Application of Emulsifier.
Lipophilic emulsifier is used as supplied by the manufacturer. It usually is applied by dipping or immersing the part in
a tank of emulsifier. Application by spraying or flowing the emulsifier is not recommended. The two major problems
with spraying are the difficulty in applying a uniform thickness and the difficulty of applying enough emulsifier
without the mechanical force of the spray scrubbing the penetrant layer. There are a few automated systems where the
emulsifier is applied as a fog. Emulsifier SHALL NOT be applied by brushing or wiping. Brushing or wiping
produces an uncontrolled and uneven mixing action.
Emulsifier Drain / Dwell.
When the part surface has been coated with emulsifier, the part SHALL be removed from the liquid and allowed to
drain. The part SHALL NOT remain in the emulsifier during the dwell period. Care must be exercised to prevent
pooling in cavities during the dwell. Immersion dwell would nullify one of the modes of emulsifier action. It was once
thought that emulsification occurred only through the chemical action of diffusion. It is now recognized that two
modes are involved. The first mode occurs as the emulsifier drains from the part surface during the dwell period. As
the emulsifier drains, the movement carries with it considerable surface penetrant. This scrubbing or mechanical
action reduces the amount of penetrant to be emulsified and also initiates the chemical or diffusion action. Without this
mixing action, emulsifier dwell time might be as long as ten or twenty minutes.
When a number of parts are being inspected, they SHALL be processed one at a
time through the emulsifier, emulsifier dwell and wash steps unless they are small
enough to be batch processed. Excessive dwell will occur when emulsifier is
applied to a number of parts and they are then individually washed.
After emulsifier has been applied and the part is draining, a period of time is allowed for diffusion. During diffusion, a
water removable colloidal mixture is being formed. This is the emulsifier dwell time and is one of the most critical
factors in the lipophilic process. A timing device is required to control this process. The objective is to stop the
diffusion when the emulsifier has just reached the part surface and before it diffuses into any penetrant entrapped in a
discontinuity. Penetrant without emulsifier resists removal. If the dwell time is too long, the emulsifier will diffuse
into entrapped penetrant that is easily removed causing loss of sensitivity and missed flaws. If the time is too short, the
thin layer of surface penetrant not emulsified will cause an excessive background that can obscure a discontinuity
indication. A number of factors, which influence the dwell times, are discussed in the following paragraphs.
Although emulsifier dwell time is critical for most defects, the large number of influencing factors makes it impossible
to develop a general dwell timetable. Optimum emulsifier dwell time must be determined on each part by experiment.
Even here, dwell times may require adjustment to compensate for local conditions. At the extreme, dwell times may
range from 10 seconds to 5 minutes; however, typical dwell times are less than 1 minute. Under no circumstances shall
the emulsifier dwell time exceed 5 minutes. The lipophilic emulsion step does not tolerate deviation from the optimum
dwell time. A relatively short over-emulsification time of 10 seconds on a 1 minute dwell period can result in failure to
indicate small flaws. Figure 2-19 are cracked chrome plate panels showing the effects of insufficient, optimum and
excessive emulsifier dwell.