Flow Coating

In flow coating, paint is pumped from a storage tank through properly positioned nozzles, onto all surfaces of parts, as they are conveyed. Excess paint drains back to the storage tank for recirculation. Paint films applied by flow coating are wedge shaped, thinner at the top and thicker at the bottom of painted parts. In flow coating, utilization of paint approaches 95% as opposed to about 50% for atomized air spraying and 70 to 80% for dipping. Properly designed flow coat machines with vapor chamber flow-out reduce solvent losses and eliminate tears, sags, and curtains.

Flow coating is used extensively to paint panels for home appliances. Flow coating is also used to coat parts with recesses inaccessible to spraying, to coat parts (such as bedsprings) for which good appearance is desirable but is secondary to complete coverage and economical application, and to coat intricate parts that are too open in design to permit efficient spray painting and too large to be practical for dip painting.

Figure 3 shows a large assembly which, because of its size and construction, is an example of a part for which flow coating is the most efficient method of painting. Spraying would be inefficient because of overspray, and dip coating would require a large quantity of paint to fill the dip tank. However, flow coating would be impractical for a similar assembly twice as large, and spray painting would be preferred.

Fig. 3 Assembly for which flow coating is an efficient painting method Equipment for Flow Coating

Basic equipment for flow coating consists of a chamber, a paint-storage tank, a pump, a drain-off section, and continuous conveyors. Because flow coating ordinarily is used only for high-volume production parts, equipment usually is set up as part of a continuous process, which may include cleaning, phosphating, drying, prime flow coating, finish flow coating, and baking. One conveyor may carry the parts through all operations. Process variables that require close control are nozzle pressure, viscosity and temperature of the paint, and hanger design and spacing of the parts.

Pressure control of paint in the circulating system is critical, particularly at the nozzle. Excessive pressure causes the paint to flow off the parts at too high a rate, resulting in high solvent losses and bubbles in the paint. Too low a pressure leaves areas on the work uncoated or improperly coated. Pressures vary from 0.02 to 0.2 MPa (3 to 30 psi). The higher pressures are used to coat difficult-to-reach or recessed areas. The lowest pressure that can be used in a particular operation is the most economical when solvent loss is considered.

Paint viscosity must be closely controlled. Viscosity that is too high causes poor flow-off of paint from the work. This results in sags, beads, blistering, and other defects associated with excessive paint thickness. Viscosity that is too low results in excessive solvent loss and inadequate film thickness. Viscosities are held within the range of 18 and 32 s (No. 2 Zahn cup), although paints with a No. 2 Zahn cup viscosity as high as 100 s have been used. Viscosity must be adjusted for each paint and each differently shaped part. Once the optimum viscosity has been determined, it should be maintained.

Temperature control of paint is essential. Too high a temperature results in excessive solvent loss and may cause instability of the paint. Too low a temperature requires increased use of solvents to maintain proper viscosity and can result in inadequate film thickness. Although temperatures as low as 16 °C (60 °F) and as high as 38 °C (100 °F) have been used, 21 to 32 °C (70 to 90 °F) is the recommended temperature range.

Drain-off chambers, enclosed tunnels immediately adjacent to the coating chamber, are incorporated in many installations. In drain-off chambers, a high concentration of solvent vapor is maintained to retard drying. This eliminates beads, bubbling, and other surface defects that result when flow-off of paint is incomplete. Some installations include an electrostatic detearing device to remove any beads or drops of paint still clinging to the edge of the part.

The equipment and production requirements for using flow coating to paint two steel production parts are discussed

below. One part is a step hanger (an angle bracket) produced from 4.8 mm (— in.) stock 50 mm (2 in.) wide, with two

legs 355 and 125 mm (14 and 5 in.) long, respectively. The other, a bracket for holding a fire extinguisher, is produced from 1.5 mm (0.060 in.) stock and has legs 305 and 75 mm (12 and 3 in.) long; a metal clamp is attached to the 305 mm (12 in.) leg to retain the extinguisher. The production requirements are as follows:

Part A

• Step hanger, 4.8 mm (— in.) thick; maximum dimensions 125 by 355 mm (5 by 14 in.)

• Production rate, 540 pieces per hour Part B

• Bracket for holding fire extinguisher, 1.5 mm (0.060 in.) thick; maximum outside dimensions, 75 by 305 mm (3 by 12 in.)

• Production rate, 720 pieces per hour

Both parts

• Clean and phosphate, using proprietary phosphate cleaner coater with 2-min immersion in solution and rinse tanks before drying; color coat with alkyd baking enamel (no prime coat)

Equipment requirements for both parts are as follows:

Work handling

Overhead conveyor(a)

Cleaning and phosphating(b)

Solution and rinse tanks

190 L (50 gal)

Solution and rinse temperature

66 °C (150 °F)

Flow coating

Flow coater

Conventional; 2 nozzles

Nozzle pressure

275 kPa (40 psi)

Drying

Infrared; 7 to 8 min; 160 °C (325 °F)

(b) Proprietary phosphate cleaner coater used; 2-min immersion in solution and rinse tanks, then drying

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