1. Pre-treatment: role and benefits of the primer
The primer (or pre-treatment) is a critical element in digital inkjet printing, especially when working on complex substrates. It acts as an “intermediate layer” that chemically levels and optimizes the surface, making it fully receptive to ink.
Reasons for using a primer
The primary function of a primer is to overcome compatibility limitations between inkjet inks and the material surface.
- Lack of adequate adhesion: many non-porous materials (metals, glass, plastics, fabrics) do not allow effective ink adhesion, with risks of flaking or deterioration.
- Porosity management: on porous substrates (papers or boards), the primer holds the ink on the surface, chemically binding pigments and preventing excessive absorption or drop spread.
- Control of dot gain/coalescence: on non-porous substrates, it prevents the ink solvent from causing excessive dot expansion or merging between adjacent drops, ensuring sharper definition.
- Printing with white ink: for printing on dark fabrics, the primer is essential to create the intermediate layer required for proper coverage and adhesion of white ink.
Operational and qualitative benefits
Applying a primer offers advantages that improve both print quality and process efficiency:
- Improved adhesion and durability: creates a receptive layer, making the print more resistant over time and, on textiles, to washing.
- Material versatility: enables printing on a wide range of substrates (glass, metal, plastics), expanding application possibilities.
- Higher print quality: provides a uniform surface, reduces defects, enhances color vibrancy, and ensures high definition by properly anchoring the drop.
- Reduced ink consumption: by keeping ink on the surface, it allows the desired saturation to be achieved with lower ink volumes.
2. Post-treatment: role and benefits of the coating (varnish)
The application of protective lacquers or varnishes immediately after digital printing (during converting or post-printing) is essential to transform a print into a durable, high-value finished product.
Primary purpose: protection
The coating creates a physical barrier over the ink film.
- Mechanical resistance: protects against abrasion and scratches during handling and transport, a crucial feature for digital inks, which are often more delicate.
- Chemical resistance: protects against oils, fats, solvents, and detergents, critical in food and cosmetic packaging.
- Weather resistance: integrates UV stabilizers that prevent fading caused by sunlight and protect against humidity.
Secondary purpose: aesthetics and processability
- Gloss control: enables glossy finishes for more vibrant colors, matte finishes for an elegant look, or satin effects.
- Tactile effects and spot coating: allows soft-touch effects or selective varnishing (spot coating) that highlights specific details, such as logos, creating both visual and tactile contrast.
- Convertibility: a smooth surface improves material transport through die-cutting and packaging equipment and facilitates subsequent processes such as hot stamping.
3. Technological choice: flexography vs. rotogravure
Primer and coating can be applied using different technologies, both off-line and in-line. However, when integrated into digital systems, the flexo unit (usually modular and equipped with a closed-chamber system) outperforms rotogravure in efficiency, cost, and flexibility.
3.1 General advantages of flexo for modern runs
Today’s market demands high job fragmentation (short and medium runs) and frequent artwork changes.
- Tooling cost: photopolymer flexo plates are inexpensive and fast to produce, while engraved rotogravure cylinders require much higher investments and long manufacturing times.
- Efficiency for short runs: the low plate cost allows start-ups to be amortized even on short runs (below 100,000 meters), where rotogravure becomes uneconomical.
- Makeready time: modern flexo presses enable rapid job changes, reducing downtime and waste and accelerating time-to-market.
- Sustainability: flexography easily uses water-based inks, offering environmental advantages over solvent-based gravure inks.
3.2 Specific advantages for primer application
For primers, flexo is particularly advantageous in coating efficiency.
- Anilox roll management: unlike the specific and costly gravure cylinder, the anilox roll is reusable for multiple jobs and provides uniform application across the web, an essential requirement in digital printing.
- Film-thickness control: the primer volume is precisely controlled by the anilox cells. Adjusting the applied amount is simple and cost-effective: replacing the roll is sufficient, with no need to engrave a new cylinder.
- Flexibility: switching from one primer to another or modifying the application area (such as for partial coatings) is fast and inexpensive thanks to low-cost plates.
3.3 Specific advantages for coating application
For post-printing varnishing, flexo excels in versatility and aesthetic performance.
- Cost-effective spot coating: flexo enables selective varnish application using a die-cut plate at low cost. Rotogravure, in contrast, would require an expensive engraved cylinder, while 3D inkjet coating systems would be slower.
- Uniformity and dosage: the anilox roll ensures consistent, streak-free finishes, whether glossy or matte.
- In-line integration: flexo towers are compact and fast, ideal for integration into high-speed digital roll-to-roll lines, enabling immediate curing (often UV/LED) and direct transition to subsequent finishing steps.
Flexo offers reduced tooling costs, fast job changes, precise primer and coating application, and wide aesthetic versatility,fully meeting the needs of packaging and label production.
Written by Gian Pio G. | Team Giugni®
