The coating revolution: high-performance blades just got even better

In 1985, BTG revolutionized coating technology with the launch of Duroblade®, the world’s first ceramic-tipped coating blade. Now, with today’s highly competitive global markets spurring further innovation, a range of new high-performance blade materials and advanced blade designs is giving mills a wider choice of specialist blades for a growing range of applications.

Micro-lines caused by traditional high performance blades
Figure 1:
Micro-lines caused by traditional high-performance blades

In the twenty years since BTG’s Duroblade transformed the coating process, efforts to further enhance blade performance were mainly focused on ways of improving the base material (aluminium oxide) – for example, techniques to eliminate surface roughness to create a smoother finished result.

While these improved versions of the original Duroblade proved popular with mills because of their much longer lifetimes and ability to deliver stable output quality, they still had some limitations for certain applications – for example, problems of micro-lines caused by the surface of the blade tip (see Figure 1). For fine paper, these micro-lines tended to ‘float out’ in the drying phase after the coaters, becoming invisible after reeling. But for applications like coating board in top coaters, micro-lines could mar the end result to a point where output was unacceptable to mills and their end customers, the print shops.

Over the last three to four years, that’s all changed, thanks to ongoing research and development efforts by BTG and others. The emergence of new high-tech materials such as chromium-based ceramics and metal- and elastomer-based materials, along with advanced blade designs like the double-bevel blade, are opening up new niches for high-performance blades.

Simple changes to blade material and modifications to tip geometry can translate into major improvements in key areas, including:

Today’s mills can now choose from a wide range of high-performance blade types to match their precise process needs and quality criteria, from board to fine paper and LWC. That’s not only helping improve their bottom line through longer blade lifetimes, reduced broke and lower energy costs, it’s also improving their competitiveness by assuring better, more consistent output quality and more flexibility over the type of papers they can produce.

New high-performance materials

Chromium-based ceramics

Chromium oxide ceramic blade tip
Figure 2:
Chromium oxide ceramic blade tip

A ceramic that’s based on chromium oxide instead of traditional aluminium oxide, the right blend of this material produces an even more homogeneous, more resistant blade tip. Chromium-oxide-based ceramics have opened up new markets for high-performance blades in mills with fast machines running under abrasive conditions, where speeds are high and coatweights low.

To give just one example, LWC single-coated is a grade long considered unsuitable for ordinary high-performance blades because of the high wear-rate it imposed on aluminium oxide blade tips. The lower wear rate of ultra-tough chromium-based ceramics has turned that around, making the use of long-life high-performance blades economically viable. Combined with double-bevel technology, chromium-oxide-tipped blades keep their original geometry much longer than traditional blades, ensuring stable quality over time.

Metal-based materials

Aluminium oxide ceramic
Figure 4:
Aluminium oxide ceramic

Metal-based blade tip
Figure 3:
Metal-based blade tip

Advanced production techniques for this family of metal-based blades creates a blade tip surface that’s smooth and homogeneous. The main benefit is better paper gloss, with the low surface roughness and even wear resulting in lower friction between blade and paper, creating a smoother flow under the blade that improves pigment orientation. Compared with aluminium oxide ceramic blades, these advanced metal-based blades also help minimize streaks and scratches on almost all coaters – although they’re not suitable for applications that produce dry friction on machines running at high speeds.

 

Metal carbides
Figure 6:
Metal carbides

Chromium oxide ceramic
Figure 5:
Chromium oxide ceramic

A typical configuration for a board machine is the use of a tougher metal compound on the pre-coaters for maximum blade lifetimes, and a finer compound on the top-coater to assure excellent quality with a minimum of streaks and scratches. Tough and highly wear-resistant, these blades keep their original geometry over time, ensuring stable production quality.

A comparison of aluminium oxide ceramics with chromium-based ceramics and new metal-based materials reveals that the surfaces of the newer materials are more dense and homogeneous. The metal carbide surface, in particular, shows a very low level of roughness compared with the ceramic surfaces.

 

Elastomer-based materials

The soft, compressible tip of an elastomer-edged blade
Figure 7:
The soft, compressible tip of an elastomer-edged blade

Comprising a steel strip with an elastomer edge, this highly-resistant blade type delivers the best fiber coverage, with a soft, compressible tip that follows paper topography more precisely than any other kind of blade.

Elastomer-based soft blade tips are highly sensitive to dry friction, and require a good, constant level of lubrication. When kept well-lubricated with coating color, however, they are extremely resistant to wear, even after very long running times.

 

 

Blade configurations

Low angle: bent blade mode
Figure 8:
Low angle: bent blade mode

High angle: stiff blade mode
Figure 9:
High angle: stiff blade mode

Depending on the desired result, coating can be successfully applied in both bent blade mode and stiff blade mode. In bent blade mode, the low angle – e.g. 5° – means the hydrodynamic forces under the blade tip dominate, while in stiff blade mode, the high working angle – up to 50° – makes contact pressure the dominant force. Both modes have advantages and disadvantages, and the papermaker’s choice will largely depend on variables like machine speed and desired coatweight and finish.

Bent blade mode

Bent blade mode
Figure 10:
Bent blade mode

Bent mode is suitable for machines running at up to 1,000m/minute and for high coatweights. Speeds above this tend to create hydrodynamic forces under the tip that are too great to manage with precision, making it difficult to control the coatweight.

Because of the narrow space between the blade and the paper, coating color flow under the blade is high. Turbulence at the blade is low, and the dewatering zone is long. This favors a better alignment of coating pigments parallel to the base paper, giving the pigment particles enough time to correctly orient themselves (see Figure 10). Good pigment orientation in turn delivers an increase in gloss, while fiber coverage is also improved because of the blade’s ability to better follow the structure of the base paper. However, it should be noted that the flexibility of the blade can make profiles harder to control.

Stiff blade mode

Stiff blade mode
Figure 11:
Stiff blade mode

Stiff blade mode is more suitable for faster machines running with lower coatweights. In stiff mode, the dewatering phase is abrupt, the coating color excess is doctored off, and the flow under the blade is lower because of the high blade angle against the web. The resulting turbulence and mixing of coating color at the blade cuts down the time pigment particles have to orient themselves properly, so gloss is normally lower (see Figure 11). Because the blade doesn’t follow the structure of the fibers, fiber coverage is lower, but profile control is easier thanks to less bending and flexibility in the blade.

 

 

The double-bevel concept

The double-bevel approach is a development aimed mainly at papermakers running in stiff mode – or in the grey zone between stiff mode and bent mode – who want to increase the quality or runnability of the coaters, but who cannot run in bent mode. Ensuring the second bevel does its job properly requires a very tough blade material, to prevent the blade wearing out after only a short time on the job.

When running a traditional steel blade in a coater, the wear rate is high. But steel blades are also much more forgiving than high-performance blades. Run-in time is shorter, because the steel blade wears and shapes to conditions quickly, and profiling is easier from the beginning of the run. Because high-performance blades exhibit much lower wear, it’s important that the correct blade tip geometry be defined from the outset.

In addition, most coaters are not perfectly straight in the CD direction. While that doesn’t pose a problem for steel blades, a blade with a much lower wear-rate can cause profile problems, because the blade will run on the heel in certain positions. Grinding a second angle on a high-performance blade effectively resolves this problem, moving the sharp heel further from the paper and substantially reducing the risk of running on the heel.

Along with a shorter run-in time and less sensitivity to the straightness of the coater (and thus a more forgiving performance when not running on completely straight coaters), other benefits of double bevel blades include:

Dewatering zones
Figure 12:
Dewatering zones

The second bevel prolongs the dewatering zone and helps ensure pigment particles orient themselves correctly, improving the gloss finish. In addition, fiber coverage is enhanced, thanks to the increased flow under the blade which allows the coating color to follow the structure of the base paper. Flow is effectively increased without bending the blade too much, while contact pressure is reduced (see Figure 12).

In addition, the risk of scratch/streak problems associated with running in stiff mode is greatly diminished. In bent mode operations, the flow has a tendency to push the impurities under the blade tip. With a double bevel, the wedge between the blade and the paper opens up, providing impurities with a greater chance of passing under the blade, while the increased flow also helps push them under the blade tip.

 

Tangible benefits

Thanks to these revolutionary new materials and the double-bevel concept, papermakers can now choose high-performance blades not merely because of their longer lifetimes, but because of their ability to best meet a range of quality and cost-saving parameters. For example:

As markets globalize, print shop demands will continue to increase. To remain at the leading edge, BTG’s R&D teams will be working to improve their understanding of the physics affecting the blade tip, developing innovative new solutions that deliver even greater levels of flexibility and performance.