Paint Booth TechnologyPosted 3/10/2003
By Charles Wilhite
Refinishing is the aspect of collision repair perhaps most examined by the customer. The overall quality of the finish is dependent on the quality of the materials, the skill of the technician and the performance of the paint booth. Paint booths are not created equal. Most painters have stories of the worst paint booth they had to paint in. What does it take to make a great paint booth?
The paint environment is what paint companies refer to when they talk about paint booths. The paint they formulate needs a very controlled environment to perform as engineered. The quality of the air is critical. Clean air with very few, if any, particles is a must. The humidity and temperature are important, as is airflow. Lighting is essential if the technician is to see the surface and colors well enough to do excellent work. Lastly, there may be state and local regulations to be met, such as fire suppression systems or VOC containment devices.
Going with the Flow
Three things control the quality of the air in the booth: flow, filtration and conditioning. Airflow affects the paint process more than many technicians realize. The paint delivery system used in refinish applications is a simple hand-held paint gun. This device uses air pressure to propel very small droplets of paint to the surface of the car. To get the paint thin enough to spray, solvents are added. These must evaporate before the paint starts to cure. For this process to happen, airflow must be only strong enough to remove the "overspray" from the booth. Too much airflow will disrupt the spray on its way to the car surface.
The temperature in the booth needs to be held constant so the solvents evaporate at a predictable rate. Once the paint is in place, more airflow and a higher temperature are desired to speed evaporation and carry the solvent, which is in the air, away from the surface. At high evaporation rates, solvents can fall back on the paint surface if not swiftly carried away. This could cause the surface to lose gloss. Once the solvents are out of the paint, the air may be recirculated to avoid having to constantly heat outside air.
So three distinct flow modes are needed for a good paint booth operation. They can be referred to as spray, purge and cure. This may be accomplished in many ways, one common method being to use ducts and movable doors to control where the air is flowing. Flow within the booth around the vehicle is also very important.
Early paint booths had large fans in one end and a few filters in the other. In these cross-flow booths, air would rush into the booth and slow down as it spread out flowing across the booth, allowing any particles in the air stream to fall out. Because the air was pulled into the booth, any leaks at the doors allowed contaminants to be pulled in.
Modern down-draft booths are pressurized, so any leaks are going out of the booth. Large masses of air are pushed through filters in the ceiling and drawn out at the floor. The larger the filter area in the ceiling, the slower the air is moving when it enters the paint environment. It must speed up to move around the vehicle. This causes particles in the air stream to stay there and any loose particles to get caught up and carried away.
Filtration was ignored in early booths. The simple goal was to get enough overspray out of the booth so the painter could see the car! Later, water falls were developed to catch paint particles leaving the booth, and simple filters were used at the intake. Today, mostly paper or plastic fiber filter mediums are used in a combination of pre-filters to catch the larger particles initially, denser filter media at the point where the air enters the booth to catch all the finer particles, and a loose filter to catch outgoing paint particles.
To maintain proper airflow, these filters must all be changed when clogged. This is best done either by tracking hours of use with a meter, or measuring airflow in the booth. In engineering, all decisions are a compromise. The filters in the ceiling are expensive. Installing a larger area will allow the air entering the booth to move relatively slowly and speed up as it moves around the vehicle. But that means changing the filters is an expensive maintenance item. A smaller filter area in the ceiling seems more cost-effective, but the air is moving faster entering the booth and thus will not accelerate as much, and may not be as effective at picking up small dust particles brought into the booth on the car or technician. Also, smaller filter packs must be replaced more often, so the annual bill may not be reduced.
Turn Up the Heat
Heat is a must for a modern paint booth. Usually a remote heater source, such as a gas burner, is used to heat the air as it flows. This is not very different from central heat in a house. Outside air passes through the heating section on the way to the booth, in paint mode. Some or all of the air is recirculated through the heating section in purge or cure mode.
How well a booth heats the air is one measure of its efficiency, but how well it heats the vehicle is the real test. Air moving too quickly through the booth may not have time to allow the heat to transfer to the vehicle surface.
How evenly that heat is distributed is also very important. Some booths are very sensitive to vehicle placement over the floor grate air exit. Shifting a vehicle too far to one end or side may cause air to move unevenly around the vehicle. This will have an effect on both spraying and drying/curing. Usually larger plenum areas help equalize airflow better than smaller ones.
From makeup artists to car paint color designers, light is one of the most important ingredients in how the color is perceived. In low light conditions subtle colors seem to fade away. Bright sunlight is where cars are most often seen. Of course, the old joke about delivering cars that don't match on an overcast day is a good example of the effect light has on color.
To properly match colors, a paint technician must have proper lighting. The Color Rendering Index (CRI) is one gauge of light quality. It indicates how much of the natural sunlight spectrum from a bright, but not harsh, sky a bulb is capable of producing. In a decent booth the CRI of the bulbs should be at least above 85. Office lighting has a CRI of about 62. These bulbs will not produce the quality of light needed to properly see colors in the booth.
Regulations covering paint booth installation and use may come from several levels. Building code will dictate distance from walls, fire suppression systems to be installed, and even little things like whether the sprinkler heads should be covered, and if so, with paper or plastic.
At the moment there are few VOC regulations on spray booth emissions. Charcoal, water trap and biological solutions are available in addition to traditional combustion-based VOC reduction technology. This may be part of the future as states look for ways to meet federal clean air requirements. Location and height of exhaust stacks may also be regulated.
Location, Location, Location
Where the booth is placed in the shop will also have an impact on efficiency. Does the shop allow for a drive-through booth so the next car is taped up and waiting to go in? Is the turn too tight getting in, requiring two people to guide the vehicle? Will the exhaust stacks be far enough from the property lines? Careful planning and booth selection will result in a highly productive paint environment.
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