Gravure press tension control is the difference between registration that holds and registration that chases itself across the roll. The machine has five tension zones — unwind, infeed, printing, outfeed, and rewind — and each one imposes different demands. Getting tension right means understanding the hardware at each zone, not just setting a number on a controller.
Unwind Zone: Controlling a Shrinking Roll
As the roll diameter decreases during a run, the brake must continuously reduce its holding force to maintain constant tension. The same torque that holds 10kg tension at full roll diameter will pull 30kg when the roll is nearly empty — if the brake doesn’t compensate. Mechanical friction brakes are cheap but can’t hold accurate torque across a changing diameter. Pneumatic brakes are common but suffer from nonlinear torque response and mechanical wear.
Magnetic powder brakes are the current standard: the braking torque is directly proportional to excitation current, giving a linear, predictable response. Combined with tension measurement — either a spring-loaded dancing roller or a micro-displacement sensor — the system can maintain constant web tension despite rapid diameter changes from acceleration, deceleration, and splicing.
Short-duration tension disturbances that originate at unwind — thickness variation, material snags, eccentric rolls, turret rotation speed changes, the momentary reverse load from the splice roller and knife — must be absorbed before they reach the print stations. A large-diameter, lightweight idler roller (typically hollow aluminum) positioned between the unwind and the infeed nip acts as a mechanical filter, smoothing these transients through its own inertia.
Infeed and Printing Zones: The Stretch Control Problem
Between the infeed nip and the first print cylinder, the web must transition from the unwind’s higher tension to the printing zone’s lower, more precisely controlled tension. Most gravure presses synchronize the infeed roller speed with the first print cylinder through a mechanical or electronic speed ratio — and then fine-tune tension by either slightly varying the nip roller diameter or adjusting its rotational speed.
The web’s transition from zero tension to set tension follows a first-order lag response, with the time constant determined by the distance between nips and the speed differential. This is called stretch control — the web stretches within its elastic limit to the target tension over a predictable distance. All subsequent tension zones — between print stations, and between the last station and the outfeed — work on the same principle.
Between the last print cylinder and the outfeed nip, the web passes through the longest dryer path. Heat softens the film and causes elongation; cooling contracts it. If this zone’s tension isn’t precisely controlled, the last-color registration drifts even when every previous station is locked in. Thin, low-tension films are also susceptible to dryer airflow disturbance and static cling — both introduce micro-tension variations that show up as registration noise.
Rewind Zone: Taper Tension
Constant-tension winding builds a roll that’s tight at the core and progressively looser toward the outside — or worse, tight enough at the core to crush the inner layers. As the roll diameter grows, the outer layers compress the inner layers. Combined with trapped air bleeding out and film shrinkage, this compression can telescope the roll sideways — the “bamboo shoot” effect — and wrinkle the material near the core.
The solution is taper tension: as the roll builds, tension decreases on a programmed gradient. Typical taper values range from 10% to 50%. At 30% taper, for example, winding starts at 10kg and ends at 7kg. The taper curve is machine-specific and material-specific — each converter should develop their own curves experimentally for each substrate and roll dimension.
Three drive technologies handle the rewind: magnetic powder clutches (good value, excellent performance), torque motors (struggle with large diameter changes), and DC motors (required for high-capacity, automatic-splice winders). Surface winding — where a driven roller contacts the outside of the roll — has advantages for constant-speed applications where roll circumference consistency matters. Center winding — driving the core directly — is more common but demands more sophisticated taper control.
Inertia Compensation
When the press accelerates from rest to running speed, the unwind and rewind rolls resist that change through their rotational inertia. The inertia moment changes nonlinearly with diameter and scales proportionally with material density and web width. If the tension controller doesn’t compensate for this inertial load during speed changes, tension spikes or sags — and registration follows. The compensation curve must be calculated for each roll configuration and built into the drive programming.
References
- Wikipedia: Web Tension Control: Fundamentals of tension zone management in continuous web processes including unwind, printing, and rewind zone interactions for registration stability.
- Wikipedia: Rotogravure: Comprehensive overview of gravure printing including press architecture, tension systems, and the relationship between tension control and color registration accuracy.
- Wikipedia: Magnetic Powder Brake: Operating principles of magnetic powder braking systems including torque-current linearity and application in unwind tension control.
- ISO 12647-4:2014 — Gravure Process Control: International standard for process control parameters including registration tolerances related to tension stability in gravure printing.
- Flexible Packaging Association (FPA): Industry resource covering web handling, tension control technology, and production optimization for gravure packaging converters.