A method of manufacturing an insulative material cellular insulator around a conductor, said insulator having a longitudinal passage in which said conductor is housed and closed cells extending longitudinally and separated from each other by radial walls, said method comprising the following steps:
- extruding said insulative material in the viscous state to impart the required shape to it by means of shaping means to form said cells,
- applying said insulative material thus shaped to said conductor, and
- cooling said insulative material to obtain said insulator,
- wherein said applying step is performed at a distance from the exit of the shaping means such that said material is drawn down sufficiently for the walls of the cells not to be thinned without pressurizing said cells.