Switching to Rotary Atomization

Justin Hooper is a product manager for Carlisle Fluid Technologies. Visit carlisleft.com

Q: Our current robotic paint process uses conventional and electrostatic air spray equipment to apply conductive primer, base, and clear coat to a variety of plastic parts. We are trying to understand what benefits, if any, we could gain by switching to rotary atomizers?

There has been a trend within the finishing industry to transition from air atomized applicators to rotary atomizers. This trend started about 20 years ago in the automotive assembly plants. Although the initial interest was related to improved transfer efficiency or reduced paint usage, additional benefits, such as improved finish quality and simplified application programming, were also realized.

Before going into greater details on the benefits, a quick review of the technology will allow a broader understanding. Air atomized applicators utilize a fluid tip and needle to turn fluid on and off along with an air cap to develop an elliptical spray pattern. When the needle is pulled back, the coating material exits the fluid tip. From the holes in the face of the air cap, atomization air exits and impinges on the fluid column, creating small droplets of coating material. Fan air is introduced through the horns on the air cap and is used to create an elliptical spray pattern. The spray gun is designed to allow the air to flow ahead of the fluid to ensure that the atomization quality is consistent. There are optimal atomization and fan air settings based on the volume of fluid that is flowing. Once the volume of fluid flow is determined, atomization air should be adjusted to get the desired droplet size. Fan air is then adjusted to get the desired spray pattern size, increased fan air develops a larger pattern. A test pattern should always be sprayed to visually inspect the overall spray pattern. Although it will vary based on coating material, flow rate and air settings, air atomization will generally produce droplets in the range of 10 – 70 microns.

Rotary atomizers utilize centrifugal force to shear the coating material into droplets. An air bearing turbine with a “bell” cup mounted on it is typically rotated in the range of 25,000 – 60,000 rpm. Coating material is introduced through a fluid tube into the backside of the rotating bell cup. The fluid column initially impacts a splash plate and then transfers to the face of the bell cup where the rotation draws it to the edge. The edge of the bell cup has serrations that the material is forced through, causing it to form into ligaments, which then shear off the edge of the rotating cup. Shaping air is introduced outside and behind the edge of the bell cup and is used to shape the spray pattern and provide additional forward momentum. A single or dual source of shape air may be used, based on the design. Although it will vary based on coating material, flow rate and rotational speed, rotary atomizers will generally produce droplets in the range of 20 – 45 microns.

Both technologies above are available with and without electrostatics. The electrostatic process introduces a negative charge to the coating material as it is being atomized. The electrically charged paint particles are then attracted to the part being coated that is at ground potential (typically through attachment to the conveyor system). The use of electrostatics drastically increases transfer efficiency and improves edge coverage due to the electrostatic attraction.

The main benefit of rotary atomization over air atomization is improved transfer efficiency with or without electrostatics. With air atomization, considerable energy is used to create the atomized droplets. This translates to high particle velocities, which are observed in the form of overspray. With a rotary atomizer, the forward particle velocity is much lower, thus increasing transfer efficiency. Transfer efficiency is a measurement of the total amount of coating sprayed, compared to how much actually goes on the part. It iscommon to see a 30-40% improvement in transfer efficiency with the use of a rotary atomizer. Generally, for every 100 gallons sprayed, 30-40 additional gallons will stay on your substrate instead of into your booth filtration system.

Finish quality is improved with a rotary atomizer because the atomized droplets are more uniform in size which reduces orange peel and improves wave scan and DOI readings. The rotary atomizer also produces a uniform spray pattern which reduces mottling or striping due to inconsistent film build created by spray pattern uniformity issues.

Application programming efficiency with a robot can also be improved by 10 – 15% with the use of a rotary atomizer. This is typically measured by looking at “trigger on” time versus total program time. With an air atomized applicator and an elliptical spray pattern, the applicator must travel off the part when triggered on and also needs to be repositioned when the robot changes direction. The elliptical pattern also needs to stay straight when spraying side to side, which can create excess rotation and strain on the wrist of the robot. Less articulation is required with a round spray pattern where the robot can simply index, change direction and even rotate while spraying and still providing a uniform film thickness across the substrate.

There are many benefits that rotary atomizers can offer to improve the finishing process. Historically, manufacturers would avoid this technology due to concerns related to color match with metallic coatings. Today’s applicators, with advanced shaping air designs, have overcome this issue. If you want to ease into this technology, it is often suggested to start in clear coat to gain experience with the equipment and process.