The Immersion Operation

INTRODUCTION TO POWDER COATING

2.5 Application Principles and Techniques

2.5.2 The Immersion Operation

In immersion phosphating plants the process and rinse solutions, called 'baths' are contained in tanks which are arranged one after the other in what is known as a (processing) 'line'. The workpieces are then moved through each of the solutions in the prescribed order using a conveyor system. The workpieces may be loaded onto jigs or placed in baskets or barrels which are secured on an overhead hoist. Immersion operations are used to coat smaller items. These workpieces are then immersed in a series of dip tanks which are situated below the hoists. ' The time the workpiece spends in each tank can be varied in immersion systems and thus the coating weight and thickness can be varied. Typical immersion times for different steps are :

• degreasing - 5 -10 minutes

• rinsing - 0.5 -1.5 minutes

• acid cleaning - 10-20 minutes

• activation -0.5-1.5 minutes

• phosphating - 3 -10 minutes and

• passivation - 0.5 minutes.

Immersion rinses can be classified into one of two categories: nonflowing or flowing.⁷² Nonflowing immersion rinses contain water that is stagnant. As a result there is a contaminant build-up in the tank as the process proceeds until a maximum tolerance level is reached. Once this level is reached the rinse water is no longer effective and the water from the tank must be dumped and the tank refilled with clean water. Flowing immersion rinses consist of water that constantly flows through the tank. This rinse tank is fitted with an inlet (which brings clean water into the tank) and an overflow pipe or weir (which provides an outlet for contaminated water to be sent to a waste stream). Running or flowing the rinse has been used to keep the concentration of contaminants at low levels for longer so as to extend the lifetime of the rinse solution. This is possible due to the continuous dilution and discharge of the tank's contents as effluent. Flowing rinses can be emptied using a single rinse tank or multiple rinse tanks placed in series. According to the literature the latter is most effective when the water is fed into the last tank and allowed to run or cascade from one tank to another in the direction opposite to that in which the workpieces move down the line. ' ' This is referred to as 'counterflow' or 'countercurrent flow'. ' ' In contrast, each tank can be arranged so that the tanks are not linked. In such cases each rinse solution operates individually with its own water supply.

Sludge build-up in immersion systems has been found to give poor quality coatings on the workpieces. This occurs when the sludge can reach a level at which it becomes deposited on the workpiece. In large plants, sludge removal systems are usually fitted into the process to continuously remove the sludge and recycle the phosphating solution to the process tank. These systems are present in the process tanks where the sludge normally forms. Thus the tanks are designed in a specific way to facilitate sludge removal. Some tanks have sloping floors at an angle ranging from 45° to 60°.

Other tanks, with horizontal floors, are fitted with a sludge box. Another method of removal involves agitating the sludge to keep it suspended while continuously removing it with a tilted plate separator or filter unit.² In small plants the solutions are decanted and the tanks desludged at regular intervals. Decanting may be partial or total. Partial decanting involves draining the solution from the tank and discarding the

solution as effluent. The sludge left at the bottom of the tank is then pumped or manually bailed out in a separate step. No literature has been published on the composition of the sludge.

Immersion is also used to apply the organic coat to the surface of the workpiece, and it is carried out using a fluidized bed (see Figure 2.13)⁷⁹'⁸⁰ and is considered to be the dry equivalent to the immersion of the workpieces in the phosphating solution. The powder is placed in a holding container called a 'hopper' and is made to flow like a fluid by passing compressed air through it. The bottom of this container contains air chambers on the sides and there is a porous plate placed at the top. The powder is placed on top of this plate. Compressed air is fed through the bottom of the container and passes through the plate. This breaks up the powder particles, removes any moisture from the powder and provides a lightweight texture that makes it easier to transport the powder. The powder is said to become 'fluidized' and electrically charged. This is achieved by applying a high voltage to the fluidized bed. The workpiece is either heated and dipped into the fluidized bed or hung above in the cloud of powder which becomes deposited on its surface as a continuous film⁵'⁶¹²' According to the literature, fluidized beds are used mainly for the application of thermoplastic powders.⁵ Fluidized bed systems should be sufficiently insulated to ensure that the powder does not easily escape into the atmosphere.

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