Abstract:
In a co-extrusion system, a primary extruder is connected to a primary flow director to direct the solid melt from the primary extruder to the co-extrusion assembly. A secondary extruder is connected to a secondary flow director to direct the melt from the secondary extruder to the co-extrusion assembly. A co-extrusion assembly produces simultaneous co-extruded melts without any significant pressure change in the melt. Dies shape the co-extruded melts to a desired shape.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/267,987, filed on Feb. 9, 2001, the entire teachings of the above application are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    An extrusion such as extruded polyvinyl chloride (PVC) pipe is formed by feeding plastic into an extruder where it is subjected to high temperatures to create a molten substrate. The substrate then proceeds through a feed tube, at the end of which additional material known as capstock can be applied if desired. The process of adding additional material to the substrate is known as co-extrusion. The substrate and capstock are known as melt. The melt proceeds through an extrusion head, at the end of which the melt passes through a die. The die contains the circular cross-sectional profile shape to be extruded. The melt hardens as it exits the die in the desired cross-sectional form. The hardened material forms a tube that can grow to arbitrary length as additional melt is extruded.  
           [0003]    Normally, PVC pipe is produced by extruding molten plastic through a single die of an extruder. In some cases, two PVC pipes are produced simultaneously by extruding molten plastic through a Y-block, a pair of extrusion heads and a pair of dies.  
           [0004]    The ultimate shape of the extrusion is determined by a melt flow passage in the die between a bushing which surrounds a pin or mandrel. For circular pipe, the pin is circular in cross-section and an opening in the bushing which surrounds the pin is circular. To obtain a non-circular cross-section, such as a square, a circular extrusion is typically reshaped through a transition bushing and pin which are circular at the inlet and square, for example, at the outlet.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, there is provided a system and method for co-extruding two simultaneous melts. A co-extrusion system contains a primary extruder, a secondary extruder, a primary flow director, a secondary flow director, at least first and second co-extrusion assemblies and extrusion dies. The primary flow director divides the solid melt from the primary extruder into two symmetrical flow paths. The secondary flow director divides the melt from the secondary extruder into two symmetrical flow paths. The first and second co-extrusion assemblies co-extrude the melt simultaneously from the primary and secondary flow directors. The co-extrusion takes place ahead of the dies so that there is no substantial pressure change in the melt, thereby providing for consistent and uniform flow control. The dies receive the co-extruded melt from their respective co-extrusion assemblies.  
           [0006]    The secondary flow director contains top and bottom plates, each of which have channels which form two symmetrical flow paths. Conduits extend from the bottom plate to feed the melt into the co-extrusion assemblies. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0008]    FIGS.  1 A-C show plan, side and perspective views of a dual co-extrusion set-up;  
         [0009]    [0009]FIG. 2A-B show assembled and exploded views of a co-extrusion assembly;  
         [0010]    [0010]FIG. 3 shows a cross-sectional view of the capstock insert, adapter flange, horizontal plate, vertical plate and plate-to-block adapter;  
         [0011]    [0011]FIG. 4A-B show bottom and cross-sectional views of the plate-to-block assembly;  
         [0012]    [0012]FIG. 5A-B show cross-sectional views of one half of the dual coex block; and  
         [0013]    [0013]FIG. 6 shows a perspective view of a co-extrusion pin. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    A description of preferred embodiments of the invention follows.  
         [0015]    In the embodiment of FIGS.  1 A-C, a dual co-extrusion setup  2  is used to extrude two simultaneous profiles (not shown) using one main extruder  3  and one co-extruder  5 . The dual co-extrusion setup  2  comprises a “Y” adapter  4 ; dual-line co-extrusion assembly  6 ; two spider pipe heads  8 ; and two adjustable profile dies  10  and is shown assembled in FIGS.  1 A-C. In the direction of flow path  12 , the main extruder injects solid substrate into the “Y” adapter  4  which divides the substrate into two separate flows. In the direction of flow path  14 , the co-extruder injects capstock (not shown) into the co-extrusion assembly  6 . Inside a co-extrusion assembly  6 , the capstock is divided into separate flows and the capstock and substrate of each flow are bonded together to form two separate flows of capstock coated substrate (CCS). Each flow of CCS flows from the co-extrusion assembly  6  into a separate spider pipe head  8  and into a respective adjustable profile die  10 . The CCS enters the two (2) adjustable profile dies  10  in circular form and each adjustable profile die transforms the CCS into a desired cross-sectional profile. The extrusions exit the two adjustable profile dies  10  simultaneously in the direction of flow path  16 .  
         [0016]    The adjustable profile dies are described in detail in application Ser. No. 09/912,250 filed Jul. 24, 2001, which is incorporated by reference in its entirety.  
         [0017]    [0017]FIGS. 2A and B show the components of the co-extrusion assembly  6 . In one embodiment of the invention, the co-extrusion assembly  6  can be made of stainless steel or another material used in the industry. The co-extrusion assembly  6  comprises the following components: capstock flow insert  20 ; adapter flange  22 ; horizontal plate  24 ; vertical plate  26 ; plate-to-block adapter  28 ; dual coex block  30 ; locating insert  32 ; co-extrusion pin  34 ; dual coex bushing  36 ; main flow insert  38 ; heat bands  42 ; heat plate covers  50 ; and thermal sensors  44 .  
         [0018]    To avoid redundancy, one half of the system will be described since the second half is a mirror image of the first. Solid substrate flows from the “Y” adapter  4  through the main flow chamber  48  (FIG. 5A) of the dual coex block  30 . As shown in FIGS. 2B and 3, the capstock flows from adapter flange  22  through horizontal plate  24 , vertical plate  26  and plate-to-block adapter  28 . As shown in FIGS. 4A, 4B and  5 A, the capstock is divided into two symmetrical flow paths by capstock divider  31  and facing channels  29  and  29 ′ in the plate-to-block adapter  28  and dual coex block  30  respectively. The plate-to-block adapter  28  is curved to allow for continuous flow of the capstock through the divider without hitting an edge and burning the melt. The capstock then flows through the dual coex block  30  at coat hanger neck  33  (FIG. 5A) at least at a 30 degree angle to allow the capstock to flow continuously into the coat hanger  35 . In one embodiment, a 45 degree angle is used to minimize the length of the flow channel reducing resonant time through the co-extrusion setup  2 . The coat hanger  35  is tapered where a proximal end connected to the coat hanger neck  33  is of greater axial length than its distal end. The capstock exits the coat hanger  35  and enters the capstock flow chamber  46  which is between the co-extrusion pin  34  and dual coex bushing  36 . The capstock flow is restricted in the portion of the capstock flow chamber  46  connected to the proximal end of the coat hanger  35  to permit the capstock to flow symmetrically around the co-extrusion pin  34  reducing resonant time through the dual coex bushing  36 . The solid substrate flows through the co-extrusion pin  34  at the same time the capstock flows around the co-extrusion pin  34 . The capstock is applied to the exterior wall of the solid substrate at a distal end of the co-extrusion pin  34  and dual coex bushing  36 . In other embodiments, the capstock can be applied to the interior of the solid substrate or to both the interior and exterior of the solid substrate.  
         [0019]    Application of the capstock to the solid substrate before entering either the spider pipe head  8  or die  10  maintains uniform equilibrium pressure. The application of the capstock to a solid substrate results in an insignificant pressure drop because it occurs at a consistent high pressure of at least 1500 psi resulting in the CCS exiting the die at the same velocity.  
         [0020]    The newly formed CCS then flows either into a spider pipe head or extrusion die. The spider pipe head  8  transforms the solid CCS into tubular form. The CCS enters the die  10  which further adjusts the shape of the extrusion. If a solid shape is preferred, the spider pipe head  8  may be removed from the co-extrusion setup  2 .  
         [0021]    The plate-to-block adapter  28  is aligned on the dual coex block  30  using alignment pins  54  and alignment holes  56 . The plate-to-block adapter  28  is then bolted to the dual coex block  30  creating the capstock flow director. The vertical plate  26  is seated in and bolted to the plate-to-block adapter  28 . The horizontal plate  24  seated in and bolted to the vertical plate  26 . The adapter flange  22  blots to the horizontal plate  24  and the capstock insert  20  is seated in the adapter flange  22 . The capstock insert  20  transitions the capstock flow from the co-extruder to the co-extrusion assembly  6 . A locating insert  32  is fitted between the dual coex block  30  and co-extrusion pin  34  to ensure proper alignment of the co-extrusion pin  34  and the dual coex block  30 . The co-extrusion pin is bolted to the dual coex block  30 . The dual coex bushing  36  is seated over the co-extrusion pin  34  and bolted to the dual coex block  30 . The dual coex bushing can further be coupled to a spider pipe head  8  or extrusion die  10 . A main flow insert  38  is seated in the dual coex block  30  and transitions the main flow from the “Y” adapter  4  to the dual coex block  30 .  
         [0022]    Heat plates  40  and heat bands  42  are attached to the horizontal plate  24 , vertical plate  26 , dual coex block  30  and dual coex bushing  36  to maintain stable temperature of the capstock, solid substrate and CCS resulting in a consistent viscosity. The heat plates  40  are covered by heat plate covers  50 . The heat plates  40  and heat bands  42  can be individually controlled to adjust viscosity of the flowing melt. Thermocouple holders  52  are bored throughout the dual co-extrusion assembly  6  so thermal sensors  44  can be inserted to sense the temperature of the capstock and solid substrate.  
         [0023]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.