Flexo Printing Accuracy: Mechanical Sources of Error and Mitigation Strategies
While substrate deformation is a major source of accuracy issues, mechanical imperfections in the press itself also contribute to printing errors. Cylinder run-out, gear backlash, bearing wear, and roller deflection cause cyclic variations in the printed mark position, leading to banding and register errors. This article identifies these mechanical sources and discusses design and maintenance solutions.
Cylinder run-out (TIR) is the deviation of the cylinder's surface from a perfect circle during rotation. This is caused by manufacturing tolerances, bearing wear, or bent shafts. A run-out of 0.01 mm on a 1-m cylinder translates to a circumferential variation of 0.03 mm, which is significant. The run-out is measured with a dial gauge during a slow rotation. To mitigate, cylinders are dynamically balanced and ground with high precision; bearings are replaced when wear exceeds limits. Sleeve technology can reduce the impact by using a compressible layer that absorbs minor run-out.
High Speed Flexo Printing Machine - Stack Flexo Flexo Printing Machine
Gear backlash: The gears that drive the plate cylinder from the main drive have a small clearance (backlash) to allow lubrication. This clearance introduces a dead zone where the cylinder can rotate slightly without the drive torque, causing a phase error. High-precision gears with zero-backlash (e.g., helical gears with preloaded bearings) are used in modern presses. Backlash compensation algorithms in the servo drive can also reduce the effect by applying a small constant torque to keep the gear teeth engaged on one side.
Bearing wear: Bearings degrade over time, increasing radial play. This allows the cylinder to shift laterally, affecting the gap between cylinders. Regular lubrication and replacement per schedule are essential. Vibration analysis can detect early bearing wear by monitoring the frequency spectrum for peaks at the ball pass frequency.
Roller deflection: Under web tension, the rollers deflect (bend) slightly, especially for wide webs. The deflection causes non-uniform nip pressure across the width, which affects the print density and also the register, because the web path length changes with pressure. To counter, rollers are designed with a crown (slight convex shape) that compensates for deflection. The crown is calculated based on the roller's material, diameter, and load. Regular inspection of the crown condition is recommended.
Frame vibration and resonance: As discussed in earlier articles, vibration can cause cyclic register errors. The press's control system can use a "vibration reject" filter that ignores register mark readings that occur during known vibration peaks (e.g., at the gear mesh frequency). This reduces the likelihood of erroneous corrections.
Measurement of mechanical errors: To diagnose mechanical errors, a test form with multiple marks is printed at slow speed, and the mark positions are measured with a precision microscope. The variation pattern reveals the source: a sinusoidal pattern indicates run-out; a step change indicates backlash; a trend indicates thermal expansion. The data guides maintenance actions.
Preventive maintenance programs: Scheduled replacement of bearings, gears, and drive belts, along with periodic laser alignment of cylinder axes, can maintain accuracy over the press's lifetime. Many presses have a built-in diagnostic routine that runs a test pattern and automatically computes the repeatability, alerting when accuracy falls below a threshold.
By addressing mechanical sources of error,
flexo printing accuracy can be maintained at the highest levels, ensuring that the press delivers consistent quality over its operational life, with minimal quality variability and waste.
