Digital Inkjet Web Press

Overview

A digital inkjet web press prints onto a continuous roll of paper instead of cut sheets, and it builds the image from digital files rather than from plates. Fixed arrays of inkjet heads span the full web width and fire ink droplets as the paper runs underneath, so every impression can carry different text, images, and barcodes. That makes the press suited to short runs, versioned packaging, transactional documents, and direct mail where each piece is unique. Output rolls off at 50 to 150 metres per minute at 1200 dpi, fast enough to replace offset on runs too short to justify plates.

The web threads from the Unwind and Infeed through the Printbar Station stations, past the Drying Unit, and onto the Rewind and Slitter. The Ink Delivery System keeps each color fed at stable pressure and temperature, the Web Transport holds the paper flat and at speed, and the Controller and RIP turns each page into the bitmap the heads jet. Everything bolts to the Machine Frame.

Printbars and heads

Each color rides on its own Printbar Station, a rigid beam carrying a row of Piezo Printhead modules butted end to end across the web. The heads are drop-on-demand piezo type: a piezo actuator behind each nozzle flexes to eject a controlled droplet, with no heat applied to the ink. Because the heads are fixed and the paper moves, the print line must be perfectly straight, so the Head Mounting Plate holds every head coplanar and the Alignment Stage uses Micro Actuator elements to stitch one head into the next without a visible seam. A Head Driver Board board shapes the piezo waveform through its Waveform Amplifier stages and clocks pixel data in time with the Print Encoder. Idle nozzles dry out fast, so the Maintenance Station caps the heads with a Capping Pad and clears blockages with a Wiper Blade and a purge into the Purge Tray.

Ink delivery

Aqueous pigment ink reaches the heads through the Ink Delivery System. Each color sits in a stainless Ink Tank and is moved by a Ink Pump toward the head and back, since constant recirculation through the Recirculation Manifold keeps the pigment in suspension and the nozzles fresh. Dissolved air is the enemy of a stable jet, so a Vacuum Degasser strips it out before the ink reaches the head. A Ink Heater holds the ink at its target viscosity under feedback from a Ink Thermistor, and a final Ink Filter traps any particle large enough to clog a nozzle. A Pressure Sensor on each line trims the slight negative pressure that holds the meniscus steady at the nozzle face.

Web transport and drying

The Web Transport pulls the paper through at constant speed using a Drive Roller pair driven by a Servo Motor, with several Guide Roller idlers routing the web. Under the heads the paper must stay dead flat, so a Vacuum Platen with a Platen Plate and a Vacuum Blower sucks the web down against cockle. Once printed, the web carries a lot of water, which the Drying Unit removes: a bank of IR Emitter Module emitters flashes off surface moisture while Hot-Air Knife nozzles, fed by a Heating Element bank and a Dryer Blower, drive out the rest. The humid air leaves through the Exhaust Hood.

Substrates and the primer unit

Aqueous ink spreads on uncoated and offset paper, which softens the dots and dulls the color. The Primer Unit solves this by laying a thin bonding layer ahead of the heads: a Primer Roller meters primer from the Primer Pan, and a Primer Pre-Dryer tacks it down before printing. On inkjet-treated coated stock the primer can be switched off. The press runs paper from roughly 40 to 250 gsm, and the finished web is slit and wound at set tension by the Rewind and Slitter using its Slitter Knife knives and a Lay-On Roller.

Control, inspection, and use

The Controller and RIP runs the show. A RIP Server rasterizes each page into a per-nozzle bitmap, which the Image Board set streams to the heads at full web speed. A Motion PLC coordinates the servos, tension, and interlocks, and the operator works from the Operator HMI. A Inspection Camera checks every print for missing nozzles and defects so faults are caught inside the roll before it ships. When the camera flags a dead nozzle, the controller can compensate by firing neighboring nozzles harder, or it can stop the press for a purge cycle at the Maintenance Station. Changeover is fast because there are no plates to make: a new file is rasterized on the RIP Server and printing resumes on the next metre of web.

Presses of this kind run book blocks, transactional statements, direct mail, and short-run folding-carton and flexible packaging work, anywhere variable content or a short run makes plates uneconomic. The economics favor inkjet on runs from a few hundred to a few thousand metres, below the point where offset plate cost pays back, while very long runs of identical work still go to plates because ink cost per page is higher. Output from the Rewind and Slitter is a printed roll that feeds a separate finishing line for cutting, folding, and binding, or goes to converting equipment for label and carton work downstream.