Powder Hoses & Ideal RH Percentage

Powder Hoses

I am a custom coater that sprays many different types of powder, including metallics. What type of powder hose should I use?

There are several things to consider when selecting a hose to convey powder from the powder pump to the spray gun. For manual spray guns, hose flexibility is important. If you will be spraying different types of powders or different colors through the same hose, it needs to be easy to clean. Abrasive materials will require the use of a very durable hose. Most powder hoses range in size from 0.375" inside diameter (ID) to 0.75" ID. Typically, 11 mm or 7/16" ID hose is used. If your application requires high powder flow, you may need a hose with a larger ID. For applications of low powder output, a smaller ID hose will give better control of the powder delivery. Many different types of powder hoses are now available, and each one is designed for a specific purpose. For your application, I would suggest a conductive powder hose. Because you make frequent color changes, using this hose will help to speed up color change a great deal. The conductive nature of the hose resists impact fusion and is easy to clean so you should not have to use cleaning cubes or a hose-cleaning gun.

Ideal RH Percentage

I've been told that I need to maintain 45 percent to 60 percent relative humidity (RH) for ideal powder coating conditions. Is this true?

The corona field ionizes the powder particles and moisture in the air. Some moisture in the air is necessary to facilitate the transfer of ions to the particle to be charged. If the moisture content is too low, the corona field is not as effective and results in lower transfer efficiency in the application process. If the humidity is too low, there is a tendency for the material to create a tribo charge, which strips away negative ions and leaves the powder with a positive charge. A typical corona spray gun uses a negative charge. Relative humidity of the environment is also important because if it is too humid, the powder resist flow and fluidization and it will be more prone to impact fuse in the delivery system. A 45 percent to 60 percent RH is the most effective range for application of most powders. As in most manufacturing processes, controlling variables within the process produces fewer rejects and reduces costs associated with rework and scrap. The bottom line is that changes in humidity can cause fluctuations in application deposition weights, thus increasing your reject rate. Maintaining an RH of 45 percent to 60 percent contributes to a consistent finish and reduced rejects. Humidity can be reduced by a condensing coil and raised with a humidifier. Most powder coating environmental control rooms are conditioned with an HVAC system (heating, ventilating, air conditioning). The RH can be controlled with a humidistat. In rare cases where temperature is low and humidity is high it may be necessary to reduce the humidity with an HVAC system and use a reheat coil to raise the temperature.