Table 3-10. Cure Times for Different Quantities of Catalyst.

Figure 3-77. Typical Use of Gaussmeter Probes

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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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.

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