The Shortest Path to the Ground
Posted on Wednesday, September 22, 2021
By John Cole
All powder coaters will eventually experience coating issues such as difficulty penetrating Faraday cage areas; back ionization; light, heavy, or uneven film builds; hook point “halos;” specialty powder failures; etc. It’s not a matter of “if” but more “when” these will creep up.
Troubleshooting the root cause of defects can be cumbersome, but based on my experience, the first place I look is the earth grounding system for the coating operation. Each of the previously listed can have their roots in other areas, but the common thread through most coating defects includes a poor ground.
One of the most fundamentally misunderstood concepts in powder coating is the need for a high-quality ground. Every plant I visit, except for very few well-managed and maintained shops, has poor ground. This is not to say they do not have their booths grounded. Almost 100% of them do this properly. Where powder coating operations fall short is providing a ground to the parts being coated. Rarely, if ever, do I see a well-grounded part being presented for powder coating. This is unfortunate because the single most effective way to guarantee transfer efficiency optimization and high- quality finishes is to have a high-quality, low resistance, path to earth ground. The concept, and science, are easy to understand, but apparently difficult to follow through on. Proper grounding is important for both safety and process efficiency.
A high-quality ground is essential to “complete” the electrical circuit necessary for current to flow from the high voltage electrode tip on your powder gun, which creates the corona charge field through which powder flows and particles pick up their ions. This is a result of Ohm's Law, which states that for any given voltage (V) and resistance (R), there is a resultant current (I): V=I x R
Having a non-existent or weak ground significantly degrades the efficiency of this relationship between voltage, resistance, and current, and leads to deficient elimination of the ions deposited on parts. This, in turn, leads to an overabundance of ions on the part surface, creating back ionization (orange peel).
Anyone who has taken a powder coating course from the Powder Coating Institute has had it drilled into them that the leading contributor to poor Faraday cage penetration
is back ionization, or the presence of excess free ions in the corona charge field. The existence of a high-quality ground path goes a long way towards minimizing these issues. Does it eliminate them? No, but it certainly helps. Proper management of gun current and operator training are also contributors.
One telltale sign of a bad ground is when there is more powder on your painter than on the parts being coated. Almost 100% of the time if the painter’s hand is the same color as the part being coated, he is presenting himself as a better ground than the part. If you see this, improve your part ground and “don’t pass go.”
Other telltale signs of insufficient grounding include:
- Halos or hook scars where the part is connected to the hanger.
- Excessive powder buildups on the booth floor and walls.
- Hearing a “snapping” sound when coating a part.
- Wide variations in film thickness over a part’s surface.
- The need to “firehose” powder to achieve minimum film thicknesses.
Several years ago, an engineering study was conducted by Kolene Corp. on the effect of bad ground on coating film thickness uniformity. This study indicated that, as part
ground is degraded, so is film-build reliability. A digital version of the Kolene study can be downloaded here (PDF - 10MB).
So, what is a good ground and how do we measure it? What defines a good grounding system?
A good ground in the powder coating world is defined in two ways. Viewed from a safety standpoint, good ground is defined as a resistance to ground of less than one megohm (1,000,000 ohms). This is well documented in the NFPA 33 standard, and many other codes referenced by Authorities Having Jurisdiction in your area. From a quality point of view, opinions range from zero ohms to one megohm. In my opinion, when speaking to quality, the target should the lowest resistance possible, zero ohms.
The best method for measuring ground is with the use of a megohmmeter. Every powder coater should have one right alongside their film thickness tester. These operate differently than a standard volt ohmmeter (VOM) or any other low-voltage continuity tester, the difference being that low-voltage devices drive the test at such low voltages that they cannot give an appropriate reading. In other words, false high resistance readings will be more common with VOMs than readings made with a megohmmeter that tests at 500 to 1,000 volts.
It is important to understand that whether you are using a VOM, commercially available ground testers, or a megohmmeter, these devices only test continuity. That is to say, they measure the resistance in a string of electrical connections. In the powder coating world that string includes the path from the part to be coated, the hook it is hanging on, the support device the hook is connected to, and the electrical connection from the support device to the ground rod. It is important to note, this is not measuring ground. It is measuring continuity between the part to be coated and a rod in the ground. Whether or not the ground rod is providing a quality ground depends on many factors.
Moist soil provides the best “ground,” while dry soil, sandy soil, or rocky soil do not provide a quality ground.
In a micro-sense, understanding your soil conditions is important. In many places you may have to install two or more ground rods and connect them in a series to get a quality ground. Since sinking a ground rod is relatively inexpensive it may be better to sink a second ground rod about 18 to 24 inches from the first one and check resistivity between the two. Low resistance indicates good grounding soil. High resistance indicates poor ground. In any case, if you sink the second ground rod, connect them together and then to your ground wire.
So, what do we do when we see, or hear, symptoms of a bad grounding system such as the dreaded “snapping” noise when spraying, or back ionization and Faraday cage issues? The first place to start looking is your hooks. If there isn’t a “clean and clear” metal-to-metal contact point from the part to the hook, to the conveyor/rack, then replacing dirty hooks with new, cleaned, or stripped hooks immediately will most likely do the trick. But I wouldn’t stop there. Follow the path from the hook to either the conveyor or rack and the point where ground is connected and make sure each connection point is metal-to-metal. And finally, follow the ground wire back to the point where it is connected to the earth ground rod and make sure the connection is continuous. If you’ve done all this and still have concerns, add a second ground rod approximately 18 inches away from the first one and measure resistance (with a megohmmeter) between the two. Low resistance indicates a good ground and high resistance means your soil is not effectively producing a good ground. Most likely you will not ever get to this point; most grounding issues are a result of lack of ground path from part to hook to conveyor or rack.
There are many reliable sources for information on powder coating ground including PCI’s Powder Coating: The Complete Finisher’s Handbook 5th Edition and PCI’s webinar series database.
Ground paths require regular inspection and testing. Without this, there is no assurance that you have a reliable ground. Wires get cut, connections come loose and, unfortunately, copper gets stolen. All lead to losing ground path.
John Cole is president of Parker Ionics and Parker Engineering.