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Revolutionizing Flexo Printing: The Impact of Reverse-Angle Doctor Blades

In the ever-evolving realm of flexographic printing, one technology has risen to prominence as a game-changer: the reverse-angle doctor blade system.

This article delves into the origins of this innovative technology and its profound impact on the printing industry.

We'll explore its foundations and the exciting advantages that have solidified its place as a cornerstone in modern flexography.

Flexographic printing

The Origin of a Game-Changer

The exact birth of the reverse-angle doctor blade concept may be shrouded in history, but its impact on flexographic printing is undeniable.

In the early 1950s, the first U.S. patent was issued to an inventor whose name remains in obscurity.

This inventor showcased an early version of the technology on a rotogravure web press, which differed from the final flexo implementation.

The Birth of Flexo Reverse-Angle Doctor Blades

The flexo version of the reverse-angle doctor blade ink distribution system took shape in 1959 when a laboratory unit was constructed for testing purposes.

This innovation was introduced at the 1960 FTA Technical Forum, where the optimal blade angle of 30° to the tangent at the point of contact with the anilox roll's "dry" side was established.

The Initial Design

Early flexo reverse-angle doctor blade units relied on a positive screw adjustment to control the blade's position and pressure against the anilox roll surface.

Several subsequent designs followed this principle, emphasizing the importance of maintaining very light blade-to-anilox roll surface pressure for extended anilox roll life.

Presses up to 60" in width typically featured rigid blade assemblies with spring, pneumatic, or hydraulic controls.

Evolution on Corrugated Presses

With the advent of flexo corrugated printing presses , which could reach widths of 120 to 240", multiple blade assembly controls became essential.

These presses commonly utilized pneumatic pressure units.

The ink fountain was positioned under the anilox roll, and the doctor blade assembly was mounted above it.

This design maintained uniform pressure across the anilox roll's surface.

Pneumatic Pressure Controls

Pneumatic pressure cylinders, often with flexible expandable tubes, were used to adjust and maintain blade assembly pressures at consistently low levels.

This approach ensured even pressure across the entire blade holder's back, enhancing the efficiency of reverse-angle doctor blade systems.

The Advantages

Reverse-angle doctor blade systems offer numerous advantages over traditional two-roll anilox ink distribution systems:

Uniform Ink Film
These systems consistently deliver a uniform ink film to printing plates and substrate surfaces across a wide range of ink viscosities.

Consistency at High Speeds
Reverse-angle doctor blades maintain superior print quality and uniformity even at high operating speeds.
Unlike two-roll units, they don't experience an increase in ink density with speed.

Top-Quality Printing
Numerous tests and production jobs have demonstrated that reverse-angle doctor blade flexo presses produce superior quality in four-color process printing, making them the choice for award-winning jobs.

Expanding Applications
The adoption of reverse-angle doctor blade/anilox roll ink distribution systems is growing, particularly in applications like newspaper printing, folding boxes, and commercial printing, as they replace traditional letterpress and lithographic processes.

The Early Days

Back in the early 1960s, the Flexographic Technical Association (FTA) reported on the development of reverse-angle doctor blade systems.

Today, as flexographers continue to explore this technology, it's worthwhile to revisit those pioneering days.

Understanding Reverse-Angle Doctor Blades

At its core, a reverse-angle doctor blade system is designed to efficiently distribute ink in flexographic printing.

In this system, a blade is positioned on the side of the press where the web travels upward. Its primary function is to "shave" or "scrape" ink from the surface of an anilox roll, similar to what a blade would do in a rotogravure press.

The Anatomy of Reverse-Angle Doctor Blade Installation

Imagine a conventional two-roll flexo press with an anilox form roll.

Figure 2 illustrates how a reverse-angle doctor blade installation might look on such a press.

Note that in this setup, a rubber-covered fountain roller isn't necessary.

This configuration ensures easy access for press operation.

doctor blade

The Evolution of Flexo Ink Distribution

To appreciate the significance of reverse-angle doctor blades, let's rewind a bit.

The earliest flexo ink distribution systems relied on two smooth rollers, one or both of which were rubber-covered.

These rollers adjusted ink distribution based on pressure, ink viscosity , and press speed.

While suitable for slower speeds and absorbent paper, they fell short when dealing with non-absorbent surfaces.

The Rise of Anilox Rolls

Around the late 1930s, the anilox ink distribution system emerged, aligning with the use of 100% pigment inks, particularly for film and foil packaging.

Initially, it featured a single anilox roll with a doctor blade.

However, issues such as wear on the anilox roll surface and accessibility prompted the adoption of two-roll systems.

Two-Roll Anilox Inking Systems

Two-roll anilox inking systems gained traction, with differential surface speed fountain rollers becoming more common.

As press speeds increased, a separate drive system for the fountain roller emerged, allowing for variable ratios between the fountain roller and the anilox roll .

At high speeds and surface speed differentials, ink film thickness control became problematic, leading to the demand for reverse-angle doctor blade systems.

The Advantages of Reverse-Angle Doctor Blades

Reverse-angle doctor blades offered several key benefits

(a) Print Quality These blades consistently delivered high-quality prints, especially when press operators correctly adjusted blade contact angles and pressure.

(b) Contact Angle Optimal blade contact angles of 30° to the tangent on the dry side of the blade, combined with light pressure, ensured uniform ink distribution.

(c) Lack of Back-Up Blade Unlike traditional systems, reverse-angle doctor blade assemblies didn't require a back-up blade, preventing ink buildup and uneven surfaces.

Modern Reverse-Angle Doctor Blades

Today, most reverse-angle doctor blades are made of Swedish blue steel for presses up to 36" printing width.

For wider presses and corrugated printing, materials like high-density polyethylene or polyester plastic blades are used.

Blades are typically ground and honed at a 45° angle on the "shaving" surface.

Conclusion In the realm of flexographic printing , reverse-angle doctor blade systems have transformed the industry.

These systems, introduced in the 1960s, efficiently distribute ink on anilox rolls, offering numerous advantages.

They provide uniform ink films, maintain print quality at high speeds, and find applications in various printing tasks.

As technology advances, reverse-angle doctor blades continue to shape the printing industry's future.