Technology Interchange—Using Nitrogen to Control Variables in Powder Coating

Posted on Thursday, January 4, 2024

By Brad Ruppert

A growing number of coating systems are being converted from liquid to powder and manufacturers are choosing to bring coating operations in-house for the first time. Alongside this trend are manufacturers seeking automation solutions to add to their finishing operations. All with the same three main goals in mind: Reduce costs (powder consumption), improve quality, and increase production. While the industry is doing a good job setting companies up for success, most are choosing to not invest in optimal environmental rooms to reduce humidity and temperature variables that affect their system. In addition, companies are struggling to keep experienced teams that know how to adapt their system to overcome day-to-day variables. This is making it hard to maintain current operating levels, much less improve upon any of them.

But what if those variables can be controlled?

Controlling Variables Starts with Understanding Electrical Resistivity
First-pass transfer efficiency (FTE) is affected by electrical resistivity, which represents the rate of change of voltage decay in the powder granule. To improve FTE, we must understand which variables in a powder system can affect changes in electrical resistivity.

The two main factors that impact electrical resistivity are relative humidity and temperature. The lower the relative humidity and temperature, the higher the resistivity and the slower the rate of change in voltage decay. The higher the electrical resistivity a powder granule can keep, the higher the transfer efficiency it will have. Let’s take a closer look at how technology can be applied to any existing coating system to control humidity and temperature.

The Polifluid technology, patented by Eurosider sas, filters standard compressed shop air to leave only nitrogen (Figure 1). Nitrogen is an inert gas that does not hold moisture. When using only nitrogen gas in a powder application, humidity can be eliminated.

Polifluid patented technology then super-chills the humidity-free nitrogen to create a constant temperature throughout the entire powder system. Using nitrogen in this way effectively reduces the two biggest variables affecting electrical resistivity, humidity and temperature. The system also adds ionization to the nitrogen to stabilize the electrostatic charge of the substrate. When the ionized, chilled, humidity-free nitrogen gas is introduced into a powder system it improves everything from the fluidized bed to the corona leaving the gun. This produces better FTE and provides consistent film build on parts (Figure 2).

Nitrogen Can Save Up to 30% in Powder Costs
Besides higher productivity rates and a more consistent finish, nitrogen has many benefits for manufacturers using powder for finishing. The easiest to measure is a reduction in powder used. Depending on the manufacturer’s variables, that can mean as much as a 30% reduction in powder consumption.

With less tendency for overcoat, efficiencies are realized due to a reduction in discharge velocity, which prevents powder buildup inside application equipment and results in less wear on nozzles and hoses. Manufacturers will also experience less overspray and booth contamination, which reduces clean-up labor.

Lab Trial Validates Polifluid Performance vs. Compressed Air
What follows are test results using Polifluid with a Venturi powder coat system and a Lesta self-learning robot at the LestaUSA Test Lab. The robot was used to create the exact same powder gun path, distance, and speed. This ensured that the carrier gas in the system was the only variable that changed.

Procedure 1
A Venturi powder pump supplied with compressed air was used to powder coat a 10 ga. steel plate hung on a conveyor set at 8 linear feet per minute. The panel was coated with a Lesta MV6 robot using a program created by a lab technician. Settings were adjusted until the resulting powder coverage was a uniform 5 mils (127 microns). The settings were documented, and three panels were coated using these settings to ensure repeatability.

Procedure 1 Recorded Data
Settings used to achieve the desired 5 mils included:
• Voltage: 100kv
• Characteristic Slope: Steep
• Powder Quantity: 80%
• Corona Current: 120 microamps
• Total Air Volume: 3.25 cubic meters/hour

Procedure 2
Using the settings from Procedure 1, a vacuum filter bag (with known tare weight) was secured to the end of the applicator and a 60-second cycle time captured and weighed the sample to the nearest gram using a digital scale. The result was recorded for later comparison.

Procedure 2 Recorded Data
In the sample collected from a 60-second trigger, one cycle of the settings produced a gross weight of 206 grams. After subtracting the tare weight of 18 grams, a net weight of 188 grams was recorded.

Procedure 3
The same powder application system was then attached downstream through Polifluid technology. The system was purged for 180 seconds to ensure the chilled and ionized nitrogen reached the applicator. An additional panel was then coated by running the same program and settings used in Procedure 1, which resulted in an excess of 7 mils (178 microns) of uncured powder being deposited on the panel. The settings were then adjusted until the desired 5 mils were achieved. These new settings were then run on three panels to ensure repeatability.

Procedure 3 Recorded Data
Settings used to achieve the desired 5 mils (127 microns) were as follows:
• Voltage: 100kv
• Characteristic Slope: Steep
• Powder Quantity: 70%
• Corona Current: 120 microamps
• Total Air Volume: 2.85 cubic meters/hour

Procedure 4
Using settings from Procedure 3, a vacuum filter bag (with known tare weight) was secured to the end of the applicator and a 60-second cycle time captured and weighed the same to the nearest gram using a digital scale. The result was recorded for comparison with the same taken in Procedure 2.

Procedure 4 Recorded Data
In the sample collected from a 60-second trigger, one cycle of the settings listed above had a gross weight of 162 grams. After subtracting the tare weight of 18 grams, a net weight of 144 grams was recorded.

Results and Conclusion
The sample weight of the compressed air system was 188 grams, whereas the sample taken from the nitrogen system was 144 grams. This showed a reduction of powder applied per minute of applicator on-time of 30.5% (Figure 3). There is also a correlation with the reduction in value of the total air volume and powder quantity settings on the Venturi controller used in Procedure 3 to achieve the 5 mil coating on the panel.

In conclusion, this test showed a definitive reduction in powder usage between compressed air and chilled, ionized nitrogen. It was also observed that the setting used with Polifluid produced a cloud that was lighter in appearance, but the measured thickness of the powder applied to the part was proven to be the same.

Real ROI From Real Companies

SAFTCART — An industry-leading manufacturer of cylinder carts, cages, and pallets, SAFTCART’s powder line, prior to adding Polifluid, consisted of three manual powder coaters in one booth and a manual liquid post touch-up station used after curing to paint tight corners that consistently were not fully coated. Polifluid technology increased FTE and reduced Faraday effect, eliminating the labor-intensive manual liquid post touch-up station altogether. “This technology provides an even coating on all surfaces and corners and reduced our powder consumption by over 20%,” said Jimmy Walker, SAFTCART President.

Engineered Fluids, Inc. — EFI added Polifluid to their manufacturing process for pumping, metering, and control systems and saw a 33% reduction in powder while increasing production capacity by 40%. They also found greatly reduced cleaning time due to the fact that more powder was adhering to parts instead of falling to the floor.

A Tier 1 Construction Equipment Manufacturer — This company was looking to increase FTE and consistency of their 60+ coaters to help with quality issues and reduce learning curves which were costing hundreds of thousands of dollars in rework per year. The equipment they installed fed all hoppers and guns in the system. Rework has dropped dramatically, increasing first pass yield (FPY). They have also seen improved FTE and a reduction in powder usage through automatic reciprocators and self-learning robot touch up.

Nitrogen Can Control Variables and Increase Production
Every company with a powder coating system wants to reduce powder consumption, improve quality, and increase production. There are many variables affecting most systems that can make consistency and improvement very hard to achieve. The path to success starts with controlling known variables.

Controlled lab tests and real-world company applications have shown that using pure nitrogen in place of standard compressed air increases electrical resistivity and FTE for robotic, automatic, and manual powder systems, resulting in increased powder savings, more consistent coating on parts, and increased production potential.

Brad Ruppert is director of operations–automation for DeGeest Corporation and LestaUSA.