How Does
Powder Coating
Stick?
From corona discharge to mirror charge, Faraday cage effect to chemical bonding in the oven — discover the physics behind powder coating, step by step.
Static electricity carries the coating
Rub a balloon on your hair and hold it near a wall — it sticks. Powder coating works on exactly the same physics, just far more powerful and precisely controlled.
The powder gun gives plastic coating particles a strong negative electric charge. The grounded metal surface (at zero potential) then attracts those charged particles like a magnet. The only force holding particles in place before the oven is this electrostatic attraction.
How the gun tip generates billions of ions
When a high negative voltage (−30 kV to −100 kV) is applied to the gun tip, the electric field becomes extremely strong and non-uniform. This triggers a chain ionisation process called corona discharge.
Free electrons naturally present in the air accelerate in this strong field and collide with air molecules. Each collision splits one molecule into 2 free electrons + 1 positive ion. Within milliseconds, the space between the gun and part fills with billions of negative ions.
The higher the ion density, the faster and more fully powder particles charge — however, excessive ions can cause back-ionisation and reduce transfer efficiency.
How powder particles acquire charge
Powder particles sprayed from the gun encounter this ion cloud. Free negative ions follow electric field lines and attach to particle surfaces. The particle keeps capturing ions until it reaches maximum charge capacity.
Maximum charge depends on particle size, the dielectric constant of the material, and field strength. Once saturated, the particle's own field repels new ions — charging stops automatically.
How particles adhere to the metal surface
As a negatively charged powder particle approaches the grounded metal surface, free electrons in the metal flee that region. The remaining area becomes positively charged — this is called the "mirror charge."
The negative particle and its positive mirror image attract each other, bonding the particle to the metal. No bounce, no slide. This electrostatic force holds particles in place until the oven — and allows adhesion from every angle, even underneath.
The mirror charge effect explains why powder coating adheres to the underside of surfaces, unlike liquid paint which relies on gravity.
Why corners and recesses are challenging
Electric field lines follow the path of least resistance. Sharp edges and corners concentrate field lines; recesses and channels receive no field at all — creating a "Faraday cage."
The result: powder builds up rapidly on edges while channel interiors remain uncoated. Modern systems use automatic current control (ACC) to dynamically adjust ion density based on gun-to-part distance, correcting this imbalance.
Chemical bonding at 180–200 °C
Particles held electrostatically on the metal enter the oven. At 160–200 °C the polymer resin melts, flows across the surface, and forms cross-linked chemical bonds.
Upon cooling, permanent chemical bonding replaces the electrostatic adhesion. The resulting film is highly resistant to scratching, corrosion, chemicals, and UV radiation.
Source: Guskov, S. — "Electrostatic Phenomena in Powder Coating", Nordson Corporation.