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Flexography, or flexo printing, is one of the most widespread and technologically advanced direct rotary printing processes in the flexible packaging sector. Its application on plastic film substrates (PET, BOPP, PVC and, more recently, highly recyclable mono-material solutions such as BOPE) involves specific challenges related to the non-absorbent and apolar nature of these materials.

Thanks to mechatronic and chemical advancements, flexographic technology has reached high levels in terms of:

• print quality and resolution
• production efficiency
• sustainability

Mechanical principles and efficiency of flexography on plastic

Flexography is based on the transfer of fast-drying or curing liquid inks through flexible relief plates (printing plates). The core of the process is the anilox cylinder, a metering roller typically ceramic-coated and laser-engraved, which picks up the ink and transfers it in a controlled and uniform manner onto the printing plate.

The use of flexography on plastic films is now the industrial standard for several reasons:

• Speed and productivity
  Modern gearless central impression flexographic presses operate steadily at speeds exceeding 600 m/min. As of the state of the art (2026), the integration of register control systems based on artificial intelligence makes it possible to maintain these speeds while almost completely eliminating start-up waste.
• Mechanical adaptability
  Plastic substrates used in food and pharmaceutical packaging offer excellent barrier properties against moisture, oxygen and light. The flexible plates used in flexo printing effectively adapt to the micro-irregularities and thickness variations typical of these materials.

Surface thermodynamics: optimization of ink transfer

The main challenge in printing on plastic materials is related to surface chemistry. Unlike paper, plastic is non-absorbent; therefore, the interaction between ink and substrate is governed exclusively by intermolecular forces.

Surface tension and the differential rule

The surface tension of the ink determines its ability to remain as a droplet or to spread uniformly. For proper adhesion and even laydown, the surface tension of the ink must be lower than the surface energy of the film.

In industrial practice, the so-called 10 dyne differential rule is applied:
if a BOPP film has a surface energy of 40 dyn/cm, the ink must have a surface tension not exceeding 30 dyn/cm.

Today, in-line optical sensor systems allow these parameters to be monitored in real time, automatically adjusting ink viscosity and rheology.

Surface energy of polymers

Surface energy represents the wettability level of the substrate. Polymers such as polyethylene (PE) and polyethylene terephthalate (PET) are chemically inert and exhibit very low values (approximately 28–32 dyn/cm). Without treatment, the ink tends to retract (beading effect), resulting in uneven prints with poor adhesion.

To ensure proper printability, the surface energy of the film typically needs to be increased to a range between 42 and 46 dyn/cm.

Wettability and contact angle

Wettability represents the macroscopic effect of the balance between surface tension and surface energy and can be measured via the contact angle of a droplet.

• high angle → poor wettability
• low angle → good wettability

Proper wettability helps prevent typical flexographic defects such as pinholing (micro-holes in the ink layer) and poor edge definition (ink retraction).

Surface treatment methods for plastic films

To increase surface energy and make the surface more reactive, plastic films are subjected to specific treatments, often performed in-line:

• Corona treatment: The most widely used method, based on high-voltage electrical discharges (cold plasma) that oxidize the surface by generating polar groups. It is a cost-effective solution but subject to decay over time.
• Atmospheric plasma treatment: A more advanced technology using gases such as nitrogen or argon. It ensures deeper, more uniform and longer-lasting chemical modification, making it particularly suitable for next-generation mono-material films.
• Chemical and photochemical treatments (UV/Excimer): Based on primers or high-energy UV radiation, these enable advanced surface modifications. They are used in high-performance applications such as sterilizable packaging.

Chemical evolution of flexographic inks

Ink formulation is a key factor in printing on plastic. Traditionally, inks are classified as solvent-based, water-based and UV.

Following updates to regulations on Volatile Organic Compounds (VOC) between 2024 and 2025, the most advanced technologies are:

• UV-LED and EB (Electron Beam) inks: UV-LED systems have replaced traditional lamps, enabling instant curing without thermal stress on the film.
  EB technology, increasingly used in food packaging, allows crosslinking without photoinitiators (chemical substances that may migrate into food), ensuring high safety and quality.

Distortion control and advanced printing plates

Traditional flexographic plates undergo deformation when mounted on the cylinder, requiring compensation in prepress.

To reduce this effect, the industry uses:

• Elastomer sleeves (In-The-Round – ITR): continuous tubular plates, directly engraved by ultra-high-resolution laser ablation (Direct Laser Engraving – DLE).

These solutions ensure:

• high dimensional accuracy
• absence of seams
• improved print quality

Surfaces are also optimized with micro-structuring to enhance ink transfer and solid ink coverage.

Surface printing and reverse printing

The ink deposition sequence is determined by the end use of the flexible packaging and its optical properties.

1. Surface Printing: Ink is deposited on the outer side of the film. The typical sequence includes a high-opacity white layer, which enhances brightness and color gamut, followed by a protective coating (Overprint Varnish – OPV) to protect the print from mechanical abrasion and chemical agents. It is widely used for labels and industrial bags.
2. Reverse Printing: The graphics are printed in mirror image on the inner side of a transparent film (e.g. PET or BOPP). The sequence is reversed: first the colors, then the opaque white. The film is then laminated with a second layer (e.g. LDPE), trapping the ink between two films (“trapped ink”), ensuring high mechanical and chemical protection.

Note (2026): In the context of the circular economy, reverse printing increasingly uses mono-material MDO-PE (Machine Direction Oriented Polyethylene) structures designed for recyclability. This requires very precise web tension control due to the high elasticity of these materials.

Flexographic printing on plastic films is now a highly advanced process in which surface chemistry, mechatronics and advanced curing technologies converge.

Control of surface tension, combined with the use of low environmental impact inks (EB/UV-LED), plasma treatments and laser-engraved plates, enables high print quality, production efficiency and full compliance

Written by Gabriele G. | Team Giugni®