The primary purposes of doctoring are the cleaning of the cylinders of the machine and the paper take off in some crucial positions, to avoid paper wrapping. The most commonly known tool, the blade, is manufactured in composite or metal materials and must have mechanical and wear resistance characteristics such as to find the right compromise between life of the blade itself and the safeguard of the cylinder on which it works.
Despite a rather fragmented offer in the world, the supply of quality blades is now the prerogative of a very limited number of suppliers such as Bonetti, who must not limit itself to supplying a “blade in a box”, but must offer customers a complete service that starts with advice on the best materials to use and the best solutions to adopt and ends with the maintenance of the complete doctoring system.
Bonetti has been successfully supplying the pulp and paper industry since more than 50 years
Established in Milano, Italy in 1923, Bonetti S.p.A has favored since the 1980s the use of blades made of composite materials – mainly glass fibers, carbon fibers and combinations of the two – which guarantee mechanical characteristics often comparable to those of metallic blades which were the only available ones at that time. Today, the full product line of the Bonetti blades include all the qualities and materials currently used in modern paper making.
A good example of Bonetti’s research for the best is the Leopard blade that was developed from a 100% carbon material, adding special additives in the resins resulting in a blade with considerable rigidity in cross direction and wear resistance superior to other materials on the market.
The success achieved on the central presses and first driers of many paper machines in the world market paid off Bonetti’s investments in research and development.
To achieve a better cleaning of the dryer cans over the years Bonetti has developed doctor blades in fiberglass – BR Abrasive – with abrasive additives that minimize the risk of sparking on dryers: these blades help keep the cylinders clean with persistent stickies and are appreciated by the most demanding paper makers.
Choosing the right blade, however, is only the first step for an efficient doctoring action: the blade is in fact the part of the doctoring system in contact with the cylinder and it is often required performance that is impossible to achieve if the other components are not in full efficiency. First of all, the doctor body or beam must be designed and built with the care and experience of a doctor specialist because it must take into account all the elements involved, such as cylinder width and diameter, available space, frame configuration, speed and frequency of the machine, etc: each of these factors differs from machine to machine and requires the attention of an engineering doctor specialist to ensure the minimum possible deflection and the lack of vibrations to hold the blade under load in the correct position and in straight line. Machining the nose bar of the doctor body in its working position is essential to guarantee an optimal result, but at the same time makes it clear how the use of the beam in a position other than that for which it was designed is highly inadvisable.
Bonetti’s success has always been based on a timely response to the needs and demands of the markets it serves
Beams in painted carbon steel or in 316L stainless steel can be single, double or twin. Bonetti doctors have a box-like shape and never an angular one to better guarantee the solidity of the structure. The design of some systems may include a tray, designed to collect debris out of internal wire rolls, upper rolls in the press section, suction rolls. Some of these trays in the press section are heated to eliminate condensation. For these and other reasons Bonetti does not use standard shapes, but designs and manufactures the doctor bodies that are tailor made for each specific position of the paper machine.
While the rigid blade holder such as Bonfirm or Bonfirm Super requires only perfect profiling (often neglected in the dryers of older machines), the flexible blade holder, in order to keep its superior advantages, must be carefully maintained by taking care of periodic cleaning and alignment to avoid sheet skipping or other doctoring problems.
In the drier section these can be caused by thermal movement when steam is injected, at the breast roll when wire is tensioned, in a press section when the nip pressure and / or a steam box profiling is applied, on calender rolls when caliper control is used, etc . Flexible or self profiling holders like Bonetti’s Bonfiflex Super are needed in these cases to ensure a correct profiling during operation, without any manual adjustment. Paper makers have lately started enjoying Bonetti’s retractable holders which have proven to be needful to solve access problems in areas like presses or dryer cans where maintenance and cleaning are often physically impossible. Bonetti strongly supports its customers with surveys that can check that the working angle is that of the project on the entire width of the roll and confirm the overall good functioning of the doctoring system.
On flexible blade holders, the hoses must be replaced with the frequency indicated by the manufacturer: they must not get hard and must not lose air pressure to ensure uniform load of the blade over its length. For this reason Bonetti offers its Bon70, Bon 140 and Bon 250 tubes, each with a temperature resistance suitable for the different positions of the paper machine. The oscillator is also an essential component of the doctoring system: stationary doctors cannot guarantee the result of cutting the material (sheet or dirt) off the roll face, nor to avoid hard particles to stick into the blade tip and cause lines of dirt or even damage the surface of the cylinder.
Electromechanical oscillators equipped with an eccentric to transform the rotary motion into linear motion are the most common ones. Their maintenance is often neglected and frequently one can find stuck oscillators with burnt motors or worn eccentrics.
On the strength of these needs Bonetti has developed a pneumatic oscillator, the BonMatic, that only requires a single air supply line and no external control panel. In both the light and heavy duty versions, it offers considerable thrust, able to satisfy the oscillation needs of the most demanding doctoring and creping systems. Papermakers do not have to worry about the air consumption which is minimal even if the BonMatic is used on each doctor of the whole dryer section.
Ease of use, compact dimensions and minimal maintenance make the BonMatic one of the most successful components supplied by Bonetti, which continually satisfies requests to replace old oscillators with these new generation ones.
Bonetti mainly uses self-aligning ball bearings that are produced in the factory north of Milan: with just a simple periodic greasing, they offer unparalleled durability and precision, so much so that they are adopted as a standard by major OEMs. Lubrication often helps the cleaning action and surely extends the life of the doctor blades.
Wherever possible Bonetti suggests the use of showers which have successfully entered the Bonetti product line after a long development in the field with the help of papermakers. Bonetti’s shower solutions can be equipped with a cleaning brush and the Hi pressure ones are oscillated with BonSync, an electronic drive which, through its control panel, allows the finest adjustments. In conclusion, the above confirms the importance of relying on a specialist to guarantee the best possible doctoring: the blade by itself cannot solve all the problems of cleaning the cylinders or detaching the paper by itself if all the components of the system are not in perfect conditions. It is clear now how essential is to use blade suppliers able to suggest the best solutions:
Companies like Bonetti which boasts decades of experience working with paper mills and paper machine manufacturers all over the world.