Shape-optimised doctor blade for consistently stable line pressure

Line pressure is the physical force with which the doctor blade and anilox roller as well as the gravure cylinder are pressed against each other at their point of contact. Keeping this contact pressure constant during print production is difficult with most of today’s doctor blade solutions and the resulting direct and indirect printing problems are numerous.

The TKM Group is one of the world’s leading suppliers of doctor blade solutions for a wide range of industrial applications. Extensive doctor blade analyses carried out as part of troubleshooting at international customers in package printing have identified specific challenges in flexo and gravure printing. Depending on the requirements of the respective print jobs, the following focal points were identified:

Problem areas in flexo printing:

• Ink mist/ink spitting
• Doctor blade wear
• Wear on the ceramic surface of anilox rollers
• Scoring lines
• Dot gain
• Back-doctoring

Problem areas in gravure printing:

• Dot gain
• Hazing / Toning
• Breakage of the lamella
• Blade crack-outs at the cylinder edge
• Cylinder wear / chromium damage
• Doctor blade wear
• Streaking / Pinlines

What is the main cause of these printing problems?

Line pressure refers to the physical pressure that occurs between the contact surface of the doctor blade and the gravure cylinder or anilox roller. It is transferred to the cylinder or roller surface by the doctor blade and cannot usually be kept constant during print production. The most common types of doctor blade are the thin-ground blade and the wedge doctor blade, whereby both types have specific strengths and weaknesses.

Thin-ground doctor blades

The blades of the thin-ground doctor blade are ground in parallel. This offers the advantage of a constant doctor blade contact zone, so that the tonal value remains unchanged throughout the entire service life. The originally set angle remains constant until the entire wear area has been ground down.

Despite its strengths, the thin-ground lamella has some serious weaknesses. One of these is its susceptibility to deflection, which results in inconsistent line printing. This characteristic requires continuous monitoring and possibly adjustments to the doctor blade pressure during ongoing print production. In addition, there is a risk of chipping in the lamella area, which can lead to massive damage to the ceramic surface of anilox rollers and the chrome surface of gravure printing plates in particular. In both cases, this leads to streaking in the print.

Wedge doctor blades

The wedge doctor blade is characterised by increased rigidity compared to the lamella doctor blade, making it a robust option for various print applications.

However, this strength also comes with some weaknesses. With this doctor blade, the line pressure decreases the longer the service life or the duration of the blade application. The physical pressure exerted on the blade remains the same, but the contact zone area increases over the course of use. Such a varying application of force can lead to inconsistent printing behaviour.

The reduction in line pressure can also result in the ink “letting through” (toning) and is usually also the point at which the doctor blade needs to be sharpened or replaced. This means that the print quality is subject to tonal value fluctuations during the print run, which do not occur with thin-ground doctor blades, as the contact zone does not change during the entire wear period.

Compared to the thin-ground doctor blade, the wedge doctor blade requires a higher line pressure to ensure efficient doctoring of the roller and cylinder surface and therefore perfect ink transfer. However, this in turn increases wear on the doctor blade, roller and cylinder.

Conclusion

The line pressure of the doctor blade has a direct influence on the tonal value behaviour and thus on the print and coating quality of the respective products. It is not possible to achieve a constant line pressure with the doctor blades available on the market. This means that constant readjustment by increasing the blade pressure during the printing process is usually necessary, which in turn depends heavily on the experience and dexterity of the machine operator.

Thomas Störte, Managing Director of TKM Meyer GmbH, comments: “Increased doctor blade pressure results in various undesirable effects, including deflection of the doctor blade and a flatter angular alignment of the contact zones, chipping of the lamella, increased doctor blade wear and increased wear of  gravurecylinders and anilox rollers. These effects can not only impair the quality of the print result, but also lead to higher operating costs and production downtime.”

Solution in sight?

As part of a three-year research project, the TKM Group has developed a shape-optimised lamella design in close cooperation with the design department at the University of Wuppertal. It aims to ensure a constant line pressure:

1. Reduced blade pressure: The optimised shape of the blade significantly reduces the blade pressure. This results in less blade deflection and reduced blade wear, which in turn increases the service life of the printing and application cylinders.
2. Continuous line printing: The new blade ensures stable line printing over the entire length of the print job without the machine operator having to readjust the doctor blade pressure. This increases production reliability, reduces susceptibility to human error and ensures consistent printing and coating behaviour.

The shape-optimised doctor blade developed by the TKM Group represents a significant advance in printing technology that will considerably increase stability and efficiency as well as quality in gravure and flexo. “The first industrial field trials are showing exceptionally good results. We will be launching a finished product range of these doctor blades at drupa 2024,” says Thomas Störte, Managing Director of TKM Meyer GmbH.

A „razor-sharp“ company

The “sharply grinded” success of the TKM Group began in 1908 with the introduction of the production of machine knives and the establishment of the company site in Remscheid. Today, the TKM Group consists of 12 individual companies in 10 countries in Europe, America and Asia as well as numerous sales and service companies and employs around 750 people worldwide. The group operates its own production sites in Germany, Austria, the Netherlands and Finland. The products are distributed by sales staff, subsidiaries and agencies. The product portfolio includes machine knives, doctor blades, saws, machine elements and spare parts for printing machines for applications in areas such as the paper, wood, plastics, rubber/tyre and recycling industries.

TKM also offers a wide range of services for knives and tools. This includes regrinding services, all repair work, re-tipping, special coatings for a longer service life and technical seminars or cutting symposia, in which the many parameters that are decisive for an optimum cut are discussed in detail.

TKM is part of the Groz-Beckert Group, a market leader in the development, manufacture and sale of process-critical precision textile tools, industrial cutting solutions and solution providers for textile construction and reinforcement solutions made of high-performance technical fibres for composite materials.