Corona, Flame or Plasma? Pre-treatment Choices for Painted Panels

If paint keeps peeling off plastic panels, the coating system usually gets the blame first — the chemical supplier reformulates, the line retunes, and the problem comes back. More often than not, the root cause sits upstream: the surface energy of the substrate was never raised (or kept) high enough for the coating to wet and anchor. The question then becomes which surface treatment to invest in: corona, flame, or plasma.

What each process actually does

All three methods oxidize the outermost molecular layer of the substrate, raising surface energy to the 38–42 dyne/cm range that most coating systems need. They differ in geometry, throughput, and aging behavior:

| Process | Best for | Watch out for | |---|---|---| | Corona | Flat panels, webs, continuous sheets; best cost/performance | Line-of-sight only; treated level decays over time; uniformity depends on electrode design | | Flame | 3D parts (moldings, bottles, dashboards); large areas fast | Open flame near sensitive materials; heat distortion on thin parts; gas handling | | Plasma | Complex geometry, heat-sensitive plastics, cleaning + activation in one step | Highest equipment cost; slower for large flat areas |

Flame-treated surfaces are known for slower dyne decay than corona — relevant when panels sit between treatment and painting.

Start with the substrate, not the brochure

• Plastic panels and moldings (PP, ABS, TPO…) are the classic candidates — their natural surface energy (PP around 30 dyne) is too low for most coatings. Corona handles flat panels economically; flame and plasma cover geometry corona can't reach.
• Bare metal panels are a different story. Metal is conductive, so conventional corona (which relies on a dielectric surface) is not the standard route — metal pre-treatment traditionally means cleaning plus conversion coating or blasting. Any supplier recommending corona for bare metal should be asked to explain the electrical setup, and the honest ones will tell you when corona is simply the wrong tool. (A meaningful share of "treatment didn't work" cases are substrate-routing mistakes, not equipment failures.)

Four questions that settle the choice

1. Geometry — flat and wide favors corona; 3D or complex favors flame/plasma.
2. Throughput — panels per hour dictates conveyor speed, which drives required power.
3. Time between treatment and coating — long storage means decay; plan re-treatment or inline activation.
4. Coating system — powder vs liquid, and the target surface energy it was formulated for.

Test before you commit

The cheapest way to choose correctly is a coupon test: send actual workpiece samples, have them treated at several parameters, and run cross-hatch adhesion checks on each. One afternoon of testing beats a mis-specified machine.