Improving Appearance of Aluminum
Q. My product uses extruded aluminum pieces that measure about 1 × 3 × 5 inches. As extruded, the parts have a dull, non-reflective finish, and I want to make them more attractive. They can be buffed to a near mirror finish, but that is an expensive process and exceeds what I feel is necessary. Can mass finishing provide a nice looking, shiny part at a reasonable cost?
A. Yes, it can. The process is known as “ball burnishing” and it can be done in either vibratory or tumble type equipment. Ball burnishing uses steel media along with compounds that contribute to a lustrous finish. While tumbling barrels are good ball burnishers, the part-on-part action leaves nicks and dents on soft metals, and you may find them objectionable. Any of the equipment manufacturers whose equipment can handle the weight of steel media will be willing to demonstrate these processes on your parts. Ball burnishing also provides a degree of deburring and edge breaking to further improve the product. If you do decide on a tumbling barrel, be sure to buy one with a variable speed drive because part-on-part damage can be reduced by lowering the RPM of the barrel. It is also reduced by using higher workloads in the barrel.
It is also possible that ceramic burnishing media will give you a satisfactory appearance; not quite as lustrous as steel ball burnishing, but still very nice. Again, you can compare barrels to vibrators with this ceramic media. One advantage to the ceramic media is the initial cost of media. Filling your machine with steel media can easily cost 15 times as much as ceramic media will cost. Another cost saving is that vibratory machines made to process with steel media are more expensive than those that are made for ceramic and plastic media. Ceramic burnishing media does better deburring and edge breaking than steel media, and it is not subject to corrosion as is hardened steel media. Of course, you can buy stainless steel media, but that is very expensive.
When selecting a burnishing media, you have the same considerations as with deburring media: Will it reach all areas of the parts that need finishing? Will it get lodged? Will it give the desired finish? And so on. Steel ball burnishing media is available in shapes other than balls, such as ballcones, and you may need one of these shapes to better reach all areas of the parts. Your vendor should be a good source of advice.
Surface finish types for commercially supplied stainless steel sheet are detailed in various standards. ASTM A480-12 and EN10088-2 are two; BS 1449-2 (1983) is still available, although no longer active. These standards are very similar in that they define eight grades of surface finish for stainless steel. Grade 7 is “buff polished,” while the highest polish—the so-called mirror polish—is designated Grade 8
Choice of equipment, media and compounds has a major impact on your finishing applications.
Precision shot peening brings an entirely new concept to the field of microabrasive blasting, and it is complementary to its larger cousin. Using glass bead media, several companies have been shot peening for years with microabrasive blasting technology.