Powder coating 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 coating application system is made up of several subsystems:
These subsystems must all work together if the goals of uniform film thickness and high material transfer are to be achieved.
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 coating gun is simply a rigid extension of the delivery rate 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 size. A soft powder cloud that gently wraps around the part is the desired mode of delivery.
Article job of the powder charging system is to put an electrostatic charge on each powder particle as it leaves the coating gun. This static charge causes the particle to be attracted to and held by any grounded conductive object within the coating spray area. The charging system is made up of the following:
Corona powder coating gun 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 60 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:
The thermoset powder coat electrostatic spraying, which is the most widely applied coating method, is not only more versatile, but generally provides good control of the powder coating properties. The electrostatic coating 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. system 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 stick and adhere to the work piece.
There are two distinct methods for building up the charge on the particle surfaces. The coating gun 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.
The powder coating recovery 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 coating booth are exhausted from the spray area into the recovery system. The recovery 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 % of the powder purchased to coat parts. It also makes it unnecessary to supply outside make-up out to the powder coating area. Several different methods of separating the powder from the air are used.
The most common primary filter methods are:
A good way to check the recovery system is to monitor and record the pressure drop across the primary filter. An increasing pressure drop trend can indicate(blindin) of the filter media or faulty operation of the "pulsing" system. Another put on system monomater. he shows filter condition.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.
The coating recovery function of a powder recycle system is to collect the extra coating powder material and render it suitable for recycling and at the same time to remove the fine powder particles from the exhaust air stream before discharge into the atmosphere.
Recovery function are two main types of collectors:
a.Cyclone Recovery Collectors
b.Cartridge Recovery Collectors(there are more designs of collection system)
The curing/stoving process allows polymer chains within the formulation to bond with one another, this process is called cross linking and is the reason powder coating is so durable. When a thermosetting powder coating is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking , requires a certain temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders curling schedule at 180 & 200 ?C (390 ?F) for 10 minutes. The curing schedule could vary according to the manufacturer's specifications.The latter demonstrates significant reduction of curing time.Unless powder is allowed to run through a full cure cycle, the cross linking process will not be complete and the durability will be compromised.
Cure schedules are based on part metal temperature( Effective metal temperature) which means the part(s) themselves must reach the specified curing temperature before the curing timer should be started.