Fine-tuning of doctor blade use in changing printing processes

As markets evolve and customer requirements change, printing processes adapt and develop. For example as packaging trends go towards smaller and more diversified packages, printing runs have become smaller. As a result, narrow web and label printing have been on the increase. Changing requirements on image quality, process efficiency and environmental impact have brought about changes in machine design, type of ink and new challenges in fine-tuning of the printing process. Studying the contact angles and surfaces of used doctor blades from the process can give many clues in this task of fine-tuning. Some examples are given below. The study of a worn doctor blade including its contact surface reveals much information about the way the blade has been used in the printing machine. The contact surface is the worn surface of the doctor blade that has been in contact with the gravure/anilox cylinder. Parameters such as blade pressure, blade alignment, condition of the ink and of the cylinder can often be revealed by a study of the worn blade tip including its contact surface. Wiping of gravure/anilox cylinder During printing, blade pressure is often increased incrementally in the effort of maintaining a clean wipe of the gravure/anilox cylinder. In rotogravure, high blade pressure and/or low contact angle will change ink transfer and decrease cylinder life. Operators often use higher pressure and/or a lower contact angle to increase density when more ink is needed, due to e.g. cylinder roughness and polishing pattern, chrome hardness, wrong lamella dimension etc.; what they get is more process problems and lower productivity. The optimal contact angle for rotogravure is 60°; this might not be possible because of cylinder engraving, ink quality or other factors, however, operators should always print with as high a contact angle as possible. n 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 re-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. Figure 1(BELOW): Used doctor blade from flexo-machine with multiple contact surfaces and sliver formation. The starting dimensions of the blade in Figure 1 were: 40 x 0,25mm radius edge (no lamella or bevel). To improve 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. If consistent blade alignment cannot be guaranteed, then it is recommended to change the doctor blade for each new demanding job, to avoid multiple contact surfaces. Used coated doctor blade from rotogravure Hard particles in the system If there are hard particles in the ink system, this will show up as defects in the blade contact surface. Some sources can be remains of polishing media or incomplete polishing of a new gravure/anilox cylinder and the absence of a magnet and filter in the ink system. Contact surface damaged by hard particle Adhesive coating and high blade pressure Is 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 degree 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 along its 1700mm length. In this case, a maintenance check of the blade holder can be recommended, as well as ensuring a higher and constant contact angle and good parallelity between blade and cylinder. Metallic inks Metallic inks on labels are being used as alternatives 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, for achieving good printing results and trouble-free contact surfaces. UV ink spitting The use of UV ink has been on the rise, especially in label printing, due to advantages including its 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 seen being used in some rotogravure machines but most appear in flexographic printing machines. UV ink has a relatively high viscosity, sometimes more than five times higher, than that of conventional water-based and solvent-based inks, and this causes different behaviour in the printing process. For example, UV ink spitting is a phenomenon occurring in many different types of machines. 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 picture. UV ink spitting has 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 different suggestions have been made by blade suppliers to solve the spitting 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 thickness 0,15mm or 0,20mm. Since UV-inks have a higher solids content compared with conventional water-based and solvent-based inks, there will be higher forces in the moving UV-ink which can cause chatter. When running at lower speeds such as 50m/min 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 0,25mm or 0,30mm are needed to handle the higher ink forces to decrease spitting. A normal lamella design for flexo such as 1,2mm/0,10mm 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/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 can give a clue to the problem: 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. Tree-barking When used flexo blades have ‘normal-looking’ contact surfaces, while at the same time parallel dark line patterns appear on the used anilox cylinder,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 paper substrate. The solution to this problem involves a use of magnets and filters in the ink system as well as, adjustment of the ink composition to avoid the binding reaction between components in the ink and in the paper. Tree-barking pattern on anilox cylinder New anilox engraving designs Over the years, anilox providers have introduced different types of engraving, generating increasing demands on doctor blades to perform clean wiping independent of line density, cell volume and printing speed. This reveals patterns from different types of anilox engraving. It also shows the contact surface of doctor blades which have been wiping aniloxes with similar line-density but different engraving pattern—a conventional 60-degree hexagonal in a channel-pattern anilox in. The varying patterns on doctor blade contact surfaces resulting from these different anilox engraving patterns are an extra parameter for users to optimise when choosing blade quality and dimension. Using incorrect quality and dimension will shorten the life of the anilox cylinder, which is very costly. The examples given above are intended to show how continuing developments and changes in printing processes bring along new challenges, and that these require re-questioning and re-thinking of former solutions. For example, a change in ink, gravure/anilox cylinder, substrate, printing speed, job length or even machine age can bring forth 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 do testing together with blade suppliers to fine-tune doctor blade selection and use for the highest possible productivity at the lowest possible total cost. For more information contact Barkev Graphics on Tel: 087 353 9740 or visit www.barkev.co.za