Differential transmission cable and method of manufacturing the same

1. A method of manufacturing a differential transmission cable, the method comprising:

providing at least one pair of inner conductors arranged side by side and extending parallel to each other;

accumulating or circulating a temperature gradient alleviating liquid of 95 degrees C. to 130 degrees C. in or through a screw of a single screw extruder so as to planarize a temperature gradient of a resin material to which a chemical foaming agent is added; and

extrusion-coating and molding the foamed resin material around the inner conductors together, to thereby arrange a foamed insulating material around the inner conductors,

wherein a variation of a degree of foaming of the foamed insulating material is within 1%.

2. The method according to claim 1 , wherein a vicinity of an extrusion opening is controlled at 190 degrees C. to 230 degrees C.

3. The method according to claim 1 , wherein the single screw extruder is being provided with a multiplicity of cylinder temperature controlling blocks.

4. The method according to claim 3 , wherein the cylinder temperature controlling blocks are controlled at their respective discrete temperatures.

5. The method according to claim 1 , further comprising:

arranging an outer conductor so as to coat the foamed insulating material; and

arranging an insulating jacket so as to coat the outer conductor.

6. The method according to claim 1 , wherein a resulting cable has a skew of not more than 3 ps/m.

7. The method according to claim 1 , wherein the variation of the degree of foaming is defined as a difference between a maximum value and a minimum value among foaming degrees of the foamed insulating material at positions in a longitudinal direction in an arbitrary part cut out from the differential transmission cable.

8. The method according to claim 1 , wherein the variation of the degree of foaming is defined as a difference between a maximum value and a minimum value among foaming degrees of the foamed insulating material at 20 positions at intervals of 50 cm in a longitudinal direction in an arbitrary part of 10 m cut out from the differential transmission cable.

9. The method according to claim 1 , wherein the foamed insulating material includes air bubbles formed by the chemical foaming agent.

10. The method according to claim 1 , wherein the extrusion-coating and molding of the foamed resin material are performed collectively by a collective extrusion.

11. The method according to claim 10 , wherein the collective extrusion includes a chemical foaming to form air bubbles by kneading the chemical foaming agent in the resin material to the foamed resin material.

12. The method according to claim 11 , wherein the collective extrusion further includes generating a gas by a thermal decomposition of the chemical foaming agent in the resin material.

13. The method according to claim 1 , wherein the extrusion-coating and molding includes a chemical foaming to form air bubbles by kneading the chemical foaming agent in the resin material to form the foamed resin material.

14. The method according to claim 13 , wherein the extrusion-coating and molding further includes generating a gas by a thermal decomposition of the chemical foaming agent in the resin material.

15. The method according to claim 1 , wherein the chemical foaming agent has a decomposition temperature of 190 degrees C. to 220 degrees C.

16. The method according to claim 1 , further comprising:

forming an inner skin layer between a surface of the inner conductors and an inner periphery of the foamed resin material.

17. The method according to claim 16 , further comprising:

forming an outer skin layer on an outer periphery of the foamed resin material;

forming an outer conductor on the outer skin layer; and

forming an insulating jacket on the outer conductor.