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OVERVIEW

1. Why use corona vs. tribo?

Corona charging is a method of powder charging that utilizes a voltage multiplier to distribute an electrostatic field at the tip of the gun. The powder particles conveyed by air pass through the electrostatic field and become negatively charged. The part to be coated is grounded and therefore is positively charged. Negative and positive ions attract - thus causing the powder to stick to the part. Tribo charging is a kinetic or frictional charge wherein the power particles make contact with the inner wall of the gun as they are conveyed by air. The powder particles then become positively charged thus causing the powder to stick to the part to be coated. Either method is sufficient for powder coating a part. While corona charging is the more widely used and commonly known method, tribo charging can work well for some parts where penetration into small areas is required - for example radiator fins - and where the electrostatic field from a corona gun may result in a Faraday Cage effect - making penetration more of a challenge.

Tribo or Corona? Here's How to Decide (pdf)

2. Why use a fluidizing hopper vs. box feeder?

Both methods work well. The decision to choose between one or the other can come down to convenience, cost, and sometimes performance depending on how well a particular powder fluidizes and flows. Feeding from powder boxes is both convenient and cost effective because the powder coater is effectively feeding from the same container that the powder is shipped in. Feeding from hoppers allows more precise control of fluidization making powder delivery less of a challenge. Powder fed from boxes typically requires vibration of the box and local fluidization to keep the powder moving toward the bottom of the plastic bag or liner inside the box. Powder fed from hoppers requires cleaning of the hoppers and/or multiple hoppers but allows for complete, uniform, and constant fluidization of the powder source if set properly. Hoppers can also be equipped with level sensors to notify the operator through some type of audible alarm or visual indicator that the powder level is getting low and the hopper needs to be refilled. Sensing the level in a powder box while not always required because the box is open and can be visually inspected can be somewhat of a challenge.

3. Why use a sieve?

The use of a sieve depends on the type of finish required, the level of potential contamination in the spray environment, and the state or quality of the virgin and/or recovered powder to be sprayed or reused. Sieves can act as both eliminators of potential contaminants that can cause defects, rejects, or rework - and classifiers that can eliminate fines caused by continuous recovery and clumping caused by moisture in the powder source or in the environment. Fines can be problematic to system and application efficiency as fluidizing and charging them can sometimes be a challenge. Two common methods of sieving include vibratory deck screens and rotary sieves. The type of sieve used often depends on the type of system and/or the required color change time (cartridge or cyclone), the level of required throughput as a function of number of guns and/or transfer efficiency, the potential level of contamination, and the degree of particle classification required - to name a few potential variables.

 

4. How often should I perform maintenance on pumps throats, gun nozzles, and other wear items?

The equipment manufacturer will typically supply recommended guidelines for preventative maintenance (PM) for more common wear parts such a pump throats and gun nozzles within their product manual. At the same time - the rate at which these parts wear depends on the abrasiveness of the powder, the pump flow rates, and the total hours of use to name a few variables. Pump throats should be inspected daily for wear as worn throats can adversely affect the amount of powder flow as well as uniformity of the flow. While they often wear at a slower rate than pump throats, nozzles too should be inspected on a regular basis for wear as this can affect uniformity of the gun fan pattern.

5. How do I know what the proper air setting is for my pumps, fluidizing, vent, etc.?

The correct air settings are very powder dependent. Some testing and/or experimentation may need to occur to find the optimum setting for a particular powder. The key to achieving optimum pump settings is to provide just enough flow air to propel the powder forward to the part, and enough atomizing air to help disperse the powder evenly, and at the same time minimize velocity. This will help to minimize overspray resulting in the highest possible transfer efficiency and lowest powder material usage. Optimum fluidization often begins with a visual check. The powder should have just enough fluidizing air to achieve the look, feel, and consistency of a liquid. However, too much fluidizing air will result in the appearance of a bubbling or boiling liquid. This results in large random bubbles and air pockets that can cause non-uniformity in the powder flow from the pump to the gun. As far as vent pressure, this can depend on the method of venting (direct, non-air-assisted, air-assisted, or combination thereof), and what device is being vented - a sieve or a hopper or both. The key is to allow just enough venting to minimize the pressure in the sieve or hopper - but not too much venting resulting in too much powder lost to the recovery system as this can cause further degradation of the powder particles increasing the rate at which fines are generated and increasing the amount of scrap powder.
 

PF Zone: Powder Coating Questions

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