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Manufacturing Processes - Powder Coating - Surface Finishing

 

Manufacturing: Surface Finishing


Finish Machining

Surface Finishing Coatings

Powder Coating

Powder coating is a dry finishing process, using finely ground particles of pigment and resin that are generally electrostatically charged and sprayed onto electrically grounded parts. The charged powder particles adhere to the parts and are held there until melted and fused into a smooth coating in a curing oven. Before coating, the parts to be coated are first pretreated similarly to conventional liquid coated parts. The pretreatment process is normally conducted in series with the coating and curing operations.

There is essentially two common ways of applying powder coating: by electrostatic spray and by fluidized bed powder coating. There are several other processes that have been developed, but they are far less used. These include flame spraying, spraying with a plasma gun, airless hot spray, and coating by electophoretic deposition.

The fluidized bed is the original powder coating technique. It is still the primary technique used for the application of thermoplastic powders. The fluidized bed is also used for the application of some thermoset powders where high film build is required. Thermoset powders designed for electrical insulation often use the fluidized bed technique. The parts are pre-heated to a temperature significantly higher than the melting point of the powder. The parts are then immersed into a "fluidized bed" of the coating powder where the plastic powder is melted onto the part.

Electrostatic spray is the primary technique used for thermoset powders. The particles of powder are given an electrical charge in the powder coating gun. The target part is attached to a fixture that is grounded. The electrically charged powder particles are attracted to the grounded part and attach themselves like little magnets to the part. The particles build-up on the surface of the part until it is covered with charged particles and the part surface is charged. At this point the oncoming particles are actually repelled by the charged particles on the part and the coating process stops. This provides an even film thickness.

Materials

There are two types of powder coatings - thermoplastic and thermosetting. A thermoplastic powder coating is one that melts and flows when heat is applied, but continues to have the same chemical composition once it cools and solidifies. Thermoplastic powders exhibit excellent chemical resistance, toughness, and flexibility. They are applied mainly by the fluidized bed application technique, in which heated parts are dipped into a vat where the powders are fluidized by air, and are used in many thick film applications. They are generally applied to a surface that has been preheated to a temperature significantly higher than the melting point of the powder. As a thermoplastic powder material is applied to the hot surface it will melt and "fusion bond" to the surface and then "flow out" into a strong, continuous film. As the film cools it develops its physical properties. Nylon powder coating materials are the most commonly used thermoplastic powders.

Thermosetting powder coatings are based on lower molecular weight solid resins, and melt when exposed to heat. After they flow into a uniform thin layer, however, they chemically crosslink within themselves or with other reactive components to form a reaction product of much higher molecular weight. These newly formed materials are heat stable and, unlike the thermoplastic products after curing, will not soften back to the liquid phase when heated. Thermosetting powders are derived from three generic types of resins: epoxy, polyester and acrylic. From these resin types, several coating systems are derived. Resins used in thermosetting powders can be ground into fine particles necessary for spray application and a thin film finish. Most of the technological advancements in recent years have been with thermosetting powders.


Epoxy - Epoxy Powder Coatings exhibit inherent toughness, corrosion resistance, chemical resistance, flexibility, adhesion and abrasion resistance. Epoxy powder is normally used where a tough durable film is required and the product will not be exposed to direct sunlight for long periods of time. An epoxy coating will form a chalk like appearance on the surface with lengthy exposure to sunlight.

Typical applications include:

- Appliances
- Business Machines
- Electrical Enclosures
- Hospital Equipment
- Office Furniture
- Oil Filters
- Power Tools
- Shelving
- Tool Boxes


Urethane Powder Coatings feature characteristics of excellent gloss retention and long-term resistance to humidity and corrosion in thin film applications.

Typical applications include:

- Agricultural Equipment
- Air Conditioners
- Construction Equipment
- Electrical Enclosures
- Lawn and Garden Equipment
- Lawn Furniture
- Light Fixtures
- Office Furniture
- Recreational Equipment
- Under-hood Automotive
- Wheels and Rims


Polyester Powder Coatings feature characteristics of long-term exterior durability, high performance mechanical properties and overbake resistance. Polyester powder is widely used for decorative components where good resistance to the ultraviolet rays from sunlight is important. Many automotive trim components and other exterior components are coated with polyester powders.

Typical applications include:

- Agricultural Equipment
- Appliances
- Construction Equipment
- Electrical Enclosures
- Lawn and Garden Equipment
- Lawn Furniture
- Recreational Equipment
- Under-hood Automotive
- Wheels and Rims


Acrylic powder is specified where the decorative requirements and resistance to ultraviolet rays from sunlight for a longer period of time is critical. Many critical automotive trim components are coated with acrylic powder.


 




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