Pre-plate Finishing with a Vibratory Finishing Machine
Competition is forcing management to eliminate costly labor operations. Many jobs can no longer justify the expense of hand polishing. Management is relying heavily on the knowledge and advice of their finishing and plating operators to help choose new systems.
One of the major sectors of mass finishing systems is the vibratory machine. While there are automatic buffers, barrel tumblers and centrifuge finishing systems, none can approach vibratory finishing for the low production cost per piece. With the proper application of media and compounds, finishes in the 10 to 4 micro-inch range can be produced in a vibrator. The system can also be automated to eliminate virtually all hand labor. Die-castings can now go from casting machines to vibrator and then to plating without being touched by human hands.
Vibratory Finishing in Pre-Plate Finishing
Generally, vibratory finishers can remove flash or burrs and refine surfaces. Vibratory finishers can also be used as a super active degreasing and chip remover or roughen plastics and metals for good adhesion of coatings.
The vibrator works by causing the media to rub against the pieces. This action is induced by shaking the tub in a circular motion or in the case of the doughnut shaped machines, wobbling. While the load is scrubbing, the vibratory action also causes the load to circulate in the tub at a slow rate. This circulation keeps the pieces apart to avoid impinging on each other. The rubbing activity is not only produced on the outer surface of the piece, but even inside a cup shape or a blind hole. Cleaners or soaps can be pumped through the tub for cleaning or burnishing operations. The action is very rapid due to the high G-force and the fact that the entire load is being cut at the same time and not just a small slide area, such as that which occurs in a barrel tumbler.
Choosing a Media
The type of finishing operation is determined mainly by the choice of media and compound. Polishing or refining operations for die-castings are usually performed in 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. This media is not a very aggressive cutting chip; therefore, do not rely on it to remove heavy flash or burrs or bad imperfections.
Heavy cutting is better done with 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, first rough cut with ceramic media and then refine the surface 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, must be 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. Costume jewelry and boutique 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.
Small die-castings can be run piece against piece, without media, in a dry operation. Parts like electrical fittings or trophy parts 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. In the vibrator, you can expect good flash removal without damaging the details or threads. This process is a good preparation for vacuum plating.
Sometimes parts to be plated have burrs or need the edges radiused. Usually, ceramic media is recommended for steel or stainless steel parts and when practical, plastic media for soft metals. Ceramic media has a good cutting rate and are available in many diverse shapes and sizes. Plastic media is not an effective cutting agent below the 1/2” size. Most of the time,non-abrasive compounds can be used, but occasionally an abrasive compound must be added to cut fast enough to avoid rolling over burrs.
The vibratory finisher is also useful as a degreaser. 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.
In order to get a good plating or bonding on plastic, it is necessary to roughen the surface to accept the conductive material or cement. The pieces can be 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 can be rather difficult to handle. The vibratory method is an attractive, uncomplicated solution since it does not produce fumes and will handle any plastic.
Since most of these processes use water to carry away dirt or lubricants, consideration must be given to the various methods of drying and rust protection.
The most common drying method is a centrifuge with forced hot air. This process is best for small parts. Water displacing dips can be used when the parts are large, but usually leave a film that must be removed before plating. Vapor degreasers can be used instead of a dip, thus avoiding the extra cleaning operation. When the parts have pockets that hold water, it is best to tumble the parts in hot corn cob grit. When the corn cob is kept hot enough, it can absorb the water rapidly without clinging to the parts.
Rust is usually not a problem when the parts go directly into plating, but when they must sit for hours or days, there must be some additional care. The displacing dip usually contains an inhibitor, such as a wax or oil. The other methods require that a chemical rust inhibitor be added to the rinse water or into the water solution in the vibrator. Kramco 1510 rust inhibitor and light cutting compound can provide rust protection for weeks, even on wet parts, and can be rinsed off in plain water. All parts run through any of the processes described here will require at least one hot rinse to remove the soap or alkali film used, with the exception of those dried in the vapor degreaser.