Using a System Audit to Improve Coating Efficiency

Q. We are looking at ways to better optimize our powder coating operation. What steps should we take to start this process, and what should we be looking for?

A. Optimizing a powder coating operation might seem like a daunting task, depending on the age and complexity of a system and how well it has been maintained over the years. And while the entire finishing process, from load station to unload station, should be evaluated—including, washer, oven, conveyor and even powder material—we will focus primarily on the powder system.

The first thing a finisher should do is contact the powder system supplier and request an audit to identify areas for improvement. Powder system suppliers are familiar with the audit process and typically look at a number of standard components to understand what changes should be made to improve the system’s daily operation and performance.

A retired veteran of the powder coating industry once shared some words of wisdom with me that I believe are particularly appropriate to this discussion. He said there is a simple formula that will aid in the optimization of a finisher’s powder system: application efficiency plus recovery efficiency equals system efficiency.

While this formula may seem overly simple, there are numerous variables within both “application efficiency” and “recovery efficiency.” This discussion will try to hit on some of the more important variables to consider when auditing a powder system. 

Application efficiency can be defined as putting enough powder on the part to achieve the desired film build while minimizing powder overspray and daily powder usage. Specifically, this means:

• Powder gun cables and multipliers should be tested to determine if they are achieving maximum kV output. If not, they should be replaced.
• Gun nozzles and pump throats should be checked for excessive wear and should be replaced if necessary. 
• Powder hoses should be replaced periodically to ensure maximum powder flow.
• Powder hoppers should be checked for powder levels and fluidization air pressure. 
• Proper powder levels will ensure enough “head pressure” in the hopper, resulting in smooth powder flow at the gun. 
• Proper fluidization air pressure is a visual check, and the powder should look like it is fluid and moving like a liquid, but not bubbling violently. If set improperly, the air bubbles will cause air pockets in the powder line, and this will result in inconsistent powder flow at the gun tip.
• Powder flow rates and electrostatic settings should be checked to ensure that the powder gun is set for maximum transfer efficiency—in other words, that it applies the least amount of powder while still achieving the desired film build. 
• The part being coated should be checked for proper earth ground. This includes checking both the part hanger and the conveyor. Conveyors can get dirty and require maintenance over time. Parts hangers also can get coated to the point that they need to be cleaned or replaced.
• A good powder system audit should include looking at maximizing “line density.” One of the worst things for a powder system’s efficiency is to have the guns spraying powder when there are no parts on the conveyor. Load the conveyor up with as much metal as possible while maintaining sufficient distance between parts so that they don’t make contact with each other.
• Finally, not all systems have or need triggering, however, if large line gaps, short parts (shorter than the tallest part) or less-than-capacity racks are common, consider adding triggering to save on powder usage. 

Recovery efficiency is the ability of the powder system to contain powder within the booth and transfer over-sprayed powder back to the guns. It also refers to the system’s ability to minimize powder in process. There are two distinctly different types of recovery systems: cartridge systems and cyclone systems. 

Cartridge systems are typically used when only one or a few colors are required, and fast color-change time is not as critical. Some things to consider when auditing a cartridge system are:

• If powder is not contained within the booth, look at the collector to understand if it is sized correctly relative to the number and size of booth openings. 
• If the collector is sized correctly but is still not containing the powder, the cartridge filters and/or the final filters may need to be replaced. Differential pressure gages on the collector and fan sections of the cartridge system, if operating properly, will typically indicate when it is time to change filters. 
• If excessive powder is building up on the cartridge filters, they may need to be pulse-cleaned more frequently.
• If powder is building up in the collector fluid beds, there may not be enough transfer pumps, the transfer pump pressure may be too low, or the fluid plates may be plugged and need to be replaced. The transfer pumps also may be plugged and in need of cleaning. 
• If powder is escaping from the final filters, they should be replaced. More importantly, the fan section of the collector should never see powder. Powder in this section might indicate that one or more cartridge filters have ruptured.
• The cartridge filters should be checked for ruptures, as should the seal between them and the clean air section of the collector.

Cyclone systems are typically used when many colors and/or the fastest possible color-change time is required. A cyclone acts as a powder classifier: Heavy particles will fall to the bottom of the cyclone and are reclaimed and resprayed. while fine particles (particles that typically do not fluidize or charge well) will escape through the top of the cyclone and ultimately end up in the after-filter, where they are eventually scrapped. Some considerations when auditing a cyclone system include:

• Whether the system is containing powder or not, cyclone efficiency should be calculated, and it should be in the mid- to high 90th percentile. (Powder system suppliers typically use a standard test to calculate cyclone efficiency.) 
• To calculate cyclone efficiency, first determine if the after-filter and cyclone were initially sized properly relative to the amount of powder being sprayed and the amount of overspray, and whether there is sufficient face velocity across the openings of the booth for powder containment.
• Additionally, visually check to determine if powder is building up in the cyclone. When a fast-color-change system is designed properly and a cyclone is running at its highest efficiency, there should be less than 1 pound of powder in the system at any given time. This includes not only in the cyclone but also on the walls and floor of the spray booth. 

While there are other items relative to gun movers (application) and sieve screens (recovery) that can and should be checked during a powder system audit, the aforementioned points are the most likely to cause a powder system to become inefficient. But, as mentioned, an audit also should not be limited to the powder system alone. It should also include the washer, oven, conveyor and even the powder material supplier. 

At the end of the day, all parties represent a partnership that has the finisher’s best interests in common: optimizing the entire finishing system’s efficiency.

John Binder is a global product manager for Nordson Corp. Visit nordson.com.