Application Equipment & Cleaning Processes
Selecting and maintaining your application equipment is necessary for achieving the best transfer efficiency possible. It is important to match your equipment with your coating selection as each combination of application equipment and coating creates its own characteristics and can affect the finished product. Additionally, implementing best practices for equipment cleaning can help lessen your waste and costs. MnTAP maintains a list of liquid coating equipment suppliers who may help you determine what coating equipment is right for your application.
In use for over 50 years, conventional, or atomized spray, uses air at high pressure (40 to 70 pounds per square inch [psi]) to atomize a liquefied stream of paint. The high-energy air stream finely atomizes paint making it easy to apply and yielding very good finishes with high-quality visible characteristics. A disadvantage to conventional spray is that a high degree of atomization is accompanied by a very fine spray that is highly susceptible to overspray. The result is more paint waste and low transfer efficiency.
As the name suggests, a high volume of air at low pressure is used to atomize paint. The air-pressure limit for HVLP is 10 psi at the spray gun air cap. The lower energy level reduces overspray and improves transfer efficiency. Generally, fluid-delivery rates up to 10 ounces per minute with low viscosities work best with the HVLP gun. At higher fluid delivery rates or with heavier materials, HVLP may not atomize well enough to achieve an acceptable finish.
This method of atomizing paint does not use compressed air. Paint is pumped at high pressure through a small opening at the spray tip to achieve atomization. Changes in airless spraying are made by adjusting the coating’s viscosity or the system’s pressure. This method has higher transfer efficiency than conventional spray. Many high-viscosity coatings can be applied without costly solvent thinning. Also, this method allows for rapidly applying a heavy paint coat—useful for keeping up with a fast-moving paint line.
This spraying system “assists” airless systems by using supplemental air jets to guide the paint spray and boost the level of atomization. Air-assisted airless technology combines the best characteristics of both air and airless spray. Benefits include: substantial material savings and reduced overspray when compared to conventional spray and improved transfer efficiency and finishing appearance when compared to airless technology. The ability to lower the fluid pressure from airless systems is the primary factor in increased finish quality. Also, operator technique is enhanced as the application rate is slowed, making product coating easier.
The electrostatics method gives the paint and the part opposite electrical charges. As paint is sprayed the electrical charge draws it to the part, which results in higher transfer efficiency. In this process, paint spray is less susceptible to drafts and air currents. Another advantage is that electrostatic coating helps wrap coatings around part edges. For safety when using electrostatics, the part should be grounded or prepped with a solution that will provide a ground path. With special equipment, waterbased paints can also be electrostatically applied.
This application system atomizes paint by dropping a stream of liquid on to a disk or bell-shaped object spinning at high speed. Rotary atomizers use electrostatics to attract paint to the part. This process is useful for high-viscosity paints and can create a spray without the use of a thinner, while also maintaining high transfer efficiency. The equipment needed for this type of application is very specialized and usually requires a major conversion of a paint line.
Dry booths and wet booths are the two basic types of enclosures used in most painting applications. The key difference is that the dry booth depends on a filter of paper, fiberglass, Styrofoam or metal to collect overspray, while the wet booth uses water with chemical additives. Decisions about this equipment should be made based on the type and volume of painting done and the type and volume of waste generated by the booth itself.
Dry filter booths generally meet the requirements of small-volume painting operations because of their low purchase cost. One disadvantage to using a dry filter booth is waste disposal. Often the majority of waste is the filter media itself, contaminated by a relatively small amount of paint. Reusable filters may decrease waste volume and reduce disposal cost. In some applications, overspray can be collected for reuse.
If overall painting volume can justify the investment, a wet booth may work to your advantage. This type of booth eliminates the need for disposal of filter media and reduces waste in weight and volume. This is achieved by chemically separating the paint from the water then settling, drying, or using a centrifuge or cyclone to remove the solids for disposal.
Paint viscosity may need adjusting before spraying. Most often this is accomplished by thinning the paint with organic solvents. But, using solvents for thinning requires the purchase of additional materials and increases air emissions. An alternative method for reducing viscosity is to use heat. Paint heaters use less solvent, have lower solvent emissions, create more consistent viscosities and produce faster curing rates. Consult your paint supplier to determine if paint heaters can be used with your coating.
Equipment cleaning may be required when painting is completed, a color change is needed or maintenance is required during operation. Equipment cleaning offers opportunities for reducing waste and air emissions.
When assessing the cleaning process, all cleaning tasks should be reviewed to determine if cleaning is necessary. While many assume that spray guns, tips and lines must be cleaned for reuse, cleaning some low-cost items may not be advisable. Costs from cleaning-solvent purchases, solvent waste disposal and solvent emissions could be higher than simply replacing the item being cleaned.
All solvents should be stored in covered containers when not in use. Leaving solvents in the open air creates unnecessary solvent waste and VOC emissions. A standard should be set to assure that used solvent is disposed of or recycled only when it loses its cleaning effectiveness, not just because it looks dirty. Consider an on-site still for solvent recovery to further reduce your costs.
For equipment that requires cleaning, methods that minimize solvent use and reduce evaporation are ideal. For example, a gun washer is a piece of equipment similar to a dishwasher designed to hold a number of spray guns and related equipment. It cleans by circulating solvent inside a closed chamber. The result is rapid cleaning and extended solvent cleaning-life while reducing solvent waste and the emissions from evaporation. For more information about automatic cleaning systems see MnTAP’s reference list, Spray Gun and Equipment Cleaning System Suppliers.
One method for line cleaning introduces turbulence to the solvent going through the line during cleaning by alternating pulses of solvent and compressed air. Payback on this equipment can come from increased production output due to faster color changes and from material savings through decreased solvent use.
Additional Cleaning Resources
- MnTAP Reference List: Aqueous Cleaning Equipment Manufacturers
- MnTAP Reference List: Solvent Recycling Equipment
- MnTAP Reference List: Solvent Degreaser Alternatives
Industry Case Studies
- Crenlo, Inc. (2009). The company reduced solvent waste by 80% by selecting appropriate spray nozzles.
- Paints and Solvents: Waterbased Finishing Systems (1998). Two companies, Automated Building Components and the US Postal Service, benefit from efficient paint line equipment.
- Ultra Image Powder Coating (2008). An intern helped Ultra Image Powder Coating reduce the amount of waste material generated through the painting process, by recommending a new powder coating booth to be installed.
- Crenlo, Inc. (2007). The facility also added a pre-soak step to the equipment cleaning process and reduced solvent waste by 55%.