T.O. 33B-1-1
3-114
3.8.2.2.4
Alginate Impression Compound Method.
The alginate impression compound method of "lifting" magnetic particle indications is a method of securing
indications in areas that are inaccessible and cannot be viewed with a black light.
3.8.2.2.4.1
Alginates are hydrocolloid polysaccharides derived from seaweed kelp. Compounds such as those used for making
dental impressions are based on mixtures of potassium alginate, calcium sulfate, sequestering agents such as sodium
phosphate and fillers such as silica, diatomaceous earth, or calcium carbonate. When the compound is mixed with the
correct amount of water it forms a soft paste which sets up to a rubbery solid in three to four minutes. This rubbery
material or gel has the property of accurately conforming to and taking an impression of the surface to which it is
applied and also absorbing or lifting traces of particulate material from the surface. This latter property is the basis for
its use as an indication lifting material.
3.8.2.2.4.2 Transfer Of Magnetic Particle Crack Indications to Alginate Impression Compound.
a. Perform the magnetic particle inspection of the area of interest in the usual manner.
b. The part does not have to be dried before taking an impression.
c. Using the plastic scoop and water measuring container, follow the directions given on the can of powder
and mix the powder with water to obtain a smooth creamy paste.
d. Transfer the paste immediately to a piece of thin polyethylene film and then apply the paste to the
inspecting area. Gently press against the film to obtain a uniform contact of the paste against the
inspection area. Avoid excessive working of the paste to avoid smearing of the indication. The plastic
film prevents the paste from sticking to the hand. For cavities such as holes, the paste can be applied
without the polyethylene film to form a plug when set.
e. After the paste has set to a rubbery gel in about 3-4 minutes, gently remove the replica from the metal
part and examine under ultraviolet light. The replica may be photographed with ultraviolet light it
desired.
3.8.2.3
Photographing Indications.
Photographs of indications can also be taken to be used for records. Enough of the part should be shown to make it
possible to recognize the part and the position of the indication. It is helpful to include in the picture some common
object to show the size of the part. Sometimes this can be done with a finger pointing at the indication or by placing a
ruler along the part to show relative size. In photographing indications on highly polished parts, care must be taken to
avoid highlights or reflections that may hide indications. Taking photographs of fluorescent indications calls for
special photographic techniques, refer to the penetrant chapter, Chapter 2, of additional information.
SECTION IX
MAGNETIC RUBBER INSPECTION METHOD
3.9
MAGNETIC RUBBER INSPECTION METHOD.
3.9.1
Description.
a. Magnetic rubber inspection (MRI) is an extension of magnetic particle inspection and is a
nondestructive inspection technique used for detecting cracks or other flaws on or near the surface of
ferromagnetic materials. Its principal applications are in certain problem areas, such as, (l) areas
having limited visual accessibility (inside holes, tubes, etc.); (2) coated surfaces, (3) complex shapes or
