Preparation of the surface starts with a process to insure that the surface is properly cleaned and free of “soils” such as manufacturing oils. Subsequent surface treatments might include blasting, if required and iron phosphate or zinc phosphate depending on the long term corrosion performance desired Thermoplastic powders normally require a primer to promote adhesion. Thermoset powders do not need a primer because they have an adhesion promoter built-in. However, primers may be used with thermosets to enhance certain properties. The priming material used must be compatible with the curing temperatures required for powder coatings.
Application of the powder is a function with many variations of two basic techniques. These techniques are fluidized bed and electrostatic spray. 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 or components are very small. Thermoset powders designed for electrical insulation often use the fluidized bed technique. The parts are preheated 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 are actually repelled by the charged particles on the part and the coating process stops. This provides an even film thickness.
Heating of the surface is normally accomplished using ovens fueled by natural gas. Thermoplastic powders normally require the part surface to be heated prior to the application of the powder via the fluidized bed technique. Thermoset powders are normally applied via the electrostatic spray technique while the part is at room temperature. The part surface and the powder are then heated at the same time.
Curing of the coating applies to thermoset powders. (Thermoplastic powders normally are not cured.) After the part surface is heated to the desired temperature (180º-200º C) it is held at that temperature for a specified period of time, normally 10 – 15 minutes, while the curing agent develops the desired film properties.
Conventional electrostatic powder application leads to large quantities of negative charges being transferred to the object. If this large surplus of electrons cannot be efficiently earthed, the coated surface rapidly develops a powerful negative charge thus repelling the negatively charged powder from the spray gun. Consequently, without sufficient earthing it becomes impossible to build up a thick layer of powder with good flow and finish, rapidly and efficiently.
GOOD EARTHING OF SPRAYING EQUIPMENT
To ensure plant safety, it is absolutely essential to fully and effectively earth spray equipment, spray booths, and related equipment. This ultimately maximizes the avoidance of high voltage discharge and the possibility of resultant electrical sparks.
Relative humidity within the working environment is of great significance for spraying efficiency. Ideal relative humidity being 45-55%. Only clean, dry compressed air should be supplied to powder coating equipment.
OTHER FACTORS INFLUENCING SPRAYING EFFICIENCY
Spraying efficiency can be expressed as the percentage of sprayed powder which adheres to the object at the first application attempt. Maximum spraying efficiency provides several advantages, both technical and economic.
The following are the representative of the most important factors that influence spraying efficiency.
- Spray gun positioning
- Hanging technique
- Powder recycling
- Addition of virgin powder