THE CHALLENGE
Why Printing Requires Corona Treatment
Most polymer films—PE, PP, PET, BOPP—have low surface energy straight off the roll. Untreated polyethylene sits at just 31 dynes/cm. Untreated polypropylene at 29. Meanwhile, water-based flexo inks have surface tensions between 28 and 45 dynes/cm, and at press speeds, the dynamic surface tension climbs even higher.
For proper wetting, the surface energy of your substrate must exceed the surface tension of your ink by 8-10 dynes/cm. Without corona treatment, most films fall well below this threshold. And because dynamic surface tension at press speed can be 8-10 dynes/cm higher than the lab value, the gap is even wider than most printers realize.
The result: ink beads instead of spreading, adhesion fails under tape test and print defects compound through the rest of your converting process. Higher treatment levels also improve ink flow-out and drying speed. Research has shown press speed can increase 50-200 fpm from proper in-line corona treatment alone.
Common Substrate Surface Energy — Untreated (dynes/cm at 25°C)
Substrate
Untreated Surface Energy
Polyethylene
31
Polypropylene
29
Polyester (PET)
43
Metallized Paper
32-50+
Target treatment level depends on your ink. The substrate surface energy must exceed the dynamic surface tension of your ink by 8-10 dynes/cm. QC Electronics sizes every system to your specific ink chemistry, substrate, and line speed.