Plastic parts can be coated with metal, a process called metallization, for both aesthetic and mechanical purposes. Visually, a metal coated piece of plastic features increased gloss and reflectivity. Other properties, such as abrasion resistance and electric conductivity, which are not innate characteristics of plastic, are often obtained through metallization. Metalized plastic components are used in similar applications as metal plated parts, but tend to be lower in weight and have higher corrosion resistance, although not in all cases. In addition, electrical conductivity can be controlled in metalized plastic components, and they are inexpensive to manufacture. To metalize a piece of plastic, several common methods are used: vacuum metallization, arc and flame spraying, or plating. It is also possible to metalize a transfer film, and use alternative methods to apply the film to the surface of the substrate.
Metallization Processes
Vacuum Metallization
Before the process can begin, the plastic component is washed and coated with a base coat, so that the metal layer is smooth and uniform. Next, a metal (typically aluminum) is evaporated in a vacuum chamber. The vapor then condenses onto the surface of the substrate, leaving a thin layer of metal coating. The entire process takes place within a vacuum chamber to prevent oxidation. This deposition process is also commonly called physical vapor deposition. Depending on the component’s application, a top coat may be applied after deposition to increase properties such as abrasion resistance. Metalized plastic components that receive their coats via this process are found in a range of applications, from automotive interior parts to certain types of foils.
Arc and Flame Spraying
In basic flame spraying, a hand-held device is used to spray a layer of metallic coating on the substrate. In flame spraying, the primary force behind deposition is a combustion flame, driven by oxygen and gas. Metallic powder is heated and melted, as a combustion flame accelerates the mixture and releases it as spray. This process has a high deposition rate and creates very thick layers, but the coatings tend to be porous and somewhat rough. Due to the nature of the application process, coatings can be applied to specific areas of components, which is useful when working with complex or unusually shaped components. The process is relatively easy and requires minimal training.
Arc spraying is similar to flame spraying, but the power source is different. Instead of depending on a combustion flame, arc spraying derives its energy from an electric arc. Two wires, composed of the metallic coating material and carrying DC electric current, touch together at their tips—the energy that releases when the two wires touch heats and melts the wire, while a stream of gas deposits the molten metal onto the surface of the substrate, creating a metal coat. Like flame spraying, the resulting coating typically suffers from high porosity.
Electroless Plating and Electroplating
Plating is typically divided into two categories, depending on the presence of electric current. In electroless plating, electric current is not used; in electroplating, electric current is used. Both processes tend to be more effective than vacuum metallization at producing metallic coats with strong adhesion, although plating tends to be more dangerous.
Electroless plating is often used to deposit nickel or copper metal onto plastic substrates. First, the surface of the plastic is etched away using an oxidizing solution. Because the surface becomes extremely susceptible to hydrogen bonding as a result of the oxidizing solution, typically increases during coating application. Coating occurs when the plastic component (post-etching) is immersed in a solution containing metallic (nickel or copper) ions, which then bond to the plastic surface as a metallic coating.
In order for electroplating (or electrolytic plating) to be successful, the plastic surface must first be rendered conductive, which can be achieved through basic electroless plating. Once the plastic surface is conductive, the substrate is immersed in a solution. In the solution are metallic salts, connected to a positive source of current (cathode). An anodic (negatively charged) conductor is also placed in the bath, which creates an electrical circuit in conjunction with the positively charged salts. The metallic salts are electrically attracted to the substrate, where they create a metallic coat. As this process happens, the anodic conductor, typically made of the same type of metal as the metallic salts, dissolves into the solution and replaces the source of metallic salts, which is depleted during deposition.
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