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.
Fine-tuning of doctor blade use
in changing printing processes
T
he 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. 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 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.
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 in Figures 2 and 3 respectively.
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.
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. An example is shown in Figure 4. 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.
Adhesive coating 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 degree
doctoring contact angle have resulted in a very fragile tip, which could be ‘ripped
Figure 1: 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).
Figure 2: Used coated doctor blade from flexo.
Figure 3: Used coated doctor blade from rotogravure.
Figure 4: Contact surface damaged by hard particle.
By Debbie Jörgensen Ågren and Håkan Olsson
Printed on 128 gsm Hi-Q Titan Gloss supplied by
I
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