Fluorescent Penetrant Inspection Systems Case Study


Fluorescent Penetrant inspection is a widely applied method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). FPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

FPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet light.

1. Section of material with a surface-breaking crack that is not visible to the naked eye.
2. Penetrant is applied to the surface.
3. Excess penetrant is removed.
4. Developer is applied, rendering the crack visible

Below are the main steps of Fluorescent Penetrant Inspection:

1. Pre-cleaning:

The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination. Surface Preparation prior to liquid penetrant application in accordance with ASTME 1417 states, "All surfaces to be examined shall be clean, dry, and free of soils, oil, grease, paint and other coatings, corrosion products, scale, smeared metal, welding flux, chemical residues, or any other material that could prevent the penetrant from entering discontinuities, suppress dye performance, or produce unacceptable background."

2. Application of Penetrant:

The penetrant is then applied to the surface of the item being tested. The penetrant is allowed time to soak into any flaws. The soak time mainly depends upon the material being testing and the size of flaws sought.

3. Excess Penetrant Removal:

The excess penetrant is then removed from the surface. Removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices. This process must be performed under controlled conditions so that all penetrant on the surface is removed (background noise), but penetrant trapped in real defects remains in place.

4. Application of Developer:

After excess penetrant has been removed a white developer is applied to the sample. Developer should form a thin, even coating on the surface. The developer draws penetrant from defects out onto the surface to form a visible indication, a process similar to the action of blotting paper. Any colored stains indicate the positions and types of defects on the surface under inspection.

5. Inspection:

The inspector will use visible light with adequate intensity (100 foot-candles is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye, but this should not be done when using fluorescent penetrant. Also of concern, if one waits too long after development the indications may “bleed out” such that interpretation is hindered.

Semi-Automated System

This Semi-Automated System consists of five steps - Penetrant, Dwell, Spray Rinse, Dry, and Developer stages . It processes a complete family of parts used for aerospace. The line is constructed from four stainless steel modular MK Systems and runs left to right. The penetrant stage, below left, is isolated and includes a long connecting conveyor with drain-back line for collecting penetrant. In this particular application two different penetrants are used and are isolated using separate penetrant tanks and dwell conveyors. The penetrant tanks are made on casters and the dwell conveyor sections free standing for ease of mobility.

The Spray, Dry and Developer Stations, photo below, is built upon a heavy duty I beam fabricated base for accuracy, stability, ease of shipping and installation. The control pedestal (far right) includes a main Nema12 panel.

Fully Automatic Dye Penetrant Systems

This RAMCO Fully Automatic Dye Penetrant System was designed for ceramic bearings. The bearings are processed in batches of 100 (shown below) in a fully automated system using RAMCO transport/oscillating elevators as the basic building block for each of the stages. The bearings are processed through penetrant application, excess penetrant removal and hot air knife dryer as per specification. Exclusive RAMCO features include Multiple Rollover, MultiPort Oscillation, RamTough Automation, and MultiZone Spray. RAMCO can design manual or fully automated systems to accommodate water soluble or oil based penetrant processes.


The operator loads a four tray carrier at the beginning of the line and unloads at the end of the line ready for inspection. A multi zone zero pressure power conveyor is used to pre-stage and feed carriers into the automation system. The entire process automated using RamTough Automation. Each stage is a standard modular RAMCO MK Series Immersion System that has been modified to fit the application. Automatic opening and closing covers are included on each stage. The bearings are immersed in a water soluble dye penetrant and allowed to drain under the closed cover for a preset cycle time.

RAMCO has built many more Fully Automated and Semi-Automated FPI systems. Please consult our factory for further information.


This Fully Automatic Dye Penetrant System incorporates a number of key RAMCO attributes. Transport/oscillating elevators are the basic building blocks for each of the stages. Exclusive RAMCO features include MultipleRollover, MultiPort Oscillation, RamTough Automation, and MultiZone Spray. RAMCO can design manual or fully automated FPI systems to accommodate water soluble or oil based penetrants