Powder Coating

Sarah Bennett

Jenny Erle

Amberlee French

Josh Hamilton

John Schmitt

April 28, 2004

1.0 Introduction

Powder coating is rapidly becoming the new standard for paint applications instead of liquid paint. Some of the reasons that powder coating is more efficient are a higher quality finish, no drying time, and over-spray recovery. The electrostatic coating process uses the technology of charged particles accelerated through a spray gun that adhere to the substrate. The problem statement is to describe the concepts necessary for electrostatic spray applications. A fishing lure, known as a spoon was chosen as the substrate for this experiment. The coating process involves the materials processing concepts of the fluidized bed, mass balance, Bernoulli equation and Stoke’s Law. The electrostatic spraying method will be evaluated in order to determine the specific elements involved in the application of the powder.

2.0 Background

Powder coatings have been used since 1950 to apply paint to various applications. Several examples of powder coating uses are appliances, automotives, boats, sports equipment, buildings, lawn furniture, electronics, and camping equipment. Several coating processes have been developed over the years which include fluidized bed, electrostatic fluidized bed, thermal spraying, and electrostatic spraying. The techniques are continuously evolving to become more efficient.

The powder coating process involves finely ground particles of pigment and resin that are sprayed onto a substrate.Once the particles are applied, the part is cured which causes a chemical reaction that bonds the powder to the workpiece. The result is a uniform, high quality, and durable finish. This finish is more attractive than conventional methods due to the elimination of runs, drips, and uneven drying associated with liquid paint. The powder that does not adhere to the part is recovered and reused, generating a maximum efficiency of 98% material usage.

The most common technique used for powder coating is the electrostatic spray application process. The process involves a feed hopper and a spray gun, which incorporates the electrostatic charge of the particles. There are two types of spray guns, a corona and a tribo charged. The corona gun uses a voltage supply to charge the powder particles, while the tribo gun uses friction generated within the gun barrel. The corona technique negatively charges the particles. This generates electric fields, which causes uneven coating. The tribo gun positively charges the particles. As a result there is no uneven coating, making it the optimal method for electrostatic spray application.

Initially, the powder particles are stored in a large fluidized bed to keep them continuously suspended. The suspension of the particles prevents clogging in the pick-up tube that leads to the powder pump. The pump pulls the particles from the fluidized bed into the delivery tube. A second burst of air accelerates the particles, increasing the number of collisions with the Teflon walls and positively charging them (Coatings). The tribostatic process is shown in Figure 2 (Tribostatic). The positively charged particles are sprayed out of the gun tip and adhered to the grounded workpiece. This process can be seen in Figure 1 (Coatings). The workpiece is then placed in a furnace and cured at 300º F for 20 minutes (TGIC). The curing process causes a chemical reaction bonding the powder coating and the workpiece.

Figure 1: Electrostatic Spray Application System

Figure 2: Tribostatic Gun Layout

3.0 Approach

A thermoset powder and desired particle properties, diameter size and density, were chosen based on the needed workpiece finish. A smaller diameter size was used to obtain a more uniform coating. In the case of a larger particle diameter size, the finish would have a coarse appearance. The fluidized bed properties were tabulated since this process was used to keep the particles suspended. The terminal velocity and the velocity of the gas in the pick-up tube were the important properties. The volume flow rates in the pick-up tube and the gun tip were compared to determine the second burst of air needed to achieve the necessary velocity to charge the particles.

4.0 Calculations

Polyester triglycidyl isocyanurate, TGIC, powder, with a diameter range of 10 microns to 30 microns, was used as the coating agent. The first calculation was the terminal velocity for the particle size range with the appropriate friction factor.

Powder Coating with Polyester/Resin Powder

The density was for a thermoset polyester resin. Air was used because it was available and cheap.

Below are the values for the smallest and largest diameter size. A small diameter range was chosen for a more uniform coating. A diameter range was needed because in reality there can not be one size in the powder mixture.

Determining the Friction Factor Range that Should be Applied:

(Laminar Flow)

(Turbulent Flow)

Find the Terminal Velocity for the Small Diameter with Laminar Flow:

The terminal velocity was found by using Stokes Law and the first friction factor.

Stokes Law:

Friction Factor Check for Laminar Flow:

The Reynolds Equation was used to determine if the assumption of laminar flow was correct.

The first friction factor works because the Re is less than 1, therefore it is laminar flow.

Check if the Large Particle will be Fluidized at the Terminal Velocity for Small Particle:

The buoyancy forces of a falling sphere were compared to the forces of a raising sphere. The Stoke's Equation was used to determine the force down and the force up.

Stokes Law:

Force up is greater than force down, therefore the vt= 4.41*10^-3 m/s will lift the particle size with a diameter of 30 m. The terminal velocity used for the fluidized bed is 4.41*10^-3 m/s.

Next, the velocity of the particles at the opening of the pick-up tube was determined with the use of the Bernoulli equation.

Determining the velocity of the powder particles at the pick-up tube opening:

(Coatings)

The volume of the fluidized bed was determined by finding the volume for 50lbs of powder without room for air. Then the volume was increased to allow for fluidization.

Dimensions for 50lbs of powder:

0.25m x 0.25m x 0.25m

Dimensions for fluidized bed:

0.4m x 0.4m x 0.4m

Volume fraction of solid for minimum fluidization:

(Volume fraction of powder particle within the air)

Velocity in the pick-up tube:

(Pressure at the bottom of the fluidized bed)

(Thickness of powder at bottom of bed before fluidization)

(Radius of pick-up tube)

The pressure change from the fluidized bed to the pick-up tube was determined by using the following equation:

(Pressure change from fluidized bed to pick-up tube opening)

The pressure within the pick-up tube was assumed to be constant throughout the length of the tube. The pressure in the tube was calculated by subtracting the pressure difference from the initial pressure within the fluidized bed of 1 atm.

(Pressure inside of pick-up tube)

The velocity inside of the pick-up tube was determined with the use of the Bernoulli equation:

Bernoulli Equation:

It was assume that there was no heat loss or gain. The pump uses a vacuum or decrease in pressure to pull up the particles, so there is no work due to the pump. We are comparing the fluidized bed pressure directly beneath the pick-up tube opening, so the height difference was assumed to be zero. According to this model, the difference in velocities just inside and outside the pipe head is assumed to be zero (SAH). These assumptions resulted in the following equation:

The friction factor (Ef) was determining using the equation below to account for the sudden contraction.

For Contraction:

(The area at the pick-up tube opening)

(The area of the fluidized bed)

It was assumed that the ratio of the pick-up tube area was so small compared to the area of the fluidized bed that is was zero. This assumption results in the following equation: (Error here, the equation below is written for a contraction and expansion energy loss summed together.)

(Velocity in pick-up tube)

The equation used to calculate the velocity in the pick-up tube was:

The above equation was solved for the velocity in the tube.

(Velocity in the pick-up tube)

Volume flow rate in the pick-up tube:

The volume flow rate was determined by multiplying the tube velocity by the pick-up tube cross-sectional area.

(Volume flow rate in pick-up tube)

Make sure that the velocity within the tube was large enough to lift the largest particle by making the tube velocity much greater than the largest particle velocity:

(Velocity of the largest particle)

Checking that the pick-up tube velocity will be large enough to pick up the particles by subtracting the largest particle terminal velocity from the velocity of the pick-up tube.

(Air velocity minus the particle velocity)

(Velocity within the tube minus the terminal velocity of the particles)

This shows that the pick-up tube velocity is large enough to pull the particles from the fluidized bed.

The velocity within the pick-up tube is 48.56 m/s.

The volume flow rate within the tube and the volume flow rate at the tip of the spray gun were compared to evaluate the volume flow rate of the second dose of compressed air before the gun. A second burst of air was needed to accelerate the particles within the gun barrel in order to produce a friction charge and to achieve a final velocity of 250 m/s out of the gun tip.

Determining the volume flow at the tip of the gun:

The velocity at the gun tip was looked up and found to be 250 m/s for a tribostatic gun.

Reference:

Nozzle:

Reference:

Area of the Nozzle:

The volume flow rate at the tip of the gun was determined by multiplying the velocity by the cross-sectional area obtained above.

(Volume flow rate at the tip of the gun)

Determining the second burst of compressed air:

The second burst of air was calculated by finding the difference between the volume flow rate at the tip of the gun to the volume flow rate within the pick-up tube. The idea of what goes in must equal what comes out was applied.

(Difference in volume flow rates)

(Volume flow rate in the pick-up tube)

(Volume flow rate at the gun tip)

(Volume flow rate of the second burst of air)

The second burst of compressed air has a volume flow rate of 0.063 m^3/s.

5.0 Conclusion

As demonstrated, the coating process involved the key concepts of the fluidized bed, mass balance, Bernoulli equation and Stoke’s Law. The specific properties calculated were the terminal velocity at 4.4x10-3 m/s, and the velocity of the gas in the pick-up tube at 48.52 m/s. The volume flow rate of the second burst of air was 0.063m3/s. These values are only for this particular experiment, but can be adapted for various powders and particle sizes. A successful electrostatic spray process for the spoon was obtained by using various materials processing concepts.

7.0 References

Coatings.de. 26 April 2004. Vincentz Network 08 April 2004 <
What Are TGIC Powder Coatings?. 26 April 2004. Caswell. 15 April 2004 <
Tribostatic Powder Spray Systems. 26 April 2004. Nordon Corporation. 15 April 2004 <
Powder Coating for Outdoors. 04 June 2002. Juhe Xing Chemical Guangzhou. 14 April 2004 <
Cold Spray Coating Process. 09 April 2004. Gordon England. 06 April 2004 <
Sure Coat Automatic Powder Spray Gun. 05 May 2003.

Nordon Corporation. 06 April 2004. <