Best practice for fine tuning doctor blades
Increased requirements for image quality, process efficiency and environmental impact have encouraged changes in machine design, the type of ink used and have provided new challenges in the fine tuning of the printing process. Studying the contact angles and surfaces of worn doctor blades used in the process can give many clues to the task of fine tuning. Some examples and best practice advices are highlighted in this article.
This article was written by Debbie Jörgensen and Håkan Olsson
The study of a worn doctor blade including its contact surface reveals a lot of information about the way the blade has been used in the printing press. The contact surface is the worn surface of the doctor blade that has been in contact with the gravure or anilox cylinder. Parameters such as blade pressure, blade alignment, condition of the ink and the cylinder can often be revealed by a study of the worn blade tip including its contact surface.
Pressure can be the enemy of the gravure or anilox cylinder
During printing, blade pressure is often increased incrementally in an effort to maintain a clean wipe of the gravure or anilox cylinder. In gravure, high blade pressure and/or a low contact angle will change the ink transfer characteristics and shorten cylinder life. Operators sometimes use higher pressure and/or a lower contact angle to increase density when, in reality, more ink is needed, due to e.g. cylinder roughness and polishing pattern, chrome hardness, wrong lamella dimension etc.; they often end up with more process problems and lower productivity.
The optimal contact angle for gravure is 60°; this might not be possible because of the cylinder engraving, ink quality or other factors, however operators should always print with as high a contact angle as possible. In flexo printing, one common reason for the increase of blade pressure is end seal quality, either too hard or too soft, generating leakage at the edges of the chamber. The optimal angle for the metering side in flexo printing is 30–35°. In older chambers the same angle is used on the containment side as on the metering side. Some newer flexo chambers have angles as low as 12° on the containment side to decrease the risk of back doctoring.
Higher metering blade pressure results in an increasingly large contact surface on the doctor blade, which can result in the entrapment of particles between the blade and cylinder causing streaks. If the same doctor blades are used for multiple jobs with variations in alignment or increase in pressure, multiple contact surfaces and slivers often form, as shown in figure 1. This can generate print quality problems.
To improve the cylinder wipe, it is preferable to start with a lamella tip on the blade to keep a smaller contact surface from the beginning. The smaller the contact surface, the lower the friction and the cleaner the wipe. In cases of extreme requirements on wiping, a coated blade is suggested to lubricate the thin tip to achieve the cleanest wipe possible. Typical contact surfaces of a coated blade from flexo printing and gravure printing are shown in figures 2 and 3 respectively. If consistent blade alignment cannot be guaranteed, then it is recommended that the doctor blade is changed for each new demanding job, to avoid multiple contact surfaces.
Hard particles in the system
If hard particles are found in the ink system, this will show up as defects in the blade contact surface. An example is shown in figure 4. Some sources can be the remains of polishing media or incomplete polishing of a new gravure or anilox cylinder and the absence of a magnet and filter in the ink system.
Adhesive coatings and high blade pressure
Figure 5 shows a blade which has been used with a very low contact angle in a hot adhesive coating unit. The high blade pressure and resulting 12° doctoring contact angle have resulted in a very fragile tip, which could be “ripped away” by the strong adhesive in the coating unit. This blade was also used far past the end of the lamella tip, and the width of the blade varied as much as 4.5 mm (0.18”) along its 1700 mm (67”) length. In this case, a maintenance check of the blade holder would be recommended, as well as ensuring a higher and constant contact angle and good parallelism between the blade and cylinder.
Metallic inks on labels are being used as an alternative to metallised (foil) paperboard. Metallic inks are also used to a degree in security printing and in printed electronics. Lamella blades with an abrasion resistant yet lubricating coating and an abrasion resistant base steel are recommended for these inks, in order to achieve good printing results and trouble free contact surfaces as in figures 2 and 3.
UV ink spitting
The use of UV ink has been on the increase, especially in label printing, due to several advantages including lower environmental impact, contribution to improved process efficiency and image quality including improved rubbing resistance and higher colour density in more intricate graphic designs. These inks are being used in some gravure machines but most appear in flexo printing presses.
UV ink has a relatively high viscosity, sometimes more than five times higher, than conventional water based and solvent based inks which causes different behaviours in the printing process. For example, UV ink spitting is a phenomenon occurring in many different types of presses. This is seen especially in the printing of solids, where “extra dots” appear in the solid areas. This ink spitting also exists in process printing, however it is not detected as easily in the process image. UV ink spitting has, in the past, limited the printing speeds for single blade holders in narrow web, so that chamber systems have been developed in their place, however still with spitting problems. Many suggestions have been made by blade suppliers to solve the problem, such as blades with thicker lamella, longer lamella, double lamella, or even non-lamella, or blades with bevel tips of varying degrees, etc.
However a fundamental fact is that new machines and chambers are often supplied with doctor blades of thicknesses 150 or 200 micron. Since UV inks have a higher solids content compared with conventional water based and solvent based inks, higher forces will occur when moving UV ink which can cause chatter. When running at lower speeds such as 50 m/min (164 fpm) there is often no problem, however as the machine speed increases, the force of the moving ink increases and ink spitting becomes an increasing problem. Sturdier blades of thickness 250 micron or 300 micron are needed to handle the higher ink forces to minimise this problem. A normal lamella design for flexo such as 1.2° /100 micron or a 2° bevel tip will give a smaller contact surface to decrease frictional forces and give good wiping function, as long as the blade body is thick enough.
Good housekeeping resulting in clean blade holders, and end seals with the right amount of sturdiness and flexibility will also help to decrease ink spitting, as will the relatively low surface tension of doctor blades with low friction coatings.
UV ink flowability
The relatively high solids fraction and viscosity of UV ink can sometimes cause printing and cleaning problems due to the lower ink flowability. Finer designs demand finer engravings on the gravure or anilox cylinder, which can result in deeper cells to achieve the correct ink volume. The deeper cells can result in more difficult release of the ink and problems with plugged cells. As an example, the blade contact surface shown in figure 6 can give a clue to the problem seen in figure 7: uneven distribution of ink along the anilox cylinder, causing “bands”, inconsistent printing and uneven wear of the doctoring blade. Fine tuning to overcome this phenomenon involves a better correlation between the ink and the anilox engraving.
When used flexo blades have “normal looking” contact surfaces as in figure 2, while at the same time parallel dark line patterns appear on the used anilox cylinder as in figure 8, then we have clues to a case of “tree barking”, in which similar lines appear in the print. The phenomenon of “tree barking” can arise for example when printing with water based inks on a paper substrate. The solution to this problem involves the use of magnets and filters in the ink system and adjustment of the ink formulation to avoid the binding reaction between components in the ink and the paper.
New anilox engraving designs
Over the years anilox manufacturers have introduced different types of engravings, generating increasing demands on doctor blades to perform clean wiping independent of line density, cell volume and printing speed. Figures 2 and 9 reveal patterns from different types of anilox engraving. The two figures show the contact surface of doctor blades which have been wiping aniloxes with similar line density but different engraving pattern, a conventional 60° hexagonal in figure 2 and a channel pattern anilox in figure 9. The varying patterns on doctor blade contact surfaces resulting from these different anilox engraving patterns is an extra parameter for users to optimise when choosing blade quality and dimension. Using the incorrect quality and dimension will shorten the life of the anilox cylinder, which is very costly.
The examples given in this article are intended to show how continuing developments and changes in printing processes bring new challenges. For example, a change in ink, engraving design of anilox cylinder, substrate, printing speed, job length or even machine age can create new requirements on printing process parameters including doctor blade selection and use. The questioning of obvious past choices can be a good investment, and printers are encouraged to carry out testing together with blade suppliers to fine tune doctor blade selection and use for the highest possible productivity at the lowest possible total cost.