Electrostatic powder coating
Powder coating is a method of applying an electrically charged powder paint to a grounded metal product, for example, aluminum profile. The powder is electrostatically attracted to the product and is deposited thinly on the surface. Then the product with a layer of powder placed in an oven, wherein the powder "baked" in a strong and durable coating.
Charging of powder
In most electrostatic painting systems, powder particles are charged using a corona discharge (Figure 1). The weighed powder is fed with compressed air to exit from the sputter gun. There is a so-called charging electrode, which is under high (up 100 kilovolt) electrical voltage, usually negative. On the edge of this electrode creates a region with a lot of negative free ions - corona discharge or "corona". The powder particles pass through this area, capture these free negative ions and is obtained due to this negative charge. Electric field strength and the flow of compressed air pushing these charged powder particles toward the grounded article.
The deposition of powder on the product
Most of the materials, used for powder coatings, are strong insulators. When such a charged particle powder suitable for metal surface, for example, aluminum profile, it induces a charge in the metal of the same size, but opposite polarity (figure 2) .
This is because, that under the effect of the negative charge of the powder particles of the conduction electrons in the metal product are repelled from the surface and go on the ground in the electric circuit ground. Near the surface of the article, a region with an excess positive charge, equal in magnitude to the negative charge of the powder particles.
The mirror charge and grounding
This positive charge is called "mirror charge". These two charges of equal magnitude and opposite polarity are arranged opposite one another on both sides of the metal, for example, aluminum, surface. They attract each other and hold a particle of powder on a metal surface .
For the rapid formation of a mirror charge must quickly free electrons and free “pushed” of the product in the ground. That is why good ground so important for electrostatic powder coating.
What is good grounding?
The problem of poor ground
A large proportion of the problems, which appear on line powder coating, It occurs due to inadequate grounding of painted products, or complete lack thereof. These issues include, including, the following :
- The heterogeneity of the coating from the product to the product, from the suspension to the suspension, from shift to shift
- Excessive consumption of paint or swing it flow
- Excessive build-up of paint equipment
- The need for constant adjustment of the technological parameters staining line.
As poor grounding affects quality
When stained with the product, which is included in the electrostatic powder spraying chamber, It has insufficient grounding, then the following characteristic phenomena occur :
- The product is not capable of effectively attract the charged powder particles, whereby the ink layer is too thin.
- The product becomes a kind of capacitor, which accumulates negative charge. Therefore, the product begins to repel the charged particles of the powder. Charge, which is contained in the product, It can cause electrical discharge, which could result in a fire under certain conditions.
- The charged powder will search for the nearest grounded objects and be attracted to them (the walls of the sputtering chamber, equipment and floor).
Reasons for lack of grounding
The search for possible causes of poor grounding of painted products consists in answering the following questions :
- Is the conveyor system reliably grounded when passing through the electrostatic spray booth?
- Are products periodically grounded prior to entering the electrostatic spraying booth?
- Is a conductive lubricant used to lubricate the conveyor rollers? Are the conveyor rollers in contact with the guide rail when passing through the electrostatic spray booth?
- How often are the conveyor chain elements cleaned (built-in cleaning brushes, periodic cleaning, replacement)?
- Are the contact points of the hangers and hooks protected from paint adhesion? What method is used to clean the contacts of the hangers and hooks, as well as suspensions in general?
One important function is to ensure safety grounding, including, fire. So, for example, The American National Fire Fighting Association (NFPA) instruction sets, coloring product that when connected to the ground should have an electrical resistance of no more than 1 MOhm. One "MEGAOhm" is equal to one million "ohm", which is a considerable amount of electrical resistance. This requirement is based on the security conditions, to reliably ensure that there are no sources of ignition for the atomized (atomized) powder .
Unpainted metal products, including, aluminum profiles, net hooks and suspension, net conveyor have a low electrical resistance, so they are good conductors. the, what spoils this ideal path of electrical charge from the workpiece to ground is:
- ink adhering to the contact point of the product, suspension and conveyor;
- rollers pollution, chains, joints and connections conveyor.
All of these contact points together and must be able to provide an electrical connection to the ground product with no more resistance 1 MOhm.
Control of grounding
Megger to measure the ground
instrument, which is used to measure electrical continuity to ground products before, It is ohmmeter, which has a scale megger. This device can be an ordinary 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 volt). This device is enough to test the conventional electrical circuit, however, it is not suitable for checking the grounding of a powder coating system .
Megger was originally designed to test the motor windings and the insulation of the wires. This device is best suited for the control of the ground powder coating system, as its power supply voltage usually gives 250, 500 or 1000 volt. This higher voltage provides the necessary current strength, which is required to measure the resistance of the circuit to ground in powder coating systems .
Before, how to apply the megohmmeter, you must carefully read the operating instructions and strictly follow it, to avoid electric shock, and to ensure correct receipt of the results of measurement of electrical resistance. To check the grounding of the entire powder coating system is typically used two sufficiently long copper wire and two clips of type "crocodile".
How to check the grounding
First of all, it is important to check the grounding of the building, To make sure, that you have a proper ground circuit. To test the connection point to the ground, you need to connect one wire to the ground rod, and the other wire to your connection point to the ground. This point connection to earth ground may be, for example, steel conveyor support structure or any metal device, which is connected to ground.
To test the resistance of the ground is connected a test lead to the audited Building Ground, and the other - to the product, mounted on the suspension of the powder coating system (Figure 3a). From a security standpoint it is an indication of grounding resistance should be less than 1,0 MOhm. This earth resistance, which includes the entire chain: product, suspension, hinges and all the components of the conveyor.
If the grounding resistance exceeds 1,0 MOhm, the need to transfer test is carried out on the product and attach it to the next element of this "circuit" – suspension contact (Figure 3b).
If the instrument reading is still above 1,0 MOhm, the need to continue to test each following point of contact of the chain - the brackets, rollers, chain, guides, etc.. P., until a positive grounding resistance measurement is obtained (Figure 3c).
Consistently performing these actions, can accurately determine the, where the ground disappears, and what part of the powder coating system requires cleaning and maintenance.
The most important technological problem, which occurs when a bad ground connection - is uneven or insufficient the thickness of the powder coating. Moreover, with poor grounding layer of paint on various product suspension will vary, and surface portions with the Faraday cell becomes more difficult to coat paint. Moreover, edge profiles can also be coated problems.
Another problem is poor grounding the low efficiency of the primary deposition of powder on the product. Since the powder "does not want" to be attracted to a product with a sufficiently high velocity, the amount of powder, who was unable to settle on the product and returned for recovery, will be significantly higher, than in the case of, when it provided a good grounding.
What is the optimal grounding resistance in terms of technology? Nordson Instruction  specifies grounding resistance requirements for powder coating systems taking into account more than just safety, but also manufacturability (figure 4) [5, 6]:
- Applied device: megohmmeter (megometer) on 500 V or 1000 In.
- Control frequency: daily.
- Required indications:
– Ideally: 0 Ohm
– Acceptable: from 200 to 300 Ohm
– Corrective action needed: ≥ 1,0 MOhm.
- optimal indications of resistance to earth in terms of efficiency technology is the interval from 0 to 300 Ohm;
- maximum permissible from the point of view of safety is an indication of resistance to earth 1,0 MOhm.
How to Maintain Reliable Grounding?
- Unpainted metal products, for example, aluminum profiles, They are naturally conductive.
- The most common cause of bad grounding contacts are contaminated with suspensions. The entire suspension as a whole can be painted, but the contact points (at both ends of the profile) must be reasonably clean, to conduct electricity to provide ground.
- conveyor components, through which the grounding circuit, also requires regular cleaning. contaminated levers, rollers, chain, guides, etc.. P. may cause insufficient ground painted products.
- 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
See. See also Quality control of powder coatings: indicators, methods, standards - Guide