Cleaning is inherently disruptive. Time, Action, Chemistry, and Temperature (TACT) combine to form the disruption to achieve optimum product cleaning. It’s easy to get stuck on a single aspect of TACT, like the chemistry. The cleaning agent alone won’t get the job done. In this article, we focus on options for Action. Physical cleaning action, including physical force, is important to dislodge soils from surfaces. Force enhances momentum of the cleaning medium (or cleaning chemistry). Cleaning action may not involve brute force; it may involve movement to increase access to surfaces.
Let’s look over the many types of cleaning action. Strategic disruption can make your current cleaning process, or a new process you may be considering, far more effective.
When we think about cleaning agents, typically we think of a liquid, be it solvent or aqueous. Even with an aggressive solvent, simply submerging a part and letting it rest is often not very effective. The cleaning agent must reach the surfaces of the part to achieve effective cleaning. Here are some physical cleaning forces that allow the cleaning agent to reach more surfaces and to remove soil faster and more completely.
Stirring, either stirring by hand with a rod or with a magnetic stirrer, provides relatively gentle agitation of the cleaning agent. We see the technique most often in lab-scale cleaning processes associated with prototype products. Typical problems are consistency and scale-up. It is not sustainable to have a roomful of techs wielding stirring bars in a valiant attempt to clean the product; and we’ve seen such situations! If you are designing a new product, design a cleaning process that is effective, repeatable, and that can be scaled-up.
The parts or basket of parts are moved circularly. Varying the orientation of components provides more effective contact with the cleaning media. It can be used in line-of-sight processes; and it can also enhance effectiveness of ultrasonic cleaning.
The parts or components are moved up and down. Stroke can provide all of the cleaning force, or it can be used with other cleaning forces. For example, stroke can reduce variation due to nodes or antinodes in ultrasonics.
Spray is a line-of-sight technique. It can be in air, like a fire hose or spray bar in a dishwasher. It can be under liquid, like the jets in a jacuzzi. The energy of spray can be used to overcome surface tension effects. One example is in-line aqueous systems, for cleaning under components of electronics assemblies.
One way to produce spray or enhanced fluid velocity is through eduction, ejecting liquid under pressure. Eduction uses the Bernoulli effect to provide directed fluid flow. Eduction can be used in air or submerged in liquid.
Intense, high frequency (>20 kHz) sound waves disrupt a liquid. At frequencies below 500Hz, the primary mechanism is cavitation, a tearing apart of the liquid, creating voids or “vapor bubbles” during the high amplitude portion of a sound wave. During the subsequent low amplitude portion of the wave, the voids collapse. The shock wave from the collapse provides the cleaning action. This is largely an omni-directional effect. Nodes and antinodes can alter the effects. These can be mitigated by stroke or rotation. They can also be mitigated by a small variation of the frequency, called “sweeping”.
Megasonics is similar to ultrasonics but at higher sound frequencies, up to one megahertz or higher. The primary cleaning mechanism is acoustic streaming, which is gentler than the shock waves from ultrasonic cavitation. Megasonics is largely a line-of-sight process. One example of a megasonics application is to remove particles from semiconductor wafers.
It can be difficult to clean the interior of very long, narrow tubes. Flushing involves forcing a fluid through a tube to clean or rinse the interior. At lower velocity and higher viscosity, fluid flow tends to be laminar, in smooth, orderly, layered pathways. In many applications, laminar flow is desirable. Under such conditions, the boundary layer effect limits the ability of the moving fluid to reach the surface to be cleaned. For cleaning, we need to achieve higher, turbulent flow.
Gas bubbles in the boiling liquid provide agitation. In a constrained space, such as inside a tube, the boiling gas creates pressure which forces any fluid to be flushed out, overcoming boundary effects, and further aiding cleaning efficacy.
Cyclic Nucleation (cyclic cavitation)
Cyclic cavitation combines a boiling effect and a flushing effect by varying the pressure above a liquid so that it cycles between the boiling state and the liquid state. The word nucleation refers to the small bubbles that mark the formation of a new thermodynamic phase, It does not involve any radioisotopes. This technique is especially useful for cleaning in small spaces and tubing. Cyclic cavitation is a different technique than is ultrasonics; both are useful depending on the application.
The technique involves having an assembler of technician spray jets of gas, (shop air or an inert gas), to dislodge particles from the product. This is a line-of-sight technique, therefore, in automating air blast, it may be necessary to vary the orientation of the product. One problem we often observe, is that shop air may be contaminated with oil from the pumps – the product may end up with increased thin film contamination after air blast.
High pressure steam is used to clean floors, engines, and precision parts. Water vapor is constrained and forced through a narrow orifice to create a jet stream that impacts a surface and dislodges surface residue, primarily particles and surface oxides. This line-of-sight technique uses the impact of water molecules, not the solubility properties of water.
Air under liquid (Jacuzzi)
The technique is sometimes referred to as Jacuzzi cleaning. Air injected into the liquid in a process bath, results in agitation not unlike what is achieved by boiling. The difference is that in boiling, the factor of heat is included.
You may think of media as inherently associated with a liquid cleaning agent, we’re looking at the aspect of cleaning associated with cleaning force. Media is a term that encompasses a very large number of solid materials of different sizes, shapes, and material composition including polymeric materials, sand, baking soda, and walnut shells. Media blast is a liquid or gas jet containing an abrasive material; it is line-of-sight cleaning. The parts can also be tumbled with media. Most media leaves residue on the part; that residue must be removed.
CO2 snow, dry ice blasting
Solid CO2 is an abrasive. The cleaning force is related to the size of the particle and the force of application. “CO2 snow” provides gentle cleaning of relatively delicate components. Dry ice blasting involves larger pellets and can be used for depainting aircraft and restoring ancient castles.
Wiping and scrubbing are associated with elbow grease cleaning. Examples of solid materials include shop towels, “cleanroom” wipes, cloths, “toothbrushes,” and bottle brushes. The solid materials have some potential to introduce contamination. With rare exceptions wiping and scrubbing are performed by people, so the cleaning technique has to be well-defined and communicated.
Manufacturers use many terms to describe cleaning force or cleaning action. If we’ve missed any techniques or terminology, let us know. In upcoming articles, we’ll take a deeper dive into techniques to achieve the right cleaning action. Critical cleaning is not about brute force, it is about achieving a balance of action, chemistry, time and temperature. We’re always pleased to help you achieve the right balance for your application.Back To Newsletter Archive