Cleaning Systems

                 When one thinks of a cleaning system, water and washing come to my mind first and I think that is typical of all people. Since childhood, one sees the washing of clothes and dishes as what is the normal method of waste material removal or the absence of anything on the surface of the material in question. Everything that had to be cleaned seemed to be able to be accomplished in soap and water. Water  was the miracle elixir that dissolved everything. Simple back then, yes? Well no, not exactly. Why? Because not all waste or contaminates can or will be removed with water.

                Next in childhood, it was learned that with a little bit of mechanical energy applied to the water and a chemical additive, your chances of material removal and cleanliness improve. Using an energy transfer device such as a cloth, brush, or wire mesh, your chance of removal were almost ascertained. However, if you are talking about waste or contaminates within the pores of the material being cleaned, then there was and is little hope of material removal unless the surface of the material being cleaned is also slightly removed or modified. Liquid surface treatments can only do so much; whereas, mechanical transfer devices, equipment, or systems deal primarily with the raw material surface profile.

                Most cleaning technologies use energy and molecules of different density, characteristics, and behavior to effect the surface of yet another density of material. One relates to the other and both effect the overall surface finish of both items even though the desired result is to modify just one item. Although the end results of all technologies using this principle is a cleaning process, it is often referred to as mechanical surface finishing when one uses solids. Because of the mass that solids have and the way energy forces are used, a mechanical process normally removes more material in a shorter period of time and has greater physical effects on the dimensions of parts than does liquid chemical processes, even with the use of electrical currents. These systems are so very aggressive that they are normally thought of as deburring or polishing systems rather than cleaning systems, but they do clean. 

Today, industry cleaning problems and methods are more complex than ever before and solutions are not easily correctable using soap and water. In fact, one must start to talk about cleaning options and the word “surface finishing”. Surface finish and surface preparation mean two different things. A surface finish is the final result of the work processes and/or the final appearance of the part. Surface preparation is the final surface appearance of the part prior to being coated with some kind of film or coating. Now, although we have just made a distinction between these two words, common usage of these words and actual processing methods confuse them. That is, we have just said earlier that “ no matter what the appearance, coatings are needed to protect the finish from oxidation”. That means that almost all parts are finished to a surface preparation condition and can mean anything from plated, painted, coated, or a surface profile, or any combination of these processes. Surface preparation is the final appearance of a part prior to another process that seals the materials surface. So the word surface finish can refer to both a part’s final surface profile before or after a finalized treatment. An easy rule to remember is, a surface preparation is a lot rougher than a surface finish.

To determine a surface finish engineers must plan on what type of material treatment is required for the parts of the product to work and hold up properly to a specific environment. Not only does the engineer have to specify the material, size, and the dimensions of the part, it must fit, form, and function. On top of all that, the engineer must realize that the machining dimensions of the part may vary from the finalized dimensions because of the surface treatment required to clean and protect the part.  Wow, that’s a lot to think about and a lot of complex set of variable conditions effecting just one part of an assembly.

As parts become smaller or where performance and reliability one must start to think more and more of surface preparation or modification in order to take into account possible dimensional tolerances. There are three main options for parts finishing. They are: 1. Surface preparation for heavy or thick protective coatings, such as paint or plastic based film products, 2. Surface preparation for thin film chemical coatings or treatments, 3. Surface porosity modification for aesthetic appearances or polished finish. These options and criteria are outline as follows:  

Surface Finishing Options

Type 1            Surface preparation for heavy thickness coatings

A.  Surface finish will be the roughest of all options and the finished    part  will exceed the parts final dimensions because of the coating.

B.  Surface finish should be as rough as possible to increase the surface area for good adhesion properties and/or wear characteristics or longevity of the coating. RMS 35 or higher.

C.  Roughness of surface should not exceed in height the profile of the thickness of the film or coating to be placed on part.

D.  Surface should be as clean as possible from debris, oils, and oxidation. Therefore, cleaning should be done immediately before coating, but part(s) should be dry.

Type 2                        Surface preparation for thin film coatings

A. Surface finish normally requires a secondary modification and that will be the final dimensions of the part, but it can be on the plus side of the tolerance depending on the film or coating.

B. Surface finish requires a smoothing or modification of the part to improve uniformity of the surface profile of the finalized processed part. Normal RMS range is 12 to 20.

C. Roughness profile is not as critical for most chemical treatments; however, the smoother the surface, the more uniform the treatment. See Type 1, C & D above for non-chemical coatings.
 

Type 3                        Polished finishes 

A. Surface finish will be the smoothest of all the options and close to the final dimensions of the part, but on the minus side of the tolerance. If a thin film coating is still required, dimensions may exceed final part size.

B. This process is not considered surface preparation, but a modification procedure or material removal process. The finalized part will either have a textured pattern or mirror finish in the RMS range of 2 to 18.

C. Surface finish is mostly a question or porosity or for appearance sake; however, coatings can still be applied for protective reasons. 

 
                All mechanical abrasive systems are material removal processes, even polishing or buffing systems. The only difference between them is the use of different abrasive size, shape, and characteristics of the particles and how the energy is applied to them. For heavy material removal, the general rule of thumb is the larger and harder the abrasive, the more material is removed in the shortest period of time; however, it also leaves the roughest surface finish. Naturally, the smaller the abrasive and the softer, the smoother the surface finish.

                Abrasive materials used for surface modifications are usually determined or used because of hardness and/or economics. In actuality, the harder the abrasive particle the more material is removed and because of the particle arrangement most hard materials do more scratching than smoothing or polishing. As hard materials break down, their edges get rounded and they become more mobile and better for polishing in a free mobile state. Therefore, for cleaning purposes one has to decide the surface preparation requirement before one selects a mechanical abrasive particle removal system.

            Hardness of minerals is an important factor in material removal rates and processing times. However, they are not the only factors. In fact, size and friability are very import elements. The larger the size of the abrasive particle the greater the amount of kinetic energy is released. Not only do you have energy from an out side source, you also get energy from an inside source. Outside forces are easy to explain. That’s what all abrasive machines systems do. Inside kinetic energy is different. The latter is similar to what happens in an earthquake. Besides the actual release of energy in the form of heat, there is also movement. Displacement and force are amplified in rapid movements of the remaining material that create tremendous pressures against whatever they are in contact with.

                Getting back to abrasives again, I do not know of any study of metal or material hardness to correspond to the mineral hardness; however, there is probably a correlation. That means that there is no scientific relationship for selecting one media over another. Most abrasive media that is used is determined by cost and availability, and that is normally determined by raw material suppliers. That also means that hands on experience knowledge is the best judge for determining media usage. The variable qualities of both materials and media make abrasive finishing systems sometimes appear unscientific, but there is a relationship. Most knowledge of abrasives, deburring, and finishing are just not taught, but are picked up with experience.

                The harder the mineral element, the greater the molecular bond. However, the bond or crystal structure does not relate to the amount of kinetic energy forces. That usually means that as abrasive particles break down and become smaller, they require greater outside energy or force to do material removal. That is both a true and a false statement. What happens is that because the particles become smaller there is actually more surface contact and friction, but generally less bulk and weight per particle. That in turn, reduces the amount of energy and size of the material being removed. That in the true part of the statement. Where the difficulties or false statement comes in is in the load characteristics of the material removal process.

Because of specific gravity, resistance, density, and molecular structure, a physical part is normally larger than the abrasive media and therefore it is more resistant to change. The larger mass will always effect the smaller mass first. That means that the ratio of the part density to the density of the abrasive particles changes.  The material removal rate maybe the same, which is false according to our first statement, but the load characteristics increase, making it more difficult to remove greater surface irregularities. This may still be a difficult concept to understand, because the ratio involved here is a relationship of the abrasive to the material’s surface profile. That is, at first larger, fewer, and heavier irregularities are remove during surface cleaning, deburring, or modification, then as the part’s surface becomes smoother, material removal size becomes smaller and smaller. That gives the appearance of little or no surface modification. In actuality, the material removal rate remains fairly constant, but physical perception is deceived. However, if you go strictly by the weight of the material being removed, then this is a false statement.

                All abrasive media will work up to a certain point then will slowly decrease, based upon the element of weight, mass, and kinetic energy. As long as there is a transfer mechanism in place to apply pressure to both the part and the abrasive, then there will be material removal. However, because we are talking about normal working conditions or processing time, an efficiency point is reached way before the life of the media is used up. As stated before, depending upon the abrasive media, the material removal rate is rapid at first then tapers off until no major material removal rate is noticeable. At some point greater energy force or pressure and smaller media is required to perform finer or smoother surface modification. The point were the performance of abrasive particle appears to decrease rapidly is when new or different abrasive particles or abrasive media should be changed in order to maintain efficient material removal rates. More energy or pressure can only be applied to the media or abrasive up to the point where that energy can be transferred in relationship to the part. 

Let me re-phase that last statement. With the exception of a flat smooth surface, irregularities in material surface profiles will always exist to some extent on a plane of reference, if only because of the porosity of the material. However, the size and/or amount of the irregularity can be reduced and will continually decrease to a point were the size of the abrasive media will not work efficiently. Weight and mass are the most important factors to remove large amounts of surface irregularities. However, a point is reached where weight and mass become a liability. At some point, which depends upon the characteristics of the abrasive media used, the breakdown rate or friability of the media becomes a more important factor than weight and mass. Technically speaking, you can not get a smoother surface feature better than the abrasive particle size you are using, because the particle, if it does not break down into a smaller particle, will actually produce irregularities that correspond to the physical abrasive particle size in use.

The only way to get around this rule of size is if the abrasive material is allowed to float or give in relationship to the pressure applied to both the abrasive and the material being worked. However, because most abrasives are fixed or bonded to a rigid surface there is little ability to float, unless you add air or water to the process. Then again, if abrasives are permitted to free float this may not produce a uniform surface; therefore, contact pressure is very important in material removal rates and surface finishes, where time is important. Now, that means that how energy is applied and transferred to the abrasive particles are also very important to material removal processes.

As mentioned, to produce finer material or edge finishes requires smaller grit sizes, longer time cycles, and more aggressive processing methods. The fastest way to get to a smooth polished finish, is to prepare or refine the edge or surface features with coarse or larger media first, then rework this same area with consecutively smaller grit sizes using multiple steps. This step procedure is a slow process, but it is also faster than using only one small abrasive size media to accomplish the same finishing task. Again, time or efficiency are factors that must be considered.

                It is absolutely necessary that the abrasive particle does change or break down in order to carry away burrs, debris, or surface irregularities. The results of not breaking down or becoming smaller is excess energy imparted unto the media causing a condition resembling an orange peel. That also means that the material surface may become more dense where impacting occurs to create a condition that is called work hardening which is similar effect produced by heat treating materials. In abrasive processes, the particles can deform the material surface being worked, creating either visual or microscopic indentations leaving the surface rougher than what its surface profile started out. This is a common condition that results from blast finishing systems.

                Naturally, the smaller the average size of the abrasive particle or media used to modify the surface, the finer or smoother the material finish. Also, the slower the movement and pressure against the part or surface, the finer or smoother the finish. Again, there is a point of economics here, so time is another factor or requirement for finishing systems to consider. I guess what I should also say here is that initially a larger abrasive to the surface irregularity is desirable at first. The next factor to consider is the hardness of the materials being worked and the abrasive particles, because there are different friable breakdown rates of abrasive materials. You need a media that will wear away and remove surface irregularities before the abrasive particle looses its transfer energy ability to abrade. Energy force, or how energy is applied to the materials surface must be considered in all surface modification systems.

                The concept of energy or force applied to abrasive particles is a major factor both in surface finish quality and the speed of the process. However, there are trade offs. Therefore, not only does one have to consider the abrasive and size to use, but he needs to know the quality and economics that can be achieved using these energy transfer systems. That basically brings us up to finishing equipment. How energy or equipment uses energy is critical to the processing element. Today, there are 5 basic energy or equipment systems that do surface finishing or material removal, 6 if you consider hand operations. Of the original 5, only 3 are mechanical systems which involve abrasives. The other 2 systems deal with liquids and temperature.

                What determines what equipment to use is largely determined by what equipment already exists in house. However, just as there are surface finishing options, so too are there equipment or processing options. Once the surface finish requirement is determined, the equipment and method to achieve that surface cleaning or preparation needs to be selected. How equipment uses energy is critical to the element of time, quality, and economics in any process. Up to now, selection of a system normally depends on in house equipment and/or ones self taught knowledge of systems. To simplify this process, I have come up with a classification system based upon how energy forces are applied to the media processing the part(s).

                   Although we are talking about cleaning systems here, the classification system was developed for all deburring or material removal systems. Initially we are talking about a single digit equipment classification system from 0 to 6, we must also take into consideration dual function systems; therefore this could be either a single number or two digits. Additionally, we must take into consideration the type or amount of material each system is capable of removing. This will be a single digit number from 0 to 4. Then most equipment systems concentrate their energy forces to work a certain geographical area of a part. This too will be a single digit from 0 to 3.  Therefore the part number system consist of either a 3 or 4 digit number, or a 0100 to 0500 for single function equipment systems or 1100 to 5500 for dual function equipment systems. An explanation of the system is outlined as follows:

Equipment Classification

Type 0           This system is for manual hand working of parts only. Energy is directed downward in a back and forward or circular pattern with an abrasive medium. The greater the force downward, the greater the abrasion.

Type 1           This system is used on relatively flat materials where the energy forces are directed down and parallel to the material being worked, via a wheel, disc, or belt. The results of action create a horizontal wiping action and the smoothing of surface features.

Type 2           This system is used primarily for surface preparation for heavy thickness coatings. This uses the abrasive blast method where the energy force is transmitted to a solid particle which is directed perpendicular or downward at a slight angle to the work piece.  The results of the action are a rough textured surface finish.

Type 3           This system is used in mass finishing type equipment. This uses abrasive particles or preform shapes in a random combination or mixed energy forces or patterns that occur in all directions relative to the part.  The results produced are modified blended surfaces and uniformly worked parts.

Type 4           This system is used in the plating industry. This is primarily  an electrical current directed through a liquid medium type energy force system. The results produced are both surface and sub surface molecular changes to parts.

Type 5           This system is an air based, high temperature heat method. This is a very selective material removal system that works primarily on surface irregularities or burrs. The results of this process vaporize and melt thin surface protrusions.

Burr Classification

Class 0          Burrs or material irregularities do not exist, but surface modification is required.

Class 1          Burrs are sharp edges which can cut one’s finger or cut wire or tubing over a period of time and/or vibration.

Class 2          Burrs are thin irregularities which can be removed from part with one’s fingernail. Material thickness approx. 0 to .010.

Class 3          Burrs or material irregularities require greater pressure to remove than by the unaided hand alone. Material thickness approx. .010 to .020.

Class 4          Burrs or material irregularities require a lot of pressure and force on media and part. Material thickness over .020.
 

Burr Location
 

0  Location    For surface modification only.

1 Location    For easy to reach outside dimensions.

2 Location    For difficult to reach inside or internal dimensions.

3 Location    For all burrs, inside and out, and surface modifications.

                                      Deburring Equipment Classification Chart

               System                                       Range                                 Option & Sequence

                                     Equipment Classification Evaluation by Category

                           Equipment                           Burr Class                      Location