Apparatus for Forming Foam-Skin Insulation on Telephone Wires

The present invention relates to an apparatus for providing telephone wires with a plastic insulation. As insulation material for telephone wires there is generally used a solid plastic or a foamed plastic insulation. Recently the use of a so called foam-skin insulation has grown rapidly wherein on the wire is formed an inner layer of foamed plastic and on top of this layer is applied a layer of solid plastic, the operation being carried out in the same extrusion tool. A telephone wire having such a foam-skin insulation has, due to the foamed plastic, good electrical properties and the outer layer of solid plastic gives the wire good mechanical properties.

This process has in the past been performed in such a way that the flow of plastic mass to be foamed and the flow of plastic mass to the solid layer, have been brought to the extruder tool by using separate extruders. A drawback of this method is the second extruder as such and the fact that this second extruder needs space and makes the connection to the extruder tool complicated.

It is the object of the present invention to provide a new apparatus for forming a foam-skin insulation which eliminates these drawbacks.

According to the apparatus of the present invention only one plastic extruder is used into which extruder a basic plastic is fed in a conventional manner. In the extruder head the homogeneous flow of basic plastic is divided into two different flows. With one of these flows is intermixed a liquid or gaseous foam agent in a manner known per se. The correct ratio between the two flows may be controlled e.g. by means of a pressure throttling device or a control valve.

In the following the invention will be described with reference to the accompanying drawing which schematically shows one embodiment of the apparatus for carrying out the invention.

FIG. 1 is a sectional view of an extruder head and of an extruder tool, according to the invention.

FIG. 2 shows a section taken along the line II--II in FIG. 1.

An extruder for plastic material to be used as an insulation for telephone wires is generally designated with reference numeral 1. The extruder comprises in a conventional manner a transport screw 2 as well as conventional heating means, not shown, for melting the plastic material. At the end of the extruder 1 is an extruder head which comprises an outer hollow cylinder 4 and mounted inside this cylinder 4 a second hollow cylinder 5. In the inner space of the cylinder 5 is arranged a mixer 6 which is at its outer surface provided with conventional means mixing means not shown in the drawing. In the cylinder 5 there is provided a separate channel 8 which bypasses the mixer 6. A foaming agent is arranged to be fed to the space 7 between the inner wall of the cylinder 5 and mixer 6 through nozzles 9, shown in FIG. 2. At the end of the above described extruder head is mounted an extruder tool, which comprises an outer casing 10 which by means of a nut 11 is mounted to the cylinder 4. Within the casing 10 is arranged a torpedo means 12 through which a telephone wire 20 to be insulated is drawn, as shown in FIG. 1. Around the torpedo 12 is arranged a second torpedo 13 and further, around the tapered parts of said two torpedos 12 and 13 is arranged a conically tapering nozzle piece 14. Channel 7 of the extruder head communicates with torpedo 12 through the casing 10 and the outer torpedo 13 to a groove 15 which terminates into an annular channel 16 between torpedos 12 and 13. Between the outer torpedo 13 and the casing 10 is likewise formed as annular channel 17 which communicates with the channel 8 through the wall of the casing 10. The annular channels 16 and 17 unite into a conical channel 18 contracting towards the wire 20. The width of the channel 18 can be adjusted by moving the conical nozzle piece 14 by means of an adjustment nut 19. Reference numeral 21 schematically designates a control valve for adjusting the flow through the channel 8. The groove 15 is preferably curvilinear and its connection to the annular channel 16 does therefore not appear in FIG. 1.

The process works as follows:

Molten plastic enters along the extruder 1 in the channel 3. The flow is divided into two parts one of which flows unchanged through the channel 8 while the other part is directed to the mixing space 7 where a foaming agent is intermixed therewith. The torpedo 12 forms the flow of plastic and foaming agent into a cylindrical inner layer, and the torpedo 13 likewise forms the flow of plastic into an outer cylindrical layer. As the pressure releases at the end of channel 18 the inner layer will foam and thus the telephone wire 20 is simulaneously coated with two layers, the inner being of foamed plastic and the outer being of solid plastic.

The extruder tool including the torpedos 12 and 13 forming the coaxial annular channels 16 and 17 emerging into the conically tapering channel 18 is known per se but previously the flow of the molten plastic intended for the outer solid insulation layer has been fed to the annular channel 17 from a separate extruder necessitating a complicated installation as, among other things, the connections shall endure high pressures and at the same time be elastic to permit different thermal expansions caused by the separate extruders.

All these problems are eliminated by the provision of the separate branch channel 8 bypassing the foam agent mixing station direct to the extruder tool. The channel 8 may of course be of different shape or position as the one shown in the drawing.