Mass Finishing Media and Compound

MASS FINISHING MEDIA
Media can be generally defined as the loose material contained in the work area of a mass
finishing machine, which, when in motion, performs the work desired on part surfaces. Media
may be natural or synthetic, abrasive or nonabrasive, random or preformed in shape. Much of
the versatility inherent to mass finishing processes can be traced to the wide array of media
types, sizes, and shapes available to industry. What follows is a rundown of the more
commonly used media types.

Natural/Mineral Media
Crushed and graded stone was once the predominant source for tumbling abrasives in the
early days on barrel finishing. Raw source material included both limestone and granite. Some
naturally sourced materials still find some barrel finishing applications today, such as
corundum and novaculite. As a general rule, problems with fracturing, rapid wear and attrition
rates, lodging, and disposal of the high amount of solid or sludge waste material created
mitigates against crushed and graded mineral materials being an effective media for most
applications.

Agricultural Media
A variety of granular media such as ground corn cob, walnut shell, pecan shell, sawdust, and wooden pegs are used in all of the equipment discussed. These dry process media are used in conjunction with various fine abrasive compounds similar to compounds that might be used in buffing applications. These media are often used in secondary cycles, after initial cutting and smoothing, to produce very fine reflective finishes. Attractive decorative finishes can be produced for jewelry and other consumer articles and, by extension, very low Ra finishes can be produced for precision industrial components.

Preformed Media
These media have largely replaced the crushed and graded mineral materials mentioned above. Media preforms are made from either extruded ceramic/abrasive shapes, which are fired, or resin-bonded, or which have been molded. The preform concept was an important
one for the finishing industry. Unlike the more random shaped mineral media, size and shape preform selection could prevent media lodging and promote access to complex part shapes.
The uniformity and predictable wear rates of the media also made it possible to prevent both
lodging and separation problems caused by undersized, worn media. A wide variety of shapes
have been developed by various manufacturers over the years to accommodate these
requirements, including cones, triangles, angle-cut cylinders, wedges, diamonds, tristars,
pyramids, arrowheads, and others.  Ceramic media are generally harder and more abrasive and are customarily used for more
aggressive applications. Plastic media, as a rule, are somewhat softer and capable of
producing finer finishes.

Burnishing Media
Media made from case hardened steel, stainless steel, and other formulations are used
widely in barrel and vibratory equipment to produce burnished surfaces. These media are very heavy (300 lb/ft3 versus 100 lb/ft3 for ceramic media) when compared with other media types and are nonabrasive in nature. It should be noted that not all vibratory equipment can turn or roll steel media. Because of the weight, enhanced or heavier duty equipment may be necessary. The media performs by peening or compressive action; surface material is not removed, as is the case with abrasive media. Burnishing processes with steel media can be used either to develop reflective decorative finishes or provide functional finishes. One attribute of burnishing processes is that part surfaces are often work-hardened, which can extend the service life of components in moving assemblies. Steel media can be extremely long lasting, if care is taken to prevent corrosion of surfaces while in use and/or storage.  Nonabrasive porcelain media are also used for some burnishing procedures and are prevalent
in some centrifugal applications.

COMPOUNDS
Many abrasive and burnishing applications use water with specially formulated compound additives. The proper selection and dosage of these additives (in either liquid or dry powder form) can have a critical effect on the viability of the process. These compounds
perform an assortment of functions including water conditioning or softening, pH control, oil/soil and metallic and abrasive fine suspension to prevent redeposition on part surfaces, rust inhibition, cleaning, foam development or control, as well as media lubricity control. Some special compounds are used to chemically accelerate finishing cycle times; some of these may be intensely caustic or corrosive and may require some special handling.