Understanding Paint Atomization
BASF coatings development expert Tim December explains how paint atomization works for both pneumatic spray applicators and high-speed rotary bell applicators.
Q: How does paint atomization work and how is the atomization process different for pneumatic spray applicators and high-speed rotary bell applicators?
In the 1800s, Joseph Binks, maintenance director at Marshall Field’s Department Store in Chicago, invented the air-atomized spray gun. In his new invention, the paint was broken into small droplets by a pressurized stream of air. He used his new atomization technique, instead of the old-fashioned paintbrush, to efficiently paint the walls of the department store.
Paint atomization is key to efficiently transferring liquid paint to a target object. The term atomization refers to the process of using an applied force to break up bulk liquids, such as a paint, into very small droplets which can then be directed by a stream of air to coat a target object.
There are two main atomization techniques commonly used. First, the innovative pneumatic method used by Joseph Binks and later modified in the mid-1900s for aerosol spray cans. And the modern high-speed rotary bell application method, which is common in industrial painting and very common for painting automobile bodies.
In the pneumatic case, atomization occurs when air mixes violently into a flowing fluid stream, breaking it into small droplets. The size of droplets varies depending on paint flow rate, paint viscosity and air pressure settings on the paint spray gun. The air pressure and paint fluid flow rate provide energy to propel paint droplets away from the spray gun nozzle to the part. The film begins to form as droplets reach the target and level. The droplets coalesce, forming a thin paint film.
The amount of applied air and the air cap geometry are critical features in the pneumatic atomization. There are two source points where a force of air is applied. One source is from the center air and we call this the atomization air for primary atomization. This will stretch the paint to atomize the liquid. The second force of air is more from the side “horn” of the air cap and we call this the horn air. The secondary atomization air “crashes” into the center atomization. These two atomization forces create a primary pattern and a secondary pattern that merge to form the overall spray paint pattern.
We can control the atomization and fan pattern by the air pressure of the horn air and the center air. When increasing the horn air and holding the atomization air constant, the particle diameter gets slightly smaller and the spray fan pattern gets broader and wider. The droplet velocity is decreased. If the horn air is constant and we increase the central atomizing air, the droplets get smaller and the spray pattern is narrower, but the droplet velocity is significantly faster.
Pneumatic applicators operate on air-impulse momentum for atomization and high-speed rotary bell applicators operate on rotary field centrifugal forces for atomization. The applied forces are quite different for these. In the pneumatic atomizer, the air velocity force is about 100-300 m/s and this air flow rate and the paint flow rate determine the degree of atomization. For the high-speed rotary applicators, the bell rotational speed (which is very high - about 15,000 to 70,000 rpm) and the paint flow rate determine the atomization. Let’s look now at the high-speed rotary bell.
The rotary bell is a very common atomizer for large industrial paint shops due to the increased efficiency. As we learned with the pneumatic gun, the amount of applied air and the air cap geometry are critical to atomization. For a rotary bell atomizer, the diameter of the bell cup, the rotational bell speed and even the type of bell cup edge geometry will have a major impact on the atomization. The bell-shaping air is also important to transfer the atomized particles to the target.
The bell applicator has a central discharge port for the paint fluid flow. The bell cup is rotating very, very fast, for example 60,000 rpm! The paint flows from the center outlet port and moves across the surface of the bell cup. The paint completely covers the bell cup surface and then, right at the very edge, liquid strings are formed. The liquid strings snap and form the droplets. This is the atomization process.
The basic operational principal of the rotary bell is that there are three types of air for operation. The bearing air holds the bearings for smooth and very fast rotation to get to 60,000 rpm. The turbine air provides the energy for rotation. The shaping air can direct the atomized droplets as they are formed from the paint flow.
There are some key parameters that control the atomization when using a rotary bell. For example, the bell speed rotation, the amount of shaping air and the distance between the atomizer and the target are all important. Increased bell rotational speed will increase the atomization. When the bell speed increases, the paint-thread diameter decreases and thus particle size decreases. This results in smaller paint droplets and very good paint flow and leveling on the part. With low bell speed, the large filaments form large paint droplets, which makes for less optimum paint flow. The shaping air at high values along with fast rotation can increase the atomization. Low shaping air and low speed will decrease the atomization. The distance to the target can impact the transfer time, however, this is very often a fixed variable.
Using either of these two atomization methods, painting efficiency can be even further improved by using an applied voltage between the atomizer and the target. This creates an electrostatic field which attracts the paint droplets to the target. A charge, Q, is applied to the paint in an electrical field of strength E, thus the force acting on the paint particles is: F=Q*E. The paint particles are attracted to the oppositely charged target part. In simple pneumatic applicators without an electrostatic field, the transfer efficiency is only about 30%. This means 70% of the paint misses the target and goes to waste. The rotary bell application process using an electrostatic field achieves about 75% to 80% efficiency — less than 20% to waste!
Since the pioneering work of Joseph Binks, we have greatly improved the quality and efficiency of the painting process. This is great news for painted consumer products like shiny automobiles and much better for our environment.
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