Choosing the right doctor blade for a paper machine application can be a daunting task. The following is a continuation of part one with an explanation of doctor blade materials and how to select a blade based on the paper machine position. 

Doctor Blade Materials Explained

Doctor blade materials have changed significantly over the years. The following examples describe different blade types and their purpose.

Metal blades were some of the first doctor blade materials used, and are still found today in many applications where a tough blade is required to clean a roll surface or where high stiffness is needed to shed a sheet. 

Cotton phenolic blades were the first generation of composite blades and are still used in wet end non-critical applications to remove water. 

Polyethylene blades are typically chosen when there is a very soft roll cover, especially with polyurethane covers. 

Glass epoxy blades are selected when a mill needs a blade with good cleaning action, as the glass fibers allow for excellent roll cleaning, as well as a need for a thin efficient blade. 

The introduction of carbon-enhanced blades serves many purposes including extending the blade life, lowering friction, avoiding damage to roll surfaces with 100% carbon blades, and providing a very stiff blade, especially when the carbon is primarily in the machine direction.  Blades with unidirectional carbon can help provide conformability to any roll surface which may not be uniform.

Numerous wear-resistant and low friction additives can be included in doctor blades to improve blade life and performance.  Additionally, abrasive particles can be added as well to maintain the roll surface and ensure excellent cleaning.

Blade Type Based on Paper Machine Position

Wet end rolls

The key blade attributes for wet end doctoring are water removal and ensuring no roll damage, so typically glass-free, soft blades as thin as possible are optimal. 

Press rolls

When doctoring a press roll, the needs are sheet shedding, long life, and maintaining roll surface roughness, so carbon blades and sometimes ceramic tipped or abrasive blades are chosen. 

Dryer and calender rolls

Contaminant removal, low energy consumption and the ability to resist high temperatures are important for dryer and calender rolls, so glass and carbon blades are typically chosen, along with coated blades. 

The doctor blade can help to ensure the efficiency of a paper machine and quality of paper produced, so it is critical to select your blade supplier and blade material carefully for optimal performance.