Abstract:
This invention is a single die by which thermoplastic insulating compounds can be co-extruded without any die for the first layer. It makes possible the extrusion of a high-density solid polyethylene skin layer over a cellular polyethylene primary insulation. Such a combination is difficult or impossible to make with a conventional co-extrusion die having a wire guide tip, one combination tip/die and a secondary extrusion die.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     With the advent of interest in foam/skin insulated telephone wire, it has been learned that satisfactory results are difficult or impossible to attain by conventional co-extrusion systems. Such conventional equipment feeds the conductor to a tip and a primary melt is discharged along the outside of the tip and into contact with the conductor before the conductor and adhered melt passes through a first extrusion die. The outside of this first extrusion die is tapered to provide another tip surface along which a secondary melt is discharged into contact with the primary melt layer as the conductor and its adhered coating advance through the second tip. The conductor with the co-extruded layers then passes through a secondary extrusion die. In order to apply a foam/skin insulation over the conductor, the first extrusion die applies a cellular polyethylene layer, usually high density polyethylene, and the second extrusion die applies a solid polyethylene skin (high density) over the cellular insulation. The principal difficulties encountered with conventional co-extrusion systems relate to a lack of smoothness of the insulation; disruption of the cellular structure of the primary layer; and lack of dielectric strength. These problems are magnified as the melt index of the compound used for the outer skin is lowered. 
     This invention overcomes these difficulties by using only one tip and one extrusion die; and by applying the two layers of insulating material successively with a melt-flow separator between the supplies of insulation as they approach the end of the tip through which the conductor passes. With this invention, the primary and secondary insulations are sized simultaneously and no cellular or surface disruption occurs. Relative wall thickness of the primary and secondary insulations are controlled by adjusting the relative speed with which the different insulations are supplied to the extruder die. 
     Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views: 
     FIG. 1 is a diagrammatic view of a conventional co-extrusion system having successive tips and dies; and 
     FIG. 2 is a diagrammatic sectional view showing the single die co-extrusion system of this invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The conventional apparatus of the prior art, illustrated in FIG. 1, has a first tip 10 through which a conductor 12 passes to the discharge end of the tip. Hot insulating material 14 is fed through an annular passage 16 surrounding the tip 10 and providing an annular clearance, with a taper corresponding to substantially that of the tip 10. The insulating material 14 contacts with the conductor 12 at the discharge end of the tip, and the thickness of the insulation applied to the conductor is determined by a first extruder die 18. The thickness of an insulation wall, made by the insulation 14, and designated by the reference character 20, depends upon the diameter at the discharge end of the extruder die 18. 
     The outside of the extruder die 18 is tapered to serve as a second tip, indicated by the reference character 22 at the outlet end of this second tip 22. 
     The second tip is spaced from a second extruder die 24 which has its inner surface tapered and spaced from the tip 22 to provide for passage of insulating material 26 to the discharge end of the second tip 22. This passage for the insulating material 26 is designated by the reference character 28 in FIG. 1. 
     The wall thickness of a second layer 30 is determined by the diameter of the opening through the extrusion die 28. As pointed out above, this equipment is unsatisfactory for applying a solid skin layer of insulation over a cellular layer. 
     FIG. 2 shows the improved apparatus of this invention. A single tip 32 is mounted in a fixed position in a frame 34 of the extruder apparatus. A melt supply passage 36 feeds the cellular melt material to a chamber 38 surrounding the upper end of the tip 32; and the cellular insulating material is supplied through the passage 36 from melt supply apparatus indicated by the reference character 40. The rate of speed of the melt material from the apparatus 40 is controlled by a motor 42 connected by conductors 44 with a control panel. 
     The structure shown in FIG. 2 has only one extruder die, designated by the reference character 46. The melt material for a second layer of insulation is supplied through a passage 48 to an annular chamber 50 in the frame 34. The rate at which the insulation is supplied through through the passage 48 is determined by the rate at which melt supply apparatus 52 is driven by a motor 54 connected by conductors 56 with a control panel 58. 
     The control panel 58 is shown diagrammatically with controls 60 and 62 for varying the speed of the motor 54 and another motor 64, the latter being used to drive a feed roll 66, which cooperates with another roll 68 to advance the insulated conductor 70, which is shown on an enlarged scale at the right hand side of FIG. 2. 
     A speed control similar to the control 60 is in the circuit of the motor 42, and the rate of speed of the motors 42, 52 and 64 are adjustable and can be subject to automatic control by programming apparatus, the details of which form no part of the present invention. 
     The cellular plastic insulation for the first layer is indicated by the reference character 72, and the passage through which it travels along the outside of the tip 32 is indicated in FIG. 2 by the reference character 74. 
     A melt flow separator 78 is secured in a fixed position surrounding the tip 32 and spaced from the tip to provide the passage 74. The outside surface of the melt flow separator 78 provides one side of a passage 80 through which insulation from the passage 48 flows toward the extruder die 46, and there is space between this outside surface and the interior of the extruder die 46 leaving a passage 82 for the insulation material that will form the second layer. 
     An important feature of the construction is that the melt flow separator merely keeps the insulating materials from merging before they are close to the discharge end of the tip 32. It should be noted that the melt flow separator 78 terminates some distance back from the end of the tip 32. The essential feature is that the melt flow separator 32 must not extend far enough to constitute an extrusion die which determines the wall thickness of the first layer of insulation and which is a factor in determining how much of the insulating material 72 is applied to the conductor per unit of length of the conductor. 
     The extruder die 46 determines the wall thickness of the composite insulation applied to the conductor 12. The thickness of the inner insulation layer 84 with respect to the outer insulation layer 86 is not determined by successive extrusion dies as in FIG. 1, but is determined by the rate at which the material 72 for the inner layer 84 and material 88 for the outer layer 86 are supplied to the chambers 38 and 50 from the melt supply apparatus 40 and 52, respectively. This results in smoothness for both layers of insulation, avoids disruption of the cellular structure of the inner layer and insures the dielectric strength of the insulation of both layers. 
     The preferred embodiment of the invention has been illustrated and described but changes and modifications can be made and some features can be used in different combinations without departing from the invention as defined in the claims.