Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to jfuchs@ctgclean.com.

Temperature Sensors Continued

July 11th, 2014

In the preceding blog we looked at the expansion and contraction of liquids as a means of measuring temperature.  The expansion and contraction of solids (usually metals) is another way to measure temperature.

Most solids expand and contract in a relatively linear way in response to temperature changes.  In most cases, solids expand as they are heated and contract as they are cooled.  The amount of expansion and contraction of solids (the coefficient of thermal expansion) is typically measured in inches per inch per degree Fahrenheit (or the metric equivalent) and, for common materials, is usually less than 100 times 10 to the minus 6th inches per inch per degree of temperature change.  Because of this very small change in dimension, using a direct observation of the expansion and contraction of a solid to measure even relatively large changes in temperature is quite difficult and impractical.  Solids, obviously, will not travel up a capillary tube as described for liquids in the preceding blog.  There are, however, ways to magnify these small changes in the dimension of solids that make them practical as a means of measuring temperature changes.

The elegant solution is to use two pieces of solid material (usually metals) with differing coefficients of thermal expansion that are fused together as shown in the illustration below.

Bimetallic sensor illustration

Metals with different coefficients of thermal expansion are fused together to make temperature sensors. As one metal expands (or contracts) at a different rate than the other, the result is deformation of the sensor.

As a bimetallic sensor is heated, the two solid materials expand at different rates which causes the composite strip or disc to deform in order to equalize internal forces.  Depending on the materials used, this deformation can be thousands of times greater than the actual growth of either of the materials the bimetallic is composed of by itself.  This larger motion makes measuring temperatures by using the expansion and contraction of solids practical.

In the case of a bimetallic strip, the motion of the bimetallic can be further amplified by forming the strip into a spiral.  With one end of the spiral fixed, the tightening and relaxation of the other end can be used to measure temperature changes directly.  If the outer end of the spiral is fixed, the center of the spiral will rotate with temperature change.  This rotation can be used directly to measure changes by adding a pointer.  An everyday example of this is a simple dial thermometer which can be purchased in any hardware store.  If the inner end of the spiral is fixed, the outer end will rotate.  Thermostats used to control home heating systems often use this motion to tilt a mercury switch which controls the heat source.

Bimetallic discs deform with changes in temperature causing the center of the disk to be displaced perpendicular to the plane of the disc.  This is the principle behind the operation of common “click disc” thermostats.  Although commonly used to indicate that a target temperature has be achieved in an on/off control system, when properly designed, the disc will remain deformed even when the heat source is removed until it is mechanically returned to the initial position or “reset” making them useful in “safety” applications.  In most cases these click devices are pre-calibrated to actuate or “click” at a single temperature.  Devices using a bimetallic strip, however, are usually adjustable and can be calibrated to indicate a range of temperatures.

In general, bimetallic sensors are best used in applications requiring the measurement of the temperature of air.  Occasionally, the disc type sensors are applied to surfaces but, in fact, are measuring the temperature of the air near the surface and not the temperature of the surface itself.

Upcoming blogs will discuss the use of changes of electrical properties of materials to measure temperature.

-  FJF  -

Temperature Sensors

June 25th, 2014

In preceding blogs I have identified temperature as probably the single most important variable in a cleaning process.  Devices that sense and control temperature, therefore, are a critical part of a cleaning system.  Let’s take a quick look at a few of the types of temperature sensors that are available and suggest where each is applicable in cleaning.

Temperature sensors can be divided into two basic categories.  Those which are mechanical and those which are electrical in function.

First, let’s look at mechanical type sensors.

Thermometer

In a thermometer of this type, expansion and contraction of the liquid in the bulb is amplified as the liquid is pushed up into the attached small diameter tube.

Most gasses, liquids and solids expand in volume as their temperature is increased and shrink in volume as their temperature is decreased (with the notable exception of water which grows a little after it freezes).  Mechanical temperature sensors rely on the expansion and contraction of a sensing element (usually a metal, or a confined liquid or gas) to measure  changes in temperature.  A mercury thermometer is probably the best known example of a temperature sensor that uses the expansion and contraction of a liquid (mercury) to indicate temperature.  Since the expansion and contraction of the mercury is relatively small, an arrangement is used which consists of a large volume of contained mercury (the bulb) which is directly connected to a very small diameter tube.  In this arrangement, the small diameter tube amplifies the result of expansion and contraction of the mercury in the bulb as the mercury is pushed up into the tube.  The result is that a very small amount of expansion of the mercury in the bulb creates a relatively larger change in how far the mercury is pushed up into the small diameter tube.  This makes the device more accurate.  With all the sensitivity about the health concerns of mercury in today’s environment, mercury is often replaced with another liquid, colored alcohol for example, for safety’s sake.

Thermometers of this type are commonly used to measure the temperature of a liquid or gas in laboratory or domestic settings.  They are simple to use and allow precise, permanent calibration making them ideal for reference use.  In this simple form, however, thermometers of this type do not offer the ability to provide feedback to control temperature.

Industrial temperature sensors utilizing the expansion of a liquid to control temperature are often called capillary thermostats.  A large bulb, usually containing a liquid, connects via a small diameter (capillary) tube to a remotely located instrument with a diaphragm which flexes in response to the amount of liquid displaced from the bulb caused by temperature changes.  Since the volume of liquid in the capillary tube is insignificant compared to the volume of liquid in the bulb, the displacement of the diaphragm is primarily the result of the expansion of liquid in the bulb and not that in the capillary.  Movement of the diaphragm or bellows can be used to close or open an electrical circuit through a switch or directly control the flow of a liquid or gas (refrigerant or steam, for example) through the use of a valve.

Illustration of a capillary bulb thermometer.

In a capillary type thermostat, expansion of liquid in the sensor bulb results in physical displacement of a diaphragm or bellows connected to it by a small diameter (capillary) tube.  This motion can be used to actuate switches, valves, etc.

In order to accurately sense temperature, the bulb of a capillary thermostat needs to be positioned in such a way that the entire bulb is heated or cooled as the temperature being sensed changes.  Immersion of the bulb in a liquid or gas, for example, will give the most accurate temperature reading.  Clamping a bulb to a tank containing a liquid, on the other hand, does not always result in an accurate temperature measurement as only one side of the bulb is seeing the heat source while the other side remains at ambient temperature.  In some cases, an insulating material is placed over the sensing bulb on the side opposite to the surface being measured to minimize this potential inaccuracy.

to be continued

-  FJF  -

Automation – Suspended Conveyors and Conveyor Belts

June 20th, 2014

A third classification of automation is a constantly moving or indexing device that conveys parts through the cleaning process.  This can either be in the form of a conveyor that suspends parts on hooks or other fixtures that hang from an overhead track or in the form of a conveyor belt on which the parts rest as they are moved through the cleaning process.  This type of automation is particularly attractive in cases where parts are already handled using a conveyor in manufacturing steps preceding or following cleaning and is also ideal in many “lean” applications.  Good examples are sheet metal parts being cleaned prior to plating or painting.  The use of a conveyor for cleaning often eliminates one or more handling steps which would otherwise be required if parts were cleaned in baskets or individual racks.

A clear benefit of using a conveyor system is that the cleaning process is continuous.  Parts can enter the system at random intervals and still receive consistent processing.  Parts progress through the cleaning process in order which means that batch integrity can be maintained if required.

Conveyors are probably best employed in applications where parts are not immersed for cleaning.  Conveying parts through a spray washer, for example, is a relatively simple matter.  Processing steps are aligned horizontally with the parts being moved by the conveyor.  In cases where changing of the elevation of parts is required (as in an immersion process), the attraction of using a conveyor may be somewhat diluted.

One excellent application for a conveyor is in the processing of parts in “near field” ultrasonic applications.  It is relatively simple to spin a part about its vertical axis using a simple sprocket arrangement similar to a rack and pinion mounted at the top of each suspension device.

  • In order to immerse a part for cleaning, a conveyor must ramp vertically into and out of the immersion tank since most conveyors can not accommodate a drastic change in elevation within a short horizontal distance.  Instead, the conveyor must gently slope to effect elevation changes.  These gentle slopes can add considerable length to a cleaning system especially in cases where the vertical profile of the part being cleaned requires a considerable change in elevation for total immersion.
  • In the case of a conveyor belt, immersion requires that the conveyor belt itself must be immersed in the cleaning process.  In essence, the conveyor belt adds to the overall load on the cleaning system as, although it may not be dirty, it still adds to tank – to – tank carryover and requires drying on each pass.

Considerations -

As is the case with walking beams and pushers (which are really just intermittently indexing conveyors with a vertical motion component), a conveyorized process is relatively inflexible.  The time in each processing step is determined by the relative horizontal length of each process step as the conveyor moves at a single speed.  Slowing down or speeding up the conveyor will result in the duration of all process steps being increased or decreased accordingly and will, of course, result in a associated change in the total throughput capability of the system.

Although not applicable in all situations, conveyors are a viable option for automation for cleaning and are worthy of consideration especially in cases where parts are already being moved through the manufacturing process by a conveyor.

-  FJF  -

Automation – Automated Hoist

June 16th, 2014

An automated hoist is probably the most commonly used means of moving parts through the steps of an automated immersion cleaning process.  These hoists come in almost unlimited variations but have in common that they can move a basket or rack of parts in two dimensions along the axis of a multi-station cleaning system.  Although the hoist suspension can be located directly over the tanks, the preferred configuration is to have a cantilevered load bar supported by a structure offset from the cleaning tanks to minimize contamination that might be introduced by the moving parts of the hoist.

Schematic illustration of an automated hoist transfer system.

An automated hoist can move baskets or racks of parts through an automated cleaning system by moving vertically and horizontally. This type of system is more versatile than any other means of automation commonly used in the automation of cleaning processes.

The above illustration shows the basic motion or an automated hoist.  Baskets or racks of parts are sequentially moved one step forward in the process.  In the illustration, the red arrows show the path of the hoist.  First, the hoist engages the carrier in process station 4 and moves it to the unload position.  The hoist then disengages that carrier and moves back to station 3 and moves the carrier in process station 3 to process station 4.  This sequence continues until all carriers have been moved forward one position.  The hoist then goes to process station 4 and repeats the process described above.

Advantages -

  • Process stations do not need to be spaced equally distant from each other nor do they have to be at the same elevation as the hoist can be programmed to account for such variations.
  • An automated hoist can be programmed to sequence carriers through the process stations in any order and with different dwell times at each process station.
  • Because the hoist only lifts one carrier at a time, the mechanical drives do not need to be as powerful as those for a walking beam transfer system.

Considerations -

  • Carriers can not be processed through the system any faster than the longest process time will allow.  This, in essence, limits an automated hoist system to the same situation of having all process steps of equal duration just as in the case of a walking beam or pusher system unless multiple stations are added where additional time is required.
  • Due to inherent limitations in the speed of movement of the hoist, a process consisting of a number of relatively short duration steps may exceed the capability of the hoist thereby limiting the throughput of the system.  This is especially true if a large number of process steps are involved.
  • As the number of process steps is increased, the demand on the automated hoist grows.  Should the total time required for all transfers to occur exceed the time required for the longest process step, throughput will drop as the hoist can not return quickly enough to make the required transfer.

Increasing Capacity -

In many cases design engineers resort to a scheme using several moving hoists mounted on a single track in an effort to meet throughput requirements.  Although one would intuitively expect to double throughput using multiple hoists, the actual benefit is usually considerably short of that goal.  Throughput rates are notoriously difficult to predict, especially in the cases of “smart” controls which have decision making capability (which carrier must be moved next to satisfy process requirements).  In a case where one or more process steps are confined by a maximum dwell time (unless it is the maximum time of all process steps), special care should be taken to assure that process requirements can be met with the proposed hoist configuration.

In summary, automated hoists, when properly designed, are a great way to move parts through a cleaning system.  Automated hoist transfer systems, despite their capability and flexibility, do have limitations which should be considered before committing to such a system.

-  FJF  -

Automation – Walking Beams and Pushers

June 13th, 2014

As in nearly any decision making process, one must consider the options available and the pros and cons of each for industrial parts cleaning automation.  The next few blogs will describe some of the automation options that are available for industrial parts cleaning along with their benefits and potential shortfalls.

Walking beams and “pushers” have been around for a long time and are relatively simple in concept.  They are both implementations of the same basic idea of simultaneously moving a number of racks or baskets of parts from one process step to the next through the use of an indexing system.

Walking Beam -

Illustration of a walking beam transfer system

A walking beam transfer system lifts all baskets or racks simultaneously and indexes them to the next position.

A walking beam is usually positioned above the process path but is offset to the side in some cases.  This beam has several hooks that can engage the items to be moved.  The beam lifts the items being processed and indexes them to the next process position.  Once the items have been moved, the beam dis-engages and returns in preparation for the next indexing event.

Illustration of a pusher sysetem

In pusher automation, parts are lifted to a height above the process tanks on roller or slide tables and are engaged by a device that pushes them to the next process station. Once moved, they are lowered by the lift tables and the “pusher” retracts in preparation for the next indexing event.

In a pusher system, baskets or fixtures are lifted by a lifting means in each tank to a height above the top of the process tanks.  They are then pushed from one process station to the next by a sliding device which engages each basket.  Rollers or sliders are usually provided to bridge the gap between process tanks.

There are many variations of walking beam and pusher systems but, in general, any device that indexes baskets or racks from one station to the next simultaneously falls into this category.  The benefits and limitations are similar in all cases.

Advantages -

Walking beam and pusher systems are quite simple and are “clockwork” in nature.  The same linear motions are repeated each time parts are indexed.  They are both very efficient and able to fully utilize process resources as there is a basket or fixture in all tanks except for the short time while the transfer takes place.

Considerations -

I have used the header of “considerations” here as not all of the below are necessarily disadvantages but, rather, are things that must be considered when thinking of a walking beam or pusher system.

Flexibility – Once a system has been installed, the only real variable is the time between indexes.  It is not possible, for example, to change the time in each individual process step making it different from the others.  The only option is to use duplicate process stations to multiply process times if needed.

Spacing – Because all indexes must be the same distance, spacing between stations must all be the same.

Structure – In the case of the walking beam, the beam must be able to lift the entire weight of all baskets or fixtures at once.  The result is that air cylinders or other mechanical drives must be of higher capacity than those of a system that lifts one basket or fixture at a time.

Walking beam and pusher automation systems have served industrial parts cleaning well for decades and continue to be popular in simple applications where processes are well-established and not likely to change over time.  In general, they are economical, require little maintenance and are easy to understand and control.

-  FJF  -

Automation – Load and Unload

June 9th, 2014

This series of blogs about automation will address automation as an “add-on” or accessory to an existing or planned cleaning system.  In many cleaning systems, automation, or at least partial automation, is an integral part of the system as it is required for effective cleaning process.  This is especially true of spray washers which often rely on part rotation to allow spray access to all surfaces of the part(s) being cleaned.  But even with systems which already have automation, adjuncts like conveyors and lift tables can be added to facilitate or simplify material handling.

Load and Unload Locations

One of the basic considerations for an automation system is where the parts will be picked up and where they will be delivered.  Depending on circumstances, load and unload stations can be arranged to facilitate the flow of parts through the manufacturing process – as long as the parts are being moved anyway, why not utilize the move to your advantage?  For example, in some instances, it may be best to have the load and unload stations immediately adjacent to one another to eliminate the need for a single operator to walk from one end of the machine to the other to load and unload parts.  In the case where there are two operators, however, delivery to a downstream location may be a desired feature.  One example of this might be when cleaned parts, once cleaned, are delivered directly into a clean room or packaging area.

To assure uninterrupted process flow, it is often beneficial to provide a conveyor at the load and/or unload positions capable of queuing 20 to 30 minutes of production ready for cleaning or transfer to the next step.  This is usually accomplished using either a simple gravity conveyor or a powered conveyor with sequencing capability.  When space is limited, load and unload conveyors can be positioned perpendicular to the flow of parts through the system to reduce the length of the overall machine.

Other considerations for the load and unload stations might be a lift table or hoist in the case of large or heavy parts or a demagnetizer for parts which might hold residual magnetism from the manufacturing process.

One thing that is often overlooked in the design of automation is that parts baskets or fixtures used to process parts through the cleaning process need to be returned to a location prior to cleaning for reloading once emptied.  In a high volume cleaning operation, a secondary conveyor might be used to return baskets from the unload end of the machine back to the load station or another point prior to cleaning.

Although the above considerations may seem trivial, they provide the starting point in the design of an automated cleaning system.  Just as important (but more obvious) are knowing the exact cleaning process as well as the required system throughput in terms of the number of baskets or fixtures of parts to be processed.  In the next several blogs I will describe a number of automation options along with their benefits and shortcomings.

-  FJF  -

Automation – Introduction

June 3rd, 2014

Many cleaning systems are automated.  There are a number of benefits that can be realized through automation -

  • Reduced labor cost
  • Increased throughput
  • Improved process consistency

Reduced Labor Cost

The cleaning process is inherently labor intensive.  Parts to be cleaned must be prepared and fixtured or put into suitable carriers.  They must then be moved through the steps of the cleaning process following a prescribed program of times and temperatures.  Features including such things as ultrasonics, sprays, rinses, etc. must be activated as required.  Finally, parts must be removed from baskets or fixtures and moved on to the next process step.  Granted, it is difficult to remove all labor from the cleaning process but automation can reduce the labor requirement significantly.  Automation, for example, can move baskets or fixtures through the cleaning process leaving the operator free to perform other tasks.  Automation can also be utilized to initiate process functions as necessary to assure proper processing.

Increased Throughput

Although automation can not increase the capacity of a cleaning machine already running at full capacity, it can assure that the ultimate capacity is met on a consistent basis.  A manually operated machine, even with automatically timed process steps, is often not attended full time.  Operators are distracted with other things, take breaks, etc. resulting in usable machine time being wasted.  Parts, for example, staying in a tank for a time beyond that required by the process will detract from overall process efficiency.  Automation can help minimize this inefficiency especially if provisions are made to queue parts prior to and after processing so that processing can continue even if the operator is absent or distracted for short periods of time.

Improved Process Consistency

Although often not considered as a major motivation for automation, when process consistency is important, automation is almost a necessity.  Properly designed automation can assure that each part processed gets the proper treatment.  This is especially true in cases where different parts processed through the same system require different treatment.  In most cases, process times migrate toward the maximum of all process steps (time, temperature, etc.) in the interest of assuring that all parts receive sufficient treatment.  In cases where over-treatment is a possibility, all parameters will migrate toward their minimums in a manually operated system to avoid the possible consequences of over-treatment.

Where to Start?

The exercise of defining an automation system can be a daunting one indeed.  The decision making process is seldom well-defined nor is it intuitive.  A good starting point is a full understanding of the process, the geometry and number of parts being processed and, finally, the “culture” of the manufacturing operation surrounding the cleaning system.  By “culture” I mean things like what is the process flow, what is the lot size, can part lots be mixed or must they be kept separate, are the parts to be processed handled in baskets or individually, is cleaning continuous or intermittent, etc.  The type and layout of an automation system will, in part, be a result of these and (often) other factors as well.  In upcoming blogs I will attempt to better define various automation options and how one goes about the decision making process for automation.

-  FJF  -

 

What is a “Closed Loop” cleaning system?

May 23rd, 2014

The ultimate “closed-loop” cleaning system would produce no effluents requiring disposal. In many manufacturing facilities, disposing of waste liquid containing chemicals and other contaminants removed from parts being cleaned is either very costly or just not possible using municipal sewer or other available facilities.  Even in the best cases, most cleaning system effluents require pre-treatment prior to disposal.  Municipalities frequently monitor waste coming from manufacturing locations on a connection by connection basis and, believe me, you want to be in compliance or you will pay!  In many instances, the only solution is to have collected waste hauled away for treatment – an expensive option unless your effluent stream happens to include precious metals such as gold or silver!

A “closed loop” cleaning system may reduce the cost of disposal of used process liquids significantly or, in some instances, completely.  The basic concept is that water is used in progressively dirtier process steps and, in the end, is disbursed by evaporation as water vapor.  In order to successfully accomplish this goal, water usage must be minimized.  The article Ten Minutes to better Rinsing demonstrates how water usage for rinsing can be significantly reduced by using a reverse cascade system.  If the required amount of water required to refresh the cascaded rinse tanks can be reduced sufficiently, then it may be possible to use the overflow from the first (final in the sequence of flow) rinse tank to makeup the cleaning tank.  If the evaporation rate from the cleaning tank is sufficient to vaporize all of the liquid received from the rinse tanks then a “closed loop” system has been achieved.  There are no effluents.

The problem with this scheme, of course, is that eventually all contaminants removed from the parts being cleaned will end up in the cleaning tank.  Although solid contaminants (particles) may be removed from the cleaning tank by filtration, water soluble or emulsified contaminants will continue to build up in the tank until cleaning is no loner effective.  Therefore, even in a well-engineered “closed loop” cleaning system removing contaminants that can mix with water, the solution in the cleaning tank will occasionally need to be dumped and replaced.  Disposal of the dumped, contaminant-laden liquid can be achieved by evaporating the liquid in an evaporator, by neutralizing it and sending it to drain, or by hauling it away for disposal.  In any event, there is a distinct benefit to reverse cascade flow even if a totally “closed-loop” system is not achieved.

Water is an expensive commodity.  Disposing if it once it’s used is often even more expensive.  Any means that can be put to use to reduce water consumption, therefore, reduces the cost of cleaning.  Although not applicable in all situations, consideration of a “closed-loop” system may be well worth the effort.  In most cases, it will mean that the initial cost of the equipment is higher, but the higher cost is repaid multi-fold by the savings in water and waste disposal.

-  FJF  -

 

Ultrasonics – Frequency vs. Exposure Time

May 13th, 2014

The blog about maximizing the effect of multiple frequency ultrasonics explained in some detail the mechanics of cleaning using multiple ultrasonic frequencies and the benefits of sequencing through a series of frequencies multiple times to achieve maximum cleaning effect.  This blog will further explore these benefits especially in cases where extended exposure of a part to certain frequencies and power intensities may cause damage to the part.

It is well known that lower ultrasonic frequencies have a higher likelihood of damaging soft substrates including aluminum, brass and others.  This is mainly because lower frequencies produce larger cavitation bubbles which implode releasing more energy than the smaller cavitation bubbles produced at higher frequencies.  It is also well known that if there is a tendency for a particular frequency to damage a substrate, the potential for damage is increased with longer exposure times.  Most substrates survive short exposures to lower frequency ultrasonics without damaging effects.  Yet, the value of using lower frequencies can not be denied.  Using multiple discreet ultrasonic frequencies allows utilization of lower frequencies while minimizing the potential for part damage by limiting the exposure time and/or intensity at lower frequencies.

For example, consider a part that is contaminated with particles of varying sizes which requires the use of multiple ultrasonic frequencies to effectively clean.  Also, let us assume that this part is susceptible to damage due to cavitation erosion if exposed to high power low frequency ultrasonic energy for an extended period of time.  A microprocessor controlled multiple frequency ultrasonic cleaning system offers several options.

1) A frequency sequence that limits the exposure time at lower ultrasonic frequencies.  This provides the benefit of lower ultrasonic frequency to remove larger-sized particles while staying below the time limit threshold for part damage.

Multi-Frequency sequency with varying time

In this example sequence, the time at lower frequencies is reduced to prevent cavitation damage while still benefiting from the full range of frequencies available.

2) A frequency sequence limiting the exposure intensity at lower frequencies that may cause part damage.

Multi-Frequency sequence with varying power

In this example sequence, the power at lower frequencies is reduced to prevent cavitation damage while still benefiting from the full range of frequencies available.

3) There may also be instances where both time and power as well as the sequence order itself is beneficial.  The following illustrates a sequence utilizing all of the variables.

Multi-Frequence sequence with all variables utilized

In this example, sequence, time, power and sequence order are all varied.

Realize that the above only illustrates the use of a three frequency system.  Today’s ultrasonic equipment offers as many as 7 frequencies depending on the manufacturer and model.

The next question, of course, is which combination of variables works best for what application.  Unfortunately, this is, as yet, uncharted territory.  There are “indicators” which suggest that particular sequences are more effective than others but the proof, as always, is in the result.  My personal guess is that frequency sequences, time and power will, in time, become as important as today’s time, temperature and chemistry options.  As cleaning requirements become more and more challenging it is good to at least know that the options are available even if not fully explored.  I am sure that diligent researchers will utilize these options to achieve the desired result.

-  FJF  -

 

Ultrasonics – Applying Multiple Frequencies for Maximum Benefit

April 9th, 2014

As described in a preceding blog, changing ultrasonic frequency has a demonstrated effect on the size of particles removed from a contaminated surface depending on frequency. Low frequencies produce large cavitation bubbles which implode with high energy.  High energy implosions are able to dislodge relatively large particles from a surface. Higher frequencies produce smaller cavitation bubbles which, although they implode releasing less energy, are able to remove smaller particles as they are able to penetrate the “barrier layer” found near the surface being cleaned.  This knowledge and the relationship of particle size vs. the ultrasonic frequency can be very useful when contaminants consist of consistently sized particles.  In the real world, however, a particle population is likely to consist of particles of varying sizes in which case the frequency/particle size relationship becomes a bit more complex.

Being aware of the benefits of using multiple frequencies for cleaning, many users employ ultrasonic equipment which is capable of delivering ultrasonic energy at a number of discreet frequencies.  The general notion is that one first subjects a part to cleaning at one frequency and then at another to derive the benefits of multiple frequencies.  A model can be put forth, however, that tells us that a better way would be to sequence repeatedly through a number of frequencies.  Let me explain – -

First, we must assume that the contaminant involved consists of a collection of particles of varying size distributed on a surface.  Further, let us assume that the particles have a natural attraction to both the substrate (surface being cleaned) and to each other.  The illustration below shows such a situation.

Illustration showing a surface contaminated with particles of varying size

The substrate (surface being cleaned) is contaminated with particles varying in size. The smaller particles in this illustration would, typically, be 1 micron or less in size.

The effect of first applying a lower frequency will be to dislodge and remove the largest particles on the surface as well as some of the smaller particles near the surface.  The remainder of the smaller particles on the surface are not affected as a result of the barrier layer which is created by the remaining larger particles below as shown in this illustration.

Showing the effect of the first application of low frequency ultrasounc

First applying a low ultrasonic frequency results in the removal of large particles on or near the surface of the contaminant layer.

Applying a higher frequency will remove some of the smaller particles remaining on the surface.  However, due to their lack of intensity, the larger particles will remain as shown below. 

After the application of low and high frequency ultrasonics

A higher frequency will remove smaller particles from the surface of the contaminant layer but leave more larger particles exposed.

The exposed layer of larger particles is removed by a subsequent application of lower frequency ultrasonic energy.  However, smaller particles thus exposed are not removed due to the barrier layer effect.

Second application of low frequency ultrasonics

A subsequent application of low frequency ultrasonic energy removes more large particles exposing more small particles below.


 Remaining small particles are removed by a subsequent application of high frequency ultrasonic energy.

Removal of smaller particles using a second application of high frequency ultrasonic energy.

Remaining small particles are removed by again applying high frequency ultrasonic energy.

The message of the above is that simply applying one, two, or more ultrasonic frequencies in succession may (likely) not produce the required cleaning effect in many situations especially if the particle population is made up of particles of differing size.  Repeated sequential application of frequencies may produce a better cleaning result.  The appropriate “recipe” of frequencies and exposure times can be developed by conducting cleaning trials and comparing the results using various combinations of frequencies.

-  FJF  -