Tiny Bubbles

Ultrasonic cleaning has been shown to be an effective pretreatment for a variety of surface finishing operations. It can effectively remove buffing compound and clean inside hidden areas of parts not accessible to spray and simple immersion technology. In order to achieve maximum cleaning results, however, the ultrasonic technology must be properly applied in a production setting.

The mechanics of the ultrasonic cleaning process are generally well understood. Briefly, high-frequency sound waves are introduced into a liquid medium by an ultrasonic transducer vibrating at a frequency above the range of human audibility. Each point in the liquid is subjected to alternating negative and positive pressure as the sound waves pass by. Negative pressure causes the liquid to fracture under tension, thereby creating cavitation bubbles. As negative pressure is replaced by positive pressure under the influence of the compression portion of the sound wave, cavitation bubbles, which actually contain a partial vacuum, collapse to create minute but intense areas of pressure and temperature due to their implosion. These mechanical effects boost the cleaning effect of chemistry and accomplish the removal of small particles from critical surfaces by mechanical force.

When ultrasonic cleaning is applied in mass finishing operations, as in metal finishing operations, it is necessary to address many concerns not typically encountered in small-batch ultrasonic cleaning. The remainder of this article will address these issues and offer suggestions on how to maximize the ultrasonic cleaning process.

Large Tanks

The first and most obvious thing in mass finishing is that, due to the volume of parts being processed, the tanks used are relatively large. Applying ultrasonic energy in a large tank is not a problem with current technology. Readily available immersible ultrasonic transducers are simply fixtured on the the tank side walls or bottom.

Each transducer requires an ultrasonic generator to supply it with electrical energy at the ultrasonic frequency. Ultrasonic generators, being somewhat sensitive to dirt, moisture and corrosive fumes, are often located in an area away from the processing tanks and are protected by appropriate electrical enclosures incorporating fan-forced filtered air and/or air conditioning. The transducers are connected to the generators using solid or flexible stainless steel tubing. An alternative scheme involves penetrating the tank wall directly behind each transducer with a gasketed bulkhead fitting. This method is less convenient as it requires draining the tank for transducer repair or replacement if needed.

Ultrasonic Power

Selecting the appropriate ultrasonic power density for a given cleaning tank and process requires some skill on the part of the ultrasonic equipment supplier. Power is commonly expressed in watts of ultrasonic energy per gallon of tank volume. Although general guidelines for ultrasonic power requirements are available, the actual power requirement in any given situation can vary widely based mainly on the size of the tank, surface area and weight of the parts and rack design and construction. Larger tanks may require less ultrasonic power than smaller tanks per unit of volume to achieve the same cleaning result in given application. The greater the part surface area and/or weight, the more ultrasonic energy density is required to achieve acceptable cleaning results. Finally, if part fixtures absorb ultrasonic energy or shield the parts being cleaned from the ultrasonic field, the application will require more energy density.

Tank Construction

Tanks used for ultrasonic processing are commonly made of double welded stainless steel for maximum strength and durability. Mild steel tanks are acceptable but may have a shorter life. Because there is an investment in fixturing a tank for ultrasonic transducers, and because the long-term effect of ultrasonic energy can cause wear on tank surfaces, the additional investment in stainless is usually considered worthwhile.

Softer materials such as plastic and rubber should be avoided in tank construction for two reasons. First, softer materials tend to absorb ultrasonic energy, necessitating an increase in ultrasonic power density to compensate for the absorbtion. In addition, there is a benefit to having sound waves reflect from the tank walls to help produce a homogeneous ultrasonic field within the tank. This effect is much less pronounced in a non-metallic tank. As an extension of the above, the use of non-metallic materials in the form of heaters, heater sheaths and immersed rubber or plastic plumbing should be avoided in an ultrasonic tank as well.

Ultrasonic Frequency

The ultrasonic frequency spectrum ranges from slightly below 20 kHz to above 200 kHz. In general, frequencies near the low end of the ultrasonic spectrum are used in pretreatment applications in the finishing industry.

Most manufacturers provide ultrasonic systems in the 20–25-kHz range as well as in the 40–45-kHz range. Higher frequencies seldom find application in the metal finishing field. Lower frequencies produce larger cavitation bubbles, because the longer wavelength puts each growing bubble under the influence of the negative-pressure portion of the sound wave for a longer period of time. Longer time gives the bubble time to grow to a larger size. When larger cavitation bubbles collapse or implode, they release greater energy than smaller ones.

Frequencies from 20–25 kHz generally provide faster and more aggressive cleaning due to higher energy release at implosion. Frequencies from 40–45 kHz are reserved for applications on substrates susceptible to damage by high-intensity cavitation and for cleaning and rinsing applications requiring enhanced penetration of complex surfaces.

Transducer Placement

Successful ultrasonic cleaning requires not only the proper energy density, but uniform distribution of ultrasonic energy throughout the cleaning tank as well. Although ultrasonics is not a “line of sight” phenomenon producing sharp shadows, there are “soft” shadowing effects as one would see by observing the shadow of a large sheet of cardboard held parallel to the surface of the earth on an overcast day. It is, therefore, important to give some consideration to the placement of ultrasonic transducers.

In most small-tank applications, the preferred transducer placement is on the bottom of the tank. Bottom placement allows use of the liquid/air interface at the surface of the liquid as a near-perfect reflector. This reflecting surface, combined with the reflecting properties of the tank sidewalls, serves to distribute sound waves throughout the liquid volume.

Tanks more than twice as deep as their width may require side mounted transducers for even energy distribution. This is especially true in cases where the critical surface of the work is located facing the side wall of the tank. In addition to the reflection phenomenon described above, the density of work in a deeper tank may cause diminished ultrasonic energy near the top of the liquid volume due to the shadowing effects of parts at the lower levels.

Low-density and “one-sided” workpieces are often accommodated with transducers placed on only one side of a cleaning tank. High-density and “two-sided” workpieces often require the placement of transducers on opposing tank side walls. Opposing transducers are often staggered to further enhance the distribution of ultrasonic energy.

In ultrasonic tanks using side-mounted transducers, sufficient space must be left between the transducer faces and the work to allow the sound field to spread out and diffuse. Work placed too close to a bank of ultrasonic transducers may exhibit bands of poor cleaning in areas not directly in front of a transducer. Further, parts inadvertently placed in critical positions at dimensions from 1/2 to three wavelengths (3/4 to 4-1/2 in.) directly in front of a transducer may exhibit cavitation burning due to the intense sound field found at these locations. As a general rule, no part should be placed closer than 6 in. from the radiating face of an ultrasonic transducer. Sizing tanks to follow this recommendation is well worth the effort.

Ultrasonic transducers are robust in their own right but are generally not up to the abuse that they can inadvertently be dealt in a high-speed automated tank line running heavy parts. For this reason, it is recommended that transducers be protected through the use of guards and/or load guides. Guards and guides are often made of bar stock and are positioned to provide minimum interference with the ultrasonic energy field. Including these protective devices into a tank design in process is relatively simple. The expenditure will be worthwhile if a single incident of damage is avoided.

Rinsing

Although generally thought of as a cleaning tool, ultrasonics can also enhance rinsing. The addition of ultrasonics to rinses is particularly effective on parts having irregular surfaces, complex internal passages, or, in the case of sheet metal, tight reverse bends or “hems.” Ultrasonic rinsing helps remove residual cleaning chemistry, which might otherwise bleed out in later finishing steps and cause rejects. It is common to realize the benefits of ultrasonic enhancement in a rinse using as little as 1/2 the ultrasonic energy density required for the initial cleaning of the same part.

In a multi-stage rinse with ultrasonics only in a limited number of tanks, ultrasonics should be employed in initial rinse(s) rather than at the end. A final ultrasonic rinse preceeded by stagnant pre-rinses will only hasten contamination of the final rinse. Contamination of the final rinse will reduce the effectiveness of the overall rinsing process.

Parts Racking

Fortunately, many finishing operations—including plating, electrocoat and many others—require the same attention to racking as is required for successful ultrasonic cleaning. As was stated earlier, part surfaces to be cleaned must be exposed to ultrasonic energy. As the medium for ultrasonic transmission is the cleaning or rinsing liquid, all surfaces to be cleaned must be in contact with the liquid for the ultrasonic energy to be effective. Parts that are likely to trap air bubbles require special positioning to eliminate or minimize the effect.

As an alternative, parts may be repositioned after immersion to “burp” out any trapped bubbles. This is a more complex solution to the problem of entrapped air, but it has been used. Yet another alternative solution for particularly difficult parts is to provide an air relief to prevent air trapping. This requires only a very small hole, which can often be placed in an inconspicuous location. In some cases this hole is actually filled in by the subsequent finishing operation.

To prevent the shadowing effect described earlier, parts must be placed with sufficient space around them to allow the ultrasonic energy to “wrap around” them. Although layers of parts can be tolerated, the position of each layer should be staggered slightly to allow the diffuse energy pattern to penetrate to all parts. In no case should parts touch one another or be allowed to stack. Not only does this prevent the ultrasonic field from reaching the surfaces that are touching, but it may also result in abrasion marks due to part against part vibration induced by the ultrasonic vibrations.

Finding the maximum part density allowable in some applications may require experimentation. In general, numbers of large, flat parts such as appliance panels and printed circuit boards are more effectively cleaned if the ultrasonic field is introduced from the edge rather than against the flat surface. The latter arrangement requires the ultrasonic energy to penetrate through a number of panels to reach those located in the center.

The best rack designs for many finishing operations are often incompatible with ultrasonics. The resists applied to racks in the form of plastisol, teflon, rubber and other insulating and/or protective materials act to severely dampen the ultrasonic effect. In many cases, these protective coatings are removed by extended exposure to ultrasonics, causing even greater problems.

Rack design requires some compromise if the resulting rack is to be compatible with both the ultrasonic cleaning and subsequent finishing operations. In some cases, minimizing the thickness of coatings and positioning supports so that they don’t shadow the parts is effective. Racks may also be built using thinner structural supports and cross members. The use of inexpensive expendable hangers has been successful in cases where there was no other acceptable solution. At the very least, the rack designer should be familiar with the requirements of both ultrasonics and the finishing operation and provide a solution that meets the needs of both.