A need to apply the best finish on our product at the most economical price is the one thing we all have in common. This is a job that must be addressed by considering three basic things: the material suppliers, the spray gun operators, and the equipment suppliers. No one would disagree that all three must work as a team in order to make this work. An unacceptable finish resulting from the improper maintenance of any of these three sources could lead to costly downtime and material rejections. We will begin this journey by examining each of the three groups separately.

Material Considerations

The material supplier partner should be a knowledgeable, reliable, and available person who will answer your questions. With the cost of materials rising, we are more aware than ever about trying to get the maximum transfer efficiencies from our equipment and making the best use of the materials that are being applied.
The material must be properly selected so that it will work best with the products and equipment being used. Material characteristics such as pot life, corrosiveness, flowability, consistency, pigmentation, and its electrical properties (in the case of electrostatic application) should be carefully considered.

Furthermore, the recommended solvent system for the respective coating must be used. Low-cost solvent blends may lead to finish problems. Some inexpensive materials may contain fillers that will not mix readily and lead to plugging in the small fluid passageways within the spray gun. The material should be properly strained or filtered to remove lumps, skins, and particles. Remember to also filter out the contaminants and not the pigments or ingredients. Your material supplier should be able to recommend the proper filter mesh size for the coatings you are using.

It is also important to realize that although coating materials are usually packaged in clean containers, they won’t always remain that way if part of the material is used one day and the rest of that material is used a month later. Rust and other contaminants may form on the container walls or lid and possibly enter into the coating. Many polyurethane coatings will begin to cure once the container is exposed to air.

An understanding of the paint’s viscosity is also important. Spray finishing equipment is very sensitive to the material’s viscosity because it will impact the selection of fluid nozzles, needles, and air caps. Proper use of Zahn cups, which are used to determine the material viscosity, can extend the use of your material and the life of your spray gun components.

Equipment

The proper use of your spray equipment can save you money by practicing some simple maintenance techniques. Let’s start by considering the proper care of your spray gun. It is recommended that the gun never be fully submerged or soaked in a cleaning solvent or in a strip tank. This practice will tend to fill the gun’s air passageways with dirt and solid paint particles. These foreign particles will ultimately dislodge and find their way into the finish, causing rejects. Fish eyes, pops in the finish, and streaking can result.

Never use solvents that contain silicone, as these will contaminate the finish. An alternative to soaking the gun is to submerge only the front end so that the fluid nozzle is cleaned. The best practice would be to remove the fluid needle, air nozzle, and fluid nozzle separately for cleaning. The outside of the gun should be cleaned with a solvent rag. The fluid passageways can be flushed with a clean solvent before the gun is stored. Commercially available ultrasonic cleaners work very well when cleaning nozzles or air nozzles.

The fundamental rule for setting spray pressures is to use the lowest pressure that will provide a satisfactory spray pattern. If this “golden rule” is applied, you will save money. If we could reduce the flow of paint from the spray gun by just one fluid ounce per minute (a one ounce shot glass), then the dollar savings will accumulate at the end of the year. Let’s consider a nitrocellulose lacquer as our material, which costs about $50 per gallon:

Multiple spray stations would add to this figure or using a more expensive material.

Using spray guns that have high transfer efficiency characteristics will put more of the material onto the object being coated instead of into the environment. HVLP (high volume, low pressure) spray guns have been designed to supply the maximum transfer efficiencies. In some cases, new spray guns that use LVMP (low volume, medium pressure) technologies can offer better results.

I previously mentioned the importance of controlling material viscosity. Viscosity levels can be controlled by the use of solvents, in the case of oil-based materials, or by the use of paint heaters. Recent environmental mandates limit the use of solvents, which may put more importance on the use of heaters. Failing to control viscosity will ultimately lead to improper pressures that could, in turn, lead to reduced transfer efficiencies. This will result in higher material and maintenance costs.

The Operator

The importance of the spray operator is often overlooked. The key to finishing excellence can be described in one word: control. Many top finishing companies have instituted statistical process controls and have begun to train both the operators and supervisors in the proper techniques that will yield correct color match and finishing quality. Constant monitoring of air and fluid pressures, line speeds, surface preparation, and operator spraying techniques will guarantee cost-effective results.

This section will deal with the proper care and maintenance of the spray gun because it is the operator who may also be in charge of scheduled equipment maintenance. Do not use anything harder than brass to unplug clogged air passageways or air nozzle holes. Doing so can damage the air nozzle beyond repair. Most spray guns can be repaired by your maintenance department if they are provided with instructional part sheets and the inexpensive gun repair kits that are available from your supplier. As previously mentioned, avoid using any silicone-based lubricants.

The spray guns one uses must have clean and dry compressed air with adequate pressures and volume in order to produce a quality finish. Check the air pressure at the spray gun by installing an air pressure gauge at the inlet of the gun. The air valve should always actuate first ahead of the fluid needle—this allows for more efficient and clean atomization of the material. Increasing the size of the air hose from 1⁄4" I.D. to 5⁄16" I.D. will mean less pressure loss and will help to save valuable compressed air. Even a simple air leak in a compressed air system can cost money and compromise your finish quality. The accompanying chart will give you an estimate of the extent of your waste. Eliminating loose air connections, faulty regulators, leaky extractors, and poorly running agitators can save money.

The cleanliness of the spray booth can also have a great effect on the finish of your products. Good housekeeping is a must in order to keep dirt off of your finished product. Dust accumulations from the booth walls should be kept to a minimum. Do not grind, sand, or polish in or near the spray area. Air intake and exhaust filters in the booth must be replaced when indicated by the manometer or draft gauge. Ensuring proper air movement through the booth is paramount to ensuring maximum transfer efficiencies of your finishing equipment. Conveyors and bake ovens can also become the primary source of dirt contamination in the finished product.

Conclusions

Coating materials must be properly selected, mixed, reduced, and strained according to the supplier’s recommendations. Training the operating personnel on how to properly spray with their equipment is very important.

Impart responsibility to the operators and maintenance personnel for taking care of their equipment. The maintenance department must be informed of the importance of cleaning and repairing the spray finishing equipment, including the guns, filters, booth, conveyors, etc. A routine preventive maintenance program can save money by minimizing rejects and maximizing performance. A good preventive maintenance program will reduce down time, extend equipment life, and result in an improvement to your return on investment.

References

1. Hund, J.P. How the Proper Maintenance of Spray Equipment Will Save You Money, 1990.
2. Adams, J. Fix It Before It Breaks, 1989.

Bio

Paul Micheli is the engineering manager of new product development for atomization products at ITW Industrial Finishing in Glendale Heights, Ill. In this capacity, he deals with atomization research and product development for industrial spray applicators and is dedicated to the goal of positioning ITW Industrial Finishing as the leader in new atomization technology. His academic credentials include a BSME from the Illinois Institute of Technology and a MSME from Northwestern University. He has a combined 32 years of industrial experience and has several U.S. patents to his name, and even more pending in the area of atomization new product development. Micheli is a registered professional engineer in the state of Illinois and resides in Glen Ellyn.