JPS6228527B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a coaxial cable with excellent characteristics by simultaneously extruding a highly foamed insulator and molding an outer conductor. More specifically, when extruding and covering the core wire with a highly foamed material, foaming of the foam and molding of the surface are performed simultaneously in the molding section of the outer conductor, thereby eliminating unnecessary gaps between the insulator and the outer conductor and creating an integrated structure. The present invention relates to a method of manufacturing a coaxial cable with excellent electrical properties, mechanical properties, and reliability during long-term use.

The conventional manufacturing method for this type of cable is to extrude and coat the core wire with an insulator, then wind it around a bobbin after cooling, and then mold and coat the outer conductor in a separate process.
It was common to apply a protective coating. As the insulator, unfoamed polyethylene or low-foamed polyethylene that has been expanded to about twice its size using a chemical foaming agent has been used.

The cables obtained by this conventional method had various drawbacks as described below. For example, in the case of a laminated sheathed coaxial cable in which thin aluminum tape is attached vertically to the outer conductor, the outer diameter of the insulator and the outer conductor should be adjusted to avoid problems such as tape breakage when forming the aluminum tape on the insulator. It is necessary to provide a gap of approximately 0.2 mm or more between the inner diameter of the cable and the insulator and the outer conductor. Therefore, if water gets into the cable from the end of the cable, the entire length of the cable will be damaged. In addition, if the outer conductor becomes perforated due to damage during use, water will also seep in, degrading the cable's transmission characteristics and making it unusable.

In addition, laminated sheathed cables are usually relatively small in diameter, but aluminum tape with a thickness of about 0.2 mm is used as the outer conductor for mechanical strength and long-term reliability (prevention of breakage). In this case, even if the outer conductor is molded using a molding die and coated on the insulator, the strong repulsive force of the tape makes it easy to recover after molding, and since the insulator is cold, the tape or sheath will be heated. It is easy for the adhesive to be taken away, especially at the weight part of the tape, and for example, if the insulator has an outer diameter of
It is known that it is extremely difficult to bond the weight part when the thickness is less than mm. Therefore, there is a continuous gap in the length direction of the cable where the outer conductors overlap, and if this cable is used for a long period of time, moisture will gradually enter the cable due to changes in temperature, etc. However, there are cases where the cable becomes unusable after several years.

In addition, as mentioned earlier, since there is a gap between the insulator and the outer conductor, no matter how uniformly and concentrically the insulator is coated on the core wire, it will not form a perfect concentric circle with the outer conductor. Since the cable is eccentric, if it is wound around a drum or installed on a utility pole, it will be locally eccentric, resulting in inferior electrical properties such as VSWR.

Furthermore, in the conventional method, after the insulator is once extruded and rolled up, the outer conductor is molded and coated, leaving curls in the insulator, and these curly parts can cause molding defects in the outer conductor.

In particular, when the wrapping ratio of the insulator is increased, the strength of the insulator decreases, so that it tends to become curly when wound onto a bobbin, and it becomes impossible to remove the curl once it has formed.

However, if the insulator is a low foam or non-foam material, extrusion molding may be extremely easy, and it is not impossible to mold the outer conductor at the same time as extruding the insulation. This solves the problem of curly hair due to excessive winding, but does not solve the other drawbacks. Recently, high foam insulated coaxial cables with a foaming ratio of 3 times or more have been used as foam insulators by using a low boiling point liquid as a foaming agent. In order to coat the wire, highly advanced technology that has not been previously used is required, for example, the surface is molded using a sizing die during foaming, so this method of manufacturing high foam insulated coaxial cables is The technology for simultaneously extruding insulation and molding the outer conductor had not yet been established.

The present invention has been made with attention to the above-mentioned points, and provides a method of manufacturing a coaxial cable with unprecedented characteristics by simultaneously performing extrusion coating of a highly foamed insulator and molding of an outer conductor. An example of manufacturing a coaxial cable according to the present invention will be described below with reference to the drawings.

In Figure 1, 1 is a core wire such as annealed copper wire, 2 is an extruder for highly foamed insulators, 3 is an extruded insulated core wire that is in the process of being foamed (called a core), and 4 is a metal tape (outer conductor). 5 is a molding device, 6 is a coaxial core coated with mold, 7 is a plastic sheathed extruder, 8 is a plastic sheathed coaxial cable, and 9 is a cooling tank. The metal tape in 4 is aluminum tape,
It may also be a copper tape or a laminate tape obtained by laminating a thermally adhesive adhesive onto these metal tapes.

The feature of the present invention is that the core extruded by the extruder 2 is guided to the outer conductor molding device 5, and the surface of the core is molded at the same time as the outer conductor is molded.The core is completely foamed after extrusion. While still in its thermoplastic state, it is introduced into a molding device together with the metal tape. The external conductor molding device usually consists of rollers and dies. First, a flat tape is sandwiched between a pair of rollers with concave and convex surfaces to form a semicircle, and then passed through a roller with a smaller curvature or through a cylindrical die. Make it more circular. Of course, in the case of a soft tape such as thin aluminum tape, it may be formed simply by passing it through a die, or the tape may be corrugated.

By doing this, the foam insulation is foamed and the surface is molded in the outer conductor molding equipment, and the highly foamed insulating core with the same surface as the outer conductor is molded using a sizing die. It is molded without any gaps inside. In addition, since the foam is guided into the outer conductor while it has thermoplasticity after extrusion and before foaming is completely completed, that is, in a high temperature state, this heat is conducted to the outer conductor and is effectively used to preheat the outer conductor. , the foam itself will be cooled to a temperature at which it no longer exhibits thermoplastic properties.

Of course, depending on the structure of the cable, there may be cases where the balance between heat dissipation from the foam and heat absorption from the outer conductor is inappropriate, but in such cases, for example, a separate core cooling device may be installed before the outer conductor molding device 5, or Device 5 can be followed by a cooling or preheating device for the outer conductor. In particular, when the insulator is a highly foamed insulator that is foamed three times or more as in the present invention, the outer diameter of the foam may easily become unstable, so a cooling device that also preforms the foam is installed before the molding device 5. If provided, work efficiency will be improved. In that case, if the preforming equipment is equipped with a measuring device that can measure the thickness unevenness of the foam and the capacitance, it will be possible to predict the electrical characteristics of the finished product (coaxial cable). This is an extremely effective means for producing good products and reducing the amount of work waste that is thrown out at the start of work.

Further, since the extruder 2 for high foaming needs to be extruded and coated with foamed plastic concentrically on the core wire, it is particularly preferable to use a non-eccentric crosshead.
Furthermore, the molding device 5 must be equipped with a centering mechanism for completely coaxially covering the outer conductor on the foamed insulator.

That is, the player can move vertically, horizontally, and horizontally parallel to the core pass line, and can freely swing his head in any direction as the case may be.

What is particularly important is that the molding device 5 must be slid back and forth depending on the state of foaming of the core so that the outer conductor can be coated at the optimal position.

In the present invention, the sheath extruder 7 and subsequent parts are not particularly different from the conventional ones.

By using the present invention, the following effects can be obtained.

(1) Since a highly foamed material is molded in the outer conductor, there are no gaps, resulting in a coaxial cable with excellent water resistance.

(2) For the same reason as above, the insulator and outer conductor are closely bonded and integrated, resulting in a cable that is resistant to bending and compression.

(3) Since the outer conductor is preheated from the inside by the heat of the foam, for example, in the case of a laminated sheath cable, the weight part of the tape can be completely fused, resulting in a cable with improved water resistance and mechanical properties. It will be done.

(4) Since the outer conductor is molded while the core has thermoplasticity after extrusion and before foaming is completely completed, there is no resistance to the tape during molding, and the outer conductor can be applied in close contact with the core.

(5) Since there is no gap between the core and the outer conductor, there is no eccentricity even when the cable is wound around a drum or installed on a utility pole, resulting in a cable with stable electrical characteristics.

(6) When molding the surface of a highly foamed material using a conventional sizing die, open cells are generated in the foam due to the resistance generated by the sizing die, but in the present invention, open cells are formed in the outer conductor, and the outer conductor Since the foam is moved and molded together with the core, open cells do not grow in the foam, resulting in a coaxial cable with excellent electrical properties, mechanical properties, and water resistance.

(7) Since it results in a cable structure with good mechanical strength and water resistance, it is particularly effective for highly foam insulated cables, where conventional manufacturing methods inevitably degrade these properties.

Example 1 In the method shown in Fig. 1, the core wire 1 is a 1.2 mm annealed copper wire, and the core wire 2 is a low density polyethylene containing 4 wt% dichlorodifluoromenthane as a blowing agent (density 0.92, MI= 2) was extruded.
The extrusion temperature was approximately 120°C, and the expansion ratio of polyethylene was targeted to be 6 times.

As the metal tape in step 4, a soft aluminum tape with a thickness of 0.15 mm was used, and one side (sheath side) of the tape was coated with 0.05 mm of ethylene-vinyl acetate-based thermal adhesive. 5 uses a die-shaped molding tool with an inner diameter of 5.4 mm, and the black polyethylene is approximately 1 mm thick from the extruder in 7.
Thickly coated. The extruder is 90mm and the extrusion temperature is 170℃.
It is. The production speed of the entire line was 30 m/min. The properties of the obtained cable (laminated sheath coaxial cable) were as follows.

Characteristic impedance 75.5Ω at 10MHz Capacitance 51nf/Km VSWR 1.1 or less at 70~800MHz Pulling force of core from external conductor 8Kg/20cm long aluminum tape Peel strength of weight part 3Kg/2mm width Open cell ratio of foam 3% Comparative example On core wire At the same time as the highly foamed polyethylene was extruded, the surface of the core was molded using a sizing die, and after cooling, it was wound onto a drum.

Next, an aluminum tape was molded on this core and a polyethylene sheath was coated to produce a laminated sheath coaxial cable of the same size as in Example-1. At the time of manufacturing this cable, an attempt was made to reduce the diameter of the aluminum tape molding device (die) so that there would be no gap between the core and the outer conductor, but the aluminum tape would break, making it impossible to manufacture the cable.
Therefore, the outer conductor was molded with a gap of 0.3 mm between it and the core.

The worst VSWR of the cable thus obtained was 1.4, which far exceeded the standard (1.2 or less). Further, the pulling force of the core from the outer conductor was less than 1 kg, and the core did not come into close contact with the outer conductor at all. Furthermore, the peel strength of the weight part of the aluminum tape was almost 0 kg, which was impossible to even measure. The open cell ratio of the foam was 28%, which was about 10 times greater than that of Example 1, and the compressive strength of the cable was about 1/2 that of Example 1.

Example 2 The same procedure as in Example 1 was carried out except that a double-sided laminated tape in which both sides of an aluminum tape were coated with a heat-sealing material was used as an external conductor. The resulting cable had a particularly strong bond (fusion) between the insulator and the outer conductor, and when the force was measured to pull out the core from the outer conductor, it was found that
It was over 20kg.

Example 3 An annealed copper tape with a thickness of 0.15 mm and not coated with a heat sealing material was used as an outer conductor. After forming the copper tape vertically, we covered the outside with iron tape and then covered it with polyester tape. Line speed is
The speed is 10m/min.

The rest was manufactured in the same manner as in Example-1. The characteristics of this copper tape vertically mounted high foam insulated coaxial cable are characteristic impedance 75.8Ω capacitance
It was suitable for data transmission and multiplex broadcasting, with a 50.5 VSWR of 1.1 or less and excellent shielding properties.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an embodiment of the manufacturing method according to the present invention, in which 1 is a core wire, 2 is an extruder, 3 is a highly foamed insulating core, 4 is a metal tape, 5 is a molding device, and 6 is a molded A coaxial core, 7 a sheath extruder, 8 a coaxial cable, and 9 a cooling tank.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing a coaxial cable in which a highly foamed insulator with a foaming ratio of 3 times or more is applied on a core wire, and a metal tape is vertically attached and molded around the insulator to form an outer conductor, after extruding the insulator, The material is introduced into a molding device together with the metal tape while it has thermoplasticity before foaming is completely completed, and the foaming is completed in the molding device, and the surface of the metal tape and the highly foamed insulator are simultaneously molded. A method for manufacturing highly foam-insulated coaxial cables. 2. The method of manufacturing a highly foamed insulated coaxial cable according to claim 1, characterized in that the metal tape for the outer conductor is a laminate tape laminated with a heat-fusible adhesive on one or both sides.