While the rubber cures, the surface inspected must stay in the required magnetized state. This can be accomplished
using a permanent magnet, a direct current yoke, an electromagnet, or some other suitable means. Whatever method of
magnetization is used, the leakage fields at any discontinuities on the surfaces inspected must be maintained long
enough to attract and hold in position the magnetic particles until a partial cure takes place. A two-step magnetizing
procedure has been developed. The first magnetization is accomplished for a short time in one direction, followed by a
second at 90° to the first for the same length of time. This procedure must be repeated for whatever period of time is
needed until the cure prevents particle mobility. Magnetization in two directions 90° apart assures formation of
indications at discontinuities in all directions.
After curing, the rubber plugs, which are exact replicas of the surfaces, are removed and visually examined for
indications, which will appear as black lines against the gray or yellow background of the silicone rubber. Location of
any discontinuities or other surface imperfections can be determined from the location of the indications on the plugs.
The magnetic rubber inspection method is covered in detail in Section 9.