Powder Coating: Spraying Techniques and Gun Settings
After you have sourced a quality supply of powder you must consider the equipment you will be using to apply your powder to you part. Having the proper powder coating equipment and techniques is as important as quality powder. Many a sales representative, properly versed in application techniques and gun settings, have persuaded clients to switch powder coating supplier on account of being able to optimize the clients equipment to make their powder look fantastic. Understanding how to spray and how to use your equipment is obviously quite important to any powder coater.
In a very simple overview a powder coating system applies paint when pressurized air fluidizes a reservoir of powder paint in a hopper. The air travels from its source through a porous fluidizing plate in the floor of the hopper and fluidizes the powder essentially acting as a solvent. The fluidized powder now behaves in the same manner as a liquid paint so long as the air supply exists. From the hopper fluidized powder is pushed by a powder pump to the powder coating gun. The powder pump is an integral component of your powder coating system and most models work via a venturi effect. Pressurized air is forced through a small opening into the pump chamber. The pump chamber is located above the supply tube from the powder reservoir where an area of low pressure is created which then draws up powder from the hopper. As powder enters the pump chamber it is expelled from the powder pump towards the gun where it can then be charged and applied to your substrate. It is more common now for powder guns to employ pressurized air to not only create a vacuum over the supply tube but also to propel the powder to the gun which lessens the occurrence of sputtering of powder.
Powder Coating Charging: Corona-Charging vs Tribostatic Charging
The two most common types of electrostatic charging are corona-charging and tribostatic. Corona-charging is by far the most commonly used method today due to its high charging efficiency , durability of equipment and low maintenance cost. Quite simply corona-charging works through the use of an electrode in the tip of the powder coating gun. The electrode emits a cloud of electrons as a plasma which can then ionize molecules in the air which then impart charge to the powder particles. Tribostatic charging relies upon friction to charge the powder particles. Collisions between the powder and the chamber of the powder gun facilitate this charging much in the same way that walking across a carpet with wool socks in the winter builds up static charge on a person, and for this reason it is of extreme importance to have an efficient ground on any powder gun used for tribo applications. Although all powders can be charged using tribo guns the effectiveness depends in large part upon the chemistry of the powder. Epoxy and polyurethane powders have better tribo charging than polyesters due in large part to their ability to donate electrons (good tribo charging) leading to a positively charged powder. However if your shop is using tribostatic applicators it is a good idea to speak with your powder supplier. Many powder coating formulators will use additives that increase a powder’s tribostatic charging ability and will result in significantly better application especially for polyester and other poor tribo chemistries.
Which Powder Coating Gun To Choose
Selecting a proper electrostatic powder coating gun depends upon a lot of factors. Cost is probably the largest and first considered by many buyers. The professional powder coater that is working on a line or paint shop would be better served by investing in a top of the line Gema or Nordson brand powder gun. The weekend warrior who only powder coats the odd pieces here and there or for setting up your own tire rim powder coating system would be better served with a more economical discount brand powder coating gun like the Craftsman Complete Powder Coating System. Although my preference is strongly in favor of Gema brand powder guns, I appreciate Nordson brand’s ability to put a lot of powder onto a piece with one pass and its intelligent auto-sensing features. The Craftsman gun although cheap is a remarkably able and handy system that has sprayed a variety of powders almost as effectively as the more professional grade brands.
Powder Coating Gun Settings and Optimization
Depending upon the powder coating gun brand your model should allow for adjustment of voltage (kV), current (µA) and air pressure. More advanced models will allow for further manipulation of parameters such as flow air and atomizing air but kV and µA play the largest effect.
Voltage in kV (1 kilovolt = 1000 volts) is the amount of electrostatic charge that a powder coating gun is capable of producing- the electrical potential or energy per unit charge. A powder coating gun produces an electric charge at an electrode that the powder must pass by as it leaves the nozzle of the gun. Increased kV results in increased charge to the powder particles. Common values of kV are in the 50 to 70 kV range for the first coat and higher kV values should be used for flat and uncomplicated work pieces, whereas complicated pieces with Faraday cages should be sprayed at lower kV values. Always spray Faraday cage areas first and “wrapping” can also be an effective technique for increased powder penetration in these tough to reach areas. Metallic powders should also be sprayed at reduced kV. Additional coats should be sprayed with reduced kV values. Reducing the kV values for subsequent coats means that paint particles from the gun will experience less electrostatic repulsion from the already present and charged film build on your substrate reducing the troubles of “back ionization”.
Current in µA (1 microamp = 0.000001 amps) is the amount of charge that is actually moving from your powder gun to the cloud of powder as it exits the gun. Think of voltage as horsepower and current as the speed your powder is travelling. Current is a very valuable indicator of how much voltage is being used to charge your powder. It is always a good idea to spray at a distance of 6-8 inches from the substrate but as the gun gets closer to the substrate the current load will increase. Most powder coating guns are intelligent and able to reduce kV as the current reaches the limit specified by the user.
Air pressure supply for your gun is highly dependent upon the model you have but typical values are in the 5 to 15 psi range. Keep in mind that if your air supply is 15 psi but connected via 20 feet of hose you will experience a significant decrease in the actual pressure at the gun. This attenuation of air pressure exists for all systems and is a good reason that your system should have an inline pressure gauge as near to your gun as possible. Only clean and dry air free of contaminants such as oil should be used for powder coating applications. It is highly recommended that a dessicating trap with filter be used between your air supply and powder gun to prevent contamination and potentially expensive repairs to you powder coating system.
Do Not Forget To Ground Your Workpiece
This simple yet very important aspect of powder coating is often neglected or over looked. Powder coatings rely on electrostatic potential to achieve adhesion to the uncured substrate. The charged powder particles like small magnets are attracted to the grounded workpiece. No ground, no powder can stick. Transfer efficiencies fall drastically with poor or compromised grounding. This is commonly seen in powder shops where multiple pieces are sprayed on the same hanger without sufficient cleaning between pieces- as the powder build-up on the hanger accumulates increased insulation happens leading to poor grounding. It is common to see even professional powder coating shops suffer from reduced transfer efficiency due to an inconsistent cleaning schedule of their hanger and hook system.
Many weekend powder coaters get by grounding their booth hangers to water pipes but a more effective and professional method is to install a dedicated ground rod or plate made of copper into the ground as close to the spray booth as possible. Contact a local electrician for details and advice regarding any electrical work although installation and can be accomplished quite cheaply with some hard work and know how.