In part 1 we covered the basics of metal manufacturing bringing you to the point of having a raw casting, this part discusses adding finishes, colors and imprints.
Pre – Finishing
Any metal product, regardless of how it is produced, comes out of the die with rough edges and dull surfaces. The quality of the finished product is completely dependent on the pre-finishing work done before plating, or color filling.
The first step in pre-finishing is to remove rough edges, or die parting lines. This is done by tumbling the pieces in special finishing media (this process is called tubbing or vibing). As the pieces tumble (or vibrate) through the media, sharp edges are dulled and metal shavings are removed. One sure fire way of gauging the quality of a piece is to check the edges for sharpness. All PinLine pieces are pre-finished to remove all sharp edges – so our pieces will never catch on clothing!
After tubbing, other pre-finishing steps are done. If a piece has many flat areas in the design, it will require polishing to bring the flat areas to a bright shine. Many manufacturers will offer polishing but give you just another tubbing operation. At PinLine our expert polishers hand polish the pieces on a special felt wheel – using methods long forgotten by our competition. The end result is a quality finish that is unsurpassed bringing the piece to a high shine.
Another common pre-finishing technique is “satin” finishing. This process produces a brushed look in the final product. The piece is rubbed against a polishing wheel which is spinning at high rpm. The wheel is coated in a special rubbing compound which produces the satin look.
Finally, after the piece is polished (or satined) it must be cleaned. Because polishing compounds are very viscous, we must use a special cleaner to remove the polishing residues. At PinLine we use a special, environmentally friendly de-greasing agent in an ultrasonic cleaning machine to remove all residues prior to plating.
Once the piece is tubbed, polished and degreased it can now be plated. Plating (electroplating) adds a thin layer of metal (e.g. gold, silver, nickel, and brass) to the piece. The piece is submerged in a liquid solution containing particles of the desired finish.
As an electric current is passed through the piece, the particles come out of solution and permanently bind to the piece. There are two main methods of plating: barrel plating and rack plating. In barrel plating the pieces are tumbled in a barrel that contains the solution. As they move around in the barrel they come in contact with electrodes which charge the pieces and bind the metal to the piece. In rack plating, the pieces are individually placed on a rack which is then submerged in the solution. The rack is attached to the electrodes and the current passes through the rack and into the pieces, binding the metal to the piece. Both processes accomplish the same thing but there are significant differences.
Barrel plating, as we described above, tumbles the pieces around. If you are looking for a high gloss finish, you would not want to use barrel plating because during the tumbling process, the pieces will bang together causing pits. However, if the piece does not have flat polished surfaces, barrel plating may be appropriate. For highly polished pieces, or for large heavy pieces, rack plating is preferred. As you might have guessed, rack plating is more expensive because it requires that the pieces be placed on the racks by hand. At PinLine we are experts at all plating finishes and will gladly help you design a piece and choose the appropriate finishing process.
Now that we have a piece that has been tubbed, polished and plated – it is time to add color. There are a number of ways to add color but for most processes there is one key concept to keep in mind. Color is applied as a liquid (like paint). Think of the color as water in a river. A river needs a left bank and a right bank or the water would spill out and you’d have a flood! It’s the same way when adding color on a metal piece. You need a wall of metal to contain the color – that means that you can’t have colors right next to each other without a wall between them or they would run together. This is a key point to remember because it determines how you’ll design the piece from the start and this principle applies to many processes (cloisonné, brass etched, jewelry cast, die cast, die struck)
There are ways to add color without having a “river bank” between colors. This can be accomplished by printing over an existing color. We do this quite often on cloisonné (called laser cloisonné) and it can be accomplished in other processes. Everyone at PinLine is able to help you decide which process will achieve your objective and will help you steer your customer in the right direction.
In addition to adding colors to achieve a specific design, you can also use color to create contrast and add depth to a piece. This is called an “antique” finish. Antiquing adds color (usually black) to a textured background to create contrast with a raised area which does not hold color. One way to accomplish this is called black wash. In the black wash process, a thin black paint is applied to the piece, allowed to semi-dry and then excess paint is wiped off. This leaves some dark color in the recessed, textured areas while leaving the top portions shiny.
A second way of antiquing a piece is through a chemical reaction called oxidation. Chemicals are used to cause a reaction between the metal (usually brass or pewter) and air. When it oxidizes, the outer part turns black. Once a uniform black cover is achieved, the top portions are hit by hand on a satin wheel (see above) or tubbed to remove most of the black, leaving the original finish. The recessed areas keep the black oxidized look and then the whole piece is lacquered to stop any further oxidation.