Selecting the Right Doctor Blade for Your Paper Machine
Choosing the right doctor blade for a paper machine application can be a daunting task. There are numerous suppliers with extensive product ranges, often making the choice difficult. Here are some tips for selecting a doctor blade supplier, finding the right doctor blade for your application, and optimizing the doctor blade runnability.
Choosing a Doctor Blade Supplier
Doctor blade suppliers should:
- Be a partner.
- Be knowledgeable about your paper machine, doctor blade holder, and application needs.
- Have experience across various machine types with varying doctor systems.
A supplier with these traits can better assist you in selecting the right doctor blade for your application.
How to Choose the Right Blade
Once you have a supportive doctor blade supplier, there are tips for determining the best doctor blade for your application.
- Determine the position needs for the application. Doctor blades typically serve multiple purposes such as removing water, cleaning contaminants off the roll, shedding the sheet, or conditioning the roll and maintaining proper surface roughness.
- Choose the appropriate material. Consider a blade material that leads to extended blade life, higher temperature resistance, lower friction, higher machine direction stiffness, and/or more cross-machine direction conformability.
- Optimize doctor blade runnability. Additionally, the blade angle and loading may be adjusted, and showering or oscillation may be introduced to provide better doctoring performance.
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 the quality of paper produced, so it is critical to select your blade supplier and blade material carefully for optimal performance.
