Flexographic Press

Overview

A flexographic printing press prints packaging by transferring fast-drying ink from a flexible relief plate onto a moving web of film, paper, or board. The image areas stand proud on a soft photopolymer plate, the plate is inked by a metering roll, and the inked plate touches the web for a fraction of a second as both turn together. Because the plate is soft and the ink is thin and quick to dry, flexo prints well on the non-absorbent plastic films that dominate flexible packaging. A press runs eight colors at four hundred to six hundred metres per minute and holds tight color-to-color register over kilometre-long rolls, which is why it prints most snack bags, labels, shrink sleeves, and corrugated liners.

This machine is the central impression type, where every color prints against one large steel drum. The web enters through the Unwind and Web Infeed, wraps the Central Impression Drum past each Print Station, passes the Drying System, and winds up at the Rewind and Web Outfeed. The Ink System system feeds each color, the Drive and Register System turns and registers every cylinder, and the Control System sequences the line. All of it bolts to a rigid Machine Frame.

The print station

The work happens at each Print Station, and the chain inside one station is short. Ink is flooded into a sealed Doctor Blade Chamber and pressed against the Anilox Roll. The anilox is the heart of flexo: its Engraved Ceramic Layer is laser-engraved with millions of tiny cells, and the Doctor Blade wipes the smooth lands clean so only the cells carry ink. That gives a fixed ink volume per area, which is what makes flexo coverage consistent. The anilox then inks the raised image on the Photopolymer Plate, a photopolymer relief plate stuck to the Plate Cylinder with foam Mounting Tape. The plate touches the web on the drum, and a Ink Pump keeps ink circulating through the chamber. Each station carries its own Servo Motor drives so the plate and anilox can be phased independently.

Central impression drum

In this layout every station prints against one Central Impression Drum. The web is held flat on the Drum Shell, a ground steel cylinder often more than a metre across, so the film cannot stretch or wander between colors. That single backing surface is the reason CI presses hold register on thin stretchy film better than stack or inline presses. The drum carries a Temperature Jacket fed through a Rotary Union, which holds the shell at a set temperature so the film dimension stays stable from the first color to the last. The drum turns on heavy Ball Bearing sets in its Drum Journal ends and is driven by its own Servo Motor.

Ink, drying, and materials

Flexo inks are low in viscosity and dry by evaporation or by UV cure. The Ink System system gives each color its own Ink Tray and a Viscosity Controller that doses solvent to hold viscosity steady as it evaporates, since a viscosity drift changes color strength. Solvent and water inks must dry between colors or they smear, so a Between-Color Dryer blows hot air on the web after each station and a Bridge Dryer cures the full ink stack before rewind, with Heating Element heaters and a LEL Monitor that keeps solvent vapor below the explosive limit. The substrate is usually a plastic film such as BOPP, PE, or PET between twelve and three hundred microns, and also paper and folding carton board. Non-absorbent films are corona treated upstream so the ink wets and anchors.

Drive, register, and control

Older presses geared all cylinders to one shaft, but a modern press gives every cylinder a Servo Motor in the Drive and Register System system. A Register Camera reads register marks at full speed, the Register Controller measures color-to-color offset, and it shifts each Plate Cylinder in angle and across the web until the colors land on top of one another. A Cylinder Encoder on each cylinder closes the servo loop. The Control System system runs the Line PLC sequence and presents the Operator HMI where the operator sets speed, ink, and register and watches the job. Tension is held end to end by the Dancer Roll and Tension Load Cell sensors so the web neither stretches nor goes slack through the print zone.

Variants and use

CI presses like this one suit thin extensible film and are the workhorse of the flexible packaging plant. Where the substrate is stiff, such as folding carton or self-adhesive label stock, presses are built instead as inline units with separate impression cylinders at each color, which trade some register stability for easier access and inline converting such as die cutting. Corrugated post-print presses use the same relief principle on much larger, slower frames. UV flexo replaces the hot-air Drying System with UV lamps that cure ink between stations and is common for labels and shrink sleeves. Across all of these, the press feeds from an Unwind and Web Infeed, splices rolls on the fly through the Flying Splice Unit to avoid stopping, and delivers a finished printed roll at the Rewind and Web Outfeed for the next converting step.