September 2008

Apply dry developers the right way

I- Introduction

Dry developers are used in penetrant testing in conjunction with fluorescent penetrants only.

In some cases, the fluorescent penetrant is nevertheless used without any developer.

It is the case, for example when looking for:

- Cracks on polished ceramics, rubbers and thermoplastic and thermosetting macromolecules of synthesis.

- Cracks, shrinkage, porosities and puckering on aluminium castings, produced in mass production in automotive industries, (such as engine blocks) and critical components (such as suspension arms and brake calipers) in automotive manufacturing.

- More anecdotal, fingerprints, method used by the Royal Canadian Mounted Police (RCMP) in addition to the cyanoacrylate ester process.

II- Why a dry developer?

Quite as with radiographic films, developer is of the utmost importance in Penetrant Testing, for a reliable detection of the finest and most critical discontinuities.

Quality is a direct consequence of the layer's application.

Several "forms" of developers may be used with fluorescent penetrants: water-based (either soluble or as a suspension in water), solvent-based (non-aqueous wet developers, i.e. NAWD), the only ones used with visible penetrants, and dry developers, which can be used only with fluorescent penetrants.

As a consequence a right application conditions the results. This paper will focus on dry developer which plays, above all, an amplifying action (1).

Fluorescent penetrants are seen against a very dark background, so that the brilliant green or yellow-green indications are easily seen by the inspector's eyes, given that the viewing conditions are good. A dry developer does not need to cover the entire surface with a white coating, as this is mandatory with visible penetrants (red-dye penetrants, colour-contrast penetrants, wording depending on the documents). An NAWD shall be sprayed as a thin, even coating. A developer used with fluorescent penetrants viewed in a UV-A booth, shall be completely invisible.

Applying dry developer is not as easy as it appears to non specialists. The coating shall be very thin on every square centimeter of the part, almost invisible in plain white light; there shall be no area without particles. NAWD are more sensitive, but when parts are complex, it is very difficult to spray them as required. That's why dry developer is generally the developer of choice in FPI (fluorescent penetrant inspection) installations.

III- Important requirements

Primes' specifications require some daily or weekly checks mainly if the powder is used in a tank: absence of fluorescence is checked using a 10 cm diameter disk (other specifications may mention a surface of 100 cm², which leads to a less severe test, due to the 78,54 cm² disk surface). This disk is processed along the parts.
When inspected, the disk shall show less than 10 fluorescent specks. This is mandatory in ASTM E 1417-05, paragraph 7.8.2.7, and specified in NADCAP questionnaire AS7114/1 paragraph 5.6, as well as in numerous other documents. The inspector shall also check any lump, which is an alert for moisture having entered the developer.

Some primes may have supplemental requirements.

IV- How to apply dry developer on parts?

Dry developers are light and fluffy powders. There are several application methods, depending on the parts shape, size, number, whether there is an automatic or manual process, etc. Following are the main methods:

-          The pear shaped powder dispenser.

-          The powder gun.

-          The electrostatic spraying.

-          The dust storm cabinet.

As a matter of fact, some primes accept parts’ immersion in the bulk dry developer. This process shows a risk of contamination of the dry developer by fluorescent flecks due to possible penetrant dragouts by parts.

V-  A survey of the different methods

V-1 Pear shaped powder dispenser and powder gun:

Both are rarely used, as:

- They do not allow for applying a very thin layer of dry developer. There is too much developer on the parts, and the excess of powder shall be removed, generally with a gentle-- but often, not so gentle!- air flow. Some developer may carry out penetrant from discontinuities; reliability may be impaired.

- They spray in the air some white particles which, not only may lay everywhere in the surroundings, but also may make breathing uneasy; hence the inspector shall wear a dust-proof mask.

So, these processes are used only for local inspection when an NAWD cannot be used, for instance due to chemical compatibility with inspected materials.

V-2 Electrostatic spraying

Initially designed for parts of large dimensions, it is now largely used, in aerospace industries, for inspections of:

- Engine components: engine casings, rotors, stators, turbine disks, etc.

- Structural parts: airframes, wings, etc.

At the end of the 80s, appeared, in aerospace industries, the first multi- sensitivity levels automatic FPI process line, controlled by a programmable controller. (2).

Diverse engine components of smaller dimensions were processed in these lines which integrated the penetrant and developer electrostatic spraying techniques. The spray guns, put in lidded-tanks, were affixed on "raise-and-lower" equipments; parts or baskets of parts were put on rotating tables.

As explained in the following chapter, this system won some criticism for unreliable results from the dry developer.

V-3 Dust storm cabinet for dry developer

Even in the first FPI installations, dry developer is applied in dust-storm cabinets.

In France the first "indexing system" automatic line was commissioned in 1973 by an aeroengines manufacturer for inspection of compressor and turbine blades.

This process line, very similar to that used by a British aeroengine manufacturer, was with a dust-storm cabinet comprising a vertical-axis fan for powder motion.

The design is very simple: a stainless steel tank with a lid. A conical basis houses the fan, a support grid for parts or baskets containing parts and a hose for air injection.

In the '70s units, the back side of the tank was fitted with an air filter, diameter ca 30 cm, 4 cm thick, made of expanded polypropylene. This filter was designed to balance inside and outside pressures, as well as to stop any particles more than 5 µm large. But, if you bear in mind that dry developer particles may be as small as 1/100 µm, filter clogging had to be considered. To prevent clogging, a thin tissue sheet was put in front of the filter and changed for a clean one on a more or less regular basis.

Nowadays three different designs for dry developer application are used:

- Electrostatic spraying.

- Dry oil-free compressed air flow in a tank.

- Fan-based tank.

The effectiveness of dry powder application is currently a VERY IMPORTANT TOPIC in the US aerospace community. Though Patrick DUBOSC draw the attention of some 100 attendees in a 2002 ASNT conference (3) about the "air-flow in a tank"  design failure, and suggested to go to the fan-based tank design, the last report by Lisa BRASCHE in Materials Evaluation(4) about dry developer application shows no test was carried out in the US on this latter design!

We explain later in this paper why the widely-used "standard" air-flow system is maybe...the worst answer to the problems met by the Aerospace industry at large in this area.

V-3.1- Electrostatic spraying (ES)

There are two ways of using ES: either in a tank, or out of any tank. In a tank, the system is always automatic. Out of a tank, it may be automatic or manual. In some installations, both ways are available: if the automatic process fails, manual application is used, to prevent loss of production. The line is then in a ‘‘downgrade’’ mode.

Using an ES system in a tank is....peculiar!!!  Keep in mind that the molecules (of developer, or of penetrant, if we enlarge the thinking) electrically charged with a30 kV to 90 kV voltage when pushed by compressed air through the gun/electrode MUST go to the CLOSEST grounded surface.

That means that, even if the parts/baskets are grounded, in fact, as the walls of the tanks are also grounded, the MAIN PART of the sprayed product is attracted first by the walls! The ES gun shall deliver MUCH MORE product than theorically needed to give rise to a "cloud" of the product which finally goes to the parts. This "cloud" could be made by a more economical means: for penetrants, a compressed-air gun would be OK! For dry developer, ES in a tank is not, in fact, so much used.

Out of a tank, but in a specific area (fluffy powder may enter the users' respiratory tract, leading to breathing uncomforted), ES may be used, on the condition that parts' shape is not too complex.

V-3.2- Dry oil-free compressed air injection

Compressed air injection has several drawbacks:

- The tank is NOT designed to withdraw the pressure due to the compressed air. Air MUST leak, must "escape", taking some white powder with it. Very often the working area around a dry developer tank is covered with a light coating of developer!

- When pressure is suddenly lowered from 600 kPa (6 kg/cm²), the standard pressure in compressed air networks, down to the atmospheric pressure, some moisture appears (for reasons linked to liquid/vapour equilibrium): this moisture degrades developer aspect and efficiency.

- Air leaks, through either rings or a specific area, balance the internal pressure with the atmospheric pressure. Air flow takes the finest particles out of the tank, hence only the heaviest particles stay in the tank: the developer formula is modified, performance is impaired.

- Some designs lead to "snowdrifts" at the bottom of the tank, with sometimes up to 1kg of unusable "powder". When tests are carried out on this "piled" powder, results parameters show it shall be discarded. Though dry developer is not expensive, better to use it the right way instead of having to get rid of it!

V-3.3- Vertical-axis fans in a tank

Dust-storm cabinets coming with vertical-axis fan(s) give reliable results either in very small installations or in very large ones.

Some "cabinets" are up to 13 meters (more than 14 yards) long, 3 meters (more than 3 yards) deep and 2.5meters (almost 3 yards) large. These have 4 vertical-axis fans at the bottom of the tank; these fans move the powder for just 10 seconds. Dry developer consumption is only 3 to 4 glasses per 8-hour shift.

The operator checks developer's efficiency with ISO 3452-3 standard Type I test panels put in different positions in several points of the tank.

The main advantages of this tank design come as follows:

- Very few particles go out of the tank.

- A very thin coating on the parts, hence very few particles taken out of the tank.

- Almost all the powder put at the beginning of the shift is used. An add-on may be needed when the quantity of parts inspected is unusually high or when parts are very complex, especially for aeroengine maintenance.

- Brand new dry developer is used for any new shift; this leads to a more stable performance and quality; very few concerns due to moisture; cheap equipment, easy to use.

VI- The end!

It is worth to point out that this design, some 25+ years old, by Patrick DUBOSC, has been substantiated by a lot of examples. Further some cabinets with the compressed-air design have been modified to build the fan system in.

Bibliography

(1) P.I.BRITTAIN, The Amplifying Action of Developer Powders, Proceedings of QUALITEST III Conference, Cincinnati. 1984.

(2) Pierre CHEMIN, Modern Engineering Achievements in the field of automatic fluorescent penetrant inspection process lines, 12th World Conference on Non Destructive Testing, Amsterdam, April 23-28, 1989.

(3) Patrick DUBOSC, Dry powder application - The French Paradox, ASNT Fall Conference and Quality Testing 2002, San Diego, California, USA, November 4-8, 2002.

(4) Lisa BRASCHE: TNT supplement in Materials Evaluation Jan 2008 issue, Materials Evaluation, 1711 Arlingate Lane, PO Box 28518, Columbus, OH 43228-0518, USA.

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We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.

We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.