Before we leave the subject of particles and particle counting, there are some things we need to touch on regarding “exceptional” particles. In an earlier blog, Particles That Don’t Count?, we discussed briefly some particles that may or may not count when it comes to analyzing the cleanliness of a part. These and several other kinds of particles as well as the sources for and bonding mechanisms that may bind these and other such particles to part surfaces fall into the category of exceptional particles. “Exceptional” particles are particles that are difficult to remove and/or are difficult to find and evaluate. Some are meant to be there, some aren’t meant to be there but won’t do any harm, and some are of critical concern but resist harvesting for particle counting and analysis using conventional means. In the next few blogs, we will delve a little deeper into particles that fall into this “exceptional” category.
Most, if not all, cleaning technologies have a difficult time overcoming magnetic attraction of particles to surfaces. If both the particle and the surface have magnetic properties and either or both are magnetized, permanently separating them in a cleaning scenario is a nearly impossible task. Even if particles are initially removed, they are often drawn back to the surface by the magnetic attraction which remains. The cleaning process most likely to remove particles held by magnetic attraction is spray washing. In spray washing, the cleaning or rinsing solution, once it had impinged on the surface being cleaned, moves away from the impingement site taking the particles with it. Separated from the surface, they can then be removed by media and/or magnetic filtration.
We all know that a body is magnetized by moving it through a magnetic field. Clearly, there are lots of sources of magnetic fields in the manufacturing environment. Good examples of magnetic fields are those created by electric current, magnetic chucks and magnetic conveyors. Less obvious is the fact that bodies can be magnetized as a result of shock, stress, differential heating and cooling and several other incidental operations that one would not normally associate with magnetization. Even the ever-present magnetic field of the earth can not be discounted in some critical applications. So, it can not be assumed that just because nothing was done to intentionally magnetize a part that magnetism is not present. By the way, even “non-magnetic” stainless steels may exhibit sufficient magnetic properties to be troublesome in critical applications.
Magnetism is detected by a “gauss meter.” A gauss meter, essentially, is a coil of wire attached to a meter that detects electrical current or voltage. When a magnetized body is moved through the coil, its magnetic field produces an electrical response in the coil of wire that can be detected by the meter. The meter is calibrated in Gauss or Lines of Magnetic Flux, both of which are measurements of magnetism. A gauss meter works well for objects with relatively large mass. However, there is no limit to how small a particle of matter can be magnetized. The magnetism of small particles can not be measured using means available to most manufacturers. Remember, however, that only one of the two bodies with magnetic properties need be “magnetized” for the two to be attracted to one another. A magnetized particle is just as attracted to a magnetic surface as it would be if the surface itself was magnetized.
In many cases, parts with magnetic properties are demagnetized after manufacturing and prior to cleaning to help prevent magnetic attraction between particles and the surface being cleaned. Demagnetizers are a common option on most cleaning systems and are sometimes called “demags.” Demagnetizers are, basically, devices with coils of wire energized by alternating current which neutralizes the magnetic effect in parts as they pass over, under or through the device. Demagnetization requires that parts to be demagnetized be properly placed with relation to the demag unit (proximity and, in some cases, orientation must be considered) and may also require a minimum time exposure to be effective. It is important to know that demagnetization does not guarantee that all potential for magnetic attraction between particles and surfaces has been eliminated. It is easy for cleaning systems to become contaminated with magnetized particles. This is especially true in cleaning systems used to clean a variety of parts, some of which may not have been demagnetized prior to cleaning.
Guarding against magnetism as a deterrent to cleaning is a critical concern and should be taken seriously. A magnetic filter for cleaning solutions and rinses used in combination with proper demagnetization procedures is good “insurance” when the potential for magnetized particles exists in a cleaning system. In some instances, the use of a demag prior to cleanliness testing, or a test like the Scotch Tape Test described in a previous blog (or a more sophisticated but similar test using an adhesive) is the best way to assure that magnetic particles have not evaded the cleaning process.
– FJF –