Curing stage. Further changes take place, permitting the powder to cure

completely.

The complete process — from flow stage to cure — generally takes less than three min-utes, which makes this an ideal process for production-line application.

12.4.4 Generic Types of Powder Thermoplastic materials: reacted curing agents and require a heat

source to convert to a liquid state. Store powders away from any heat source until just before application. In warm and hot mates or during shipping in war and hot cli-mates, store the powders in refrigerated containers.

The range of application temperatures for powder varies by manufacturer. Thermo-plastic powders normally require lower application temperatures; always consult the manufacturer’s data sheet for the proper temperature range.

12.4.6 Preheat

Preheat the surface or object to be coated either by a high-frequency induction coil or in a direct gas-fired oven.

12.4.7 Application Methods

Powders are applied by one of the following methods:

• Electrostatic spray

• Fluidized bed (dip method)

• Flame spray

• Roto-lining

12.4.7.1 Electrostatic Spray

The most common and efficient method to spray apply powders is using an electrostatic spray handgun (Figure 12.12). The powder is conveyed under pressure into the gun in fluidized form.

12.4.7.2 Fluidized Bed

An application method known as the fluid-ized bed (analogous to dipping in the liquid coatings field) was originally developed in Germany in 1953 (Figure 12.13).

Figure 12.12 Electrostatic Spray

Figure 12.13 Fluidized Bed Dipping

When a finely divided stream of air passes through a powder, a solid in gas dispersion forms and behaves like a liquid. A fluidized bed is a tank with a false bottom made of porous material. Air pressure is applied from below the porous false bottom to lift the powder above it and force it into suspension.

12.4.7.3 Flame Spray

Low air pressures blow thermoplastic pow-der particles through a high-temperature, open-flame torch similar to an oxyacetylene blowtorch. Simultaneously, particles melt and the coating surface is heated.

12.4.7.4 Roto-lining

To roto-line, charge a pre-weighed amount of powder into a hollow mold (Figure

(Figure 12.15), then rotate the mold around two axes while heating the mold and the powder. When the interior metal surface is hotter than the melting point of the powder, the powder melts on contact with the metal.

Upon cooling, the powder forms a protective coating (Figure 12.16).

Examples of items that can be lined by roto-lined powder coating include: drums, car-boys, storage and process vessels, pipes, pipe flanges, valves, flow meters and pumps, as well as other equipment.

Figure 12.14 Charging a Pre-Weighed Amount of Powder into a Hollow Mold

Figure 12.15 Placing a Mold into a Heated Oven

12.4.8 Inspection Concerns

Inspectors in the powder coating industry work in a relatively safe environment.

Inspection criteria are similar to the liquid coating industry including the quality

Figure 12.16 The Powder Forms a Protective Coating when Cooled

of surface preparation. The requirements for surface preparation in immersion service are more critical than for atmospheric ser-vice. Ensure preparation is suitable for the powder coating and that it meets the specifi-cation requirements.

12.4.9 Inspection Checklist Check and record:

• Ambient conditions — air and substrate temperatures, relative humidity, and dew point

• The dehumidification system — ensure it performs properly and will “hold the blast”

• Fabrication defects — such as rough welds, skip welds, pits, crevices, particu-larly in hard-to-reach or even inaccessible areas

• Soluble chemical salts

• Surface cleanliness

• Surface profile meets specification

• Residual abrasive dust

Carefully document each inspection item and note any potential problem areas to bring to the attention of the client for review and/or correction before coating operations proceed.

12.5 Special Application Equipment

12.5.1 Introduction

Along with the continuing development of high performance coatings there is a need for new and improved application equip-ment. This section discusses some of the more common specialized equipment. Keep up to date on new equipment as a member of NACE; read the monthly magazines and attend conferences where new techniques are discussed and new materials and equip-ment are exhibited and demonstrated.

12.5.1.1 Plural-Component Spray Systems

While plural component spray equipment is not new, it has been greatly improved in the past few years (Figure 12.17, Figure 12.18).

Computerized proportioning systems have greatly improved the accuracy of the mix ratio and contractors can use the same machine with various products without have to rebuild it or change the pump legs. The machines are also much smaller and easier to maintain. A trained technician is still needed to set up and operate the equipment.

Coating inspectors need to understand the ratio and heat check methods that are built into the machine.

12.5.1.2 Equipment Types

There are two basic types of plural compo-nent spray equipment: fixed- and adjustable-ratio machines. Fixed-adjustable-ratio systems have two pumps that operate with a fixed throw on each leg. To change the ratio, the techni-cian manually changes one or both of the legs (pistons) on the pump. On the variable-ratio systems, the variable-ratio is controlled

auto-matically by the machine (it controls the dis-tance each piston travels in its cylinder),

Figure 12.17 Plural Component Spray System

Figure 12.18 Plural Component Spray System

thereby controlling the amount of material pushed with each movement (Figure 12.19).

Plural spray units have two types of feed mechanisms: one type that blends the com-ponents in a manifold and mixes them in an inline static mixer, and another type that mixes the components at the spray gun tip (Figure 12.20). Select the type of machine based on the pot life of the coating being sprayed. Polyureas and their hybrids, which can have only a 10 second pot life, must be

Figure 12.19 Plural Component Spray Setup

mixed just outside of the spray tip, while materials such as solvent free epoxies, with a 20 minute pot life, can be mixed in the manifold. It is important to know the correct set up of the hose connections and the neces-sary connections to be able to verify the sys-tem is set up properly.

Figure 12.20 Mixing Block for Plural Component Spray Unit with Insulated Hoses

12.5.1.3 Hot-Spray Systems

Polyureas (and some other products) require temperatures of 110F (43C) or higher to lower the viscosity enough to make the material sprayable. A heated system uses the combination of a drum heater to preheat the product with an inline heater built into the pump mechanism to ensure the product reaches the required temperature (Figure

Figure 12.21 Heated System with Insulated Hoses

12.5.1.3.1 Advantages and Disadvantages

Plural component spray equipment has sev-eral major advantages over single piston pumps:

• Accurate automatic material mixing

• The ability to spray apply very thick sol-vent-free materials without thinning

• The ability to spray materials with very short pot lives

Of course, this equipment also has disadvan-tages:

• The cost of the equipment is much higher than the cost of a single piston pump

• The education requirement for the mechanic is higher

• The heaters require high voltage electric-ity

• This type of equipment may have as many as five hot hoses attached to the gun, mak-ing the applicator’s job more difficult 12.5.1.3.2 Inspection Concerns

Even though the machine controls the mix ratio, check the ratio manually at intervals during spray operations because numerous things can go wrong to cause an off-ratio sit-uation. All modern machines have a built in method to manually check the ratio. The