Education

Daily preventative maintenance is a must in all powder systems. If powder guns, hoses, venturis, ground etc. are not properly maintained, the ratio of virgin/reclaim can change on a daily basis. This results in reclaim powder building up in the collector. Causing fines to increase in the reclaim powder, which adversely affects film build consistency, electrostatics, transfer efficiency (T.E.), cured film appearance and increases reject rates. The difference as I see it between virgin and reclaim powder is the recycled powder in filter type collectors will be lower in particle size and more likely to pick up lint and dirt contaminants through the collection process. A rule of thumb managing recycled powder is to USE IT AS YOU GENERATE IT. By doing this, the chance of contaminating the reclaim powder is reduced. The longer the powder is in the collector, the more chance it will get contaminated. And, if by chance there is a problem, less powder is scrapped. This will save money. Regular sweeping of the walls and floors into the reclaim collector is always a good practice. Attention to the reclaim usage will also enhance overall system efficiency.

First of all knowing your first pass transfer efficiency will help in determining what pressures are ultimately set on the transfer pumps. Your powder or equipment supplier can assist with setting up an experiment to determine your first pass T.E. Poor grounding will affect your T.E. and cause reclaim to build up. Routine maintenance of the hangers is a must. Logging all your application settings daily is always a good practice. Let’s say one configuration of part is coated in your system. If you find the need to constantly adjust the settings, there is something causing this to happen. If the particle size is changing in the feed hopper out of a pre-determined range, the need to adjust gun settings will result.

Virgin powder is manufactured to a specific particle size distribution. The powder suppliers control the distribution to a pre-determined specification. Typically the range on the median size of the powder is plus or minus 2-3 microns from the nominal value of the median or approximately 10%. In theory you should be able to control the particle size in the feed hopper in a 4-6 micron range. The range is typically 8-10 microns lower than the virgin median specification. You will need to take powder samples from the virgin, feed hopper and reclaim and have the samples analyzed for particle size to find where your system ultimately runs. (The powder supplier can run the analysis and help determine the range for your system)

A transfer efficiency test can be run to establish the T.E. percentage. Once you know your T.E., the next step will be setting your transfer pump pressures. Let’s say your first pass T.E. is 55%. At this percentage the transfer rate would be 55% virgin to 45% reclaim.

The next step will be to run a flow rate test on the virgin and reclaim pumps. Items needed to run the test are; vacuum cleaner bags or a fabric type filter bag to measure the output of each pump, a stop watch, and a scale (preferably with a measurement in grams). The bag used should be porous enough to allow the air to migrate through the bag, but not allow powder to escape and cause a mess. Weigh the bags before and after running the test for each transfer pump. Take the transfer hose off the cyclone and hold inside the bag. Turn on the pump for thirty seconds, and then weigh the bag. Subtract the weight of the bag and multiply by 2, which will give you grams per minute of each pump. I would suggest running the test at 3 different pressures for each pump (in increments of 10-PSI) if adjustments are needed to achieve the prescribed mixture based on your T.E. Once you complete the test I think you’ll find that at the same pressures set on the virgin and reclaim pumps, the reclaim output will be more, typically 2 to 1. The reason, reclaim powder is fluidized in the color module and transfers better than virgin powder in a container that uses vibration to feed the pumps. There may be times when the mixture becomes unbalanced, and reclaim begins to build up in the collector. Knowing what adjustments to the pump pressures are needed will get the system back on track quickly.

Because of the abrasive nature of powder, powder equipment requires routine maintenance to ensure proper fluidization and atomization of the coating. Worn applicator nozzles, pumps and guns can clog and hamper flow, leading to higher material usage and finishing defects.

Experts from PPG coatings services business recommend daily checks, frequent replacement of worn equipment and reliance on OEM parts. While aftermarket parts often come at a discount, they also tend to be inferior and use more material, offsetting any initial cost benefits.

One of the most common causes of insufficient film build is poor grounding of the metal substrate. The first thing we recommend for troubleshooting is a full inspection of the application setup to ensure hooks are clean and all grounding wires are properly attached. Proper grounding is essential for efficient build-up of powder onto the part during the electrostatic spray process.

Measuring Ground

A megohmmeter with a power source of at least 500 volts is the proper device for checking part-ground resistance. A volt-ohm-meter with a low voltage power source, is accurate for checking electrical circuits in the megohm range, but does not provide sufficient electrical current to accurately check part-ground resistance.

When testing ground, one lead of the megohmmeter should be connected to a known ground source (earth ground), such as a system ground rod to earth, by means of a long test lead. Starting with the parts and working backward toward a known ground, the other lead is used to complete the circuit and establish ground resistance readings.

If part resistance is less than 1.0 megohm and less than the threshold resistance values established for optimum TE, readings can be noted without further testing. If readings are more than the threshold values, the technician will work backward from the part to the hanger, swivel, load bar, S-hook, and conveyor until satisfactory ground resistance readings are discovered. This approach isolates the contact points where ground is lost and facilitates proper problem identification and corrective action.

Safety

Ungrounded parts, system components, and work piece fixtures can accumulate a charge much like a capacitor. The worse the ground and the larger the part area, the greater the amount of energy the object can store. When such a charged object approaches a ground object, the charge buildup can arc over or spark, and the intense heat of the arc may be sufficient to ignite a fire or an explosion. If the grounded object is a person, the person can be injured.

The National Fire Protection Association (NFPA) states that ground resistance should not exceed 1.0 megohm (1 million ohms) on all parts, fixtures, and system components. The NFPA defines this upper threshold as sufficient to eliminate the potential for static electrical discharge, arcs, sparks in the powder coating system.

The corrosion protection performance of powder coatings is well known but what are some additional benefits of powder?  

Aside from better edge coverage and corrosion- and weather-resistance, powder has two notable advantages over liquid coatings. The first is that it is more sustainable since it can be reclaimed, reused and recycled and is formulated without VOCs added, making it safer for workers and the environment. 

The second is greater design capabilities. Textured finishes like hammertones, wrinkles, ripples and other natural looks like stone or terracotta are easier to achieve with powder than with liquid coatings.

Beyond storing powder in a tightly sealed container to lock out moisture and contaminants, there are steps that help to maximize the shelf life of powder and its performance.

Since high humidity can negatively influence powder’s fluidization and flow, the recommended temperature range for storing powder is 60°F (15°C) - 80°F (27°C) with 40%- 60% relative humidity. 

One of the advantages of powder coating technology is the ability to reclaim and reuse. While it’s optimal to recycle as much powder as possible from a sustainability and material cost standpoint, there are some circumstances that make it less feasible.

For many coaters, it comes down to a cost versus time savings equation. Color swaps can be time consuming since a full cleaning of guns, hoses, pumps is needed before a new color module is added. In some cases, coaters may reclaim only the highest volume of powder applied. 

Other times, the decision to reclaim powder depends on the limitations of the equipment. For instance, a cartridge booth with a single removable color module makes reclaim more difficult since the operator must separate the color module for each reclaimed color. A full cleaning is needed before being replaced by the next color module.

Metallic and bonded powders also make reclaim more difficult due to their color shifting properties and the need for a precise balance of virgin and reclaim powders.

But leading equipment manufacturers are making reclaiming more efficient with newer quick color change cyclone systems that make rapid color changes and reclaim more feasible, even for smaller volumes of powders.

Have questions about reclaim? PPG Coatings Services can help. Click HERE.