BRUSSELS, Nov 1, 2014 (PPI Magazine) - Doctoring is an essential function in papermaking often overlooked. Doctoring systems include three major components: the doctor back, doctor holder, and doctor blade. Together, these components are used to remove excess water and contaminants from the roll surface. In some positions, doctors are used to deflect the web from the roll in a controlled manner. Because a doctoring failure in a critical position can cause significant down time and irreparable damage to roll surfaces and dryer fabrics, properly designed, adjusted, and maintained doctors using the right combination of blade material, application angle, and pressure can significantly enhance machine efficiency and operating costs.
Fig. 1 - Before: Brand X doctor blade after two days; Fig. 2 - After: Kadant blade after two days
Throughout the history of modern papermaking, significant advances have been made in roll cover and press and dryer fabric materials. Likewise, the materials used to make doctor blades have also evolved over the past century to reflect other technology advance in papermaking. As one of the pioneers in doctoring systems, Kadant's rich history includes doctoring innovations from Vickery and Lodding that have become known as the "gold standard" for doctor blade materials and performance.
In the 1930s, Vickery led the market with the introduction of composite materials to replace metals which were no longer appropriate as speeds increased and new roll surface materials and finishes were introduced.
The consistent demand for new and better-performing doctoring materials has accelerated in recent years as machine speeds pushed higher and recycled fiber increased in popularity and usage worldwide.
(ABOVE: Fig. 3 - The leading edge must be strong to ensure maximum performance and operating life)
The recent product introduction of Kadant's latest doctoring development shifts from a conventional product upgrade to a step-change in doctor blade materials featuring patented and patent-pending nanotechnology-enhanced resins applied to a range of doctor blades.
The nanotechnology enhanced resins allow the composite resin matrix to be manipulated on a molecular scale to produce high-performing, wear-resistant doctor blades. According to Michael Draper, research and development manager at Kadant UK, "The results from an extensive range of tough, arduous mill trials on press rolls and high-temperature calender, drying cylinders, and early dryers in Asia, Europe, and North America alongside our own laboratory testing have exceeded our expectations with respect to nano-blade performance and operating characteristics."
Kadant's latest nanotechnology-enhanced doctor blades have been shown to deliver exceptional results in field trials on many previously "notoriously difficult to doctor" applications, including heavily-contaminated lead-in dryers and hard-rubber rolls. These advanced products are characterized by increased stiffness, increased resistance to chipping, improved inter-laminar bonding, and a lower coefficient of friction compared to traditional doctor blades. As a result, roll surface cleaning is improved and doctor blade life has been shown to increase significantly compared to conventional doctor blades. For the papermaker, this means longer blade life, fewer blade changes, and the potential for reduced operating costs while enhancing overall machine efficiency.
Case study: Center press roll application
At one mill trial in Europe, the center press roll doctor featured a dual-beam doctoring system and was underperforming in a critical position. Frequent blade changes and the associated downtime led the mill to seek an alternative doctoring solution that would reduce maintenance downtime and the frequency of blade changes.
To address the performance issue, a Kadant nanotechnology-enhanced doctor blade was installed in the first position and a SynTek 5 PlusTM doctor blade in the second position. While other materials failed to reach the desired 14-day run time on this high-speed modern machine, this blade combination enabled the mill to successfully achieve the 14-day run time target on the first attempt.
Fig. 4 - A poor leading edge and a large amount of pitchmark a competitor's blade; Fig. 5 - Kadant's nanotechnology enhanced blade still has a strong leading edge and no visible wear
Because of the new blade material, the increased blade life led to a 30% reduction in procurement costs and provided a solid platform for the mill to attain its internal cost saving target earlier than expected. In addition, the subsequent increased uptime between maintenance shutdowns helped the mill to increase production efficiencies and annual tonnage.
Case study: Drying cylinder application
A high level of contamination in the early dryers were negatively affecting production due to excessively short blade life and poor performance. Even with blade changes every other day, the contamination issues continued to create runnability issues. Following an assessment of the operating conditions and contamination problems, Kadant recommended and installed the nanotechnology-enhanced blade on these early dryers.
The performance life of the nanotechnology-enhanced blade was more than 10 times that of the conventional blade and after 20 days, no edge wear was visible on the nanotechnology-enhanced blade. The mill has installed additional nanotechnology-enhanced blades and found a dramatic decrease in the frequency of blade changes with a corresponding increase in machine runnability. In addition, heat transfer and product quality is expected to increase because of the cleaner cylinder. Figures 1-3 illustrate the before and after conditions of the doctor blade and cylinder cleanliness.
Because a doctor blade's cleaning effectiveness depends on a strong leading edge, special attention was given to the new nanotechnology-enhanced resin matrix to ensure maximum longevity. Figures 4-5 show the difference between the leading edge of the Kadant nanotechnology-enhanced blade and a competitor blade after both had run on the same position. Note the poor leading edge and considerable amount of pitch "passing" on the back of the competitor's blade. The nanotechnology-enhanced blade, after running in the same position, shows a strong leading edge, light dusting, and no visible wear.
Case study: Unirun chrome dryer felt return roll application
A linerboard mill in China using locally-sourced OCC was experiencing a high failure rate and poor operating life of its conventional doctor blades. The highly abrasive nature of recycled furnish and stickies on this aggressive application was limiting blade life to two days. The mill had excessive downtime due to forced and repeated doctor blade changes. Examples of the damage experienced by conventional composite doctor blades after several hours of operation are shown in Figs. 6-7.
A variety of composite doctor blades were offered by multiple doctor blade suppliers to improve the doctoring performance on this position. However, the aggressive nature of the application proved too much for composite blades and something more was needed.
The nanotechnology-enhanced blade was installed on the same position. After three days of running against the chrome dryer felt return roll there were no visible signs of wear or delamination. The mill replaced its composite blades with the new nanotechnology-enhanced blade and benefited from reduced downtime attributed to fewer blade changes.
(ABOVE: Fig. 6 - Stock penetration into conventional blade structure)
Nanotechnology - A radical innovation
As illustrated in these case studies, the nanotechnology-enhanced doctor blades have introduced a step change in blade performance and are now setting a new bar for blade performance. Not since the early 1900s when composite blade materials were introduced to address new roll surface materials and finishes has there been a radical innovation in blade material to cost-effectively address contaminants removal and machine runnability.
As a new standard in doctor blade performance is being established using nanotechnology, paper mills around the world are benefiting. Cylinder cleanliness, machine runnability, heat transfer, and product quality are all being affected in various ways, and papermaking operations are finding a new source of value from their doctoring system.
Fig. 7 - Severe damage caused by stickies
For more information on Kadant's latest doctoring solutions, visit www.kadant.com/nano-blade. Or contact your local Kadant doctoring specialist today and ask how your machine could benefit from nanotechnology-enhanced doctor blades.
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