Abstract:
A crosshead assembly for forming extruded shapes from molten polymer. The assembly includes body sections, a mandrel, a choke ring, and a tip holder for admitting, turning, and accelerating molten polymer toward a novel rotatable die sub-assembly. The die sub-assembly includes a hub, axially-loaded duplex ball bearings, and a pulley for rotating the sub-assembly. The choke ring extends into the die sub-assembly, and a cylindrical seal element is disposed therebetween. A changeable extrusion tip is threaded onto the second cylindrical portion of the tip holder. A die in the sub-assembly is specific to the shape to be extruded. For forming a spiral ribbon and a skin layer on core material, the die opening is a circular central aperture and a radial slot. Multiple ribbons may be extruded simultaneously by using multiple slots.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for extrusion forming of molten polymer material; more particularly, to crossline extrusion heads for continuous extrusion coating of hollow or solid shapes; and most particularly, to an extrusion crosshead having a shaped die opening for applying extrudate material to a cylindrical core material, wherein the die is rotated during extrusion such that the extrudate forms a spiral pattern on the core material. 
     BACKGROUND OF THE INVENTION 
     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. 
     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, known in the art as “inline,” or at an angle, typically 90°, to the axis of the head, known in the art as “crosshead.” Except when coating highly flexible core materials such as wire, the coating of a sheath layer onto a core stock requires passing the core stock axially through a die and injecting the molten polymer into the die head in a crosshead relationship. 
     In a typical prior art extrusion crosshead, a generally cylindrical body element concentrically surrounds a generally cylindrical mandrel, a first annular flow space being provided therebetween. Molten polymer injected orthogonally from a screw extruder enters an annular reservoir provided in either the body element or mandrel and then flows from the reservoir along the annular flow space. Contiguous with the annular flow space is a conical flow space, formed between a conical choke ring and a conical portion of the mandrel, wherein the diameter of annular flow is decreased and the velocity of flow is increased. Downstream of the conical flow space is a second annular flow space formed between a second cylindrical region of the extruder body and a second cylindrical region of the mandrel. This flow space leads into a flow shaping region formed between an extrusion die and an extrusion tip, from whence the formed shape is extruded. 
     When it is desired to form a coating on a core element, the mandrel and extrusion tip are provided with an axial passage through which the core element is passed as extrusion proceeds. 
     When it is desired to provide a spiral element in a coating, the extrusion die may be made rotatable of the extrusion body. 
     Several problems exist in prior art extrusion heads having rotatable dies. 
     First, it has been found to be difficult to provide a rotatable seal to prevent leakage of molten polymer from the head between axial faces of the stationary and rotating components. Typically, such leakage causes continuous polymer buildup on the outside of the head, resulting eventually in failure of the head and requiring shutdown of the process to clean and restart. 
     Second, polymer may leak into the bearings, causing failure of the head. 
     Third, the extrusion die and tip must be heated externally to prevent freeze-up at the start of operation. Such heat is provided typically via a blowtorch, which a) is a crude means of heating, b) requires undesirably a substantial open flame which can damage or melt some core materials such as other plastics, and c) can adversely affect the temper of head elements including the die itself. 
     Fourth, the bearing assemblies are poor transferors of heat from external blanket heaters into the hub, melt, and extrusion tip. 
     Fifth, the large surface areas of die and tip within the rotation chamber create high viscous drag, imposing large torque requirements on the driving apparatus. 
     Sixth, in the prior art it is not known to form a raised spiral ribbon element on a core material by using continuous rotational extrusion of a ribbon element. 
     It is a principal object of the present invention to prevent leakage of molten polymer from an extrusion head having a rotating die. 
     It is a further object of the invention to provide a raised spiral ribbon element on a core material by using continuous extrusion of the ribbon element material. 
     SUMMARY OF THE INVENTION 
     Briefly described, a polymer extrusion crosshead in accordance with the invention includes conventional components as described above for admitting, turning, and accelerating molten polymer toward a novel rotating die assembly. An annular reservoir is provided for receiving molten polymer from a supply means such as a progressive screw extruder. A generally cylindrical body element concentrically surrounds a generally cylindrical mandrel, a first annular flow space being provided therebetween. Molten polymer injected orthogonally from a screw extruder enters an annular reservoir provided in either the body element or mandrel and then flows from the reservoir along the annular flow space. Contiguous with the annular flow space is a conical flow space, formed between a conical choke ring and a conical portion of the mandrel, wherein the diameter of annular flow is decreased and the velocity of flow is increased. Downstream of the conical flow space is a second annular flow space formed between a cylindrical extension of the choke ring and a second cylindrical portion of the mandrel. 
     The body element preferably comprises first and second sections to facilitate installation of the conical choke ring. The second body section includes an axial well for receiving a rotatable die assembly including duplex ball bearings that preferably are axially-loaded to prevent radial runout. The cylindrical extension of the choke ring extends through the second body section and into a bore in the rotatable die assembly, and a cylindrical seal element defining a radial-surface seal is disposed therebetween. 
     A changeable extrusion tip is threaded onto the second cylindrical portion of the tip holder. 
     The rotatable die assembly includes a hub having a stepped bore for receiving the cylindrical seal element, for cooperating with the extrusion tip to form the second annular flow space, and for receiving a die specific to the shape to be extruded. The cylindrical outer surface of the hub receives the inner races of the duplex bearings which are secured and loaded by a loading nut threaded onto the hub. A drive element, for example, a pulley, is mounted on the hub such that the assembly may be rotated in known fashion during extrusion of polymer through the die. 
     Preferably, a band heater is mounted at the distal end of the hub for pre-heating the hub and die before extrusion is begun. Because of its cable connections, the heater must be disconnected and/or removed before beginning extrusion. 
     For forming a spiral ribbon on a rod- or tube-shaped core material, the die opening includes a circular central aperture and a radial slot. The aperture preferably has a diameter slightly larger than the diameter of the core material, such that a continuous skin layer is formed on the core material as it passes through the die. The thickness of the skin layer may be controlled by the diameter of the aperture and the pressure in the extrusion head. Further, the extruding skin helps to hydrodynamically center the core material in the die. The ribbon is extruded simultaneously through the slot integrally with the skin layer. The ribbon dimensions may be controlled by controlling the pressure in the extrusion head. The pitch of the ribbon spiral is a function of the axial speed of the core material through the die and the rotational speed of the die assembly. When the molten skin and ribbon extrusions have cooled and set, the skin layer holds the spiral element in place to prevent axial collapse of the spiral and to aid in bonding the extrusions to the core material. 
     Obviously, multiple ribbons may be extruded simultaneously by providing a plurality of slots radiating from the central aperture. Further, the cross-sectional shape of each ribbon may be controlled by varying the shape of the ribbon slot in the die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an elevational cross-sectional view of a spiral ribbon extrusion crosshead assembly having a rotatable die in accordance with the invention; 
         FIG. 2  is an entrance end view of the crosshead assembly shown in  FIG. 1 ; 
         FIG. 3  is an exit end view of the crosshead assembly shown in  FIG. 1 ; 
         FIG. 4  is a longitudinal view of an exemplary spiral-ribbon extruded element produced by an extrusion crosshead assembly in accordance with the invention; and 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  in  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 through 3 , there is shown an exemplary embodiment  10  of an improved extrusion crosshead assembly in accordance with the invention. Assembly  10  includes a fixed member  21  and a die sub-assembly  82 . Fixed member  21  includes body element  13 , mandrel  22 , tip holder  46 , choke ring  54  and extrusion tip  66 . A first body section  12  of body element  13  is substantially cylindrical on an inner surface  14  and outer surface  16  thereof. A radial bore  18  therein is receivable of supply means  20  for providing molten polymer to assembly  10  in use thereof. Mandrel  22  is disposed within section  12  and secured therein via bolts  24  extending through a radial mounting flange  26 . Mandrel  22  includes a cylindrical outer surface portion  28  that is close-fitting to inner body surface  14 , a first tapered portion  30  having a pattern of decreasing helicoid flow channels  31 , and a second tapered portion  32 , all defining a progressive annular flow space  34 . A radial passage  36  in body section  12  connects bore  18  with an annular reservoir  38  formed in portion  28  for receiving molten polymer into the head assembly. By means of reservoir  38  and flow space  34 , polymer flow through head assembly  10  is converted from columnar flow orthogonal to assembly axis  40  to annular flow through space  34 . Preferably, body section  12  is surrounded by a band heater  42 . Preferably, mandrel flange  26  is provided with a thermal probe  44 . 
     Disposed coaxially and snugly within mandrel  22  is extrusion tip holder  46  which extends beyond mandrel tapered portion  32 , having its own portion  32   a  tapered at substantially the same taper angle to continue progressive annular flow space  34 . Tip holder  46  includes an annular mounting flange  48  for securing tip holder  46  to mandrel  22  via bolts  50 . A cylindrical portion  52  of tip holder  46  extends from tapered portion  32   a.    
     Surrounding tapered portion  32   a  and cylindrical portion  52  is choke ring  54  having a tapered inner surface  56  and a cylindrical portion  58  having an axially-extending surface  108 , portion  58  cooperating with tip holder portion  52  to define an annular flow space  60 . 
     Tip holder  46  terminates in an enlarged portion  62  which defines an additional choke region in flow space  60 , of particular significance in the invention as described below. Portion  62  includes a threaded counterbore  64  for receiving extrusion tip  66 . Preferably, a set screw  63  is also provided in portion  62  for securing tip  66  in counterbore  64  and preventing the tip from being unscrewed by viscous drag during rotation of the die assembly when filled with molten polymer. An important additional purpose of enlarged portion  62  is to provide high back pressure of polymer at the seal entrance, as described below. 
     Tip holder  46  is provided with a stepped axial bore  65  throughout that mates with a similar stepped bore  67  in extrusion tip  66 . The narrowest portion  69  of tip  66  has a diameter selected for snug but slidable support of core material to be spiral coated. The axial length of tip  66  is selected to optimize the opposed requirements of a) maximal length for core material support to prevent vibration or chattering, and b) minimal length to minimize surface area for viscous drag during rotation of die sub-assembly  82 . 
     Choke ring  54  is mounted to first body section  12  via a centering counterbore  68  therein and is secured to section  12  via second body section  70  and clamp  72 . Second body section  70  of body element  13  includes a stepped well  71  having a central opening  74  through which ring portion  58  protrudes, walls  76 , and bottom surfaces  78 , 80 . 
     Rotatable die sub-assembly  82  is disposed in well  71  of body element  13 . Sub-assembly  82  includes a hub  84  having a cylindrical outer surface  86  for receiving first and second inner ball bearing races  88   a,   88   b  and a threaded portion  90  of surface  86  for receiving a loading nut  92  for axially loading the inner races against hub flange  94 . A locking nut  96  secures loading nut  92 . First and second outer ball bearing races  98   a,   98   b  of duplex ball bearing assembly  99  are received against wall  76 , first and second ball sets  100   a,   100   b  being disposed conventionally between the inner and outer races. The outer races are retained by retaining ring  102 . A currently preferred axially-loadable duplex ball bearing assembly  99  is Fafnir #7319WN MBR-DU, available from The Timken Company, Canton, Ohio, USA. A grease fitting  75  may be provided for periodic lubrication of the bearing assembly. A resilient rotary seal  77 , for example, a Teflon O-ring, is disposed in an annular groove in the outer surface of flange  94  for preventing grease from working along gap  93  into seal element  104  from whence contamination of polymer within the head assembly would be possible. Seal  77  further redundantly eliminates any possibility of polymer entering the ball bearing assemblies. 
     Cylindrical rotary seal element  104  is disposed against axially-extending surface  106  in hub  84 . Preferably, seal element  104  rotates with hub  84 , forming a sliding seal with surface  108  of choke ring cylindrical portion  58 . A currently preferred seal element is a porous bronze oil-filled bushing such as Bunting #EP364432, available from Bunting Bearings Corp, Holland, Ohio, USA. During operation of the apparatus, the pores in seal element  104  become filled with molten polymer that acts as a lubricant of the seal and also assists in forming a hydrodynamic blockage of significant leakage past element  104 . As noted above, an important purpose of enlarged portion  62  is to provide high back pressure in the region including the entrance to seal element  104  to ensure that polymer beneficially enters and fills seal element  104 . 
     Hub flange  94  is off-spaced by gap  93  from bottom surface  78  of body element  13  preferably by about 0.010 inches to assure rotational clearance therebetween. Further, a weep hole  95  is provided in second body section  70  such that any small amount of polymer that may leak by seal element  104 , as can happen as the seal wears over time, will be diverted to the exterior of the cross-head assembly and will not find its way into the bearings. 
     Outboard of walls  76 , second body section  70  is surrounded by a heating element  110 . Because body section  70  is closely contiguous with hub flange  94  and is in extended contact with the outer races  98   a,   98   b , there is excellent heat transfer from heating element  110  into the interior of hub  84 , a significant improvement over the prior art. 
     An extension  112  of hub  84  is receivable of a die  114  having an extrusion opening  116  suitable for an intended extrusion shape. Die  114  is secured within hub  84  by a threaded retainer  115  disposed on a threaded outer portion of hub  84 . In the example shown in  FIGS. 1 and 3 , die  114  has a central circular opening  118  and a radial slot  120  communicating with opening  118 . 
     Referring to  FIGS. 1 through 5 , when a rod-shaped core material  122  is fed through the crosshead assembly  10  at a predetermined and fixed linear speed, and die sub-assembly  82  is rotated about axis  40  at a predetermined and fixed rotational speed without rotating core material  122 , a spiral ribbon  124  and integral skin coating  126  of polymer are applied to core material  122  at a predetermined pitch  128  and height  130  of ribbon  124  and a predetermined thickness  132  of skin coating  126  to produce an extrusion-coated element  127 . Of course, core material  122  may be tubular rather than solid, as may be desired. 
     Referring again to  FIGS. 1 and 3 , a preferred drive means for die sub-assembly  82  is a pulley  134  mounted on hub  84  via a wedged bushing  136 . Obviously, all other means of driving sub-assembly  82 , as may occur to one of skill in the art, are fully comprehended by the invention although not shown here. 
     Die retainer  115  is surrounded by a band heater  138  for cooperating with heater element  110  for heating the entire rotatable die sub-assembly  82  to a suitable temperature prior to introducing molten polymer into head assembly  10 . This is an important advance over the prior art method of heating the apparatus with a blowtorch. Immediately before beginning extrusion activity, heater  138  is electrically disconnected to permit rotation of sub-assembly  82 ; if desired, heater  138  may be provided in two separable pieces to permit removal from the extrusion head after extrusion activity has begun. The outer portion of sub-assembly  82  is heated satisfactorily by the molten polymer during operation of the apparatus. 
     In summary, an improved extrusion crosshead having a rotatable die sub-assembly in accordance with the invention provides at least the following benefits over prior art crossheads:
         1. An electric preheater at the die provides controlled heating prior to initiating extrusion.   2. Axially pre-loaded duplex ball bearings reduce the torque requirement and provide accurate alignment of the rotating and fixed elements of the head.   3. The secondary choke in the second cylindrical flow region both provides additional smoothing of polymer flow and creates back pressure to assist in providing polymer for seal bushing lubrication.   4. The rotatable assembly is readily removable to change extrusion tips and dies; the basic crosshead assembly remains intact and uninvolved in such changes.   5. The rotating seal between the fixed body and rotating die assembly is changed from being an axial face seal in the prior art to a cylindrical barrel seal.   6. The ball bearing assemblies are redundantly protected from contamination from polymer by the barrel seal, a weep hole, and a rotating seal on the hub flange.   7. A spiral-wrapped ribbon may be readily formed on a continuous core element.       

     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.