Providing plastic insulation on an electrical conductor in which the conductor, enclosed in an oversize tube, is passed through a fluid and through an annular crystal of a piezoelectric material immersed in the fluid. During passage of tube and conductor, the crystal is subjected to electric current to cause alternating expansion and contraction of the crystal. Pressure shockwaves are produced by this expansion and contraction and the shockwaves are focussed upon the pass line of the tube and conductor to cause localized inward deformations of the tube into engagement with the conductor to form spaced annular depressions in the tube. In preferred arrangements, apparatus has an extruder immediately upstream of the crystal so that the plastic tube is in a heated and softened condition during its deformation. A focussing means is required in the apparatus for focussing shockwaves on the pass line. In a practical arrangement, the focussing is provided by an annulus of wave transmitting material which has a concave inner surface facing the pass line.