Types of powder coatings systems
Powder coating is praised by industrial, commercial, and consumer customers alike for its durability and aesthetic appeal. It’s attractive across many production settings because it’s an easily controllable process carried out in shop settings.
What’s more, powder coatings are environmentally friendly because they’re virtually 100% solids and contain no VOC-laden solvents.
But production managers have choices to make when choosing powder coating systems. What follows are the stages of powder coating, common powder coating methods, and a comparison of automated vs. batch setups.
The powder coating process
Powder coating occurs in three stages: Pretreatment, coating, and curing:
- In pre-treatment, parts are cleaned and undergo surface preparation as recommended by the powder manufacturer. SSPC-SP 5 and SSPC-SP 10 are the most commonly recommended surface prep specs.
- Powder coatings are applied during the coating stage. (Further discussed below.)
- Curing is the heating stage that seals powder coatings into place.
(While powder formulation and coating methods are discussed here, a more detailed examination is available in Painting Manual Vol.1: Good Painting Practice.
Powder formulation and common powder coating methods
To produce powder coatings, polymer granules are mixed with hardening agents, pigments and other ingredients and then heated so they blend together. After that, the mixture is extruded and rolled flat. This sheet-like material is broken up into chips and further milled and sieved into a fine powder. There are two types of powder coating formulations:
- Thermoset formulations include cross-linked polymers that fuse together when a part is cured. This chemical fusion is irreversible; thermosetting coatings cannot melt. These coatings typically provide better impact, wear, and corrosion resistance.
- Thermoplastic formulations do not undergo chemical changes during curing, so they can be softened or melted if desired after curing. Thermoplastic coatings are more easily blended than thermosetting formulations and are ideal for low-stress applications.
Electrostatic application is the industry norm but flocking and dipping are the most common alternatives and are usually chosen in scenarios where electrostatic isn’t as effective.
Electrostatic coating
In this method, powder particles are sprayed onto metallic parts attached to a grounding wire. In addition to powder, charged particles also issue from the application gun. These particles cause the powder to stick to the surface of the grounded part. Note that parts with complex geometries or small recesses can make electrostatic coating difficult. That’s due to the Faraday effect, caused by a buildup of free ions in confined spaces that repels powder instead of attracting it. Lowering the voltage on the application gun can overcome the Faraday effect.
Electrostatic coating is the most common powder coating method and is preferred for the widest range of parts. The two other methods mentioned below are used in special circumstances.
Flocking
It’s also known as hot coating, in which parts are heated above the powder’s gel-out temperature. The powder is then applied as before, but without the accompaniment of an electric charge. The powder simply gels to the part surface on contact. Without the electrical charge, the risk of the Faraday effect is eliminated.
Flocking is an alternative coating method for complicated parts where free ions gather and repel powder. The method is chosen when lowering the voltage on an electrostatic application gun still is not enough to overcome the Faraday effect.
Dipping
In this method, parts are heated to above the powder gel-out temperature and then submerged in a fluidized bed of powder particles. As with flocking, the powder particles gel to the surface of the heated part on contact. Then the part is lifted out of the powder bed and excess powder is pneumatically driven off.
Dipping is a preferred method for specialty parts like valves or other items that need to be coated inside and out. It’s much easier to achieve a uniform coating for specialty parts via dipping.
Curing
After parts are coating, they must be cured. This involves heating parts (or reheating after flocking or dipping) above the coating’s gel-out temperature. This causes the coating to transition from a dry, coarse powder to a liquid. Drying the part after curing seals the coating into place.
Batch vs. automated powder coating systems
Another choice managers must make is whether to implement batch or automated powder coating systems. In some scenarios, both may be needed.
Batch powder coating systems
For those who coat a wide variety of parts or deal with very complex parts, batch powder coating systems are the best fit. These setups are less specific than their automated counterparts by design—if daily projects range from coating watering cans to bicycle frames to desk lamps, consider a system that can easily accommodate such a variety.
The tradeoff when choosing a batch powder coating scheme is that the lack of repeatable automated processes means projects take longer. And while batch systems are great for coating a variety of parts, they may hinder the ability to coat high volumes of parts on tight deadlines.
Automated powder coating systems
On the other hand, automated powder coating systems are great for operations that feature high part volumes, tight deadlines, and require consistency across production runs. (It’s the difference between coating a handful each of watering cans, bike frames and desk lamps vs. coating a couple hundred bike frames a day.)
An added benefit of automated powder coating systems is that they increase output without a corresponding increase in labor cost.
The drawback with automated coating systems is that they are much harder to customize. They’re great at coating and curing what they’re designed to coat and cure, but they cannot accommodate wide varieties of parts unless they’re engineered to be flexibly automated. Flexible automation is gaining traction across manufacturing and finishing operations, but managers must weigh the cost of designing and building those systems against the more immediate ease of using batch powder coating.
Which powder coating system is better?
Neither batch nor automated is better than the other. It’s about choosing the system that best the customers’ needs and suits the parts that need to be coated. Some operations choose to specialize in jobs well suited for one system or the other; some implement a mixture of both types so they can stay flexible.