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Precision Cleaning and Verification: A Practical Guide

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The last several years have seen a ground swell trend among large manufacturers toward flowing down cleanliness requirements on parts and assemblies to their sub-tier suppliers. Many vendors have been caught ill prepared or perhaps even totally unaware of the issues involved in precision cleaning and verification.


Long ignored, and still somewhat underestimated, the importance of part cleanliness is rapidly coming to the forefront in many of today.s high-tech products. Historically many producers have dismissed part cleaning as an insignificant part of their manufacturing process until they start experiencing field failures and are faced with a barrage of customer complaints. Many learn all too late that their highly engineered, closely toleranced device is rendered inoperable by a tiny particle, often times so small that it can.t be seen with the naked eye. Suddenly they are faced with a steep learning curve, for a myriad of equipment, chemistry, staffing and environmental issues awaits them. This article is written with an eye toward guiding you through this maze of converging technologies to help you decide how to best achieve the cleanliness level that your customer is suddenly requiring of you.


The field of parts cleaning is a wide one, ranging from .gross. cleaning of heavy industrial components to .critical. cleaning for the space program in class one clean rooms and a wide spectrum in between.


In general, my comments will be made with a bent toward small to medium sized .aerospace quality. parts. Please bear in mind, there are many factors that must be considered when deciding which processes best fit your particular situation. For the purposes of this article, I have chosen to address what I can consider to be the top three; they are: man power, equipment and environmental concerns.


The first factor to evaluate is manpower. To many, precision cleaning and verification is a relatively new and mysterious subject; people experienced in the field are scarce. In the event that your customer has imposed a cleanliness specification on you, you at least have someplace to start. It.s now a matter of compliance. If not, you will need someone who is experienced in selecting an appropriate cleaning process and verifying that it.s capable. One of the biggest questions will become; just how clean do your parts need to be and who is going to determine this? If you do not have the expertise, may I suggest a job shop specializing in this field; they have experts on staff that can determine this for you.


A method that I.ve used over the years with a good success is to send parts out for evaluation. They can be quantitatively tested and the existing level of cleanliness (or lack thereof) determined. Based on the material and the geometry of the part, a cleaning process can be recommended and an experimental run made. The part is then tested to quantify the cleanliness level achieved by the selected process. You then test the functionality of the part in your application. If satisfactory performance is achieved, the level of cleanliness and the methods of how it was obtained become the benchmark. You now have a model from which you can setup a duplicate process in house or subcontract it.


The next factors to be considered are equipment and process, which will ultimately be determined by the volume and the type of contamination coupled with the material and complexity of the part. There are several options; most of them entailing significant capital expenditures. But before we go too far, it is always best to determine the nature of the contaminant to see if it can be eliminated from the manufacturing process.


Again, if you do not have this capability in-house there are labs that can do this evaluation for you. Even if you can reduce the contaminant, precision cleaning may still be necessary to ensure smooth operation of a closely toleranced mechanical assembly. There are many options: vapor degreasing, pressure washing, ultrasonics, CO² snow, even mechanical blasting followed by a filtered rinse are common ways to achieve a high level of cleanliness.


For removing the smallest and most tenacious particles, particularly on parts with complex geometries, ultrasonic are very effective. Many frequencies are available but as a rule, .the finer the particles, the higher the frequency.. Frequency also directly influences how the parts survive the process. Low frequencies (in the 25 KHz range) can literally shatter delicate material or worse yet, introduce micro cracks that may escape detection.


Most job shops and certainly all manufacturers of this equipment have labs and can develop a process for you. Chemistry is as important as the equipment. There are untold numbers of manufacturers that will say that their product is the best for your application. So how do you choose?


Being from a job shop environment, many of my customers have dictated what chemistry they want me to use, consequently I have used many different brands. In my experience they are all relatively good under certain conditions but the variables are many; concentration, temperature, and time, to name a few. I recommend that you work with a company that is service oriented and willing to demonstrate the effectiveness of their products. As with the equipment makers, most have labs to do this type of development work and of course job shops by their very nature have a wide variety of chemistries on hand.


You.re now halfway there; but don.t underestimate the importance of the remaining processes. Rinsing, drying, preserving and packaging, all present their own little nuances. If you need spot free parts, you will need to install a deionized water system; it should be to a purity of at least 50,000 ohms and filtered to one micron for good particle control. Drying of the parts can be the most time consuming part of the whole operation. Options range from blowing down with shop air to vacuum bake out ovens. A device which has proven itself to be very convenient and cost effective is a portable dryer consisting of small electric turbine which produces heat via mechanical friction where the intake is filtered to three microns to ensure purity at the output end. You simply blow the parts dry and you don.t have to worry about re-contaminating your parts because the oil trap on your shop air compressor isn.t working properly.


What comes next is what is fast becoming the prominent factor in the whole cleanliness industry and that.s the quantitative verification of the cleaning process. Automated laser-based particle counters, microscopes coupled to dedicated PC.s, microbalances and nano scales along with all the peripheral sample collection equipment including the lab to house them will run into many tens of thousands of dollars.


There are two commonly accepted methods of obtaining cleanliness samples. One entails spraying the test part with sub micron filtered solvent from a pressurized spray can, collecting the effluent and running it through a vacuum filter funnel depositing the debris onto a pre-weighed and or gridded filter. The second method is to sonicate the parts in solvent, remove the parts from the bath, and rinse them with additional solvent. Then collect and filter the effluent as in method one.


The most commonly called out particle analysis criteria are count, maximum particle size and weight, and it.s not uncommon to see a combination of two or even all three. Reading the following specifications will give you a very good understanding for this entire process in much greater detail than I can go into here. ASTM F 303 .Sampling Aerospace Fluids from Components. and ASTM F 312-97 . Microscopical Sizing and Counting Particles from Aerospace Fluids. will give you a good understanding of how to obtain the sample. National Aerospace Standard 1638 .Cleanliness Requirements of Parts used in Hydraulic Systems. will tell you how to categorize them. They do not tell you how to clean the parts. They govern the test procedures, cleanliness levels and the environment for the testing of them.


I.ve made environmental issues the last of the three, but perhaps this should be your first concern because they are the ones that can give you the most heartburn. Nothing is worse than developing a process and having it installed only to find out that your municipality won.t issue you a permit to run it. What will you be removing from the parts and what are you using to do it with? Do you need local, state or federal permits to store, use and dispose of your chemistry? What will you do with your spent cleaning solutions and rinse water, both of which will contain the contaminates removed from the parts? Will one or both of them be reportable as .hazardous. in your waste stream.


In most cases it is cost effective to purchase an evaporator which will greatly reduce the volume of wash and rinse waters. What about the effect of this chemistry on your employees? Is it safe? What are the exposure limits? Some alkaline cleaners are highly caustic and can burn the skin and irritate the respiratory system with very limited exposure. I strongly advise you to investigate this area fully before committing too much time or money on equipment and manpower.


As I stated in the beginning of this article, there are innumerable details that you need to research with regard to precision cleaning. I.ve tried to cover the most important ones to get you started down the right path. Once you.ve waded through all the aforementioned questions for yourself (and some I may have forgotten) you have to come to terms with the final question; who is best to do all this? Can you justify setting up to clean in-house or do you out source your cleaning to someone who specializes in the field leaving you to concentrate on what you do best?