A. Polishing and Buffing
Polishing and buffing are operations that are frequently performed on the workpiece prior to the plating operation. This is because after fabrication the article may contain numerous surface defects in the form of scale, pits, mold marks, grinding lines, tool marks, or scratches. These defects must be removed prior to plating if a satisfactory finished part is to be expected. The article is processed in the following sequence as may be required depending upon the surface defects that may be present.
Grinding may be considered a machining operation; it is used to remove a substantial amount of metal from the surface. Grinding operations will generally leave the surface with a finish of 20 μin. rms or rougher.
Polishing is a continuation of the abrasion process; it is used to remove grinding lines as well as numerous other defects. Polishing is accomplished with either an abrasive belt or with a cloth wheel to which a suitable abrasive has been bonded. Polishing will also remove a substantial amount of metal from the surface but will leave the articles virtually free from undesirable defects. Common abrasives used in polishing include silicon carbide and aluminum oxide with grain sizes ranging from a #60 to # 320 depending upon the amount of metal to be removed.
Buffing is also an abrasion process but with a much finer abrasive used. It has long been known that surface metal flow accompanies metal removal. The purpose of buffing is to produce a bright, smooth, and scratch-free surface, and it is used just prior to electroplating to provide a finished product of suitable surface quality. The buffing operation uses cloth wheels to which a compound is applied. These compounds usually contain a glue or binder as well as an abrasive. The abrasives used in buffing may be similar to those used in polishing but are of a much finer particle size. Typical abrasives used in buffing range down to 8 μm particle size. The selection of proper compounds, abrasives, wheel type, wheel speed, and pressure is of paramount importance in order to produce the desired finish. Some of the newer plating solutions have the ability to level out and cover surface defects, thereby avoiding the polishing and buffing operations.
Most polishing and buffing operations are performed by an operator who processes each individual part. Automated equipment is used where the volume of a particular run warrants the expense. For the most part polishing and buffing is expensive and time consuming, thereby adding substantially to the cost of the finished article. Minimizing this cost is an important design objective.
B. Mass Finishing
Mass finishing is an operation used to remove surface defects and is similar to polishing and buffing in that it produces a like effect. Mass finishing is used to process parts in bulk as opposed to individual piece part operation. The parts to be processed are placed in either rotating barrels or in vibrating tubs. In addition to the parts, these vessels also contain an abrasive and a compound to produce the desired effect. Depending upon the abrasive used, a grinding, polishing, or buffing action may be achieved.
The only limiting factors in mass finishing are the size of the parts, their mass, and effect that sharp edges and other factors may have on other parts as they tumble or ate against each other and against the abrasive.
Where it can be used, mass finishing is a much less expensive way to remove surface defects than manual polishing and buffing. Generally used on smaller parts, mass finishing can an lower costs by yielding higher productivity with decreased rejects.