle assembly illustrated in  FIG. 1 . 
       FIG. 11  is a cross section of an exemplary object capable of being formed in accordance with the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   The ability to quickly end or break the tail of the material forming the interior layer of a molded plastic object leaves a region extending from a stream combination area in a nozzle to a gate of a mold cavity substantially free of the interior layer material to avoid the need to clean any surfaces in this region prior to a subsequent controlled volume shot. The stretching and eventual breaking of the interior layer material are achieved by controlling at least the flow characteristics of the inner and outer layer materials through the nozzle assembly. One such flow characteristic is velocity. The present invention increases the velocity of the material streams entering the area of a nozzle where simultaneous or near simultaneous combination of material streams occurs. The increased velocity and the simultaneous or near simultaneous combination of the material streams provide a quicker more abrupt breaking of the tail of the material forming the interior layer of the molded plastic object. 
   The present invention discloses methods, systems, and apparatuses for combining three material flows in a nozzle assembly cavity (e.g., stream combination area or combining means) to result in select nozzle assembly surfaces free of the interior layer material after the injection of a controlled volume shot of the materials into a mold cavity. Practicing of the invention disclosed herein provides techniques that avoid a need to clean selected surfaces in a region extending from a stream combination area of a nozzle to a gate of a mold cavity to form an object. Moreover, the desirable material flow characteristics provided by the methods, systems, and assemblies described herein improve the volume control of the interior layer material flowing into a mold cavity. This improved volume control allows for improved distribution of interior layer material in the molded object. The improved distribution of the interior layer material allows for a reduction in an amount of such material used to form the molded object without detracting from the performance, quality, or reliability of the resulting object. 
   Additionally, when using the inner and outer layer materials to quickly end or break the tail of the material forming the interior layer of a molded object, it is important to minimize the quantity of inner and outer layer material required to stretch and break the interior layer material. When this is accomplished the interior layer ends abruptly, allowing the tail of the interior layer to be moved closer to the gate of the resulting object. 
   The present invention minimizes the quantity of inner and outer layer material required to stretch and break an interior layer material by realizing a reduction in the cross sectional area of select outer and inner layer material orifices in a nozzle assembly. The nozzle assembly of the present invention reduces the volume of inner and outer layer material required to stretch and break an interior layer material when forming a molded object. By reducing the cross sectional area of select orifices in the nozzle assembly for the inner and outer layer materials, the interior layer material can be stretched and broken by a desired quantity of inner and outer layer materials (i.e., skin material) thus creating an abrupt interior layer material trailing edge in the molded object. Consequently, the nozzle assembly of the present invention achieves the goal of improving the volume control of material forming the interior layer of a molded object, which, in turn, advantageously improves the ability to extend the interior layer closer to the gate of the resulting part. 
   The present invention advantageously discloses an optimum total cross sectional area at the point of combination for the inner and outer layer materials in an exemplary nozzle assembly is between about 70 mm 2  and about 160 mm 2 . Within this optimum range of total cross sectional area, the inner and outer layer materials at selected times are substantially free of the interior layer material. That is, the inner and outer layer materials flowing from the nozzle assembly of the present invention are well suited for stretching and abruptly breaking the interior layer material at a desired length and, in turn, avoid the need clean at least one surface of the nozzle assembly of the interior layer material. As such, for example, at the initial moments of injecting a controlled volume shot of material into a mold cavity the inner and outer layer materials are free of the interior layer material. Moreover, this reduced cross sectional area can create an interior layer material having a tail length of between about 10 mm and about 12 mm in a preform with a wall thickness of about 4 mm. Consequently, the tail length of the inner layer material achievable with the methods, systems and apparatuses of the present invention beneficially improves the full thickness length of the interior layer material (as measured between leading tail and trailing tail) in a selected preform sidewall by approximately 10 mm. 
   Additionally, if the interior layer material is offset from a substantially centered annular position with respect to the inner and outer layer materials as a result of adjusting the ratio of the inner layer material to outer layer material volumetric flow, the outer layer material orifices of the illustrative nozzle assembly can be proportioned mathematically to match the volumetric flow rates in order to maintain the advantageous cleaning properties of the inventive nozzle assembly. 
     FIG. 3  illustrates an exemplary system suitable for practicing the present invention. Co-injection molding system  10  is configured to inject at least two materials into a mold cavity. Materials suitable for use with the present invention include polymer based materials such as, polyethylene terephthalate (PET), ethylene vinyl alcohol (EVOH), polycarbonates and the like. Co-injection molding system  10  includes a first material source  12 , a second material source  14 , and a manifold  16 . Co-injection molding system  10  further includes nozzle assemblies  18 A- 18 D and mold  24 . Mold  24  includes gates  20 A- 20 D and cavities  22 A- 22 H. 
   In operation, first material source  12 , second material source  14 , and manifold  16  cooperatively operate to deliver at least two material streams to nozzle assemblies  18 A- 18 D upstream of gates  20 A- 20 D. Nozzle assemblies  18 A- 18 D combine the material streams and feed gates  20 A- 20 D with a combined material stream for delivery to cavities  22 A- 22 H. 
   In one embodiment of the present invention, first and second material sources  12  and  14  are reciprocating screw injection units and manifold  16  is a hot runner having separate flow channels for each material and being arranged such that the material flow through each flow channel is balanced and equal. 
     FIG. 4  illustrates an exemplary nozzle assembly suitable for practicing the present invention. Nozzle assembly  18  includes an inner combining means  30 , a middle combining means  32 , and an outer combining means  34 . Nozzle assembly  18  further includes nozzle body  36  and nozzle tip  38 . Inner combining means  30 , middle combining means  32 , outer combining means  34 , nozzle body  36 , and nozzle tip  38  cooperatively combine to form a number of conical, annular, and axial passages and channels in nozzle assembly  18 . The nozzle assembly  18  is well suited for use in a co-injecting system, for example system  10 , for forming a plastic object having two or more layers. 
   Inner combining means  30  includes a first inlet  46  to receive a first polymeric material  64 , such as a skin material (i.e., inner and outer layer material), and a second inlet  44  to receive a second polymeric material  66 , such as a core material (i.e., interior layer material). The inner combining means  30  further includes a through bore  40  configured to receive a valve pin  42 . The through bore  40  extends through the middle combining means  32 , and through a portion of the outer combining means  34  to allow the valve pin  42  to move in an axial direction along a longitudinal axis of the nozzle assembly  18 . Through bore  40  has an inner wall diameter that varies along a central longitudinal axis of the nozzle assembly  18 . Valve pin  42  is movable in an axial direction along the central longitudinal axis of nozzle assembly  18  to assist in controlling the flow of the first polymeric material  64  and second polymeric material  66  through nozzle assembly  18  and into mold  24 . 
   Middle combining means  32  cooperatively engages with the inner combining means  30  form a portion of the plurality of annular flow channels in nozzle assembly  18 . Middle combining means  32  receives from channel  37  the first polymeric material  64  and receives from channel  41  the second polymeric material  66  to manipulate the flow of each of the polymeric materials through a plurality of annular fluid carrying passages or channels. The flow manipulation carried out by middle combining means  32  initiates the creation of an outer material stream  58  and an inner material stream  56  that together encapsulate an interior material stream  60 . 
   The middle combining means  32  when coupled with the inner combining means  30  forms a wrapped-coat-hanger die  31  that circumferentially extends around the through bore  40  and valve pin  42 . Wrapped-coat-hanger die  31  provides annular fluid flow passage  48  with a uniform melt distribution of the first polymeric material  64 . Annular fluid flow passage  48  channels an annular flow stream of the inner material stream  56  into stream combination area  54  through orifice  80 .  FIG. 7  illustrates orifice  80  with more detail. 
   Outer combining means  34  cooperatively engages with middle combining means  32  to form one or more fluid carrying passages or channels to manipulate the second polymeric material  66  forming an interior layer of the resulting plastic object. The outer combining means  34  when coupled with the middle combining means  32  forms a wrapped-coat-hanger die  33  that circumferentially extends around inner material stream  56 , through bore  40 , and valve pin  42 . Wrapped-coat-hanger die  33  provides conical fluid flow passage  52  with a uniform melt distribution of the second polymeric material  66 . Conical flow passage  52  feeds an annular stream of the second polymeric material  66  into stream combination area  54  through orifice  82 .  FIG. 7  illustrates orifice  82  with more detail. 
   The outer combining means  34  cooperatively engages with nozzle body  36 . The outer combining means  34  when coupled with the nozzle body  36  forms wrapped-coat-hanger die  35  that circumferentially extends around the interior layer stream  52 , the inner layer stream  56 , the through bore  40 , and the valve pin  42 . Wrapped-coat-hanger die  35  provides radial fluid flow passage  50  with a uniform melt distribution of the first polymeric material  64 . Radial fluid flow passage  50  feeds stream combination area  54  with a flow of first polymeric material  64  through orifice  84 . The first polymeric material  64  fed into the stream combination area  54  through orifice  84  forms the outer layer of a resulting molded object. 
   Fluid flow passages  48 ,  50 , and  52  feed stream combination area  54  with the outer material stream  58 , the inner material stream  56 , and the interior material stream  60 . A portion of the nozzle tip  38 , a portion of the outer combining means  34 , a portion of the middle combining means  32 , and a portion of the valve pin  42 , in combination form the stream combination area  54 . Stream combination area  54  has an inner passageway diameter of between about 6.7 mm and about 17.2 mm. Stream combination area  54  combines in a simultaneous or near simultaneous manner the outer material stream  58  received from the fluid flow passage  50 , the inner material stream  56  received from the fluid flow passage  48 , and the interior material stream  60  received from the fluid flow passage  52  to form annular output stream  49 . Stream combination area  54  is discussed in more detail relative to  FIGS. 7 and 8 . 
   The annular output stream  49  flows from the stream combination area  54  through fluid flow passage  62  to output portion  39  of nozzle assembly  18 . Fluid flow passage  62  has an annular inner passage that radially extends about through bore  40  and axially extends from the stream combination area  54  to the output portion  39 . The output portion  39  communicates with a gate of a mold, such as one of gates  20 A- 20 D. 
   The annular output stream  49  formed by the stream combination area  54  has an outer annular skin layer and an inner annular skin layer formed of the first polymeric material  64 , and an interior or core annular layer formed of the second polymeric material  66 . The inner and outer skin layers of the first polymeric material  64  each have a substantially like cross sectional area as the materials flow through the fluid flow passage  62  to the output portion  39 . The inner and outer skin layers of the first polymeric material  64  encapsulate the interior layer of the second polymeric material  66 , which forms a core portion of a resulting plastic object. 
   The ability of the nozzle  18  to form an annular output stream  49  with an inner and outer annular skin layer of a first polymeric material  64  having uniform cross sectional area that encapsulates an annular interior layer of a second polymeric material  66  allows a co-injection system employing such a nozzle assembly to improve distribution of a volume of material forming the core portion of the resulting plastic piece. For example, use of the nozzle assembly  18  allows a co-injection system to lengthen a barrier region in the resulting plastic object without increasing the risk of contaminating each initial portion of a controlled volume shot with core material. The result of lengthening the barrier region in a preform results in improved barrier performance of the resulting plastic object. Furthermore, the ability of the nozzle assembly  18  to form the annular output stream  49  with annular inner and outer skin layers of the first polymeric material  64  having substantially like cross sectional areas that encapsulate an annular interior layer of the second polymeric material  66  allows the interior layer or core layer to be stretched and eventually broken in a quicker more abrupt manner leaving a region of the nozzle assembly  39  between the combination area  54  and the output portion  39  substantially free of the second polymeric material  66  at completion of each controlled volume shot. This provides the nozzle assembly  18  with an advantageous quick clean feature where an amount of skin material needed to break the interior layer material and the trailing edge of the interior layer material from the combination area  54  to output portion  39  is significantly reduced. 
   As a result of this quick clean ability, subsequent shots or fills of a mold cavity are not contaminated with the interior layer which, if present, flows into the mold cavity, catches the flow front of the shot and flows toward the inside, the outside, or both of the molded object depending on the location in the melt stream, forming an extra layer close to the inside, the outside, or both of the molded object. This extra layer, known as scale, is a defect in the part. The ability of the nozzle assembly  18  to have a self-cleaning action allows a mold cavity and the output portion  39 , and any other processing elements therebetween to remain substantially free of the first polymeric material after injection of a controlled volume shot. 
     FIG. 5  illustrates the effect of not fully cleaning the interior layer material or the second polymeric material  66  from the region in the nozzle assembly  18  extending from the stream combination area  54  to the gate  20  of a mold cavity  22  associated with nozzle assembly  18 . When interior layer material remains in this region it catches the flow front of the initial volume of material of a subsequent controlled volume shot, flows toward the inner or outer surfaces of the resulting plastic object to form an extra layer, which is referred to in the art as scale. In  FIG. 5 , the interior layer material that remained in this region caught the flow front of a subsequent shot and is illustrated as having flowed towards the outside surface of the plastic object  130  and created extra layers or scale  132  in the plastic object  130 . Scale  132  is considered a part defect and can cause a blemish in the plastic object if the plastic object is further manipulated. 
     FIG. 6  is an exemplary cross section of annular output stream  49 . Annular output stream  49  includes a substantially equal volume of outer annular skin layer  51  and inner annular skin layer  53 . The outer annular skin layer  51  and inner annular skin layer  53  encapsulate the interior annular core layer  55  at select times during the flow of the annular output stream  49  from nozzle assembly  18 . 
     FIG. 7  is a partial cross sectional view of nozzle assembly  18 .  FIG. 7  illustrates stream combination area  54  in detail. Radial fluid flow passage  50  feeds stream combination area  54  through orifice  84  with a uniform distribution of the outer material stream  58 . Annular fluid flow passage  48  feeds stream combination area  54  through orifice  80  with a uniform distribution of the inner material stream  56 . Conical fluid flow passage  52  feeds stream combination area  54  through orifice  82  with a uniform distribution of the interior material stream  60 . Stream combination area  54  combines the outer material stream  58  from orifice  84 , the inner material stream  56  from orifice  80 , and the interior material stream  60  from orifice  82  to form annular output stream  49 . That is, stream combination area  54  combines the inner material stream  56 , the interior material stream  60 , and outer material stream  58  to form the inner annular skin layer  53 , the interior annular core layer  55 , and the outer annular skin layer  51 , respectively, of annular output stream  49 . 
   Radial fluid flow passage  50  enters the stream combination area  54  substantially perpendicular to the central longitudinal access of through bore  40 . Annular fluid flow passage  48  enters the stream combination area  54  substantially parallel to the central longitudinal access of through bore  40 . As such, outer material stream  58  enters the stream combination area  54  through orifice  84  substantially perpendicular to inner material stream  56 . Conical fluid flow passage  52  enters stream combination area  54  between orifice  80  and orifice  84  at an acute angle relative to a longitudinal axis of through bore  40 . 
     FIG. 8  illustrates a portion of the stream combination area  54  in more detail. Those skilled in the art will recognize that stream combination area  54  circumferentially extends around valve pin  42  to form annular output stream  49 . Orifice  80  as measured along line “A 2 -B 2 ” has a cross sectional area of between about 22 mm and about 76 mm 2 . Orifice  82  as measured along line “B 2 -C 2 ” has a cross sectional area of between about 17 mm 2  and about 23 mm 2 . Orifice  84  as measured along line “C 2 -D 2 ” has a cross sectional area of between about 28 mm 2  and about 102 mm 2 . In one embodiment of the present invention, orifice  80  as measured along line “A 2 -B 2 ” has a cross sectional area of about 51 mm 2 , orifice  82  as measured along line “B 2 -C 2 ” has a cross sectional area of about 23 mm 2 , and orifice  84  as measured along line “C 2 -D 2 ” has a cross sectional area of about 71 mm 2 . The cross sectional areas of orifices  80 ,  82 , and  84  are considered smaller than the prior art orifices. A result of the smaller cross sectional areas of orifices  80 ,  82 , and  84  is an increase in the velocity profile of the outer material stream  58 , the inner material stream  56 , and the interior material stream  60  at the entrance to stream combination area  54  without decreasing the volume of material that can flow through stream combination area  54 . 
     FIG. 9  is a partial cross section of nozzle assembly  200  discussed in relation to  FIG. 1 . The entrance to combination volume  220  is defined by orifices  222 ,  224 , and  226  in the plane defined by line “A 1 -B 1 -C 1 -D 1 ”. Combination volume  220  receives inner material stream  230  through orifice  236 , interior material stream  232  through orifice  224 , and outer material stream  228  through orifice  222 . In this manner, the inner material stream  230 , the interior material stream  232 , and the outer material stream  228  enter combination volume  220  substantially parallel to a longitudinal axis of valve pin  218 . As such, combination volume  220  receives three material flow fronts flowing substantially parallel to one another for combination into output stream  244 . In nozzle assembly  200 , orifice  222  along line “C 1 -D 1 ” has a cross sectional area of about 102 mm 2 , orifice  224  along line “B 1 -C 1 ” has a cross sectional area of about 28 mm 2 , and orifice  226  along line “A 1 -B 1 ” has a cross sectional area of about 76 mm 2 , 
     FIG. 10  graphically illustrates a simulated velocity profile plot  150  of the inner material stream  56 , the interior material stream  60 , and the outer material stream  58  entering the stream combination area  54  of nozzle assembly  18  at orifice  80 , orifice  82 , and orifice  84 , respectively.  FIG. 10  also graphically illustrates a simulated velocity profile plot  152  of the inner material stream  230 , the interior material stream  232 , and the exterior material stream  228  entering the combination volume  220  at orifice  226 , orifice  224 , and orifice  222 , respectively. The Y-axis of  FIG. 10  represents the flow velocity in “mm/s” for each respective material stream as the material exits a respective orifice to enter either stream combination area  54  or combination volume  220 . The X-axis of  FIG. 10  represents each respective orifice at the entrance to either stream combination area  54  or combination volume  220  as measured along lines “A 1 -B 1 -C 1 -D 1 ”. 
   Plot  150  graphically represents the velocity profile of each respective material stream entering stream combination area  54 . That is, plot  150  between “A 2 -B 2 ” represents the velocity profile of the inner material stream  56  as it passes through orifice  80  to enter stream combination area  54 . In similar fashion; plot  150  between “B 2 -C 2 ” represents the velocity profile of the interior material stream  60  as it passes through orifice  82  to enter stream combination area  54 . Likewise, plot  150  between “C 2 -D 2 ” represents the velocity profile of the outer material stream  58  as it passes through orifice  84  to enter stream combination area  54 . 
   Plot  152  graphically represents the velocity profile of each material stream entering combination volume  220 . That is, plot  152  between “A 1 -B 1 ” represents the velocity profile of the inner material stream  230  as it passes through orifice  226  to enter combination volume  220 . In similar fashion, plot  152  between “B 1 -C 1 ” represents the velocity profile of the interior material stream  232  as it passes through, orifice  224  to enter combination volume  220 . Likewise, plot  152  between “C 1 -D 1 ” represents the velocity profile of the outer material stream  228  as it passes through orifice  222  to enter combination volume  220 . 
   As  FIG. 10  graphically illustrates, the smaller cross sectional area of each orifice  80 ,  82 , and  84  feeding stream combination area  54  with a material stream advantageously increases the velocity of each material stream. The increase in the velocity for each material stream provided by orifices  80 ,  82 , and  84  allow nozzle assembly  18  to achieve greater distribution control an interior layer material being injected into a mold cavity. This increase in material flow velocity advantageously allows nozzle assembly  18  to abruptly end the interior layer of a controlled volume shot which allows the thickness of the interior layer material to be positioned closer to the gate portion of the molded object. The increased volume of the interior layer material and the abrupt manner of breaking the interior layer material allows nozzle assembly  18  to produce an interior layer having a tail of between about 3 mm and about 12 mm in a preform having a wall thickness of about 4 mm. 
     FIG. 11  illustrates a cross section of an exemplary plastic object  100  formed in accordance with the illustrative embodiment of the present invention. The exemplary plastic object  100  is a preform for a container such as, a bottle. Although the illustrative embodiment is discussed in relation to the exemplary plastic object  100 , those skilled in the art will appreciate that the ability to control distribution of an interior layer when forming a plastic object is applicable to other types of plastic objects and the processes for forming those plastic objects. Other types of plastic objects include, but are not limited to shingles, bumpers, containers such as beverage, food, medical, pharmacological, containers having properties relating to gas permeability, gas scavengability and other multiple material co-injected parts. Other types of processes for forming plastic objects include, but are not limited to multiple layer extruded products. 
   Plastic object  100  includes an interior core portion  110  encapsulated by a skin portion  116 . The interior core portion  110  is formed from the second polymeric material  66  and the skin portion  116  is formed from the first polymeric material  64 . The interior core portion  110  includes a leading edge  112  and a trailing edge  114 , or tail. The interior core portion  110  has a substantially annular shape that extends circumferentially about a central longitudinal axis of the plastic object  100  from a neck portion  120  to a gate portion  122 . The region between the neck portion  120  and the gate portion  122  is referred to as core distribution  118 . That is, the core distribution  118  in the plastic object  100  extends from the leading edge portion  112  to the trailing edge portion  114  of the interior core portion  110 . In one illustrative embodiment of the present invention, the core distribution  118  has a length of between about 35 mm and about 45 mm, with the trailing edge portion  114  having a length of between about 3 mm and about 12 mm when the plastic object  100  has a wall thickness of about 4 mm in at least the region of the core distribution  118 . 
   While the present invention has been described with reference to the above illustrative embodiments, those skilled in the art will appreciate that various changes in form and detail may be made without departing from the intended scope of the present invention as defined in the appended claims.