When it comes to evaluating particle residues remaining on parts after manufacturing, the “go to” is often particle counting. Particles remaining on parts after normal manufacturing operations are collected using a secondary, more aggressive washing method. Collected particles are then tallied by size, number and, in some cases, type and probable origin as a measure of cleanliness. This evaluation can be accomplished using any of several established means which are generally described in ISO 16232, a 180+ page behemoth which, although comprehensive, leaves a LOT of room for interpretation. The specification is actually entitled Road Vehicles – Cleanliness of Components and Systems. It was initially intended and written for assessing cleanliness of road vehicle systems and components. However, it is now being applied in many industries with the requisite “tweaks,” although necessary, sometimes diverting from the intent of the original document. In the next several bogs I am going to try to distill parts of this document that pertain to optical particle counting (“optical” meaning using light) into something which I hope is a little more helpful to those who prefer not to read it in it’s entirety. My goal here is to make the reader aware of the facets of particle counting that may not be immediately obvious to those “skim-reading” the specification or that may only manifest themselves through hands-on experience.
Know What You Are Looking For
Previous blogs (1) (2) described the procedures for gravimetric analysis of non-volatile (particle) residues. Gravimetric analysis provides the weight of the non-volatile residues remaining on the tested surfaces. It is a valuable tool, especially if the residues being evaluated are relatively consistent in composition, size and weight or in cases where the detail is just not important. Grains of sand, glass beads and skin cells are examples of contaminants where gravimetric analysis will tell you just about everything you need to know. Gravimetric analysis can is a good tool to verify that a cleaning process is working effectively from day to day when the contaminants are consistent. Evaluation by particle counting can provide a lot more information.
The most common goal of particle counting is to determine the size and number of particles present. This is of concern because particles that exceed a particular size can, for example, prevent the free operation of closely toleranced parts like pistons and bearings or may have the potential to become lodged in and clog small orifices. In addition to their number and size, the composition of particles may also be of concern. Examples of this include instances where fibers are not of concern while particles that are metallic, and/or those that are magnetic, abrasive or conduct electricity etc. are of concern. The shape of particles (spiral, “C” shaped, flakes, needles, etc.) may be another important concern. In fact, microscopic analysis of a particle can often lead to the identification of its source provided the manufacturing process is known.
The first critical step in particle counting is to know what you are looking for and what makes a difference. “No particles over 400 microns in any dimension” and “no metallic particles over 400 microns” are two completely different requirements. Similarly, if the specification requires no particles over 200 microns in any length then why filter out and count particles that are 10 and 20 microns in size. Particle counting is a time consuming and tedious task as it is, why bother with the things that don’t make a difference?
How About Cost
Before we move on, you should know that the equipment required for particle counting ranges over several orders of magnitude. If you are looking for pea-sized particles, a kitchen strainer will do the job. If the size range of concern is down to about 500 microns and slightly smaller you might want to consider just a good microscope and scaled reticle. Down to 50 microns (and maybe a bit smaller depending the need for accuracy), a good optical microscope (preferably with a motorized stage and particle counting software) will do. Considerably below 50 microns is electron microscope territory.
Finally, particle counting is not an “entry level” job. It is tedious and exacting and requires personnel who will devote time effort to the task. There is also some thinking and decision making involved – unlike running a punch press. Consistency and dedication are important! We’ll talk about that next.