Reservoir chamber from Tresu

 

The doctor blade performs a vital function in the ink transfer process as it ensures only the correct amount of ink reaches the plate from the anilox roller. On wide web presses, chambered doctor blade systems seal in the ink and these are now also increasingly used on narrow web inline presses. By David Atkinson.

Chambered doctor blade systems were initially developed to enable reverse angle doctor blades to meter the ink on the inboard as well as the outboard units of satellite CI presses in an effective and accessible manner. It was soon recognised that they offered a number of additional benefits such as the ability to run a press at high speed without the risk of ink fly, a reduced volume of ink required to prime the print unit and, being a sealed system, they reduced evaporation from solvent and water-based inks thus offering better colour control. They are now fitted as standard to all CI presses and the press manufacturers have developed auto wash systems to work in conjunction with them to minimise down time.

The chamber is sealed by having two doctor blades, one on either side, in conjunction with two end seals. One of the blades is the ink metering blade while the other is the sealing blade. Most printers tend to use the same blade material on the metering and the sealing side of the chamber, but it is possible to use two different grades of blade.

 

Expanding use of chambers

Until fairly recently, chambers were not fitted to narrow web inline presses because the running speed of these presses did not warrant their use. Instead the ink was typically metered by a single reverse angle doctor blade in a holder. However, with the increased speed capability of the latest generation of narrow and mid web inline presses, this situation is changing, and an increasing number of these presses are now fitted with chambered doctor blade systems. 

On a narrow or mid web inline press, all the anilox rollers rotate in the same direction and as there is no issue of evaporation with UV cured inks, auto wash units tend not to be used with these machines. As a result, it is not necessary for the chamber to be a sealed unit and this offers greater scope for the design of the chamber.

Delpro Ltd, for example, has developed a hopper fed carbon fibre chamber for short run narrow web applications and Tresu A/S has developed the FlexiPrint Reservoir Chamber Doctor Blade system for use with water-based and UV cured inks. This has a ‘closed cassette’ design, no moving parts and also works without a pump. Ink levels can be topped up manually when the ink level has seen to have visually fallen, or alternatively, a float can be used to open a valve to replenish the system, a bit like a household header tank. Allison Systems Corporation offers a reverse angle blade system and also specialises in custom designed chamber systems that can be retrofitted to presses.

It is possible to fit standard chambers with a pumped ink supply, but with UV cured inks and in the absence of an auto wash system, press downtime is likely to increase during job changes as more clean up is required.

 

Chamber designs

It is advantageous to build chambers for use on wide web presses out of lightweight materials to facilitate removal from the press by a single operator. At the same time, they need to be rigid enough not to flex when in operation, and the material from which they are constructed needs to be resistant to the inks and coatings they come into contact with.

Delpro produced the first carbon fibre chambered doctor blade a number of years ago and this material has proved to be a good choice for constructing the body of the chamber as it fulfils all the above requirements. Carbon fibre has significantly greater stiffness and rigidity than aluminium, a material that used to be used extensively for the chamber body. It is also 70% lighter and does not suffer from corrosion with water-based or solvent-based inks or coatings.

Absolute Engineering Ltd also produces woven carbon fibre chambered doctor blade systems for a range of applications, and the company states that it produces both the widest and fastest production speeds of any chambers available. These chambers have achieved significant penetration into the high quality post print market, and as a result, Bobst has selected the company as its supply partner for woven carbon fibre doctor blade systems. By working together, the two companies have produced a range of chambers that are interchangeable with those on the Masterflex post printers and incorporate the Bobst patented tool free rapid blade change facility.

Stainless steel offers similar benefits with regard to weight, rigidity and resistance to inks and is a good alternative to carbon fibre for manufacturing the chamber body. With its Nova XLS system, AkeBoose Graphic Products AB offers an all stainless steel option for all flexo applications. These chambers have an asymmetric design with optimised angles for both the ink metering and the sealing blade and there are seven chamber profiles available. This means that the profile can be chosen according to the diameter of the anilox roller so as to achieve the optimum blade angles.  

Another important aspect of chamber design is the speed with which the blades and seals can be changed and most chambers now employ various tool-less methods for changing both these components, the method depending on the manufacturer.

 

Doctor blades

Doctor blades are available in a number of different materials, edge profiles, thicknesses and width so as to suit the chamber design and meet the demands of various applications. Blade materials can normally be bought either on a roll or as cut lengths and are available from specialist blade manufacturers and suppliers such as Swedev, MDC Max Daetwyler, Allison Systems Corporation and PrimeBlade. It is also possible to obtain blades from chamber manufacturers and general flexo suppliers.

The ink metering blade runs in contact with the ceramic surface of the anilox roller which results in wear to the blade, the rate of which is dependent on the anilox engraving, the nature of the ink, the type of blade material and the pressure applied to the blade. Under some conditions, blades can wear very rapidly, which results in the consumption of a lot of doctor blade material and also a lot of blade changes thereby increasing downtime on the press. As a result, most printers are interested in blades that offer greater longevity and the suppliers have put a lot of effort into developing blades with enhanced wear properties.

Swedev AB supplies the SwedCut brand of blades, which covers a wide range of blades made from composite materials and steel. To meet the requirements of printers with demanding applications, the company has developed a patented surface coating to reduce friction and increase blade life.

The PrimeBlade AB range of blades consists predominantly of steel blades made from a number of different types of steel each of which has different benefits to offer. For extended blade life, the company has developed the PrimeBlade 900 Nano blade. This is not a coated blade but a patented metallurgic-treated steel blade that has been produced by using the latest Nano technology.

As a result of developing a number of blades for demanding applications, MDC Max Daetwyler has a very large range of predominantly steel blades. Its Gold Star Plus blade has a special coating that reduces friction and wear at the point of contact and improves blade life. In order to reduce the risk of anilox roller scoring, the company introduced a soft blade, which is made of carbon steel with a corrosion resistant coating, into the flexo market and says that the softer point of contact has greatly reduced the incidence of score lines.

Allison Systems Corporation supplies blades made from a number of materials, including various steels, plastic and composites, as well as chamber end seals and complete chambered blade systems. For extreme wear properties, ceramic coated blades are available from Rolf Meyer GmbH under the name of RMB Duroblade. These ceramic coated blades have a lifespan 10 times or more greater than a standard blade but do raise the question ‘if the blade is not wearing then does that mean that the anilox is?’

 

Seals

Seals are a critical component of a chambered doctor blade system. Printers often run the doctor blades under too much pressure because they have forced on the chamber to prevent ink leaking from the end seals. This causes premature blade wear, increases anilox wear, can compromise ink metering and increases the risk of surface damage to the anilox roller. With a good chamber design and perfectly fitting end seals it should be possible to use the bare minimum pressure when setting up the chamber.

Even with well fitting end seals, if there is no lubrication, this can cause the seals to drag on the surface of the anilox roller resulting in leaks. If a smooth area of unengraved ceramic is left at the point where the seal contacts the anilox, friction between the seal and the anilox can be minimised. It is also possible to use specially impregnated seals such as those supplied by Print-Tech Solutions. The impregnated material acts as both a sealant and a lubricant that reduces drag from the surface of the anilox roller.

An alternative to working with impregnated seals, and recommended by some chamber manufacturers, is to grease the seals at the end of the chamber. If this approach is used it is important to ensure that no grease enters the chamber and mixes with the ink.

 

Retrofits

Chambered doctor blade systems can be customised fairly easily for most flexo presses and applications. This makes it possible to upgrade the chambers on existing equipment by retrofitting the latest chambered doctor blade systems and thus capitalising on the benefits they have to offer with regard to ease of operation, cleaner running and enhanced ink metering.