Ultrasonic Cleaning Process
An ultrasonic cleaning tank is simply a metal tank [stainless steel] that has pizeo ceramic transducers bonded to the bottom or side. These transducers have a unique property of changing size almost instantly when excited by an electrical signal. When excited the transducer increases in size and causes the tank bottom or side to move. This creates a compression wave in the liquid of the tank.
By using an electrical generator that puts out a high frequency signal [20 to 50 kHz] the transducer rapidly induces compression and rarefaction waves in the liquid. During the rarefaction cycle the liquid is torn apart. This creates a vacuum cavity within the liquid. These cavities will grow larger and smaller as the compression waves are continued. When the cavity reaches a certain size [based on the frequency and the wattage of the signal] the cavity can no longer retain its shape. The cavity collapses violently and creates a temperature of 5,000 degrees centigrade and a jet of plasma that impacts against whatever object is in the tank. There are millions of these bubbles created and collapsing every second in an ultrasonic tank.
Compression & rarefaction waves in an ultrasonic tank :
It is these collapses that clean the part. The jet will explode the dirt or any other material off the surface of the part. By adding soap or other chemical to the water in an ultrasonic tank, you can increase the effectiveness of the cleaning operation. Heat also improves ultrasonic cleaning by eliminating entrapped air in the water and making the detergent more effective. The best temperature to clean with is 80% of the boiling temperature of the solution. You should always use a basket to hold the parts you are cleaning. Never put parts directly on the bottom of an ultrasonic cleaner
Basic Types of Ultrasonic Cleaning Systems
A single tank cleaning system : A single tank cleaning system or single tank ultrasonic cleaners is most often used when only a small number of parts are cleaned. It is important to remember that you should use an industrial grade ultrasonic cleaning system, not one of the standard table top types. Most table top systems are very lightly constructed and have small transducers, with the generator, tank and heater in the same shell. These systems are used primarily for cleaning small parts, once or twice a day. They are not designed to provide high throughput or a high end clean part. Industrial table top tanks always have the generator as a separate unit; this prevents damage by the heat generated in the ultrasonic tank and allows more room for larger transducers on the tank bottom. Single industrial table top tanks can be set up to clean parts as well as a multiple tank system at far less cost. Of course there is no free lunch. A single tank system requires a manual cleaning procedure with full operator attendance. Another restriction is that you are limited by the size of a table top to smaller loads, especially if you are using a multiple beaker cleaning system where 3 or 4 beakers are placed in a holder and filled with cleaning and rinsing solutions. The parts are manually transferred from one beaker to another. Single tank systems are ideal for low volume, lab type or prototype cleaning lines.
Industrial single tank cleaning systems : Large single tank floor mount ultrasonic cleaning tanks can be used for in line cleaning of parts in plating or industrial cleaning applications, there is no limit to the size of the tank.
Multi tank cleaning systems : A multi tank cleaning system consist of an ultrasonic detergent tank, spray rinse tank, and an ultrasonic rinse tank, at the minimum. Additional stations such as multiple, detergent and spray rinse tanks, ultrasonic rinse tanks can be added to the basic system for quicker throughput and higher cleanliness levels. With multiple tanks you can process a lot of parts fast and still maintain a high level of cleanliness. Usually these systems have many automated features such as auto fill,drain, filtration, DI water heaters, etc. The exact configuration of the system can be worked out after discussing the cleaning requirement.
Automation : Automation in a cleaning system eliminates the inconsistency in process. By automating your cleaning system parts will always be cleaned in the same manner from lot to lot. Choosing an automation system is a decision that will be worked out between you and the manufacturer.
Why Use Ultrasonic Cleaning?
Ultrasonic cleaning has become increasingly popular for Casted Aluminum Components. One reason is that it's an environmentally friendly process capable of replacing some vapor degassing processes. The second reason is that it enhances aqueous cleaning processes in many applications. The ultrasonic cleaning action is able to infiltrate the smallest crevices and hard-to-reach surfaces, penetrating areas normally inaccessible to other cleaning methods. Oil skimmers, automatic handling systems, jet spray washing machines etc used in cleaning process.
What is "degassing", and why should it be done?
"Degassing" is the initial removal of gases present in the solution. Useful cavitation occurs after gasses have been removed from the cleaning solution, leaving a vacuum in the formed bubble. When the high pressure wave hits the bubble wall, the bubble collapses; it is the energy released by this collapse that will assist a detergent in breaking the bonds between parts and their soils.
Can ultrasonic cleaning damage my parts?
With certain cautions, ultrasonic cleaning is considered safe for most parts. While the effects of thousands of explosions per second are very powerful, the cleaning process is safe since the energy is localized at the microscopic level. The most important cautionary consideration is the choice of cleaning solution. Potentially adverse effects of the detergent on the material being cleaned will be enhanced by the ultrasonics. Ultrasonic cleaning is not recommended for the following gem stones: opal, pearl, emerald, tanzanite, malachite, turquoise, lapis and coral.
How do I get the best ultrasonic cleaning?
There are many considerations important to ultrasonic cleaning. Optimizing these variables will produce the best cleaning. The most important decisions to be made are choosing the proper cleaning solution, cleaning at the right temperature for the correct amount of time, and choosing the right size and type of ultrasonic cleaner.
What is "direct" and "indirect" cleaning?
Direct cleaning occurs when the parts are cleaned in a cleaning solution which fills the cleaner, usually inside a perforated tray or mesh basket. The limitation of direct cleaning is that a solution must be chosen that will not damage the ultrasonic cleaner. Indirect cleaning involves placing the parts to be cleaned in an inner non-perforated tray or beaker that often contains a solution that the user may not want directly filling the ultrasonic tank. When choosing indirect cleaning, make sure that the water level inside the tank itself is maintained to the fill line (about 1" from the tank top) at all times.
Why is a special solution required for cleaning?
Soils adhere to the parts... if they didn't, the soil would just fall off the parts! The purpose of the solution is to break the bonds between parts and their soils. Water alone has no cleaning properties. The primary purpose of the ultrasonic activity (cavitation) is to assist the solution in doing its job. An ultrasonic cleaning solution contains various ingredients designed to optimize the ultrasonic cleaning process. For example, increased cavitation levels result from reduced fluid surface tension. An ultrasonic solution will contain a good wetting agent or surfactant.
What cleaning solution should I use?
Modern ultrasonic cleaning solutions are compounded from a variety of detergents, wetting agents and other reactive components. A large variety of excellent formulations are available, designed for specific applications. Proper selection is crucial for acceptable cleaning activity and to preclude undesirable reactivity with the part being cleaned. Transonic can help you to identify either the optimal 'stock' cleaning formula, or likely candidates to test and evaluate.
More Cleaning Tips
When should solutions be changed?
Cleaning solutions should be replenished when a noticeable decrease in cleaning action occurs, or when the solution is visibly dirty or spent. A fresh batch of solution at each cleaning session is usually not required.
Why must I keep solution at the tank's level indicator?
The solution level should always be maintained at the level indicator in the tank, with trays or beakers installed. The ultrasonic cleaning system is a 'tuned' system. Improper solution levels will change the characteristics of the environment, can affect the system frequency, decrease effectiveness, and potentially damage the cleaner. Maintaining the proper solution level provides optimum circulation of solution around parts, and protects heaters and transducers from overheating or stress.
What is the length of cleaning time?
Cleaning time will vary, depending on such things as soil, solution, temperature and the degree of cleanliness desired. Highly visible removal of soils should start almost immediately after the ultrasonic cleaning action begins. Cleaning time adjustment is the easiest (and most often misapplied) factor used to compensate for process variables. Although new application cycle an experienced operator can approximate duration, it usually must be validated by actual use with the chosen solution and the actual soiled parts.
How do I perform the "foil" test?
Cut three small pieces of aluminum foil about 4" x 8" each. Fold each piece over a rod that you will use to suspend the foil in the tank. A clothes hanger works well. Your cleaner should be filled with an ultrasonic cleaning solution, degassed, and brought up to normal operating temperature. Suspend the first "square" in the center of the tank and the other two a couple of inches from each end of the tank. Make sure that the tank is filled to the fill line, and turn on the ultrasonics for about ten minutes. Remove the foil and inspect: All three pieces of aluminum foil should be perforated and wrinkled to about the same degree.
Why must trays or beakers be used?
Items being cleaned should never be placed directly on the tank bottom. Transducers (which produce the ultrasound) are bonded to the bottom of the tank. Items resting directly on the tank bottom can damage the transducers and/or reduce cavitation. Additionally, a tray or beaker will position the item within the optimal cleaning zone of the tank. The tray or beaker will also hold the load together and allow for easy, no-touch removal, draining and transport of the items to the next step in the cleaning process.
What is the optimum cleaning temperature?
Heat usually enhances and speeds up the cleaning process, and most detergent solutions are designed to work best at an elevated temperature. The best way to find the optimum temperature, which will give you the fastest, cleanest and safest results, is to run tests. Usually, the best results are within the 50¡C to 65¡C range.
Is rinsing required after cleaning cycles?
Rinsing is recommended to remove any chemical residue, which could be harmful to the part. Parts can be rinsed right in your ultrasonic cleaner, using a clean water bath, or in a separate tub containing tap, distilled or de-ionized water