Wet or Dry Finishing Systems?
By A. F. Kenton
President Nova Finishing Systems Inc.
To accomplish surface deburring or polishing, one normally thinks in terms of wet processing using mass finishing systems. That is because barrel tumbling systems have been around since ancient times and these mechanical systems duplicate the natural action of rivers and rocks. Smooth rocks normally relate to bright burnished finishes on metal parts and these are relatively easily accomplished over a long period of time provided one uses hard nearly non abrasive materials and a lot of water. The selection of a non abrasive is important for a bright finish, but not necessarily a smooth finish. The greater the particle size, its weight, and harder the abrasive, normally the better and faster it is for deburring or metal removal.
With today’s alloys and the speed of cutting machinery there is a need for the assistance of special chemical cutting fluids. These chemical products improve all types of machining operations but because of these fluids, that means that water and rocks aren’t good enough any more. Technology has improved the look of rocks so that they now appear in uniform geometric sizes and shapes composed of different abrasive grits with bonding agents holding them together and it is now refer to as media. Neat looking stuff and it is a great improvement over nature in its material removal capabilities. It also behaves in a very predictable manner making mass finishing systems a very reliable secondary operation.
Now, as for the water part of our analogy, we have a problem. Water is water, but the chemicals used to assist cutting operations, is another problem altogether. Chemicals are formulated to perform specific tasks of either carrying away heat, material chips, oils, or other debris. That means that they react with both solids and liquids. That also means that when chemicals dry they leave these chemical residues and soils on parts. Because mass finishing systems are secondary operations, they are not normally done in the area of the primary cutting operations that also means that the composition of the cutting fluids may not be known. Even if they are known, some of the chemicals used are proprietary and their composition are still not known. If multiple machining operations take place prior to finishing operations, it is also possible to be confronted with an equal number of different chemical fluids that can combine and or react with one another to create yet another chemical compound altogether.
To do proper deburring or polishing, one needs to know what one is dealing with. This is important both from the production stand point of the part and the human factor. That is, not only does the product have to clean the part, it has to do it with no harm to the human overseeing the operation or who comes in contact with the parts afterwards. If the chemical used in the deburring operation doesn’t clean the material properly, the surface finishing process will take more time and still may not achieve the desired appearance of the final product desired. Not only does one have to be concerned about the chemicals used in the actual machine operations and finishing processes, the human factor, but also the disposal of all of the debris both solid and liquid of the affluent. Consequently that means that more care or consideration should be given to the selection of the chemicals used throughout the making of a part from a stand point of both speed and safety.
The problems associated with the wet processing and mass finishing systems are not complete even if we can control all the uses of the chemical additives in both the machining and the finishing operations. As mentioned, one of the biggest problems is what to do with the disposal of the liquid waste water. Most chemical additives used today are water based and biodegradable. That means that if you do nothing with the affluent liquids after the cutting or finishing operations, they will eventually decompose and become inert. Simple right! No, not exactly. There are certain metals or material contaminates that can go into solution and don’t readily settle out. Example: If you are mass finishing a lot of stainless steel parts, there is a concern by EPA that the chromium in this material can go into solution and this is a carcinogenic agent. If that happens, even expensive filtration systems may not remove enough of these molecules to be considered safe for humans.
O.K., so what does all of this mean. The bottom line here means that there are a lot of hidden costs and operations associated with wet parts processing or surface finishing that are not readily thought of or considered. Where does the cost of chemicals come in and/or their treatment? What equipment or accessory supplies are needed in the treatment of these chemicals and bi-products? Is there a need of protective or safety ware? What are the costs of all the supplies and indirect labor needed to sustain these operations? What about the costs to dry or treat the parts against oxidation. Remember when things were a lot easier? Questions, questions, questions! Answers? Maybe!
There is an alternative to wet processing. Dry processing media has been know for some time in the mass finishing industry to primarily dry or polish parts. When inorganic pumice is added to organic materials, it is a very effective deburring media. However, the deburring process still takes too long and is not considered a serious option for large volume production. But, everything that can be done in a wet process can be done in dry process. The only difference is the element of time. The main reason for that time difference is because most ceramic deburring media weights about 80 to 110 pounds per cubic feet and plastic materials weight in at around 50 to 85 pounds per cubic foot. The more weight of the abrasive the greater the pressure on the parts, the shorter the processing time. Dry organic materials run between 20 to 35 pounds per cubic foot, consequently their time cycle is at least 2 to 5 times that of ceramic materials in its granular saw dust form using the same equipment.
You may have noticed that the time cycle of dry organic materials can be more than double that of ceramics or plastic media. Ceramic materials are relatively heavy, very rigid and work by exposing new sharp rigid particles of inorganic material; whereas, dry organic materials in their random small size particle form are light weight and very flexible, yet friable. Another reason for these longer time cycles is because the dry organic materials, up to now, have only been available in its common granular lose state of smaller sized particles even with the addition of inorganic materials of like size. That means that there is less energy, pressure, and mass working the part than that of abrasive preform media shapes. All of this means that dry organic media materials behave similar to water and the resistance of the part is significantly greater than that of the media. This is both good and bad. Instead of a breakdown of the parts material only and some of the media, there is a significant fluidic motion similar to cleaning or a polishing action of the organic material in a give and take relationship. This is the same basic problem that abrasive blast systems have in material removal processing, except the breakdown rate of the media is not as great with mass finishing systems.
Processing times for lose dry organic materials can take anywhere from 2 to 5 days in a barrel tumbler, to 24 hours in a vibratory machine system and from 1 to 4 hours in a high energy centrifugal mass finishing machine. To speed up time cycles using dry organic materials, the common practice has been to add wood shapes to the small granular organic materials. In fact the recommended blend is to put 5 parts wood shape to 1 part of the small gain particles. Porcelain or non abrasive ceramic and to a lesser extent plastic materials can also be added to lose particles for greater weight and/or processing than wood. The greater the amount and size of preform shape the faster the material removal or process. However, the more loose particles the finer the surface finish.
Inorganic materials alone can be used dry to deburr parts, but that leave the parts with an extremely rough surface finish and the media itself has a very short life, probably equal to that of abrasive blast systems. However, when mixed with lose dry organic material, the surface finish and life of the media is greatly improved. Typically, pumice is added to the dry organic materials along with a sticky binder to hold the two together. Other inorganic materials such as aluminum oxide, zirconium, can also be used . Polishing additives or rouge can also be added to create a mix or blend for better looking parts. Pretreated dry organic materials are readily available.
Recently, there have been some new developments regarding this dry organic media. Within the last 5 years, a way has been found to make random dry organic particle materials into shapes. This is a patented process that gives this media more bulk and mass similar to the wood shapes. When inorganic materials are blended with the organic materials to create a preform shape, the added weight puts them on par with ceramic materials at about 65 to 85 pounds per cubic foot. The weight factor makes them basically behave and perform the same as ceramic or plastic except there is not need or use of water and chemicals. These new shapes are formed using organic materials and the same inorganic abrasives that ceramics and plastics use, but they are held together in a patented resin bond process. The results are outstanding. You get the same performance of the preform shape, but because of the flexibility of the organic materials, you get greater life of the media itself. According to the manufacturer of the media, life span is 5 to 20 times that of standard ceramic media.
Now, the down side to these new organic media shapes is the cost. Right now the cost of this new material runs around $12.00 per pound. This seems high, but even at this price, the over all benefits and comparable costs may out weight these higher prices. When you take into consideration the cost of water, water waste treatment, the need for chemicals, maintenance of the processing equipment, labor, your costs are compatible. Surprisingly, one of the biggest cost savings is in freight. If your wear rate is 5 to 20 times that of ceramic or plastic, you will not use as much media; therefore, you will also incur a whole lot less transportation costs. Besides costs, you have some other advantages of the dry process itself. Without water, there are no immediate rust problems and no sticking together of flat or light weight parts, your parts come out clean, dry and ready for the next operation and to some extent, reduced surface work hardening.
The negative side of dry processing is the dust or air born particles created during the actual processing of the parts. Because organic material is light weight and friable, a closed loop processing chamber is suggested and this does add some extra expense to all mass finishing systems. This is not a problem for all barrel machines because these systems run completely enclosed. Vibratory systems require some form of cover as do disc machines or an air flow collection system.
Right now, this new organic shaped media is an excellent alternative deburring process; however, because of the inorganic additives, it does not or can not produce the smooth polished finishes of the lose pretreated organic materials. To accomplish nearly buffed, smooth polished surface finishes still requires the use of organic materials and additives in a preferred closed loop system to generate extra heat. There is no doubt that both wet and dry systems have their advantages and disadvantages, but as more and more rules and regulations are imposed, I am sure you will see the use of more dry mass finishing systems. Dry systems, by their very nature are cleaner more easily controlled than wet systems.
The distributor of these supplies in North America is Finishing Associates Inc. a Sinto America group company located in Huntingdon Valley, PA. They also make high energy equipment and special machine systems that have their own dust collection or venting features to handle this new media.
For more information, contact A.F. Kenton at Nova Finishing Systems Inc.,Huntingdon Valley, PA., or call 1-800-444-4159
• 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:
PO Box 185, Hatboro, PA 19040 * 1610 Republic Rd. Huntingdon Valley, PA. 19006
215-942-4474 * 800-444-4159 * Fax 215-444-9982