Q.1122: What is doctoring?
Doctoring is an important element in paper machine functionality, sheet quality and productivity. Doctors are basically scraping equipment required in many positions on a paper machine and coater to remove contaminants or water from the rolls and for special functions like creping and coating.
Sheet Removal: The Doctoring system if located and installed properly then it helps in the removal of sheet from the surfaces of cylinders and rolls. The system also assists the machine operators in transporting the paper from Wet-End to Dry-End of the paper machine.
Roll Cleanliness: A correctly designed doctor blade ensures the cleanliness of the rolls and cylinders. If the design is not good enough then stickies/debris might build up on the rolls and transferred to paper resulting in poor quality of paper. If the Doctor is not set up properly i.e. if blade is not touching each and every part of the roll then the roll will not be clean. This is known as doctor missing.
Condition Rolls: For conditioning of the roll a correct doctoring system design is a must i.e. the right selection of holder, blade and other doctor components. Selection of blade material is very important. If a right choice is not made then it might result in damaging the roll e.g. if the roll cover material if soft rubber and a mill uses a metal blade, then the roll will be damaged. On a soft rubber one should use soft blade i.e. synthetic blade. If the selection were right then the blade would ensure roll condition automatically.
Water Removal: This is a part of roll cleanliness, but is mentioned separately because now a days it has become a major function of a doctor. At wet end of paper machine (Wire + press), the water removed from the sheet, stick to the roll. If this water is not scrapped, it will go back to the wire, felt and/or wet web.
Doctoring System:The doctor blade is held in a holder which in turn is mounted to a cross machine beam type structure called doctor back. The doctor back is mounted on journals that are supported by self aligning bearings and custom mounted brackets. Each doctor is loaded against the roll by an air cylinder system, adjustable torque arms or spring loaded rig. Further, an electromagnetic or pneumatic oscillation system provides improvement in performance of doctor. While selecting a unit or a system, the application requirements and the details of the duty should be well understood. The application variables include blade loading, roll temperature, sheet shedding, cleaning requirement, expected life and wear and overall system reliability.
Material: Doctor blades can be metallic, composite or of plastic material. The common metallic blades are bronze, steel and Monel. The bronze blades find use in burnishing of new dryers and the steel blades are mainly used for intermediate dryer doctor applications. The composite blades could be made of fibreglass, carbon fibre or a combination. Abrasive blades containing silicon carbide are used when dryer surface is dirty. Plastic blades do not carry fibre reinforcement and due to temperature limitation are not used on paper dryers. At high speeds, steel blades have spark risk and hence the composite blades find more acceptance