Erasing Particles with Ultrasonics

Removing particles from machined and fabricated parts is a must for productivity and profitability. Particles interfere with coating adhesion and appearance, and can also decrease product quality and performance. An exercise conducted by participants at the recent Product Quality Cleaning Workshop sponsored by Gardner Business Media illustrates simple, effective methods to test how well cleaning processes can remove particles from high-value products.

The exercise used a graphite removal test to indicate ultrasonics functionality, and showed that ultrasonic cleaning can erase particles from surfaces. Participants marked the frosted end of a glass slide with a pencil, then submerged the marked slide in an aqueous cleaning agent in the bath of an ultrasonics tank heated to 40ºC. Immersion alone did not have any immediate, visible effect, however, the mark—particles of graphite—disappeared within seconds after the ultrasonics cleaner was activated.

An aluminum foil test is most commonly used to detect ultrasonic functionality, especially at lower frequencies, but it is generally a visual, qualitative indicator that the ultrasonics work. We know of no standard test method for the foil test. Typically, for 40-kHz tanks, an “orange peel” erosion pattern is visible on regular weight, commercial aluminum foil after 30 seconds of exposure.

While the graphite removal test is probably less well -known than the aluminum foil test, a number of participants in the workshop latched on to this idea. Why would using ultrasonics essentially as a pencil eraser produce such interest among rather sophisticated participants? One reason is simply that particles on product surfaces are a persistent pain in the posterior. Another is that, at frequencies above 100 kHz, the foil test ceases to be a reliable indicator of ultrasonic activity, but the graphite removal test still works well.

Particles on product surfaces must be minimized in applications ranging from metal finishing to optics, in industries from aerospace to automotive to medical devices. Large particles can result in visible defects in coatings, can interfere with movement of bearings or critical components, or can clog nozzles. Small particles, even submicron size, can render critical products non-functional.

Particles sprinkled on a clean surface can sometimes be dusted or rinsed off, especially if the surface is otherwise clean and if the action is taken immediately. But will this happen reliably, and will all particles be removed? Nope.

In actual manufacturing conditions, particles have an unfortunate desire to cling to surfaces. Small particles are more difficult to remove than large ones. This is in part because of the influence of non-polar (dispersive) forces. This influence may seem counter-intuitive, especially in applications such as plating, in which electrochemical processes do the work, or in electronics assembly, where conductivity is an issue, and in high-purity water. To understand the importance of non-polar forces, consider the gecko. A gecko climbs walls and travels across ceilings not because its feet exude some sort of adhesive, but rather through the non-polar forces between the tiny cilia or hairs on its feet and the wall. Fine particles (of 5 microns and smaller), especially dried-on particles, behave like gecko feet and adhere to surfaces tenaciously through non-polar forces.

In addition, manufacturing processes often involve mixed soils of varying composition with particles embedded in thin film. The chemical nature of the soils and the size distribution of the particles may not be well-characterized. The types of soil may vary for a given product, especially if a number of subcontractors/fabricators are involved. This adds to the challenge of particle removal, and it is why some manufacturers choose to clean with both solvents and aqueous cleaners. The polarity of the surface itself is important, so some cleaning protocols involve alternating between cleaning with acids and bases to “wiggle” the particles off the surface.

Proving that all (or almost all) particles have been removed is an even more vexing challenge. With any solvent, repeated extractions will continue to remove particles, although usually at a lower rate. Eventually (although sometimes right away), parts of the product may be extracted; that really muddies the waters (pun intended).

Switching to another extraction solvent may result in an upswing in the number of particles removed, as a different solvent may have a different mix of forces and may be more effective at removing certain particles. For this reason, current Food and Drug Administration guidance for residue on medical devices includes extraction with multiple solvents (at least two) that differ in polarity. Choosing the right extraction solvents (including water as a solvent) becomes a judgment call; that judgment call has to be based on logic and at least a modicum of testing.

This brings us back to testing and building methods. Ultrasonic cleaning, including cleaning at high frequencies (100+ kHz) are important for fine particle removal. Simple tests, such as graphite removal, are visual and subjective. They are also practical, quick, inexpensive and therefore very useful.