| Does one pressure test before or after impregnation?
The answer to this is dependent upon the experience gained from a batch of actual castings. Obviously, pressure testing involves the use of labor and, therefore, adds to the cost. It may, therefore, be decided to forego the initial pressure test, impregnate all of the batch of castings, and then test after impregnation or eliminate testing completely, depending upon percentage of porosity experienced. For instance, a well known manufacturer of brake parts has a raw casting leak rate of 20%. He impregnates 100% and with 95-100% sealing of leakers, this results in 0-1% final reject rate. Where oil is used as a testing medium, it is best used after impregnation as it is extremely difficult to remove the oil from the porosity in a casting.
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