Grounding for powder coating
Electrostatics of aluminum powder coating
Powder coating
Powder coating is a method of applying electrically charged powder paint to a grounded metal product, such as an aluminum extrusion. The powder is electrostatically attracted to the product and settles in a thin layer on its surface. Then the product, along with the applied layer of powder, is placed in an oven, where this powder is “baked” into a strong and durable coating.
Charging powder
In most electrostatic painting systems, the powder particles are charged using a corona discharge (Figure 1). The suspended powder is supplied by compressed air to the outlet of the spray gun. Here is the so-called charging electrode, which is under high (up to 100 kilovolts) electrical voltage, usually negative. At the tip of this electrode, an area with a large number of negative free ions is created – a corona discharge or “corona”. Powder particles pass through this area, capture these free negative ions and thereby acquire a negative charge. Electric field forces and a flow of compressed air push these charged powder particles towards the grounded workpiece.
Figure 1 – Principle of electrostatic powder spraying using the “corona” method [1]
Deposition of powder onto the product
Most materials used for powder coatings are strong dielectrics. When such a charged powder particle approaches a metal surface, for example, an aluminum profile, it induces a charge in the metal of the same magnitude, but of the opposite polarity (Figure 2) [2].
Figure 2 – Mirror charge [2]
This happens because, under the influence of the negative charge of the powder particles, conduction electrons inside the metal product are repelled from its surface and go along the electrical grounding circuit to the ground. Near the surface of the product, an area with an excess positive charge is formed, equal in magnitude to the negative charge of the powder particles.
Mirror charge and grounding
This positive charge is called the “mirror charge”. These two charges of equal magnitude and opposite polarity are located opposite each other on either side of a metal surface, such as aluminum. They attract each other and hold the powder particle on the metal surface [2].
For the rapid formation of a mirror charge, free electrons must be quickly and freely “pushed” out of the product into the ground. This is why good grounding is so important for electrostatic powder coating.
What is good grounding?
Poor grounding problems
A significant proportion of powder coating quality problems that occur on a powder coating line are due to insufficient grounding of the product being painted or its complete absence. Such problems include, but are not limited to, the following [3]:
- Heterogeneity of coating from product to product, from suspension to suspension, from shift to shift
- Excessive paint consumption or fluctuations in paint consumption
- Excessive paint adhesion to equipment
- The need to constantly adjust the technological parameters of the dyeing line.
How does grounding affect quality?
When the product to be painted, which enters the electrostatic powder spraying chamber, has insufficient grounding, the following characteristic phenomena occur [3]:
- The product is not able to effectively attract charged powder particles, resulting in a too thin layer of paint.
- The product becomes a kind of capacitor that accumulates a negative charge. Therefore, the product begins to repel charged powder particles. The charge contained in the product may cause electrical discharge which, under certain conditions, may result in fire.
- The charged powder will search for nearby grounded objects and be attracted to them (the walls of the spray chamber, equipment and the floor).
The causes of insufficient grounding
The search for possible causes of poor grounding of painted products consists of answering the following questions [3]:
- Is the conveyor system properly grounded when passing through the electrostatic spray booth?
- Is there periodic monitoring of the grounding of products before they enter the electrostatic spraying chamber?
- Is conductive grease used to lubricate conveyor rollers? Are the conveyor rollers in contact with the guide rail as they pass through the electrostatic spray booth?
- How often are conveyor chain elements cleaned (built-in cleaning brushes, periodic cleaning, replacement)?
- Are the contact points of hangers and hooks protected from paint adhesion? What method is used to clean hanger contacts and hooks, as well as hangers in general?
Safe grounding
One of the important functions of grounding is to ensure safety, including fire safety. For example, the instructions of the American National Fire Protection Association (NFPA) state that the product to be painted, when connected to ground, must have an electrical resistance of no more than 1 MOhm. One “megaohm” equals a million “ohms”, which is a considerable amount of electrical resistance. This requirement is based on safety conditions in order to reliably ensure the absence of ignition sources for the atomized (atomized) powder [4].
Unpainted metal products, including aluminum profiles, clean hooks and hangers, and clean conveyor have low electrical resistance, so they are all good conductors. What spoils this ideal path of electrical charge from the part to ground is:
paint sticking to the contact points of the product, suspension and conveyor;
contamination of rollers, chains, hinges and conveyor joints.
All these contact points together must be capable of providing an electrical connection between the product and ground with a resistance of no more than 1 MOhm.
Grounding check
Megohmmeter for measuring grounding
The device that is used to measure the continuity of the electrical circuit to the product before grounding is an ohmmeter, which has a megaohm scale. This device can be a regular volt-ohmmeter or a megohmmeter (megger). To measure the electrical resistance of electrical circuits, a conventional volt-ohmmeter uses a low voltage power source (about 9 volts). This device is quite sufficient for testing a regular electrical circuit, but it is not suitable for testing the grounding of a powder coating system [4].
The megohmmeter was originally designed to test electric motor windings and wire insulation. This meter is better suited for ground monitoring of a powder coating system as its power source typically provides 250, 500 or 1000 volts. This higher voltage provides the necessary current required to measure circuit resistance before grounding in powder coating systems [4].
Before using this megohmmeter, you must carefully read the instructions for its use and strictly follow them in order to avoid electric shock, as well as to ensure that correct electrical resistance measurement results are obtained. To check the grounding of the entire powder coating system, two sufficiently long copper wires and two alligator clips are usually used.
How to check grounding
First and foremost, it is important to check the building’s grounding to ensure that you have a proper grounding circuit. To test the ground connection point, you need to connect one wire to the ground rod and the other wire to your ground connection point. This ground connection point could be, for example, the conveyor’s steel support structure or any metal device that is connected to ground.
To test ground resistance, connect one test lead to a proven building ground and the other to the product mounted on the powder coating system hanger (Figure 3a). From a safety perspective, this ground resistance reading should be no more than 1.0 MΩ. This is the ground resistance, which includes the entire circuit: the product, hangers, hinges and all conveyor components.
If the grounding resistance exceeds 1.0 MOhm, then you need to move the test wire from the product and connect it to the next element of this “electrical circuit” – the suspension contact (Figure 3b).
If the device reading is still above 1.0 MOhm, then you need to continue testing each subsequent contact point of this circuit – brackets, rollers, chains, guides, etc., until a positive ground resistance measurement is obtained (Figure 3c).
c
Figure 3 – Measurements of electrical grounding resistance:
a – on the product, b – on the suspension, c – on the conveyor
By following these steps in sequence, you can pinpoint where the ground is failing and which part of the powder coating system requires cleaning and maintenance.
Technological grounding
The most important process problem that occurs with poor grounding is uneven or insufficient powder coating thickness. In addition, if the grounding is poor, the paint layer on different pendant products will vary, and areas of the surface with Faraday cages will become even more difficult to coat with paint. In addition, profile edges may also have coating problems.
Another problem with poor grounding is the low efficiency of initial powder deposition onto the workpiece. Since the powder “does not want” to be attracted to the product at a sufficiently high speed, the amount of powder that could not settle on the product and returned for recovery will be significantly higher than in the case when good grounding is provided.
What grounding resistance is optimal from a technology point of view? Nordson’s instructions [5] establish requirements for grounding resistance for powder coating systems, taking into account not only safety conditions, but also manufacturability (Figure 4) [5, 6]:
- Device used: megohmmeter (megger) 500 V or 1000 V.
- Monitoring frequency: daily.
- Required readings:
– Ideal: 0 Ohm
– Acceptable: 200 to 300 ohms
– Corrective action required: ≥ 1.0 MΩ.
Figure 4 – Control measurement of grounding resistance
powder coating systems [5]
Thus:
- The optimal readings of grounding resistance from the point of view of technology efficiency are the range from 0 to 300 Ohms;
- The maximum permissible from a safety point of view is a ground resistance reading of 1.0 MOhm.
How to maintain reliable grounding?
- Unpainted metal products, such as aluminum profiles, are naturally conductive.
- The most common cause of poor grounding is dirty pendant contacts. The entire pendant can be painted, but the contact points (at both ends of the profile) must be clean enough to conduct electricity to provide grounding.
- Conveyor components through which the ground circuit passes also require regular cleaning. Dirty levers, rollers, chains, guides, etc. may cause insufficient grounding of the work being painted.
Sources:
- Application Variables for Powder Coating Systems / Ken Kreeger – Nordson Corporation – 1994
- Electrostatic Phenomena in Powder Coating / S. Guskov – Nordson Corporation – 2017
- Electrostatics: Better Understanding for Better Results /John Tomaro -Nordson Corporation – 2004
- Finding Solid Ground / N. Liberto – Powder Coating, September 2011
- Electrostatic System Installation, Checks, and Troubleshooting. Customer Product Manual – Nordson Corporation – 2003
- https://www.powdercoatguide.com/2012/12/7-grounding.html