Flexographic Printing · The Complete Guide

Flexographic printing is a form of relief printing, the image areas on the plate are raised above the non-image areas. Ink is applied to the raised surface and transferred directly to the substrate. It is the printing equivalent of a rubber stamp, but at high speed with precision-engineered plates, controlled ink metering, and high-quality results.

Flexo dominates flexible packaging (pouches, films, bags), self-adhesive labels, folding cartons in some markets, corrugated board printing, and large-format bags and sacks. It is the correct process whenever the substrate cannot be fed as individual sheets through an offset press, particularly films, thin foils, and corrugated board.

Key differences from offset printing

• Direct contact, in flexo, the plate contacts the substrate directly. There is no intermediate blanket cylinder. The plate must be soft enough to conform to substrate surface irregularities without the compliance aid of an offset blanket.
• Low-viscosity inks, flexo inks are much thinner (lower viscosity) than offset inks. They are liquid rather than paste, which allows them to flow into the anilox cells and transfer efficiently at high press speeds to non-absorbent substrates like film and foil.
• Anilox ink metering, ink quantity is controlled by the anilox roller's cell volume rather than by ink duct keys. This is a more controllable and consistent metering system for liquid inks than the offset ink train.
• Web-fed operation, almost all flexo presses run from a continuous reel of substrate. This makes flexo ideal for high-speed, high-volume production on films, foils, and papers that cannot be cut into sheets for sheetfed presses.

The flexo plate is the element that defines what prints. Unlike offset plates (flat aluminium with a chemical image), flexo plates are three-dimensional, the image is physically raised above the non-image areas. This relief structure means the plate must be manufactured with precision to ensure uniform print pressure across all image elements.

Photopolymer plates · the industry standard

Modern flexo printing uses photopolymer plates, a light-sensitive polymer material that hardens when exposed to UV light and washes away where it was not exposed, leaving the raised image relief. Photopolymer plates produce the highest quality flexo printing and are used for all quality-critical label and flexible packaging work.

There are two types of photopolymer plate:

• Analogue photopolymer (conventional), a film negative is placed over the plate and exposed to UV light. The plate hardens under the transparent (image) areas and remains soft under the opaque (non-image) areas. The soft material is washed away with a solvent or water-based wash, leaving the raised image. Film-based plate making is being replaced by digital in most Indian quality label and flexible packaging printers.
• Digital photopolymer (LAMS / direct laser ablation), a laser ablates (burns away) a thin black mask layer on the plate surface in the non-image areas, then the plate is exposed to UV light. No film negative is required. Digital plates produce sharper dots, finer highlight reproduction, and more consistent results than analogue plates. This is now the standard in premium flexo production in India.

Rubber plates

Rubber flexo plates (moulded or engraved) are used for simpler applications, corrugated board printing, basic bag and sack printing, and simple one- or two-colour packaging. They are less expensive than photopolymer and more durable for abrasive corrugated surfaces, but cannot reproduce fine halftone detail or photographic images. Rubber plates are the correct choice for corrugated box outer printing where the substrate surface is too rough for fine photopolymer work.

Plate mounting and sleeve technology

Photopolymer plates are mounted on the plate cylinder using double-sided mounting tape. The compressibility and thickness of this tape affects the overall print impression and must be specified correctly for the plate thickness, cylinder circumference, and substrate combination. Modern flexo presses increasingly use sleeves, lightweight composite cylinders onto which the plate is mounted off-press. Sleeves improve changeover time, reduce press downtime, and improve repeatability of plate mounting.

The anilox roller is arguably the most critical component in a flexo printing unit. It is a steel or ceramic roller with millions of microscopic engraved cells covering its surface. These cells pick up ink from the fountain, are wiped clean on their surface by the doctor blade, and then transfer a precisely controlled volume of ink to the raised areas of the flexo plate. The cell volume and geometry determine how much ink is applied, and therefore the density and consistency of the printed colour.

Anilox cell geometry

Anilox cells are typically hexagonal in shape (for maximum packing efficiency) and are described by two measurements:

• Lines per centimetre (lpcm) or lines per inch (lpi), the number of cell rows per unit of distance. Higher lpcm means smaller, more numerous cells. Fine-screen process colour flexo uses 180–360 lpcm anilox; bold solid colour printing may use 60–100 lpcm anilox.
• Cell volume (BCM, billion cubic microns per square inch), the total ink-carrying capacity of the cells. Higher BCM delivers more ink. Fine process colour work requires low BCM (2–4 BCM) to prevent over-inking of fine halftone dots; solid coverage may use 8–15 BCM.

The anilox-to-plate ratio

A fundamental rule of flexo: the anilox screen ruling should be at least 4–5 times the halftone screen ruling of the print job. If the job is printed at 60 lines per centimetre halftone, the anilox should be at least 240–300 lpcm. This ratio ensures that the anilox cells are small enough to support the finest halftone dots on the plate, if the cells are too large, they deposit ink inconsistently across fine dots, producing mottled or speckled highlights.

Anilox maintenance

Anilox cells become partially or fully blocked with dried ink over time, particularly with UV-curable and water-based inks that dry rapidly. Blocked cells reduce ink transfer, producing lower ink density and inconsistent coverage. Anilox rollers must be cleaned regularly using appropriate cleaning methods for the ink system in use. Ultrasonic cleaning is the most effective method for recovering heavily blocked rollers. Anilox cell condition should be checked periodically with a microscope, blocked cells are visible as dark patches in the cell pattern.

Flexo inks are liquid inks with much lower viscosity than offset inks, typically 20–200 centipoise compared to 40,000–100,000 centipoise for offset paste inks. This low viscosity allows them to flow into and out of anilox cells rapidly at press speeds, and to transfer from non-absorbent surfaces like films and foils. There are three main flexo ink systems, each with different chemistry, drying mechanism, and application profile.

The flexibility of the flexo plate, and the use of low-viscosity liquid inks, makes flexo suitable for a very wide range of substrates, including many that offset cannot print on. This substrate versatility is the primary reason flexo dominates flexible packaging.

Flexo has historically had a reputation for lower quality than offset, a reputation that was entirely justified for conventional rubber-plate flexo in the 1980s and 1990s, and is increasingly unfair for modern digital photopolymer flexo. The quality gap between premium flexo and offset has narrowed significantly, but differences remain that designers and buyers must understand.

Where modern flexo performs well

• Solid colour panels, bold brand colours in solid coverage are excellent in flexo, particularly with UV inks and quality anilox selection
• Large type and bold graphics, flexo excels at clean, sharp reproduction of text above 8pt and bold graphic elements
• Extended run consistency, flexo ink delivery via anilox is inherently more consistent than offset ink duct adjustment for long runs
• Process colour photography on quality flexible packaging, modern digital plate flexo with fine-screen anilox (240+ lpcm) and expanded colour gamut inks produces genuinely impressive photographic reproduction on BOPP and PET films

Where flexo has limitations

• Highlight dot reproduction, the minimum printable dot in flexo is typically 3–5% depending on the plate and anilox combination. Below this, dots either fill in (print as solid) or disappear entirely. Offset can hold 1–2% dots on good coated paper. This means smooth gradients that fade to white are more difficult in flexo, a hard cut-off is required at the minimum printable dot.
• Solid-to-highlight transitions, the kiss impression (light touch) required for fine highlight dots is different from the slightly heavier impression needed for clean solid coverage. Printing both in the same job requires careful plate construction with different floor depths for solid and halftone areas.
• Fine reversed text, small white text reversed out of a dark background in flexo can fill in due to ink spread. Minimum reversed text is typically 8–9pt for flexo versus 5–6pt for offset.
• Registration accuracy, flexo registration is generally less precise than offset, particularly on extensible film substrates that stretch under web tension. Multi-colour register tolerance in flexo is typically ±0.1–0.3mm versus ±0.05–0.1mm in quality offset.