Correct Use and Maintenance of Anilox Rolls in Flexographic Printing

In traditional multi-color letterpress and lithographic printing machines, the inking system consists of a series of rollers, including the ink fountain roller, ink transfer roller, ink distribution roller, ink oscillating roller, and ink form roller, among others, totaling up to twenty or more rollers. While this system generally meets the requirements for ink transfer, it struggles to maintain a constant ink supply. In contrast, flexographic printing machines utilize a short inking system, where the anilox roller provides a consistent and uniform ink supply through its engraved cells. The anilox roller plays a pivotal role in flexographic printing, and understanding its proper use and maintenance is crucial for avoiding operational errors, improving product quality, and enhancing production efficiency.

I. Types and Characteristics of Anilox Rolls Anilox rolls are categorized into two main types: metal engraved anilox rolls and ceramic anilox rolls. The surface of an anilox roll is engraved with ink cells, which facilitate precise and uniform ink transfer. This allows for accurate control of ink volume, a key advantage of flexographic printing’s short inking system. The structure and line count of the cells determine the ink transfer performance and application range of the anilox roll. The uniformity of the cells affects ink distribution, while their density (line count), shape, size, and depth determine the volume of ink transferred.

1. Metal Anilox Rolls: The manufacturing process for metal anilox rolls is complex. Typically, a seamless steel tube is precision-machined and then copper-plated to create a workable surface. The surface is then engraved with uniform ink cells, followed by a final chrome plating to enhance wear resistance. The durability of the chrome layer directly impacts the roll’s lifespan and ink transfer performance. If the plating is not wear-resistant, the cells may wear out, affecting ink transfer.

2. Ceramic Anilox Rolls: Ceramic anilox rolls are created by applying a synthetic ceramic coating (Cr₂O₃) to the roll surface using plasma technology, followed by laser engraving of the ink cells. Ceramic rolls offer superior wear resistance compared to metal rolls and can achieve cell line counts as high as 1200 lpi, making them suitable for high-quality color printing. The thin and uniform ink film formed by ceramic rolls reduces ink consumption, speeds up drying, minimizes dot gain during high-speed printing, and improves print quality.

II. Cell Shape and Angle of Anilox Rolls The shape and angle of the cells significantly influence the performance and results of anilox rolls.

1. Cell Shapes: Common cell shapes include pyramid, truncated pyramid, hexagonal, and trapezoidal with a 45° spiral angle.

. Pyramid Cells: Widely used but have small cell bottoms, making complete ink transfer difficult and cleaning challenging. Suitable for rubber ink fountain roller systems.

. Truncated Pyramid Cells: Offer better ink release and flexibility, suitable for both rubber ink fountain rollers and doctor blade systems. They provide thin and uniform ink transfer, enhancing color printing quality.

. Hexagonal Cells: Provide excellent ink transfer and release, reduce moiré patterns, and are ideal for halftone printing. They also offer high strength, reducing doctor blade wear.

. Trapezoidal Cells with 45° Spiral Angle: Ensure good ink flow and transfer, suitable for high-viscosity inks and coating applications.

2. Cell Angles: The cell angle affects print quality, especially in color printing, where mismatched angles can cause moiré patterns. Common angles include 30°, 45°, and 60°.

. 30° Hexagonal Cells: Offer good ink transfer but are prone to wear due to horizontal cell walls. Suitable for coating and non-doctor blade systems.

. 45° Diamond Cells: Effective in avoiding moiré patterns but may trap ink at the cell bottom, making cleaning difficult. Not ideal for fine color printing.

. 60° Hexagonal Cells: Provide the highest surface area utilization, with wide cell openings and shallow depths, ensuring smooth ink transfer and easy cleaning.

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III. Proper Use and Maintenance of Anilox Rolls

1. Selecting Anilox Rolls Based on Substrate: . For rough, absorbent substrates, use rolls with lower line counts (larger cells) to transfer more ink.

. For smooth, non-absorbent substrates, use rolls with higher line counts (smaller cells) for less ink transfer.

2. Matching Anilox Rolls to Plate Screen Ruling: To ensure accurate ink transfer, the anilox roll’s cell area should be smaller than the plate’s dot area. The anilox roll’s line count should be 3 to 4 times higher than the plate’s screen ruling, with a 45° cell angle to prevent moiré.

3. Selecting Anilox Rolls Based on Print Design: For fine halftones or small text, use rolls with higher line counts for precise ink transfer.

For large areas or bold text, use rolls with lower line counts for uniform ink coverage.

4. Matching Anilox Rolls to Inking Systems: . Pyramid cells are suitable for rubber ink fountain roller systems.

. Truncated pyramid and hexagonal cells are ideal for doctor blade systems, requiring larger cell volumes and line counts above 500 lpi.

5. Ensuring Quality Manufacturing: Rounded cell edges reduce damage during use, enhancing the roll’s durability.

6. Proper Cleaning and Maintenance: . Use appropriate cleaning methods and solutions to avoid damaging the anilox roll.

. Ultrasonic cleaning should be used cautiously, especially for ceramic rolls, to prevent surface damage.

. Handle rolls carefully during installation and storage to avoid scratches or deformation.

IV. Precautions for Using Anilox Rolls

1. Controlling Roller Pressure: Maintain uniform and moderate pressure between the anilox roll and plate cylinder to prevent surface damage. Adjust pressure using a feeler gauge to ensure even contact.

2. Proper Doctor Blade Adjustment: Ensure the doctor blade is parallel to the anilox roll and apply minimal pressure to avoid excessive wear.

Avoid dry contact between the blade and roll to prevent “grooving.”

3. Maintaining Machine Cleanliness: . Inspect and clean the anilox roll before use to remove debris that could cause damage.

. Regularly check and lubricate gears to prevent roll vibration and damage.

In conclusion, the anilox roll is a critical component of the flexographic printing process. Its precision, proper use, and maintenance are essential for achieving high production efficiency and print quality. By following these guidelines, operators can maximize the performance and lifespan of anilox rolls, ensuring consistent and high-quality printing results.