Fluorescent Penetrant Inspection - Inspection Steps

Inspection Steps

See the following main steps in a Fluorescent Penetrant Inspection Process:

1. Initial Cleaning:

Before the penetrant can be applied to the surface of the material in question one must ensure that the surface is free of any contamination such as paint, oil, dirt, or scale that may fill a defect or falsely indicate a flaw. Chemical etching can be used to rid the surface of undesired contaminates and ensure good penetration when the penetrant is applied. Sandblasting to remove paint from a surface prior to the FPI process may mask (smear material over) cracks making the penentrant not effective. Even if the part has already been through a previous FPI operation it is imperative that it is cleaned again. Most penetrants are not compatible and therefore will thwart any attempt to identify defects that are already penetrated by any other penetrant. This process of cleaning is critical because if the surface of the part is not properly prepared to receive the penetrant, defective product may be moved on for further processing. This can cause lost time and money in reworking, overprocessing, or even scrapping a finished part at final inspection.

2. Penetrant Application:

The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or defects in the material. The process of waiting for the penetrant to seep into flaws is called Dwell Time. Dwell time varies by material and the size of the indications that are intended to be identified but is generally around 30 minutes. It requires much less time to penetrate larger flaws because the penetrant is able to soak in much faster. The opposite is true for smaller flaws.

3. Excess Penetrant Removal:

After the identified dwell time has passed, penetrant on the outer surface of the material is then removed. This highly controlled process is necessary in order to ensure that the penetrant is removed only from the surface of the material and not from inside any identified flaws. Various chemicals can be used for such a process and vary by specific penetrant types. Typically, the cleaner is applied to a lint-free cloth that is used to carefully clean the surface.

4. Developer Application:

Having removed excess penetrant a contrasting developer may be applied to the surface. This serves as a background against which flaws can more readily be detected. The developer also causes penetrant that is still in any defects to surface and bleed. These two attributes allow defects to be easily detected upon inspection. Dwell time is then allowed for the developer to achieve desired results before inspection.

5. Inspection:

In the case of fluorescent inspection, the inspector will use ultraviolet radiation with an intensity appropriate to the intent of the inspection operation. This must take place in a dark room to ensure good contrast between the glow emitted by the penetrant in the defected areas and the unlit surface of the material. The inspector carefully examines all surfaces in question and records any concerns. Areas in question may be marked so that location of indications can be identified easily without the use of the UV lighting. The inspection should occur at a given point in time after the application of the developer. Too short a time and the flaws may not be fully blotted, too long and the blotting may make proper interpretation difficult.

6. Final Cleaning:

Upon successful inspection of the product, it is returned for a final cleaning before it is either shipped, moved on to another process, or deemed defective and reworked or scrapped. Note that a flawed part may not go through the final cleaning process if it is considered not to be cost effective.