Understanding Media Supplies
For Surface Finishing Using
Mass Finishing Systems
By A. F. Kenton
Nova Finishing Systems Inc.
This article is not a beginners introduction to mass finishing media supplies; otherwise, we would have to explain the variations and differences of the types of equipment available to do surface finishing of parts. All mass finishing systems are designed to do uniform edge or surface modification to machined or cast parts. The major difference in equipment is the application of energy forces which relates to speed or time necessary to produce the desired surface modifications. This article is a comprehensive version of the basic fundamentals of media supplies used in mass finishing equipment.
Next to equipment, the abrasive is the most important element effecting speed or time as well as actual surface texture and smoothness. Just as all equipment will eventually do the job, so too will all abrasives. However, there are literally hundreds, maybe thousands, of media compositions, sizes, and shapes. Therefore, some guidelines maybe helpful to speed you in the proper direction of selecting media supplies for your parts and applications.
The most typical application of mass finishing equipment is to deburr parts en mass. The cheapest and fastest way to do this is to go with the largest size abrasive possible, yet small enough to get into all of the areas that have to be worked without getting stuck. Simple yes! No, not really. To begin with, there are different metals or materials that have a worked and these too have different hardness and finishing requirements. That means that what abrasive is hard to one part, may not be hard to another part and/or capable of achieving the desired end results. Non-ferrous or soft metals normally never get worked with the largest, coarsest abrasive, because the metal will have a rougher finish that it did before processing.
Before we go into depth on abrasive media, we should clarify the term abrasive media. Normally the cheapest form of any abrasive product is random, naturally occurring compounds classified by screen sizes. Surprisingly, little of this material is actually used in mass finishing equipment, but it is popular with blast finishing systems. Why random media is not commonly used is because of the lodging or sticking problem. That is, media that varies in size and shape is more likely to get stuck in parts than a pre-determined shape and size which is more controllable. Hence, such man made media is called a preform shape. In the trade, it is just referred to as the shape it comes in. Also noted is the size of the preform shape and its abrasive composition. The latter description takes into account the abrasive size within the man made bonded preform. Unlike its random counter part, which can be over an inch in size, the largest abrasive used in a preform shape rarely exceeds a particle size of .060 inches. The large the particle size the faster it works.
Typically, the most common abrasive particles used in almost all preform shapes is aluminum oxide, and some have silicon carbide, silicon, zircon, and porcelain. Aluminum oxide comes in many variations and densities. It is about the cheapest material available and handles most of the common deburring applications. Silicon carbide is much more expensive, harder, heavier and is used on most products were parts are welded or joined together. If not cleaned properly, a part worked with an aluminum oxide preform may leave a trace which in any amount can cause welds to fail or bond properly. Zirconia is becoming very popular especially in plastic bonded media. It is the heaviest and hardest of the abrasives and for that reason is very aggressive. In some cases, plastic media can now be used on some ferrous metal products in high energy equipment with good results. Silicon is nothing more than sand and is relatively cheap. The problem is than it is not very aggressive and therefore not very popular, but it does not contain aluminum oxide making it good for silver soldering or brazing of jewelry. Porcelain has no abrasive. It is clay and it is really considered a burnishing media rather than an abrasive. Meaning that it laps or smoothes surface features rather than remove or deburr materials.
You may have noticed that weight was mentioned a couple of times in the description of abrasives. Besides equipment being the most important factor that determines the speed of processing parts, abrasives is second and then weight is a close runner up. Weight or specific gravity normally relates to the density of the materials being used in deburring media supplies. The greater the pressure that can be exerted onto the part the greater the material removal. Technically, if you use a very light weight media shape, but have a machine with a work chamber that is extremely deep, you can achieve the same results as a very aggressive media in a very shallow machine system. The more parts and media you can get into a machine system, the faster they work or process parts. However, the ideal percentage of parts to media by volume is 60% media to 40% parts. For polishing, that percentage changes to 80 or 90% media to parts.
Getting back to the media, most preform shapes used in mass finishing systems are primarily made with a ceramic and plastic binder, but there are other shapes such as steel, stainless steel, and dry organic compositions that are also used mainly for surface modification rather than material removal. An exception to this will be noted below. Ceramic shapes are made with inorganic materials and abrasive particles bonded together with a type of clay matrix. After being formed and cut to shape, the materials are baked to achieve an over all hardness. The clay binders are formulated to decompose at a predetermined rate to expose new sharp abrasive particles. The harder the bond, the longer the shape lasts, but normally the finer the surface finish of the part being worked. The faster the bond breaks down the faster the media removes material and the coarser the finish on the end product.
Ceramic preform shaped media is relatively hard, very rigid and therefore is normally used on hard carbon steel parts or where a lot of material needs to be removed; whereas, plastics are softer, more flexible and therefore normally used exclusively on non-ferrous parts. Other than the bonding materials used in plastic media shapes, the manufacturing process of plastic shapes is the same as ceramics and they behave the same. One exception to this rule is the fact that plastic comes in two different types of bonds and many more shapes because they are molded rather than extruded like their ceramic counterparts. The two types of bonding materials used are polyester and urea. Again, they behave almost identical. The cost of urea plastic shapes maybe a little cheaper and softer than the polyester.
One good feature of plastic media is that it is formulated with colors for easy identification of compositions or coarseness of the abrasive shape. Unfortunately, there are no uniform standards between manufacturers; therefore, the colors of one supplier do not necessarily represent the same composition that other suppliers make. That leads to a lot of confusion by both suppliers and end users. To get around this problem you need to stick to the verbal description of the product rather than a color code.
Another possible problem of both ceramic and plastic media is the water content of the preform shape which effects its weight. We discussed weight earlier to indicate its importance in processing times; however, water content is a different problem. Because we are dealing with solids that have a porosity and a shape that is composed of a matrix, moisture content does and can vary from product to product and also it will change with its environment and/or age. Therefore, weights can and will vary. That also means that when comparing one media performance against another the weight factor is a real wild card that can play havoc with all forms of testing. Again, because there are no uniform standards in the industry, the end user is basically left up to his own to determine performance and costs. There is almost no way to compare apples and apples accurately.
Now that we have discussed some of the criteria concerning preform shapes, let’s talk about the shape in regards to the deburring application. Using my own terminology, there are two different philosophies or theories regarding shapes and parts. Both work. It is only a question of time and there have been no or few conclusive studies to indicate one shape is better than the other. Basically you have what I call stream rollers and bulldozers. One shape predominately rolls and is very mobile. The other shape is very rigid and behaves more like a pusher or scraper.
Uniform cylinder shapes, spheres/balls or curved shapes like cones are very mobile and behave something like both a fixed wheel and a mobile wheel or roller. As these shapes move they rotate en mass and by themselves. This shape works extremely well where you have a lot of holes and the shape can poke itself slightly into the hole and rotate there a little before moving on. Because this shape is so mobile en mass, it also does not hold, support, or restrict parts from reaching the bottom of the work chamber. That means the full weight of the media mass is utilized to put pressure on the parts. The only negative of this shape is that you can not work inside corners or angles very well and that can develop a shadow effect.
Given the same machine system and energy force, the more geometric shapes such as a triangle moves or rotates more slowly, because they are restricted in movement due to alignment to edges that resist movement. That means that that shape basically pushes like a bulldozer as the mass rotates. It also means that it transmits more energy force or pressure to the parts on a more constant basis. This shape is especially good for working edges on parts and because of this restricted movement, parts of this shape remain relative or maintain their position to one another or their original position within the mass and are less likely to have part on part contact.
There are some preform media shapes that utilize both the geometric and spherical shape. Of these shapes the best general purpose shape is accomplished by a pie shaped cylinder called either a cylinder wedge, V-cut cylinder, or tri-cylinder. What is so good about this shape is that unlike all the other shapes that have their center of gravity in the middle of the form, this shape has its center of gravity on the outside edge. A shape with its center of gravity in the middle of its form has a tendency to resist movement and this encourages media to lodge or stick within the inside dimensions of a part. Media with its center of gravity on the outside edge of the shape has the greatest mobility of all shapes and because of its geometric edges and point, it is usually the best of all worlds for deburring purposes.
Now, earlier I made a comment about an exception to deburring or material removal other than ceramic or plastic bonded preform shaped media. Up to this point, all of the media we have discussed, even the random shape material, is normally used in wet processing systems, because dry inorganic materials do not leave parts clean and may also produce parts rougher than before they were processed. Also, used in a dry state, inorganic materials become contaminated and there is no way to rid the media of the contaminates. They just keep getting used over and over again. Therefore, water and a chemical compound are needed and used in most deburring processing.
A dry process alternative is available, but because of the light weight nature of organic materials in its random small size form, the time element is extremely long and almost prohibitive. When dry organic materials in random form is used or combined with pumice it is a very good deburring media for small batches of parts, light weight parts, or jewelry, but not normal on larger volumes of parts. To gain a little faster results, larger precut wood shapes, porcelain, or plastic shapes have or can be added to the predominately organic sawdust like mix of either wood, corn cob, or nut shells and inorganic abrasives.
Now, there is a relatively new media bonding process that produces media shapes from organic materials that are run dry but is difficult to classify as either organic or inorganic. In the last 5 years, a company has developed a patented resin bonded dry finishing media that looks and behaves the same as wet preform ceramic and plastic shapes, but it is used without water and compound. It looks and feels like ceramic media, but because of the manufacturing process, the media leaves the parts smoother and cleaner than traditional wet processing methods and media.
There are four formulations or grades of this resin bond media and it can actually have more inorganic materials than organic. So, how do you classify it? Even at extremely high percentage rates of inorganic abrasive this media should be run dry and it does not have the same dust problems as straight inorganic random or preform shapes when run dry. Presently, the product is produced in different sizes and formulations of cylinders, rollers, and the pushers, the cube and triangle shapes. The key to its use as a dry media seems to be the resin bond. The unique manufacturing and bonding process is suppose to give the media outstanding life. In fact, it is suppose to out last (low attrition rate) standard wet preform products from 5 to 20 times their life expectancy
Because the resin bonded material is run as a dry process, there is a major advantage in waste treatment or the lack of a need to treat wastes. Considerable savings can be realized by the removal of water from any process. In this case cost savings from chemicals and the elimination of maintenance associated with of rinsing, oxidation problems of parts and equipment, inhibitors, and/or drying processes. Another advantage is that parts come out clean and can go directly into another operation without waiting. On the down side is the problem of dust or air born particles; however, with simple procedures and proper care most equipment can use this new media. That means that drains should be closed or sealed and either a cover or a suction air flow system should be utilized with older equipment. Because of the one source and the newness of the product, it is a little expensive, but the cost savings and low attrition rate more than justifies the initial higher costs.
This new resin bonded media is only distributed in the USA by Finishing Associates Inc. a Sinto Amnerica group company located in Huntingdon Valley, PA. This company also make and distribute new high energy equipment designed and built to handle this new dry media process. They offer conversion packages designed to convert existing wet units to operate as dry processing systems.
• Nova Finishing Systems Inc., manufactures small, heavy-duty bowl finishers that stack up to most of the big equipment on the market, but cost much less. Nova series vibratory equipment also comes with the same warranties of the larger machines. Form more information on this equipment line, contact:
NOVA Finishing Systems
559 Crook Street
Hampton, TN 37658
980 429-5773 Tel, 704 665-5658 Fax