Uncoated metal parts, clean hangers and clean conveyors have little resistance to ground and are excellent conductors of electricity. What causes this desirable ground path to drop is the accumulation of powder at the contact points of the hangers or conveyor belts. Conveyor belts may also have rotating or indexing components that could prevent grounding if contaminated. The total resistance of all these to ground must not exceed 1 megohm. Ground continuity is usually measured with a Megger (ohmmeter with a megohm scale) and a 500 volt or higher power source.
In fact, all powder coatings are not hazardous waste as defined by the Resource Conservation and Restoration Act (RCRA). In most states, waste toner is disposed of in the same way as non-hazardous waste. However, there may be some exceptions and your powder supplier should be contacted for proper disposal. Always renew disposal requirements with local authorities to manage your facility location.
The powder is usually stored for several months. Desirably, the powder should be stored in very good conditions below 80°F and about 50-60% relative humidity. Under these conditions, most powders should last at least a year. Avoid placing powder stocks near any heat sources such as ovens, washing machines, furnaces, space heaters, etc. Powder packaging is designed to keep the contents from being compressed so that there are no lumps that can break easily. Do not stack damaged or folded packages.
Some powders with special appearance, performance or curing properties may be more sensitive to storage conditions and should be stored with more care to ensure satisfactory results.
For powder coatings to cure properly, two conditions need to be met. The first is temperature, called metal temperature, and the second is time. Time/temperature requirements for specific powder materials need to be met for full cure. Cured thermoset powder coatings will not remelt on further heating. To ensure proper time and temperature, it is usually better to test with a Datapaq or similar device to provide actual performance figures.
Particle size plays an important role in the application process. The size distribution affects many other properties that cannot be overstated as important to maintaining consistent performance, these include: fluidization, powder transport in the hose, uniform cloud density, powder movement in the air, electrostatic charge, Deposition and accumulation rates, transfer efficiency, Faraday cage penetration, encapsulation and film smoothness. The particle size distribution produced by the powder manufacturer needs to take into account the design of the coating system and the expected variation in order to provide a suitable product. The final cured surface appearance can also be affected by particle size distribution and flow rate.
One of the important considerations in defining end-use performance is weatherability. Outdoor exposure absorbs UV energy. This energy can attack organic binders and cause loss of gloss and color changes. Due to the tendency to chalk chalk, epoxies and epoxy-containing mixes are generally not recommended for exterior use when aesthetics are a major concern. On the other hand, polyester and acrylic resins offer great UV stability and are commonly used in the construction, automotive, lawn and garden, and outdoor furniture markets.
Adhesion is largely dependent on cleanliness of the substrate, proper selection and maintenance of pre-treatments and proper melting and curing of the powder coating. Adhesion can be tested by several different methods. Commonly used techniques are cross-hatching and taping. In this method, cross cuts are formed through the coating. Tape is then applied to the cut coating and quickly removed. The adhesion of the coating was then measured by its ability to remain on the substrate and not be removed by the tape.
Powder coatings vary in their ability to provide intercoat adhesion when recoated. Epoxy powders tend to be harder and more prone to intercoat adhesion issues, especially if overbaked. Polyester and epoxy-polyester hybrids, on the other hand, generally recoat well. Recoat chemistry is also important and should be tested with the powder supplier.
Coating selection should be a joint effort between the end user and the powder formulator (supplier). Formulators need to consider many factors when developing a coating for a specific application. For example, if the coating requires high pencil hardness and mar and weather resistance, then flexibility may be compromised. If excellent chemical resistance is required, then weatherability and bake-over yellowing may be compromised. Other considerations may be cure time and temperature, substrate type and treatment, and method of application. Only when all requirements of a particular application are considered can the formulator recommend the appropriate coating type.
Oven cycle times include prep time and dwell time for proper powder curing. Lift-off time is the time required to reach the desired substrate cure temperature for the part. Dwell time is the time required to maintain the substrate at the curing temperature. These times and temperatures can be obtained from the cure schedule for powder coatings. In some applications, oven cycle times can be shortened by rapidly heating the substrate to a higher cure temperature and shortening the dwell time.
In addition to being properly grounded, it is important to clean the compressed air. Oil and water will not mix with the powder. In powder systems, air plays an important role. The powder is fluidized by the air, pumped by the air to the Spray Gun, and the filter elements are recoiled by the air. Air quality should be consulted with equipment and powder manufacturers for best performance. Typically, a minimum of 35°F dew point and less than 0.1 ppm of contaminants or oil is a good starting point.
You should have good oil and water eliminators, and a refrigerant or desiccant drying system. Regardless of the system type chosen, an oil coalescing filter and particulate filter with automatic drain are recommended.
Drain the extractor daily and check the internal filter monthly. Replace the desiccant beads, if indicated, and have your refrigerant dryer serviced annually. Depending on size and volume, automatic indicators can be used to ensure correct performance. Again, always consult the manufacturer's manual.
The dryer should be placed as close to the room as possible, not in the powder room. This will give the compressed air little chance of moisture forming in the pipes as it travels to the booth.
If the room is too cold, condensation can form in the supply lines and vents and drip onto parts or the spray booth.
This is the air supplied at the nozzle to distort the pattern and close the donut hole pattern common with diffusers or round nozzles. This allows for better penetration of the powder directly into the corners. The extra air will increase the pattern size, potentially reducing the need to change tips.
The Spray Gun should be cleaned after each color change. At the end of the day, you should purge the gun, pump, and hose to prevent moisture from settling in the powder and clogging things up for the next use. Depending on the amount of powder sprayed and the time between color changes, deep cleaning and worn parts replacement should be done weekly.
It is recommended that you replace the powder hose at least once a year, depending on wear caused by the powder or if there are contamination issues. Check frequently for cracks or impact fusion, which can shorten hose life.
Drill pipe is driven vertically into the ground, and grounding straps are connected directly to the work cell. Check with your local electrical contractor for a standard ground rod, usually copper.
Prepainted or repainted parts may already be insulated with a 1-2 mil coat. The powder begins to self-repel about 4 to 5 mils. Reduce gun voltage (Kv) by at least half to prevent repulsion or back-ionization of fresh powder material. Also check with powder supplier to ensure proper intercoat adhesion.