Electrostatic Applications – Key Points to Consider
There are several key points that you need to be successful in the use of electrostatic spraying equipment.
Electrostatic equipment can be a very effective application device but only if you have selected the right atomization technology for your coatings and for the products you are coating. The following are other key points that should be considered.
Once you have selected the right atomization technology for your coating, your production, products and for your operators, you will need to ensure that your coating is properly formulated for electrostatic spraying. In general, the coating must be conductive enough to take a charge but not be too conductive which could result in the gun shorting out. In addition, the material should not dry too fast as these coatings do not hold the electrostatic charge as well as a slower drying coating.
The next thing that is critical to the success of using electrostatic technology is to ensure that what you are spraying can be positively grounded. This is absolutely necessary for the system to work and to prevent shocks and sparks that can bother your finisher or start a fire! Ground to the part must be well maintained and hooks that are used to hang parts must be routinely stripped clean, especially at the point where the hook makes contact with the part and with the conveyor.
In addition, anything in the spaying area must be grounded, especially any cans of paint or solvent. However, if you are spraying a water base material you will need to isolate the fluid supply system and the container of coating from ground to prevent the system from shorting out.
Some thought should also be given to part spacing. If you hang your parts too close to one another, this may prevent the charged paint from getting in between the parts or limit the potential for the coating to wrap around the back side of the part. This could lead to excessive material build up on the face of the part and insufficient material build on the sides or back of the part. Also, how you face your parts when hanging them on a conveyor can also be an issue. It is easier for an electrostatic gun to spray the back side of a piece of angle iron if you spray into the angle rather than trying it the other way around. The most difficult areas should face the operator and should be sprayed first.
Angles and recesses present a challenge for all spray guns but can be very pronounced when using electrostatic guns. For most guns that use a lot of atomizing air, such as air spray or HVLP, the air exiting the gun creates a dam or back flow of air which pushes back on the incoming paint particles. With electrostatic guns, a phenomenon known as the “Faraday Cage Effect” prevents the charged particles from penetrating into deep or tight recesses. Since we know that like charges repel from each other it is easy to see that once some charged particles get into the recess they repel additional charged particles from also entering the recess or angles.
In some cases it is necessary to turn the electrostatic gun off in order to get the coating into these recesses first, before turning the gun back on and spraying the rest of the part. In other cases you might want to use less atomizing air or use an atomization technology that uses little or no atomizing air. In other cases adjusting the fan pattern down so that it is narrow can also be helpful. Also, bringing the gun very close to the part will automatically reduce the voltage and this can help reduce the Faraday Cage Effect. Sometimes shaking the gun very slightly can also help disturb the electrostatic field just enough to allow some angles and recesses to be coated sufficiently.
Regardless of which type of electrostatic system you use, it is always better to use the least amount of pressure, either atomizing air, fluid pressure or both in order to maximize transfer efficiency. However, setting these pressures too low could result in poor atomization. You will have to set these pressures just to the point where an acceptable finish quality is obtained and high enough to meet your productivity requirements. If you cannot achieve acceptable atomization at low pressures, you may need to try a different type of electrostatic atomization technology.
However, be sure to contact your equipment supplier first, since they may be able to recommend a different nozzle, air cap or tip for your existing equipment that could significantly improve atomization while also improving the efficiency. For some materials an in-line paint heater can also help by reducing the coatings’ viscosity, which in turn, would allow you to use lower fluid and/or air pressure.