Cleaning vs Material Removal
By A.F. Kenton
Nova Finishing Systems Inc.
Where does cleaning begin and material removal take over, or is there a difference?
Does it really matter? Well, yes it does. Cleaning is the material removal process of foreign matter from parent material. It can also be considered the removal of mostly organic compounds from inorganic materials; however, it can also be foreign inorganic contamination. These organic compounds or contaminates are considered a secondary substance which does nothing for the part, but can and does eventually effect the primary parent materials fit, form, or function.
What constitutes a contaminate depends mostly on the end use or the environment in which that parent material part will work and/or for how long a period of time will it safely perform its function. That is, dirt or contaminates are subjective to arbitrary conditions imposed by the end user of the part and based upon acceptable levels of engineering reliability. Under microscopic conditions, all materials will still maintain some form of contamination, it is just a matter of how much is acceptable.
Because of the nature and physical properties of organic materials, cleaning can become a material removal process when the parent material and contamination bond to one another. This is a condition where the parent material is porous or has a rough surface finish. It can also be a result of a chemical reaction caused by contamination or environment working conditions, or if there are adhesion problems. Foreign matter can not always be dissolved and removed with a liquid system alone and surface finishing may be required to do proper cleaning. In some cases we are talking about oxidation rather than organic material deposits and/or their removal.
There are a lot of factors controlling a cleaning or material removal process. Most cleaning systems use chemicals to dissolve, attract, or dislodge foreign matter. Cleaning can also be a molecular bonding or electrical conveyance process. It helps to know exactly what that foreign matter is composed of so that an appropriate cleaning action system and equipment can be selected to remove the contamination. Once a part has been successfully cleaned, the cleaning process itself creates another problem. Kind of like good news, bad news. In a sense, the original contaminates serve as a protective coating against oxidation and by cleaning the part, one opens up the porosity of the parent material to oxygen in the air which starts to deteriorate the part and the contamination process all over again.
So, when is a part clean? I can't answer that question and I'm not sure anyone else can either. A lot has to do with the usage of the part and or its working environment, or in short, specifications that someone has determined will serve the life of the part in its working environment. Most company's just specify the measurement dimensions of the part and its surface finish. The final part finish can not only specify measurements, but preparation steps and sealant coatings. Rarely does an engineer or company specify the amount of time between processes and/or environmental conditions for holding that part until the final surface finish is achieved and/or applied. Normally, the part design and materials used to make the part exceed most working conditions and time lapses in manufacturing are not a problem. Fortunately, common sense prevails in most manufacturing processes and few people question anything about the finished part, except fit, form, and function
Now, getting back to our problem of foreign contamination matter. We know that liquid systems work well on most organic materials and normally achieve accepted cleaning results. However, if a liquid can not get into or at the contamination problem area it can not remove the foreign matter without the use of more physical means or greater pressure systems. Liquid systems can be improved by adding heat to the cleaning process or a pressurized flow can achieve material removal by force, within limitations. At too high a heat a part can be distorted or at too great a force or not enough movement of the part, a pressurized system can actually cut the part. So, there are limitations to liquid systems, but they also cover a greatest range of cleaning capabilities and achieve more acceptable results on most contaminates than other system.
Earlier we mentioned non-soluble problems caused by porosity, reactions, and adhesion. Now, these problems may be all solved, or at least diminished, by liquid systems, but solid mechanical abrasive systems will also work and maybe a more cost effective process, especially where surface and cleaning requirements are important. Most people are familiar with abrasive wheel and blast systems for material removal and cleaning, but mass finishing systems are normally better for smoothing the surface profile of the part. A part's surface profile is very important and determines the proper cleaning method to use, because this is where and how contaminates can be held in place. The smoother the surface profile, the easier it is to clean a part.
The difference between cleaning and material removal can be considered the amount and type of material being removed. Again liquids primarily dissolve contaminates and abrasive methods use a solid to solid method of grinding away and smoothing of all surface materials of the abrasive, the contaminate, as well as the parent material to achieve cleanliness. However and because abrasives are solids and involve a particle break down process of both the medium and the part, a liquid cleaning system is normally still a recommended secondary procedure to insure all loose material removal and/or cleanliness of the part.
Unlike liquid systems, mechanical abrasive systems use a number of equipment variations or versions to achieve material removal. All of these differences involve the use of how the energy or pressure is applied to the medium to achieve the end results. Each system has its own advantage and disadvantage depending on the size of the part, the material, and the configuration of the part. We will not go into these differences and/or applications in this article; however, for single or small quantities of parts, the most common method people use or prefer is the wheel or blast method for cleaning parts. Blast systems are still the best process for parts that will eventually receive a heavy sealant coating. However, if the surface finish is critical or the metal coating is a treatment rather than a coating, this may not be an option. Smoother, more uniform thin plating or coatings can be better achieved using mass finishing methods. Another advantage is that larger volumes of parts can be better handled more efficiently and cost effective by mass finishing systems.
Increasingly, the medical and high tech industries are requiring smoother, cleaner surfaces because of bacteria and harsh environmental working conditions. Machining processes and liquid systems can only do so much and that may not be enough to prevent microscopic particles from becoming a potential problem. So, that means that a mechanical material removal process is required and/or that means the need for abrasives or greater mechanical energy systems. Without the use of strong chemicals, liquid systems will not produce the results that can normally be accomplished using abrasive methods.
Most machined parts and raw materials have a surface finish of about a 35 RMS prior to any secondary process. About the best surface finish one can achieve using blasting methods is about a 35 RMS or higher, meaning rougher. By creating a rougher surface feature, one is increasing the amount of surface area and this surface finish is desirable for coatings to adhere too. A 35 RMS surface finish is actually about the starting point for most mass finishing systems and they can get down to a 2 or 4 RMS. Abrasive wheel systems are used a lot on small parts and they produce results anywhere in between these two methods, but the surface finishing results from wheel systems will never be rougher or greater than what the surface RMS profile starts with.
The biggest visual difference of a blast finished part to a mass finished part initially is its physical appearance. Both processes produce a more uniform surface texture and both processes do remove the oxidation and contaminates that cover a part; however, as mentioned, the rougher texture of the blast process does improve molecular bonding for thick coating materials. For precision dimensional purposes a smoother surface profile of the parent material is more desirable for thin uniform plating treatments and this can best be achieved using mass finishing systems.
To do proper plating or chemical treatment of metal parts, a surface finish of from 12 to 18 is about the best range to apply a uniform plating, sealant coatings, or treatments and this surface finish can easily be accomplished by most mass finishing systems. Finer surface finishes can also be achieved using mass finishing, wheel systems, or lapping methods; however, such surface finishes are not normally required and are not cost effective for the over all need of the part. Any reflective surface finish is normally for the aesthetic appearance of the part, but can be desired for medical cleanliness purposes.
The bottom line here is that cleaning processes may have to include material removal and the best, most effective method to do this in volume is by using mass finishing systems that use abrasives in a mechanical process. The trend in industry is to improve surface finishing standards and/or cleaning for safety reasons and that usually means smoother surface profile finishes. * ( Hopefully, this is the first article in a series to introduce mass finishing systems to those of you who need cleaner smoother parts.)
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NOVA Finishing Systems
559 Crook Street
Hampton, TN 37658
980 429-5773 Tel, 704 665-5658 Fax