Finishing Media

CHOOSING THE RIGHT MEDIA AND COMPOUND The choice of media and compound determines the type of finishing operation. Polishing or refining operations for die-castings are usually performed using plastic media with a mild, alkaline compound. The plastic media can provide a 4 RMS finish on soft metals. The media, being soft and lightweight, can cut without gouging or scratching. Even though the parts have a low RMS, there is not much glitter and almost no reflected image. Copper plate is heavy enough to fill the fine microscopic scratch pattern of this media. After chrome plating, the parts appear to have been hand rubbed.

HEAVY CUTTING WITH CERAMIC MEDIA Heavy cutting is done using ceramic media. It is more aggressive, but does not produce the flatness achieved with plastic media. To obtain a maximum finish on parts with heavy flash or burrs or bad imperfections, rough cutting is first done with ceramic media and then the surface refined with a short run in plastic media. Die-castings or rubber-molded parts that are to have finishes other than chrome, such as paint or vacuum plating, are treated differently. The thin coatings will not show up well over the dull finish of plastic. Ceramic media is used in a heavily glazed condition. When run with the proper soap, the stones take on a glossy surface, causing the stones to burnish somewhat like steel shot, while maintaining a slight degree of cutting ability. Decorative items with a large degree of detail are run in this manner. Steel parts, such as seat buckles, requiring a good finish prior to plating, are also run this way, but with highly alkaline compounds. Harder metals will be polished to a very high luster and be in the 10 RMS range. Soft metals will have a good shine, but not as bright as the steel parts.

FINISHING SMALL DIE CAST PARTS Small die-castings can be run piece against piece, without media, in a dry operation. Parts like electrical fittings or jewlery components are run in this manner. The pieces are run in a vibrator with a small amount of corn cob or sawdust to pick up dirt. You can expect good flash removal without damage to the details or threads. This process is a good preparation for vacuum plating. Corn Cob or walnut shells can be used for cleaning and polishing.

PROVIDING EDGE RADIUS AND BURR REMOVAL Sometimes parts to be plated have burrs or need the edges radiused. Usually, ceramic media is used for steel or stainless steel parts and when practical, plastic media for soft metals. Ceramic media has a good cutting rate and offers a diversity of shapes and sizes. Plastic media is not an effective cutting agent below the 1/2 inch size. Most of the time, non-abrasive compounds are used, but occasionally an abrasive compound is added to cut fast enough to avoid rolling over burrs.

DEGREASING CAPABILITIES Vibratory finishing is also effective as a degreasing process. The energy developed in a vibratory finisher far exceeds the power range of ultrasonic cleaners. Soils, such as silicone oil or lapping compounds, can be removed, defying an ultrasonic or a vapor degreaser’s capabilities. The pieces can be run with media or, if the part permits, without media. While the parts vibrate, a cleaning compound is circulated through the tub. The vibrating action rubs the parts while churning the cleaner into all crevices. The chips and dirt are flushed out of the tub as the machine is running.

IMPROVING PART TO SURFACE BONDING To achieve a good plating or bonding on plastic, it is necessary to roughen the surface to accept the conductive material or cement. Pieces are sometimes self-vibrated with quartz or pumice added as an abrasive. Ceramic media is added when the part has recesses or pockets. The run is normally only about ten to fifteen minutes long. The parts can be thoroughly rinsed in the vibrator before dumping. This process is an alternative to chemically etching the surface which may be impractical. The vibratory method is an optimal solution since it does not produce fumes and will handle any plastic.