Surface Finishing

‘Flash free’. That’s the main goal to accomplish with all our products. There are many ways to ensure a flash free finish, but when needed, our secondary operations will ensure a clean component ready to accept an applied surface finish.

Surfaces of die casting may be classified as one of 5 grades. Where customers specify their requirements, this provides assistance to Alumess in the selection of appropriate tooling and process design.

Choose from one of the many finishes offered at Alumess:

 Decorative Finish  Conductivity  Protection  Electroplating  Design od Components for Electroplating  Electroplating of Copper from Cyanide Solutions  Electroplating of Nickel  Electroless Nickel  Electroplating of Chromium (Chrome) – Types: Bright, Satin and Black  Electroplating of Brass  Antique Finishes  Hex-free Chromate Treatments  Painting  Anodizing  Impregnation  Wear Resistance  Cataphoretic Painting  Passivating  Powder Coating

Decorative Finish All decorative finishes require a good quality surface and internal die cast structure to yield the desired effect. The reason for this is that during plating and painting processes, the die casting is heated, which may expose and highlight blemishes on the surface. These effects may be minimized by good component and tooling design.

During the assessment of visual impact during component design, requirements are sometimes over- specified, leading to higher costs. These can be avoided through a greater understanding of the finishes

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and textures available from Alumess and their likely impact on the final product. Texturing applied to the tooling cavity can dramatically improve the repeatability and yield of plated components.

All castings exhibit small levels of internal porosity, so for components requiring decorative finishes, designs and processes that produce denser castings are necessary. Alumess has developed proprietary gating and overflows designs resulting in reduced levels of porosity. The smoother and shinier the surface requirements, the denser the die casting will need to be. For the most mirror-like finishes, such as chrome plating, buffing operations may be required between plating stages (first copper, then nickel and finally chrome in this case). Satin finishes can be achieved by brushing the die casting. Other effects may be achieved by designing them into the tooling or additional operations.

Special considerations must be included in part design for electroplating and painting of components. Sharp corners, edges and posts increase electrical current, producing thicker coverage, while blind holes, deep channels and internal features receive less current and therefore less coverage. Another design consideration related to blind holes and deep channels, is that they restrict drainage, which could lead to excess coverage and restrictions in features. The temperature of the process must be known during the design process so that component and tooling design are matched to it, minimizing potential porosity and surface finish problems.

Conductivity For some electrical and electronic applications, increased electrical conductivity is required. This is achieved by plating such as nickel, tin, silver or gold. To insure adhesion of precious metals to the die casting surface, an initial copper 'strike' is applied. This process is critical to the success of the plating operation Nickel is then applied, occasionally followed by another appropriate conductive metal.

Appropriate design considerations are outlined in Decorative Finishes.

Protection Zinc, aluminum and magnesium alloys are relatively active metals that react chemically with the atmosphere. This potential weakness is actually a great strength, as the oxidation forms a formidable barrier coating against further deterioration. Most die cast applications need no further surface treatment to function in their environment.

One vital factor in keeping corrosion resistance of untreated parts low is the purity of the alloys from suppliers and subsequent internal housekeeping. Alloy composition must be controlled tightly, as the presence of out of specification material will affect not only corrosion resistance, but also many other mechanical and physical properties.

Where components are exposed to an external environment, salt spray tests are often specified. A common treatment for these instances is Hex-Free chromate.

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Electroplating Electroplating is a relatively inexpensive and very effective means of obtaining good corrosion protection while at the same time enhancing the appearance of a component. Die castings are usually electroplated for corrosion resistance, decorative purposes, electrical conductivity, solder ability and for wear resistance. The most frequently applied electroplated finishes on die castings are:

 Copper-Nickel  Copper-Nickel-Chromium  Copper and Antique Copper  Brass and Antique Brass  Tin

There are 2 common electroplating methods: barrel plating and rack plating.

Barrel plating is a bulk method limited to relatively small and light castings. Components produced using Alumess's multi-slide process and small-sized conventional die castings are ideally suited to this method. This is the most effective method of plating parts that do not require individual handling. The components, in effect, become part of the plating equipment as they are in electrical contact with the other parts in the load. This feature greatly contributes to the higher efficiencies attained in barrel plating since the entire surface of the workload is in cathode contact. This relatively large cathode contact allows faster, larger volume production when ample current is supplied.

The barrel is a closed drum with perforated walls into which the work is placed. The barrel is then immersed in the plating bath and mechanically rotated. Inside the barrel, at each end, is an electrode called a 'dangler'. This is mingled with the tumbling workload, making electrical contact and forming the cathode. Control of the tumbling speed, plating time and current determine the plating thickness for a load of given surface area.

Rack plating differs from barrel plating in that the die castings must be attached to racks or fixtures before plating. This requires either some form of universal rack or construction of special fixturing, depending upon the shape and complexity of the part. The rack becomes the cathode when it is immersed in the plating bath. Rack plating is suited to large, heavy or complex parts, which cannot be bulk tumbled without causing damage.

Before plating thicknesses are specified, customers must give consideration to environmental and service conditions. These factors include atmospheric, thermal, corrosive, moisture and any other factors, which could adversely affect the integrity of the casting.

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Design of Components for Electroplating

When die castings are to be plated, the design constraints imposed by the plating process must be considered. Alumess has developed design guidelines and standards to minimize finishing and plating costs, promote an even distribution of plating metals and ensure adhesion. Departures from these may impose cost penalties and reduce performance.

The configuration of the casting significantly affects plating costs because electric current is not distributed equally throughout the plating tank, but is concentrated along paths of least electrical resistance. The complexity and shape of the casting influences the current density, and hence the build- up of plating material. Good uniformity of plating thickness is obtained only on gently curving, convex surfaces. Corners, edges, fins, ribs and other protruding features attract more than an equal share of electrodeposited material. Conversely, grooves, serrations, holes, cavities and deep recesses are starved of their share of electrodeposited material. The area that receives the thinnest electroplate is commonly the area of first failure. Specifications for exterior automotive parts recognize this fact, by requiring a minimum thickness of electroplate on any area exposed to view during service.

Specific features that influence the distribution of electroplate are illustrated in our website section dealing with component design.

Electroplating of Copper from Cyanide Solutions The first step in the electroplating of zinc die castings, is the deposition of copper from a copper cyanide solution to cover the surface sufficiently, so that the substrate is not attacked in the later processing in more corrosive solutions. Although copper is also electroplated from acid solutions, zinc die castings should never be immersed in acid electrolytes without first covering them with a deposit from a copper- cyanide solution. The acid attacks the zinc die castings as soon as they enter the solution and they become badly corroded. A thin layer of copper deposited from a copper-cyanide solution is essential for the protection of zinc die castings prior to further plating in an acid copper or bright nickel plating solution.

Electroplating of Nickel Nickel is deposited onto zinc die castings after they have been plated with sufficient thickness of copper. Nickel deposition can be followed by chromium electroplating to obtain a decorative finish and improve resistance to corrosion, wear and tarnishing under indoor and outdoor service conditions. Other decorative finishes on zinc die castings utilizing nickel plating include nickel/copper, nickel/bronze, nickel/brass and nickel/tin-nickel alloy. Nickel may be electroplated bright, semi-bright or in satin finish. For outdoor service conditions, such as for automotive parts, two or three layers of nickel containing different amounts of sulfur are electroplated to improve corrosion resistance in severely polluted atmospheres. Multi-layer nickel deposits combined with chromium electroplate provide excellent corrosion resistance on electroplated zinc die castings.

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Electroplating of Chromium (Chrome) – Types – Bright, Satin, and Black Chromium electroplate on copper and nickel-plated die castings provides resistance to scratching, abrasion and wear. In addition, nickel is protected from tarnishing or blackening. Chromium is highly resistant to corrosion in all environments except acid solutions containing a high concentration of chloride ions. Furthermore, rainfall or condensed water tends to run off chromium surfaces in situations that leave corrosive water films on other metals for longer periods. These characteristics are attributed to the passive oxide film formed on chromium surfaces soon after plating.

Chromium deposits from conventional solutions can only be applied to a limited thickness i.e. 0.3 to 0.5 microns. The deposits begin to crack when the thickness of plating is increased.

Electroplating of Brass Brass electroplating is used on zinc die castings for decorative purposes, as well as a substitute for solid brass where casting and relative cost are factors. Brass plating is also used for the production of antique finishes. Most of the decorative brass plated finishes are applied as a flash deposit over a bright nickel or bright copper coating. Brass is an alloy of copper and zinc, the composition of which can vary considerably. Brass containing higher concentrations of copper is reddish in appearance, and those containing higher concentrations of zinc are yellowish to white in appearance. In conventional brass plating for decorative purposes, the ratio of copper to zinc is usually kept in the region of 60 to 40, or 70 to 30. In this region, a very attractive and rich yellow colour is obtained.

Antique Finishes Zinc castings, electroplated with copper or any of its alloys, may be covered with a layer of copper sulfide or any other colored compounds, and relieved to impart an antique effect on them. The process of relieving involves the removal of some of the colored layer from the highpoints of the work to uncover the underlying reddish copper or yellowish brass. Castings treated in this way are lacquered with cellulose or similar lacquer to protect them from tarnishing.

Electroplating of zinc die castings with copper or its alloys, and oxidizing the latter, is generally the method adopted for the production of antique finishes. This is because copper is a very satisfactory metal to color chemically, by virtue of the ease with which its color can be changed, and the variety of other colors which can be available. Any shade of a desired color, or combination of colors, can be obtained since copper compounds are colored.

Hex-Free Chromate Treatments Hex-Free Chromate treatments for zinc die castings have been developed over the years to provide a variety of processes yielding a wide choice of finishes. These finishes range from a bright, clear, chemically polished surface, to the heavy olive-drab coating which offers the greatest corrosion protection. Chromating ZA-27 requires a modification to the process.

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Painting Paints, enamels and lacquers are applied to zinc die castings for decorative and protective purposes, because of their effectiveness and ease of application. The methods of application are not peculiar to zinc die castings. Precautions should be taken to remove any oil from the castings, but as the cast surface has good paint adherence characteristics, special pre-treatment is not required for most applications. It is not uncommon however, to do a pretreatment on zinc castings by applying a conversion coating such as chromating or phosphating, which significantly improves paint adhesion and additional corrosion protection. Anodizing Anodic oxide coatings on aluminum die castings provide surface finishes that may be used to increase resistance to corrosion and abrasion, or to make a good base for paint. Aluminum anodizing is an electrochemical process that converts the naturally formed oxide coating on the outer surface of the aluminum part, to a tightly adhering layer of aluminum oxide of specified thickness. In general, the part will grow dimensionally by an amount equal to the total anodized film thickness since the film grows outward and inwards by approximately equal amounts. The process is accomplished by suspending the casting in an electrolytic cell and making it the anode. The oxide coat is porous, making it necessary in most cases to seal the surface. This is achieved by immersing the casting in boiling deionized water for 15 to 30 minutes, which hydrates the oxide, causing it to swell and sealing the pores. The corrosion resistance of anodized aluminum depends largely on the effectiveness of the sealing operation. Different sealing solutions and treatments are also used.

Hard-coat anodizing is a highly abrasion-resistant, non-conductive coating of aluminum oxide that is harder than tool steel (typically 65 - 70 Rc). Designers should note that anodizing, especially hard-coat anodizing, does compromise the fatigue resistance of an aluminum alloy component. Also, as the silicon content in the alloy increases, it becomes more difficult to anodize. Impregnation For applications subjected to high pressure, there is a possibility that miniature internal porosity may provide leak paths in applications involving air, gas or hydraulic fluids under high pressure (even when the casting is structurally sound). The surface 'skin' of die castings is very dense and provides much of its strength. To avoid potential leakage, the best solution is not to machine the surface, where possible. Alumess's technical expertise means that we are able to design solutions and cast to tight tolerances, avoiding this potential problem. Machining allowances as low as 0.25mm mean that part of this porosity-free layer is left intact, alleviating any potential problems. Our process knowledge and controls on casting machines also reduces porosity.

Where machining cannot be avoided, impregnation can offer a solution. A variety of systems is available, with anaerobics and methacrylates often being used. These impregnates are applied using vacuum, pressure or vacuumpressure methods. Wear Resistance In most applications, the properties of die cast alloys provide good wear resistance. In some cases however, higher resistance is required. Two options are hard-coat anodizing and chrome plating. For specific suggestions on your application, contact us for a detailed analysis. You can also send us your files by hitting contact us.

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