Ultrasonic Cleaning 101
There are important considerations when choosing or designing an ultrasonic cleaning system. All of these items help determine the tank size, frequency (generator and transducer) and other retrofits required, as well as whether a solution, with or without heat, may be effective during cleaning.
Parts to be Cleaned
What the parts are, what they do, their size and shape, how intricate or delicate they are and what they are made of all come into play. Generally, smaller tanks require more watt density, or watts per-gallon of fluid. The larger the tank, the less watt density is needed. Tanks larger than 25 gallons usually will work with 25 watts RMS/Gallon or 50 watts peak.
Plastic absorbs sound, so you may need more ultrasonic power to cavitate through the parts. If items are large, you may need more power to overcome that extra mass. If heavy mass parts have most of the threaded holes on vertical surfaces, then an ultrasonic cleaner unit with side-mount transducers is best.
If you are talking about a plating line application that is very deep with small parts hanging from a tree-style bracket, putting transducers on the bottom will perform very well on the items toward the bottom of the tank. However, these same transducers can be somewhat ineffective on the items on the top. Some applications are complex enough to warrant side- and bottom-mount transducers.
Contaminant to be Cleaned
If the job to be done is cleaning oils, water-based coolants and light dirt, either a system with bottom-mount transducers or bottom-mount direct bond transducers will be effective because oil floats and there isn’t much dirt to settle to the tank bottom. However, direct bond transducers are fixed to the tank bottom so when you look at the inside bottom of the tank, it is flat.
With immersibles, the transducers are mounted inside a housing that is 4 inches thick and bolted to the bottom of the tank. If you are cleaning heavily soiled items with a lot of dirt, grime, carbon, etc., the dirt falls to the bottom of the tank.
In a direct-bond system the dirt will lie on top of the transducers and muffle the sound. This causes the cleaning performance to erode rapidly. Heavy dirt falling to the bottom of the tank with immersibles will accumulate around the transducer sides and will affect performance for a longer period of time.
Typically ultrasonic generators come in single-phase 120-volt AC or single-phase 240-volt. If the facility supply is a 3 phase 240-volt or if the heating circuit necessitates so much heat that a 3 phase is required, the equipment can be built so that it is fed with 3 phase/240VAC, but internally the ultrasonic cleaner generators tap off from 2 legs of the 3 phase legs.
If a 3 phase 480-volt power system is the only voltage available, then a step down transformer is necessary. If a single-phase/208VAC is the only thing available, the 240V ultrasonic system will operate without harm, though some manufacturers’ products will operate 10-17 percent below peak performance.
This can be remedied with a buck-boost transformer, which is not terribly expensive. Generators with universal input and a unity power factor will have no degradation of the output power waveform from 120 – 240-volt AC.
Basket choice is important because flat solid surfaces absorb sound and decrease the performance of the ultrasonic technology. Baskets should be made from round rod material to more efficiently reflect sound waves.
Most applications make use of immersion baskets with 1″x1″ mesh. Smaller parts will necessitate either a tighter mesh basket or a sub-basket with a tighter mesh to fit inside the larger basket.
Also, if parts are easily scuffed and cannot have metal-to-metal contact, baskets can be lined with vinyl. If parts can’t touch each other, the basket will require spacers or guides to avoid parts contact.
Depending on the cleaning job to be done, heat may be required. This can be controlled through either analog or digital controls. Ultrasonic cleaner activity heats liquid, so if a maximum temperature cannot be exceeded, the ultrasonic circuit, the heat circuit or both must shut off. Another option is a constant flow into and out of the tank of cooler flowing fluid.
With some contaminants, especially oils, filtration systems may be necessary. When using an emulsifier solution, oils and other particulates will float suspended in the tank solution or sink to the bottom.
A pump must pull them from the main tank. If using a separator solution and only floating oils are present, an overflow weir is necessary to push fluid in from the surface of the bath through a set of filters in a closed loop process. A diverter valve can accommodate filtering from the overflow weir, the main tank or both simultaneously, depending upon the position of the valve.
Primary filters typically clean 10 to 50 micron to remove dirt particles before passing into a charcoal secondary filter to eradicate oils. Charcoal filters have a 3-micron discharge so it is optimal to pre-filter as much dirt as possible before beginning the cleaning process. In applications dealing with primarily suspended dirt, it is best to use micron filters in both cartridges.
Control systems give users the ability to control the volume or intensity of the ultrasonic action in the cleaning unit. This is sometimes necessary if wide assortments of parts are being cleaned.
Portability and maneuverability functions may be required. With smaller units, wheels or other specially designed retrofits can be added. But for larger machinery, moving the unit for cleaning, rinsing and draining typically requires a forklift, or possibly a pallet jack.
Users may require automation. Options such as bells, alarms, and dual output timers that automatically cycle a filtration or flush after an ultrasonic cycle will work if the application calls for it. However, basic batch machines are best left to be simple, low option units as these make them easier to work with.
Location & Environment Considerations
An ultrasonic generator should never be in a room that has strong atmospheric acids, such as a plating line, which will corrode internal components. In this case, generators should be fitted with extended coax cables and be housed in a separate room.
Equipment must be built to handle the overall moisture in the room where it is located. Circuit boards should be blown off occasionally, and generators also should be housed in a climate-controlled electrical housing.
Many applications require more precise cleaning and rinsing that can only be accomplished in more than one stage. The number of stages required is determined by the application, the type of filtration necessary in each stage and whether a final hot air dryer stage is necessary. Robotics also can be used to move parts baskets seamlessly from stage to stage with minimal labor.
Two-stage systems are a common design for multi-stage systems, though three- and four-stage systems are very popular. Two-stage units don’t affect simplicity of use or portability. The first stage is an ultrasonic wash using a detergent followed by either a heated rinse to remove soap residue or a heated ultrasonic water rinse to ensure better final cleanliness.
Because of part complexity or the chemical make-up of the contamination, some applications may be more effective using multiple wash cycles differing in chemistries or rinse cycles.
Complex industrial items with intricate multi-threaded blind holes or requiring a higher degree of final cleanliness should also go through a rinsing process, preferably heated. Depending upon the level of cleanliness needed, the secondary rinse may be helped by adding ultrasound to remove residual debris and remaining soap residue.