CI Press Web Path & Process Guide: From Unwind Tension to Rewind Slitting

A CI printing press handles raw roll feeding at the unwind and delivers converted, slit rolls at the rewind in a continuous pass. Maintaining color registration across synthetic films requires calibrating each mechanical stage to substrate dynamics rather than relying on default settings.

Below is a shop-floor walkthrough of web path dynamics, inline converting configurations, and tooling maintenance for the RS Series CI Full Rotary Printing Machine.

The video below demonstrates narrow-web rotary press operation and web tracking adjustments across the machine line:

3:11

Flexographic Printing Basics

YouTube · Label Technology Inc

1. Web Path Control: Infeed to Impression

[Unwind Station] ➔ [Surface Treatment] ➔ [CI Drum Printing] ➔ [Inline Converting] ➔ [Rewind Station]
  • 800 mm Max Roll   • 2.0 kW Corona       • 2-12 Color Decks    • Cold Foil / Lamination • Razor/Shear Slitting
  • Taper Tension     • Static Discharge    • Interstation UV Pin • Rotary Die-Cutting     • Dual Shaft Rewind

Stage 1: Unwind & Taper Tension

Registration errors frequently originate at the unwind spindle. Master rolls stored on their sides or exposed to container heat develop telescoping and core blocking that press adjustments cannot fix.

Roll Capacity: Accepts parent rolls up to 800 mm in diameter.

Taper Tension Adjustment: As the parent roll depletes from 800 mm down to the core, maintaining a constant brake torque increases web tension ($T = \frac{\tau}{r}$). Operators must engage active diameter-compensated taper control to gradually reduce brake torque, preventing web necking and first-station register drift.

Stage 2: Web Pre-Treatment & Static Neutralization

Non-porous synthetic materials (BOPP, PET, PE) resist ink wetting due to low surface energy.

Corona Treatment: The built-in 2.0 kW corona system oxidizes the film face to raise surface energy above 38–42 dynes/cm.

Static Discharge: High-speed roll unwinding generates electrostatic charges that pull airborne debris and cause ink misting. Dual static bars neutralize the web before it hits the infeed nip.

Stage 3: CI Drum Stability & Color Laydown

The Impression Cylinder: The web locks onto a 600 mm linear CI drum (Taiwan Dongting, 92–95 Shore A), driven by a 4.4 kW Inovance servo motor and a 600 mm Quanduo master gear.

Repeat Range: Accommodates repeat lengths from 190.5 mm (7.5 in) to 457.2 mm (18 in).

Interstation Pinning: Each ink layer must pin or cure before entering the next print station. Inadequate UV/LED pinning allows wet ink to pull back onto downstream photopolymer plates, contaminating anilox rolls.

Standardizing Plate Thickness: Plate thickness directly affects dot gain under pressure. Thicker photopolymer plates compress more under impression. Standardize plate and cushion tape thickness plant-wide so press curves remain consistent across repeat jobs.

2. Inline Converting & Finishing Setups

Flexo Varnishing & Cold Foil

Flexo Varnish Unit: Uses a 350 LPI ceramic anilox roll to meter flood protective coats or spot gloss/matte finishes without flooding highlight areas.

Inline Cold Foil: A flexo station deposits UV-curable adhesive in the image area, nips a metallic foil layer against the substrate, and peels away the polyester carrier inline. This applies fine metallic linework at press speed without heated brass dies.

OPP Overlamination: Driven by magnetic powder tension control across widths of 250 mm, 335 mm, or 460 mm, running both self-wound and backed films.

Rotary Die-Cutting: Preventing Matrix Breaks

Skeletons stripping from complex shapes frequently snap under tension.

Corner Radius Correction: Sharp internal corners on label die-lines concentrate stripping stress. Adding a subtle 0.5 mm radius to internal die corners distributes tension evenly, stopping web breaks on high-speed synthetic runs.

Slitting Setup: Preventing Adhesive Bleed

Adhesive accumulation and edge dusting during slitting point to blade geometry rather than steel hardness:

Blade Penetration: Over-penetration increases cutting friction.

Heat Management: Friction heats the web edge and softens pressure-sensitive adhesives, leading to gummed blades and roll-edge bleed. Set minimal blade overlap and clean shear angles to keep contact points cool.

3. Tooling Care: Preserving Photopolymer Plates

Photopolymer plates degrade faster from improper storage than from on-press mechanical impression:

UV & Ozone Damage: Ambient daylight, shop fluorescent lighting, and airborne ozone from corona units cross-link the polymer base, making relief dots brittle and prone to micro-cracking.

Handling Protocol: Wash plates with neutral solvent solutions immediately after removal, dry thoroughly, and store them flat inside light-tight, opaque sleeves at 15–20°C and 50–60% RH.

4. Job Ticket & Rewind Verification

Before starting high-speed rewinding on rolls up to 800 mm in diameter, cross-check the job ticket against customer requirements:

Core Inner Diameter: Verify core dimensions (typically 76 mm / 3 in) against the customer's high-speed applicator spindle.

Wind Orientation: Confirm unwind direction (face-in vs. face-out; top-first vs. bottom-first). A roll wound in the wrong direction will be rejected by automated labeling lines regardless of print quality.