Transfer Efficiency and Spray Technique
Transfer efficiency is the measurement of the percentage of the paint material sprayed that provides coating on the parts versus the percentage that is lost in the spray process. The type of spray gun is very important, but it is only part of the picture. The extent of automation and electronic control equipment, the cleanliness and maintenance of the system, the skill and training of the operator, the coating material, the product being sprayed, the flow rates, and spray booth air velocities will affect the transfer efficiency of any spray equipment. Table 7.1 provides a comparison of different technologies in field and lab tests, and shows the relative transfer efficiencies of each.
SPRAY GUN CONTROLS
There are several related adjustment points on a gun that allow the operator to control the size and shape of the pattern, the amount of atomization, and the amount of fluid coming out of the gun.
For manual guns, there are typically two adjustments located at the rear of the gun handle. One knob will adjust the amount of air going to the air nozzle, providing control of the size and shape of the pattern. The other knob adjusts the fluid needle travel, allowing more or less fluid flow.
The fluid volume should not be adjusted with the knob on the gun unless the gun is being used with a siphon system. Adjustment by fluid needle travel is unstable and causes premature wear to the needle and seat. If a spray gun is used with a pressure system, then the fluid flow should be controlled with the fluid regulator, and the knob should be left in the full open position.
Adjustment starts by following the manufacturer’s recommendations for needle and nozzle selection from their charts. The charts will indicate the proper range of fluid and air pressures.
The fluid pressure is set from the chart and the atomization pressure is added until the pattern is uniform and consistent. A good starting point for the atomization pressure could be as low as 5 psi. The pattern can then be tested to see if the paint is properly atomized and additional pressure can be added if it is needed. Excess atomization air will generate more velocity and reduce transfer efficiency.
If the atomization is not fine enough, the air pressure should be increased about two or three pounds at a time and tested each time it is increased. Once the correct amount of atomization is reached the pattern size and shape can be adjusted with the pattern adjustment knob on the spray gun.
The normal position for the ears on the air cap is parallel to the floor to produce a vertical fan pattern. This assumes spray gun movement from left to right. To create horizontal fan spray pattern, the operator can turn the ears or turn their wrist 90◦.
Usually, when the fluid pressure is increased it is necessary to increase the atomizing air pressure. The two air pressure controls are adjusted to provide the correct volume of material with uniform atomization and a minimum of velocity. The best overall delivery pressure is always the lowest pressure that will adequately provide the correct volume of paint and the correct atomization.
If the atomizing air is too high relative to the amount of fluid being delivered, it may produce excess velocity and dry spray. If it is too low, the fluid droplet size may be too large (inadequate atomization) and the surface may have orange peel.
If too much fluid is applied, the film may be too heavy with more risk of sags. If the amount of fluid applied is too low, it may cause lightly coated areas on the part and dry spray.
It is possible to do several things wrong and still produce a good finished product.
However, proper technique will improve the quality of the finish, reduce overspray,
reduce labor, and increase the life of the spray equipment.
Proper technique begins with an examination of the spray gun.Abrief inspection
of the system may find some detail that needs to be adjusted before the gun is triggered.
Some of the things to look at include
1. Is the trigger easy to pull back, and does it return smoothly and quickly
2. Are the air and fluid nozzles properly installed?
3. Are the fluid and air hoses the correct size?
4. Is the fluid hose free of pinch points?
5. Are the hose connections tight?
After the system inspection has confirmed that the gun is ready for spraying, a test pattern should be sprayed on cardboard. A full oval shape with an even distribution of paint is needed to produce a quality finish. Spray enough paint on the cardboard to allow it to run. The runs should be equal all across the pattern. If the runs are heavier on one side of the pattern, the air cap may be defective. Rotate the air cap 180◦ and respray the cardboard to see if the heavy runs shift sides. If they do, replace the air cap.
The amount of paint delivered to the gun tip is determined by the amount of product to be painted and the time available to spray it. The amount of atomizing pressure is determined by how much energy it takes to break up the paint stream into a uniform pattern. The best spray pressure is the lowest possible level at which the job can be done.
A spray gun needs to be held at a constant angle to the part to help apply uniform film buildup. If the spray gun is tilted, the distance between the nozzle and the part surface is not consistent, and film deposition will not be uniform. More paint at a greater velocity will contact the part in the one area of the pattern. This causes poor hiding and dry spray at one end of the pattern and heavy film at the other. This tilted position is sometimes referred to as heeling or toeing the gun (see Figure 7.1).
Heeling/toeing is an easy way to recognize technical error and one that is easily corrected. Figure 7.1 shows the incorrect position. Proper application requires a level stroke.
Another common error is arcing or fanning the spray gun from side to sideb (see Figure 7.2). This will occur when the operator hinges the spray gun from the elbow instead of the shoulder and wrist. The pattern is not kept at the same distance from the product during the entire spray stroke, and the film deposits heavy in the center and light at the edges.
Proper spray application requires the use of the wrist, elbow, and shoulder working together during the stroke. The average spray stroke is 36′′. The arm moves side to side in a horizontal motion. The wrist must be flexed as the arm strokes from side to side, attempting to maintain a consistent gun-to-target distance (see Figure 7.3). The proper spray stroke will produce the most uniform film with the least risk of runs, sags, or dry spray.
Gun distance and gun speed are also very important factors in spray accuracy and uniform paint application. The distance from the nozzle tip to the product affects the wetting of the paint and the appearance of the coating.
Solvents evaporate as the coating leaves the gun. The correct gun-to-target distance works in harmony with the specific solvent blend in the coating and provides the proper “wetness” when the coating hits the target. If the gun is too far from the product, there is more risk of dry spray. If the gun is too close to the target, the film may sag (see Figure 7.4).
If the distance between the spray gun and the target increases or decreases, the wetness of the paint film will be affected by the evaporation rate of the solvents in the paint blend. The typical distance from the target for hand spray is 6′′–12′′. Each coating may have a slightly different distance that seems to work best, and the spray gun and gun adjustment will also affect the wetness of the film. It is important to be as consistent as possible while stroking the gun.
Another important factor in accurate film deposition is speed of the stroke. The stroke speed of the spray gun should be kept consistent for the best material utilization and to conserve the energy of the operator.
The “gyroscopic painter” technique should be avoided. Excessive pivoting of the spray gun uses more paint material than necessary, makes a mess of the booth walls and ceiling, and wastes energy.
The correct speed allows for an even wet film of material to be applied in one stroke. If the stroke is too slow, it may cause sags and runs, solvent pop may occur, and it is more difficult to keep up with production. If the stroke is too fast, it may cause dry spray, uneven film buildup, and exhaustion of the operator.
Gun speed should be maintained at a comfortable and even pace. The fluid pressure and the air nozzle pressure are set to work in harmony with the speed of the stroke.
The gun stroke should be overlapped in a precise pattern to ensure complete and uniform coverage. The amount of the spray pattern that is overlapped should be 50% of the total pattern width. If the gun delivers a 12′′ pattern for each spray stroke and each stroke overlaps by 6′′, it is called a 50% overlap. The center of the spray pattern is aimed at the wet edge of the previous stroke (see Figure 7.5). Sometimes a spray pattern is “crosshatched” to build additional film thickness and to provide uniform thickness and coverage. Crosshatching refers to a pattern that is stroked vertically and then horizontally.
The spray gun should be triggered before the pattern reaches the part and released after the pattern leaves the part to avoid a heavy area. Triggering on the part will leave a large deposit of material in one area, because the coating will not be fully atomized for a split second after the trigger is pulled.
The size of the spray pattern should be related to the part being coated. Large panels can be efficiently coated with a full pattern. For slender work, the pattern should be small enough to avoid excessive overspray.
In order to apply an even film, the spray pattern must be uniform. It is useful to spray a sample pattern on a piece of cardboard to make sure that there is no problem with the fluid nozzle or air cap. Some examples of faulty spray patterns are shown in Table 7.2.
When coating a large panel, the conventional method is to cover the surface with horizontal strokes. The stroke starts about 6′′ off the part, and the trigger is pulled
just before the gun tip reaches the edge of the part. The trigger is released just after the gun tip moves off of the other end, and a new stroke begins in the other direction with the gun moved down to produce a 50% overlap.
The strokes can be vertical if the operator is comfortable with the motion and able to produce consistent and uniform film build.
Coating the edge of the panel with parallel strokes can reduce the amount of overspray. The gun does not have to be moved off the edge of the part. An illustration in Figure 7.6 shows this technique.
Abox shaped part should be coated on the corners first, and then the flat surfaces are covered with the same technique as a panel (see Figure 7.6).
The addition of an in line paint heater can provide several advantages, particularly considering the high-solids content of a compliant liquid coating. The heater will not only help to achieve a correct spray viscosity, but will also provide consistency so that the paint is less sensitive to temperature, and the operator can apply a more uniform film without adding additional solvents.
The spraying of heated paint enables the low boiling solvents to evaporate between the spray gun and the surface, leaving the high boilers to level the material. The increased viscosity of the paint will allow more material to be deposited without sagging.
Paint heaters can be fitted for different types of application systems; they can be attached to a booth, or they can be portable for field applications.
There are several types of paint heaters that can provide consistent paint temperatures at a specific flow rate. A typical heater circulates heated water or oil through a heat exchanger. The paint is heated as it passes through the heat exchanger on its way to the spray device (see Figure 7.7).
Electric paint heaters are often used in medium to small paint supply and circulation systems. Paint heaters are typically capable of raising the paint temperature to 70◦F from the temperature on the inbound side of the heater.
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