Abstract:
A polymer extrusion head for forming continuous shapes having one or more lumens therein. The head includes an extrusion mandrel and die. At least one lumen pipe is disposed radially through a port in the die to intersect the polymer flow in the annular region between the tip and the die. The lumen pipe is a welded assembly having a 90° turn formed as a mitered joint with zero bend radius. The lumen pipe is held in a pin vise engaged in the die such that position of the lumen pipe may be readily adjusted. Axial and radial positions are established during assembly by means of a gauge block. A plurality of lumen pipes may be accommodated for a plurality of lumens by a manifold distribution system including individual lumen pipe assemblies.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to method and apparatus for extrusion forming of molten polymer material; more particularly, to extrusion heads for continuous extruding of hollow or solid shapes; and most particularly, to method and apparatus for forming longitudinal voids or inclusions in such extruded shapes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Extrusion heads for continuous extrusion forming of continuous plastic elements having specific cross-sectional shapes are well known. Such extruded elements may include, for example, pipes, rods, moldings, tubings, and the like.  
         [0003]     In a typical prior art extrusion system, solid pellets of the thermoplastic material to be used are fed into a progressive-screw extruder wherein the pellets are liquefied under high pressure and are injected into an extrusion head. Such injection may be made axially of the extrusion head, as is commonly done for extrusion of solid shapes and tubes, or transversely of the extrusion head, as is commonly done for coating of a core stock being passed axially through the head. Axial injection is known in the art as “inline,” and transverse injection is known in the art as “crosshead.” In cross-head injection, the molten extrudate enters the extrusion head at an angle, typically 90°, to the axis of the head. Crosshead extrusion is especially useful in applying coatings to rigid core forms such as rods, wire, and lumber.  
         [0004]     Prior art extrusion heads are known to be used in forming longitudinal voids, inclusions, and stripes in extruded shapes such as rods, tubes, and core material coatings. For example, the colored insulative coating on electric wire such as bell wire typically includes a different colored stripe to aid in wire identification in a wire bundle.  
         [0005]     A particularly demanding extruded form is tubing having a central passage and one or more smaller passages formed in the tubing wall. Such passages are known in the art as “lumens.” The lumens may be open passages or may be filled with different colors of the wall material or may be filled with entirely different material. For example, a medical catheter may require one or more open wall lumens and may also require a lumen filled with a radio-opaque material such as barium sulfate. The formation of such tubing and lumens requires a high level of precision in the extrusion dies and in the placement of die elements for forming the lumens.  
         [0006]     In a prior art cross-head extrusion head for forming tubing having lumens, molten polymer is injected into the head and is shaped into an annular flow around a cylindrical mandrel. The flow is then squeezed along a conically tapered section of the mandrel and then along another, smaller diameter cylindrical section of the actual extrusion tip and die. An axial pipe extends through the mandrel and the die as an extrusion tip for forming and supporting the principal axial passage in the extruded tubing. Typically, air is injected through the central pipe to keep the tubing properly inflated as it is extruded.  
         [0007]     A linear pipe of smaller diameter, such as a hypodermic needle and referred to herein as a “lumen pipe,” is disposed in an off-axis passage formed through the mandrel parallel to the central pipe and extends to the extrusion end of the die. During tubing extrusion, air is injected through the lumen pipe to continuously form a void (lumen) in the tubing wall. If a filled lumen is desired instead, an appropriate material may be injected through the lumen pipe instead of air.  
         [0008]     At least two significant problems are known in forming lumens in the prior art. First, because the lumen pipes are disposed in bores in the mandrel, the positions of the pipes are fixed. This means that the position of the pipe within the die cannot be adjusted, although such adjustment may be highly desirable to compensate for errors in bore location or in the pipes themselves. Also, a user may desire to change the radial location of a lumen. Further, because the lumen pipes become part of the mandrel, their radial position is fixed with respect to the inner diameter of the extrusion.  
         [0009]     Both the mandrel and the die must be changed to change over to a different diameter extrusion.  
         [0010]     Second, because the lumen pipes extend from the mandrel, they must necessarily intersect the conical portion thereof. It is known that flow disturbances around the pipes at their angular exit from the mandrel can remain in the wall of the extruded tubing and can be both cosmetically and functionally undesirable.  
         [0011]     Third, because the diameters of the off-axis bores are fixed and selected for a given diameter lumen pipe, each mandrel is dedicated to extrusions having that size lumen. The diameter of a lumen cannot be changed without changing the mandrel.  
         [0012]     Thus, for a facility producing a large number of different extrusions, an equally large number of different mandrels is required.  
         [0013]     What is needed in the art is a means for extruding a shape having at least one wall lumen wherein the lumen pipe may be accurately and adjustably positioned when an extrusion head is assembled.  
         [0014]     What is further needed is a means whereby the position of the lumen within the wall may be readily adjusted as desired after assembly of an extrusion head.  
         [0015]     What is further needed is a means whereby a single mandrel may be used for a plurality of lumen-forming extrusion dies. What is further needed is a means for preventing or minimizing flow disturbances in the walls of lumen-containing shapes.  
         [0016]     What is further needed is a means for changing a lumen pipe without having to change either the die or the mandrel.  
         [0017]     It is a principal object of the present invention to provide apparatus for extruding lumen-containing shapes wherein the location of a lumen within the shape may be accurately established prior to beginning of extrusion and may be readily adjusted as desired during extrusion.  
         [0018]     It is a further object of the present invention to provide apparatus for extruding lumen-containing shapes wherein a plurality of such shapes may be generated without requiring a change in the extrusion mandrel.  
       SUMMARY OF THE INVENTION  
       [0019]     Briefly described, a polymer extrusion head in accordance with the invention for extruding continuous shapes having one or more lumens or stripes therein includes an inner extrusion mandrel and an outer extrusion body and die. An annular flow path for polymer is formed therebetween, similar to the prior art. Preferably, the mandrel is provided with an integral cylindrical tip rather than a separate axial pipe, as in the prior art. At least one lumen pipe is disposed radially through a port in the die to intersect the polymer flow in the smaller-diameter annular region of flow between the tip and the die.  
         [0020]     The lumen pipe is a welded assembly of first and second portions having a 90° turn therebetween which preferably is a mitered joint with zero bend radius, such that the first portion is disposed radially of the die through a bore in the die wall, and thus is orthogonal to the material flow through the die, and the second portion is disposed axially of the die within the annular flow region, and thus is disposed parallel to the material flow through the die. The first portion is held in a chuck-type pin vise that is sealably engaged in a counterbore in the wall of the die such that the radial position of the lumen pipe assembly may be readily adjusted as desired. Preferably, the axial alignment and radial position are established during assembly by means of an appropriately shaped gauge block that is inserted into the extrusion opening between the tip and the die, after which the pin vise is tightened. In a first embodiment, the wall bore is formed along a joint between planar faces of the extrusion body and the extrusion die such that the 90°-angled welded lumen pipe assembly may be positioned on the die face and then captured by the body face. Any number of separate bores and welded lumen pipes may be provided as needed for a desired number of lumens.  
         [0021]     In a second embodiment, wherein it is desired to form a plurality of lumens using a common injectant, the die includes a distribution manifold formed in the die face, and individual lumen pipe assemblies are disposed radially as required between the distribution manifold and the die annular flow region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0023]      FIG. 1  is a cross-sectional view of a portion of a prior art lumen-forming extrusion head;  
         [0024]      FIG. 2  is an exploded cross-sectional view of a first embodiment of a lumen-forming extrusion head in accordance with the invention;  
         [0025]      FIG. 3  is a cross-sectional view of a portion of the head shown in  FIG. 2 , showing use of a gauge block to position lumen pipes correctly;  
         [0026]      FIG. 4  is an axial cross-sectional view of a gauge block;  
         [0027]      FIG. 5  is an end view of the gauge block shown in  FIGS. 3 and 4 ;  
         [0028]      FIGS. 6 through 8  are cross-sectional views of three different and exemplary lumen-containing extrusions formable by apparatus in accordance with the invention;  
         [0029]      FIG. 9  is an exploded half isometric view of a second embodiment of a lumen-forming extrusion head in accordance with the invention; and  
         [0030]      FIG. 10  is an exploded full isometric view of the embodiment shown in  FIG. 9 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Referring to  FIG. 1 , there is shown a portion of a typical prior art apparatus  10  for extruding a tubing extrudate having a central opening and two diametrically opposed lumens. Apparatus  10  includes a mandrel  12  having a cylindrical portion  14  (for forming a cylindrical flow passage with an outer cylindrical body, not shown) and a tapered portion  16 . An extrusion die  18  includes a conically tapered entrance  20 , for forming with portion  16  a conically tapered flow chamber  22  for extrudate material  23 , typically a molten polymer, and a non-tapered passage  24 , which may be cylindrical or any other cross-sectional shape required in the extrusion. An axial central bore  26  in mandrel  12  supports a pipe  28  extending to the end  30  of die  18  for providing air  32  to support the central opening in the extrusion. Pipe  28  also functions as an extrusion tip for forming a non-tapered flow chamber  34  terminating at die end  30 .  
         [0032]     Mandrel  12  is further provided with first and second bores  36   a , 36   b  parallel with central bore  26  and extending through tapered section  16  for receiving first and second lumen pipes  38   a , 38   b  which also extend through flow chamber  34  to end  30 . Bores  36   a , 36   b  are located in fixed radial and azimuthal relationship to central bore  26 . The lumen pipes are thus parts of mandrel  12 . Lumen-forming material  40   a , 40   b , which may be air or other liquid polymer or slurry, is provided through pipes  38   a , 38   b , respectively.  
         [0033]     Referring to  FIG. 3 , an improved lumen-forming extrusion head  10 ′ in accordance with the invention (linearly exploded view) comprises a hollow mandrel  12 ′ including a cylindrical portion  14 ′ and tapered portion  16 ′. Mandrel  12 ′ also preferably includes a tip portion  28 ′ analogous to prior art pipe  28  for providing supporting air  32 . Of course, alternatively, a continuous solid core material can be supplied for coating with a lumen-containing jacket through tip  28 ′. As in the prior art example shown in  FIG. 1 , a cylindrical body surrounding mandrel  12 ′ is omitted and should be assumed. Prior art die  18  is replaced by first and second die elements  18   a , 18   b  joinable along planar axial faces  19   a , 19   b , respectively, as by bolts  21 . Preferably, faces  19   a , 19   b  are surface ground to produce a tight seal therebetween. Element  18   a  includes a tapered entry  20 ′ and a cylindrical portion  24   a ′. Die element  18   b  includes a portion  24   b ′ that joins smoothly with portion  24   a ′ and combines with tip portion  28 ′ to form a flow passage  34 ′.  
         [0034]     Axial faces  19   a , 19   b  are provided with mating features  42   a , 42   b  that define threaded radial wells and passages, when the faces are joined, for receiving first and second pin vises  44  and first and second welded lumen pipe assemblies  46 . The pin vises are sealed by compression O-rings  48 . Assemblies  46  each include first and second elements  50 , 52  meeting at a precise  900  joint  54  formed by microwelding of metal tubing along a 45° miter. Thus, joint  54  has a bend radius of zero. The pin vises are hollow chucks for receiving elements  50  to position lumen pipes  46  within the apparatus. After assembly, lumen pipes  46  extend beyond vices  44  ( FIG. 3 ) for receiving lumen-forming materials  40   a , 40   b  (not shown here) as in the prior art.  
         [0035]     Note that the lumen pipes enter the polymer flow stream in the cylindrical portion  34 ′ thereof rather than in the conical portion  22 ′ as in the prior art. The pipes thus present only a circular profile (element  50 ) transverse of polymer flow and a laminar profile (element  52 ) parallel to polymer flow through the die. Flow around element  50  heals without substantial continuing disturbance, and flow around and along element  52  smoothly opens the polymer for formation of a lumen at the downstream tip of each element  52 .  
         [0036]     Referring to  FIGS. 2 through 5 , during assembly of improved extrusion head  10 ′, the lumen pipe assemblies are first positioned on either of faces  19   a  or  19   b  with pipe elements  52  extending into die element  18   b  in approximately the correct final position.  
         [0037]     Die elements  18   a , 18   b  are then joined, assembly elements  50  remaining both radially translatable and rotatable therebetween. The O-rings  48  and pin vises  44  are then passed over elements  50 , and the pin vises are threaded into the die. The correct location and orientation of the portions of pipe assemblies  46  extending into the die are established through use of a dedicated gauge block  56  having a shape and size closely matching the extrudate shape of die portion  24   b ′. In the example shown in  FIG. 5 , gauge block  56  is substantially cylindrical. Block  56  is formed to be easily and close-fittingly insertable into portion  24   b ′ and is provided with precisely-machine axiaily-extending channels  58  on the outer surface thereof for receiving and positioning the lumen pipes precisely within portion  24   b ′. The bottoms of channels  58  define the radial positions of the lumen pipes, and the channels themselves precisely align the pipes with the axis of die element  18   b . When the lumen pipes are properly positioned for use, pin vises  44  are tightened, and the gauge block is removed.  
         [0038]     Assembly is completed by installation of mandrel  12 ′ into the extruder body (not shown) and insertion of mandrel  12 ′ into die elements  18   a , 18   b  to form the desired flow passages  22 ′, 34 ′. Of course, if adjustment or a change in radial position of a lumen is desired, the pin vise may be loosened and the position of the lumen pipe altered and re-secured, even during extrusive use of the head.  
         [0039]     In an alternative assembly procedure, lumen pipe elements  52  may be placed in appropriate grooves  58  in the gauge block  56 , and the gauge block then inserted into portion  24   b ′ from the upstream side, until pipe element  50  are entered into their respective radial half-passages  42   b  in die section  18   b . Section  18   b  is then bolted to section  18   a , the pin vises are installed and tightened, and the gauge block removed from the downstream (exit) end  30 ′ of the head.  
         [0040]     The cross-sectional shape of a lumen may be easily controlled by selecting the proper tip shape for the lumen pipe and properly locating the pipe radially within the die by means of its pin vise.  FIGS. 6 through 8  illustrate three extrusion cross-sectional forms  60   a , 60   b , 60   c  including lumens  62   a , 62   b , 62   c  that may be readily provided by an extrusion head in accordance with the invention. Note that the lumens need not be identical in shape or content; lumen  62   d  in  FIG. 6  differs from lumens  62   a  and may, for example, be filled with a radio-opaque material such as barium sulfate during extrusion.  
         [0041]     Note also that, although the diameter of the central passage  64  is the same in all three examples, to form these extrusions using prior art extrusion means such as is shown in  FIG. 1  would required three separate mandrels  12  having three different patterns of bores  36 . In improved extrusion head  10 ′, only dies  18   a , 18   b  and lumen pipe assemblies  46  must be changed; mandrel  12 ′ is unchanged.  
         [0042]     Referring to  FIGS. 9 and 10 , it may be desirable to form and supply a plurality of lumens from a single material source in applications wherein non-air materials are to be emplaced in the lumens. This is readily accomplished within the scope of the invention by including a manifold distribution system  75  within either or both of the die elements  18   a , 18   b . In a currently preferred embodiment  10 ″, the distribution means is formed solely within die element  18   b . An entry  64  is formed in the outer surface  66  of element  18   b  spaced apart from mating surface  19   b . A distribution tank  68 , preferably hemispherical, is formed in surface  19   b  and is connected via appropriate passages  70  with entry  64 . A distribution runner  72  leads from tank  68  toward passage  24   b ′ for each lumen intended. Preferably, the volume of tank  68  is at least ten times greater than the total volume of passages and runners  70 , 72 . Runner  72  preferably is curved to end in a direction radial of passage  24   b ′. A smaller and shallower connecting passage  74  extends between each runner  72  and portion  24   b ′ for receiving element  50  of a welded lumen pipe assembly  46 , element  52  thereof extending appropriately into and along passage  34 ′. Preferably, passage  74  is so formed that element  50  is slightly compressed by face  19   a  during assembly of the head to effect a seal against leakage.  
         [0043]     In embodiment  10 ″, no pin vises are employed, so preferably, the head is assembled with the appropriate removable gauge block (not shown) in place in portion  24   b ′ to assure proper positioning of the lumen pipes. No adjustment of the pipes is possible after the head is assembled and operating. One skilled in the art will recognize that various combinations of entries  64 , tanks  68 , and passages and runners  70 , 72 , 74  are possible to provide a very large possible number of lumen arrangements of varying lumen numbers, shapes, sizes, positions, and material contents. For example, two lumens having identical contents will be formed by the arrangement shown in  FIG. 10 . All such possible combinations are fully comprehended and anticipated by the invention.  
         [0044]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.