DE Ionized Water

De Ionized water is water that has had all of the minerals, particulates, and dissolved ions removed by a filtration process. It is extremely pure water. The level of purity is expressed in Meg Ohms [resistance to an electrical current DI water is extremely active. It will pull ions from any surrounding metal, including stainless steel. To heat DI water in a stainless steel vessel is possible though if it remains standing in the vessel it will eventually pick up ions and deteriorate. A better method is to heat the DI water only when it is needed. 

Ultra pure DI water is rated at 18 Meg Ohms this is extremely pure water. Lesser grades of DI water are also used for cleaning. [10 meg ohm etc.] It is so clean that if you drank it would pull the ions from your body chemistry and would cause serious health problems.

DI water has a great capacity for ions; it will pull them from the contaminants that are on your parts. It is an extremely active cleaner in itself. The use of DI water will greatly improve your cleaning results. The fact that DI water does not contain ions, or other particulates makes it a very effective rinse agent. This will avoid spotting on your parts when drying.

Distilled water is also a good cleaner when you do not need the full benefit of DI water. Another good reason to use distilled or DI water is that the rinsing process is greatly improved and it leaves no residue to cause spotting on your parts. Hot DI water is even more effective in cleaning and rinsing.

Solutions

Solutions are the most single influential variable in ultrasonic cleaning. Properties of the specific fluid interact greatly, that is, some fluids operate quite well at ambient temperatures while others operate better at 140 to 160 degrees F. Somefluids require wetting agents [surfactants, detergents] to effectively transfer the ultrasonic energy into the solution. Water always requires a wetting agent and operates better at the higher temperatures. The choice of a detergent is dependent on the type of soils to be removed. This is one of the more important choices to make in any ultrasonic cleaning process.

 The intensity with which cavitation takes place in a liquid medium varies greatly with the Colligative properties of that medium which include vapor pressure, surface tension, viscosity, and density as well as any other  property that is related to the number of atoms, ions or molecules in the medium. In ultrasonic cleaning applications, the surface tension and the vapor pressure characteristics of the cleaning fluid play the most significant part in determining cavitation intensity and cleaning effectiveness. The energy required to form a cavitation bubble in a liquid is proportional to both surface tension and vapor pressure. The higher the surface tension of a liquid, the greater will be the energy that is required to produce a cavitation bubble and the greater will be the shock wave energy that is produced when the bubble collapses, In pure water whose surface tension is about 72 dynes/cm sq. , cavitation is produced only with great difficulty at ambient temperatures. It is easilyproduced when a surface-active agent is added to the liquid, reducing the surface tension to about 30dys/cm sq. When the vapor pressure of a liquid is low, as is the case with cold water, cavitation is difficult to produce but becomes less and less so as the temperature is increased. Every liquid has a characteristic temperature relationship in which cavitation exhibits maximum activity within a fairly narrow temperature range.  

The flow characteristics or reological properties of the cleaning applications static fluid conditions are highly conducive to the formation of the standing wave pattern that characterize intense ultrasonic fields, and hence it would seem likely that cavitation intensity would be maximized under such conditions. Optimum performance is seldom achieved in static fields since continuous purification of the cleaning fluid either by overflow or by recyclefiltration [filtering up to 50% of the tank volume per minute] is a prerequisite to effective cleaning.  When the filtered liquid is properly introduced into the bath little or no cavitation is lost.  In fact, improvement in overall surface impingement and homogeneity of cleaning can be attained with this method.

Aqueous Cleaning Solutions

Aqueous cleaners are designed to reduce the surface tension of the water and also to provide a chemical reaction with the type of soils it is designed to remove.

The chemicals in an aqueous cleaner may vary from soaps to surfactants to acids or chelating agents, builders, saponofiers, alkaline or combinations of the above. The cleaning solutions may be Ionic or non-Ionic dependent on the application.

De ionized water itself can be an effective cleaning agent in some circumstances. It is always preferable to use DI water as the major portion of the cleaning fluid as it is pure water and does not have minerals or other contaminants in it. It provides an excellent vehicle for the detergent and there no chance of depositing minerals on the substrate. This will aid in the rinsing of the detergent and will provide spot free drying.

Aqueous Cleaners/Soaps and Detergents

Aqueous cleaning works by the detergent actually bonding to the dirt (oil, grease, or particulate) and the mechanical action of the ultrasonics flushing this new compound into solution. This reaction of alkaline detergent with fatty acids is the saponification of dirt emulsifying in to solution. 

A typical detergent is made up of several agents which work in combination to accomplish cleaning, Surfactants  (wetting agents), reduce the surface tension of the dirt, allowing the cleaning agents to penetrate, Saponifiers combine with the fatty acids and the flocculent to disburse the dirt into tiny particles, the mechanical action of the cleaning system flushes away the contaminants, one microscopic layer at a time. 

Acidic type cleaners are typically used for removal of scale, rust and calcium deposits. Heavy acid cleaners are used for pickling parts as they are cleaned, since in a good ultrasonic system, the parts are striped to the bare metal and all natural oxides are removed the part will oxidize rapidly after extraction unless protected either by pickling or by addition of rust inhibitors to the cleaning solution 

Solvent Cleaners

Solvent cleaners generally have a lower surface tension than water and are much denser. Solvents work on the basis of dissolving the contaminant. The extremely low surface tension of a solvent permits it to penetrate fine cracks or blind holes and dissolve organic, oils and other contaminants. The solvents penetrating action as well 
as its chemical action remove Inorganics. Solvents can be blends, azeotropes or mixtures of both solvent and water. Solvents require special handling and design in selecting the correct ultrasonic system. In most cases since solvents are denser than water additional ultrasonic power is required to induce cavitation. Most solvents are specific in type of soil removed and care must be used in selecting the correct solvent.  HFC solvents contain carbon, hydrogen and fluorine and are not ozone depleting. 

Flammable solvents such as isopropyl, turpines and ethers are also usable in an ultrasonic system, however specific, expensive  modifications must be made to any system that uses flammable solvents. Consult with the manufacturer before using flammable solvents in an ultrasonic tank.