The Ultimate Guide To Plastic Coating Metal: Techniques And Applications

There are several ways to coat metal with plastic, including dip coating, electroplating, and metalizing. Dip coating involves dipping metal components into a fluidised bed of thermoplastic powders, which then melt onto the metalwork to form a smooth, continuous coating. Electroplating, meanwhile, involves depositing metal ions onto a plastic surface, allowing the part to take on properties of both materials. Metalizing, or vacuum metalizing, involves evaporating a metal, usually aluminium, so that it condenses onto a plastic substrate, leaving a thin layer of metal coating.

Electroplating

The Process

The process of electroplating on plastic involves several steps. Firstly, the plastic surface is prepared by cleaning it and removing any surface contaminants. Then, the plastic is treated with a solution to enhance its adhesive properties. This can be done by etching the plastic in a chromic acid-based solution or by applying a conductive paint. After this, a solution of palladium and tin salts is applied, followed by a coating of nickel or copper. At this point, the plastic is ready for electroplating using standard techniques.

Benefits

Applications

Dip coating

Plastic dip coating is a process that involves immersing a preheated metal component into a liquid plastic solution to form a plastic coating. This process can add protection and aesthetic value to most metal parts. The coating can enhance the appearance of the part, provide a soft grip, and protect the part from chemicals, abrasion, and corrosion.

There are several methods for plastic dip coating, including:

• Hot dip coating in a fluidized bed of polymer powder: The metal is thoroughly cleaned and placed in an oven where it is heated to a high temperature. This heat melts the polymer powder, and the metal is then dipped into the fluidized bed. The polymer adheres to the surface, and the metal is given time to cool, leaving a smooth, even plastic coating.
• Hot dip coating in a vinyl plastisol: A very similar process to the above, except the object is dipped into a liquid that contains PVC particles.
• Spraying polymer powder onto a heated product: This method, also known as flock spraying, is used for larger objects that are impractical to coat by dipping. The part is heated so that the spray melts and adheres to it on contact.

The process of plastic dip coating typically involves four main steps:

• Pretreatment: The part is cleaned and a primer may be applied if desired.
• Preheating: The part is heated to a temperature that will determine the thickness of the coating.
• Immersion: The part is immersed in the liquid plastic solution for a set time to build the required thickness.
• Baking/curing: After removal from the solution, the part is baked or cured at the required temperature, depending on the formula.

Plastic dip coating can be applied to a variety of components, including transmission and gearbox components, automotive springs and brackets, outdoor furniture, hand tools, toys, medical instruments, and electrical equipment.

Vacuum metalizing

The process involves heating the metal coating material, typically aluminium, until it vaporises inside the vacuum chamber. The lack of pressure in the chamber lowers the boiling point of the metal, allowing it to change from a condensed phase to a gaseous phase. This vapour then adheres to the surface of the substrate, creating a metallic coating.

Arc and flame spraying

Arc Spraying

Arc spraying, also known as twin wire arc spray or thermal arc spray, is a process that uses an electric arc to melt wires. The wires are pushed or pulled to the gun to be sprayed, where they are forced together to form an electric arc, melting the wire. Compressed air then passes through a nozzle, atomising the molten metal and spraying it onto the workpiece. The wires are interchangeable and can be used in combinations such as one wire zinc and one wire aluminium. The higher the current rating of the system, the higher the spray rate.

Arc spraying is a cost-effective method of applying metallic coatings and is one of the most versatile of all thermal spray processes. It is a simple process that can provide corrosion protection coatings to steel fabrication or engineering coatings to rebuild or change the surface properties of the sprayed objects. Coatings of zinc, aluminium, steels, copper, bronze and other materials can be sprayed for corrosion protection, engineering reclamation, surface modification and decoration.

Arc spraying can be applied to almost any material, including metal, wood, polystyrene, plastic, glass, paper, ceramics and even chocolate. It is known for its low heat input when spraying, which is useful when it comes to thermally sensitive substrates as it eliminates the risk of component distortion.

Flame Spraying

Flame spraying is a century-old process that involves a gun nozzle in which a mixture of oxygen and fuel such as acetylene, propane or hydrogen is injected and combusted to create a flame. The selection of the nozzle design depends on the coating material, which can be in the form of powder or wire. A cold air sheath is used to collimate the flame to control the spray pattern.

In the powder feed system, the coating material is gravity-fed to the gun from a hopper, where it is carried by aspirating gas through the flame. The powder particles melt during transit and deposit on the substrate. In the wire feed system, the wire tip is continually fed into the flame by a roller drive mechanism, where it is fragmented and propelled to the substrate by compressed air injection. The wire diameter and feed rate, air and gas flow rates, flame temperature and gas pressure determine the coating properties.

Flame spraying is a versatile, portable and low-cost method of coating metal onto plastic. It can be used to coat a wide range of materials, from polymers to ceramics and refractory metals.

Roughening the surface

When it comes to coating metal with plastic, there are several methods available, including electroplating, Fluidised Bed dipping, and 3D printing. Regardless of the method chosen, preparing the metal surface is crucial for achieving a successful coating. One essential aspect of surface preparation is roughening the surface, which can be achieved through various techniques.

One common technique for roughening metal surfaces is abrasive blasting, which can be done through methods such as sandblasting or shot blasting. This process involves propelling abrasive materials at high speeds onto the metal surface, creating a rough texture. Abrasive blasting is effective in removing contaminants and creating a suitable profile for the plastic coating to adhere to. It is important to note that this method can result in a smooth to rough surface profile, depending on the specific requirements.

Another technique for roughening metal surfaces is acid etching. This process involves using acids, such as phosphoric acid, to chemically etch the surface and create a rough texture. Acid etching is often used for light abrasion and is suitable for preparing surfaces for sealants, films, and coatings. However, it may not be effective in removing certain contaminants, such as petroleum-based products or animal vegetable oils. Therefore, it is crucial to assess the type of contaminants present on the metal surface before selecting the appropriate roughening technique.

For large or challenging surfaces, mechanical profiling is typically the preferred method for roughening. This process involves using mechanical tools, such as grinding machines, to create a uniform and consistent rough texture on the metal surface. Mechanical profiling ensures that the surface is adequately prepared for the subsequent coating process and can help achieve the desired level of roughness.

Additionally, water jetting can be used to roughen metal surfaces. This technique involves using a high-pressure water jet to dislodge coarse aggregates and create a rough profile. Water jetting is often employed when thicker coatings or repair overlays are required.

It is worth noting that the choice of roughening technique depends on various factors, including the size and complexity of the surface, the desired level of roughness, and the presence of specific contaminants. Each technique has its advantages and limitations, so selecting the most suitable one is essential for achieving a successful plastic coating on metal.

Frequently asked questions