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Basic conditions for successful powder coating
Good Earthing of the Object
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.
Powder application by friction charging results in powder leaving the spray gun positively charged; that is it will have a deficiency of electrons. To build up the powder layer on an object, it is therefore necessary to partially neutralize the powder by drawing electrons from earth. Should this not occur quickly enough, the coated surface would develop a strong positive charge that subsequently repels positively charged powder on leaving the gun. The resultant effect being the same as for high voltage equipment; an insufficient powder coating thickness.
Inadequate earthing can easily be recognized by little or no powder accumulation around the hanging point of the object being coated.
Good Earthing of Spraying Equipment
To ensure plant safely, 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.
With friction charging equipment, good earthing of the spray gun is decisive for successful application. As the powder becomes positively charged, the electrons that are stripped away must be earthed. Without effectively earthing the spray gun, negative charges will accumulate and powder will then be allowed to pass through the gun without being charged.
Air Quality
Relative humidity within the working environment is of great significance for spraying efficiency. Ideal relative humidity being 45-55%. A quicker build-up of the powder layer and more even coating thickness are two of the advantages achieved by controlled humidity. Relative humidity can be measured with simple and inexpensive equipment.
Compressed Air Quality
Only clean, dry compressed air should be supplied to powder coating equipment. The quality of the compressed air can change the electrostatic charging and transporting properties of the powder. Impure compressed air may also cause visual defects in the coating.
Pressurized air must be free from oil, water and as dry as possible. Chillers are often used in addition to mechanical water/oil separators for removing moisture from compressed air. Air leaving the cooler/dryer at a temperature of 3°C or lover is an indication that it is dry and well suited for powder coating.
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 representative of some of the most important factors that influence spraying efficiency.
Spray Gun Positioning
All powder coating processes necessitate the powder, suspended in its air flow, being as close as possible to the object. The force of electrostatic attraction between powder particles and the object decreases by the square of the distance between them (D2), and only when that distance is just a few centimeters will the powder be drawn towards the object. Careful positioning of the spray gun also assures that small and large particles are deposited on the object in the same proportions found in the virgin powder.
Hanging Technique
In order to increase spraying efficiency it remains advantageous to suspend objects as close together as possible along the conveyor line. This reduces the amount of powder that is recycled thus preventing an excess of finer particles returning to the powder reservoir. To achieve the same coating thickness on all objects, however, spacing must be adapted according to the size of the objects, as the following diagrams illustrate:
- When the distance is too small, objects are not evenly coated:
- By increasing the distance, the coating thickness is even on all objects:
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A small object will produce a higher concentration of fields and subsequently receive a thicker coating than a larger object adjacent to it.
It is advantageous to hang objects of equal size next to each other along the conveyor.
Hanging objects correctly on the conveyor is of greater importance to successful conventional electrostatic spraying than it is to friction charged powder spraying.
Powder Recycling
Spraying booth with cyclone for recycling powder
A major advantage of powder coatings over liquid paint systems is the ability to reuse powder that does not adhere to an object (overspray).
Air and powder are separated by either a cyclone or a filter system, as illustrated.
Powder that is recycled will be recollected in the powder hopper. Particles which are more easily charged adhere themselves to the object whilst the proportion being recycled are of another particle distribution size than that of virgin powder. The relationship between small and large powder particle size has significant meaning regarding spraying properties and consequently the end result.
Spraying booth with filter recycling
Recycled powder will always have properties that differ from virgin powder. In order to maintain a constant composition it is always an advantage to keep the volume of reused powder to a minimum and continuously return any recycled powder to the powder hopper.
Addition to Virgin Powder
To further reduce variations in particle size distribution due to recycling, virgin powder should be added frequently to the powder hopper. Without the addition of virgin powder, the volume level of fine-grained particles will tend to increase to an extent, which ultimately makes particle charging and transporting within the spray equipment more difficult.
