|
Developers
The role of the developer is to pull the trapped penetrant material
out of defects and spread it out on the surface
of the part so it can be seen by an inspector. The fine developer
particles both reflect and refract the incident ultraviolet light,
allowing more of it to interact with the penetrant, causing more
efficient fluorescence. The developer also allows more light to
be emitted through the same mechanism. This is why indications
are brighter than the penetrant itself under UV light. Another
function that some developers perform is to create a white background
so there is a greater degree of contrast between the indication
and the surrounding background.
Developer Forms
The AMS 2644 and Mil-I-25135 classify developers into six standard
forms. These forms are listed below:
- Form a - Dry Powder
- Form b - Water Soluble
- Form c - Water Suspendable
- Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)
- Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)
- Form f - Special Applications
The developer classifications are based on the method that the
developer is applied. The developer can be applied as a dry powder,
or dissolved or suspended in a liquid carrier. Each of the developer
forms has advantages and disadvantages.
Dry Powder
 Dry
powder developer is generally considered to be the least sensitive
but it is inexpensive to use and easy to apply. Dry developers
are white, fluffy powders that can be applied to a thoroughly
dry surface in a number of ways. The developer can be applied
by dipping parts in a container of developer, or by using a puffer
to dust parts with the developer. Parts can also be placed in
a dust cabinet where the developer is blown around and allowed
to settle on the part. Electrostatic powder spray guns are also
available to apply the developer. The goal is to allow the developer
to come in contact with the whole inspection area.
Unless the part is electrostatically charged, the powder will
only adhere to areas where trapped penetrant has wet the surface
of the part. The penetrant will try to wet the surface of the
penetrant particle and fill the voids between the particles, which
brings more penetrant to the surface of the part where it can
be seen. Since dry powder developers only stick to the area where
penetrant is present, the dry developer does not provide a uniform
white background as the other forms of developers do. Having a
uniform light background is very important for a visible inspection
to be effective and since dry developers do not provide one, they
are seldom used for visible inspections. When a dry developer
is used, indications tend to stay bright and sharp since the penetrant
has a limited amount of room to spread.
Water Soluble
 As
the name implies, water soluble developers consist of a group
of chemicals that are dissolved in water and form a developer
layer when the water is evaporated away. The best method for applying
water soluble developers is by spraying it on the part. The part
can be wet or dry. Dipping, pouring, or brushing the solution
on to the surface is sometimes used but these methods are less
desirable. Aqueous developers contain wetting agents that cause
the solution to function much like dilute hydrophilic emulsifier
and can lead to additional removal of entrapped penetrant. Drying
is achieved by placing the wet but well drained part in a recirculating,
warm air dryer with the temperature held between 70 and 75°F.
If the parts are not dried quickly, the indications will will
be blurred and indistinct. Properly developed parts will have
an even, pale white coating over the entire surface.
Water Suspendable
Water suspendable developers consist of insoluble developer particles
suspended in water. Water suspendable developers require frequent
stirring or agitation to keep the particles from settling out
of suspension. Water suspendable developers are applied to parts
in the same manner as water soluble developers. Parts coated with
a water suspendable developer must be forced dried just as parts
coated with a water soluble developer are forced dried. The surface
of a part coated with a water suspendable developer will have
a slightly translucent white coating.
Nonaqueous
 Nonaqueous
developers suspend the developer in a volatile solvent and are
typically applied with a spray gun. Nonaqueous developers are
commonly distributed in aerosol spray cans for portability. The
solvent tends to pull penetrant from the indications by solvent
action. Since the solvent is highly volatile, forced drying is
not required. A nonaqueous developer should be applied to a thoroughly
dried part to form a slightly translucent white coating.
Special Applications
Plastic or lacquer developers are special developers that are
primarily used when a permanent record of the inspection is required.
|
