Abstract:
A polymer extrusion head for forming extruded elements includes a tapered supply section and a one-piece helicoid manifold comprising an entrance cone, a spider element for splitting the flow into multiple streams, a cylindrical transition and flow-turning zone, a flow space for re-mixing the streams, and a conical zone. A conical choke ring surrounds the conical zone. An extrusion tip and die are disposed downstream of the manifold and body. The body and die are provided with heaters, and the manifold and extrusion tip may include internal heaters. The extrusion tip may be accessed for maintenance or changeover to another shape or size. The die surrounding the tip may be readily centered with respect to the tip. An axial surface in the die acts as a seat for the choke ring.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to apparatus for extrusion forming of molten polymer material; more particularly, to inline extrusion heads for continuous extruding of hollow or solid shapes; and most particularly, to an inline spiral extrusion head wherein the flow of material is cleanly divided by multi-funnel shaped spider into a plurality of annularly arranged streams which then are spirally recombined to eliminate longitudinal knit lines.  
       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. Typically, such injection is made axially of the extrusion head; hence, the term “inline” in the art, and as used herein, as opposed to “crosshead” wherein the molten extrudate enters the extrusion head at an angle, typically 90°, to the axis of the head.  
         [0004]     Prior art extrusion heads typically are formed of a collection of individual sections, each section being responsible for a particular manipulation of the flowing stream. The single, axial stream first encounters a male conical section having the conic apex pointing upstream, such that the axial stream is converted into an annular stream.  
         [0005]     Leaving the conical section, the annular stream is divided by a spider section into a plurality of individual laminar streams, the purpose of the spider being to provide mechanical integrity while providing an annular flow path.  
         [0006]     Downstream of the spider section, the individual streams are recombined in a mandrel section into a single annular stream of improved thickness uniformity. The annular stream is then typically directed through a female frusto-conical section, known generally in the art as a choke ring or wedge ring, wherein the flow is accelerated, the annular stream thickness is reduced, and the outside diameter of the annular stream is reduced or increased. The choke ring can further smooth out non-uniformities in the annular extrudate.  
         [0007]     The annular stream is then directed into a male and female die section wherein the stream is shaped into a desired cross-sectional form and extruded for cooling, setting, and further treatment as may be necessary.  
         [0008]     Several problems are known to exist both in operation of a prior art extrusion head and in product extruded therefrom.  
         [0009]     First, the conical section, spider section, and mandrel section are formed as individual units which are then joined together. The joints therebetween define small discontinuities in the flow surfaces which cause small areas of flow stagnation. Molten polymer in these areas can become degraded to form either unwelcome hard protrusions into the flow stream or slugs that break loose and create defects in the finished product.  
         [0010]     Second, a prior art spider includes a plurality of radially-arranged fins, typically air-foil shaped, around which the extrudate flows and is divided and then recombined. As the extrudate enters and flows through the spider it undergoes compression as the total flow cross-section is progressively reduced. However, upon passing the broadest dimension of the fins, the extrudate undergoes decompression as the flow cross-section increases. Again, the lee of the fins is a known area of flow stagnation.  
         [0011]     Third, downstream of the spider in the manifold section, the individual streams must recombine along a plurality of mutual joining surfaces, known in the art as knit lines or weld lines. The quality of such knitting is of great concern and can be compromised by stagnation at the spider fins. Further, in general the knit lines are visible in the final extruded product, which can be visually undesirable. Further, longitudinal knit lines are lines of minimum burst strength, which is a serious concern in extruded piping and is a cause for otherwise excessive wall thickness, which is a waste of material.  
         [0012]     Fourth, the pressures to which the molten polymer is subjected within the apparatus can cause plastic leakage at joints in the extrusion head, especially at the transition from the choke collar to the die.  
         [0013]     Fifth, the quality of the final extruded shape requires very accurate placement of the extrusion tip and extrusion die elements with respect to each other. Adjustment of either one in prior art extrusion heads is difficult and time-consuming.  
         [0014]     Sixth, at start-up of a prior art extrusion head, and especially a relatively large head having a large thermal mass, the extrusion dies must be heated externally, typically via a blowtorch, to prevent the first extrudate entering the die from setting therein and causing the entire process to seize.  
         [0015]     Seventh, the extrusion tip is an integral part of some prior art mandrels, or if removable, it requires extensive and time-consuming unthreading.  
         [0016]     It is a principal object of the present invention to provide extruded polymer elements having a high degree of polymeric structural uniformity.  
       SUMMARY OF THE INVENTION  
       [0017]     Briefly described, an inline polymer extrusion head in accordance with the invention includes a first tapered supply section for receiving molten polymer from a supply means. An integral helicoid manifold downstream of the supply section includes an entrance cone, a non-stagnating spider element, a cylindrical transition and flow-turning zone, a helical progressive flow-mixing zone, and a conical zone. A conical choke ring concentrically surrounds the conical zone. The mandrel and choke ring are disposed within a generally cylindrical body. An extrusion tip and die are disposed downstream of the manifold and body. The body and flange are provided with resistance heaters. Optionally, the extrusion tip and the manifold include an internal resistance heater and the die external heater.  
         [0018]     The helicoid manifold is formed as a single entity to eliminate joints in the polymer flowpath, as in the prior art. The entrance cone surface leads smoothly into the spider section, which comprises a plurality of annularly-arranged funnel-shaped passages that meet in knife edges at their upstream ends. Each funnel-shaped passage leads smoothly and without stagnation zones into a generally cylindrical passage leading in an axial direction into the transition and flow-turning section of the manifold. Here, the outer surface of the manifold is cylindrical and close-fitting to the surrounding body and is formed into a plurality of passages. These passages are smoothly turned from axial to become helical along the manifold surface, which becomes conical. Thus the passages become progressively shallower and the clearance between the manifold surface and the cylindrical body becomes progressively greater. As the passage depth becomes zero, the manifold surface becomes a smooth frusto-cone from which the conical choke ring is off-spaced.  
         [0019]     The extrusion tip fits into a well in the end of the manifold and may be readily accessed for maintenance or changeover to another shape or size. The tip surface makes a smooth juncture with the manifold surface. The tip may be hollow and may be provided with an internal resistance heater for facilitating extrusion start-up.  
         [0020]     The die surrounding the tip is fitted loosely into a well in the end of the body and is secured by a plurality of radial positioning screws in the body such that the die may be readily centered or otherwise positioned with respect to the extrusion tip. An internal axial surface in the die acts as a seat and seal for the choke ring which is urged against the seat by the force of polymer flowing through the head.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0022]      FIG. 1  is an isometric view of a complete extrusion head assembly in accordance with the invention;  
         [0023]      FIG. 2  is an axial vertical cross-sectional view of the extrusion head assembly shown in  FIG. 1 ;  
         [0024]      FIGS. 3   a - 3   c  are various isometric views of a helicoid manifold suitable for use in an extrusion head assembly,  FIG. 3   b  being partially in cut-away;  
         [0025]      FIG. 4  is a horizontal cut-away view of the assembly shown in  FIG. 1 ; and  
         [0026]      FIG. 5  is a quarter cut-away view of the assembly shown in  FIG. 1 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Referring to  FIGS. 1 through 5 , there is shown an improved inline polymer extrusion head  10  in accordance with the invention. A supply section  12  includes a flange adapter  14  for connecting to a source (not shown) of molten polymeric material, for example, a conventional progressive-screw extruder. Section  12  is adapted to sealingly mate with an extruder body  16  at an interface  18  and being secured thereto by bolts  17 . Body  16  preferably is surrounded by a conventional band heater  15 . A supply passage  22  in section  12 , preferably conically tapered, connects to a supply passage  24 . Passage  24  opens onto an expanding conical region  26  within section  12  which in turn opens onto a cylindrical bore  28  within and extending toward the end  30  of body  16 . Preferably, a counterbore  32  is provided in end  30  for receiving an extrusion die, as described hereinbelow.  
         [0028]     Radial access ports  20  may be provided in supply section  12 , opening onto passage  24  as shown in  FIG. 2 , for receiving, for example, a pressure rupture safety disk  36  and a temperature sensor assembly  38 . Preferably, a sleeve heater  40  surrounds the cylindrical portion of section  12 .  
         [0029]     A conical section  42  on helicoid manifold  35  is disposed coaxially within conical region  26 , forming a conical flow chamber  44  wherein polymer flow is converted from axially columnar to axially annular. Adjacent to conical section  42  is a spider section  45  including a radial flange  46  captured between supply section  12  and body  16  to secure and accurately center manifold  35  coaxially within body  16 . Inboard of flange  46 , spider section  45  includes a plurality of annularly-arranged funnel-shaped channels  48 , adjacent of which meet at their upstream ends in knife-edges  50 . Because the channels taper in the direction of flow, polymer flow is accelerated. The smooth transition from conical section  42  into and through spider section  45  ensures that no stagnation regions are created.  
         [0030]     Preferably, helicoid manifold  35  ( FIGS. 3   a - 3   c ) includes a plurality of radial passages  52  extending through spider section  45  into communication with an interior bore  54 . Passages  52  also communicate with radial passages  56  formed in interface  18  between body  16  and supply section  12 , such that the interior of the manifold is accessible from the exterior of the extrusion head assembly without passing through the polymer flowpath. Thus, wiring  58  may be inserted via passages  52 , 56  to energize an internal resistance heater  60  in the manifold, and wiring  62  may be inserted to energize an internal resistance heater  64  in extrusion tip  66 . Also, compressed air  68  may be provided via an inlet fitting  70  ( FIG. 5 ) and passages  52 , 56  to support pneumatically the interior of hollow forms such as pipe and tubing being extruded by head assembly  10 .  
         [0031]     Downstream of spider section  45  for a short distance, the outer surface mandrel section  34  of helicoid manifold  35  is cylindrical  72 , then becomes conical  74 . Cylindrical portion  72  is close-fitting to body bore  28 . Within this cylindrical region, a plurality of axially-directed semi-cylindrical flow channels  76  are formed in cylindrical portion  72 , each of which is smoothly connected to one of the funnel-shaped channels  48  in spider section  45  such that there are no stagnation points. In a currently preferred embodiment, there are eight such funnel-shaped channels  48  and eight such flow channels  76 . Also within this cylindrical region, channels  76  are turned from axial to helical via smooth elbow bends  78 . As the channel direction is changed from axial to helical, the surface of cylindrical portion  72  is changed to conical in portion  74  defining lands  80  between helical flow channels  76 . Thus, a progressively deeper flow cavity  82  is formed between lands  80  and cylindrical bore  28 . Further, the locus of bottoms of channels  76  define a virtual cylindrical surface such that channels  76  become progressively shallower as cavity  82  becomes deeper, and eventually the channels disappear altogether, leaving a smooth, unfigured, conical surface extending almost to the end of manifold  35 . Preferably, a short portion  84  of the manifold surface is again cylindrical.  
         [0032]     It is an important element of an extrusion head in accordance with the invention that helicoid manifold  35  is formed in a single piece, from a single blank of material. Thus, the internal interfaces known in the prior art from assembly of cone, spider, and mandrel are eliminated, resulting in a manifold having very uniform heat distribution, no cold spots, and no discontinuities to result in stagnation and slugging of polymer. Manifold  35  may be formed, preferably from a rod of suitable tool steel, by a combination of lathe turning, ball milling, and electric discharge machining. All flow surfaces of the manifold (and all other components exposed to the flowing polymer) are polished and may be plated via electroless nickel plating.  
         [0033]     Within bore  28  and downstream of the disappearance of channels  76  is inserted a choke ring  86  having a cylindrical outer surface  88  and a conical inner surface  90 . Preferably, the included cone angle of surface  90  is greater than that of conical portion  74  such that a conical annular flow space  92  formed therebetween is progressively shallower and is of a progressively smaller average radius. Preferably, a portion  94  of choke ring  86  is also formed as a cylinder, defining with portion  84  an annular flow space  96 .  
         [0034]     Manifold  35  is provided with a threaded counterbore  98  for coaxially receiving a threaded boss  100  extending from extrusion tip  66 . Preferably, the female threads in counterbore  98  and the male threads  104  in tip  66  are interrupted to extend circumferentially in sections of threads and interruptions  106  of about 45° each. Thus, the tip may be secured in the manifold by inserting the boss into the counterbore and rotating it through 45° to fully engage the male and female threads, thereby permitting simple and rapid changing of extrusion tips as desired. Such attaching action further serves to securely anchor and center the extrusion tip to the manifold.  
         [0035]     Extrusion tip  66  includes a central chamber  108  contiguous with interior bore  54  in manifold  35 , chamber  108  opening at the outer end  110  of the tip. Preferably, tip  66  includes a tip heater  64  as recited above for bringing the tip outer surface to or near operating temperature at start-up, thereby preventing non-uniform flow or seizing of polymer within the extrusion head. Preferably, the opening of chamber  108  is fluted  112  to facilitate rotation and removal of the tip by a fluted tool (not shown).  
         [0036]     The outer surface  114  of extrusion tip  66  is conical over most of its length, having a first short cylindrical portion  116  for mating with cylindrical portion  84  of manifold  35 , and a second short cylindrical portion  118  adjacent tip end  110 .  
         [0037]     Surrounding tip  66  is extrusion die  120  having a conically tapered inner surface  122  preferably having substantially the same cone angle as tip surface  114  and also being cylindrical  124  around cylindrical portion  118 , defining an extrusion annulus  126  therebetween. Die  120  is preferably provided with band heaters  128  for pre-heating of the die prior to start-up.  
         [0038]     Die  120  is disposed in counterbore  32  in body  16  and is secured therein by a retaining ring  130  and bolts  132 , ring  130  having slotted holes  135  for quick removal of the ring without full removal of the bolts, to change the die and tip for different sizes and shapes of extruded product. Die  120  is radially loose-fitting in counterbore  32  and is engaged by a plurality of positioning screws  134  threadedly engaged in radial bores in body  16 . Thus, extrusion annulus  126  may be simply and very accurately adjusted by screws  134  after assembly of the extrusion head, and even during operation.  
         [0039]     Referring to  FIG. 5 , extrusion die  120  is provided with a counterbored step  136  which serves as a seating surface for a sealing face  138  on choke ring  86 . During extrusion operation, ring  86  is urged axially against step  136  by the pressure of molten polymer within conical annular flow space  92 , thus effectively sealing against leakage at the entrance to the die, a common problem in prior art extrusion heads.  
         [0040]     In operation, polymer is liquefied as by a conventional progressive-screw extruder (not shown) and is introduced into conical supply passage  22 .  
         [0041]     Polymer flows through passage  24  in columnar flow wherein the temperature of the melt is monitored by temperature sensor assembly  38 .  
         [0042]     Polymer engages conical section  42  and is spread in conical flow channel  44  into annular flow. The annular flow is divided by knife edges  50  between funnel-shaped channels  48  in spider section  45  into a plurality of axial flow streams which enter flow channels  76  without stagnation. Up to this point, flow velocity and pressure are continuously increased by the geometry of the passages in the head.  
         [0043]     In cylindrical portion  72 , the axial flow streams are turned by elbow bends  78  to become helical flow streams, thereby obviating longitudinal knit lines resulting from prior art extruders.  
         [0044]     In flow cavity  82 , the polymer progressively overflows lands  80  as the height of flow cavity  82  increases and the depth of channels  76  decreases, forming thereby a conically annular flow at the entrance to choke ring  86 . At each point along channels  76 , a portion of the polymer is overflowing axially into the next channel while the remainder is flowing helically along its own channel. The flow from the region of decreasing channel height thus comprises a complex multitude of very thin concentric “onion-skin” layers of polymer, the layers being indistinguishable after full passage through the extrusion head and the extruded element having a very high degree of polymeric structural uniformity. The flow velocity is continuously decreased in this section without any stagnation.  
         [0045]     An important benefit of forming pipe in this way is that there are no longitudinal knit lines, and further, that the resulting pipe is stronger than prior art pipe. Thus, in many applications, pipe wall thicknesses may be reduced,. at an immediate savings in polymer consumed (although for drain/waste/vent pipe the wall thicknesses are fixed by industry schedules dictated by prior art pipe technology).  
         [0046]     Polymer enters flow space  92  and is squeezed and accelerated again by passage through choke ring  86 . The cone angles of portion  74  and surface  90  may be varied independently in manufacture to produce a desired pressure and flow profile through this section without stagnation. Polymer then enters the die proper wherein it is further accelerated and shaped to the desired cross-sectional profile by the mechanical relationship between die  120  and tip  66 , and is extruded for cooling and/or further processing from extrusion annulus  126 .  
         [0047]     An extrusion head as just described is useful primarily for forming solid or tubular plastic elements. Of course, it will be readily seen that flexible core forms such as wires may also be coated by introducing such core forms into the extrusion head via passages  56 , 52  and providing suitable extrusion tips and dies  66 , 120 .  
         [0048]     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.