A heated ultrasonic cleaner is recommended for tough cleaning jobs generally involving deposits of heavy grease, grime, residues in plastic injection molds, tough carbon deposits on engine parts and similarly difficult cleaning challenges. Just as hot water is better for cleaning greasy dishes, pots and pans, a heated ultrasonic cleaning solution often “works best” solving tough cleaning challenges.
When Not to Use a Heated Ultrasonic Cleaner
There are certain applications where heat is not necessarily desirable when using an ultrasonic cleaner.
An example is the use of an ultrasonic cleaner as a pre-sterilization or disinfecting step for medical and surgical instruments.
Blood and tissue should be removed from instruments at a temperature below 40⁰C (104⁰F). That’s because elevating the temperature can cause protein in blood to harden and become more difficult to clean. For more details on this see our post on cleaning surgical instruments.
Heat may also cause warping when cleaning printed circuit boards and can damage similarly delicate products.
For other jobs involving heat-sensitive materials or running long ultrasonic cycles cooling coils connected to an outside water source and a temperature regulating device can be employed to keep temperatures in check.
An important point to keep in mind is that heat is automatically created during the operation of an ultrasonic cleaner. This is covered later in this post.
The Role of Heat in Ultrasonic Cleaning
Fast, safe and efficient ultrasonic cleaning cycles call for a judicious blend three variables:
- cleaning cycle time
- cleaning solution temperature
- cleaning solution chemicals
The time required to effect efficient cleaning depends largely on the products being cleaned: composition, complexity and nature of contaminants to be removed.
Cleaning temperature is often recommended by cleaning solution manufacturers.
Cleaning solution chemicals are selected based on the project at hand in order to provide efficient removal of contaminants. We cover this topic in our post on ultrasonic cleaning solution guidelines.
How Heat is Created in an Ultrasonic Cleaner
At the beginning of this post we note that heat is a natural byproduct of ultrasonic cleaning.
How is this so?
Ultrasonic cleaners use generators and transducers that cause the bottom (and sides in some models) of ultrasonic cleaner solution tanks to vibrate as a membrane.
This vibration, measured in kHz such as 37,000 cycles per second, results in the creation of billions of microscopic-sized vacuum bubbles that implode through a complex process beyond the scope of this post.
The implosion of these bubbles against surfaces to be cleaned loosen and carry away contaminants far faster and, in most cases safer, that can be achieved by manual cleaning techniques.
So, you ask, where does the heat come from?
Frankly, it’s a bit mind blowing when you think about it.
When cavitation bubbles implode they create shock waves that range from 15,000 to 150,000 psi against the surface of products being cleaned.
Now, before you get carried away, we’re talking about microscopic-sized bubbles imploding on a microscopic-sized surface area, and not the pressure of water at increasing depths. For example, the water pressure at the depth of the Titanic is about 5,500 psi.
These shock waves also create heat. As the cavitation bubbles implode they create enormous temperatures on the order of 5,000⁰C (9,032⁰F) which is believed to be similar to the surface of the Sun.
But: as with the pressure of imploding bubbles, the size of the bubbles is so small that heat dissipates rapidly and cleaning solution temperatures are relatively unaffected.
Nevertheless, cleaning solution temperature does increase during long cleaning cycles. In fact, during permanent operation cleaning bath temperatures exceeding 60°C (140⁰F) can be reached simply by the use of ultrasonic energy. This, as well as for other reasons, makes it is unwise to reach into an operating ultrasonic cleaning bath.
The Role of Temperature in a Heated Ultrasonic Cleaner
Of the variables that apply to the ultrasonic cleaning process temperature ranks high, with time and cleaning solution chemistry close after. This is because the temperature of the ultrasonic cleaning solution impacts the properties of the cleaning solution’s ability to perform efficient cleaning.
As mentioned earlier, cleaning dishes with hot water is easier and better than with cold water.
Two factors relating to cleaning solution temperature are:
- Viscosity. Warmer solutions are less viscous than cold or room-temperature solutions. Low viscosity solutions result in more effective cavitation.
- Removal of entrapped gasses. Dissolved air inhibits cavitation and is removed by a degassing cycle prior to cleaning, and by increasing the temperature of the solution.
Recommended cleaning solution temperatures are provided by cleaning solution manufacturers.
Generally heated ultrasonic cleaners find widespread use when removing grease, grime, coolants and other tenacious contaminants from cast, machined and fabricated metal parts.
An example of a biodegradable cleaning solution concentrate ideal for this application is elma tec clean A4 ultrasonic cleaning solution all purpose cleaner with a recommended cleaning temperature of 50⁰ to 80⁰C.
Ultrasonic Cleaner Heat: More is Not Always Better
When it comes to a heated ultrasonic cleaner there are two important points to keep in mind. First of all, there’s a limit to when increasing the temperature contributes to the cleaning process. As the cleaning solution temperature increases cavitation action actually decreases.
That is why heater-equipped ultrasonic cleaners such as the Elma line offered by Tovatech have thermostats with a maximum temperature setting of 80⁰C (176⁰F).
There are, of course, exceptions. Some highly caustic cleaners used for cleaning iron, steel and stainless steel are used at higher temperatures. An example is cleaning jet engine fuel injectors at a temperature of 90⁰C for one hour.
Setting the heated ultrasonic cleaner thermostat to the highest point (80⁰C) and allowing the unit to run continuously will through cavitation allow the solution to reach the desired temperature.
Many ultrasonic cleaners are designed to initiate the cavitation process when the set temperature is reached, and provide LED indicators showing the target and actual solutions temperature.
Other heated ultrasonic cleaners may be equipped to automatically shut off when solution temperatures get too high. The 90⁰C cited above is the limit for the unit used in this application.
Other Factors Impacting Ultrasonic Cleaning Efficiency
At the beginning of this post we stated that time, temperature and chemicals are key to safe, fast and efficient ultrasonic cleaning.
But that’s not all.
For these three factors to work best, the correct ultrasonic frequency comes into play. Most commercially available ultrasonic cleaners operate at a single frequency.
Elma offers other models that operate at dual frequencies allowing operators to select the best frequency for the job at hand. Examples of dual-frequency heated ultrasonic cleaners include the Elma Ultrasonic P series, also a benchtop ultrasonic cleaner, allowing operators to switch between 37 and 80 kHz. Industrial benchtop cleaners, exemplified by the TI-H series, are offered at 25/45 or 35/130 kHz. And so on.
The point we make is that ultrasonic frequency determines the size of the cavitation bubbles (not that you’d notice by looking at them). The lower the frequency the larger the bubbles, the higher the frequency the smaller the bubbles.
Large bubbles implode more violently against surfaces being cleaned, smaller bubbles are better able to penetrate small cracks, crevices and blind holes to remove contaminants otherwise impossible to reach by manual methods.
The final point for this post is a brief on yet another key to efficient cleaning: ultrasonic power.
This too is a complex subject. Simplified, while more power usually indicates faster and more effective cleaning, more power is not always better. Too much power can damage electronic parts, the surface finish on a soft metal (e.g. polished aluminum), and other delicate items. For cleaning extremely sensitive items, adjustable power is a useful feature. Examples are the Elma TI-H and P series of heated ultrasonic cleaners.
A Summary on Heated Ultrasonic Cleaners
In summary bath temperature plays a key role in ultrasonic cleaning. The optimum temperature is determined by the ultrasonic cleaning solutions employed and the items being cleaned. But as suggested in this post there is much more to be considered. That’s why you should call (908) 402-7258 to contact the scientists at Tovatech who are ready to help you establish procedures to maximize the efficiency of your cleaning operations.