Powder Coating Advanced - Application
2nd February 2011This section will deal only with the electrostatic spray application of powder coatings because it is the most widely used method of applying thermosetting powders.
The electrostatic spray powder application system is made up of several subsystems:
1. Powder delivery system
2. Powder charging system
3. Electrostatic spray process
4. Reclaim system
5. Curing/Stoving
These subsystems must all work together if the goals of uniform film thickness and high material utilisation are to be achieved.
The powder delivery system delivers powder particles from a delivery hopper to the part to be coated. In some systems, the delivery hopper is a fluidised bed, in others it is a hopper with some form of mechanical agitation. A pump is located near the bottom of the delivery hopper. The pump draws powder from the hopper, mixes it with a controllable volume of air, and sends the air/powder mixture through a delivery hose toward the spray gun.
The powder spray gun is simply a rigid extension of the delivery hose with some means of controlling the shape of the powder cloud as it exits the end of the gun. Some guns use a diffusion cone at the exit end of the gun tip to gain control of the cloud shape. The mounting position of the gun and the shape of the powder cloud are both calculated to deliver powder particles close to the part to be coated and, at the same time, minimise the forward velocity of the powder particle. A soft powder cloud that gently wraps around the part is the desired mode of delivery. This is the exact opposite of the liquid spray gun that blows the wet paint toward the part with great velocity.
The job of the powder charging system is to put an electrostatic charge on each powder particle as it leaves the spray gun. This static charge causes the particle to be attracted to and held by any grounded conductive object within the spray area. The charging system is made up of the following:
1 Electrode
2 Current limiting resistor string
3 High voltage source
4 Controllable low voltage source
The electrode is located near the gun tip. On some guns, the electrode extends from the diffuser cone. Other guns employ a number of electrodes just inside the gun tip. The electrode is a needle-like wire that juts into the delivery powder / air stream. It is connected to the high voltage source through the resistor string and normally operates at a potential of 50 kV to 100 kV. The high voltage applied to the needle electrode creates an ionising field at the electrode tip. As the powder / air mixture exiting the gun passes through this field, a high static charge is transferred to the stream.
The items that need to be monitored in the charging system are:
1 Is the electrode present (not broken or bent over)?
2 Is the electrode clean (not coated with powder)?
3 Is high voltage actually present at the gun tip (independent meter measurement, not just system meter reading)?
4 Is the resistance string intact? If the resistance string is either open or shorted, the gun must be replaced.
Electrostatic spray coating process
Electrostatic spraying, which is the most widely applied coating method, is not only more versatile, but generally provides better control of the powder coating properties. The electrostatic spray process makes use of electrically charging the powder particles.
The powder is contained in a hopper in a fluidised state and is held adjacent to the application booth. It is delivered by a powder pump and a transport air flow system to the electrostatic spray gun. The particles are charged on emission from the gun and with the help of the transport air move in the direction of the grounded work piece. As the charged particles come close to the grounded work piece, electrostatic attraction causes the particles to deposit and adhere to the work piece.
There are two distinct methods for building up the charge on the particle surfaces. The Corona charging method makes use of a high voltage generator (80-100 kV) to bring an electrostatic charge (mostly negative) onto the powder particles through the intermediate process of creating oxygen ions.
In the Tribo method the electrostatic charge (positive particles) is built up by the particles rubbing with increased velocity along a specially selected material (e.g. Teflon) inside the spray gun for sufficient time, without the use of a high voltage generator.
For more information see Corona vs Tribo page.
The reclaim system is the housekeeping subsystem in the powder application system. The powder that does not cling to a grounded object in the spray area and the air entering the spray booth are exhausted from the spray area into the reclaim system. The reclaim system separates the air from the powder. It cleans the powder by passing it through a fine screen and then returns it to the delivery hopper. It cleans the air by passing it through an absolute filter and returns it to the spray room. This automatic housekeeping feature makes it possible to use almost 100 percent of the powder purchased to coat parts. It also makes it unnecessary to supply outside make-up out to the powder painting area. Several different methods of separating the powder from the air are used.
The most common primary filter methods are:
1 Filter belts
2 Bag filters
3 Cartridge filters
4 Cyclones
5 Some combination of the above
A good way to check the reclaim system is to monitor and record the pressure drop across the primary filter. An increasing pressure drop trend can indicate "blinding" of the filter media or faulty operation of the "pulsing" system. Another good check is to observe the inlet side of the absolute filter. If there is any significant amount of powder build-up on this filter, it indicates leakage through (or around) the primary filter.
For more information on recovery see Powder Recovery page.
Curing/Stoving
To polymerise the powder applied to the substrate we must heat both to a high temperature for a few minutes. This is the curing process.
As powder coatings do not contain solvents (unlike liquid coatings) a flash zone is not required in the curing oven. The volume of exhaust gases is also substantially lower which can also substantially lower operational costs.
The formulation of the powder coating material determines to a large extent the curing time and temperature of the coated object. It will also result in the specified film properties.
At present we can select from several curing oven types:
Convection oven: Is most frequently used and can be divided into directly fired and indirectly fired ovens. Fuel options are natural gas, propane, oil or electricity. In the case of a directly fired oven combustion gases can interfere seriously with the powder during curing and with film properties thereafter.
In any oven care should be taken that no high air velocities exist or are created that could damage the virgin, or not yet solidified, powder coating layer. Acceptable air velocities are in the 1 to 2 m/sec range.
Infra-Red oven: Uses radiant energy to heat a product through electromagnetic waves. Infrared heating works very quickly. There are three types of emitters – short, medium and long wave length. Their main performance differences are operating temperature (some 2000, 1050 and 600° C respectively) and radiation efficiency (80, 60 and 50% respectively). Higher temperatures result in faster heating rates and a lower efficiency which results in a higher loss through convection heat.
Dual or combination oven: Both infra-red and convection are applied. In the infra-red section the powder film is melted to avoid powder being blown off in the convection section where additional time is available to complete the entire cross-linking process.
Induction oven: Heat is generated in the metal object through induction of eddy currents. The disadvantage being, as with infra-red, the powder coating can start reacting before contact with gas combustion components can take place.
Medium temperature radiation offers the most effective source of heat for curing thermosetting powders. Gas fired emitter panels present a panel surface temperature of 900° C. Electric panels give a surface temperature of approximately 800° C. The work pieces should be maintained at a distance of approximately 300 mm from the emitter panels. Substrates coated with darker colour powder absorb more infra-red radiation, while objects with light coloured powder do not heat up as quickly. It is therefore recommended that each individual powder is tested in combination with the curing oven to evaluate the curing performance.
