Air Spray vs. Air-Assisted Airless Technology

Q: I work for a company that refurbishes aircraft and am involved in the painting side of the business. We are currently using air-assisted airless electrostatic applicators, and I have been asked to look at any other available technologies for finishing. What options are available, and what are the advantages/disadvantages of our current technology?

A: Let’s review your current process and review the advantages and disadvantages before discussing alternatives.

Air-assisted airless (AAA) applicators utilize hydraulic atomization to mechanically shear liquid coatings into small droplets, which involves moving the coating from its supply container to the applicator using a high-pressure pump. Most AAA applications have fluid pressures ranging from 500 - 2500psi (34 - 170 bar), which relates to material viscosity, length of fluid hose, and fluid hose size. The material is forced under high pressure to a fixed narrow orifice just before the exit of the applicator. As the material moves through this orifice, the mechanical transfer from high pressure to ambient pressure causes the material to shear into smaller droplets, effectively atomizing the coating. The angular cut tip, commonly referred to as the cat-eye based on its shape, guides the material to its overall pattern size at a spray distance from the part. When balanced with fluid flow rate, adjusted by fluid pressure on the pump, the fluid tip ultimately dictates the overall atomization quality during spray. Air is then added via the air cap to “assist” the atomization uniformity across the full spray pattern and slightly reduce pattern width. Depending on the manufacturer, the fluid tip part number will have 3-4 digits to indicate the orifice size and the expected spray pattern size. For example, XXXXX-1315 indicates a 13in (330mm) spray pattern at a designated target distance with a 0.015in (0.38mm) orifice. Please refer to the applicator’s service literature for more detailed information when selecting the proper fluid tip for each application.

Advantages of Air-Assisted Airless applicators include:

• Low particle velocity. Hydraulic atomization yields a relatively soft spray pattern compared to other atomization techniques with low forward velocity of the sprayed coating exiting the applicator. This softer spray characteristic helps reduce overspray and increase transfer efficiency.
• Speed of application. Faster material builds can be achieved due to the wide range of fluid flow rates achievable, as high as 33 oz/min (1000ml/min). Generally speaking, optimal performance relative to appearance and transfer efficiency are typically realized in the 12-17 oz/min (350-500 ml/min) range.
• Fixed spray pattern size. The spray pattern is determined by the fluid tip selected. Other than using the assist air to improve the uniformity, it will not vary across multiple applicators.

Disadvantages of Air-Assisted Airless applicators include:

• Fixed spray pattern size. Although this is listed above as an advantage, it can also be viewed as a disadvantage if variability is needed in the shape of the spray pattern when coating smaller and/or wider areas of parts with complex geometries. AAA technology, fixed spray pattern could be considered a disadvantage as the operator would need to change out the fluid tip to achieve these spray patterns.
• Atomization quality. The atomization quality, or achievable spray droplet size, is considered “good” compared to other forms of atomization. Typically, this technology is not used when the application demands a “Class A” finish (the hood of a car has a “Class A” finish, little to no orange peel, and high gloss).
• Difficulties achieving low material builds. To achieve lower material builds, the fluid flow rate needs to be reduced (under 10 oz/min [300 ml/min]) or the fluid tip’s orifice size needs to be decreased. The common problem with doing this is lower fluid pressures can yield coarser atomization, and smaller orifice sizes are more prone to plugging up by contents in the material.
• Cost of Ownership. There is a more significant investment in high-pressure pumps, service parts, maintenance, and other related equipment to keep the applicator functioning. There are also additional safety training requirements related to high-pressure equipment.

Regarding alternative spray technologies with manual applicators, the other option you have is air atomization, sometimes referred to as conventional spray or air spray. Air atomization technology uses air pressure to impinge a fluid stream that breaks the material apart, effectively atomizing the coating. Additional fan air is then used to shape the spray pattern into a wider elliptical shape. With independent air controlling atomization and pattern, this technology offers a wider range of spray patterns. This technology uses low fluid pressures, typically under 100 psi (7 bar), and can support multiple fluid feed techniques: gravity-fed and suction-fed for small batch/low-production applications and pressure-fed for high production requirements.

You noted that you were using electrostatics with your AAA applicator and I waited to address this last. The electrostatic process itself is relatively straightforward; a negative charge is applied to the coating material as it is being atomized by the applicator. The electrically charged paint particles are attracted to the part being coated, which is set up to be at ground potential (grounded via the fixture supporting the part in the booth). Regardless of the atomization technology, the use of electrostatics will improve transfer efficiency and aid in edge coverage and overall uniformity of the spray finish.

In conclusion, both solutions can optimize your application. If you are currently obtaining acceptable finish quality, you may want to maintain your current AAA process. Suppose you have exhausted all options with your current equipment and want to improve your finish quality or need variability in your spray pattern and flow rate, you may want to look at air atomized technology.

Important Note: Before making any adjustments to your spray application process, please make sure to check your local regulatory requirements for hazardous material application in your facility. Local regulations can influence what you can and cannot use depending on multiple factors, including yearly material usage.