T.O. 33B-1-1
3-121
Table 3-10. Cure Times for Different Quantities of Catalyst.
CATALYST
DROPS PER 30 CC OF MAGNETIC
RUBBER BASE
ESTIMATED
POT LIFE
ESTIMATED
CURE TIME
Stannous octoate
2
7 minutes
30 minutes
Dibutyltin dilaurate
2
7 minutes
30 minutes
Stannous octoate
1
10 minutes
60 minutes
Dibutyltin dilaurate
1
10 minutes
60 minutes
(2)
Permanent Magnets. Appropriate bar magnets are placed to obtain the field strengths
recommended by Table 3-9.
i. Mix, measure and deaerate (only if bubbles in replica are a problem) magnetic rubber base material as
follows:
(1)
Mixing. The magnetic rubber base material must be thoroughly mixed prior to use. Materials
that have settled should be agitated on a mechanical shaker (paint shaker or equivalent). . Steel
balls may be placed in the container containing the magnetic rubber to facilitate thorough
mixing. Prior to measuring or weighing a quantity of magnetic rubber, it should be thoroughly
mixed with a wooden tongue depressor or a spatula. Mixing should continue until the material
contains no streaks or color variations.
(2)
Measuring. The magnetic rubber base material may be weighed or measured volumetrically into
paper cups or other suitable containers. One gram of magnetic rubber base material is equal to
one cubic centimeter (cc) of base material. The number and size of the batches measured must
be based on the area to be inspected. Do not measure more material per batch than can be
poured and magnetized within the pot life of the formula selected. To determine the pot life at
the time of inspection, measure a small trial batch and time the pot life in the mixing cup before
the inspection batch is mixed and poured.
(3)
Deaerating. For the inspection of horizontal holes or upside down surfaces, the magnetic rubber
base material is placed in a vacuum chamber and pumped down to 25 to 30 inches of mercury for
one to two minutes. This will remove excess air and help prevent the formation of bubbles on the
upper surfaces of the cured replicas.
NOTE
The magnetic rubber will begin to thicken when curing agents are added.
Therefore, magnetization must begin immediately and the entire batch must be
magnetized before the pot life of the formula has expired.
Magnetic rubber material, catalyst addition, and cure time are based on a room
temperature of 76°F. The cure times are very unpredictable when the temperature
is below 60°F or over 90°F.
When inspecting holes with scored surfaces, holes that are deep and of small
diameter or unusual configuration, the inspection area may be coated with a thin
layer of petrolatum to aid in removal of replica.
j. Add to the magnetic rubber base material the correct number of drops of both catalysts using the criteria
in Table 3-10.
k. Using a tongue depressor or equivalent, thoroughly stir the mixture. Avoid whipping air into the
mixture.
