Finishes for Precision Die Cast Parts
Known for being one of the most advanced, world class die casting suppliers, ChinaSavvy offers various finishing services for precision die cast parts, including:
- Decorative Finishes
- Electroless Nickel
- Hex-Free Chromate Treatment
- Powder Coating
- Wear Resistance
Because copper compounds are colored, the electroplating of zinc die castings with copper or any of its alloys is the most general method of antiquing, as copper is a chemically colorable metal.
In order to create an antique look, zinc castings electroplated with copper (or any other of its alloying agents), can be covered in a layer of colored components, such as copper sulfide. This casting is then 'relieved' - a process which entails the removal of some of the colored layers on highpoints in order to present the underlying layer of the yellowish brass or the reddish copper.
Die cast parts are then treated with a lacquer in order to prevent tarnishing. This is usually done with a cellulose lacquer or a lacquer similar to it.
Anodic oxide coatings on aluminum precision die cast parts are used for various reasons, including the preparation of a part for a base paint, or to increase the part's resistance to abrasion and corrosion.
Hard-coat anodizing is a coat that is highly resistant to abrasion and non-conductive. This hard-coat layer is harder than tool steel and, especially hard-coat anodizing, does affect the aluminum part's resistance to fatigue. Note that as the silicon content of aluminum increases, so does its difficulty to anodize.
You can learn more about the process of anodizing aluminum and the associated benefits by visiting our Anodizing Aluminum Fabrication page.
Conductive plating (such as gold, nickel, silver and tin) is used for electronic and electrical parts where electrical conductivity is necessary. In order to ensure adhesion, a copper 'strike' is applied in the initial stages, where after nickel is applied and, in some cases, followed by another conductive metal.
Because the die cast part is heated during decorative finishing processes (such as plating and painting), it is vital for the die cast part to have both a good internal structure and good surface finish. This is achieved through a good design for both the tool and the component.
As a world-class die casting supplier, ChinaSavvy is able to deliver a wide array of decorative finishes, adapting designs and processes in the earliest stages in order to achieve the best possible results.
In cases where a smooth and shiny surface is required, the density of the casting itself must be higher. Desired finishes can then be achieved through chrome plating, with buffing operations between each plating stage.
In terms of plated components, a textured tooling cavity will also be able to yield an improved repeatability.
For satin finishes, brushing can be used or, depending on the type of decorative finish required, can be designed into the tool itself and coupled with additional operations.
The temperatures used in the finishing processes must be available during the design process, so that both the tooling and component designs are matched to it. This consideration in the design stages can help to potentially minimize surface finish problems as well as porosity problems.
Special considerations must be taken in the part design stage when components are to be electroplated. In order to ensure finishing costs are kept low, plating costs are kept at an economically viable level, an even distribution of plating metals are achieved and adhesion is sufficient, ChinaSavvy follows strict standards and design guidelines.
How a casting is configured will affect the cost of plating. During the plating process, an electrical current is distributed along the paths that offer the least electrical resistance. The shape as well as the complexity of the casting that is being plated, will influence:
- The density of the current being distributed.
- The build-up of the plating material on the casting.
Protruding features (such as fins, corners, edges and ribs) attract more electrodeposited material and deep recesses, holes, grooves and so on, are starved of electrodeposited material. First failure will occur at sections where the thinnest electroplating was distributed to.
Thus, electroplating is best used on gently curving and convex surfaces as a uniform plating thickness can be obtained.
Electroplating of Brass
This type of electroplating is used on zinc die cast parts as:
- A substitute for solid brass (usually in cases where cost plays a factor).
- Or for decorative requirements (also used in order to create an antique finish).
In decorative brass plated finishes, brass is applied in what is called a flash deposit. This is usually done either over a bright copper coating or over a bright nickel coating.
Brass, being an alloy of zinc and copper, has variable compositions. In cases where brass contains higher consecrations of zinc, a yellowish to white appearance is achieved, whereas brass with a higher concentration of copper produces a reddish appearance.
In conventional brass plating, used for the purpose of a decorative finish, the zinc to copper ratio is kept at a 30 to 70, or a 40 to 60 ratio, resulting in a rich, yellow colored finish.
Electroplating of Nickel
In the electroplating of nickel, a zinc die casting will first be plated with an appropriate thickness of copper, after which nickel is then deposited into the zinc die casting.
This nickel deposit can then be followed by chromium electroplating, which serves as a decorative finish and has the following benefits under both indoor and outdoor conditions:
- An improved resistance to corrosion.
- Improved resistance to wear and tarnishing.
Other decorative finishes applying nickel plating on zinc die cast parts include:
- Nickel / Brass
- Nickel / Copper
- Nickel / Tin-Nickel Alloy
- Nickel / Bronze
A combination of chromium electroplating and multi-layer nickel deposits on zinc die cast parts is able to yield an excellent resistance to corrosion. In some cases, such as seen in automotive parts, two to three layers of nickel (which contain different levels of sulfur) are electroplated. This improves the part's resistance to corrosion in tough and polluted environments.
Electroplating of Chrome / Chromium
The use of chromium electroplating on nickel and copper plated precision die cast parts, yields beneficial results, such as a better resistance to wear, a netter resistance to scratching and a better resistance to abrasions.
Additionally, nickel also benefits from protection against blackening and tarnishing.
Except in environments where it is subjected to acid solutions with a high concentration of chloride ions, chromium has a high resistance to corrosion.
Condensed water or rain is prone to run off chromium surfaces, leaving behind corrosive water films.
Note that chromium deposits from a conventional solution is only able to be applied to a limited thickness of 0.3 microns to 0.5 microns. When the thickness of the plating is increased beyond these points, the chromium deposits will begin to crack.
The most widely used form of electroless plating in the industry today, it offers a range of very unique deposit properties, which includes an uniform deposit within:
- Blind holes
- Deep recesses
Commercially, deposition is done by using an acid phosphorous bath, giving the following unique and beneficially excellent physical characteristics:
- Resistance to corrosion
- Resistance to wear
- Resistance to abrasion
- Electrical properties
- Soldering abilities
There are three types of electroless nickel:
With a superior resistance to corrosion and an excellent adhesion, this type yields compressively stressed deposits.
With a bright and uniform appearance, this type is used for both electronic applications as well as for decorative purpose. This is also the most commonly used deposit.
Comparable to baron electroless nickel , it is mainly used for its beneficial wear and hardness properties. It is a hard deposit where no heat treatment is required.
Hex-Free Chromate Treatment
A treatment that is applied to zinc die cast parts, it is used for protection against corrosion, a decorative finish or as a base for paint finishes. Finishes here range from a olive-drab coating (yielding the best results in terms of resistance to corrosion) to a clear and chemically polished surface.
This type of treatment, combined with a chemical polishing solution, has replaced the more expensive finishing operations used for zinc die casted parts within the die casting suppliers' industry. Completing zinc plating processes before applying chromate treatments are able to yield an inexpensive and highly decorative finish.
In applications where high pressure occurs (such as in applications that involve gas, air or hydraulic fluids), there will always be the possibility of miniature internal porosity causing leak paths, even if the casting is structurally sound.
In order to avoid this possible leakage, it is best to avoid machining when and wherever possible, with impregnation offering a solution in cases where machining cannot be avoided.
Impregnation, using methacrylate and anaerobic systems, can be applied using:
- Pressure methods,
- Vacuum methods, or
- Vacuum-pressure methods
For both protective and decorative purposes, lacquers, paints and enamels are easily applied to die cast parts. Precautions are taken in order to remove any oils from the cast surfaces, but no pre-treatments are required simply because die cast parts have such good adhesion characteristics towards paints.
Zinc castings are however commonly be pre-treated (the application of a conversion coat - phosphating or chromating), resulting in a better protection against corrosion and better paint adhesion. Note that, when painting components, special considerations must be taken at the design stages of the part.
This finishing process is used (instead of wet paints) because it yields a better color consistency and a higher uniform thickness.
Powder coating is commonly used as a decorative finish because it is able to hide minor flaws in the surface, has a durable high gloss and has a great color consistency. In terms of functionality, powder coating yields a high anti-corrosion finish and is harder, lending itself to a better resistance against dings and scratches.
Cured at a high temperature in order to produce a tough finish, it is also an environmentally friendly method, producing no hazardous waste byproducts and has no associated hazards on air quality.
Because aluminum, zinc and magnesium are metals that react chemically to their environment, they need no further protective treatments in order to function in their environments as oxidation forms a barrier against deterioration.
Note that the purity of the untreated part will have an effect on its resistance to corrosion. All alloy compositions must thus be tightly controlled, as it not only plays a role on its mechanical properties, but its resistance to corrosion and physical properties as well.
In cases where precision die cast components are exposed to external environments, salt spray tests are specified and the common treatment used in cases such as these, are Hex-free Chromate Treatments.
Cast alloys have the ability to yield a good resistance to wear, but in cases where a higher wear resistance is required, chrome plating and hard-coat anodizing will be recommended as viable options by professionally skilled die casting suppliers.
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Further Suggested Reading:
- Metals That Can Be Cast
- Precision Die Casting
- Wall Thicknesses in Die Cast Parts
- Assembly Methods for Die Cast Parts
- Die Casting Tolerances