Die Casting Porosity Guidebook

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The finish is very hard and durable and, in many cases, produces a true black color which the other methods cannot match. It is used most often as an extension of the brass plating operation. Since the parts are already racked for electrolytic deposition of the brass, they are ready for a second electrolytic operation - in this case, blackening, after thorough rinsing only.

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The bath can be operated as a permanent plating bath in the line, with periodic titration and replenishment and excellent tank life. Many experienced platers find that they can mix their own solution using commodity chemicals, rather than purchase a pre-blended proprietary product. When operated in this way, the operating costs can be quite low. Black nickel works best on racked parts. Bulk or barrel handling methods work less well and usually result in more difficulty in achieving a uniform deposit, due to the continual interruption of electrical contact between the parts.

As a result, the black nickel finish is best suited for use on high value parts which are rack-plated. Verde Green Patinas — Also called "verdegris", this finish is a soft, pale green color, similar to that seen on the aged copper roofs of older buildings. Actually, the authentic green patina formed on these roofs is a mixture of many different copper compounds, including oxides, carbonates, sulfates, sulfides and more. The composition is directly related to the purity of the air in the area. For example, some copper roofs are more black than they are green, due to a higher concentration of sulfur in the air from a coal-burning power plant in the vicinity.

Others are more green, owing to a concentration of nitrates in the air from automobile exhaust. Consequently, the color varies widely. Artificial green patina solutions are, in simplest terms, mildly acidic corrosive copper solutions. They work by slowly tarnishing or corroding the surface of the brass or copper substrate, and forming some of these same green or bluish colored copper compounds.

These finishes can be quite attractive when properly applied. They have two inherent disadvantages, however: the finish takes several hours to form and it is only loosely adherent to the metal surface. Consequently, the green patina solutions sold commercially tend to be workable only in small volume process lines where the finisher can afford to let the parts hang and corrode as they dry. And because, the finish is loosely adherent, it depends on the lacquer topcoat to provide the adhesion to the substrate to form a clean final finish. Once the parts have been colored, or oxidized, to the desired finish, they are ready to be highlighted, or burnished.

This operation can take several forms, depending on the final appearance requirements of the part. The essence of the operation is the removal of some or most of the colored finish to reveal portions of the underlying base metal in order to make it appear worn. In other words, the colored finish is polished off the high points, or highlights, of the parts, and allowed to remain in the recessed areas. The only way to accomplish this task is to mechanically remove the coating from these areas — there is no chemical treatment available to do this job.

There are several proven methods which work well:. Hand-buffing on an abrasive or polishing wheel — The buffing wheel is constructed of many discs of cotton fabric, sewn together to form a single buffing wheel, about half an inch thick. These can be stacked together on a single spindle to form a buffing wheel up to inches wide, depending on what is needed to cover the part most effectively.

Once the wheel is assembled, it can be loaded with different compounds, ranging from abrasive to fine polishing compounds, depending on the type of contrast desired on the part's surface. For example, some parts have designs which have well-defined edges to the details or have sharp corners, etc. These parts generally would be highlighted with a fairly abrasive compound in order to clean off the colored coating completely from the highlights and allow the coating to remain almost entirely in the recesses.

Or, a dry, non-metallic abrasive flapwheel might be used to achieve a sharp contrast. On the other hand, the part may have a rounder shape, with softer curves and no clear-cut, sharp edges.

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This part may look better with a softer contrast burnishing than with sharp contrast abrasive buffing. If so, the cotton wheel would be loaded with a less abrasive compound in order to achieve a softer shading or "feathering" of the colors on the part. Some parts go one step further, requiring no actual removal of the antique finish, but only a softening or burnishing of the coating to blend tones. This type of part might be buffed on a soft, brass wire wheel rather than a cotton wheel and a compound.

This softer wire wheel would not really remove any coating, but merely smooth it out a bit, or impart a soft directional grain to the surface. An alternative method might be to use a wet burnishing wheel - a brass wire wheel wetted with a slow dribble of water to soften the abrasive action. It is easy to see that the hand buffing operation is more art than science. Just as cleaning is important to the integrity of the deposit on the surface, so buffing is critical to the final appearance of the finish and can even determine the market value of the piece. Since the decorative hardware business is all about appealing to "the eye of the beholder", it is important to appeal to the eye of the buffer first.

Automated buffing machines — As in many other aspects of the finishing process, higher production volumes also produced a need for automatic buffing capabilities in order to reduce labor costs and rely less heavily on the human factor in the buffing operation. Larger volume production lines often use very little hand buffing and have come to rely on automatic machines which can be programmed to follow the shape of almost any part.

These machines often take the form of a turntable, surrounded by several buffing heads, each of which is oriented to buff just one aspect of the part as it passes by. Alternatively, some machines can index the part, or rotate it so that a single buffing head does the entire job. The shape of the part will determine which type of machine will be most suitable.

Tumbling and Vibratory Methods — Just as hand buffing is most often suitable for high value pieces, lower value parts can often be effectively highlighted in bulk. Parts such as certain cabinet hardware, fasteners or other small parts would typically be brass plated or antiqued in bulk handling methods.


If so, it is desirable to burnish or highlight in bulk as well. To do this, the parts can be burnished in a tumbler or in a vibratory mill. A tumbler is a rotating drum which rolls the parts against each other like a cement mixer. The parts can be burnished either wet or dry, using a plastic or ceramic media, with an abrasive or a polishing compound.

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Selecting the desired combination of these effects will produce a variety of different burnishing possibilities. The parts can generally be taken right off the process line, without drying, and loaded directly into the tumbler. Vibratory finishers operate in a similar manner, but use a vibrating bowl rather than a rotating drum. As mentioned, the vibratory bowls can also be charged with different types of media and compounds to achieve the type of contrast desired.

Both the tumbler and vibratory mill will produce a non-directional pattern on the part surface and cannot really reproduce the effect achieved by a hand buffing operation. However, they operate at much lower cost and can be pre-programmed to produce the identical result batch after batch. Consequently, they are less dependent on the human factor for consistent quality. For certain parts, compromising on quality a bit in order to control the cost allows the manufacturer sell the finished piece at the desired price point and still make a profit.

After coloring and highlighting are completed, the part is ready to be topcoated to protect it from corrosion. Even though the parts may look completely finished, the decorative antique finish is quite susceptible to corrosion or tarnish unless protected. The products most often used to accomplish this are clear lacquers. As in all the previous operations, there can be many options open to the finisher, depending on the durability required of the final finish, operating cost, equipment cost, environmental concerns, etc.

In actual practice, there are a few options which provide the most benefits:. Air-dry lacquers — These products can be water-based or solvent-based and commonly utilize acrylic or urethane polymers to form a protective film. The acrylics are the lower cost option and can provide an effective topcoat for many parts used indoors only, such as light fixtures, wall sconces, etc. Generally, solvent based lacquers are more protective than water based products, but also present a potential solvent fume problem in terms of discharge into the atmosphere.

Very often, they are cured in an oven, at degrees for ten to twenty minutes, to speed drying. These products are suitable for high value parts or surfaces which must be exposed to outdoor weathering elements. It is also possible to use lacquers containing corrosion inhibitors which specifically protect copper alloys. The most widely used is benzotriazole and its related compounds. These materials can be blended into many types of lacquers in small concentrations and provide an extra measure of corrosion resistance, making them particularly well suited for use on items such as marine hardware, building components, etc.

Clear powder coats — Relatively new on the scene, these topcoats produce coating thicknesses of mils, and offer extremely high protection levels. They are applied like any other powder coat - in a dry, electrostatic spray, followed by degree oven bake. Powder coats are not suitable for all parts. They work best on parts which have an open shape, with few or shallow recessed areas and can be susceptible to the Faraday Cage Effect.

This is commonly seen with any electrostatic or electrolytic operation including plating and prevents deposition in deep recesses. Consequently, it is difficult to powder coat the inside surfaces of many parts. Electrophoretic liquid lacquers — These products are not new, but they are just now coming into popular use. They are liquid lacquers, to be used as an electrophoretic immersion at the end of the plating line, followed by an oven cure.

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Though not commonly used on parts that are highlighted after coloring, they do find use as a clear sealant over a solid black finish, like a black nickel finish. In this setting, the part is racked and taken through the plating operation, then black nickel, then electrophoretic lacquer - all requiring the use of current to do the coating. Paste wax and oil finishes — Some parts do not require a permanent antique finish, but are designed to allow the surface to age naturally in service. For example, brass hand rails, building fascia panels, elevator panels, and other parts can be initially sealed with a temporary protective film such as paste wax or oil.

Then, when installed, they will be handled during normal use and be constantly "burnished" by this contact. Over time, they will develop a natural, soft patina which will ultimately be permanent because it is being constantly developed. This area is of critical importance to the Metal Finishing Industry because a chemical process line cannot operate without proper treatment of waste products, as mandated by the Federal EPA and appropriate State or Local agencies.

Since these process lines utilize a variety of different chemical products, it is impossible to offer a simple overview of the waste treatment picture. A few comments are in order, however, about the types of wastes generated in these lines and the waste treatment methods commonly employed to achieve compliance with the regulations:. Alkaline Cleaning residues: These residues are primarily composed of non-hazardous alkaline salts such as sodium hydroxide, sodium carbonate, sodium phosphates, wetting agents and other compounds which are not specifically regulated.

By virtue of their operating pH, they tend to dissolve metals from the parts being processed — in most cases, copper and zinc. Simple pH adjustment is very effective, in precipitating much of the metal content and bringing the effluent into the acceptable pH range of pH 5 - 9. Download citation file: RIS Zotero.

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